Mesomorphic Order in Polymers - American Chemical Society

9 Applications of Thermotropic Mesophase Reactions RAJ N. GOUNDER

Downloaded by CORNELL UNIV on July 24, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch009

Corporate Research and Development, Lord Corporation, Erie, PA 16512

Recent r e s e a r c h in t h e areas o f m o l e c u l a r / m i c r o structural e n g i n e e r i n g o f polymer solids has resulted in a number o f t e c h n o l o g i c a l b r e a k t h r o u g h s . As a result, we a r e now c l o s e t o realizing t h e theoretically p o s s i b l e maximum performance from these microstructurally engineered polymeric m a t e r i a l s . Table 1 lists a number o f t o o l s t h a t are now a v a i l a b l e f o r t h e microstructural d e s i g n i n g o f u l t i m a t e polymer solids. The potentials and limitations o f t h e s e v a r i o u s tools f o r m o l e c u l a r / m i c r o s t r u e t u r a l e n g i n e e r i n g o f polymer s o l i d s are t h e s u b j e c t s o f a monograph c u r r e n t l y under preparation. Some o f t h e s e t e c h n o l o g i e s offer g r e a t p o t e n t i a l s for producing polymeric s o l i d s with ultimate behaviors in all t h r e e crystallographic directions. Thus, t h e t e c h n i q u e o f solid s t a t e p o l y m e r i z a t i o n has shown t h e practicality of p r o d u c i n g d e f e c t - f r e e s i n g l e c r y s t a l s o f c h a i n extended p o l y m e r i c systems, p r o v i d e d c e r t a i n stringent criteria may be satisfied . U l t i m a t e properties along specific d i r e c t i o n s o n l y , however, a r e o b t a i n e d more easily by a variety o f t e c h n i q u e s . Thus, a number o f t e c h n i q u e s utilizing certain extrinsic v a r i a b l e s have been-very w i d e l y i n v e s t i g a t e d . These i n c l u d e t h e c o n t r o l o f m e c h a n i c a l and t h e r m a l p a r a meters on polymer solids , c o n t r o l of mechanical ( h y d r o s t a t i c ) and t h e r m a l parameters on polymer melts and solutions , as w e l l as t e c h n i q u e s utilizing the effects o f electric and magnetic fields i npolym e r i c systems. A number o f t e c h n o l o g i e s have a l s o been d e v e l o p e d utilizing t h e effects o f intrinsic parameters f o r controlling the m i c r o s t r u c t u r a l a r c h i t e c tures o f polymeric solids. Examples o f these t e c h n i q u e s utilize polymer-solvent interactions , polymerpolymer interactions , and interchain f e a t u r e s — 7 . Two t e c h n i q u e s w i t h maximum s u c c e s s utilize thermotropic mesophase systems and lyotropic liquid crystalline 1

2-5

6-7

8-10

11-12

13

14-15

1-6

0-8412-0419-5/78/47-074-108$05.00/0 © 1978 American Chemical Society

Blumstein; Mesomorphic Order in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

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Thermotropic Mesophase Reactions

109

systems r e s p e c t i v e l y . The Kevlar® f i b e r s were d e v e l oped from l y o t r o p i c l i q u i d c r y s t a l l i n e systemsi-L; t h e e q u a l l y i n t e r e s t i n g u l t r a - h i g h modulus g r a p h i t e f i b e r s are made from t h e r m o t r o p i c mesophase p i t c h e s . The a p p l i c a t i o n o f t h e r m o t r o p i c l i q u i d c r y s t a l l i n e o r the s o c a l l e d mesophase r e a c t i o n s f o r the c o n t r o l of m o l e c u l a r / m i c r o s t r u c t u r a l a r c h i t e c t u r e s i n macro-molecular systems w i l l be the s u b j e c t o f t h i s r e v i e w .


