Introduction to Polymer Science and Technology - ACS Symposium

2 Introduction to Polymer Science and Technology C H A R L E S E. C A R R A H E R , JR.1, and R A Y M O N D B. S E Y M O U R

2

1Department of Chemistry, Wright State University, Dayton, O H 45435 Polymer Science Department, University of Southern Mississippi, Hattiesburg, MI 39406-0076

Downloaded by UNIV OF MINNESOTA on October 1, 2013 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch002

2

Structure of Polymers K i n e t i c s of Polymerization Property-Molecular Weight Relationships Interchain and Intrachain Forces Crystalline-Amorphous Structures Transitions End Uses of Polymers as Related to Structure Physical Characterization and Testing Educational Aspects Nomenclature

Polymer science and technology are i n t e r d i s c i p l i n a r y i n that they borrow and c o n t r i b u t e to other fields of s c i e n c e . They borrow i n the sense t h a t the laws t h a t s e r v e as the b a s i s of c h e m i s t r y , physics, and engineering are e q u a l l y a p p l i c a b l e to macromolecules. They c o n t r i b u t e i n a similar manner, t h a t is, b a s i c principles formulated within the framework of polymer science and technology are applicable t o c h e m i s t r y and o t h e r disciplines. Thus, the technological p r i n c i p l e s a p p l i c a b l e to the processing of metals are a p p l i c a b l e to the processing of polymers and v i c e versa. Briefly, polymer s c i e n c e is the s c i e n c e t h a t d e a l s w i t h l a r g e molecules wherein the chemical bonds are l a r g e l y covalent. Polymer technology is the practical application of polymer s c i e n c e . The word "polymer" is d e r i v e d from the Greek " p o l y " (many) and "meros" (parts). The word "macromolecule" ("macro" meaning large) i s often used synonymously for polymer, and v i c e versa. Some s c i e n t i s t s tend to d i f f e r e n t i a t e between the two terms with macromolecule being used to describe large molecules such as DNA and proteins that cannot be depicted as b e i n g (exactly) derived from a single, simple (monomeric) u n i t , and polymer is used to describe l a r g e r molecules such as p o l y s t y r e n e t h a t can be d e p i c t e d as being composed of styrene units. This d i f f e r e n t i a t i o n i s not always observed and will not be in this t e x t . The process of forming a polymer is called polymerization. 0097-6156/85/0285-0013S09.75/0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

14

A P P L I E D P O L Y M E R SCIENCE


The d e g r e e o f p o l y m e r i z a t i o n (DP) o r a v e r a g e d e g r e e o f p o l y m e r i z a t i o n (UP) i s the number (or average number) of u n i t s (mers) composing a c h a i n ( s ) . The term " c h a i n l e n g t h " i s used as a synonym f o r DP. The DP of a dimer i s 2, t h a t of a t r i m e r i s 3, t h a t of a tetramer i s 4, e t c . Chains w i t h DP v a l u e s below 10 t o 20 a r e referred to as oligomers (small units) or telemors. Many polymer p r o p e r t i e s are dependent on c h a i n l e n g t h , but f o r most commercial polymers the change i n polymer property with change i n DP i s s m a l l when the DP i s greater than 100.

Structure of Polymers Two terras, c o n f i g u r a t i o n and conformation, are often confused. Configuration refers to arrangements fixed by chemical bonding that cannot be a l t e r e d except by primary bond breakage. Terms such as h e a d - t o - t a i l , d- and c i s , and trans refer to the configuration of a chemical species. Conformation refers to arrangements around s i n g l e primary bonds. Polymers i n s o l u t i o n or i n melts continuously undergo conformational changes. Monomer units i n a growing chain u s u a l l y form what i s referred to as a h e a d - t o - t a i l arrangement where the repeating polymer chain for a v i n y l monomer rL^C = CHX can be described by HHHHHHHHHHHHHH

I I I I III I I II1I I

HH

II

etc. A-C-C-C-C-C-C-C-C-C-C-C-C-C-C-A etc. or, simply, -(C-C)

lliilllllll[1l

II

X H X H X H X H X H X H X H

H X

n

A head-to-head configuration would be represented by -(CH2CHXCHXCH2) . Even with a h e a d - t o - t a i l configuration, a v a r i e t y of possible s t r u c t u r e s e x i s t s . For i l l u s t r a t i v e purposes, we w i l l c o n s i d e r possible combinations derived from the homopolymerization of A and the c o p o l y m e r i z a t i o n of A w i t h B. The f o l l o w i n g are types of polymers t h a t can be prepared. Homopolymerization ( i n v o l v e s one monomeric unit i n the chain): n

Linear A

> -A-A-A-A-A-

Branched


2.

Introduction to Polymer Science and Technology

C A R R A H E R A N D SEYMOUR

Cross-linked A

-A-A-A-A-A-A-A-A| and/or | -A-A-A-AC

>

1 -A-A-A-Awhere C = a c r o s s - l i n k i n g agent


C o p o l y m e r i z a t i o n ( i n v o l v e s more than one raonomeric u n i t i n the chain): Linear-Random -A-A-B-A-B-B-A-A-B-A-BLinear-Alternating -A-B-A-B-A-B-A-BLinear-Block -A-A-A-A-B-B-B-B-B-B-A-A-A-A-B-BGraft -A-A-A-A-A-A-A-A-A

i

i

I

I

1

1

B

B

B

B

B C r o s s - l i n k e d or Network ( t h r e e - d i m e n s i o n a l ; wide v a r i a t i o n possible structures -A-A-AJ B

I

-A-A-A-A-A-A-

I

J

B

B

I

J

B

B

J

I

-A-A-A-A-A-A-


in

15

16



Polymers w i t h the above s t r u c t u r e s can be t a i l o r e d to e x h i b i t d e s i r e d p r o p e r t i e s by u s i n g combinations of many of the common monomers. Configuration a l s o refers to s t r u c t u r a l r e g u l a r i t y with respect to the s u b s t i t u t e d carbon w i t h i n the polymer c h a i n . For l i n e a r homopolymers d e r i v e d from monomers such as s t y r e n e and v i n y l c h l o r i d e of the form H2C = CHX, configuration from monomeric unit to monomeric unit can vary somewhat randomly (atactic) with respect to the carbon to which the X i s a t t a c h e d , or can vary a l t e r n a t e l y (syndiotactic), or can be a l i k e such that a l l of the X groups can be placed on the same side of a backbone plane ( i s o t a c t i c ) . H

fx

X

X H X H H X X j/H L H l/X U H | ^ X J ^ X U H U H

H

X

,xl

H X ,H| ^X[

H

X 'HLx L H

Syndiotactic

Atactic

X

X

X

X

X

X X

W W I sW " otactic Polymerization of 1,3-dienes such as 1,3-butadiene can occur by 1,2-polymerization or 1,4-polymerization as f o l l o w s : n H C=CH-CH=CH 2

> 4CH -CH}

2

2

CH=CH2

1,2-polymerization n H C=CH-CH=CH 2

N

> £CH -CH=CH-CH }

2

2

2

N

1,4-polymerization The 1,2-products can e x i s t i n the s t e r e o r e g u l a r i s o t a c t i c or s y n d i o t a c t i c forms and the i r r e g u l a r a t a c t i c forms. The s t e r e o r e g u l a r forms are r i g i d , c r y s t a l l i n e m a t e r i a l s , and the a t a c t i c forms are soft elastomers. In the case of 1,4-polymers, rotation i s r e s t r i c t e d because of the double bond i n the chain. The cis isomer of 1,4-polybutadiene i s a s o f t elastomer w i t h a T of -108 ° C , but the t r a n s isomer i s harder w i t h a T of -83 ° C . g

g

H f ^ C=C h^^C=C H N

\

Butadiene

H \

H / C=C

>

1,4-polymerization ^ H /C 2

H \

C=C

\ C H 2 ) 4CH /

1,4-cis polymer

CH2> /

2

H\ 1,4-trans polymer


2.




K i n e t i c s of Polymerization General Considerations. The terms a d d i t i o n and condensation polymers were f i r s t used by Carothers and are based on whether the r e p e a t i n g u n i t , mer, of a polymer c h a i n c o n t a i n s the same atoms as the monomer (1.-^3). A d d i t i o n polymers have the same atoms as the monomer i n the repeat u n i t , with the atoms i n the backbone t y p i c a l l y being only carbon. Condensation polymers t y p i c a l l y c o n t a i n fewer atoms w i t h i n the repeat u n i t than the r e a c t a n t s because of the formation of byproducts during the polymerization process, and the polymer backbone t y p i c a l l y contains atoms of more than one element. P o l y s t y r e n e , p o l y ( v i n y l c h l o r i d e ) , p o l y e t h y l e n e , and p o l y ( v i n y l a l c o h o l ) are i l l u s t r a t i v e of addition polymers, and polyesters and polyamides (nylons) are i l l u s t r a t i v e of condensation polymers. The c o r r e s p o n d i n g p o l y m e r i z a t i o n s are then c a l l e d a d d i t i o n and condensation polymerizations. Stepwise, or step-growth, k i n e t i c s refers to polymerizations i n which polymer molecular weight increases i n a slow, stepwise manner as reaction time increases. Thus, for a polyamide, polymerization begins with one d i c a r b o x y l i c acid molecule reacting with one diamine and the r e s u l t i n g formation of an amide l i n k a g e . T h i s a m i n e - a c i d u n i t can now r e a c t w i t h e i t h e r another a c i d or amine to produce a chain capped with either two acid or two amine groups. This process continues throughout the reaction system wherever molecules with the c o r r e c t f u n c t i o n a l i t y , necessary a c t i v a t i o n energy, and c o r r e c t geometry c o l l i d e . The a c t i v a t i o n energy f o r each step-growth reaction i s about 20 to 50 k c a l / m o l and i s approximately constant throughout the reaction. Chain growth through chain k i n e t i c s requires i n i t i a t i o n to begin the growth. For f r e e - r a d i c a l processes, the i n i t i a t i o n step produces a free r a d i c a l d e r i v e d from l i g h t or p e r o x i d e s . For c a t i o n i c p o l y m e r i z a t i o n s , the i n i t i a t i o n step produces a c a t i o n t y p i c a l l y d e r i v e d from a c a t a l y s t - c o c a t a l y s t complex such as H [BF30H~]. Anionic polymerizations begin through i n i t i a t i o n w i t h metal a l k y l s , a l k a l i amines, etc. that form carbanions. Polymerization r a p i d l y occurs only with chains possessing a free r a d i c a l , cation, or anion (referred to as a c t i v e chains), with rapid addition of u n i t s and subsequent chain growth. This process r e s u l t s i n a r e a c t i o n mixture l a r g e l y composed of polymer and monomer throughout the e n t i r e t y of the p o l y m e r i z a t i o n p r o c e s s . P o l y m e r i z a t i o n occurs u n t i l the r e a c t i v e end i s terminated by chemical or p h y s i c a l means. The a c t i v a t i o n energy f o r each chain-growth r e a c t i o n i s o n l y a p p r o x i m a t e l y 0 to 5 k c a l / m o l . Thus, the d r i v i n g f o r c e f o r d i f f e r e n t l y observed k i n e t i c processes i s d i r e c t l y r e l a t e d to the ease of addition of subsequent units during polymerization. Most a d d i t i o n p o l y m e r s a r e formed from p o l y m e r i z a t i o n s e x h i b i t i n g chain-growth k i n e t i c s . Such processes i n c l u d e the t y p i c a l polymerizations of the vast majority of v i n y l monomers such as ethylene, styrene, v i n y l c h l o r i d e , propylene, methyl a c r y l a t e , and v i n y l a c e t a t e . Furthermore, most condensation polymers are formed from systems e x h i b i t i n g stepwise k i n e t i c s . I n d u s t r i a l l y , such systems include those used for the formation .pa of polyesters and polyamides. Thus, a l a r g e o v e r l a p e x i s t s between the terms +


17

18


a d d i t i o n p o l y m e r s and c h a i n - g r o w t h k i n e t i c s and the t e r m s condensation polymers and stepwise k i n e t i c s . Although the overlap of terms i s great, many exceptions e x i s t . For example, the formation of polyurethanes t y p i c a l l y occurs through stepwise k i n e t i c s . The polymers are c l a s s i f i e d as condensation polymers, and the backbone i s heteratomed, yet no byproduct i s released when the isocyanate and d i o l are condensed. The formation of nylon 6, a condensation polymer, from the corresponding i n t e r n a l lactam occurs through chain-growth k i n e t i c s . O H HO -> {.fi-A-R-N-i-ft'-0}


