Multiphase Polymers - ACS Publications - American Chemical Society

5 Thermoplastic Polyurethane Elastomer

Downloaded by UNIV MASSACHUSETTS AMHERST on September 9, 2012 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0176.ch005

Structure—Thermal Response Relations C. S. S C H O L L E N B E R G E R B. F. Goodrich Co., Research and Development Center, 9921 Brecksville Rd., Brecksville, O H 44141

Thermoplastic

polyurethane

elastomers are high

perform-

ance materials that have found a variety of uses including solution applications as coatings and adhesives,

extrusion

applications as tubing and jacketing, film applications, and molding applications, particularly flexible injection-molded automotive parts such as front ends, sight shields, fascia, etc.

These polymers have a segmented structure

gives rise to heterophase morphology.

which

This morphology is

responsible for the fascinating and useful "virtually crosslinked" state of the polymers, and determines other important

polymer

properties.

This

chapter

describes

thermal analysis, namely differential scanning

how

calorimetry

(DSC), can be applied to detect and investigate the contribution of the separate phases in such polymers.

H P h e w i d e s p r e a d use of p l a s t i c a n d r u b b e r parts w i t h i n v e h i c l e passenger a n d e n g i n e c o m p a r t m e n t s has b e e n p r a c t i c e d f o r some t i m e n o w b y t h e a u t o m o t i v e i n d u s t r y a n d continues to g r o w . requiring

specific

front-

e x t e n d e d t h e use of b o t h structural members.

a n d rear-end flexible

vehicle

H o w e v e r , legislation i m p a c t resistance has

plastics a n d r u b b e r s t o b o d y - e x t e r i o r

F l e x i b l e p l a s t i c a n d r u b b e r f r o n t ends, sight shields,

a n d fascia have appeared o n p r o d u c t i o n m o d e l automobiles for approxim a t e l y 10 years a n d are n o w w e l l e s t a b l i s h e d . A n i m p o r t a n t r e q u i r e m e n t of the materials u s e d f o r these a p p l i c a t i o n s is

flexibility,

w h i c h enables

i m p a c t e d parts t o d i s t o r t w i t h o u t b r e a k i n g t h e n t o c o m p l e t e l y t h e i r o r i g i n a l shape u n d a m a g e d . 0-8412-0457-8/79/33-176-083$05.00/0 © 1979 American Chemical Society

In Multiphase Polymers; Cooper, S., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

recover

MULTIPHASE POLYMERS

84

N o w a n o t h e r f a c t o r has a p p e a r e d o n the scene. It is the w o r l d - w i d e e n e r g y shortage w h i c h r e c o m m e n d s the use of c o n s t r u c t i o n m a t e r i a l s less dense t h a n m e t a l w h e r e v e r p r a c t i c a l to p e r m i t the s u b s t a n t i a l r e d u c t i o n of o v e r a l l v e h i c l e w e i g h t i n the interest of i m p r o v e d f u e l e c o n o m y .

This

d e v e l o p m e n t c o m b i n e s w i t h i m p a c t r e q u i r e m e n t s to s t r e n g t h e n the case f o r m u c h b r o a d e r use of s y n t h e t i c p o l y m e r s i n a u t o m o b i l e s .

So, the t i m e


w o u l d seem to b e r i p e to c a p i t a l i z e m o r e f u l l y o n t h e use of l i g h t - w e i g h t , flexible

p o l y m e r i c materials i n m a k i n g exterior a u t o m o b i l e - b o d y

tural members.

struc-

I n d e e d , w e a l r e a d y h e a r of the " f r i e n d l y f e n d e r , "

the

n o n d e n t i n g d o o r p a n e l , etc. It w o u l d seem

that

such broadened

use of

flexible

plastics

and

r u b b e r s i n e x t e r i o r - b o d y s t r u c t u r a l m e m b e r s w i l l c o m p e l the c o n c l u s i o n that h i g h p e r f o r m a n c e m a t e r i a l s are n e e d e d . relatively light,

flexible,

fabricated

finished,

possible.

and

T h e y must not only

s e r v i c e a b l e at t e m p e r a t u r e extremes,

b u t t h e y m u s t b e as s t r o n g a n d as t o u g h

T h e p u b l i c w i l l d e m a n d this.

O n e t y p e of

be

a n d easily

flexible

as

synthetic

p o l y m e r w h i c h meets these r e q u i r e m e n t s v e r y w e l l i n d e e d is the t h e r m o plastic urethane flexible

elastomers

w h i c h h a v e a l r e a d y p r o v e d themselves

in

f r o n t ends, sight shields, a n d f a s c i a i n p r o d u c t i o n m o d e l auto-

mobiles. T h e r m o p l a s t i c p o l y u r e t h a n e elastomers w e r e i n v e n t e d a n d d e s c r i b e d a l m o s t 20 years ago (1,2).

T h e y w e r e first c o m m e r c i a l i z e d b y the B . F .

G o o d r i c h C h e m i c a l C o . as " E s t a n e " i n 1959.

I n this c h a p t e r I w i l l discuss

the u s e f u l b u t r a t h e r c o m p l e x T P U elastomers i n terms of t h e i r c h e m i c a l c o m p o s i t i o n , c h e m i c a l structure, a n d m o l e c u l a r w e i g h t , a n d h o w these factors affect t h e i r i n t e r n a l p h y s i c a l s t r u c t u r e (i.e., m o r p h o l o g y ) , n a t u r e , and properties. Specifically, I w i l l show h o w differential scanning calorim e t r y ( D S C ) , a m e t h o d of t h e r m a l analysis, c a n b e u s e d to l e a r n m u c h a b o u t a n d to p r e d i c t p r o c e s s i n g a n d p e r f o r m a n c e characteristics of T P U elastomers,

s u c h as are b e i n g u s e d c u r r e n t l y i n a u t o m o t i v e

injection-

m o l d i n g a p p l i c a t i o n s . T h e effects of changes i n T P U c h e m i c a l c o m p o s i t i o n a n d m o l e c u l a r w e i g h t o n t h e r m a l transitions also w i l l b e d i s c u s s e d .