A p p l i c a t i o n s o f Carbonaceous Mesophase P i t c h The f o r m a t i o n o f mesophase s t r u c t u r e s from the l i q u i d p y r o l y s a t e d u r i n g the heat t r e a t m e n t o f p o t e n t i a l l y g r a p h i t i z a b l e m a t e r i a l s such as a r o m a t i c h y d r o carbons, c o a l - t a r , and p e t r o l e u m p i t c h e s was f i r s t r e p o r t e d by Brooks and T a y l o r L i z i J . The t o t a l l y i s o t r o p i c t a r o r p i t c h t r a n s f o r m s t o an almost comp l e t e l y a n i s o t r o p i c mesophase p i t c h a t temperatures r a n g i n g from 300-50Q°C by the p a r a l l e l a l i g n m e n t o f l a r g e p l a n a r m o l e c u l e s d e v e l o p e d by a r o m a t i c p o l y m e r i z a t i o n a t such températures» Hot s t a g e m i c r o s c o p i c s t u d i e s — ~JLï. have r e v e a l e d t h a t on h e a t i n g a f u l l y i s o t r o p i c t a r o r p i t c h under q u i e s c e n t c o n d i t i o n s a t a temperature of about 350-450°C, s m a l l i n s o l u b l e l i q u i d spheres b e g i n t o appear ( F i g u r e 1 ) . These spheres g r a d u a l l y i n c r e a s e i n s i z e as h e a t i n g i s c o n t i n u e d . These spheres a r e found t o be h i g h l y a n i s o t r o p i c i n c h a r a c t e r . P o l a r i z e d l i g h t m i c r o s c o p y as w e l l as e l e c t r o n d i f f r a c t i o n s t u d i e s show t h a t t h e s e spheres c o n s i s t o f l a y e r s of o r i e n t e d s h e e t - l i k e molecules a l i g n e d i n n e a r l y the same d i r e c t i o n ( F i g u r e 2 ) . These s h e e t l i k e o r p l a n a r m o l e c u l e s were d e v e l o p e d by a r o m a t i c p o l y m e r i z a t i o n upon h e a t i n g a t temperatures over 300°C ( F i g u r e 3 ) . Based upon t h e i r e l e c t r o n d i f f r a c t i o n and p o l a r i z e d l i g h t a b s o r p t i o n s t u d i e s , Brooks and T a y l o r proposed a model f o r the a n i s o t r o p i c spheres as shown i n F i g u r e 4. A c c o r d i n g to t h i s model, the l a y e r a t the e q u a t o r i a l p l a n e i s p e r f e c t l y f l a t whereas the l a y e r s away from the e q u a t o r i a l p l a n e shows i n c r e a s i n g amounts of curvature. As h e a t i n g i n the range o f 300-500°C i s c o n t i n u e d , t h e s e spheres c o n t i n u e t o grow u n t i l they come i n t o c o n t a c t w i t h one a n o t h e r , whereupon they g r a d u a l l y c o a l e s c e w i t h each o t h e r t o produce l a r g e masses o f a l i g n e d l a y e r s ( F i g u r e 5 ) . Thus domains o f a l i g n e d m o l e c u l e s much l a r g e r than t h o s e o f the o r i g i n a l spheres are formed. These domains come t o g e t h e r t o form a b u l k mesophase wherein the t r a n s i t i o n from one o r i e n t e d domain t o another sometimes o c c u r s smoothly ©Dupont t r a d e mark f o r p o l y a r a m i d f i b e r s


M E S O M O R P H I C

ORDER

I N

P O L Y M E R S

TABLE 1

MOLECULAR/MICROSTRUCTURAL ENGINEERING OF POLYMER SOLIDS Reactions in Pre-Oriented Media Solid State Reactions Mesophase Reactions Thin Layers


Effect of Intrinsic Parameters Solvent-Polymer Interactions Polymer-Polymer Interactions Interchain Features Effect of Extrinsic Parameters Mechanical and Thermal Parameters on Polymer Solids Mechanical and Thermal Parameters on Polymer Melts Mechanical Parameters on Polymer Solutions Electric and Magnetic Fields

Metallography

Figure 1.

Photomicrograph of mesophase spherules (41 )




GOUNDER

(I)

(II)

(HI)

Figure 3. Formation of planar molecules by aromatic polymerization



112

MESOMORPHIC

ORDER I N P O L Y M E R S

Figure 4. (left) Mesophase sphere with section including polar diameter; (right) cross section of mesophase sphere

Molecular Crystals and Liquid Crystals

Figure 5.

