Liquid-Crystalline Polymers - ACS Publications - American Chemical

Chapter 4 All-Aromatic Liquid-Crystalline Polyesters of Phenylhydroquinone with Ether and Ketone Linkages 1

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: August 24, 1990 | doi: 10.1021/bk-1990-0435.ch004

Michael H. B. Skovby, Claus A. Heilmann , and Jørgen Kops Institut for Kemiteknik, Technical University of Denmark, DK-2800 Lyngby, Denmark The modification of polyterephthalates of phenylhydro quinone by s u b s t i t u t i n g the terephthalic a c i d f o r kinked 3,4'- and 4,4'-dicarboxydiphenylether and -ketone is a suitable way of obtaining melt processable thermotropic polyesters with melting t r a n s i t i o n s i n the range 200-300°C. The all-aromatic polyesters with ether linkages show excellent thermal stability surpassing those with ketone linkages. Fibers were spun from the l i q u i d c r y s t a l l i n e melts. E' moduli ranging from 30-50 GPa and break t e n a c i t i e sinthe range 400-600 MPa were found f o r these f i b e r s . The melt flow properties were independent of the type of linkage, 3,4' or 4,4'. A large increase in the v i s c o sities were found in case of high degrees of sub s t i t u t i o n when i s o t r o p i c melt state were approached. Recent work i n the f i e l d of thermotropic l i q u i d c r y s t a l (LC) polymers, and the introduction into the commercial market of various polymers of t h i s type has shown that i n order to achieve high p e r formance c h a r a c t e r i s t i c s with regards to mechanical and thermal properties the polymers should most often have an a l l - a r o m a t i c structure. In our studies, we have been interested i n i n v e s t i g a t i n g the e f f e c t of compositional v a r i a t i o n s i n some closer defined f u l l y aromatic polyesters on the properties. The main objective has been to achieve thermotropic behavior at processing temperatures normal for engineering p l a s t i c s and at the same time thermal s t a b i l i t y under processing conditions and i n connection with use at elevated temperatures. Linear 1,4-1inked aromatic polyesters are known to have too high melting points for melt processing. Various changes i n the structure may lower the melt t r a n s i t i o n and the basic s t r u c t u r a l modifications i n t h i s respect have been summarized by G r i f f i n Current address: Rohm Gmbh, Kirschenallee, Postfoch 4242, D-6100 Darmstadt 1, Federal Republic of Germany

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O097-6156/90/0435-O046$06.00/0 © 1990 American Chemical Society Weiss and Ober; Liquid-Crystalline Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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LC Polyesters with Ether and Ketone Linkages

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and Cox ( I ) . The commonly used method to b u i l d - i n f l e x i b l e chain segments, most often c o n s i s t i n g of methylene groups, into an aromat i c polyester (2) was not considered to be s u i t a b l e i n our case. Thus, i f the melt processing range for the aromatic polyester was to be lowered to a desirable range of less than 300°C, while maintaining high performance c h a r a c t e r i s t i c s , including good thermal s t a b i l i t y , the f l e x i b l e chain segments should be avoided, since they would represent weak l i n k s i n the polymers. S u b s t i t u t i o n of the aromatic rings was chosen as a way of depressing the melting ranges. In t h i s connection the work by W.J. Jackson, J r . (3) i s e s s e n t i a l concerning substituted hydroquinones. I t was obvious from t h i s work that p a r t i c u l a r l y phenylhydroquinone (PHQ) i s able to lower the melt t r a n s i t i o n while good thermal s t a b i l i t y i s maintained. This monomer has been used throughout the present work as the d i o l component i n the polyester syntheses. Concerning the d i a c i d component, terephthalic a c i d (TA) was chosen due to good properties and general a v a i l a b i l i t y , however, i n order to lower the melting to below 300°C for the polymers kinked d i c a r b o x y l i c acids have been substituted for the terephthalic a c i d to various extents. We already have reported on the replacement of the terephthalic a c i d with kinked diphenylether d i c a r b o x y l i c acids (4). 3 , 4 * - and 4,4 -Dicarboxydiphenylether (3,4*-0 and 4,4*-0) were synthesized and a l l - a r o m a t i c polyesters were prepared represented by structure 1. These polyesters were thermotropic with melt t r a n s i tions decreasing to about 200°C with increasing replacement of the terephthalic a c i d with the kinked monomers. The polymers generally were thermally stable without measurable weight loss u n t i l well over 400°C. We wish here to supplement our previous studies with Theolog i c a l measurements and f i b e r spinning of the polymers, including some measurements of f i b e r properties. In addition 3 , 4 ' - and 4,4 -dicarboxydiphenylketone (3,4*-K and 4,4'-K) have also been synthesized as reported i n preliminary fashion (5). The thermal properties of the polyesters prepared with these monomers, represented by structure 2, w i l l be reported and compared with those of polymers 1^. ,

