wholly aromatic thermotropic liquid crystalline polyesters of 4,4′-biphenol, substituted...

Wholly Aromatic Thermotropic liquid Crystalline Polyesters of 4,4’-Biphenol, Substituted Biphenols, and 1,l’-Binaphthyl- 4,4’-diol with 3,4’-Benzophenone Dicarboxylic Acid

HAESOOK H A N and PRADIP K. BHOWMIK”

Department of Chemistry, University of Detroit Mercy, 4001 W. McNichols Rd., P.O. Box 19900, Detroit, Michigan 4821 9-0900

SYNOPSIS

Wholly aromatic, thermotropic homopolyesters, derived from 4,4’-biphenol, substituted biphenols, or l,l’-binaphthyl-4,4’-diol and 3,4’-benzophenone dicarboxylic acid, and two copolyesters, each of which contained 30 mol % of 6-hydroxy-2-naphthoic acid, were prepared by acidolysis polycondensation reactions and characterized for their liquid crystalline properties. The solubility behavior of these polymers has also been investigated. The two homopolymers of phenyl-substituted biphenols with 3,4‘-benzophenone dicarboxylic acid were soluble in many common organic solvents. All of the homopolymers had lower T,,,/Tf values than those with terephthalic acid, which was attributed to the incorporation of the asymmetric 3,4‘-benzophenone dicarboxylate units in a head-to-head and head-to-tail fash- ion along the polyester chain. Two copolymers had lower T,,, values than those of the respective homopolymers, as expected. They formed nematic phases which persisted up to 400°C, except those of phenyl-substituted biphenols with 3,4‘-benzophenone dicarboxylic acid. Each of these two polymers also exhibited an accessible Ti transition, and had a broad range of LC phase. They had glass transition temperatures, T,, in the range of 139-209°C and high thermal stabilities in the temperature range of 465-511OC. 0 1995 John Wiley & Sons, Inc. Keywords: liquid crystalline polymer thermotropic nematic synthesis characterization

solubility

INTRODUCTION

Wholly aromatic, thermotropic polyesters are among the most important classes of liquid crystalline polymers ( LCPs) because of their ease of processing in the nematic phase to obtain either high strength fibers or engineering thermoplastics. In general, however, they have high melting transitions, T,,,, because of their high enthalpy change and low en- tropy change at the crystal-to-nematic transition. Moreover, they have very low solubility in all but aggressive solvents.

There are several kinds of structural modifi- cation~’-~ to decrease the T,,, values of this class of

* To whom all correspondence should be addressed. Journal of Polymer Science: Part A Polymer Chemistry, Vol. 33,211-225 (1995) 0 1995 John Wiley & Sons, Inc. CCC 0&37-624X/95/020211-15

polymers to a convenient level to prevent thermal degradation during melt processing. Several structural modifications8-12 have also been developed to increase their solubility in common organic solvents. The solubility of this class of polymer is desirable for the ease of characterization, for the ease of preparation of films by solution casting, and for the in- vestigation of polymer blends.

A large number of ~tudies’~-~l has been devoted to wholly aromatic, thermotropic polyesters based on substituted hydroquinones (HQs), substituted terephthalic acids (TAs), and substituted 4,4‘-biphenols (BPs), but few studies have been performed to prepare polymers based on 3,4’-benzophenone dicarboxylic acid (3,4’-BDA).22-24 Skovby and co- w o r k e r ~ ~ ~ reported a series of copolyesters based on phenylhydroquinone (PhHQ) with mixtures of TA and 3,4’-BDA. All of the copolyesters in this series, except the copolymer containing 90 mol % of 3,4‘-

211

212 HAN AND BHOWMIK

BDA, exhibited anisotropic melts in the temperature range of 185-372”C, suggesting that the 3,4’-benzophenone dicarboxylate unit was capable of de- creasing the T,,, values considerably with the in- creased amount of substitution for TA. Recently, we reported26 a series of thermotropic homopolyesters of 3,4’-BDA with various aromatic diols. These were HQ, 2,6-, 1,4-, 1,5-, 2,3- and 2,7-naphthalenediol isomers. All of the homopolymers of 3,4‘-BDA, except that from 2,7-naphthalenediol, formed nematic phases in the temperature range of 305-360OC.

In this article, we describe the preparation and characterization of a series of homopolyesters of 3,4’- BDA with BP, 3,3’,5,5’-tetramethyl-4,4’-biphenol (TMBP), 3-phenyl-4,4’-biphenol (MPBP), 3,3‘- bis(phenyl)-4,4’-biphenol (DPBP), and l,l’-binaphthyl-4,4’-diol as part of an extensive study on

this class of polymer. Two copolyesters of 3,4’-BDA with either BP or BND and 30 mol % of 6-hydroxy- 2-naphthoic acid (HNA) were also included. The structures and designations of the polyesters are shown in Scheme 1.