OCN-R-NCO + H0-R»-0H

n Polyurethane

0 H ^CH^.N} z 5 n

^

Nylon-6 Stepwise Polymerization. Although condensation polymers account for only about one-fourth of synthetic polymers (bulkwise), most natural polymers are of the condensation type. As shown by Carothers i n the 1930s (2^, 3), the chemistry of condensation p o l y m e r i z a t i o n s i s e s s e n t i a l l y the same as c l a s s i c condensation reactions that r e s u l t i n the s y n t h e s i s of monomeric amides, urethanes, e s t e r s , e t c . ; the p r i n c i p l e d i f f e r e n c e i s t h a t the r e a c t a n t s employed f o r polymer formation are b i f u n c t i o n a l (or h i g h e r ) instead of monofunctional. A l t h o u g h more c o m p l i c a t e d s i t u a t i o n s can o c c u r , we w i l l c o n s i d e r o n l y the k i n e t i c s of s i m p l e p o l y e s t e r i f i c a t i o n . The k i n e t i c s of most other common condensations f o l l o w an analogous pathway. For u n c a t a l y z e d r e a c t i o n s i n which the d i c a r b o x y l i c a c i d and d i o l are present i n e q u i m o l a r amounts, one d i a c i d m o l e c u l e i s experimentally found to act as a c a t a l y s t and leads to the f o l l o w i n g k i n e t i c expression. Rate of polycondensation = -d[A]/dt = k[A]2[D] = k[A]3

(1)

where [A] i s the d i c a r b o x y l i c acid concentration and [D] i s the d i o l concentration. When [A] = [D], rearrangement gives -d[A]/[A]3 = kdt

(2)

I n t e g r a t i o n over the l i m i t s of A = AQ to A = A and t = 0 to t = t gives t

2kt = l / [ A ] t

2

- 1/[A ] 0

2

= l / [ A ] 2 + constant t

(3)

I t i s c o n v e n i e n t to express E q u a t i o n 3 i n terms of e x t e n t of reaction, p, where p i s the f r a c t i o n of functional groups that have reacted at time t. Thus 1-p i s the f r a c t i o n of unreacted groups and

A

t

= Ao(l-p)


(4)

2.



S u b s t i t u t i o n of the e x p r e s s i o n f o r A from Equation 4 i n t o Equation 3 and rearrangement y i e l d s 2 2A kt = l / ( l - p ) + constant (5) t

2

0

2

A p l o t of l / ( l - p ) as a function of time should be l i n e a r with a slope of 2A~k from which k i s determinable. Determination of k as a f u n c t i o n of temperature e n a b l e s the c a l c u l a t i o n of a c t i v a t i o n energy. The number average degree of p o l y m e r i z a t i o n , DP^, can be expressed as DP = number of o r i g i n a l molecules/number of m o l e c u l e s at a s p e c i f i c time t i s given by


N

DP = N / N - A A - A / A ( 1 - p) - 1/1 - p N

Q

Q

t

0

Q

(6)

This r e l a t i o n s h i p , Equation 6, i s c a l l e d the Carothers equation because i t was f i r s t found by C a r o t h e r s w h i l e working w i t h the s y n t h e s i s of n y l o n 66. (1_, 3>). Because the value of k at any temperature can be determined from the slope (2A k) of the l i n e when l / ( l - p ) i s p l o t t e d against t, DP can be determined at any time t from the expression 2

2

2

DP = 2 k t [ A ]

2

Q

+ constant

(7)

F r e e - R a d i c a l Chain P o l y m e r i z a t i o n . In contrast to the t y p i c a l l y slow stepwise polymerizations, chain' r e a c t i o n p o l y m e r i z a t i o n s are u s u a l l y rapid with the i n i t i a t e d species r a p i d l y propagating u n t i l termination. A k i n e t i c chain reaction u s u a l l y consists of at l e a s t three steps, namely, i n i t i a t i o n , propagation, and termination. The i n i t i a t o r may be an a n i o n , c a t i o n , free r a d i c a l , or c o o r d i n a t i o n catalyst. Because most s y n t h e t i c p l a s t i c s , e l a s t o m e r s , and f i b e r s are prepared by f r e e - r a d i c a l chain polymerizations, t h i s method w i l l be d i s c u s s e d here. I n i t i a t i o n can occur through decomposition of an i n i t i a t o r such as a z o b i s i s o b u t y r o n i t r i l e (AIBN), l i g h t , heat, s o n i c s , or other technique to form a c t i v e f r e e r a d i c a l s . Here i n i t i a t i o n w i l l be considered as occurring by decomposition of an i n i t i a t o r , I , and i s described as f o l l o w s . k

d

I

> 2R-

(8)

Rate of i n i t i a t o r decomposition = R^ = - d [ I ] / d t = k ^ f l ]

(9)

I n i t i a t i o n of polymerization occurs by addition of the generated i n i t i a t o r free r a d i c a l R* to a v i n y l molecule, M. k

R- + M

i > RM*

(10)

R = -d[M]/dt = d[RM-]/dt = k [R-] M = 2 k f [ I ] ±

±

d

(11)

where f i s an i n i t i a t o r e f f i c i e n c y factor, e s s e n t i a l l y the f r a c t i o n of decomposed i n i t i a t o r fragments, R*, that s u c c e s s f u l l y s t a r t chain growth. Propagation i s a bimolecular reaction i n which the r a d i c a l RM* In Applied Polymer Science; Tess, R.,setl ial.; adds to another monomer molecule. Although g h t changes e x i s t i n ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

19

20


the propagation r a t e c o n s t a n t , k i n the f i r s t few s t e p s of c h a i n growth, the rate constant i s g e n e r a l l y considered to be independent of c h a i n l e n g t h . Thus, a l l of the propagation steps can be described byY a s i n g l e s p e c i f i c rate constant, kp. a

k

p p

M- + M — --> M-

(12)

The rate of decrease of monomer with time i s described by -d[M]/dt = k [M-][M] + k [R-][M] Downloaded by UNIV OF MINNESOTA on October 1, 2013 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch002

p

(13)

±

For l o n g c h a i n s the amount of monomer consumption by the i n i t i a t i o n step i s n e g l i g i b l e and permits E q u a t i o n 13 to be rewritten as Rp = -d[M]/dt - k [M-][M] (14) p

Termination t y p i c a l l y occurs by c o u p l i n g of two m a c r o r a d i c a l s (Equations 15 and 16) or through d i s p r o p o r t i o n a t i o n (Equations 17 and 18) M* + M-

R

t

> M-M

(15)

= -d[M-]/dt = 2 k [ M - ]

M- + Mas 2M-CH -CH' 9

2

t

(16)

I

> 2M

(17)

> M-CH = CH + M-CH -CH 9

|

R R

2

t

2

R

= -d[M-]/dt = 2 k [ M - ]

I

9

2

R

2

(18)

t

Although Equations 14, 16, and 18 are t h e o r e t i c a l l y important, they contain [M*], which i s d i f f i c u l t to experimentally determine. The f o l l o w i n g approach i s used to render a more usable form of these equations. The rate of monomer change i s described by d[M']/dt = k j R ' H M j - ^ k j M - ]

2

(19)

E x p e r i m e n t a l l y the number of growing c h a i n s i s found to be approximately constant over most of the reaction. This s i t u a t i o n i s c a l l e d a "steady state" and r e s u l t s i n d[M*]/dt = 0 and MR-][M] = 2k [M']

2

(20)

t

Furthermore, experiments i n d i c a t e t h a t i f the temperature, amount of l i g h t , e t c . are c o n s t a n t , then the g e n e r a t i o n of R* and number of growing chains i s constant and leads to a steady state for R with #

d[R-]/dt = 2 k f [ I ] - k [R-][M] = 0 d

±


(21)

2.


C A R R A H E R AND SEYMOUR -

S o l v i n g for [M ] from Equation 20 and [R*] from Equation 21 and s u b s t i t u t i n g i n t o Equation 20 the expression for [R"] from Equation 21 gives an expression for [M ] that contains r e a d i l y determinable terms. #

[M-] = ( k f [ I ] / k ) d

(22)

1 / 2

t

U s e f u l r a t e and k i n e t i c c h a i n l e n g t h e x p r e s s i o n s obtainable from Equations 14, 16, 18, and 20. R

= k [M][M-] - k [ M ] ( k f [ I ] / k )

p

p

p


f

where k = ( k R

2 p

k f/k )

= 2k [M-]

t

d

2

d

1 / 2

t

1 / 2

t

1

= k'tMHl] /

are

2

(23)

.

= 2k f[I]

t

(24)

d

1

2

ff

1

DP = R p / R = R / R = k p [ M ] ( k f [ I ] / k ) / / 2 k f [ I ] = k [ M ] / [ I ] / i

p

then

t

d

t

d

2

(25)

1 / 2

where k" = k / ( 2 k k f ) . Because of the great i n d u s t r i a l importance of f r e e - r a d i c a l polymerizations, these reactions are the most studied reactions i n a l l of chemistry. d

t

F r e e - R a d i c a l C o p o l y m e r i z a t i o n . Although the mechanism of c o p o l y merization i s s i m i l a r to that described for homopolymerizations, the r e a c t i v i t i e s of monomers may d i f f e r when more than one monomer i s present i n the feed and lead to polymer chains with varying amounts of the r e a c t a n t monomers. The d i f f e r e n c e i n the r e a c t i v i t y of monomers can be expressed w i t h r e a c t i v i t y r a t i o s , r . The copolymer e q u a t i o n , which expresses the c o m p o s i t i o n of growing chains at any reaction time t, was developed i n the 1930s by a group of i n v e s t i g a t o r s i n c l u d i n g W a l l , Mayo, Simha, A l f r e y , D o r s t a l , and Lewis (for instance, 1_ and ,4-6). Four chain extension reactions are possible when monomers M-^ and M are present i n a polymerization reaction mixture. Two of these steps are self-propagating steps (Equations 26 and 28) and two are c r o s s - p r o p a g a t i n g steps (Equations 27 and 29). The d i f f e r e n c e i n the r e a c t i v i t y of the monomers can be expressed i n terms of r e a c t i v i t y r a t i o s that are r a t i o s of the propagating rate constant where r^ = k j j / k ^ * 2 22^21 2

a n c

k

M

M

r + i— >

k

#

M

w

M

i r

M

+ 2 -"---> M M « 1

k

M- + M 2

M- + M

x

2

(27)

22

2

> MM-

(28)

> M M|*

(29)

2

k

2

=

ll

M

f

r

2

21 2

E x p e r i m e n t a l l y , as i n the case of c h a i n p o l y m e r i z a t i o n s , the s p e c i f i c rate constants are found to be e s s e n t i a l l y independent of


21

22

APPLIED POLYMER SCIENCE

chain length, with monomer addition p r i m a r i l y dependent only on the adding monomer u n i t and the growing end. Thus, Equations 26-29 are s u f f i c i e n t to describe the polymerization. The r a t e of monomer consumption can be d e s c r i b e d by the following: Rate of consumption of M = - d [ M ] / d t = k [ M * ] [ M ] + k [ M * ] [ M ] (30) 1

1

11

1

1

21

2

1

Rate of consumption of M = - d [ M ] / d t = k [ M - ] [ M ] + k [ M ' ] [ M ] (31)


2

2

22

2

2

12

1

2

A g a i n , experiments i n d i c a t e t h a t the number of growing c h a i n s remains e s s e n t i a l l y constant throughout much of the copolymerization r e a c t i o n and g i v e s a steady s t a t e c o n c e n t r a t i o n of M*. The concentration of M^* and M * can then be described and inserted i n t o Equation 32 which describes the r a t i o of monomer uptake i n growing chains, d [ M j ] / d [ M ] , and leads to Equation 33. 2

2

d[M ] x

k ^ M ^ H M i ] + k [M -][M ]

=

2 1

2

1

= d[M ]

(32)

n

k [M -][M ] + k [M '][M ]

2

1 2

[M ] x

1

2

2 2

r [M ]+[M ]