TPU

Elastomer

Composition

T P U elastomers are u s u a l l y m a d e f r o m three c h e m i c a l s : ( 1 ) cyanate, (2)

a diiso-

a high-molecular-weight macroglycol, a n d (3) a low-molec-

u l a r - w e i g h t c h a i n extender g l y c o l .

E x a m p l e s of these t h r e e c o m p o u n d s

a n d t h e i r structures are s h o w n i n F i g u r e 1. N o w , i t h a p p e n s that the r e l a t i v e a m o u n t s of the a b o v e r e a c t a n t types u s e d to m a k e the T P U elastomer a n d the o r d e r i n w h i c h t h e y c h e m i c a l l y r e a c t w i t h e a c h other is v e r y i m p o r t a n t , f o r this d e t e r m i n e s the s t r u c t u r e of T P U p o l y m e r c h a i n s , w h i c h m u s t b e

"segmented."


5.

SCHOLLENBERGER

Thermoplastic Polyurethane Typical Example

Component


Structure

DIISOCYANATE

MDI

OCN -O~ 2-"C)"~

MACROGLYCOL

PTAd

H0(CH ) +0C0(CH ) CO 0(CH > - OH

(HIGH

(LOW

NC0

CH

2

4

2 4

2

4

n

MW,-IOOO-2000)

CHAIN EXTENDER GLYCOL

1,4 BDO

HO(CH )

OH

2

MWÎ

4

Figure 1.

TPU

85

Elastomers

Elastomer

Chemical components of TPU elastomer

Structure

W h a t I mean b y segmented

structure i n T P U elastomer

a p p a r e n t - i n the s c h e m a t i c of F i g u r e 2.

chains

is

T h e structure i n F i g u r e 2 shows

t h e T P U elastomer to consist of l i n e a r p o l y m e r p r i m a r y c h a i n s .

These

p r i m a r y chains are s e g m e n t e d i n c o m p o s i t i o n . S p e c i f i c a l l y t h e y are m a d e u p of a l t e r n a t i n g h a r d a n d soft segments w h i c h are joined, e n d to e n d t h r o u g h strong, c o v a l e n t c h e m i c a l l i n k a g e s . Soft Segments. T h e soft segments are t h e l i n e a r r e a c t i o n of the d i i s o c y a n a t e c o m p o n e n t ,

products

e.g., M D I ( d i p h e n y l m e t h a n e - p , p ' - d i i s o -

c y a n a t e ) , a n d the m a c r o g l y c o l c o m p o n e n t , e.g., P T A d [ p o l y ( t e t r a m e t h y l ene a d i p a t e ) g l y c o l ] , as is seen i n F i g u r e 3. B e c a u s e of t h e d e l i b e r a t e l o w m e l t i n g o r l i q u i d n a t u r e of t h e m a c r o g l y c o l c o m p o n e n t , w h i c h has a m o l e c u l a r w e i g h t of a b o u t 1000-2000 a n d so c o m p r i s e s a p p r o x i m a t e l y 75-95% of t h e soft segments, t h e soft segments i n the d e r i v e d T P U chains t e n d to b e l o w m e l t i n g a n d l a r g e l y amorphous.

A t a n y a p p r e c i a b l e m o l e c u l a r w e i g h t , the soft s e g m e n t b y

itself tends to b e l i k e a g u m . W i t h this e x p l a n a t i o n w e m a y n o w e q u a t e the

exemplary

soft-segment c h e m i c a l

structure

of

Figure

3 with

s c h e m a t i c r e p r e s e n t a t i o n ( / ^ ^ ^ ) i n F i g u r e 2.

MDI +1,4 BD0

i

SOFT

HARD

SOFT

HARD

SOFT

HARD

SOFT

HARD

SOFT

MDI+PTAd Figure

2.

Schematic of TPU-elastomer chains

polymer primary


its

86

MULTIPHASE

POLYMERS

0 C N - ^ - C H - ^ - N C 0 + HO(CH ) |oCO(CH ) COO(CH )^OH-» 2

2

(MDI)


"*

c o

N H

"O"

4

2

4

2

(PTAd)

CH

2 "Ό"

NHC0

°(CH ) [OC0(CH ) COO(CH ) j - 0 - 2

4

2

4

2 4

(SOFT SEGMENT) Figure 3 .

