Liquid Crystalline Oligoester Diols as Thermoset Coatings Binders

Chapter 22

Liquid Crystalline Oligoester Diols as Thermoset Coatings Binders

Downloaded by RUTGERS UNIV on February 6, 2017 | http://pubs.acs.org Publication Date: April 18, 1988 | doi: 10.1021/bk-1988-0367.ch022

Adel F. Dimian and Frank N. Jones Polymers and Coatings Department, North Dakota State University, Fargo, ND 58105

Liquid crystalline (LC) oligoester diols were synthesized and characterized. They were crosslinked with a hexakis(methoxymethyl)melamine resin (HMMM) to form enamels of high crosslink density. Crosslinking was effected at temperatures between T and T . Enamels made from LC diols are far harder and tougher than enamels made from non-LC diols. They are hardened without substantially increasing Tg, and they retain the elasticity associated with low Tg. Reasons for the observed property enhancement are unclear, but the effect is substantial. m

i

The mesophases formed by l i q u i d c r y s t a l l i n e (LC) polymers a r e w e l l known t o impart s t r e n g t h , toughness, and t h e r m a l s t a b i l i t y t o p l a s t i c s and f i b e r s ( 1 - 5 ) . W h i l e LC polymers have been w i d e l y s t u d i e d , t h e i r p o t e n t i a l u t i l i t y as c o a t i n g s b i n d e r s seems t o have been o v e r l o o k e d . Among the v e r y few r e p o r t s t h a t may d e s c r i b e LC polymers i n c o a t i n g s a r e p a t e n t s c l a i m i n g t h a t p - h y d r o x y b e n z o i c a c i d (PHBA), a monomer commonly used i n LC p o l y m e r s , enhances the pro p e r t i e s o f p o l y e s t e r powder c o a t i n g s ( 6 - 9 ) . S t u d i e s o f c r o s s l i n k e d networks o f LC polymers have been propos ed ( 1 0 ) , and prepared w i t h o u t d i s r u p t i o n o f the mesophase (11-14). C r o s s l i n k e d e l a s t o m e r s were shown t o r e t a i n the mesophase up t o c e r t a i n c r o s s l i n k d e n s i t i e s (15,16). Here we r e p o r t s y n t h e s i s , p r o p e r t i e s , and c r o s s l i n k i n g o f t h e r m o t r o p i c LC o l i g o e s t e r d i o l s , and comparison o f t h e p r o p e r t i e s o f c r o s s l i n k e d enamels t o those o f enamels made from c r y s t a l l i n e and amorphous d i o l s . LC d i o l s ( l a - l g ) were s y n t h e s i z e d by r e a c t i n g 4 , 4 ' - t e r e p h t h a l o y l d i o x y d i b e n z o y l c h l o r i d e (TOBC) w i t h excess a l i p h a t i c d i o l s H0(CH2) 0H, η = 4-12. The s e m i - r i g i d , r o d l i k e TOBC segments are w e l l known t o impart LC c h a r a c t e r t o h i g h p o l y m e r s ( 1 7 ) . n

0097-6156/88/0367-0324$06.00/0 © 1988 A m e r i c a n C h e m i c a l Society

Dickie et al.; Cross-Linked Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

22.

Liquid

DIMIAN AND JONES

Crystalline

Oligoester

Diols

325

0

II c

/

Ph 0 2

0

0

^δ/^ ""·^^^" "^^"

+ H

\

CI

la-lg

CH

°( 2)nOH

190-200°

CI TOBC

Two t y p e s o f non-LC o l i g o e s t e r d i o l s were p r e p a r e d f o r compari son. I n one t y p e , 2a-2g, l i q u i d c r y s t a l l i n i t y was d i s r u p t e d by s u b s t i t u t i n g a d i p o y l c h l o r i d e f o r t e r e p t h a l o y l c h l o r i d e i n TOBC. The s t r u c t u r e s o f l a - l g and 2a-2g may be r e p r e s e n t e d t h u s : 0 Downloaded by RUTGERS UNIV on February 6, 2017 | http://pubs.acs.org Publication Date: April 18, 1988 | doi: 10.1021/bk-1988-0367.ch022

M /p=\

H0(CH ) O - -C Λ W > 2

n

0 /p^n O - C - R - C - 0 - ^ Π >C-0( CH ) O - Η 0

H

II

2

la 2a A

2, R = - ( C H ) 2

0

i

n

l b l e Id l e I f l g 2b 2c 2d 2e 2 f 2g 5 6 7 8 10 12

A second s e r i e s o f non-LC d i o l s was amorphous g l y c o l a d i p a t e s 3a-3g. These were a l s o p r e p a r e d by Shotten-Baumann o l i g o m e r i z a t i o n : 0