2

2

_ r ( [ M ] / [ M ] ) + l _ r^+1 " [M ]+r [M ] r ([M ]/[M ])+l (r /X)+l 1

n = [M ] 2

1

1

2

2

2

1

1

2

2

2

1

(33)

2

where X = [M^]/[M ] = molar r a t i o of the feed composition, and n i s the molar r a t i o of monomers i n c o r p o r a t e d i n t o growing polymer chains, that i s , the composition of copolymer chains formed. V a l u e s of r have been determined f o r most of the i n d u s t r i a l l y important v i n y l monomers and a l l o w a quick p r e d i c t i o n of polymer c o m p o s i t i o n . Thus, v a l u e s of r^ g r e a t e r than one s i g n i f y t h a t r a d i c a l M^' w i l l tend to add monomer M | r a t h e r than Mo, e t c . When both r^ and r are near 1, a random (not a l t e r n a t i n g ; c h a i n w i l l r e s u l t . An a l t e r n a t i n g copolymer i s produced when both r^ and r are near zero. The presence of r values i n excess of one indicates the formation of block copolymers and/or mixtures. The c o p o l y m e r i z a t i o n e q u a t i o n (Equation 33), a l l o w s one to a d j u s t the monomer feed to produce a copolymer of d e s i r e d compos i t i o n . The copolymer e q u a t i o n i s found to be v a l i d except when strong s t e r i c or polar r e s t r i c t i o n s are present. 2

9

2

Property-Molecular Weight Relationships P o l y m e r i z a t i o n r e a c t i o n s , both s y n t h e t i c and n a t u r a l , l e a d to polymers w i t h a heterogeneous m o l e c u l a r weight, t h a t i s , polymer c h a i n s w i t h a d i f f e r e n t number of u n i t s . M o l e c u l a r weight d i s t r i b u t i o n s may be r e l a t i v e l y broad as i s the case f o r most synthetic polymers and many n a t u r a l l y occurring polymers, r e l a t i v e l y narrow as occurs f o r c e r t a i n n a t u r a l polymers (because of the imposed s t e r i c and e l e c t r o n i c c o n s t r a i n t s ) , or may be mono-, b i - , t r i - , or p o l y m o d a l . A bimodal c u r v e often c h a r a c t e r i z e s a p o l y m e r i z a t i o n o c c u r r i n g under two d i s t i n c t pathways o r environments. Thus, most s y n t h e t i c polymers and many n a t u r a l l y o c c u r r i n g polymers c o n s i s t of m o l e c u l e s with different molecular weights and are s a i d to be p o l y d i s p e r s e . In c o n t r a s t , s p e c i f i c



2.

CARRAHER AND SEYMOUR


p r o t e i n s and n u c l e i c a c i d s c o n s i s t of m o l e c u l e s w i t h a s p e c i f i c molecular weight and are said to be monodisperse. Because t y p i c a l s m a l l m o l e c u l e s and l a r g e m o l e c u l e s w i t h m o l e c u l a r weights l e s s than a c r i t i c a l v a l u e r e q u i r e d f o r c h a i n entanglement are weak and are r e a d i l y a t t a c k e d by a p p r o p r i a t e reactants, these properties are r e l a t e d to molecular weight. Thus, m e l t v i s c o s i t y and c h e m i c a l r e s i s t a n c e of amorphous polymers i s dependent on the m o l e c u l a r weight d i s t r i b u t i o n . In c o n t r a s t , d e n s i t y , s p e c i f i c h e a t c a p a c i t y , and r e f r a c t i v e i n d e x a r e e s s e n t i a l l y independent of the molecular weight at molecular weight values above the c r i t i c a l molecular weight, t y p i c a l l y above chain lengths of 100 u n i t s . The melt v i s c o s i t y i s u s u a l l y proportional to the 3.4 power of the average molecular weight at values above the c r i t i c a l molecular weight r e q u i r e d f o r c h a i n entanglement, t h a t i s , X) = M^.4, Thus, the m e l t v i s c o s i t y i n c r e a s e s r a p i d l y as the m o l e c u l a r weight i n c r e a s e s , and more energy i s r e q u i r e d f o r the p r o c e s s i n g and f a b r i c a t i o n of these large molecules. However, as shown i n Figure 1, the s t r e n g t h of polymers i n c r e a s e s as the m o l e c u l a r weight increases and then tends to l e v e l off. Thus, a l t h o u g h a v a l u e above the t h r e s h o l d m o l e c u l a r weight v a l u e (TMWV) i s e s s e n t i a l f o r most p r a c t i c a l a p p l i c a t i o n s , the a d d i t i o n a l c o s t of energy r e q u i r e d f o r p r o c e s s i n g e x t r e m e l y h i g h m o l e c u l a r weight polymers i s seldom j u s t i f i e d . A c c o r d i n g l y , a commercial polymer range i s customarily established above the TMWV but below the e x t r e m e l y h i g h m o l e c u l a r weight range. However, because toughness i n c r e a s e s w i t h molecular weight, extremely high m o l e c u l a r weight polymers, such as u l t r a h i g h m o l e c u l a r weight p o l y e t h y l e n e , are used f o r the p r o d u c t i o n of tough a r t i c l e s i n c l u d i n g f i l m s employed as trash bags. Oligomers and other low molecular weight polymers are not useful for a p p l i c a t i o n s for which high strength i s required. The value of TMWV w i l l be dependent on the g l a s s t r a n s i t i o n temperature, Tg, the cohesive energy density (CED) of amorphous polymers, the extent of c r y s t a l 1 i n i t y i n c r y s t a l l i n e p o l y m e r s , and t h e e f f e c t o f reinforcements i n p o l y m e r i c composites. Thus, a l t h o u g h a low molecular weight amorphous polymer may be satisfactory for use as a coating or adhesive, a polymer with a DP value of at l e a s t 1000 may be r e q u i r e d i f i t i s used as an elastomer or p l a s t i c . With the e x c e p t i o n of polymers w i t h h i g h l y r e g u l a r s t r u c t u r e s , such as isotactic polypropylene, polymers w i t h s t r o n g hydrogen i n t e r m o l e c u l a r bonds are r e q u i r e d f o r f i b e r s . Because of t h e i r higher CED values, polar polymers having DP values lower than 1000 are s a t i s f a c t o r y f o r use as f i b e r s . In a l a t e r s e c t i o n of t h i s chapter, molecular weight and i t s determination w i l l be discussed i n some d e t a i l . Interchain and Intrachain Forces The f o r c e s present i n m o l e c u l e s are often d i v i d e d i n t o primary f o r c e s ( t y p i c a l l y g r e a t e r than 50 k c a l / m o l of i n t e r a c t i o n ) and secondary forces ( t y p i c a l l y l e s s than 10 kcal/mol of i n t e r a c t i o n ) . Primary bonding f o r c e s can be s u b d i v i d e d i n t o i o n i c bonds (not t y p i c a l l y present i n polymer backbones and characterized by a lack of d i r e c t i o n a l bonding); m e t a l l i c bonds (often considered as charged


23



F i g u r e 1. R e l a t i o n s h i p of polymer p r o p e r t i e s t o m o l e c u l a r w e i g h t . (Reproduced w i t h p e r m i s s i o n from M c G r a w - H i l l . C o p y r i g h t 1971.)


2.

C A R R A H E R AND SEYMOUR


atoms surrounded by a p o t e n t i a l l y f l u i d sea of electrons, not found i n polymers); and covalent bonds ( i n c l u d i n g coordinate and dative). Covalent bonds are d i r e c t i o n a l and the major means of bonding within polymers. The bonding l e n g t h s of primary bonds are u s u a l l y 9 to 20 nm, and the carbon-carbon bond l e n g t h i s a p p r o x i m a t e l y 15 t o 16 nm. Secondary forces, frequently c a l l e d van der Waals forces because they are the forces responsible for the van der Waals correction to the i d e a l gas r e l a t i o n s h i p s , i n t e r a c t over l o n g e r d i s t a n c e s and g e n e r a l l y e x h i b i t s i g n i f i c a n t i n t e r a c t i o n between 25 and 50 nm. The force of these interactions i s i n v e r s e l y proportional to some power of r , g e n e r a l l y 2 or g r e a t e r [ f o r c e l / ( d i s t a n c e ) ] , and thus i s dependent on the d i s t a n c e between the i n t e r a c t i n g m o l e c u l e s . T h e r e f o r e , many p h y s i c a l p r o p e r t i e s of polymers are dependent on both the conformation (arrangements r e l a t e d to r o t a t i o n about s i n g l e bonds) and c o n f i g u r a t i o n (arrangements r e l a t e d to the a c t u a l c h e m i c a l bonding about a g i v e n atom) because both a f f e c t the proximity of one chain r e l a t i v e to another. Atoms i n i n d i v i d u a l polymer molecules are joined to each other by r e l a t i v e l y s t r o n g c o v a l e n t bonds. The bond e n e r g i e s of the carbon-carbon bonds are approximately 80 to 90 k c a l / m o l . Polymer m o l e c u l e s , l i k e a l l other m o l e c u l e s , are a t t r a c t e d to each other (and i n long-chain polymer chains they are attracted to each other even between segments of the same chain) by i n t e r m o l e c u l a r , secondary forces. I n t e r m o l e c u l a r f o r c e s are a l s o p a r t l y r e s p o n s i b l e f o r the i n c r e a s e i n b o i l i n g p o i n t s w i t h i n a homologous s e r i e s such as the a l k a n e s , f o r the h i g h e r than expected b o i l i n g p o i n t s of p o l a r organic molecules such as a l k y l c h l o r i d e s , and for the abnormally high b o i l i n g points of a l c o h o l s , amines, and amides. Although the f o r c e s r e s p o n s i b l e f o r these i n c r e a s e s i n b o i l i n g p o i n t s are a l l c a l l e d van der Waals f o r c e s , these f o r c e s are s u b c l a s s i f i e d i n accordance w i t h t h e i r source and i n t e n s i t y . Secondary, i n t e r m o l e c u l a r forces include London dispersion forces, induced permanent forces, and d i p o l a r forces, i n c l u d i n g hydrogen bonding. Nonpolar molecules such as ethane H(CH2)2H and polyethylene are a t t r a c t e d to each other by weak London or d i s p e r s i o n f o r c e s r e s u l t i n g from induced d i p o l e - d i p o l e i n t e r a c t i o n . The temporary or transient d i p o l e s i n ethane or along the polyethylene chain are due to instantaneous f l u c t u a t i o n s i n the density of the e l e c t r o n clouds. The energy range of these f o r c e s i s about 2 k c a l per u n i t i n nonpolar and p o l a r polymers a l i k e , and t h i s force i s independent of temperature. These dispersion forces are the major forces present between chains i n elastomers and soft p l a s t i c s . Methane, ethane, and ethene are a l l gases; hexane, octane, and nonane are a l l l i q u i d s (under standard conditions); and polyethylene i s a waxy s o l i d . T h i s trend i s p r i m a r i l y due to both an i n c r e a s e i n mass per m o l e c u l e and to an i n c r e a s e i n the London f o r c e s per m o l e c u l e as the c h a i n l e n g t h i n c r e a s e s . With the assumption t h a t the a t t r a c t i o n between methylene or methyl units i s 2 k c a l / m o l or 3 x 10~24 k c a l / m o l e c u l a r i n t e r a c t i o n , the i n t e r a c t i o n per molecule can be c a l c u l a t e d as 3 x 10~24 k c a l / m o l e c u l e for methane, 2 x 10 23 k c a l / m o l e c u l e for hexane, and 6 x 10~21 k c a l / m o l e c u l e for a polyethylene chain of 2000 repeating u n i t s .