Typical TFU-elastomer soft-segment

structure

S e v e r a l types of m a c r o g l y c o l s f o r u s e i n f o r m i n g p o l y u r e t h a n e soft segments are possible a n d p r a c t i c a l . E x a m p l e s of some of these are l i s t e d i n F i g u r e 4. T h e p o l y m e r c h e m i s t makes his m a c r o g l y c o l selection b a s e d o n s u c h considerations as t h e d e s i r e d m e c h a n i c a l properties, l o w - t e m perature flexibility, e n v i r o n m e n t a l resistance, a n d economics o f t h e d e r i v e d T P U . E v e n b y h i s c h o i c e of m a c r o g l y c o l m o l e c u l a r w e i g h t , t h e p o l y m e r chemist c a n f u r t h e r regulate s u c h T P U elastomer properties as low-temperature flexibility. H a r d Segments. T h e h a r d segments a r e t h e l i n e a r r e a c t i o n p r o d u c t s of t h e diisocyanate c o m p o n e n t a n d t h e t h i r d m o n o m e r t y p e i n t h e T P U elastomer r e c i p e , t h e s m a l l g l y c o l - c h a i n - e x t e n d e r c o m p o n e n t . I n F i g u r e 5 w e see a t y p i c a l T P U hard-segment structure w h i c h is f o r m e d f r o m M D I a n d 1 , 4 - B D O ( 1,4-butanediol ). T h e p o l y m e r chemist selects h i s d i i s o c y a n a t e a n d chain-extenderg l y c o l c o m p o n e n t s to p r o d u c e h i g h - m e l t i n g h a r d segments i n t h e d e r i v e d T P U chains. H a r d segments d i s p l a y some b u t n o t a l l of t h e classical characteristics of t h e c r y s t a l l i n e state a n d f o r this reason h a v e b e e n c a l l e d " p a r a c r y s t a l l i n e . " A t a n y a p p r e c i a b l e m o l e c u l a r w e i g h t t h e h a r d segment b y itself w o u l d t e n d to b e h a r d a n d n y l o n l i k e i n character. W i t h this e x p l a n a t i o n w e m a y n o w equate t h e e x e m p l a r y h a r d - s e g m e n t structure of F i g u r e 5 w i t h its schematic representation ( l l ) i n F i g u r e 2. S e v e r a l types of diisocyanates ( a r o m a t i c , a l i p h a t i c , c y c l o a l i p h a t i c ) a n d m a n y different g l y c o l - c h a i n extenders ( o p e n - c h a i n a l i p h a t i c , c y c l o a l i p h a t i c , a r o m a t i c a l i p h a t i c ) c a n b e u s e d to p r o d u c e T P U - e l a s t o m e r h a r d segments. I n t h e m o r e c o n v e n t i o n a l a n d p r a c t i c a l f o r m u l a t i o n s o n l y a single diisocyanate c o m p o n e n t is u s e d to m a k e a T P U , so t h e d i i s o cyanate is c o m m o n to b o t h t h e h a r d a n d soft segments. T h e p o l y m e r c h e m i s t makes h i s diisocyanate a n d g l y c o l - c h a i n - e x t e n d e r c o m p o n e n t selections b a s e d o n s u c h considerations as d e s i r e d T P U m e c h a n i c a l properties, u p p e r service temperature, e n v i r o n m e n t a l resistance, s o l u b i l i t y characteristics, a n d e c o n o m i c s .


In Multiphase Polymers; Cooper, S., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979. 4

PQLYHYDRQCARBON GLYCOL

POLYETHER GLYCOL

Figure 4.

Some macroglycol types

(a) POLYBUTADIENE GLYCOL

2

n

2

2

CH II CH

-|-CH -ÇH

5

f CH -CH = CH~CH ^ L and

Γ.

CH*

2

f0CH-CH }

POLY( 1,2 ~OXYPROPYLENE)GLYCOL

(b)

2

fo(CH ) j -

6

(a) POLYfTETRAMETHYLENE OXIDE) GLYCOL

2

£(CH ) ~ O-CO-O^

5

(c) POLY(HEXAMETHYLENE CARBONATE)GLYCOL

2

t(CH ) -C00l

POLY(CAPROLACTONE)GLYCOL

2

(b)

4

(a) POLYfTETRAMETHYLENE ADIPATE)GLYCOL

POLYESTER GLYCOL 2

£oCO(CH ) -CO-0(CH ) --^

Example

M a c r o g l y c o l Type

Repeat Unit Structure


88

MULTIPHASE

0CN

"O~ 2^O" CH

NC0

POLYMERS

+ HO(CHg) OH

(1,4 BDO)

(MOD

+ CONH-0-CH -0^NHCOO(CH ) -o| L Jη (HARD SEGMENT) 2


Figure 5. Typical TPU-elastomer hard-segment structure

2

4

Just as the p o l y m e r c h e m i s t c a n s i g n i f i c a n t l y r e g u l a t e T P U elastomer, l o w - t e m p e r a t u r e flexibility b y h i s c h o i c e of m a c r o g l y c o l m o l e c u l a r w e i g h t a n d thus soft-segment l e n g t h ( m o l e c u l a r w e i g h t ) , h e also c a n regulate o t h e r i m p o r t a n t T P U - e l a s t o m e r properties b y r e g u l a t i n g h a r d - s e g m e n t l e n g t h ( m o l e c u l a r w e i g h t ) . S u c h r e g u l a t i o n is easily a c c o m p l i s h e d b y v a r y i n g t h e amounts of the d i i s o c y a n a t e a n d g l y c o l - c h a i n - e x t e n d e r c o m ponents ( r e l a t i v e to the m a c r o g l y c o l a m o u n t ) c h a r g e d i n the T P U elastomer f o r m u l a t i o n . I n c r e a s i n g m a t c h e d a m o u n t s of t h e d i i s o c y a n a t e a n d g l y c o l - c h a i n - e x t e n d e r c o m p o n e n t s , relative to the m a c r o g l y c o l a m o u n t , increase h a r d - s e g m e n t c h a i n l e n g t h ( m o l e c u l a r w e i g h t ) a n d T P U - u r e thane c o n c e n t r a t i o n . T h i s i n t u r n increases T P U m o d u l u s (hardness, stiffness, l o a d - b e a r i n g c a p a c i t y ) , p r o c e s s i n g temperature, u p p e r service temperature, etc., w h i l e r e d u c i n g T P U s o l u b i l i t y .