0

0

0

u

u

u

ir -0(CH ) 0H 2

0(CH ) 0-C-(CH )4-C-0(CH ) 0-C ^ 2

η =4 "3a

n

3b

2

3c

3d

2

8 3e

n

n

^ C

10 12 3 f 3g

C l e a r c o a t i n g s c o n t a i n i n g o l i g o e s t e r d i o l s 1, 2, and 3 c r o s s l i n k e d w i t h a hexakis(methoxymethyl)melamine (HMMM) r e s i n were p r e p a r e d , baked and t e s t e d . Experimental D e t a i l s M a t e r i a l s . Reagent grade m a t e r i a l s were used as r e c e i v e d . The hexakis(methoxymethyl)melamine (HMMM) r e s i n was Cymel 303 (American Cyanamid). B o n d e r i t e 1000 p r e t r e a t e d c o l d - r o l l e d 3"x9"x24GA s t e e l p a n e l s ( P a r k e r ) were used f o r enamel t e s t i n g . Q-Panel Type DT 3"x6" 0.010" t i n - p l a t e d s t e e l p a n e l s were used f o r p r e p a r a t i o n o f free films. Preparation of l a - l g . TOBC was p r e p a r e d from t e r e p h t h a l o y l c h l o r i d e and PHBA as d e s c r i b e d by B i l i b i n e t a l . ( 1 7 ) . TOBC (0.005 m o l ) , d i o l (0.025 m o l ) , and d i p h e n y l o x i d e (10 mL) were p l a c e d i n 100 mL s i n g l e - n e c k e d round-bottomed f l a s k equipped w i t h a magnetic s t i r r i n g bar, a d i s t i l l a t i o n a d a p t e r , and a septum. The f l a s k was f l u s h e d w i t h argon f o r 15 min, and was s t i r r e d and heated i n an o i l b a t h a t 190-200° under slow argon f l o w . The r e a c t i o n m i x t u r e became homo geneous a f t e r 5 min and t h e e v o l u t i o n o f HC1 was observed. The


CROSS-LINKED POLYMERS

326

r e a c t i o n was c o n t i n u e d u n t i l the e v o l u t i o n o f HC1 was no l o n g e r d e t e c t a b l e by moistened l i t m u s paper (4-5 h r ) . The hot r e a c t i o n m i x t u r e was poured c a u t i o u s l y i n t o 100 mL o f t o l u e n e and c o o l e d . The o i l y r e s i d u e t h a t s e p a r a t e d was d i s s o l v e d i n CH2CI2» washed 3x w i t h w a t e r , and d r i e d over anhydrous MgSO^. The s o l u t i o n was f i l t e r ed and c o n c e n t r a t e d u s i n g a r o t o r y e v a p o r a t o r . The r e s i d u e was p r e c i p i t a t e d from methanol/water. Y i e l d s were 87-92% based on TOBC. lH NMR f o r l c i n CDCI3: 1.4 ( b r ) , 3.6 ( t ) , 4.2 (m) 6.8 ( d ) , 8.1(m) ppm. FT-IR f o r l c : 3420, 2960, 2938, 1720, 1606, 1512 cm" . LC d i o l s l a - l g had s i m i l a r s p e c t r a .


1

P r e p a r a t i o n o f 2a-2g. The d i a c i d c h l o r i d e p r e c u r s o r was prepared by substituting adipoyl chloride for terepthaloyl chloride i n B i l i b i n ' s procedure (17)· R e a c t i o n o f t h i s p r e c u r s o r w i t h d i o l s was c a r r i e d out as d e s c r i b e d f o r l a - l g e x c e p t t h a t the p r o d u c t s were not poured i n t o t o l u e n e . D i o l s 2a-2g were r e s i n o u s s o l i d s w h i c h s o l i d i f i e d on standing. P r e p a r a t i o n o f 3a-3g. A two-step Schotten-Baumann s y n t h e s i s was performed by the method o f B i l i b i n e t a l . ( 1 8 ) . D i o l (0.04 mol) and a d i p o y l d i c h l o r i d e (0.01 mol) were p l a c e d i n 100 mL s i n g l e - n e c k e d round-bottomed f l a s k equiped w i t h magnetic s t i r r i n g bar and d i s t i l l a t i o n a d a p t e r and septum. The system was f l u s h e d w i t h argon f o r 15 minutes and p l a c e d i n an o i l b a t h a t 90-100°. The r e a c t i o n was c o n t i n u e d u n t i l the e v o l u t i o n o f HC1 was no l o n g e r d e t e c t a b l e by moistened l i t m u s paper (2-3 h r ) . The r e a c t i o n m i x t u r e was a l l o w e d to c o o l and p h t h a l o y l d i c h l o r i d e (0.01 mol) was added. The system was f l u s h e d w i t h argon f o r 15 min and heated i n an o i l b a t h a t 160170° The r e a c t i o n was c o n t i n u e d u n t i l t h e e v o l u t i o n o f HC1 was no l o n g e r d e t e c t a b l e by moistened l i t m u s paper (2-3 h r ) . The product was not f u r t h e r p u r i f i e d . GPC o f o l i g o e s t e r d i o l 3c: M 840, M 1180, PDI 1.4. n