Coalescence of mesophase spheres (1977) 38,177


9.

113


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and c o n t i n u o u s l y through g r a d u a l l y c u r v i n g l a m e l l a e and sometimes through more s h a r p l y c u r v i n g l a m e l l a e . The d i f f e r e n c e i n o r i e n t a t i o n between the domains c r e a t e s a complex a r r a y o f p o l a r i z e d l i g h t e x t i n c t i o n c o n t o u r s i n the b u l k mesophase c o r r e s p o n d i n g t o v a r i o u s t y p e s of l i n e a r d i s c o n t i n u i t y i n m o l e c u l a r a l i g n m e n t . The m o l e c u l a r arrangements w i t h i n the mesophase p i t c h e s thus o b t a i n e d resemble t h a t of c o n v e n t i o n a l nematic l i q u i d c r y s t a l s . I t has been shown t h a t l i k e c o n v e n t i o n a l nematic l i q u i d c r y s t a l s ~ the mesophase p i t c h e s p o s s e s s p l a s t i c p r o p e r t i e s - ! , h i g h degrees of a n i s o t r o p y L i , magnetic f i e l d e f f e c t s - ! , conf i n i n g s u r f a c e s - ! , and e u t e c t i c e f f e c t s ! - ! . However, the mesophase p i t c h e s p o s s e s s o t h e r p r o p e r t i e s which are d i f f e r e n t from the b e h a v i o r s o f c o n v e n t i o n a l nematic liquid crystals. F o r example, the mesophase i s formed i r r e v e r s i b l y ; i t i s r e l a t i v e l y i n s o l u b l e i n benzene and p y r i d i n e — . Mesophase p i t c h e s e x h i b i t h i g h a c t i v a t i o n e n e r g i e s (38-85 K C a l m o l " ) which c o r r e s p o n d t o c h e m i c a l r a t h e r than p h y s i c a l processes- -!"" — . Further, increases i n molecular w e i g h t r and C/H r a t i o s r and the k i n e t i c s i - i of mesophase format i o n i n p i t c h e s a l l i n d i c a t e chemical processes of mesophase f o r m a t i o n . These are n o t p r o p e r t i e s o f conv e n t i o n a l nematic l i q u i d c r y s t a l s . Thus, the mesophase p i t c h e s w h i l e b e i n g l i k e nematic l i q u i d c r y s t a l s i n the s t r u c t u r e s they p o s s e s s , are u n l i k e nematic phases i n t h e i r b e i n g formed through c h e m i c a l p r o c e s s e s . S i n c e the d i s c o v e r y o f mesophase p i t c h e s by Brooks and T a y l o r i n 1965, numerous s t u d i e s have been c a r r i e d out to u n d e r s t a n d and o p t i m i z e the e f f e c t s o f v a r i o u s parameters on the mesophase n u c l e a t i o n , growth, and coalescence. T a b l e 2 l i s t s a number o f parameters t h a t e f f e c t the f o r m a t i o n o f mesophase spheres - t h e i r n u c l e a t i o n , growth and c o a l e s c e n c e as w e l l as the p r o p e r t i e s of the r e s u l t i n g mesophase p i t c h . For example, use o f h i g h p r e s s u r e s d u r i n g heat t r e a t m e n t t o produce mesophases has been shown -! t o i n h i b i t mesophase c o a l escence and produce a l i q u i d c r y s t a l system composed o f uncoalesced spheres. Mesophase sphere growth and c o a l escence are a l s o h i n d e r e d by the p r e s e n c e o f i n s o l u b l e carbon p a r t i c l e s ! ! and m e t a l o x i d e p a r t i c l e s ! * . Of c o u r s e , the s t r u c t u r e and s i z e of mesophases o b t a i n e d by the heat treatment of carbonaceous p i t c h e s v e r y much depend upon the c h e m i s t r y o f p a r e n t s u b s t a n c e s ! ! and the n a t u r e o f r a d i c a l i n t e r m e d i a t e s o f the p y r o l y t i c process!- . The amount and the c o m p o s i t i o n o f the mesophase as w e l l as any c o - e x i s t i n g i s o t r o p i c phase i n a p y r o l y s a t e p i t c h depend upon temperature -**, . The 2 2