Polymer Chain Organization F i g u r e 2 d e p i c t e d the T P U - e l a s t o m e r p r i m a r y c h a i n s c h e m a t i c a l l y and, I believe, accurately. B u t i n the solid polymer, the p o l y m e r p r i m a r y chains d o n o t r e a l l y exist separately as s h o w n i n F i g u r e 2. R a t h e r , a l l e v i d e n c e points to t h e fact that the h a r d segments of the T P U chains strongly attract a n d t e n d to associate w i t h e a c h other t h r o u g h u r e t h a n e - u r e t h a n e h y d r o g e n b o n d i n g a n d a r o m a t i c ττ-electron attractions. A s a result, t h e h a r d segments i n t h e p o l y m e r p r i m a r y chains f o r m aggregates ( d o m a i n s ) i n t h e m o b i l e soft-segment m a t r i x a n d a t w o - p h a s e p o l y m e r system results. T h i s n e w o r g a n i z a t i o n of the p o l y m e r p r i m a r y chains is d e p i c t e d s c h e m a t i c a l l y i n F i g u r e 6. T h e separate hard-segment aggregates ( d o m a i n s ) are seen to tie the l i n e a r p o l y m e r p r i m a r y c h a i n s together i n l a t e r a l fashion, i n effect c r o s s l i n k i n g t h e m . It is also a p p a r e n t that the same p h e n o m e n o n extends t h e chains i n l i n e a r f a s h i o n . T h e c o m b i n e d l a t e r a l a n d l i n e a r effects p r o d u c e a g i a n t c r o s s l i n k e d n e t w o r k w h i c h accounts f o r t h e elastic character of T P U elastomers. H o w e v e r , these crosslinks w h i c h h o l d t h e T P U n e t w o r k together c a n b e o v e r c o m e b y heat or b y s o l v a t i o n b y processes w h i c h m o r e o r less regenerate the p o l y m e r p r i m a r y chains. T h i s is a r e a d i l y reversible p r o c ess, as F i g u r e 6 shows, a n d o n c o o l i n g o r o n d r y i n g free of solvent, t h e



5.

SCHOLLENBERGER

Thermoplastic Polyurethane Elastomers

89

VIRTUALLY CROSSLINKED/EXTENDED NETWORK of POLYMER PRIMARY CHAINS J J Δ or SOLVENT

POLYMER PRIMARY CHAINS

Figure 6.

Schematic of chain organization in solid TPU elastomer

p o l y m e r p r i m a r y chains r e f o r m t h e T P U - n e t w o r k s t r u c t u r e i n t h e i r n e w l y f a b r i c a t e d f o r m . T h e f o r e g o i n g l a b i l i t y o f T P U - e l a s t o m e r n e t w o r k s leads us to r e f e r to t h e m as b e i n g " v i r t u a l l y c r o s s l i n k e d " ( 2 ) . T h a t is, t h e y a r e c r o s s l i n k e d i n effect b u t n o t i n fact, a n d t h e h a r d - s e g m e n t d o m a i n s a r e the v i r t u a l crosslinks. Hard-Segment

Domain

Morphology

A s c a n b e seen, h a r d - s e g m e n t d o m a i n s a r e a necessary s t r u c t u r a l feature o f T P U elastomers since t h e y t i e t h e p o l y m e r p r i m a r y c h a i n s together.

W i t h o u t these d o m a i n s T P U w o u l d l a c k elastic c h a r a c t e r a n d

w o u l d b e g u m l i k e i n nature. M u c h s t u d y o f t h e n a t u r e o f T P U h a r d - s e g m e n t d o m a i n s has b e e n m a d e (3-11 ), p a r t i c u l a r l y since t h e y m i g h t b e e x p e c t e d t o b e c r y s t a l l i n e b u t d o n o t a p p e a r t o b e b y c o n v e n t i o n a l c r y s t a l l i n i t y tests s u c h as w i d e a n g l e x-ray d i f f r a c t i o n ( W A X D ).

I t a p p e a r s that t h e s t r o n g m u t u a l

a t t r a c t i o n o f t h e h a r d segments (e.g., u r e t h a n e - u r e t h a n e h y d r o g e n b o n d i n g ) restricts t h e i r m o b i l i t y a n d thus t h e i r a b i l i t y to o r g a n i z e themselves w e l l into a crystalline lattice.

P e r h a p s t h e s i t u a t i o n is s i m i l a r t o t h a t

e n c o u n t e r e d i n c o n c e n t r a t e d solutions o f t h e sugars w h i c h c r y s t a l l i z e r e l u c t a n t l y , a g a i n l i k e l y b e c a u s e o f t h e restrictions o f h y d r o g e n b o n d i n g . T h e u n o r i e n t e d p o l y a m i d e s w o u l d seem t o b e a s i m i l a r case. T h e p o s s i b i l i t y also exists that t h e h a r d - s e g m e n t d o m a i n s a r e r e a l l y c r y s t a l l i n e , b u t t h e crystals a r e so s m a l l that t h e y a r e n o t d e t e c t e d b y WAXD.

I n this r e g a r d , s m a l l angle

x-ray d i f f r a c t i o n ( S A X D )

does

d e t e c t t h e d o m a i n s , so w e k n o w t h a t t h e y exist, t h a t t h e y h a v e some o r d e r ,


90

MULTIPHASE POLYMERS

a n d w h a t t h e i r size a n d s e p a r a t i o n i n t h e soft-segment m a t r i x a r e . T h e s u b c r y s t a l l i n e state o f p o l y u r e t h a n e elastomer h a r d - s e g m e n t d o m a i n s has b e e n r e f e r r e d t o as t h e " p a r a c r y s t a l l i n e state"

Phase (Hard

Segment, Soft Segment)

(4).

Segregation


A final p o i n t o n T P U s t r u c t u r e . Studies h a v e s h o w n (11 ) that m e l t i n g a T P U elastomer, as d u r i n g p r o c e s s i n g , results i n t h e r e m i x i n g o f its h a r d a n d soft segments.

T h i s is f o l l o w e d b y t h e i r t e n d e n c y to d e - m i x ,

o r segregate, i n t h e c o o l e d s o l i d p o l y m e r . I t c a n b e a p p r e c i a t e d f r o m F i g u r e 6 that a T P U - e l a s t o m e r s a m p l e w h o s e h a r d segments

have not

a g g r e g a t e d f a i r l y w e l l has n o t d e v e l o p e d its u l t i m a t e p r o p e r t y p o t e n t i a l . O f t e n t h e segregation o f h a r d a n d soft segments increases w i t h t i m e . I n some T P U c o m p o s i t i o n s t h e i n t e r m o l e c u l a r forces f a v o r i n g t h e m i x e d o r the d e - m i x e d states seem to l a r g e l y b a l a n c e e a c h other, a n d d e - m i x i n g is a p p r e c i a b l y r e t a r d e d . I n a n y case, phase segregation i n T P U elastomers is a t i m e - d e p e n d e n t p h e n o m e n o n w h o s e consequences

c a n n o t b e over-

l o o k e d i n t h e s t u d y a n d u s e o f these p o l y m e r s . T h e r m a l analysis p r o v e s to b e a n excellent t o o l f o r i n v e s t i g a t i n g t h e p r o p e r t y - d e t e r m i n i n g m o r p h o l o g i c a l state a n d tendencies o f t h e T P U elastomers.