r

-


25

26


P o l a r m o l e c u l e s such as e t h y l c h l o r i d e , H 3 C - C H 2 C I , and p o l y ( v i n y l c h l o r i d e ) (PVC), {CH2-CHCl-) ,are attracted to each other by d i p o l e - d i p o l e i n t e r a c t i o n s r e s u l t i n g from the e l e c t r o s t a t i c a t t r a c t i o n of a c h l o r i n e atom i n one molecule to a hydrogen atom i n another molecule. Because t h i s d i p o l e - d i p o l e i n t e r a c t i o n , which ranges from 2 to 6 k c a l / m o l repeat u n i t i n the m o l e c u l e , i s temperature dependent, these forces are reduced as the temperature i s i n c r e a s e d i n the p r o c e s s i n g of .pa polymers. A l t h o u g h the d i s p e r s i o n f o r c e s are t y p i c a l l y weaker than the d i p o l e - d i p o l e f o r c e s , they are a l s o present i n p o l a r compounds such as e t h y l c h l o r i d e and PVC. Strongly polar molecules such as ethanol, p o l y ( v i n y l a l c o h o l ) , and c e l l u l o s e are a t t r a c t e d to each other by a s p e c i a l type of d i p o l e - d i p o l e i n t e r a c t i o n c a l l e d hydrogen bonding i n which the oxygen atoms i n one molecule are attracted to the hydrogen atoms i n another m o l e c u l e . These a t t r a c t i o n s are the s t r o n g e s t of the intermolecular forces and may have energies as high as 10 k c a l / m o l repeat u n i t (the H-F hydrogen bond i s h i g h e r ) . Intermolecular hydrogen bonds are u s u a l l y present i n fibers such as cotton, wool, s i l k , n y l o n , p o l y a c r y l o n i t r i l e , p o l y e s t e r s , and p o l y u r e t h a n e s . I n t r a m o l e c u l a r hydrogen bonds are r e s p o n s i b l e f o r the h e l i c e s observed i n starch and g l o b u l a r proteins. The h i g h m e l t i n g p o i n t of n y l o n 66 (265 °C) i s the r e s u l t of a combination of d i s p e r s i o n , d i p o l e - d i p o l e , and hydrogen bonding f o r c e s between the polyamide c h a i n s . The hydrogen bonds are decreased when the hydrogen atoms i n the amide groups i n nylon are replaced by methyl groups and when the hydroxy1 groups i n c e l l u l o s e are e s t e r i f i e d or e t h e r i f i e d . In addition to the contribution of intermolecular forces, chain entanglement i s a l s o an important c o n t r i b u t o r y f a c t o r to the p h y s i c a l p r o p e r t i e s of polymers. A l t h o u g h p a r a f f i n wax and h i g h d e n s i t y p o l y e t h y l e n e (HDPE) are horaologs w i t h r e l a t i v e l y h i g h m o l e c u l a r w e i g h t s , the c h a i n l e n g t h of p a r a f f i n i s too s h o r t to permit entanglement, and hence i t l a c k s the s t r e n g t h and other c h a r a c t e r i s t i c properties of HDPE.


n

Crystalline-Amorphous Structures General Considerations. There are numerous theories associated with c r y s t a l l i z a t i o n t e n d e n c i e s and t h e f o r m ( s ) and m i x ( e s ) o f crystalline-amorphous regions within polymers. Here we w i l l only b r i e f l y consider a few contributing factors. A three-dimensional c r y s t a l l i n e polymer often can be described as a f r i n g e d m i c e l l e ( c h a i n s packed as a sheaf of g r a i n ) or as a folded chain. Regions where the polymer chains e x i s t i n an ordered array are c a l l e d c r y s t a l l i n e domains. These c r y s t a l l i n e domains i n polymers are t y p i c a l l y smaller than c r y s t a l l i n e portions of s m a l l e r molecules. Furthermore, i m p e r f e c t i o n s i n polymer c r y s t a l l i n e domains are more frequent, and one polymer c h a i n may r e s i d e both w i t h i n a c r y s t a l l i n e domain and w i t h i n amorphous r e g i o n s . These c o n n e c t i v e c h a i n s are r e s p o n s i b l e f o r the toughness of a polymer. Sharp boundaries between the ordered ( c r y s t a l l i n e ) and d i s o r d e r e d (amorphous) p o r t i o n s are the e x c e p t i o n but do occur i n some instances such as with c e r t a i n proteins, p o l y ( v i n y l a l c o h o l ) , and c e r t a i n c e l l u l o s i c materials. Highly c r y s t a l l i n e polymers e x h i b i t



2.



h i g h m e l t i n g p o i n t s and h i g h d e n s i t i e s , r e s i s t d i s s o l u t i o n and s w e l l i n g i n s o l v e n t s , and have high moduli of r i g i d i t y (are s t i f f ) r e l a t i v e to the polymers with l e s s c r y s t a l l i n i t y . The amount and kind of c r y s t a l l i n i t y depends on both the polymer structure and on i t s treatment. The l a t t e r point i s i l l u s t r a t e d by n o t i n g t h a t the p r o p o r t i o n of c r y s t a l l i n i t y can be e f f e c t i v e l y r e g u l a t e d f o r many common polymers by c o n t r o l l i n g the r a t e of formation of c r y s t a l l i n e segments. Thus, p o l y p r o p y l e n e can be heated above i t s m e l t i n g range and c o o l e d q u i c k l y (quenched) to produce a product w i t h o n l y a moderate amount of c r y s t a l l i n e domains. Y e t , i f i t i s c o o l e d a t a s l o w e r r a t e ( s u c h as 1 °C/10 m i n ) , the r e s u l t i n g p o l y p r o p y l e n e w i l l be l a r g e l y crystalline. Polymer properties are d i r e c t l y dependent on both the inherent shape of the polymer and on i t s treatment. Contributions of polymer shape to polymer properties are often complex and i n t e r r e l a t e d , but can be broadly divided into terms dealing with chain f l e x i b i l i t y , chain r e g u l a r i t y , interchain forces, and s t e r i c effects. Chain F l e x i b i l i t y . F l e x i b i l i t y i s r e l a t e d to the a c t i v a t i o n energies required to i n i t i a t e r o t a t i o n a l and v i b r a t i o n a l segmental chain motions. For some polymers, as f l e x i b i l i t y i s increased, the tendency toward c r y s t a l l i n i t y i n c r e a s e s . Polymers c o n t a i n i n g r e g u l a r l y spaced s i n g l e C - C , C - N , and C-0 bonds a l l o w r a p i d c o n f o r m a t i o n a l changes t h a t c o n t r i b u t e to the f l e x i b i l i t y of a polymer c h a i n and to the tendency toward c r y s t a l l i n e f o r m a t i o n . Yet, c h a i n s t i f f n e s s may a l s o enhance c r y s t a l l i n e formation by permitting or encouraging only c e r t a i n "well-ordered" conformations to occur within the polymer chain. Thus p-polyphenylene i s a l i n e a r chain that cannot f o l d over at high temperatures. Such species are h i g h l y c r y s t a l l i n e , high melting, r i g i d , and i n s o l u b l e . Intermolecular Forces. C r y s t a l l i z a t i o n i s favored by the presence of r e g u l a r l y spaced u n i t s t h a t permit s t r o n g i n t e r m o l e c u l a r interchain associations. The presence of m o i e t i e s t h a t c a r r y d i p o l e s or that are h i g h l y p o l a r i z a b l e encourages strong interchain attractions. This tendency i s p a r t i c u l a r l y true for s i t u a t i o n s i n which interchain hydrogen bonds are formed. Thus, the presence of r e g u l a r l y spaced c a r b o n y l , amine, amide, and a l c o h o l m o i t i e s encourages c r y s t a l l i z a t i o n tendencies. S t r u c t u r a l Regularity. S t r u c t u r a l r e g u l a r i t y enhances the tendency for c r y s t a l l i z a t i o n . Thus, l i n e a r p o l y e t h y l e n e i s d i f f i c u l t to o b t a i n i n any form other than a h i g h l y c r y s t a l l i n e one. Low density, branched polyethylene i s t y p i c a l l y l a r g e l y amorphous. The l i n e a r polyethylene chains are n o n p o l a r , and the c r y s t a l l i z a t i o n tendency i s mainly based on the f l e x i b i l i t y of the chains to achieve a regular, t i g h t l y packed conformation that takes advantage of the s p e c i a l r e s t r i c t i o n s inherent i n dispersion forces. M o n o s u b s t i t u t e d v i n y l monomers can produce polymers w i t h d i f f e r e n t c o n f i g u r a t i o n s — t w o r e g u l a r s t r u c t u r e s ( i s o t a c t i c and s y n d i o t a c t i c ) and a random ( a t a c t i c ) form. For polymers w i t h the same chemical makeup, those forms derived from regular structures e x h i b i t greater r i g i d i t y , are higher melting, and are l e s s s o l u b l e r e l a t i v e to the a t a c t i c form.


27

28



Extensive work with condensation polymers and copolymers f u l l y confirms the importance of s t r u c t u r a l r e g u l a r i t y on c r y s t a l l i z a t i o n tendency, and c o n s e q u e n t l y on a s s o c i a t e d p r o p e r t i e s . Thus, copolymers containing r e g u l a r a l t e r a t i o n of each copolymer u n i t , e i t h e r ABABAB type or b l o c k type, show a d i s t i n c t tendency to c r y s t a l l i z e , and corresponding copolymers with random d i s t r i b u t i o n s of the two are i n t r i n s i c a l l y amorphous, l e s s r i g i d , lower melting, and more s o l u b l e . Steric Effects. The e f f e c t of s u b s t i t u e n t s on polymer properties depends on a number of items i n c l u d i n g l o c a t i o n , s i z e , shape, and mutual i n t e r a c t i o n s . Although methyl and phenyl substituents tend to lower chain m o b i l i t y , they do permit good packing of chains, and t h e i r presence produces d i p o l e s t h a t f u r t h e r c o n t r i b u t e to the c r y s t a l l i z a t i o n tendency. The presence of aromatic s u b s t i t u e n t s , even comparatively large substituents such as anthracene, further c o n t r i b u t e s to i n t r a c h a i n and i n t e r c h a i n a t t r a c t i o n t e n d e n c i e s through the mutual i n t e r a c t i o n s of the aromatic s u b s t i t u e n t s . Although t h e i r bulky s i z e discourages c r y s t a l l i z a t i o n by increasing the i n t e r c h a i n d i s t a n c e s , they do encourage r i g i d i t y . Thus, polymers containing bulky aromatic s u b s t i t u e n t s tend to be r i g i d , high melting, l e s s s o l u b l e , yet f a i r l y amorphous. E t h y l to h e x y l s u b s t i t u e n t s tend to lower the tendency f o r c r y s t a l l i z a t i o n because t h e i r major c o n t r i b u t i o n i s to increase the average distance between chains and thus decrease the contributions of secondary bonding f o r c e s . I f the s u b s t i t u e n t s become l o n g e r (from 12 to 18 carbon atoms) and remain l i n e a r , a new phenomenon occurs—the tendency of the side chains to form c r y s t a l l i n e domains of t h e i r own. Transitions Polymers can e x h i b i t a number of d i f f e r e n t conformational changes with each change accompanied by differences i n polymer properties. Two major t r a n s i t i o n s occur at Tg, which i s associated with l o c a l , segmental chain m o b i l i t y i n the amorphous regions of a polymer, and the m e l t i n g p o i n t ( T ) , which i s a s s o c i a t e d w i t h whole c h a i n m o b i l i t y . The T i s c a l l e d a f i r s t - o r d e r t r a n s i t i o n temperature, and Tg i s o f t e n r e f e r r e d t o as a s e c o n d - o r d e r t r a n s i t i o n temperature. The values for T are u s u a l l y 33 to 100% greater than for Tg, and Tg values are t y p i c a l l y low for elastomers and f l e x i b l e polymers and higher for hard amorphous p l a s t i c s . V i s c o s i t y i s a measure of the resistance to flow. Flow, which i s the r e s u l t of cooperative movement of the polymer segments from h o l e t o h o l e i n a m e l t o r s o l u t i o n , i s impeded by c h a i n entanglement, intermolecular forces, c r o s s - l i n k s , and the presence of r e i n f o r c i n g agents. The f l e x i b i l i t y of amorphous polymers above the glassy state, which i s governed by the same forces as melt v i s c o s i t y , i s dependent on a w r i g g l i n g type of segmental motion i n the polymer chains. This f l e x i b i l i t y i s i n c r e a s e d when many methylene groups are present between s t i f f e n i n g groups i n the chain and when oxygen atoms are present i n the c h a i n . Thus, the f l e x i b i l i t y of a l i p h a t i c polyesters u s u a l l y increases as the number of methylene groups i s increased. In contrast, the f l e x i b i l i t y of amorphous polymers above m

m

m


2.