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1: A = -O- , 2: A = - 0 0 - and x = molfraction Experimental Monomers 3,4* and 4,4*-Dicarboxydiphenylether ( 3 . 4 - 0 and 4.4*-0) have been prepared by oxidation of the corresponding dimethyldiphenylether as described (4). 3.4*- and 4.4 -Dicarboxydiphenylketone (3.4'-K and 4 , 4 - K ) have been synthesized by dichromate oxidation of the corresponding dimethyldiphenylketones which i n turn were obtained as described (5). f

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American Chemical Society Library 1155 15th St., N.W. Weiss and Ober; Liquid-Crystalline Polymers Washington, O.C. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

LIQUID-CRYSTALLINE POLYMERS

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Terephthalic a c i d (purity >99%) was always used without further p u r i f i c a t i o n . Phenylhydroquinone (Aldrich) was d i s t i l l e d and acetylated and r e c r y s t a l l i z e d from a 80/20 pentane/chloroform mixture (4). Polymers A l l polymerizations were c a r r i e d out by melt a c i d o l y s i s based on a procedure described i n a U.S. Patent (6). A general procedure was used for a l l the experiments (4). Three larger lOOg batches of polymers with the composition PHQ, TA, 4 , 4 - 0 = 0.5/0.35/0.15 were prepared for f i b e r and melt flow studies. The polymerization procedure for these was as f o l l o w s : T e m p ( ° C ) / t i m e ( m i n ) / p r e s s u r e (mmHg) = 290/60/760; 320/45/760 ; 320/70/10; 340/60/0.1. The three batches were ground and melt blended i n a Brabender at 300°C for 15 min under nitrogen purge.


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Thermal Analysis The thermal t r a n s i t i o n s of l i q u i d c r y s t a l polymers are normally studied by a combination of d i f f e r e n t i a l scanning calorimetry (DSC) and v i s u a l observations on a hot-stage p o l a r i z i n g microscope. While DSC i s run i n a recording manner and the actual t r a n s i - t i o n s are determined from the recorded curve, a n a l y s i s by microscopy i s generally based on subjective evaluations and descriptions of the observations. It appeared to us that c e r t a i n advantages could be derived from running the hot-stage p o l a r i z i n g microscopy i n a r e cording manner i n order to obtain a l i g h t transmission curve with r e l a t i o n to the birefringence and determine the t r a n s i t i o n temperatures on the basis of t h i s curve. Although subjective evaluations s t i l l i s required i f the texture of the thermotropic phase i s to be established, a recording of the degree of l i g h t transmission could be much more r a t i o n a l and lead to better and perhaps standardized ways of determining the t r a n s i t i o n s temperatures. Despite the apparent advantages of such a technique only very few references are found i n the l i t e r a t u r e . This i s s u r p r i s i n g i n view of the very large number of p u b l i c a t i o n s i n recent years on l i q u i d c r y s t a l polymers and of course a l s o on low molecular weight l i q u i d c r y s t a l s . We wish here to i l l u s t r a t e t h i s thermo-optical a n a l y s i s (TOA) as a tool for c h a r a c t e r i z a t i o n of l i q u i d c r y s t a l polymers. O r i g i n a l l y , TOA was devised and used for analyses of chain m o b i l i t y i n polymers (7) and polymer blends (8,9) by monitoring birefringence disappearance during programmed heating i n scratches scribed on f i l m surfaces. Lenz (10,11) has reported the use of TOA i n the study of l i q u i d c r y s t a l polymers, and recently we have used the technique i n determining melt- and i s o t r o p i z a t i o n temperatures for thermotropic f u l l y aromatic l i q u i d c r y s t a l polymers ( 4 , 5 ) . DSC thermograms were recorded on e i t h e r a DuPont 900 or a Stanton Redcroft STA 785 + CPC 706 instrument with a heating rate of 20°C/min. TGA thermograms were determined with the Stanton Redcroft instrument. Thermal t r a n s i t i o n s were a l s o studied using a Linkam