These wholly aromatic, polyesters were characterized by a variety of experimental techniques, including solution viscometry, gel-permeation chromatography (GPC), elemental analysis, photo- acoustic infrared (IR) spectroscopy, ultraviolet (UV) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, polarized light microscopy (PLM), dif- ferential scanning calorimetry (DSC), thermomechanical analysis (TMA), thermogravimetric analysis (TGA), and wide angle x-ray diffraction (WAXD). Their solubility in various common organic solvents had also been investigated.

Polymer No. Polymer Structure

Homopolymers

1-1

1-2

1-3

1-4

1-5

Copolymers

1-6

1-7

Scheme 1.

LIQUID CRYSTALLINE POLYESTERS 213

EXPERIMENTAL

Monomers

BP and TMBP were received from Aldrich Chemical Company and Dow Chemical Company, respectively, and used without further purification. A previously reported procedure was applied for the Frie- del-Crafts acetylation reaction on these two di01s.~~ The acetylated compounds were recrystallized twice from toluene and ethanol, respectively. The two substituted biphenols, MPBP and DPBP, and BND were received as diacetates from Schnectady Chem- icals Incorporated, NY.

The preparation of HNA was performed accord- ing to a previously reported procedure.28 Subse- quently, it was acetylatedZ7 and recrystallized twice from ethyl acetate to obtain very small needles.

The preparation of 3,4'-BDA was performed from aqueous permanganate oxidation" of 3,4'-dimethyldiphenyl ketone. The crude oxidation product was recrystallized twice from dimethylsulfoxide, (DMSO) and water (90/10) to give a powder. Traces of recrystallization solvent could not be removed even at high temperature under vacuum, therefore, the product was extracted with dichloromethane in a Soxhlet apparatus.

The 3,4'-dimethyldiphenyl ketone was prepared from the corresponding ketimine,30 which was hy- drolyzed in 1.OM HC1 solution to the ketone. Re- crystallization twice from ethanol gave the desired ketone as needles.

The purity of all diacetate monomers was checked by elemental analysis and that of 3,4'-BDA and 6- acetoxy-2-naphthoic acid was checked by 'H-, 13C- NMR, and elemental analysis.

Polymer Synthe~is*~*~'

All of the homopolyesters, 1-1-1-5, in this study were synthesized by a melt polycondensation, without an added catalyst, from 3,4'-BDA reacted with the individual diacetate derivatives of biphenol, substituted biphenols, and BND. The two copolyesters, 1-6 and 1-7, with 30 mol % HNA were prepared from 6-acetoxy-2-naphthoic acid and ap- propriate amounts of the other two comonomers. At the end of polymerization, the solidified polyester was processed by the following procedure: the product was chipped out of the reactor, ground, and washed successively with n-propanol or iso-propanol and acetone several, times to remove any residual monomers. It was dried under vacuum at 85°C for

24 h. The usual polymerization time was 24 h and the yield of polyester generally varied between 94-9696.

Polymer Characterization

Inherent viscosities of the polyesters, when possible, were measured in p-chlorophenol at 50°C at a polymer concentration of 0.2 g/dL with a Cannon Ubb- elohde-type viscometer. Those of two polymers, I- 3 and 1-4, were also measured in chloroform at 35°C at the same concentration with the same type of viscometer. Their molecular weights were determined using a Waters 804E gel-permeation chromatography devise calibrated with monodisperse polystyrene standards through "Ultrastyragel" col- umns with 500, lo3, lo4, and lo6 A pore sizes at 50°C with THF as a solvent. Elemental analyses were performed by the Microanalytical Laboratory of the University of Massachusetts a t Amherst. Photo- acoustic IR spectra of the polyesters were recorded on a Nicolet 60 Spectrometer with a MTEC pho- toacoustic attachment. UV-absorption spectra of a polyester solution both in chloroform and in tetrahydrofuran were obtained on a Hewlett-Packard 8452A diode array spectrophotometer. The 'H- and I3C-NMR spectra were recorded with a Bruker AM 300 Spectrometer, operating at 300 and 75.48 MHz, respectively, in CDC13 using TMS as an internal standard.

Phase transition temperatures were measured on a DuPont 2100 DSC under a nitrogen flow with both heating and cooling rates of 20"C/min. The temperature axis of the DSC thermogram was calibrated prior to use with a reference standard of high purity indium and tin. Polymer samples usually weighing 8-10 mg were used for this analysis. The peak max- imum was recorded from both the heating cycles, whenever possible. Glass transition temperatures were taken as the inflection point of that transition either during the first or second heating cycle, whichever was more prominent. Tgs were also measured with a DuPont 2100 TMA with a heating rate of 20"C/min under a load force of 100 mN. The liquid crystalline textures of the polyesters were observed on an optical polarizing microscope (Leitz, Model Ortholux) equipped with cross polarizers and a Mettler hot stage. Thermogravimetric analyses, TGA, were obtained with a DuPont 2100 instrument at a heating rate of 20°C/min either in nitrogen or in air. A Statton I1 camera and point collimated nickel-filtered CuK, radiation were used for the WAXD studies.