2 5

2

2


2

1

3

1 9

2 1

3

5

30

3

8

39

1


114

MESOMORPHIC

ORDER IN

v i s c o s i t y o f the p y r o l y s a t e v e r y much c o n t r o l s the c o a l e s c e n c e o f mesophase spheres and hence the forma t i o n o f a n i s o t r o p i c domains. V a r i o u s s t u d i e s ~ * have been performed t o s t u d y the e f f e c t s o f temperature, h e a t i n g time and v i s c o s i t y on the mesophase sphere f o r m a t i o n , growth and c o a l e s c e n c e . Yamada, e t a l — have shown t h a t removal o f low m o l e c u l a r weight mole c u l e s from p a r t i a l l y t r a n s f o r m e d p i t c h e s by s o l v e n t e x t r a c t i o n a f f e c t s the v i s c o s i t y and hence p r e v e n t s sphere c o a l e s c e n c e d u r i n g a d d i t i o n a l h e a t i n g . Thus, a body o f knowledge has been d e v e l o p e d e x p l o r i n g the e f f e c t s o f v a r i o u s parameters on the n a t u r e of meso phases formed i n carbonaceous p i t c h e s . D u r i n g t h i s same p e r i o d , the t e c h n o l o g y o f p r o d u c i n g carbon f i b e r s from v a r i o u s p i t c h e s was a l s o b e i n g established. ~ The a l r e a d y e x i s t i n g body o f know l e d g e c o n c e r n i n g mesophase f o r m a t i o n i n p i t c h e s has g r e a t l y impacted on the development o f p i t c h p r e c u r s o r c a r b o n f i b e r t e c h n o l o g y . Thus, about the same time Brooks and T a y l o r o f A u s t r a l i a p u b l i s h e d t h e i r r e s u l t s on the mesophase f o r m a t i o n i n carbonaceous p i t c h e s , O t a n i U o f Japan p u b l i s h e d h i s work on the m e l t s p i n n i n g o f carbon f i b e r s from c o n v e n t i o n a l i s o t r o p i c carbonaceous p i t c h e s . O t a n i s process involved several s t e p s as shown i n F i g u r e 6. F i r s t , carbon f i b e r s were spun from p i t c h e s s p e c i a l l y p r e p a r e d by p y r o l y z i n g p o l y v i n y l c h l o r i d e a t a temperature of 400-415°C f o r 30 minutes i n a n i t r o g e n atmosphere. The melt-spun f i b e r was made i n f u s i b l e by o x i d i z i n g i n ozone o r a i r and was then c a r b o n i z e d a t a temperature i n the range o f 500-1350°C. The f i b e r s p r e p a r e d i n t h i s manner e x h i b i t e d a t e n s i l e modulus and s t r e n g t h o f 8 χ 10^ p s i and 256 χ 1 0 p s i r e s p e c t i v e l y . L a t e r l , carbon f i b e r s w i t h s i m i l a r p r o p e r t i e s were a l s o p r e p a r e d from n a t u r a l l y o c c u r i n g p i t c h e s such as p e t r o l e u m a s p h a l t and c o a l - t a r p i t c h e s . A few y e a r s l a t e r , Hawthorne and c o w o r k e r s L i m Canada showed t h a t the m e c h a n i c a l p r o p e r t i e s o f p i t c h f i b e r s c o u l d be g r e a t l y enhanced by s t r e t c h i n g the f i b e r d u r i n g the c a r b o n i z a t i o n s t e p and c a r r y i n g out the c a r b o n i z a t i o n a t a much h i g h e r temperature o f 2000-2800°C (see F i g u r e 7 ) . The f i b e r s thus o b t a i n e d by s t r a i n i n g d u r i n g c a r b o n i z a t i o n e x h i b i t e d an e l a s t i c modulus as h i g h as 70 χ 10^ p s i and a t e n s i l e s t r e n g t h of almost 375 χ 10 p s i . Such f i b e r s were shown to e x h i b i t a h i g h degree o f c r y s t a l l i t e o r i e n t a t i o n p a r a l l e l t o the l o n g i t u d i n a l f i b e r a x i s . However, the i n d i v i d u a l c r y s t a l l i t e s were found t o be t u r b o s t r a t i c and e s s e n t i a l l y d e v o i d o f t h r e e - d i m e n s i o n a l o r d e r c h a r a c 39


POLYMERS

h k

I

5 0

1

3

2


2

9.