the g l a s s y s t a t e i s decreased when s t i f f e n i n g groups such as phenylene, sulfone, and amide are present i n the backbone. The f l e x i b i l i t y of amorphous polymers i s reduced d r a s t i c a l l y when they are c o o l e d below Tg. At temperatures below Tg, no segmental motion e x i s t s , and any dimensional changes i n the polymer chain are the r e s u l t of temporary d i s t o r t i o n s of the primary valence bonds. Amorphous p l a s t i c s perform best below Tg, but e l a s t o m e r s must be used above the b r i t t l e point, or Tg. The Tg value of i s o t a c t i c polypropylene i s approximately -10 °C, yet because of i t s high degree of c r y s t a l l i n i t y , i t does not r e a d i l y flow below i t s T of approximately 150 °C. Thus, p h y s i c a l flow tendencies are r e l a t e d to both the T and T values and to the r e a l p h y s i c a l nature of the product ( p r o p o r t i o n and type of crystallinity). Because the s p e c i f i c volume of polymers increases at Tg i n order to accommodate the increased segmental chain motion, Tg values may be estimated from p l o t s of the change i n s p e c i f i c volume w i t h temperature. Other p r o p e r t i e s s u c h as s t i f f n e s s ( m o d u l u s ) , r e f r a c t i v e i n d e x , d i e l e c t r i c p r o p e r t i e s , gas p e r m e a b i l i t y , X - r a y a d s o r p t i o n , and heat c a p a c i t y a l l change at T . Thus, Tg may be estimated by n o t i n g the change i n any of these v a l u e s such as the increase i n gas permeability. m


g

m

g

End Uses of Polymers as Related to Structure General Discussion. The usefulness of polymers depends not only on t h e i r p r o p e r t i e s but a l s o on t h e i r abundance and r e a s o n a b l e c o s t . Polymer properties are r e l a t e d not only to the chemical nature of the polymer, but a l s o to such factors as extent and d i s t r i b u t i o n of c r y s t a l l i n i t y and d i s t r i b u t i o n of polymer c h a i n l e n g t h s . These f a c t o r s i n f l u e n c e p r o p e r t i e s such as hardness, comfort, c h e m i c a l resistance, b i o l o g i c a l response, weather resistance, tear strength, d y e a b i l i t y , f l e x l i f e , s t i f f n e s s , e l e c t r i c a l p r o p e r t i e s , and flammability. Elastomers. Elastomers are characterized by the a b i l i t y to elongate upon a p p l i c a t i o n of s t r e s s and to r e t u r n q u i c k l y to the o r i g i n a l length upon release of stress. They are high polymers that possess chemical c r o s s - l i n k s . For i n d u s t r i a l a p p l i c a t i o n they must be used above Tg to a l l o w for f u l l chain m o b i l i t y . The normal unextended state of elastomers must be amorphous. The restoring force, after elongation, i s l a r g e l y due to entropy effects. As the material i s e l o n g a t e d , the random c h a i n s are f o r c e d to occupy more ordered p o s i t i o n s . Upon r e l e a s e of the a p p l i e d f o r c e the c h a i n s tend to return to a more random state. Gross, a c t u a l m o b i l i t y of c h a i n s must be low. The c o h e s i v e energy f o r c e s between c h a i n s s h o u l d be low and permit r a p i d , easy expansion. In i t s extended s t a t e a c h a i n s h o u l d e x h i b i t a h i g h t e n s i l e s t r e n g t h , whereas at low e x t e n s i o n s i t s h o u l d have a low t e n s i l e strength. Polymers with low c r o s s - l i n k e d density u s u a l l y meet the d e s i r e d p r o p e r t y requirements. The m a t e r i a l a f t e r deformation s h o u l d r e t u r n to i t s o r i g i n a l shape because of the cross-linking. T h i s p r o p e r t y i s often r e f e r r e d to as rubber "memory."


29


30


F i b e r s . F i b e r p r o p e r t i e s i n c l u d e h i g h t e n s i l e s t r e n g t h and h i g h modulus (high s t r e s s f o r s m a l l s t r a i n s , i . e . , s t i f f n e s s ) . These p r o p e r t i e s can be o b t a i n e d from h i g h m o l e c u l a r symmetry and h i g h c o h e s i v e e n e r g i e s between c h a i n s , both r e q u i r i n g a f a i r l y h i g h degree of polymer c r y s t a l l i n i t y . F i b e r s are n o r m a l l y l i n e a r and drawn ( o r i e n t e d ) i n one d i r e c t i o n to produce h i g h mechanical properties i n that d i r e c t i o n . T y p i c a l condensation p o l y m e r s , such as p o l y e s t e r and n y l o n , often e x h i b i t these p r o p e r t i e s . I f the f i b e r i s to be i r o n e d , i t s Tg s h o u l d be above 200 °C; i f i t i s to be drawn from the m e l t , i t s Tg s h o u l d be below 300 °C. Branching and c r o s s - l i n k i n g are undesirable because they disrupt c r y s t a l l i n e formation even though a s m a l l amount of c r o s s - l i n k i n g may increase some p h y s i c a l properties i f effected after the material i s s u i t a b l y drawn and processed. P l a s t i c s . M a t e r i a l s with properties intermediate between elastomers and f i b e r s are grouped t o g e t h e r under the term " p l a s t i c s . " Thus, p l a s t i c s e x h i b i t some f l e x i b i l i t y with hardness with varying degrees of c r y s t a l l i n i t y . The molecular requirements for a p l a s t i c are the f o l l o w i n g : i f i t i s l i n e a r or branched, w i t h l i t t l e or no c r o s s l i n k i n g , then i t s h o u l d be below i t s T i f amorphous and/or below i t s m e l t i n g p o i n t i f c r y s t a l l i z i b l e when i t i s used; or i f i t i s c r o s s - l i n k e d , the c r o s s - l i n k i n g must be s u f f i c i e n t to s e v e r e l y r e s t r i c t molecular motion. g

A d h e s i v e s . A d h e s i v e s can be c o n s i d e r e d as c o a t i n g s between two surfaces. The c l a s s i c adhesives were water-susceptible animal and vegetable glues obtained from hides, blood, and starch. Adhesion may be defined as the process that occurs when a s o l i d and movable material ( u s u a l l y i n a l i q u i d or s o l i d form) are brought together to form an i n t e r f a c e , and the s u r f a c e e n e r g i e s of the two substances are transformed i n t o the energy of the interface. A u n i f i e d s c i e n c e of adhesion i s s t i l l being d e v e l o p e d . Adhesion can r e s u l t from mechanical bonding between the adhesive and adherend and/or primary a n d / o r secondary c h e m i c a l f o r c e s . Contributions through chemical forces are often more important and i l l u s t r a t e why nonpolar polymeric materials such as polyethylene are d i f f i c u l t to bond, a l t h o u g h p o l y c y a n o a c r y l a t e s are e x c e l l e n t adhesives. Numerous types of a d h e s i v e s are a v a i l a b l e such as s o l v e n t - b a s e d , l a t e x , p r e s s u r e - s e n s i t i v e , r e a c t i v e , and hot-melt adhesives. The combination of an a d h e s i v e and adherend i s a l a m i n a t e . Commercial laminates are produced on a large s c a l e with wood as the adherend and phenolic, urea, epoxy, r e s o r c i n o l , or polyester resins as the a d h e s i v e s . Many wood l a m i n a t e s are c a l l e d plywood. Laminates of paper or t e x t i l e include items under the trade names of Formica and M i c a r t a . Laminates of p h e n o l i c , n y l o n , or s i l i c o n e r e s i n s w i t h c o t t o n , a s b e s t o s , paper, or g l a s s t e x t i l e are used as mechanical, e l e c t r i c a l , and general purpose s t r u c t u r a l m a t e r i a l s . Composites of f i b r o u s g l a s s , mat or sheet, and epoxy or p o l y e s t e r resins are widely employed as reinforced p l a s t i c (FRP) structures. C o a t i n g s . The t r a d i t i o n a l uses of c o a t i n g s f o r d e c o r a t i v e and p r o t e c t i v e purposes are expanding i n t o future concepts of coatings as energy c o l l e c t i v e devices, burglar alarm systems, and other novel


2.




end uses. Even so, such p r o p e r t i e s as adhesion, c o r r o s i o n p r o t e c t i o n , w e a t h e r a b i l i t y , c o l o r s t a b i l i t y , water and c h e m i c a l resistance, toughness, hardness, and a p p l i c a t i o n properties continue to be major requirements. Recent emphasis i s on coatings that can be used with low quantities of s o l v e n t or can be d i l u t e d with water. A c r y l a t e s , e p o x i e s , and urethanes are important types of r e s i n s used, although alkyds are s t i l l popular. Polyblends and Composites. Polyblends are made by mixing components together i n e x t r u d e r s or m i x e r s , on m i l l r o l l s , e t c . Most are heterogeneous systems c o n s i s t i n g of a p o l y m e r i c m a t r i x i n which another polymer i s imbedded. Although the units of copolymers are connected through primary bonds, the components of polyblends adhere only through secondary bonding forces. In contrast to polyblends, composites consist of a polymeric matrix i n which a foreign material i s dispersed. Composites t y p i c a l l y contain f i l l e r s such as carbon b l a c k , wood f l o u r , t a l c , or r e i n f o r c i n g m a t e r i a l s such as g l a s s f i b e r s , hollow spheres, and g l a s s mats. Physical Characterization and Testing Testing Societies. P u b l i c acceptance of polymers i s u s u a l l y a s s o c i a t e d w i t h an assurance of q u a l i t y based on a knowledge of successful long-term and r e l i a b l e tests. In contrast, much of the d i s s a t i s f a c t i o n with synthetic polymers i s r e l a t e d to f a i l u r e s that possibly could have been prevented by proper t e s t i n g , design, and q u a l i t y c o n t r o l . The American S o c i e t y f o r T e s t i n g and M a t e r i a l s (ASTM), through i t s committees D - l on p a i n t and D-20 on p l a s t i c s , has developed many standard tests that should be referred to by a l l p r o d u c e r s and consumers o f f i n i s h e d p o l y m e r i c m a t e r i a l s . C o o p e r a t i n g groups i n many other t e c h n i c a l s o c i e t i e s a l s o e x i s t : the American N a t i o n a l Standards I n s t i t u t e , the I n t e r n a t i o n a l Standards Organization, and standards s o c i e t i e s such as the B r i t i s h Standards I n s t i t u t i o n i n England, the Deutsche Norraenausschuss i n Germany, and comparable groups i n every developed nation throughout the entire world. Thus, standard t e s t s account f o r much of the t e s t i n g done i n industry and are used to ensure product s p e c i f i c a t i o n s . These t e s t s measure s t r e s s - s t r a i n r e l a t i o n s h i p s , f l e x l i f e , t e n s i l e strength, a b r a s i o n r e s i s t a n c e , moisture r e t e n t i o n , d i e l e c t r i c c o n s t a n t , hardness, thermal c o n d u c t i v i t y , e t c . New t e s t s are c o n t i n u a l l y b e i n g d e v e l o p e d and s u b m i t t e d t o ASTM, and a f t e r a d e q u a t e v e r i f i c a t i o n through "round robin" t e s t i n g are f i n a l l y accepted as standard t e s t s . Some t e s t s are d e v e l o p e d w i t h i n a g i v e n company t o measure a c e r t a i n p r o p e r t y p e c u l i a r to t h a t company, and these t e s t s may or may not be submitted to ASTM for v e r i f i c a t i o n and acceptance. Data obtained from such t e s t s may be quite v a l u a b l e to that company, but such data s h o u l d not be used f o r comparative r e s u l t s w i t h other t e s t s u n t i l adequate precautions are taken to ensure that conditions of the two s e t s of t e s t s are the same. Furthermore, i t i s not always c l e a r what p a r t i c u l a r property or other p r o p e r t y - s t r u c t u r e r e l a t i o n s h i p i s being t e s t e d w i t h many standard and nonstandard tests because the tests are more often use oriented. Even so, such