I n t h e b a l a n c e of

this c h a p t e r , I w i l l d e m o n s t r a t e t h e t h e r m a l responses c o m m o n l y seen i n T P U b y D S C , w h a t they m e a n , a n d h o w t h e y c a n a i d i n u n d e r s t a n d i n g and improving polymer performance.

Thermal

Analysis

of TPU

Differential Scanning Calorimetry

(DSC).

T h e D S C instrument

heats o r cools a s m a l l ( 1 0 - 1 5 m g ) s a m p l e u n d e r v e r y c a r e f u l l y c o n t r o l l e d and reproducible conditions.

W h i l e this is g o i n g o n a v e r y sensitive

t h e r m o c o u p l e c o n t i n u o u s l y m o n i t o r s changes i n t h e heat c a p a c i t y of t h e s a m p l e , w h i c h are r e c o r d e d as a t h e r m o g r a m . changes

whenever the temperature

S a m p l e heat

capacity

reaches a p o i n t w h e r e i t causes a

c h a n g e i n t h e o r g a n i z a t i o n o f t h e m o l e c u l e s i n t h e s a m p l e . S u c h detecta b l e changes a r e c a l l e d t h e r m a l responses o r t h e r m a l transitions. D S C curves are p l o t t e d as i n F i g u r e 7. I n F i g u r e 7, t h e v e r t i c l e axis is t h e d i f f e r e n t i a l heat scale w h i c h is p r o p o r t i o n a l to t h e heat

flow

(mcal/sec/in. ) into or out of the T P U

s a m p l e i n a s m a l l a l u m i n u m p a n ( s a m p l e system ) w i t h respect t o a n i d e n t i c a l b u t e m p t y p a n ( r e f e r e n c e s y s t e m ) , as t h e t o t a l system ( reference system, s a m p l e system ) t e m p e r a t u r e is r a i s e d o r l o w e r e d u n i f o r m l y . T h e h o r i z o n t a l axis is t h e t e m p e r a t u r e scale ( ° C ) f o r t h e a c t u a l t e m p e r a t u r e at t h e s a m p l e .



5.

SCHOLLENBERGER

Thermoplastic

Polyurethane

SAMPLE TEMPERATURE, C — e

Figure

7.

91

Elastomers

DSC thermogram of an injection-molding urethane elastomer

-

thermoplastic

poly-

T h e T P U s a m p l e tested i n F i g u r e 7 w a s i n j e c t i o n m o l d e d f r o m a tough, strong, high-urethane-content contains p o l y ( t e t r a m e t h y l e n e

injection-molding polymer w h i c h

a d i p a t e ) - M D I soft segments

a n d 1 , 4 - b u t a n e d i o l - M D I h a r d segments

( Figure 3)

( F i g u r e 5 ) . O u r test p r o c e d u r e

w a s t o : ( 1 ) r a p i d l y c o o l the s a m p l e to — 1 2 0 ° C i n o u r D S C i n s t r u m e n t (duPont, M o d e l 990);

( 2 ) heat i t f r o m -

120°C to + 2 5 0 ° C at 1 0 ° C /

m i n u n d e r n i t r o g e n ; ( 3 ) i m m e d i a t e l y c o o l i t to 2 5 ° C at t h e same rate. GLASS-TRANSITION TEMPERATURE ( T ) . g

A s w e s l o w l y heat t h e T P U

s a m p l e ( t o p c u r v e ) , a n increase i n its rate o f heat a b s o r p t i o n is n o t e d a t A w h e r e t h e s l o p e o f t h e u p p e r D S C c u r v e increases. persists u n t i l B , w h e r e i t reverts t o its o r i g i n a l rate.

T h i s n e w rate

T h e A - B region is

t h e t e m p e r a t u r e r a n g e i n w h i c h the T P U soft segments g o f r o m a r i g i d " g l a s s y " c o n d i t i o n t o a flexible " r u b b e r y " c o n d i t i o n . T h i s i s c a l l e d t h e g l a s s - t r a n s i t i o n r e g i o n o f t h e T P U soft segments, a n d t h e c h a n g e o c c u r s w h e n the a p p l i e d thermal energy

is a d e q u a t e

to overcome

a set o f

r e l a t i v e l y w e a k , i n t e r c h a i n a t t r a c t i v e forces w h i c h i m m o b i l i z e t h e T P U c h a i n segments, C

allowing them to move.

B y convention, t h e midpoint

( — 3 0 . 5 ° C ) o f A - B is c a l l e d t h e g l a s s - t r a n s i t i o n t e m p e r a t u r e , T . g

T has p r a c t i c a l s i g n i f i c a n c e i n T P U elastomers since i t i n d i c a t e s t h e g

t e m p e r a t u r e at w h i c h the p o l y m e r w i l l lose a p p r e c i a b l e

flexibility


as i t

92

MULTIPHASE POLYMERS

cools.

I n T P U elastomer, soft segments that p r o d u c e l o w T

g

desirable, for they indicate g o o d low-temperature MELTING TEMPERATURE ( T ) M

values

are

flexibility. As uni-

AND H E A T OF FUSION (àH ). t

f o r m h e a t i n g of the T P U samples continues b e y o n d B , n o f u r t h e r i n t e r p r é t a b l e t h e r m a l events are n o t e d u n t i l D ( 1 0 6 . 0 ° C ) , w h e r e a n i n c r e a s e d rate of heat a b s o r p t i o n a g a i n c o m m e n c e s .