Weiss and Ober; Liquid-Crystalline Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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PR600 hot stage (HS) i n connection with a Reichert-Jung Microstar 110 microscope equipped with a Photodyne 22xL photometer and a r e corder. The heating rate was set to 20°C/min. The c r y s t a l l i n e to nematic t r a n s i t i o n i s reported as the temperature corresponding to a 20% increase i n l i g h t transmission and s i m i l a r l y the nematic to i s o t r o p i c t r a n s i t i o n temperature as that corresponding to an 80% decrease i n l i g h t transmission from the maximum value. This was based on obtaining the best correspondance with values determined by DSC.


Rheological Measurements V i s c o s i t y measurements at low shear rates were made with a Rheomet r i c s System 4 or Rheometrics RMS-800/RDS-II spectrometer i n e i t h e r cone and p l a t e (cone angle = 0 . 1 , radius = 25 mm) or p l a t e and p l a t e (radius = 25 mm, distance = 1 mm). For higher shear rates v i s c o s i t y measurements were made with an Instron c a p i l l a r y viscometer. A c a p i l l a r y with L/D = 51.4 and D = 0.889 mm was used. Sample d i s c s were prepared by compression molding of the predried (at least 3 days at 125°C i n vacuum) copolyesters at 250 C. Prepared d i s c s were d r i e d at 125°C i n vacuum for 5 hr before use. The c o r r e c t i o n for non-Newtonian behavior was applied, but entrance pressure c o r r e c tions were neglected. Melt spinning of f i b e r s Melt spinning of two polyesters: a) 15 mol% 4,4'-dicarboxy diphenyl ether modified poly(phenyl-l,4-phenylene therephthalate) and b) 20 mol% p-hydroxybenzoic a c i d modified poly(pheny1-1,4-phenylene terephthalate) was c a r r i e d out using an Instron c a p i l l a r y v i s c o meter. The die had a diameter of 1.24 mm and L/D = 40.99. The draw r a t i o s (DR) were calculated as surface area r a t i o s of the f i b e r s to the Instron plunger area (0.7125 cm ). The r e s u l t i n g f i b e r was cooled by a continuous stream of dry nitrogen approx. 2.5 cm below the c a p i l l a r y d i e . In order to avoid moisture, the poly-ester was added to the b a r r e l through a closed box purged with a continuous stream of nitrogen. The DR values were varied by varying the speed of a system of s p e c i a l l y constructed take-up r o l l s , and f i b e r diameters were determined by o p t i c a l microscopy. 2

Mechanical properties Dynamic mechanical properties were determined with a Polymer Laborat o r i e s Dynamic Mechanical Thermal Analyzer (DMTA) using the t e n s i l e mode. The f i b e r length was i n a l l cases 20 mm at an i n i t i a l 0.5% elongation. The heating rate was set to 5°C/min. Break t e n a c i t i e s were measured on an Instron t e n s i l e tester model 1130 using a sample length of 25.4 mm and a s t r a i n rate of 0.508 mm/min (0.02 in./min). A l l reported break-tenacities and moduli are the mean values of four measurements.