w

Enamel P r e p a r a t i o n . S o l u b l e o l i g o e s t e r d i o l s ( l b - I d , 2a-2g, and 3a-3g), HMMM, m e t h y l i s o b u t y l ketone (MIBK) and p _ - t o l u e n e s u l f o n i c a c i d (p_-TSA) were t h o r o u g h l y mixed i n a 70/30/30/0.3 wt. r a t i o . The s o l u t i o n was c a s t on p a n e l s and baked a t 150° f o r 30 min. Less s o l u b l e LC d i o l s l e - l g were m e l t e d , d i s p e r s e d i n MIBK, mixed w i t h HMMM and p_-TSA i n the above p r o p o r t i o n s and immediately c a s t as f i l m s . O l i g o e s t e r d i o l l a was t o o i n s o l u b l e f o r enamel f o r m a t i o n . NMR s p e c t r a were r e c o r d e d a t 34° on a V a r i a n A s s o c i a t e s EM-390 90 MHz NMR s p e c t r o m e t e r , u s i n g Me^Si as i n t e r n a l s t a n d a r d . IR s p e c t r a were r e c o r d e d a t 25° on a M a t t s o n Cygnus FT-IR u s i n g f i l m s c a s t on N a C l p l a t e s w i t h p o l y s t y r e n e as s t a n d a r d . A DuPont model 990 t h e r m a l a n a l y z e r was used f o r d i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y (DSC) a t h e a t i n g r a t e s o f 10°/min. A f t e r T the temperature was h e l d f o r 1 min b e f o r e the scan was resumed. C a p i l l a r y m e l t i n g p o i n t s were used to c o n f i r m the t h e r m a l d a t a . M and M were determined by g e l permeation chromatography (GPC) w i t h a Waters model 510 pump equipped w i t h a model R401 r e f r a c t i v e index d e t e c t o r , a model M730 d a t a a n a l y z e r , and U l t r a s t r a g e l 100 Â, 500 Â, 1 0 Â, and 10^ Â columns. GPC was c a l i b r a t e d w i t h p o l y s t y r e n e s t a n d a r d s . M was a l s o d e t e r mined by vapor p r e s s u r e osmometry (VPO) on a Wescan model 233 m o l e c u l a r w e i g h t a p p a r a t u s a t a c u r r e n t s e t t i n g o f 50/aamp i n t o l u e n e m

n

w

3

n


22.

Liquid

DIMIAN AND JONES

Crystalline

Oligoester

Diols

327

at 50°. The i n s t r u m e n t was c a l i b r a t e d w i t h s u c r o s e o c t a a c e t a t e and s o l u t i o n s o f unknowns r a n g i n g from 2.4 t o 12.0g/L were used t o determine t h e y - i n t e r c e p t . Mass a n a l y s i s was performed by M i c A n a l , Tucson AZ. A L e i t z L a b o l u x m i c r o s c o p e equipped w i t h a p o l a r i z i n g f i l t e r was used f o r o p t i c a l m i c r o g r a p h s a t 500x m a g n i f i c a t i o n ; d i o l s were observed i m m e d i a t e l y a f t e r h e a t i n g t o T , enamels were observed at room t e m p e r a t u r e . S w e l l i n g t e s t s were performed by t h e method o f Jones and Lu (19). m

Results


O l i g o e s t e r D i o l s . The p h y s i c a l p r o p e r t i e s o f LC o l i g o e s t e r d i o l s l a - l g measured by GPC, VPO, DSC, and p o l a r i z i n g o p t i c a l m i c r o s c o p y are summarized i n T a b l e I . Table I . diol

η

M h

a

T

la lb lc Id le If ig a

c

4 5 6 7 8 10 12

550 578 606 634 662 718 774

P h y s i c a l P r o p e r t i e s o f LC D i o l s Mn

u

D

570 625 690 720 780 810 910

Mn

n

M

c

480 530 570 610 650 680 720

c

720 740 810 850 910 950 1130

PDI

C

1.5 1.4 1.4 1.4 1.4 1.4 1.4

Τm

la-lg T i

d

d

i

204 207 349 300 302 231 220

110 58 75 47 82 80 90

texture

smectic smectic smectic smectic nematic smectic

t h e o r e t i c a l m o l e c u l a r weight f o r x = l ; ^determined by VPO; d e t e r m i n e d by GPC; d e t e r m i n e d by DSC.