GOUNDER

115

TABLE 2


EFFECT OF PARAMETERS ON MESOPHASE FORMATION

Isotropic Pitch

1.

Pressure

2.

Insoluble Particles

3.

Temperature

4.

Parent Material

5.

Nature of Radical Intermediate

6.

Viscosity

7.

Heating Rate and Time

Melt Spinning

Spun Fiber

Oxidize in Ozone

Infusible Fiber

w

Εj

= 8 x 1 0 psi

Oj

= 256 χ 1 0 psi

Carbonize 500-1350° C

Carbon Fiber

6

3

Carbon

Figure 6.

Isotropic Melt ^ — Pitch Spinning

w

Fibers from isotropic pitches (51)

Spun Oxidize Infusible ^ >» Fiber in Ozone Fiber Εj

= 70 χ 1 0 psi

σ

= 375 χ 1 0 psi

τ

Strain & Carbonize Oriented _ >. Carbon 2000 - 2800 C p i b e r

6

3

Oriented Fibers Turbostratic Devoid of 3D Order Nature

Figure 7.

Fibers from strained isotropic pitches (53)


MESOMORPHIC


116

ORDER IN

POLYMERS

t e r i s t i c of p o l y c r y s t a l l i n e graphite. Only r e c e n t l y , the u s e f u l n e s s o f m e l t - s p i n n i n g carbon f i b e r s from a carbonaceous p i t c h i n i t s meso phase s t a t e has been r e a l i z e d and patented!!!!!! . According to these p a t e n t s , petroleum p i t c h , c o a l t a r p i t c h o r a c e n a p h t h y l e n e p i t c h i s f i r s t c o n v e r t e d t o the mesophase s t a t e by h e a t i n g under q u i e s c e n t c o n d i t i o n s and under vacuum o r n i t r o g e n atmosphere a t a tempera t u r e o f about 3 5 0 - 4 5 0 ° C (see F i g u r e 8 ) . The c a r b o n aceous p i t c h h a v i n g a mesophase c o n t e n t o f 5 5 t o 6 5 p e r c e n t thus produced i s spun i n t o f i b e r s by c o n v e n t i o n a l t e c h n i q u e s such as m e l t s p i n n i n g , c e n t r i f u g a l s p i n n i n g o r blow s p i n n i n g . F o r 5 5 t o 65 p e r c e n t mesophase c o n t e n t , s u i t a b l e s p i n n i n g temperatures range from 3 4 0 ° C to 3 8 0 ° C . The t h e r m o p l a s t i c carbonaceous f i b e r s thus o b t a i n e d are g i v e n a t h e r m o s e t t i n g treatment by h e a t i n g i n an o x y g e n - c o n t a i n i n g atmosphere a t temperatures below the s o f t e n i n g p o i n t o f the f i b e r s but above a t l e a s t 2 5 0 ° C f o r 5 to 60 minutes. The i n f u s i b l e f i b e r s o b t a i n e d by t h e r m o s e t t i n g are then c a r b o n i z e d by h e a t i n g i n an i n e r t atmosphere a t a temperature r a n g i n g from 1 5 0 0 ° C t o 1 7 0 0 ° C f o r about 1 to 5 m i n u t e s . The carbon i z e d f i b e r s are f u r t h e r heat t r e a t e d i n an i n e r t atmos phere a t a temperature of 2 8 0 0 ° C t o 3 0 0 0 ° C f o r about 1 minute. The f i b e r s thus o b t a i n e d p o s s e s s the t h r e e dimensional order of p o l y c r y s t a l l i n e graphite. The f i b e r s c o n s i s t o f h i g h l y o r i e n t e d g r a p h i t i c domains o f 5 0 0 0 A to 4 0 , 0 0 0 A i n s i z e . As a r e s u l t , the f i b e r s obtained e x h i b i t extremely high mechanical p r o p e r t i e s w i t h the Young's modulus i n the range of 7 5 χ 1 0 ^ p s i to 1 2 0 χ 1 0 p s i and t e n s i l e s t r e n g t h i n the range o f 250 χ 1 0 p s i , to 350 χ 1 0 p s i . B e s i d e s the a p p l i c a t i o n s i n the p r o d u c t i o n o f u l t r a h i g h modulus c a r b o n f i b e r s , the mesophase p i t c h e s are used i n making t h e r m a l - s h o c k - r e s i s t a n t aerospace g r a p h i t e s — and g r a p h i t e e l e c t r o d e s u f o r the s t e e l industry. Needle cokes are produced by d e f o r m a t i o n t h r o u g h t u r b u l e n c e and bubble p e r c o l a t i o n o f c a r b o n aceous mesophase p i t c h e s ' . Such a n i s o t r o p i c n e e d l e cokes form f i l l e r m a t e r i a l s i n the p r o d u c t i o n o f t h e r m a l - s h o c k - r e s i s t a n t a e r o s p a c e g r a p h i t e s and g r a p h i t e e l e c t r o d e s . The n e e d l e cokes are shown t o con s i s t o f a c i c u l a r - s h a p e d , c o r r u g a t e d a r o m a t i c mesophase l a y e r s t h a t are t i g h t l y f o l d e d and entwined. Increased c o n t e n t of the a c i c u l a r phase i n g r a p h i t e s i s shown to d e c r e a s e the t h e r m a l e x p a n s i o n î l z l l and i n c r e a s e the r e s i s t a n c e t o c l e a v a g e and f r a c t u r e - ! . 6