T h i s d i p ( e n d o t h e r m ) is q u i t e


p r o n o u n c e d a n d contains a series of m i n i m a at Ε ( 1 7 5 . 5 ° C ) , F ( 1 8 5 . 5 ° C ) , a n d G ( 1 9 8 . 0 ° C ) , e n d i n g at H ( 2 0 4 . 5 ° C ) .

T h e D - H endotherm repre

sents the m e l t i n g / d i s r u p t i o n of the t o t a l 1 , 4 - b u t a n e d i o l - M D I m e n t d o m a i n s i n the T P U elastomer.

hard-seg

T h e m u l t i p l e m i n i m u m values, E ,

F , a n d G , are c a l l e d m e l t i n g temperatures

(T ) m

a n d are b e l i e v e d

to

reflect the s e q u e n t i a l m e l t i n g / d i s r u p t i o n of h a r d - s e g m e n t d o m a i n s h a v i n g different degrees of o r g a n i z a t i o n w h i c h w e r e d e v e l o p e d d u r i n g p r i o r T P U thermal a n d processing history. B y m e a s u r i n g the area of the entire m e l t i n g e n d o t h e r m , w h i c h is b o u n d e d b y D , E , a n d H , one c a n c a l c u l a t e the heat of f u s i o n (àH )

of

the s a m p l e a n d express i t as m i l l i c a l o r i e s p e r m i l l i g r a m

of

f

sample.

AH

t

(mcal/mg)

t h e n is a measure of the " s t r e n g t h " of the p o l y m e r v i r t u a l

crosslinks, or i n o t h e r w o r d s , of the degree of m o l e c u l a r o r g a n i z a t i o n i n the h a r d - s e g m e n t d o m a i n s . I n F i g u r e 7, ΔΗ

{

O n e likes to see h i g h T

m

indicates

is 3.20 m c a l / m g .

values f o r T P U h a r d segments,

f o r this

s t r o n g v i r t u a l crosslinks w h i c h w i l l a l l o w t h e m o l d e d T P U

parts to pass t h r o u g h p a i n t - d r y i n g ovens a n d to s t a n d i n the h o t s u n w i t h o u t sag o r d i s t o r t i o n . O f course if T

m

is too h i g h , m o l d i n g p r o b l e m s

c o u l d be encountered. F u r t h e r h e a t i n g of the s a m p l e to I i n t e r p r é t a b l e t h e r m a l response.

( 2 5 0 ° C ) produces no

further

A n d since another test, t h e r m o g r a v i m e t r i c

analysis ( T G A ), w h i c h I w i l l not discuss i n this c h a p t e r , shows that t h e p o l y m e r s a m p l e loses a l i t t l e w e i g h t ( suggestive of d e c o m p o s i t i o n )

at

a p p r o x i m a t e l y 250 ° C , w e h e a t t h e s a m p l e n o f u r t h e r b u t n o w i m m e d i a t e l y b e g i n to c o o l it at 1 0 ° C / m i n . CRYSTALLIZATION TEMPERATURE ( T ) AND H E A T OF CRYSTALLIZATION c

A t t e n t i o n is n o w d i r e c t e d to the l o w e r ( c o o l i n g )

(AH ). C

c u r v e i n the

F i g u r e 7 t h e r m o g r a m . N o t h e r m a l response is n o t e d i n t h e c o o l i n g s a m p l e until J

( 1 4 5 . 0 ° C ) , w h i c h is r e a c h e d after a b o u t 10 m i n u t e s of c o o l i n g .

H e r e h e a t s u d d e n l y evolves f r o m the s a m p l e i n a s h a r p e x o t h e r m , p e a k i n g at Κ ( T , 1 3 1 . 5 ° C ) a n d e n d i n g at L ( 1 0 5 . 5 ° C ) . T h i s response c

represents

c r y s t a l l i z a t i o n i n the s a m p l e a n d is a s c r i b e d to the r e f o r m a t i o n of hard-segment

domains.

H o w e v e r , the

accepted

methods

of

the

detecting

c r y s t a l l i n i t y d o not s h o w c r y s t a l l i n i t y i n s u c h T P U , as m e n t i o n e d earlier. B y m e a s u r i n g the area of the entire " c r y s t a l l i z a t i o n " e x o t h e r m w h i c h is b o u n d e d b y J , K , a n d L , one c a n c a l c u l a t e the h e a t of c r y s t a l l i z a t i o n (Δ/ίο mcal/mg)

of t h e s a m p l e , as w a s d o n e to get AH . t

I n F i g u r e 7,


5. AH

SCHOLLENBERGER

C

93

Thermophstic Polyurethane Elastomers

is 3.11 m c a l / m g .

I d e a l l y , t h e heat a b s o r b e d t o m e l t t h e s a m p l e

s h o u l d e q u a l t h e heat e v o l v e d w h e n t h e m e l t e d s a m p l e c r y s t a l l i z e s , so that AHf = AH . B u t as F i g u r e 7 shows, AH is n o t q u i t e as large as AH , C

C

f

s h o w i n g that t h e d e g r e e of c r y s t a l l i n i t y / o r d e r present

i n the sample

b e f o r e m e l t i n g w a s n o t r e e s t a b l i s h e d c o m p l e t e l y d u r i n g its f o u r - m i n u t e crystallization exotherm.


soft-segment =

H o w e v e r , after s t a n d i n g a w h i l e , h a r d - s e g m e n t -

phase s e g r e g a t i o n

tends t o increase

(11 ) a n d o f t e n

AH

C

ΔΗ{. N o t i c e also that the c o o l e d s a m p l e started to c r y s t a l l i z e at J ( 1 4 5 . 0 ° C ) ,

w h i c h is u n d e r n e a t h the m e l t i n g e n d o t h e r m b u t b e y o n d its center a n d w e l l b e l o w H ( 2 0 4 . 5 ° C ) , t h e p o i n t at w h i c h m e l t i n g w a s c o m p l e t e o n h e a t u p . T h i s c r y s t a l l i z a t i o n l a g is r e c o g n i z e d as s u p e r c o o l i n g . T o o m u c h of i t means l o n g m o l d i n g cycles, d i s t o r t e d parts, etc. F o r i n j e c t i o n m o l d i n g T P U , one likes to have the crystallization exotherm

( J - K - L ) sharp

a n d of a b o u t e q u a l area a n d c e n t e r e d f a i r l y w e l l u n d e r a large, r e a s o n a b l y high-temperature melting exotherm

(D-E-H).