214 HAN AND BHOWMIK

Table I. Dicarboxylic Acid

Solubilities of Thermotropic Homopolyesters of MPBP and DPBP with 3,4'-Benzophenone

Solubilityb in Polymer

No. Monomer' PCP DCA TCE CHC13 CH,Cl, Dioxane THF

1-3 MPBP + 3,4'-BDA + + + + + + + 1-4 DPBP + 3,4'-BDA + + + + + + + a Monomer Designation: MPBP = 3-phenyl-4,4'-biphenol, DPBP = 3,3'-his(phenyl)-4,4'-biphenol, and 3,4'-BDA = 3,4'-benzophenone

Investigated at ca. 0.1 g/dL a t room temperature (not the highest achievable concentration): +, soluble; PCP, p-chlorophenol; dicarboxylic acid.

DCA, dichloroacetic acid; TCE, 1,1,2,2-tetrachloroethane; THF, tetrahydrofuran.

RESULTS AND DISCUSSION

Table I shows a very useful property for the homopolymers of MPBP and DPBP with 3,4'-BDA, 1-3 and 1-4, namely, that both polymers are soluble in many common organic solvents, including chloroform, methylene chloride, and tetrahydrofuran. A large number of wholly aromatic thermotropic polyesters have been prepared and characterized, but only a few polyesters of this type have been reported to have solubility in common organic solvents. To date, the structural that are re- quired to have the solubility of these polymers are those that incorporate: ( a ) 2,2'-bis (trifluoromethyl) - substituted para-biphenylene in combination with trifluoromethyl-substitutedpara-phenylene units; ( b ) 2,2'-bis (methyl) -substituted para-biphenylene along with methyl-, bromo-, and tert-butyl-substituted pura- phenylene units; (c) bulky substituents, such as tert- butyl, phenyl, phenoxy, phenylalkyl, and 4-biphenyl in both the aromatic diacid and aromatic diol moieties; and ( d ) a high percentage (> 25%) of ortho-phenylene units from 1,2-dihydroxybenzene (catechol) in the polyester chain. Therefore, the incorporation of either 3-phenyl- or 3,3'-bis (phenyl) - substituted para-biphenylene in combination with

the flexible 3,4'-benzophenone dicarboxylate moiety provide structural variants having solubility in common organic solvents.

Because of the solubility properties of polymers 1-3 and 1-4, it was possible to evaluate their molecular weights by gel-permeation chromatography (GPC) ; the data obtained are collected in Table 11. A GPC plot for polymer 1-4, as shown in Figure 1, indicates that a single peak with an average-molecular weight (M,) of 128,000, compared with polystyrene standards, was obtained. Similarly, the GPC plot for polymer 1-3 also indicated that a single peak with M , of 36,000, compared with polystyrene standards, was obtained. The molecular weight distributions, as indicated by the ratio of the weight- to number-average molecular weights, M,/M,, were 7.2 and 12.8, respectively, for these polymers. It was observed that a high M , sample has higher molecular weight distributions than those of a low M , sample which is in agreement with the results as reported by Heitz and N i e s ~ n e r . ~ ~

All of the polyesters were prepared by melt polycondensation reactions, and they, except polymers 1-1, 1-6, and 1-7, were soluble in p-chlorophenol. The data in Table I11 show that, for this series of polymers, inherent viscosity (IV) values were in the range of 0.58-1.88 dL/g despite the in-

Table 11. Molecular Weights of Polymers 1-3 and 1-4 by GPC Measurements

Polymer No. Monomer" M,b MU" M,d M," M W I M I I

1-3 MPBP + 3,4'-BDA 5000 36,000 90,000 28,000 7.2 I- 4 DPBP + 3,4'-BDA 10,000 128,000 900,000 44,000 12.8

a See footnote in Table I for monomer identification.

' M,: weight-average molecular weight.

eMp: peak molecular weight.

M,: number-average molecular weight.

M,: z-average molecular weight.


Figure 1. GPC plot of polymer 1-4 in THF at 50°C.

clusion of a flexible 3,4'-benzophenone dicarboxylate moiety as a nonlinear component; this suggests that the polymers have sufficiently high molecular

weights so that their thermal properties, optical textures, and other properties can be compared ne- glecting the effect of molecular weight on these properties.

The homopolymer of BP with 3,4'-BDA, I- 1, and its copolymer with 30% HNA, 1-6, were soluble neither inp-chlorophenol (PCP) nor in a mixture (60/ 40 by weight) of PCP/ 1,1,2,2-tetrachloroethane (TCE) . However, the homopolymer of BND with 3,4'-BDA, 1-5, was soluble in PCP, but its copolymer with 30% HNA, 1-7, was soluble neither in PCP nor in a mixture of PCP/TCE (60/40 by weight). The incorporation of 30% 6-oxy-2-naphthoate units presumably may increase the rigidity of the co- polyester chain resulting in a complete loss of solubility.