3

3

60

6 2

6


9.

GOUNDER


Development o f L i q u i d C r y s t a l l i n e

Polyesters

Another recent development i n v o l v i n g the use o f t h e r m o t r o p i c mesophases i s the d i s c o v e r y o f p o l y esters exhibiting l i q u i d c r y s t a l l i n e melts. These p o l y e s t e r s are prepared by s y n t h e s i z i n g polymers cont a i n i n g m o i e t i e s known t o l e a d t o l i q u i d c r y s t a l l i n i t y i n non-polymeric materials. The s t r u c t u r a l r e q u i r e m e n t s f o r the l i q u i d c r y s t a l f o r m a t i o n h a s been w e l l - s t u d i e d by GrayiL .. Most c o n v e n t i o n a l nematic and smectic l i q u i d c r y s t a l s approximate a general s t r u c t u r e as shown i n F i g u r e 9. F i g u r e 9 a l s o shows a f e w e x a m p l e s of t h el i n k a g e groups and t e r m i n a l groups t h a t f a c i l i tate theformation o f l i q u i d c r y s t a l s . The r e c e n t d e v e l o p m e n t o f l i q u i d c r y s t a l l i n e p o l y e s t e r s make u s e o f s u c h m o i e t i e s t h a t a r e known t o f a c i l i t a t e l i q u i d c r y s t a l formation. Thus, Kuhfuss and coworkers successfully prepared a l i s t of polyesters containing moieties l i s t e d i n F i g u r e 1 0 . Many o f t h e s e p o l y e s t e r s a n d c o p o l y e s t e r s are f o u n d t o be l i q u i d c r y s t a l l i n e i n t h e i r m o l t e n s t a t e . For example, these workers prepared a c o p o l y e s t e r o f PET w i t h a n u m b e r o f l i q u i d c r y s t a l f o r m i n g m o i e t i e s . T h e s e c o p o l y e s t e r s , i n a d d i t i o n t o t h e PET s e g m e n t s I I I , c o n t a i n e d t h e l i q u i d c r y s t a l f a v o r i n g segments I I and IV (see F i g u r e 1 1 ) . These c o p o l y e s t e r s were prepared by t h e a c i d o l y s i s o f P E T w i t h p - a c e t o x y b e n z o i c a c i d and polycondensation through t h eacetate and c a r b o x y l groups. Thus a l i s t o f c o p o l y e s t e r s c o n t a i n i n g v a r i o u s m o l e f r a c t i o n s o f p a r a o x y b e n z o y l m o i e t i e s were o b t a i n e d . I t was f o u n d t h a t m e l t s o f c o p o l y e s t e r s c o n t a i n i n g 40-90 m o l e p e r c e n t p a r a o x y b e n z o y l s e g m e n t s e x h i b i t e d behaviors s i m i l a r t o those o f nematic l i q u i d c r y s t a l s . As s h o w n i n F i g u r e 