NMR and IR s p e c t r a o f d i o l s l a - l g , 2a-2g, and 3a-3g were consistent with the assigned structures. S l i g h t l y high Η analyses (0.84% h i g h e r than t h e o r e t i c a l f o r x=2, 0.11% f o r x = l ) suggested t h a t s m a l l amounts o f u n r e a c t e d H0(CH2) 0H were p r e s e n t i n t h e p r o d u c t s . The LC n a t u r e o f d i o l s l a - l g was demonstrated by DSC ( F i g . 1 ) . Two f i r s t o r d e r t r a n s i t i o n s were observed. The lower t r a n s i t i o n temperature appeared t o be t h e c r y s t a l l i n e - m e s o p h a s e t r a n s i t i o n ( T ) , and t h e h i g h e r t r a n s i t i o n temperature t h e m e s o p h a s e - i s o t r o p i c t r a n s i t i o n ( T ^ ) . Other t r a n s i t i o n s were n o t e v i d e n t i n t h e DSC. The t h e r m a l d a t a r e v e a l e d an odd-even s p a c e r e f f e c t f o r T . The mesophases o f LC d i o l s l a - l g were a l s o observed d i r e c t l y i n p o l a r i z e d o p t i c a l micrographs t a k e n i m m e d i a t e l y a f t e r m e l t i n g t h e sample. T e x t u r e s were i d e n t i f i e d o n l y by comparison w i t h p u b l i s h e d micrographs ( 2 0 ) , and a r e t h e r e f o r e t e n t a t i v e . A nematic t e x t u r e i s observed f o r I f ( F i g . 2 ) , w h i l e more h i g h l y o r d e r e d s m e c t i c t e x t u r e s are observed f o r l b - l e and l g ( F i g . 3 ) . In c o n t r a s t , d i o l s 2a-2g and 3a-3g a r e n o t LC m a t e r i a l s . Diols 2a-2g were shown t o be c r y s t a l l i n e by m i c r o s c o p y and by t h e e x i s t e n c e of s i n g l e f i r s t o r d e r t r a n s i t i o n s i n t h e DSC. D i o l s 3a-3g appeared amorphous i n t h e micrographs and had no f i r s t o r d e r t r a n s i t i o n s i n the DSC. n

m

m



328


loô

2οσ

300

Temperature °C

e

F i g u r e 1. D S C thermograms o f L C diols l a - l g , heating rate 10 C / m i n .

Figure 2. P o l a r i z i n g optical micrograph o f l g , 400 X magnification.


22.

Liquid

DIMIAN AND JONES

Crystalline

Oligoester

Diols

329

C r o s s l i n k e d Enamels--70:30 r a t i o . A l l enamels d e s c r i b e d i n t h i s s e c t i o n were o b t a i n e d u s i n g a 70:30 wt. r a t i o o f diol:HMMM. D i o l s l b - l g , 2a-2g, and 3a-3g were c r o s s l i n k e d w i t h HMMM a t 150°. Cure temperature was w i t h i n t h e mesophase temperature range f o r l b - l g . Enamel f o r m a t i o n o f l a was not f e a s i b l e because o f poor m i s c i b i l i t y . The p r o p e r t i e s o f t h e enamels a r e summarized i n T a b l e I I . T a b l e I I . P r o p e r t i e s of Enamels P r e p a r e d from l b - l g and 2a-2g. Diol:HMMM:p.-TSA 70:30:0.3 by wt., c u r e c y c l e 150°/30 min Enamels from LC d i o l s le lc Id If

lb

12 10 8 7 6 55 50 50 50 >80 50 50 50 50 >80 5H-6H 6H 6H 6H 5H 5B 5B 5B 5B 5B 200 200 200 200 200 100% 100% 100% 100% 100% 015 0.5 0.5 0.5 0.5 22° 15° 16° 35° 23° t r a n s p a r e n t , glcDssy

s p a c e r l e n g t h (n ) 5 r e v e r s e i m p a c t >80 direct impact >80 p e n c i l hardness* 6H adhesion 5B acetone rubs* 200 flexibility 100% dry f i l m t h i c k . ^ 0.5 T g 17° appearance 3

3


lR

5

0

1

e

g

non-LC 3a-g 2a-R 4-12 8-15 10-15 H-2H 5B 200 100% 0.5 17-28°

4-12 30-45 30-45 HB-H 5B 200 100% 0.5 16-25°

a

ASTM-D 2794, u n i t s a r e i n - l b , impact t e s t e r l i m i t was 80 i n . l b . ; ASTM-D 3363; ASTM-D 3359,5B i s 100% c r o s s - h a t c h a d h e s i o n ; double r u b s ; ASTM-D 522; ASTM-D 1400, G e n e r a l E l e c t r i c t h i c k n e s s gauge t y p e B, u n i t s a r e 1/1000 i n . ; Sonset o f t r a n s i t i o n , determined by DSC.