31


32


t e s t s form the basis of product r e l i a b i l i t y and r e p r o d u c i b i l i t y and are benchmarks of industry. Each standardized t e s t i s specified by a unique combination of l e t t e r s and numbers and c o n t a i n s s p e c i f i c a t i o n s r e g a r d i n g data g a t h e r i n g , i n s t r u m e n t d e s i g n , and t e s t c o n d i t i o n s . These s p e c i f i c a t i o n s make i t p o s s i b l e for laboratories across the country to compare data w i t h some c o n f i d e n c e . Thus, the Izod t e s t , a p o p u l a r impact t e s t , i s g i v e n the ASTM number D256-56 (1961), the l a t t e r number being the year i t was f i r s t accepted. The ASTM i n s t r u c t i o n s for the Izod t e s t specify test material shape and s i z e , t e s t equipment, t e s t procedure, and r e s u l t r e p o r t i n g . A l i s t of a v a i l a b l e t e s t s a l o n g w i t h b r i e f d e s c r i p t i o n s of the use of each t e s t i s contained i n "Compilation of ASTM Standard D e f i n i t i o n s " (7). Molecular Weight. A polymer i s a polymer because of i t s large s i z e , and i t i s necessary to o b t a i n p h y s i c a l measurements of t h i s s i z e . The p h y s i c a l parameter t y p i c a l l y used to describe polymer s i z e i s m o l e c u l a r weight. With the e x c e p t i o n of a few n a t u r a l o c c u r r i n g polymers, polymer samples consist of chains having varying length and v a r y i n g m o l e c u l a r weights as d e p i c t e d i n F i g u r e 2. S e v e r a l mathematical moments can be described by using t h i s d i f f e r e n t i a l or frequency d i s t r i b u t i o n curve, and these moments can be described by equations and determined p h y s i c a l l y by using various techniques. The f i r s t moment i s c a l l e d the number-average molecular weight, M . Any measurement t h a t l e a d s to the number of m o l e c u l e s , functional groups, or p a r t i c l e s that are present i n a given weight of sample a l l o w s the c a l c u l a t i o n of FT . Most thermodynamic properties are r e l a t e d to the number of p a r t i c l e s present and thus are dependent on M . C o l l i g a t i v e p r o p e r t i e s dependent on the number of p a r t i c l e s present are obviously r e l a t e d to M . M values are independent of m o l e c u l a r s i z e and are h i g h l y s e n s i t i v e to the presence of s m a l l molecules i n the mixture. Values for M are determined by Raoult's techniques and are dependent on c o l l i g a t i v e p r o p e r t i e s such as ebulliometry ( b o i l i n g point e l e v a t i o n ) , cryometry ( f r e e z i n g p o i n t depression), osmometry, and end-group a n a l y s i s . The number-average molecular weight, M , i s c a l c u l a t e d l i k e any other n u m e r i c a l average by d i v i d i n g the sum of the i n d i v i d u a l molecular weight values, M^N^, by the number of molecules, N^. n

n

n

n

n

n

n

00 t o t a l weight of sample M

n

.

. no. of molecules of N i

Z i=l

w

00 i=l

.

M

i N i

00

(34)

i=l

Weight-average m o l e c u l a r weight, M , i s determined from experiments i n which each molecule or chain makes a contribution to the measured r e s u l t . This average i s more dependent on the number of heavier molecules than i s the number-average m o l e c u l e weight, which i s dependent simply on the t o t a l number of p a r t i c l e s . w





F i g u r e 2. M o e l c u l a r weight d i s t r i b u t i o n s . (Reproduced w i t h p e r m i s s i o n from M c G r a w - H i l l . C o p y r i g h t 1971.)


34


The weight-average molecular weight, M , i second power average as shown mathematically: w

00

if1 R

w

the second moment or

s

2 M i N ±

=

(35)


1=1 B u l k p r o p e r t i e s a s s o c i a t e d w i t h l a r g e deformations, such as v i s c o s i t y and toughness, are p a r t i c u l a r l y r e l a t e d to M values. fl i s d e t e r m i n e d by l i g h t s c a t t e r i n g and u l t r a c e n t r i f u g a t i o n techniques. Melt e l a s t i c i t y i s more c l o s e l y dependent on M (the z-average molecular weight) which can a l s o be obtained by u l t r a c e n t r i f u g a t i o n t e c h n i q u e s . K i s the t h i r d moment or t h i r d power average and i s shown mathematically as w

w

z

3 M

z

if1 =-

M i N i

(36)

00

2 Z M.

N i

i=l

A l t h o u g h z + 1 and h i g h e r average m o l e c u l a r weights may be c a l c u l a t e d , the major i n t e r e s t s are i n M" , M" , and E , which are l i s t e d i n order of increasing s i z e i n Figure 2. Because ^ L i s always greater than M except i n monodisperse systems, the r a t i o M ^ / M i s a measure of p o i y d i s p e r s i t y and i s c a l l e d the p o l y d i s p e r s i t y index. The most probable d i s t r i b u t i o n for polydisperse polymers produced by condensation techniques i s a p o l y d i s p e r s i t y index of 2.0. Thus, for a polymer mixture that i s heterogeneous with respect to molecular weight, M > M > M . As the heterogeneity decreases, the various molecular weight values converge u n t i l for homogeneous mixtures M = M = M . The r a t i o s of such molecular weight values are often used to describe the molecular weight heterogeneity of polymer samples. Viscometry i s the most w i d e l y used method f o r the c h a r a c t e r i zation of polymer molecular weight because i t provides the easiest and most rapid means of obtaining molecular weight r e l a t e d data and r e q u i r e s a minimum amount of i n s t r u m e n t a t i o n . A most o b v i o u s c h a r a c t e r i s t i c of polymer s o l u t i o n s i s t h e i r high v i s c o s i t y , even when the amount of added polymer i s s m a l l . Even so, because viscometry does not y i e l d absolute values of M, one must c a l i b r a t e the viscometry r e s u l t s with values obtained for the same polymer and s o l v e n t by u s i n g an a b s o l u t e technique such as l i g h t s c a t t e r i n g photometry. A l l c l a s s i c molecular weight determination methods require the p o l y m e r t o be i n s o l u t i o n . To m i n i m i z e p o l y m e r - p o l y m e r i n t e r a c t i o n s , s o l u t i o n s e q u a l to and l e s s than 1 g of polymer per 100 ml of s o l u t i o n are used. To f u r t h e r minimize s o l u t e i n t e r a c n

w

n

z

w

n

z

w

n


2.




t i o n s , e x t r a p o l a t i o n of the measurements to i n f i n i t e d i l u t i o n i s normally practiced. Molecular Weight D i s t r i b u t i o n . Polymer properties are dependent on both average chain length and d i s t r i b u t i o n of chain lengths. Curves such as F i g u r e 2 are now u s u a l l y determined by g e l permeation c h r o m a t o g r a p h y (GPC). P r i o r to the i n t r o d u c t i o n o f GPC, p o l y d i s p e r s e polymers were f r a c t i o n a t e d by the a d d i t i o n of a n o n s o l v e n t to a polymer s o l u t i o n , c o o l i n g a s o l u t i o n of polymer, s o l v e n t e v a p o r a t i o n , zone m e l t i n g , e x t r a c t i o n , d i f f u s i o n , or centrifugation. The m o l e c u l a r weight of the f r a c t i o n s may be determined by any of the c l a s s i c a l techniques mentioned e a r l i e r . The l e a s t s o p h i s t i c a t e d but most c o n v e n i e n t technique i l l u s t r a t i n g polymer f r a c t i o n a t i o n i s f r a c t i o n a l p r e c i p i t a t i o n , which i s dependent on the s l i g h t change i n the s o l u b i l i t y parameter w i t h m o l e c u l a r weight. Thus, when a s m a l l amount of m i s c i b l e nonsolvent i s added to a polymer s o l u t i o n at a constant temperature, the product w i t h the h i g h e s t m o l e c u l a r weight p r e c i p i t a t e s . This procedure may be repeated after the p r e c i p i t a t e i s removed. These f r a c t i o n s may a l s o be r e d i s s o l v e d and f r a c t i o n a l l y precipitated. The shape of the d i s t r i b u t i o n c u r v e i s then c o n s t r u c t e d from the f r a c t i o n a l amount of each sample after chain length determination. Rheology. The b r a n c h of s c i e n c e r e l a t e d t o the s t u d y o f deformation and f l o w of m a t e r i a l s was g i v e n the name r h e o l o g y by Bingham, who has been c a l l e d the father of modern rheology (1). The p r e f i x "rheo" i s d e r i v e d from the Greek term "rheos," meaning c u r r e n t or f l o w . The study of r h e o l o g y i n c l u d e s two v a s t l y d i f f e r e n t branches of mechanics c a l l e d f l u i d and s o l i d mechanics. The polymer chemist i s u s u a l l y concerned with v i s c o e l a s t i c materials that act as both s o l i d s and f l u i d s . The e l a s t i c component i s dominant i n s o l i d s , hence t h e i r mechanical properties may be described by Hooke's law (Equation 37) which s t a t e s t h a t the a p p l i e d s t r e s s (S) i s p r o p o r t i o n a l to the r e s u l t a n t s t r a i n (y) but i s independent of the r a t e of t h i s s t r a i n (dy/dt). S = Ey

(37)

Stress i s equal to the force per unit area, and s t r a i n or elongation i s the extension per unit length. For an i s o t r o p i c s o l i d , that i s , one having the same properties independent of d i r e c t i o n , the s t r a i n i s defined by P o i s s o n ' s r a t i o , V = Y i / y , which i s the percentage change i n l o n g i t u d i n a l s t r a i n , y\, per percentage change i n l a t e r a l strain, y . When t h e r e i s no volume change, as when an e l a s t o m e r i s stretched, Poissons's r a t i o i s 0.5. This value decreases as the Tg of the substance i n c r e a s e s and approaches 0.3 f o r r i g i d PVC ana ebonite. For s i m p l i c i t y , the polymers can be c o n s i d e r e d to be i s o t r o p i c v i s c o e l a s t i c s o l i d s with a Poisson r a t i o of 0.5, and only deformations i n tension and shear w i l l be considered. Thus, a shear modulus (G) w i l l u s u a l l y be used i n p l a c e of Young's modulus of e l a s t i c i t y (E). Hooke's law for shear i s g i v e n i n Equation 38. E i s approximately 2.6 G at temperatures below T . w

w

g


35

36


s =

(38)

The v i s c o u s component i s dominant i n l i q u i d s , thus t h e i r f l o w p r o p e r t i e s can be d e s c r i b e d by Newton's law (Equation 39) which s t a t e s t h a t the a p p l i e d s t r e s s S i s p r o p o r t i o n a l to the r a t e of s t r a i n dy/dt, but i s independent of the s t r a i n y or a p p l i e d v e l o c i t y gradient.


S = n(dy/dt)

(39)

Both Hooke's and Newton's laws are v a l i d f o r s m a l l changes i n s t r a i n or r a t e of s t r a i n , and both are u s e f u l i n d e s c r i b i n g the effect of stress on v i s c o e l a s t i c materials. The i n i t i a l elongation of a stressed polymer below Tg i s the r e v e r s i b l e elongation due to the s t r e t c h i n g of c o v a l e n t bonds and d i s t o r t i o n of bond a n g l e s . Some of the e a r l y stages of elongation by disentanglement of polymer chains may a l s o be r e v e r s i b l e . However, the r a t e of f l o w , which i s r e l a t e d to s l o w d i s e n tanglement and s l i p p a g e of polymer c h a i n s past one another, i s i r r e v e r s i b l e and i n c r e a s e s as the temperature increases i n accordance with the Arrhenium r e l a t i o n s h i p r,

=

AeE/RT

(40)

A s i n g l e w e i g h t l e s s Hookean, or i d e a l , e l a s t i c s p r i n g w i t h a modulus of G and a s i m p l e Newtonian ( f l u i d ) dash pot or shock absorber having a l i q u i d with a v i s c o s i t y are convenient to use as models i l l u s t r a t i n g the deformation of an e l a s t i c s o l i d and an i d e a l liquid. Because p o l y m e r s are o f t e n v i s c o e l a s t i c s o l i d s , combinations of these models are used to demonstrate deformations r e s u l t i n g from the a p p l i c a t i o n of s t r e s s to an i s o t r o p i c s o l i d polymer. P h y s i c a l Tests. Numerous p h y s i c a l t e s t s are r o u t i n e l y employed to p r e d i c t the performance of polymers. Many of these can be termed "use tests," to indicate that some r e l a t i o n s h i p e x i s t s between the t e s t r e s u l t s and some performance o p e r a t i o n . The f o l l o w i n g are d e s c r i p t i o n s of s e v e r a l of the more r o u t i n e l y performed p h y s i c a l tests. T e n s i l e strength, which i s a measure of the a b i l i t y of a polymer to withstand p u l l i n g stresses, i s u s u a l l y determined by p u l l i n g a dumbbell-shaped specimen (ASTM-D638-72). These test specimens, l i k e a l l o t h e r s , must be c o n d i t i o n e d under standard c o n d i t i o n s of humidity (50%) and temperature (23 °C) before testing. The ultimate t e n s i l e strength i s equal to the load that caused f a i l u r e divided by the minimum c r o s s - s e c t i o n a l area. F l e x u r a l strength, or cross-breaking strength, i s a measure of the bonding s t r e n g t h or s t i f f n e s s of a bar t e s t specimen used as a simple beam (ASTM-D790-71). The f l e x u r a l strength i s based on the load required to rupture a simple beam before i t s d e f l e c t i o n i s 5%. Compressive s t r e n g t h , or the a b i l i t y of a specimen to r e s i s t c r u s h i n g f o r c e s , i s measured by c r u s h i n g a c y l i n d r i c a l specimen (ASTM-D695-69). The ultimate compression strength i s equal to the l o a d t h a t caused f a i l u r e d i v i d e d by the minimum c r o s s - s e c t i o n a l area.