1 2 , m e a s u r e m e n t s o f t h e m e l t v i s c o s i t i e s o f c o p o l y e s t e r s c o n t a i n i n g v a r y i n g mole p e r c e n t p a r a o x y b e n z o y l r e v e a l e d v e r y i n t e r e s t i n g f e a t u r e s . Up t o 30 m o l e p e r c e n t p a r a o x y b e n z o y l c o n t e n t t h e m e l t viscosity increased. H o w e v e r , h i g h e r t h a n 30 m o l e p e r cent para oxybenzoyl r e s u l t e d i n a decrease i nthe melt v i s c o s i t y l e a d i n g t o a m i n i m u m a r o u n d 60 m o l e p e r cent para oxybenzoyl. Further, copolyesters containing i n t h e r a n g e o f 0-30 m o l e p e r c e n t p a r a o x y b e n z o y l e x h i b i t e d c l e a r m e l t s , whereas c o p o l y e s t e r s c o n t a i n i n g g r e a t e r t h a n 40 m o l e p e r c e n t p a r a o x y b e n z o y l l e d t o opaque m e l t s . T h i s was a c l e a r i n d i c a t i o n o f l i q u i d c r y s t a l f o r m a t i o n a t g r e a t e r than 4 0 mole p e r c e n t p a r a oxybenzoyl contents. The c o p o l y e s t e r s p r o c e s s e d f r o m s u c h l i q u i d c r y s t a l forming melts e x h i b i t very i n t e r e s t i n g mechanical properties. F i g u r e 13 s h o w s t h e e f f e c t o f p a r a o x y b e n z o y l content on the m e c h a n i c a l p r o p e r t i e s ( i n t h e 2


117

66- 69


118

MESOMORPHIC

ORDER IN P O L Y M E R S

Isotropic Pitch Inert A t m . , Quiescent Cond. 400 - 450° C 55 - 6 5 % Mesophase Pitch Melt Spin 340 - 380° C


Spun Fiber Thermoset in Oxygen 300 - 390° C

U.S. Patent 4,005,183

Infusible Fiber Carbonize 1 5 0 0 - 1700° C Graphitize 2 8 0 0 - 3 0 0 0 ° C

Figure 8. Carbonfibersfrom meso Ultrahigh Modulus Carbon Fibers phase pitches (58)

- C H = CH -

Ν =Ν — C - O -

il Ο -C = C -

— CH = Ν

Ο CH

3

-

(CH ) 2

RO

-

II C

Ο

Figure 9. (top) Structure of liquid crystals; (middle) examples of linkage groups; (bot tom) examples of terminal groups




GOUNDER

m Figure 10.

Ο

Polymers containing moieties known to lead to liquid crystalUnity

Ο r s

CH C—O 3

C — O H

I 0

-o—f

0

V-c—ο—f

V-c-

-OH + C H C — O H 3

(1)

II "0

0

0 -OCH CR,02

+

€—OH-y

CH C—0 3

(2)

III Ό

0

0

0 C—0CH CR,0-f2

0

7

0

0

C—OCH CH 02

2

+

CH^OH

(3)

IV Figure 11.

Synthesis of copolyesters containing liquid crystalline segments



120

MESOMORPHIC

ORDER IN

POLYMERS

Figure 12. Effect of p-OB content on the melt viscosity of copolyesters



9.