b

C

e

f

A l l enamels had e x c e l l e n t a d h e s i o n , s o l v e n t r e s i s t a n c e , and f l e x ibility. Enamels made from LC d i o l s l b - l g were f a r s u p e r i o r t o t h o s e made from c o n t r o l d i o l s 2a-2g and 3a-3g i n b o t h hardness (5H-6H vs. H-2H; 5H c o r r e s p o n d e d t o about 20 KHN on a Tukon t e s t e r ) and impact r e s i s t a n c e (50 t o >80 i n - l b v s . 8 t o 15 i n - l b ) . Odd s p a c e r s l b and I d a f f o r d e d t h e b e s t p r o p e r t i e s i n t h e s e f o r m u l a t i o n s . Spacer v a r i a t i o n s d i d not measurably a f f e c t enamel p r o p e r t i e s i n t h e c o n t r o l oligoesters. DSC thermograms o f t h e c r o s s l i n k e d enamels r e v e a l e d o n s e t o f g l a s s t r a n s i t i o n s (Tg) r a n g i n g from 15° t o 35° f o r a l l t h r e e t y p e s o f enamels. Attempts t o d e t e c t f i r s t o r d e r t r a n s i t i o n s i n t h e DSC c o r r e s p o n d i n g t o T o r T-^ were u n s u c c e s s f u l due t o l a r g e exotherms s t a r t i n g a t about 90°. An odd-even p a t t e r n was n o t o b s e r v e d . P o l a r i z e d o p t i c a l m i c r o g r a p h s r e v e a l e d biréfringent h e t e r o geneous r e g i o n s i n t h e c r o s s l i n k e d enamels o f l b - l g ( F i g . 4 ) . Enamels o f 2a-2g and 3a-3g appeared amorphous. IR s p e c t r a o f t h e baked LC and amorphous enamels had bands a t t r i b u t a b l e t o u n r e a c t e d -OH groups a t 3420 cm" (-0H s t r e t c h ) . m

1

C r o s s l i n k e d E n a m e l s - - V a r i a b l e Wt. R a t i o . A l l enamels d e s c r i b e d i n t h i s s e c t i o n were p r e p a r e d from LC d i o l l c w i t h t h e diol:HMMM r a t i o v a r i e d from 90:10 t o 60:40. R e s u l t s a r e summarized i n T a b l e I I I . The r a t i o o f t h e maximum absorbances o f -OH t o -C=0 i n t h e FT-IR s p e c t r a and s w e l l i n g t e s t s p r o v i d e rough e s t i m a t e s o f t h e e x t e n t o f the c r o s s l i n k i n g r e a c t i o n s .



330


Figure 3. Polarizing optical micrograph of lc, 400 X magnification.

Figure 4. Polarizing optical micrograph of enamel made from lc, 400 X magnification.


22.

Liquid

DIMIAN AND JONES

Crystalline

Oligoester

Diols

331

FT-IR d a t a i n d i c a t e t h a t a t 80:20 and 70:30 r a t i o s a s i g n i f i c a n t f r a c t i o n o f t h e -OH groups a r e u n r e a c t e d even though more than a s t o i c h i o m e t r i c r a t i o o f HMMM i s p r e s e n t . The s t o i c h i o m e t r i c r a t i o i s e s t i m a t e d a t about 84:16. S w e l l i n g data i n d i c a t e t h a t c r o s s l i n k d e n s i t y i n the continuous phase o f t h e 70:30 and 60:40 networks i s h i g h . C r o s s l i n k d e n s i t i e s were e s t i m a t e d from t h e d a t a i n T a b l e I I I by t h e method o f H i l l and K o z l o w s k i ( 2 1 ) . R e s u l t s were: f o r 80:20, -v = 1 0 " moles o f e l a s t i c a l l y e f f e c t i v e network c h a i n s / c m ; f o r 70:30, -v = 2.5 χ 1 0 " c h a i n s / c m ; f o r 60:40, -v = 4.3 χ 1 0 " c h a i n s / c m . These e s t i m a t e s suggest t h a t t h e c r o s s l i n k d e n s i t i e s a r e w i t h i n t h e range r e p o r t e d f o r c o n v e n t i o n a l , h i g h l y c r o s s l i n k e d acrylic:HMMM and p o l y e s t e r :HMMM enamels (19,20). F i l m p r o p e r t i e s o f t h i s s e r i e s o f enamels i s shown i n T a b l e I I I . Enamels made w i t h a 90:10 lc:HMMM r a t i o were t a c k y . M i c r o s c o p i c biréfringent heterophases were v i s i b l e i m m e d i a t e l y a f t e r b a k i n g , b u t t h e y d i s a p p e a r e d w i t h i n a week. Enamels w i t h h i g h e r p r o p o r t i o n s o f HMMM a l s o had m i c r o s c o p i c h e t e r o p h a s e s , and t h e y remained i n t h e f i l m s i n d e f i n i t e l y . The b e s t o v e r a l l p r o p e r t i e s were a t t a i n e d w i t h t h e 80:20 r a t i o . T h i s r a t i o a l s o had t h e h i g h e s t p o p u l a t i o n o f v i s i b l e heterophases. 3

e

3

3

e

3

3

3


e

Table I I I . diol:HMMM 90:10 80:20 70:30 60:40

IR 0H/C=0 0.745 0.464 0.250 0.099

a

Enamel P r o p e r t i e s w i t h V a r y i n g HMMM C o n c e n t r a t i o n s Prepared from LC D i o l l c %swell _

pencil _

35.1% 14.2% 7.5%

3H-4H 5H 7H

film properties impact r e s . f l e x _

heterophasesb

a

none most few least

_

>80in.lb 25in.lb. lOin.lb.