The IV values of the homopolymers of MPBP and DPBP with 3,4'-BDA, 1-3 and 1-4, in chloroform at 35°C were 0.22 and 0.24 dL/g, respectively, which were much lower than those inp-chlorophenol at 5OoC (Table 111). This observation suggests that the hydrodynamic chain properties of these polymers are quite different in these two solvents. The length of the statistical Kuhn segment of each of these polymers, which characterizes the equilibrium

Table 111. Properties of Thermotropic Polyesters of 3,4'-Benzophenone Dicarboxylic Acid

Polymer IVb T: Tmd Tfg T,d T d

Monomer" (dL/g) ("C) ("C) ("C) ("C) ("C) Texture by PLM WAXD Patterns ~ ~ ~~~

No.

1-1 BP + 3,4'-BDA - - 37ge 390' Nematic Crystalline (highly)

1-2 TMBP + 3,4'-BDA 0.58 - 380' 400' Nematic Crystalline (highly)

1-3 MPBP + 3,4'-BDA 0.77 152 - 195 310e 497' Nematic Two diffuse halos

1-4 DPBP + 3,4'-BDA 0.64 170 - 210 300h 510' Nematic Two diffuse halos 1-5 BND + 3,4'-BDA 1.88 203 343" 465' Nematic Crystalline (v. low) 1-6 BP + 3,4'-BDA + HNA - 139 307e 511' Nematic Crystalline (v. low) 1-7 BND + 3,4'-BDA + HNA - 209 301' 487' Nematic Crystalline (v. low)

378'

349f

282'

a Monomer designation: B P = 4,4'-biphenol, TMBP = 3,3',5,5'-tetramethyl-4,4'-biphenol, BND = l,l'-binaphthyL4,4'-diol, and HNA = 6-acetoxy-2-naphthoic acid; 30 mol % HNA was included in each copolymer. See footnote in Table I for MPBP, DPBP, and 3,4'- BDA identification.

Measured in p-chlorophenol a t 50OC.

Crystalline melting transition, T,, and isotropization transition, Ti , were determined from the DSC thermogram and also verified

From DSC thermogram for first heating cycle. From DSC thermogram for second heating cycle. Fusion temperature, T,, was determined from the temperature of onset of flow as observed on a hot-plate melting point apparatus

and verified with a polarizing light microscope on observation of a typical nematic texture on the edge, the thinnest part of the sample. Isotropization temperature, Ti, was determined in the hot-plate melting point apparatus a t the temperature a t which loss of stir

opalescence occurred and also verified by PLM studies. 'Thermal stability, T d , the temperature ("C) at which a 5% weight loss occurred, was determined from TGA measurement in air

a t a heating rate of 20"C/min. J Thermal stability, Td, was determined from TGA measured in nitrogen a t a heating rate of 20°C/min.

' The T, was also verified from TMA measurement.

by polarizing light microscopy studies.

216 HAN AND BHOWMIK

4.5000 -

3.6000 -

W

z 4

= 0 v)

0 2.7000 - m

2 1.8000 -

0 90000 -

0.m1 I I I I 1 200 300 400 500 600

W A V E L E N G T H

Figure 2. The UV-absorption spectrum of polymer 1-4 in CHC13.

rigidity of the chain, is presumably different in the two solvents. In contrast, the hydrodynamic chain properties and the length of Kuhn segment of each of the homopolymers of MPBP and DPBP with 2- phenylterephthalic acid ( PhTA ) were essentially identical in these two solvents." Similarly, the properties of the homopolymer of 2-phenylhydroquinone ( PhHQ) with PhTA were also identical in dichloroacetic acid and dioxane, as reported by Tsvetkov and c o - ~ o r k e r s . ~ ~

The results of elemental analysis were in good agreement with those of calculated values of the structures of the polyesters. Each of the IR spectra of the polyesters showed the following characteristic absorption bands: 3050-3150 ( = C - H, aromatic), 1735 (C=O, from an ester group), 1665 (C=O, from benzophenone moiety), 1265, 1155 (C-0) , and 3330,3470 cm-' (overtones of C = 0). The UV- absorption spectra of polymer 1-4, both in CHC13 and in THF, are shown in Figures 2 and 3, respectively. The spectrum of 1-4 in CHC13 showed a broad absorption band, 240-360 nm, with no fine structure and a shoulder in the visible region at 444 nm. Its spectrum in THF showed only a broad absorption band with no fine structure, but no shoulder in the visible region. The UV-absorption spectra of polymer 1-3 in these two solvents showed essentially identical absorption bands to those of polymer 1-4 except that its shoulder was at 424 nm. Each of the polymers formed a yellow orange solution in CHC13, but a colorless solution in THF.