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LIQUID-CRYSTALLINE POLYMERS

Results and Discussion Thermal Properties


The thermal t r a n s i t i o n s for polymers of structure .1 and 2 are summarized i n Tables I and II and the t r a n s i t i o n temperatures for f o r mation of nematic melts (Tj^) are p l o t t e d i n Figure 1. The kinked dicarboxy compounds obviously has considerable c a p a b i l i t y for decreasing the melt t r a n s i t i o n s with increasing subs t i t u t i o n for the terephthalic a c i d . A decrease to below 300 C i s already achieved with the diphenyl ether d e r i v a t i v e s - ( 3 , 4 ' - 0 and 4,4'-0) at a degree of s u b s t i t u t i o n corresponding to 30%. For the corresponding diphenyl ketone derivatives (3,4*-K and 4,4'-K) a somewhat higher degree of s u b s t i t u t i o n i s required, 35-40%, to get a comparable melt depression. The more s t i f f structure of the ketones compared to the ethers may be connected with t h i s kind of behaviour. However, further increase i n the amounts has a very d r a s t i c e f f e c t on the melt t r a n s i t i o n . The actual type of kinked monomer with regards to the connecting linkage between the phenyl rings or the p o s i t i o n s of s u b s t i t u t i o n has l i t t l e influence on the quantitative e f f e c t on the t r a n s i tions at high l e v e l s of a d d i t i o n . Previously, we have found (4) that the melt t r a n s i t i o n of the homopolymer of hydroquinone and 3 , 4 ' - 0 was much lower than the homopolymer made with 4 , 4 ' - 0 . This may, however, be due to an e f f e c t from the unsymmetrical monomer 3 , 4 ' - 0 which may be incorporated i n a h e a d - t o - t a i l or a random fashion i n contrast to the symmetrical monomer 4 , 4 ' - 0 i n combination with the unsubstituted hydroquinone. The e f f e c t of the diphenyl ether linkage has been investigated before i n case of preparation of polyesters of chlorohydroquinone and 4,4 -0/TA i n d i f f e r e n t r a t i o s ( i , 12-13). For t h i s polymer system i t was a l s o p o s s i b l e to obtain nematogenic compositions that were melt processable i n the range 25O-300°C. The thermal s t a b i l i t y have been investigated for the polymers and t h i s property i s very important i n r e l a t i o n to the study of the rheological properties which are reported i n t h i s paper. Generally, the diphenyl ether modified polymers were more thermally stable than the corresponding polymers with ketone structure, since a s i g n i f i c a n t weight loss occurred at around a 50° C higher temperature. Typical TGA curves are i l l u s t r a t e d i n Figure 2. An example on a TOA thermogram i s shown i n Figure 3 . The polymer was prepared with TA/4,4'-K = 0,285/0,215. By DSC a melt t r a n s i t i o n of 272°C was recorded. The TOA curve indicates a phase change i n the thermotropic i n t e r v a l , which could not be detected by DSC. The i n i t i a l melting to the nematic state corresponds to the f i r s t increase i n the l i g h t i n t e n s i t y . However, a second t r a n s i t i o n i s c l e a r l y indicated at a higher temperature. The various curves were recorded for d i f f e r e n t s t a r t i n g l i g h t i n t e n s i t i e s corresponding to d i f f e r e n t sample thicknesses. ,

Polyesters for f i b e r s and flow studies We report here studies on polyesters A, A l , A2, B l , B2 (see Table and the b i g batch c a l l e d BB1. Polyester BB1 had the same compos i t i o n as B l , but an inherent v i s c o s i t y could not be determined


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Table I.

Thermal Transitions of Polyesters .1

Composition Mol F r a c t i o n Nr.


Hot Stage, TOA ^C

DSC

TA

A

T _

T

g

T

342 326 281 210 -

T *NI 423 425 325 -

350 333 278 205 198

T NI 400 >490 350 405 -

415 350 288 -

320 275 255 210

480 331

Liquid-Crystalline Polymers - ACS Publications - American Chemical

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