100% 100% 0%

b

f l e x i b i l i t y , ASTM-D 522; f r o m o p t i c a l micrographs t a k e n a f t e r 1 week.

Discussion O l i g o e s t e r s D i o l s . The s y n t h e t i c method used t o make o l i g o e s t e r d i o l s l a - l g was adapted from B i l i b i n ' s method (17) f o r making main c h a i n LC h i g h polymers. A f i v e - f o l d excess o f H0(CH2) ° used to suppress molar mass. S p e c t r a l , chromatographic and a n a l y t i c a l e v i d e n c e i n d i c a t e d t h a t t h e expected p r o d u c t s were o b t a i n e d . GPC, VPO and a n a l y t i c a l d a t a suggested t h a t t h e s t r u c t u r e s w i t h x=2 and x = l predominate; s m a l l e r amounts o f s t r u c t u r e s w i t h x>2 and o f H0(CH2) 0H are p r o b a b l y p r e s e n t i n l a - l g and 2a-2g. The d i s crepancy between GPC and VPO d a t a i s not s u r p r i s i n g i n v i e w o f t h e l a r g e s t r u c t u r a l d i f f e r e n c e s between t h e p o l y s t y r e n e GPC s t a n d a r d s and t h e o l i g o m e r s s t u d i e d . The t h e r m a l b e h a v i o r o f l a - l g observed by DSC ( F i g . 1) c o n f i r m s t h e presence o f mesophases and i s t y p i c a l o f low m o l e c u l a r w e i g h t t h e r m o t r o p i c LC m a t e r i a l s ( 2 0 ) . The lower T f o r l b and I d a r e c o n s i s t e n t w i t h t h e h i g h e r entropy o f a c t i v a t i o n f o r c r y s t a l l i z a t i o n of odd-n s p a c e r s , demonstrated i n s e v e r a l main c h a i n LC polymers (23). The apparent absence o f n e m a t i c - s m e c t i c t r a n s i t i o n s i n t h e DSC H w

a

s

n

n

s

m



332

s u g g e s t s t h e observed morphology i s s t a b l e throughout t h e mesophase temperature r e g i o n . The nematic t e x t u r e o f o l i g o m e r i c LC d i o l I f i s t h e same as r e p o r t e d f o r t h e homologous LC h i g h polymer ( 2 4 ) . The mesophases o f t h e u n c r o s s l i n k e d LC d i o l s can be observed below T a f t e r m e l t i n g because f o r m a t i o n o f f u l l y o r d e r e d c r y s t a l s i s slow. The mesophases were n o t o b s e r v a b l e a f t e r one week a t room temperature.