The homopolymer of B P with 3,4'-BDA, 1-1, showed an endotherm in each of the heating cycles of the DSC thermograms. This endotherm corresponded to the crystal-to-nematic transition confirmed by the polarizing light microscope (PLM) studies. Similarly, the homopolymer of TMBP with 3,4'-BDA, 1-2, had crystal-to-nematic transitions, T,, at 380 and 349°C in the first and second heating cycles, respectively. The T, values of these two polymers were rather high, but they were lower than those of respective homopolymers with TA, which undergo thermal decomposition before their trans- formation into LC phase^.^^,^^

The homopolymer of MPBP with 3,4'-BDA, I- 3, had a glass transition temperature, Tg, and an endotherm in both the heating cycles. There was correspondingly an exotherm in each of the subse- quent cooling cycles of the DSC thermograms. This exotherm underwent a low degree of supercooling of 10°C which suggested that the endotherm in the heating cycle corresponded to the nematic-to-isotropic transition, Ti, as confirmed with PLM studies. Moreover, the AHi value at Ti of this polymer was 0.52 kcal/mol of the repeating unit from the second heating cycle of the DSC thermogram which suggested that the endotherm corresponded to the Ti transition. In general, AHi values a t Ti transitions are in the range of 0.30-0.85 for a nematic and 1.5- 5.0 kcal/mol of the repeating unit for smectics depending on the nature of the phases.36 This homopolymer had a fusion temperature, Tf , at 195OC. Its

LIQUID CRYSTALLINE POLYESTERS 2 17

4.5000

3.6000

w

z U

Lz 0 v)

0 2.7000

m

m u 1.8000

0.90000

O.Wo0 I I I I 1 200 250 300 350 400

W A V E L E N G T H

Figure 3. The UV-absorption spectrum of polymer 1-4 in THF

DSC thermograms in the second heating and cooling cycles are shown in Figure 4. The low T, value of the homopolymer may be attributed both to the random arrangements of the bulky phenyl group of MPBP units, and to the random arrangements of 3,4’-BDA units in the head-to-head and head-to-tail fashions along the polyester chain, which can se- verely disrupt the packing in the crystal lattice of this polymer. These random arrangements of two asymmetric units along the polyester chain were confirmed with 13C-NMR spectroscopy. Figure 5 shows the 13C-NMR spectrum covering the chemical shift range ca. 120-200 ppm of polymer 1-3. The two distinct resonances of carbonyl carbon peaks at 194.5 and 194.8 ppm were attributed to the carbonyl carbons of ketone group derived from the 3,4’-benzophenone dicarboxylate units. These two resonances presumably arose from the head-to-head and head-to-tail arrangements of MPBP units, which was also confirmed by the comparison of 13C-NMR spectrum of the homopolymer of DPBP with 3,4‘- BDA, 1-4, discussed in the following section. The carbonyl carbons of the ester groups gave rise to four distinct resonances ca. at 164 ppm covering a total chemical shift of 0.5 ppm, which are consistent with both the head-to-head and head-to-tail arrangements of the two asymmetric MPBP and 3,4‘- BDA units. Therefore, it was found that the carbonyl carbon chemical shift of ester group is more sensitive not only to the nature of the next neighboring rings but also to the distant ones than that of carbonyl

carbon of ketone group of a 3,4‘-benzophenone moiety. The sensitivity of carbonyl carbon chemical shift of ester group to the sequence effects of aromatic copolyesters has been demonstrated in the l i t e ra t~re .”*~~-~’ The C4 and C4’ of an asymmetric MPBP unit showed resonances at ca. 147.5 and 151.2 ppm and their lineshapes also indicated that these carbons are sensitive not only to the nature of next neighboring rings but also to that of the more distant ones. This result is consistent with the head-to-head and head-to-tail arrangements of two asymmetric units, and is in excellent agreement with the results reported by other^.^',^^-^' The C3 and C3’ of the same unit showed resonances at 123.3 and 122 ppm, respectively, as expected. Similarly, C1 and C1‘ showed two peaks at 134.2 ppm covering a total chemical shift range of 0.4 ppm. The C1 and C1‘ of 3,4’-BDA unit present in polymer 1-3 showed four distinct resonances at ca. 133 ppm covering a total chemical shift of 0.6 ppm, which suggested that these carbon signals are sensitive to the random arrangements of two asymmetric units. All other aromatic carbons showed their resonances at expected peak positions. The ‘H-NMR spectrum appeared broad and unstructured, except that the four hydrogens of one of the rings of the MPBP unit showed crudely a four- line pattern in the range of 8.5-8.7 ppm.

In contrast, the homopolymer of DPBP with 3,4’- BDA, 1-4, showed a Tg, and there was neither a melting endotherm, T,, nor an isotropization endotherm, Ti, in both the heating cycles of the DSC

218 HAN AND BHOWMIK

- p 0.0- I

L 0 P I LL U 6 -0.5-

-

P

-1.0-

1.0

-1.5 ! I I I I I I 1

0 50 100 150 200 250 300 350 0 DSC V4.08 DuPont 2100 Temperature ("C)

Figure 4. 2H and 2C, respectively.