GOUNDER


121

Figure 13. Effect of p-OB content on the mechanical properties of copolyesters


MESOMORPHIC


122

ORDER IN

POLYMERS

i n j e c t i o n molding d i r e c t i o n ) o f i n j e c t i o n molded c o polyesters. The c o n t i n u o u s l i n e r e p r e s e n t s the f l e x u r a l modulus and the d o t t e d l i n e denotes the t e n s i l e strength. The p l o t i n d i c a t e s t h a t maximum i n t h e s e p r o p e r t i e s are a c h i e v e d around 60 mole p e r c e n t p a r a o x y b e n z o y l c o n t e n t , thus i n d i c a t i n g the e f f e c t o f l i q u i d c r y s t a l f o r m a t i o n on the m e c h a n i c a l p r o p e r t i e s o f r e s u l t i n g polymer s o l i d . T a b l e 3 summarizes the m e c h a n i c a l p r o p e r t i e s o f an i n j e c t i o n molded l i q u i d c r y s t a l l i n e polymer cont a i n i n g 60 mole p e r c e n t p a r a o x y b e n z o y l . The mechani c a l p r o p e r t i e s were measured p a r a l l e l t o the i n j e c t i o n m o l d i n g d i r e c t i o n ( l o n g i t u d i n a l ) as w e l l as t r a n s v e r s e t o the i n j e c t i o n m o l d i n g d i r e c t i o n ( t r a n s v e r s e ) . The f l e x u r a l moduli, t e n s i l e s t r e n g t h s , e l o n g a t i o n to break and impact s t r e n g t h o f c o p o l y e s t e r s e x h i b i t i n g l i q u i d c r y s t a l l i n e m e l t s are found to be s i m i l a r t o o r h i g h e r than t h o s e o f commercial g l a s s f i b e r r e i n f o r c e d polyesters. M c F a r l a n e and coworkers -! have s y n t h e s i z e d o t h e r polymers c o n t a i n i n g o t h e r moitiés t h a t are a l s o known to l e a d to l i q u i d c r y s t a l l i n i t y i n n o n - p o l y m e r i c forms. Thus, a wide v a r i e t y o f h i g h m o l e c u l a r weight copolymers e x h i b i t i n g l i q u i d c r y s t a l l i n e m e l t s may be p r e p a r e d . S t r u c t u r a l shapes c o n s i s t i n g o f a h i g h degree o f molec u l a r o r i e n t a t i o n s and c h a i n e x t e n s i o n s may be e a s i l y f a b r i c a t e d from such l i q u i d c r y s t a l l i n e m e l t s . 6

Polymers

w i t h L i q u i d C r y s t a l l i n e S i d e Chains

E x t e n s i v e work has been done i n a number o f l a b o r a t o r i e s - on the s y n t h e s i s and c h a r a c t e r i z a t i o n o f s e v e r a l polymers w i t h l i q u i d c r y s t a l l i n e s i d e c h a i n s . V a r i o u s o f the p a p e r s i n t h i s monograph d e a l w i t h v a r i o u s a s p e c t s o f such polymers c o n t a i n i n g mesomorphic s i d e c h a i n s . A r e v i e w of the p o t e n t i a l a p p l i c a t i o n s of such s i d e c h a i n l i q u i d c r y s t a l l i n e polymers t h e r e f o r e , w i l l be redundant. 70

75

Conclusion In c o n c l u s i o n , i t may be s a i d t h a t the thermotropic l i q u i d c r y s t a l l i n e reactions offer a highly e f f i c i e n t t e c h n i q u e f o r the c o n t r o l of the m o l e c u l a r / m i c r o s t r u c t u r a l a r c h i t e c t u r e s i n macromolecular s y s tems . A v a r i e t y o f n o v e l p o l y m e r i c systems e x h i b i t i n g unique m e c h a n i c a l and p h y s i c a l p r o p e r t i e s have been p o s s i b l e through s p e c i a l m o l e c u l a r / m i c r o s t r u c t u r a l d e s i g n s o b t a i n e d by u t i l i z a t i o n o f t h e r m o t r o p i c mesophase r e a c t i o n s .


9.


GOUNDER

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