2.



Impact s t r e n g t h i s a measure of toughness or the a b i l i t y of a specimen to withstand a sharp blow, such as the a b i l i t y to withstand a g i v e n o b j e c t being dropped from a s p e c i f i c h e i g h t . Impact resistance can be determined by measuring the energy of a pendulum necessary to break a p l a s t i c specimen. An unnotched specimen i s used i n the Charpy t e s t (ASTM-D256-73), and a notched specimen i s used i n the Izod impact t e s t . Because the r e s u l t s of these impact tests are c o n t r o v e r s i a l , other t e s t s simulating a c t u a l use have been developed, such as dropping specimens from s p e c i f i c heights. Shear s t r e n g t h i s a measure of the l o a d r e q u i r e d to cause f a i l u r e i n the area of the sheared specimen i n accordance with ASTM-D732-46. The shear s t r e n g t h i s e q u a l to the l o a d d i v i d e d by the area. Hardness i s a g e n e r a l term t h a t can d e s c r i b e a combination of p r o p e r t i e s i n c l u d i n g r e s i s t a n c e to p e n e t r a t i o n , a b r a s i o n , and scratching. Indentation hardness of thermosets can be measured by a Barcol Impressor [ASTM-E-2583-67(1972)]. Rockwell hardness tests [ASTM-D785-65 (1970)] measure hardness i n progressive numbers on different s c a l e s corresponding to the s i z e of the b a l l indentor used. S c r a t c h hardness may be measured on Mohs s c a l e , which ranges from 1 for t a l c to 10 for diamonds, or by scratching with p e n c i l s of specified hardness (ASTM-D-3363). Hardness may a l s o be measured by the number of bounces of a b a l l or the amount of rocking by a Sward hardness rocker. Abrasion resistance may be measured by the l o s s i n weight caused by the rubbing of the wheels of a Taber abraser (ASTM-D-1044). The tests for change i n dimensions of a polymer under long-term s t r e s s , c a l l e d creep or c o l d f l o w (ASTM-D74-56), are no l o n g e r recommended by ASTM. ASTM-D-671 d e s c r i b e s suggested t e s t s f o r fatigue or endurance of p l a s t i c s under repeated f l e x u r e . E l e c t r i c a l Measurements. The e l e c t r i c a l properties of polymers have much i n common with mechanical properties. They can be divided i n t o s t a t i c p r o p e r t i e s e q u i v a l e n t to d i r e c t c u r r e n t p r o p e r t i e s and dynamic properties r e s u l t i n g from a l t e r n a t i n g current measurements. The most used p a r a m e t e r i s the v o l u m e or b u l k r e s i s t i v i t y (ASTM-D257-75b) which i s the resistance i n ohms of a material 1 cm t h i c k and 1 cm^ i n area. B u l k r e s i s t i v i t y i s one of o n l y a few p r o p e r t i e s t h a t vary n e a r l y 10^5 i t y p i c a l use ( m a t e r i a l s w i t h values above 1 0 ^ ohm-cm for polystyrene to 10~5 ohm-cm for copper). Most p o l y m e r i c a p p l i c a t i o n s c a l l f o r m a t e r i a l s w i t h h i g h r e s i s t i v i t i e s that act as i n s u l a t o r s of e l e c t r i c a l wire, etc. More recently a number of polymeric semiconductors and conductors have been developed. Conductivity i s the r e c i p r o c a l of r e s i s t i v i t y . The e l e c t r i c a l p r o p e r t i e s can be a l t e r e d g r e a t l y by a d d i t i o n of i m p u r i t i e s and fillers. Thus, the b u l k r e s i s t i v i t y of ABS ( t e r p o l y m e r of a c r y l o n i t r i l e , butadiene, and styrene) can be decreased from 1 0 ^ to 10 1 ohm-cm by addition of s i l v e r powder. Several t e s t s are e s s e n t i a l for the e v a l u a t i o n of p l a s t i c s i n e l e c t r i c a l a p p l i c a t i o n s . These t e s t s include d i e l e c t r i c constant ( p e r m i t t i v i t y ; ASTM-D150-74), which i s the r a t i o of the capacitance of the polymer compared to a i r , d i e l e c t r i c strength, and d i e l e c t r i c breakdown voltage (ASTM-D149-75). D i e l e c t r i c breakdown voltage i s n

_


37

38


the maximum a p p l i e d v o l t a g e a polymer can w i t h s t a n d f o r 1 min divided by the thickness of the sample i n m i l s ( l C T i n . ) . The power factor i s the energy required for the r o t a t i o n of the d i p o l e s of a polymer i n an applied e l e c t r o s t a t i c f i e l d of increasing frequency. These values, which t y p i c a l l y range from 1.5 x 10* for polystyrene to 5 x 10~2 f r p l a s t i c i z e d c e l l u l o s e acetate, increase at Tg because of increased chain m o b i l i t y . The l o s s factor i s the product of the power f a c t o r and the d i e l e c t r i c c o n s t a n t . The a r c r e s i s t a n c e , or r e s i s t a n c e to t r a c k i n g , i s c o n s i d e r e d the minimum time r e q u i r e d for a h i g h - v o l t a g e d i s c h a r g e to f i n d a c o n d u c t i n g carbonized path across the surface of a polymer as evidenced by the disappearance of the arc into the test specimen (ASTM-D495-73). 3


0

Thermal A n a l y s i s . Thermal and r e l a t e d properties of polymers can be determined by v a r i o u s procedures i n c l u d i n g thermal g r a v i m e t r i c a n a l y s i s (TGA), d i f f e r e n t i a l scanning c a l o r i m e t r y (DSC), d i f f e r e n t i a l thermal a n a l y s i s (DTA), t o r s i o n a l b r a i d a n a l y s i s (TBA), thermal mechanical a n a l y s i s (TMA), and p y r o l y s i s gas chromatography (PGC). One of the s i m p l e s t techniques i s PGC i n which the gases r e s u l t i n g from the p y r o l y s i s of a polymer are a n a l y z e d by gas chromatography. T h i s technique may be used f o r q u a l i t a t i v e and q u a n t i t a t i v e a n a l y s i s . Quantitative a n a l y s i s requires c a l i b r a t i o n w i t h known amounts of standard polymer p y r o l y z e d under the same conditions as the unknown. S e v e r a l d i f f e r e n t modes of thermal a n a l y s i s are d e s c r i b e d as DSC. DSC i s a technique of nonequilibrium calorimetry i n which the heat flow into or away from the polymer i s measured as a function of temperature or time. This technique i s different than DTA i n which the temperature d i f f e r e n c e between a r e f e r e n c e and a sample i s measured as a function of temperature or time. Currently a v a i l a b l e DSC equipment measures the heat f l o w by m a i n t a i n i n g a thermal b a l a n c e between the reference and sample by changing a c u r r e n t p a s s i n g through the h e a t e r s under the two chambers. For example, the h e a t i n g of a sample and r e f e r e n c e proceeds at a predetermined r a t e u n t i l heat i s emitted or consumed by the sample. I f an endothermic occurrence takes p l a c e , the temperature of the sample w i l l be l e s s than t h a t of the r e f e r e n c e . The c i r c u i t r y i s programmed to maintain the reference and the sample compartments at the same temperature by r a i s i n g the temperature of the sample to t h a t of the r e f e r e n c e . The c u r r e n t necessary to m a i n t a i n the temperature of the sample at that of the reference i s recorded. The area under the r e s u l t i n g c u r v e i s a d i r e c t measure of the heat of transition. P o s s i b l e d e t e r m i n a t i o n s from DSC and DTA measurements include heat of t r a n s i t i o n ; heat of reaction; sample p u r i t y ; phase diagram; s p e c i f i c heat; sample i d e n t i f i c a t i o n ; r a t e of c r y s t a l l i z a t i o n , melting, or reaction; and a c t i v a t i o n energy. In TGA, a s e n s i t i v e balance i s used to f o l l o w the weight change of a polymer as a f u n c t i o n of time or temperature. In making both TGA and thermocalorimetric measurements, the same heating rate and f l o w of gas s h o u l d be employed to g i v e the most comparable thermograms. TGA can a l l o w determination of the f o l l o w i n g : sample p u r i t y , material i d e n t i f i c a t i o n , s o l v e n t retention, reaction rate, a c t i v a t i o n energy, heat of reaction, and polymer thermal s t a b i l i t y .



2.



TMA measures the mechanical response of a polymer as a function of temperature. Typical measurements as a function of temperature include the f o l l o w i n g : expansion properties, that i s , expansion of a material to c a l c u l a t e the l i n e a r expansion c o e f f i c i e n t ; tension properties, that i s , the measurement of shrinkage and expansion of a m a t e r i a l under t e n s i l e s t r e s s , f o r example, e l a s t i c modulus; d i l a t o m e t r y , t h a t i s , v o l u m e t r i c expansion w i t h i n a c o n f i n i n g medium, for example, s p e c i f i c volume; s i n g l e f i b e r properties, that i s , t e n s i l e response of s i n g l e f i b e r s under a s p e c i f i c l o a d , f o r example, s i n g l e - f i b e r modulus; and compression properties, such as measuring softening, or penetration under load. Compressive, t e n s i l e , and s i n g l e - f i b e r properties are u s u a l l y measured under some l o a d and y i e l d i n f o r m a t i o n about s o f t e n i n g p o i n t s , modulus changes, phase t r a n s i t i o n s , and creep p r o p e r t i e s . For compressive measurements, a probe i s p o s i t i o n e d on the sample and loaded with a given stress. A record of the penetration of the probe i n t o the polymer i s o b t a i n e d as a f u n c t i o n of temperature. T e n s i l e p r o p e r t i e s can be measured by a t t a c h i n g the f i b e r to two fused quartz hooks. One hook i s loaded w i t h a g i v e n s t r e s s . E l a s t i c modulus changes are recorded by m o n i t o r i n g a probe displacement. In TBA the changes i n t e n s i l e strength as the polymer undergoes thermal t r a n s i t i o n i s measured as a f u n c t i o n of temperature and sometimes a l s o as a function of the applied frequency of v i b r a t i o n of the sample. As thermal t r a n s i t i o n s are measured, i r r e v e r s i b l e changes such as thermal decomposition of c r o s s - l i n k i n g are observed, i f p r e s e n t . In g e n e r a l , a change i n Tg or change i n the shape of the curve (shear modulus versus temperature) during repeated sweeps through the region, such as a region containing the Tg, i s evidence of i r r e v e r s i b l e change. The name TBA i s derived from the fact that measurements are made on f i b e r s that are "braided" together to g i v e t e s t samples connected between or onto v i c e l i k e attachments or hooks. DSC, DTA, TMA, and TBA analyses are a l l i n t e r r e l a t e d and s i g n a l changes i n thermal behavior as a function of heating rate or time. TGA i s a l s o r e l a t e d to other a n a l y s e s i n the assignment of phase changes associated with weight changes. The polymer s o f t e n i n g range, a l t h o u g h not a s p e c i f i c thermodynamic p r o p e r t y , i s a v a l u a b l e "use" property and i s n o r m a l l y a simple and r e a d i l y obtainable property. Softening ranges g e n e r a l l y l i e between T and T . Some polymers do not e x h i b i t a s o f t e n i n g range but r a t h e r undergo a s o l i d s t a t e decomposition before softening. Softening ranges depend on the technique and procedure used to determine them. Thus, l i s t i n g s of s o f t e n i n g ranges s h o u l d be accompanied by the s p e c i f i c technique and procedure employed for the d e t e r m i n a t i o n . The f o l l o w i n g are techniques o f t e n used f o r the determination of polymer softening ranges. The c a p i l l a r y technique i s analogous to the technique employed to determine m e l t i n g p o i n t s of t y p i c a l o r g a n i c compounds. The sample i s placed i n a c a p i l l a r y tube and heated, and the temperature i s recorded from beginning to end of m e l t i n g . C o n t r o l of the h e a t i n g r a t e g i v e s more s i g n i f i c a n c e to t h e measurements. Instruments such as the F i s h e r - J o h n s m e l t i n g p o i n t apparatus are useful i n t h i s respect. g