DSC thermograms of polymer 1-3 in the second heating and cooling cycles,

thermograms. However, it had a low Tf at 210°C and a Ti at 300°C as determined with PLM studies. The low Tf value of this homopolymer, 1-4, may be attributed to the random arrangements of asymmetric 3,4'-BDA units in the head-to-head and head- to-tail fashions along the polyester chain. The random arrangements of the asymmetric units were also confirmed with 13C-NMR spectroscopy. Figure 6 shows the 13C-NMR spectrum covering the chemical shift range ca. 120-200 ppm of polymer 1-4 - In contrast to the two distinct resonances of ketone car- bony1 carbons of polymer 1-3, this homopolymer had a single ketone carbonyl carbon resonance at 194.6 ppm. The observation of a single peak suggested that there is no change in the resonance of ketone carbonyl carbons because of the random arrangements of asymmetric 3,4'-BDA units along the polyester chain. The carbonyl carbons of the ester groups gave rise to two distinct resonances at ca. 164 ppm covering a total chemical shift of 0.5 ppm. These two peaks are consistent with the head-to- head and head-to-tail arrangements of the asymmetric 3,4'-BDA units. It is also important to note that the other symmetric DPBP unit of this polymer lacks in these type of arrangements. Both the C4

and C4' of symmetric DPBP units showed a distinct resonance at 147.5 ppm and their lineshapes indicated that they are sensitive to the random arrangements of asymmetric 3,4'-BDA units. The C3 and C3' of the same unit showed a single peak at 123.3 ppm, as expected. Similarly, a single peak at 134.2 ppm was observed for the C1 and Cl'. The C1 and C1' of 3,4'-BDA unit present in polymer 1-4 showed a crude resonance at 132.8 ppm as opposed to the expected two resonances. All other aromatic carbons exhibited resonances at their expected peak positions. The 'H-NMR spectrum of this polymer also appeared broad and unstructured, except that the six hydrogens of the DPBP unit showed clearly a singlet at 8.5 ppm.

The Tf values of these polymers, 1-3 and 1-4, were much lower than the T, values of the respective homopolymers with TA." They not only had low Tf values but also had a broad range of LC phase of 115 (87) and 9O"C, respectively, for their convenient melt processing in the LC phase. The ranges of their LC phases were comparable to those of the homopolymers of both MPBP and DPBP with PhTA,'' and much higher than those25 of a series of copolymers based on PhHQ, TA, and 3,4'-BDA, except


m7mrF 194.0

I " " 1 " " I ' !E5.3 ! C 4 . 0 !E.5.0 PPM

200.0 190.0 180.0 170.0 160.0 150.0 140.0 130.0 I 2 0 . U PPM

Figure 5. 13C-NMR spectrum of polymer 1-3 in CDC13 solution.

that of a copolymer of PhHQ with 90 mol % of TA and 10 mol % of 3,4'-BDA, which had a LC phase range of 137 (162) 0C.25 The observed high LC phase ranges of these polymers may be attributed to the substituted, long rigid 4,4'-biphenylene units. Fur- thermore, it is worth mentioning that the observation of a nematic-to-isotropic transition is not fre- quently encountered in wholly aromatic, thermotropic polyesters. A large number of thermotropic

polyesters of this class had high Ti values > 4OO0C, at which thermal decomposition can take place si- multaneously with this tran~ition. '~ Those wholly aromatic, thermotropic polyesters that have accessible Ti values, < 400°C, have been compiled in the literat~re.~'

The homopolymer of BND with 3,4'-BDA, 1-5, exhibited a Tg, and there was a T, at 343OC in the first heating cycle only, which suggested that crys-

220 HAN AND BHOWMIK

- 194.0

I I I z0h.o 19A.0 . 1sb .0 1,i.e 16;. 0 1 s i . e 140.0 130.0 120.0

PPlp

Figure 6. 13C-NMR spectrum of polymer 1-4 in CDCIB solution.

tallization did not occur a t a cooling rate of 2OoC/ min. The T, value of this polymer was also lower than that of respective homopolymer with TA.41

The copolymer of BP with 3,4'-BDA and 30 mol % of HNA, 1-6, showed a Tg at 139°C and a broad melting endotherm, T,, at 307°C in the first heating cycle. Similarly, the copolymer of BND with 3,4'- BDA and 30 mol % of HNA, 1-7, had a Tg at 209°C

and a broad T, at 301°C in the first heating cycle. The absence of T, in the second heating cycle for each of the copolymers indicated that there was no development of any measurable amount of crystallinity on cooling at a rate of 20"C/min. The copolymer 1-6 had a lower T, than that of a copolymer of BP, TA, and 33 mol 9% of 4-hydroxybenzoic acid (HBA) , which is a member of Xydar family of co-


0 Temperature (‘C) TMA ~ 5 . 0 ~ DuPont 210c

Figure 7. force of 100 mN.