T h e p o l y m e r of F i g u r e

7 is seen to a p p r o x i m a t e these r e q u i r e m e n t s f a i r l y w e l l . Effects of Composition and Molecular

Weight

T h e p o l y m e r of F i g u r e 7 is, as n o t e d earlier, a h i g h - u r e t h a n e - c o n t e n t p o l y m e r w h o s e p r o p e r t i e s , i n c l u d i n g t h e r m a l responses, suited

for injection-molding applications.

representative

of t h e t h e r m o g r a m s

t h e r m a l differences differences. macroglycol,

are a t t r i b u t a b l e

make

However, Figure

encountered

it well

7 is n o t

for a l l T P U , whose

primarily to polymer composition

T h i s e v e n i n c l u d e s p o l y m e r s b a s e d o n t h e same d i i s o c y a n a t e , a n d chain-extender

h a r d - s e g m e n t con c e nt ra t i ons.

components

b u t containing

different

T h i s is i l l u s t r a t e d i n F i g u r e 8.

F i g u r e 8 is t h e t h e r m o g r a m of a g e n e r a l - p u r p o s e T P U m o r e s u i t a b l e for extrusion, calendering, a n d solution applications than f o r injection molding.

L i k e t h e p o l y m e r of F i g u r e 7, i t w a s m a d e b y m e l t p o l y m e r i z

ing M D I , P T A d , and 1,4-BDO.

B u t its h a r d - s e g m e n t

content

1 ι 1 ι 1 1 1 ι 1 ι I ι 1 ι I ι -100 - 6 0 - 2 0 0 20 6 0 100 140 180

TEMPERATURE,^

Figure 8. General-purpose thermoplas tic, polyurethane elastomer thermogram


is o n l y

94

MULTIPHASE POLYMERS

a b o u t one-half, a n d its m a c r o g l y c o l m o l e c u l a r w e i g h t o n l y a b o u t

one-

t h i r d those of the F i g u r e 7 p o l y m e r values, m a k i n g i t a less-segmented p o l y m e r . T h e F i g u r e 8 t h e r m o g r a m w a s ran i n the same w a y as t h a t of Figure

7,

but

the

c o o l d o w n is

not

shown

since

it

was

essentially

featureless. Inspection

of F i g u r e 8 s h o w s :

to b e

T

g

— 35°C

(lower than i n


F i g u r e 7 ) ; a d i s t i n c t m e l t i n g e n d o t h e r m at a p p r o x i m a t e l y 6 7 ° C i n F i g u r e 7 ) ; a n d a b r o a d , s h a l l o w e n d o t h e r m at 8 0 ° - 1 8 0 ° C

(absent

(at

106°-

2 0 5 ° C , a n d m u c h m o r e p r o n o u n c e d i n F i g u r e 7 ) w h i c h contains a s m a l l , s h a r p e n d o t h e r m at 1 6 5 ° C (at 1 7 8 ° , 1 8 6 ° , a n d 1 9 8 ° C i n F i g u r e 7 ) . The lower T

v a l u e of the F i g u r e 8 vs. the F i g u r e 7 p o l y m e r p r o b a b l y

g

reflects : a l o w e r p o l y m e r m o l e c u l a r w e i g h t ( M f r e e - v o l u m e effect; its l o w e r h a r d - s e g m e n t

w

48,000) a n d

concentration

attendant

( reduced hard-

s e g m e n t - s o f t - s e g m e n t p h a s e m i x i n g ) ; a n d its u n u s u a l l y l o n g ( a p p r o x i m a t e l y 500 days) e q u i l i b r a t i o n p e r i o d d u r i n g 2 5 ° C shelf storage (increased hard-segment-soft-segment

p h a s e segregation ).

T h e 6 7 ° C e n d o t h e r m i n F i g u r e 8 is s u s p e c t e d of r e p r e s e n t i n g m e l t i n g of

soft segment

( macroglycol

lengthened

to

high

w e i g h t v i a d i i s o c y a n a t e c o u p l i n g ) at a t e m p e r a t u r e

appreciably

t h a t of t h e

macroglycol

relatively low-molecular-weight parent

the

molecular above itself

(~46°C). T h e b r o a d , shallow, 8 0 ° - 1 8 0 ° C endotherm i n F i g u r e 8 a n d the small s h a r p e n d o t h e r m w i t h i n it at 1 6 5 ° C are b e l i e v e d to c o r r e s p o n d to

the

106°-205°C

the

D - E - H e n d o t h e r m of F i g u r e 7 a n d also to represent

t h e r m a l d i s r u p t i o n of the (less extensive) h a r d segments of t h e

Figure

8 polymer. I n a d d i t i o n to p o l y m e r c o m p o s i t i o n , p o l y m e r m o l e c u l a r w e i g h t also c a n exert a significant effect o n the t h e r m a l response of p o l y m e r s i n c l u d i n g those h a v i n g i d e n t i c a l c h e m i c a l m a k e u p a n d c o m p o s i t i o n .