m

C r o s s l i n k e d Enamels. P r o p e r t i e s o f enamels made from LC d i o l s o p t i m i z e d w i t h s l i g h t v a r i a t i o n s i n HMMM c o n c e n t r a t i o n gave 5H h a r d ness (20 KHN Tukon t e s t e r ) and >80 i n - l b . impact r e s i s t a n c e . The best p r o p e r t i e s o b t a i n a b l e f o r enamels made from non-LC d i o l s were 2H hardness and 45 i n - l b . impact r e s i s t a n c e . The g r e a t l y enhanced hardness and impact r e s i s t a n c e o f enamels made from LC d i o l s i s n o t s i m p l y e x p l a i n a b l e by t h e monomer r a i s i n g the Tg o f t h e c o a t i n g . I n f a c t , T g o f t h e c r o s s l i n k e d enamels o f l b - l g a r e a b n o r m a l l y low f o r hard c o a t i n g s and a r e s i m i l a r t o t h e much s o f t e r c o n t r o l enamels ( T a b l e I I ) . D i r e c t e v i d e n c e f o r t h e n a t u r e o f t h e m i c r o s c o p i c biréfringent domains i n t h e cured enamels i s l a c k i n g . They may be c r y s t a l l i n e , l i q u i d c r y s t a l l i n e , o r mixed. The s u b s t a n t i a l f r a c t i o n o f u n r e a c t e d -OH groups i n t h e 80:20 and 70:30 lc:HMMM enamels s u g g e s t s t h a t t h e -OH groups may be b u r i e d i n heterogeneous domains i n a c c e s s i b l e t o t h e HMMM. The b u l k p r o p e r t i e s o f such domains a r e p r o b a b l y s i m i l a r t o those o f t h e o r i g i n a l o l i g o e s t e r d i o l s . I f s o , they a r e c r y s t a l l i n e a t room temperature and l i q u i d c r y s t a l l i n e a t e l e v a t e d t e m p e r a t u r e s . I n t h e i n t e r p h a s e between domains and t h e c r o s s l i n k e d c o n t i n u o u s phase t h e r e may be r e g i o n s t h a t a r e i m m o b i l i z e d by t h e network and a r e l i q u i d c r y s t a l l i n e a t room t e m p e r a t u r e . I t i s d i s a p p o i n t i n g t h a t DSC exotherms, presumably caused by f u r t h e r c r o s s l i n k i n g r e a c t i o n s , i n t e r f e r e d w i t h t h e d e t e c t i o n o f LC phase t r a n s i t i o n s i n t h e cured enamels. These exotherms were b r o a d , and r e c u r r e d when t h e samples were c o o l e d and r e h e a t e d . This b e h a v i o r may be a t t r i b u t a b l e t o slow consumption o f -OH groups i n t h e domains o r t o slow s e l f - c o n d e n s a t i o n o f HMMM (21,22). I f these r e a c t i o n s o c c u r t o any a p p r e c i a b l e e x t e n t , t h e i r exotherms a r e l i k e l y to obscure t h e r e l a t i v e l y weak endotherms produced by t h e low conc e n t r a t i o n o f t h e LC phase. I t i s i n t e r e s t i n g t o s p e c u l a t e about what happens d u r i n g c r o s s l i n k i n g . A t 25° t h e 70:30 diol:HMMM m i x t u r e appears t o be a h e t e r o geneous d i s p e r s i o n o f LC d i o l i n l i q u i d HMMM. As t h i s m i x t u r e i s heated t o 150° two p r o c e s s e s presumably o c c u r : [1] p a r t i a l o r complete d i s s o l u t i o n o f t h e d i s p e r s e d LC d i o l i n HMMM and [2] r e a c t i o n of t h e -OH groups w i t h HMMM t o form a c o v a l e n t l y c r o s s l i n k e d network (22). As c r o s s l i n k i n g proceeds a stage w i l l be r e a c h e d , p r o b a b l y near t h e g e l p o i n t , a t w h i c h t h e m a t e r i a l i s i m m o b i l i z e d . D i s s o l u t i o n w i l l v i r t u a l l y s t o p , b u t t h e c r o s s l i n k i n g r e a c t i o n can c o n t i n u e . The morphology o f t h e cured f i l m w i l l p r o b a b l y be f i x e d a t t h e stage where d i s s o l u t i o n o f LC p a r t i c l e s s t o p s . A f t e r c o o l i n g , u n d i s s o l v e d LC p a r t i c l e s would p r o b a b l y c r y s t a l l i z e . P r o p e r t i e s w i l l be a f f e c t e d by t h e r e l a t i v e r a ^ e s o f t h e d i s s o l u t i o n and c r o s s l i n k i n g p r o c e s s e s . R e l a t i v e r a t e s w i l l be governed by f a c t o r s such as diol:HMMM r a t i o , s i z e o f t h e d i s p e r s e d p a r t i c l e s , c a t a l y s t l e v e l s and h e a t i n g r a t e s . S



22.

DIMIAN AND JONES

Liquid

Crystalline

Oligoester

Diols

333

A l t e r n a t i v e l y , i t i s p o s s i b l e t h a t t h e LC d i o l s c o m p l e t e l y d i s s o l v e d u r i n g c r o s s l i n k i n g and t h a t m i c r o s c o p i c biréfringent heterophases form when t h e c r o s s l i n k e d network c o o l s . This p o s s i b i l i t y seems u n l i k e l y s i n c e m o b i l i t y w i t h i n t h e h i g h l y c r o s s l i n k e d network i s r e s t r i c t e d , p r o h i b i t i n g a s s o c i a t i o n o f l a r g e numbers o f mesogenic u n i t s . Finkelmann and Rehage have s t a t e d t h a t LC domains cannot form i n h i g h l y c r o s s l i n k e d networks ( 1 5 ) . The r e a s o n f o r t h e observed p r o p e r t y enhancement remains a matter o f c o n j e c t u r e . The o b s e r v a t i o n t h a t p r e s e n c e o f m i c r o s c o p i c h e t e r o p h a s e s i n the enamels i s a s s o c i a t e d w i t h enhanced p r o p e r t i e s s u g g e s t s a cause and e f f e c t r e l a t i o n s h i p , b u t t h i s c o r r e l a t i o n may be i n c i d e n t a l . P r o p e r t y enhancements c o u l d a l s o be a t t r i b u t e d t o s u b m i c r o s c o p i c heterogeneous r e g i o n s o r t o t h e i n h e r e n t s t i f f n e s s o f the o l i g o m e r s t r u c t u r e . An i n t e r e s t i n g p o s s i b i l i t y i s t h a t p r o p e r t i e s may be enhanced by mesogenic u n i t s t h a t d i s s o l v e d u r i n g c r o s s l i n k i n g and then a s s o c i a t e w i t h i n t h e network i n t o v e r y s m a l l a g g r e g a t e s , perhaps c o n t a i n i n g as few as two o r t h r e e mesogenic units. Conclusions LC o l i g o e s t e r d i o l s can be c r o s s l i n k e d w i t h HMMM by c a r r y i n g o u t the c r o s s l i n k i n g r e a c t i o n a t temperatures between T and T-^. C r o s s l i n k d e n s i t y i s h i g h . C e r t a i n f i l m p r o p e r t i e s o f enamels made from LC d i o l s a r e f a r s u p e r i o r t o t h o s e o f enamels made from non-LC d i o l s . F i l m s are hardened w i t h o u t s u b s t a n t i a l l y i n c r e a s i n g Tg, and t h e y r e t a i n the e l a s t i c i t y a s s o c i a t e d w i t h low Tg. The mechanism o f t h i s p r o p e r t y enhancement i s u n c e r t a i n , b u t t h e e f f e c t i s s u b s t a n t i a l . I n c l u s i o n o f LC d i o l s i n enamels o f f e r s v a s t p o s s i b i l i t i e s f o r m a n i p u l a t i o n and improvement o f f i l m p r o p e r t i e s . m