The TMA plot of polymer 1-7 with a heating rate of 20°C/min and a load

polyester^.^^ The copolymer 1-7 had also a lower T,,, than that of the respective homopolymer 1-5. The reason for the lower T,,, values of the copolymers compared to those of the respective homopolymers was due to the copolymerization effect of 6-oxy-2- naphthoate units in the copolymer chains.

The Tg values of the homopolymers, except for I- 1 and 1-2, including two copolymers, are collected in Table 111. The various polymers had Tg values in the range of 139-209°C. The absence of Tg in the two homopolymers was presumably because of high crystalline order or perfection as determined by WAXD studies, discussed in the following section. Both the homopolymer of BND and its copolymer, 1-7, had higher Tg values than those of other polyesters in the series. This result is in excellent agreement with the recent result41 for another series of copolyesters of BND. The high Tg values for the polyesters of this monomer are presumably related to the resistance of binaphthalene units to backbone rotation. The Tg values for all of the polyesters were also verified with a thermomechanical analyzer. A typical TMA plot of polymer 1-7 is shown in Figure 7. There were two regions of softening: the first one corresponded to the Tg, and the second one corresponded to the T,,, for this polymer.

All of the polyesters of 3,4’-BDA formed turbid melts that exhibited stir opalescence. This property

can be taken as the first indication of their liquid crystalline behavior. For further characterization of their melt morphology, they were evaluated by visual observations with polarizing light microscope studies. All polyesters above their T,,,/Tf values exhibited a typical nematic appearance with either the so- called polished marble texture or a threaded texture depending on the thickness or temperature of the sample. The two homopolymers of MPBP and DPBP with 3,4’-BDA, 1-3 and 1-4, had also a nematic-to-isotropic transition, as confirmed with the following microscopic observations. On cooling from the isotropic phase, the so-called nematic droplets were observed.43 Such droplets characterize uniquely the texture of a nematic phase because they occur in no other LC phases. Other characteristics of a nematic phase were an intense movement within the melt and scintillation effects due to directly ob- servable Brownian motion.44 A photomicrograph of polymer 1-4 at the isotropic-to-nematic phase is shown in Figure 8 which shows clearly nematic droplets along with a number of inversion walls of the first kind.45,46 An inversion wall, sometimes, is very thin giving rise to a very narrow line which is also called an inversion line. Inversion lines may form closed curves or start from disclination strength, S = + f or - 4.

As discussed earlier, each of the homopolymers

222 HAN AND BHOWMIK

Figure 8. 295’C (magnification 200X).

The photomicrograph of polymer 1-4 at the isotropic-to-nematic transition,

of 3,4‘-BDA and two copolymers of 3,4’-BDA formed a nematic phase in the melt. In other words, 3,4‘- BDA is a potential mesogenic diacid for the preparation of thermotropic polyesters, due to the asymmetric structure of the 3,4‘-BDA unit. This unit can be incorporated in head-to-head and head-to-tail fashions into the polyester chain, imparting an internal copolymerization effect. This effect can decrease the crystallinity and, thus, the T, of the resulting polyester, as discussed in the following section. One additional structural feature of this diacid deserves special comment. Despite the presence of the nonlinear or kinked carbonyl group in this diacid, it was found that both the present series of homopolymers and the previously reported series of homopolymersZ6 exhibited thermotropic behavior. Presumably, the kinked carbonyl group is compen- sated for by the asymmetric 3-position of the COOH group present in this diacid. Thus, one kink off-sets the other, so that the net effect from end-to-end of the diacid are chain extending bonds approximately 180’. Consequently, the polymer chain can be rod- like, imparting liquid crystallinity.

All of the thermotropic polyesters of 3,4’-BDA were examined by wide angle x-ray diffraction methods. Comments on WAXD patterns are collected in Table 111. The original powder homopolyesters of BP and TMBP with 3,4’-BDA, 1-1 and 1-2, showed diffraction patterns which are typical of semicrystalline polymers. Each diffraction pattern contained quite a number of symmetrical circular rings of high intensity. Presumably, the symmetri- cally substituted four small sized methyl groups were

not effective in reducing the crystalline order or perfection, thus T,, of the homopolymer of BP with 3,4’-BDA. This result is consistent with the result reported by Lenz and Behm.34