This can

b e seen i n F i g u r e s 9 A a n d 9 B . T h e 10 p o l y m e r s i n these figures w e r e a l l of

the

same composition,

materials,

and

preparation

method

F i g u r e 8 p o l y m e r , w h i c h reappears at t h e h e a d of F i g u r e 9 A . molecular-weight 48,000-367,000.

levels

of this series w e r e

T h e thermograms

v a r i e d i n the

as

the

But

the

range,

M

w

w e r e a l l m e a s u r e d as i n the case of

the F i g u r e 7 p o l y m e r . C o m p a r i s o n of F i g u r e 9 A a n d 9 B t h e r m o g r a m s shows several interesti n g changes w i t h i n c r e a s i n g M

w

t h a t J F values i n c r e a s e w i t h M

from -

g

at M M

w

w

w

i n this p o l y m e r series. 3 5 ° C at M

w

It c a n b e seen

of 48,000 to -

27°C

of 183,000, w h e r e t h e y h o l d t h r o u g h the final m e m b e r of the series,

of 367,000.

We

polymer free-volume

interpret

this p a t t e r n

(chain end)

mobility w i t h increasing polymer M

to reflect

the

influence

of

reduction on polymer-chain segment w

. It appears that p o l y m e r f r e e - v o l u m e

decreases w i t h i n c r e a s i n g p o l y m e r m o l e c u l a r w e i g h t u p to M

w

of a p p r o x i -



5.

SCHOLLENBERGER

Thermoplastic

Polyurethane

TEMPERATURE, °C

Figure 9.

95

Elastomers

TEMPERATURE,°C

(a, left; b, right) Effects of M on general purpose TPU elastomer thermograms w

m a t e l y 160,000, t h e r e a f t e r c e a s i n g to c h a n g e s i g n i f i c a n t l y o r to f u r t h e r influence polymer T

g

with additional M

w

increase.

N o t i c e that t h e 67 ° C e n d o t h e r m shows n o M b e c a u s e t h e smallest T P U of t h e series ( M

w

w

dependency, possibly

of 48,000) a l r e a d y consists

of soft segments ( d i i s o c y a n a t e - c o u p l e d p o l y e s t e r chains ) l o n g e n o u g h to d i s p l a y t h e m a x i m u m t e m p e r a t u r e f o r this t r a n s i t i o n i n this system. T h e b r o a d , s h a l l o w , 8 0 ° - 1 8 0 ° C e n d o t h e r m of t h e 4 8 , 0 0 0 - M

w

polymer

( F i g u r e s 8, 9 A ) n a r r o w s , d r a w i n g t o w a r d t h e l o w e r t e m p e r a t u r e , a n d also thins as p o l y m e r m o l e c u l a r w e i g h t increases, e n d o t h e r m at M M

w

w

w h i l e the distinct

165°C

of 48,000 g r a d u a l l y s h r i n k s , essentially d i s a p p e a r i n g at

of 117,000. H a r d - s e g m e n t d o m a i n d e v e l o p m e n t w o u l d a p p e a r to b e

p r o g r e s s i v e l y i m p e d e d i n t h e T P U as M

w

increases.

A g a i n , w e suggest

that this is a t t r i b u t a b l e to a decrease of p o l y m e r free v o l u m e a n d thus


96

MULTIPHASE POLYMERS

c h a i n - s e g m e n t m o b i l i t y . C o n s e q u e n t l y , less p r o n o u n c e d e v i d e n c e o f h a r d segment t h e r m a l response is a p p a r e n t i n t h e t h e r m o g r a m s o f t h e h i g h e r M

w

p o l y m e r s o f this T P U series. O t h e r i n f o r m a t i o n c a n b e o b t a i n e d f r o m t h e r m a l studies o f T P U

elastomers w h i c h is also h e l p f u l i n u n d e r s t a n d i n g a n d i m p r o v i n g these h i g h p e r f o r m a n c e , easily p r o c e s s e d p o l y m e r s . B u t i t w a s t h e i n t e n t i o n to l i m i t this c h a p t e r t o t h e b a s i c D S C t h e r m a l responses w h i c h

forecast


strengths a n d weaknesses i n T P U p r o c e s s i n g a n d p e r f o r m a n c e c h a r a c t e r istics a n d t o i n d i c a t e the parts o f t h e T P U s t r u c t u r e that a r e r e s p o n s i b l e f o r these t h e r m a l responses. Literature

H o p e f u l l y , this has b e e n a c c o m p l i s h e d .

Cited

1. Schollenberger, C . S., assignor to the B. F. Goodrich Company, U.S. Patent 2,871,218, "Simulated Vulcanizates of Polyurethane Elastomers," (Appl. Dec. 1, 1955, Issued Jan. 27, 1959). 2. Schollenberger, C . S., Scott, H., Moore, G . R., "Polyurethane V C , a Vir tually Crosslinked Elastomer," Rubber World (1958) 137(4), 549. 3. Cooper, S. L., Tobolsky, Α. V., J. Appl. Polym. Sci. (1966) 10, 1837. 4. Bonart, R., J. Macromol. Sci., Phys. (1968) B 2 ( 1 ), 115. 5. Clough, S. B., Schneider, N . S., J .Macromol. Sci., Phys. (1968) B2(4), 553. 6. Miller, G. W., Saunders, J. H., J. Appl. Polym.Sci.(1969) 13, 1277. 7. Harrell, L . L., Jr., Macromolecules (1969) 2(6), 607. 8. Rausch, K. W . , Farrissey, W . J., Jr., J. Elastoplastics (1970) 2, 114. 9. Morbitzer, L., Hespe, H . , J. Appl. Polym. Sci. (1972) 16, 2697. 10. Wilkes, C. E., Yusek, C. S., J. Macromol. Sci., Phys. (1973) B 7 (1), 157. 11. Wilkes, G . L., Bagrodia, S., Humphries, W., Wildnauer, R., J. Polym. Sci., Polym. Lett. Ed. (1975) 13, 321. RECEIVED April 14, 1978.


Multiphase Polymers - ACS Publications - American Chemical Society

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