m


39


40


Another technique r e q u i r e s a p l u g or f i l m (or other s u i t a b l e form) of the polymer to be stroked along a heated surface for which the temperature i s i n c r e a s e d u n t i l the polymer s t i c k s to the surface. A modification of t h i s uses a heated surface containing a temperature gradient between the ends of the surface. The Vicat needle method consists of determining the temperature at which a 1-mm penetration of a needle (having a point with an area of 1 mm) occurs on a standard sample (0.32 cm t h i c k w i t h a minimum w i d t h of 1.8 cm) at a s p e c i f i e d h e a t i n g r a t e (often 50 ° C / h ) under s p e c i f i c stress (generally l e s s than 1 kg). This determination i s r e l a t e d to the heat d e f l e c t i o n point. In the r i n g and b a l l method the s o f t e n i n g range of a sample i s determined by n o t i n g the temperature at which the sample, h e l d w i t h i n a h o r i z o n t a l r i n g , i s forced downward by the weight of a standard s t e e l b a l l supported by the sample. The b a l l and r i n g are g e n e r a l l y heated by i n s e r t i n g them i n a bath. Softening range data are useful i n s e l e c t i n g proper temperatures f o r melt f a b r i c a t i o n , such as m e l t p r e s s i n g , m e l t e x t r u d i n g , and molding. They a l s o indicate the thermal s t a b i l i t y of products. Other T e s t s . F l a m m a b i l i t y t e s t s f o r polymers i n c l u d e t e s t s f o r i g n i t i o n (ASTM-D-1929-68), r a t e of burning of c e l l u l a r p l a s t i c s (ASTM-D-1692-74), many f l a m m a b i l i t y t e s t s such as ASTM-D-635-74, measurements of smoke density (ASTM-D-2843-70), and the oxygen index t e s t (ASTM-D-2863-74). The oxygen i n d e x i s t h e minimum c o n c e n t r a t i o n of oxygen i n an o x y g e n - n i t r o g e n m i x t u r e t h a t w i l l support c a n d l e l i k e combustion. Flammability t e s t s are useful for comparative purposes, but because of the presence of many v a r i a b l e s i n a c t u a l f i r e s , they are not r e l i a b l e f o r a s s u r i n g l a c k of flammability i n l a r g e - s c a l e f i r e s . Because many polymers are r e s i s t a n t to a t t a c k by c o r r o s i v e s , t e s t s f o r c h e m i c a l r e s i s t a n c e of polymers are p a r t i c u l a r l y i m p o r t a n t . ASTM-D-543-67 (1977) measures weight and d i m e n s i o n a l changes of test samples immersed for 7 days i n many different t e s t s o l u t i o n s . These t e s t s may be c o u p l e d w i t h t e n s i l e t e s t s . Other ASTM tests include those under accelerated s e r v i c e conditions [ASTMD756-76 (1971)], water a b s o r p t i o n [ASTM-D570-63 (1972)], and environmental s t r e s s cracking of e t h y l e n e p l a s t i c s [(ASTMD1693-70)]. Other t e s t e d p r o p e r t i e s i n c l u d e those associated with o p t i c a l properties, a c o u s t i c a l properties, c o l o r a b i l i t y , and degradation. Spectroscopy. The u s u a l s p e c t r a l i n s t r u m e n t a t i o n a p p l i c a b l e to smaller molecules are e q u a l l y a p p l i c a b l e to macroraolecules w i t h l i t t l e m o d i f i c a t i o n . For example, the o l d axiom t h a t good IR spectra cannot be obtained f o r p o l y m e r s , a l t h o u g h untrue (because p o l y s t y r e n e f i l m i s used to s t a n d a r d i z e s p e c t r a ) , does have some factual basis. S p e c t r a of amorphous polymers, or polymers c o n t a i n i n g an abundance of amorphous r e g i o n s , are t y p i c a l l y l e s s sharp r e l a t i v e to spectra of s o l i d , well-ordered organic c r y s t a l l i n e compounds. Thus, "background" c o n t r i b u t i o n s and l a c k of band sharpness are responsible for " h i d i n g " many p e r t i n e n t v i b r a t i o n a l bands within polymers. This phenomena i s r e a d i l y overcome by using Fourier transform IR spectrophotometry. Thus, although bands, etc.


2.



tend to be l e s s sharp for many polymers, t h i s lack of sharpness can often be overcome with proper instrumentation. S p e c t r o s c o p i c i n s t r u m e n t a t i o n t h a t has been w i d e l y and s u c c e s s f u l l y applied to polymers includes IR, NMR, e l e c t r o n spin resonance, UV, X - r a y , near IR, SIMS (secondary i o n mass s p e c t r o metry), MS (mass spectrometry), photoacoustic, Raman, and microwave spectroscopy, and e l e c t r o n spectroscopy for chemical a n a l y s i s .


Educational Aspects Because the m a j o r i t y of s c i e n t i s t s and engineers are employed i n some aspect of the polymer industry, a number of s o c i e t i e s have been a c t i v e i n the education areas. These groups include the Society of P l a s t i c s Engineers, P l a s t i c s I n s t i t u t e of America, the Society of the P l a s t i c s I n d u s t r y , and the American Chemical S o c i e t y (ACS). With the e x c e p t i o n of the ACS, these groups have focused on continuing education by offering short courses under a wide v a r i e t y of formats. Much of the a c t i v i t y w i t h i n the ACS has focused on the J o i n t Polymer Education Committees l a r g e l y composed of members from the D i v i s i o n s of Polymer Chemistry and Polymeric M a t e r i a l s : Science and Engineering (formerly Organic Coatings and P l a s t i c s Chemistry). The f i r s t s t a n d a r d i z e d ACS examination i n polymer c h e m i s t r y was developed i n 1978. A model s y l l a b u s was generated i n 1980. Probably the most s i g n i f i c a n t s i n g l e event i n polymer education occurred i n 1978. The l a t e s t e d i t i o n of "Undergraduate Professional Education i n Chemistry: C r i t e r i a and E v a l u a t i o n Procedures" by the ACS Committee on P r o f e s s i o n a l T r a i n i n g s t a t e s , "In view of the current importance of inorganic chemistry, biochemistry, and polymer c h e m i s t r y , advanced courses i n these areas are e s p e c i a l l y recommended and students should be strongly encouraged to take one or more of them. Furthermore, the b a s i c a s p e c t s of these t h r e e important areas s h o u l d be i n c l u d e d at someplace i n the core m a t e r i a l s . " A f t e r almost 30 years as a s t a l w a r t of the s c i e n c e s , polymer chemistry has been recognized as e s s e n t i a l core material i n the t r a i n i n g of a l l ACS accredited undergraduate majors. The f u l l impact of these new p r o v i s i o n s i s yet to be f u l l y r e c o g n i z e d . Educators are a d v o c a t i n g t h a t polymer c h e m i s t r y be recommended as advanced work, and p o s s i b l y of g r e a t e r importance t h a t " b a s i c a s p e c t s " of polymer c h e m i s t r y be i n c l u d e d i n the core m a t e r i a l . The e d u c a t i o n committees of a number of d i v i s i o n s and s o c i e t i e s a s s o c i a t e d w i t h polymer s c i e n c e are working toward adopting recommendations i n v o l v e d w i t h these two major r e l a t e d points. Nomenclature The International Union of Pure and Applied Chemistry (IUPAC) formed a Subcommission on Nomenclature of Macromolecules i n 1952 and has proceeded to study v a r i o u s t o p i c s r e l a t e d to c y c l i c polymers, blends, composites, c r o s s - l i n k e d polymers, b l o c k copolymers, e t c . IUPAC p e r i o d i c a l l y reports i t s decisions regarding nomenclature (1_, Tj, and 8}. Even so, these r u l e s have not been g e n e r a l l y accepted f o r common polymers by the m a j o r i t y of those i n polymer s c i e n c e ,


41


Sample Names of Some Common Polymers IUPAC Poly(methylene) Poly(1-phenylethylene) Poly(iminohexamethylene iminoadepoly) Poly(oxy-l,4-phenylene) Poly(oxyethyleneoxyterephthaloyl Poly(l-chloroethylene) Poly[(2-propy1-1,3-dioxene4,6-diyl)methylene] Poly[l-(methoxycarbonyl)crylate)1-methylethylene]

Common Polyethylene Polystyrene Poly(hexamethylene adipamide) Poly(phenylene oxide) Poly(ethylene t e r e phthalate Poly(vinyl chloride) P o l y ( v i n y l butyral) Poly(methyl methaacrylate

Polyethylene

Polystyrene

Polyhexamethylene adipamide

Polyphenylene oxide

Polyethylene terephthalate

Polyvinyl chloride

P o l y v i n y l butyral

Polymethyl methacrylate

Industrial

Table I .



2.

CARRAHER AND SEYMOUR


such as textbook and monograph a u t h o r s , a l t h o u g h many j o u r n a l s require conformity to IUPAC r u l e s . A l t h o u g h a wide d i v e r s i t y e x i s t s i n the p r a c t i c e of naming polymers, three approaches represent the most used systems. Table I g i v e s the names of some common polymers to i l l u s t r a t e the t h r e e systems. The o n l y f o r m a l system i s the IUPAC system J7, and 8). The second system i s r e f e r r e d to as s i m p l y the i n d u s t r i a l system because i t i s used by a number of i n d u s t r i a l s o c i e t i e s f o r t h e i r p u b l i c a t i o n s . The t h i r d system i s referred to as the common system because of past h i s t o r i c a l use. The l a t t e r two systems are informal (semisystematic) and are o n l y u s e f u l f o r the more common, s i m p l e polymers and t y p i c a l l y d i f f e r from one another only by the absence or presence of parentheses. The majority of undergraduate texts use the i n d u s t r i a l system, and a few of the polymer t e x t s have adopted the IUPAC system f o r common polymers. An IUPAC r e p o r t (9} s t a t e s "The Commission recognized that a number of common polymers have semisystematic or t r i v i a l names that are w e l l established by usage; i t i s not intended that they be immediately supplanted by the s t r u c t u r e - b a s e d names. Nonetheless, i t i s hoped that for s c i e n t i f i c communication the use of semisystematic or t r i v i a l names f o r polymers w i l l be kept to a minimum." Nevertheless, the trend i s toward usage of the i n d u s t r i a l system. In summary, c u r r e n t l y a d i v e r s i t y of polymer nomenclature e x i s t s with regard to common polymers with none being u n i v e r s a l l y accepted as c o r r e c t . L i t e r a t u r e Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

Seymour, R.; Carraher, C. "Polymer Chemistry: An Introduction"; Plenum: New York, 1981. Marvel, C.; Carraher, C. CHEMTECH 1985, 716-20. Carothers, W. Chem. Rev. 1931, 8, 353-426. Alfrey, T.; B o h r e r , J.; M a r k , H. "Copolymerization"; Interscience: New York, 1952. Wall, F . J. Am. Chem. Soc. 1944, 66, 2050-7. Dostal, H. Monatsh. Chem. 1936, 69, 424-6. "Compilation of ASTM Standard Definitions"; American Society for Testing and M a t e r i a l s : P h i l a d e l p h i a . E l i a s , H. G.; Pethrick, R. "Polymer Yearbook"; Harwood Academic: New York, 1984; Chap. 1. Hall, C. "Polymer Materials"; Macmillan: New York, 1981; pp. 184-7. Macromolecules 1973, 6(2), 149-54.

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