In contrast, each of the WAXD patterns of the homopolymers of MPBP and DPBP with 3,4’-BDA, 1-3 and 1-4, contained two diffuse halos (inner and outer) indicative of its “frozen” nematic phase as obtained from the melt polycondensation reaction.47 The WAXD pattern of polymer 1-3 is shown in Fig- ure 9. The absence of T, in the DSC thermogram for each of these two polymers, as mentioned earlier, can be explained because of its “frozen” nematic phase. In other words, there was no crystalline order or perfection in these two homopolymers. However, it is worth mentioning that the homopolymers of MPBP and DPBP with T A had diffraction patterns typical of semicrystalline polymers. The crystallinity and T, of the homopolymer of DPBP with TA were much higher than those of the homopolymer of MPBP with TA.48,49 The higher T, and crystallinity of the former compared to that of the latter is most likely a result of the symmetric structure of DPBP in contrast to the asymmetric structure of MPBP. That is, the symmetric DPBP allows the units of its homopolymer to pack well giving rise to a higher crystallinity and T, for this homopolymer. In ad- dition, it lacks the internal copolymerization effect. Therefore, it was found that 3,4’-BDA, either with an asymmetric monomer, MPBP, or with a symmetric monomer, DPBP, was capable of forming a low Tf thermotropic homopolymer. This result is presumably related to the asymmetric structure of


Figure 9. The WAXD pattern of “as-made’’ polymer 1-3.

a 3,4‘-benzophenone dicarboxylate unit as opposed to a symmetric terephthalate unit. The incorporation of this asymmetric unit in head-to-head and head-to-tail fashions into the polyester chain caused an internal copolymerization effect, which was confirmed with 13C-NMR spectroscopy, as discussed earlier.

The homopolymer of BND, another symmetric monomer, with 3,4’-BDA, 1-5, had less crystalline order or perfection than that of the respective homopolymer with TA, because the diffraction pattern of the former contained fewer symmetric circular rings of moderate intensity when compared with that of the latter.49 Each of the copolymers, 1-6 and I- 7, also had less crystalline order or perfection than that of its respective homopolymer, as expected. The results of WAXD studies for this series of polyesters were in agreement with the DSC measurements.

The thermal stabilities of all of the thermotropic polyesters were determined by TGA either in air or in nitrogen, and the results obtained are collected in Table 111. A typical TGA plot of polymer 1-6 is shown in Figure 10. As expected, the thermal stabilities of polymers 1-1 and 1-2 in air were lower than those of other polymers in the series in nitrogen. The temperature a t which 5% weight loss occurred varied from 465-511°C in nitrogen, indicating

that the polymers had good thermal stabilities for melt processing at elevated temperatures. They had about 50°C higher stability than that of a series of analogous copolyesters of PhHQ, TA, and 3,4‘- BDA.50 The higher stability may be related to the incorporation of the more rigid BP, substituted BP, and BND moieties in the present series of polyesters than that of PhHQ moiety.

CONCLUSIONS

The homopolymers of BP, TMBP, and BND with 3,4’-BDA including two copolyesters had no solubilities in common organic solvents. In contrast, the homopolymers of MPBP and DPBP with 3,4’-BDA (1-3 and 1-4, respectively) had solubilities in many common organic solvents including chloroform, methylene chloride, tetrahydrofuran, and dioxane. The solubility of these two polymers may be attributed both to the presence of a bulky substituent ( s) , a phenyl group, in each of the substituted 4,4‘-biphenols and to the presence of flexible 3,4’-BDA moieties.

All of the homopolymers of 3,4’-BDA had either lower T,,, (343-380°C) or lower Tf (195-210°C) values than those of TA, which is a well known mesogenic diacid. Their low T,,,/ Tf values arose from the incorporation of the asymmetric 3,4’-BDA unit in the head-to-head and head-to-tail fashions along the polyester chain, which was confirmed from the 13C-NMR spectra of the homopolymers of MPBP and DPBP with 3,4‘-BDA (1-3 and 1-4, respectively). Furthermore, the internal copolymerization effect of this diacid moiety had also been investigated by the WAXD studies. Two copolymers, 1-6 and I- 7, had lower T,,, values than those of the respective homopolymers, as expected, because of the copolymerization effect of 6-oxy-2-naphthoate units.

The polymer 1-3 formed a nematic phase at or above 195°C and showed a Ti at 310 (282) “C, as determined from the DSC, PLM, and WAXD studies. The polymer 1-4 also formed a nematic phase at or above 210°C and also exhibited a Ti at 300”C, as determined from both the PLM and WAXD studies. Their LC phase ranges were much higher than those of a series of copolymers of PhHQ, TA, and 3,4’-BDA, except that of one copolymer com- position. Other homopolymers including the two copolymers formed nematic LC phases at above their T,,, values, which persisted up to an accessible temperature of 400°C. They had also high thermal sta-

224 HAN AND BHOWMIK

0 int 210

Figure 10. Thermogravimetric analysis, TGA, of polymer 1-6 in nitrogen.

bilities in the temperature range of 465-511°C in nitrogen.

P. K. B. wishes to acknowledge the Donors of the Petro- leum Research Fund, administered by the American Chemical Society, for support of this research. The authors also gratefully acknowledge the Schenectady Chemical Company, New York, for supplying MPBP, DPBP, and BND monomers. We are grateful to Professor Robert W. Lenz for a number of critical comments and suggestions.

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Received March 21, 1994 Accepted July 22, 1994

wholly aromatic thermotropic liquid crystalline polyesters of 4,4′-biphenol, substituted...

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