Acknowledgment F i n a n c i a l s u p p o r t by t h e U.S. E n v i r o n m e n t a l P r o t e c t i o n Agency (Grant No. R-811217-02-0) i s g r a t e f u l l y acknowledged.

Literature Cited 1. Flory, P.J. In Advances in Polymer Science, Liquid Crystal Polymers I; Gordon, M., Ed.; Springer-Verlag: New York, 1984; Vol. 59. 2. Schwarz, J. Macromol. Chem. Rapid Commun. 1986, 7, 21. 3. Shibaev, V.P.; Plate, N.A. Polym. Sci. U.S.S.R. 1977, 19, 1065. 4. Kwolek, S.L.; Morgan, P.W.; Schaefgen, J.R.; Gulrich, LW. Macromolecules 1977, 10, 1390. 5. Dobb, M.G.; McIntyre, J.E. In Advances in Polymer Science, Liquid Crystal Polymers II/III; Gordon, M., Ed.; Springer-Verlag: New York, 1984; Vol. 60/61. 6. Maruyama, K. et al., Japan. Kokai 75 40,629, 1975; Chem. Abstr. 1975, 83, 133572y. 7. Nakamura, K. et al., Japan. Kokai 76 56,839, 1976; Chem. Abstr. 1976, 85, 110175y. 8. Nogami, S. et al., Japan. Kokai 76 44,130, 1976; Chem. Abstr. 1976, 85, 79835n.




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9. Nogami, S. et al., Japan. Kokai 77 73,929, 1977; Chem. Abstr, 1978, 88, 8624u. 10. De Gennes, P.G. Phys. Lett. A 1969, 28(11), 725-726. 11. Bhadani, S.N.; Gray, D. G. Mol. Cryst. Liq. Cryst. 1984, 102, 255-260. 12. Tsutsui, T.; Tanaka, R.; Tanaka, T. J. Polym. Sci., Polym. Lett. Ed. 1979, 17(8), 511-520. 13. Tsutsui, T.; Tanaka, R. Polymer 1981, 22(1), 117-123. 14. Aviram, A. J. Polym. Sci., Polym. Lett. Ed. 1976, 14(12), 757-760 15. Finkelmann, H.; Rehage, G. In Advances in Polymer Science, Liquid Crystal Polymers II/III; Gordon, M., Ed.; Springer-Verlag: New York 1984; Vol. 60/61. 16. Zentel, R.; Rechert, G. Makromol. Chem. 1986, 187 1915-1926. 17. Bilibin, A.Y.; Tenkovtsev, A.V.; Piraner, O.N.; Skorokhodov, S.S. Polym. Sci. U.S.S.R. 1984, 26, 2882. 18. Bilibin, A.Y.; Pashkovsky, E.E.; Tenkovtsev, A.V. Macromol. Chem. Rapid Commun. 1985, 6, 545-550. 19. Jones, F.N.; Lu, D.L. J. Coat. Technol. 1987, 59(751), 73-79. 20. Noel, C. In Polymeric Liquid Crystals; Blumstein, Α., Ed.; Plenum Press: New York, 1984. 21. Hill, L.; Kozlowski J. Coat. Technol. 1987, 59(751), 63-71. 22. Jones, F.N. Polym. Mat. Sci. Eng. 1987, 55, 222-228. 23. Ober, C.K.; Jin, J.; Lenz, R.W. In Advances in Polymer Science, Liquid Crystal Polymers I: Gordon, M., Ed.; Springer-Verlag: New York, 1984; Vol. 59. 24. Lenz, R.W. J. Polym. Sci., Polym. Sym. 1985, 72, 1-8. RECEIVED October

7, 1987


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