Transient flow of thermotropic liquid crystalline polymers in step strain experiments

Abstract

SynopsisThe transient response of two thermotropic liquid crystalline polymers (a copolyester of 60 mol % p-hydroxybenzoic acid, PHB, and 40 mol % poly(ethylene terephthalate), PET, and a copolyester of PHB and 2-hydroxy-6-naphthoic acid, following step strains up to 20 strain units was measured. The relaxation curves typically show an initially rapid decay followed by a long relaxation tail. The lower the temperature, the more remarkable is the length of the long relaxation tail. This behavior makes the LCP uniquely different from most flexible chain polymers such as PET and polystyrene, which show a relaxation modulus which decreases continuously having no discontinuity in the slope. This behavior is probably due to unmelted solid phase which exists in the melt up to quite high temperatures. When the LCP's are heated to temperatures well above their melting points as determined by DSC, then the long relaxation tails are eliminated. Furthermore, on cooling the sample rapidly down from a temperature well above the melting point the relaxation modulus resembles that at the higher temperature as a result of the supercooling effect. The relaxation modulus was also determined for samples subjected to lubricated squeezing flow. Whereas for polystyrene the relaxation modulus determined in lubricated squeezing flow was equal to that determined in step shear strain experiments, this was not the case for the LCP's. It is not known whether the behavior reported here is common to nematic LCP's or to the multiphase structure (crystallites, nematic phase, isotropic phase) which might be present. INTRODUCTIONThermotropic liquid crystalline polymers (LCP's) have been of considerable interest in the last few years because they can be processed to form high modulus materials. For example, the flexural modulus of Copyright by the American Institute of Physics (AIP). Done, D; Baird, DG, "transient flow of thermotropic liquidcrystalline polymers in step strain experiments," J. Rheol. 34, 749 (1990); http://dx.doi.org/10.1122/1.550149 750 DONE AND BAIRD injection molded plaques can exceed that found for fiber reinforced materials. 1 Furthermore, the modulus of fibers processed from thermotropic LCP's is similar to that reported for steel.' The exceptional properties are related to the high degree of molecular orientation generated during melt processing. Hence, it is apparent that polymer systems with liquid crystalline order (LCO) can be oriented much more readily than isotropic polymer systems.It is suggested by theories such as those of Leslie and Ericksen 3 ,4 and Doi 5 that the change in transient rheological properties is due to changes in the orientation of the rod-like molecules. Although under some conditions it is predicted by Ericksen's theory that the rod-like molecules will tumble, in general it is predicted that the molecules will orient in both shear and extensional flow. In studies by Viola and Baird 6 it was found for a thermotropic copolyester that in isothermal shear flow there was no evidence of molecular orienta...

Rheology and relaxation processes in a melting thermotropic liquid–crystalline polymer

“…Moreover, the relaxation behaviors of TLCPs are much more complicate than those of flexible polymers due to the presence of polydomain structure, whose equilibrium state is hard to define. Numerous investigations have been made on the rheological properties and relaxation behavior of TLCPs in nematic state 1–18…”

“…Wissburn8 points out that liquid–crystalline polymers typically possess long relaxation times, up to 100 times longer than those of isotropic polymers of similar molecular weight. Step strain experiments show a fast initial stress relaxation followed by a slower relaxation period, and a tail in the relaxation curve is observed, which has led to the speculation that some structure or unmelted solid phase exists 9. Amundson et al10 have used proton and C 13 NMR to study the microstructure of the HBA/PET(80/20) polymer (provided by Tennesses Eastman).…”

Development of Recyclable and High-Performance In Situ Hybrid TLCP/Glass Fiber Composites

“…Each sample was equilibrated to the experimental temperature for 5 min. First, pure TLCP pellets were loaded directly into the rheometer and then tested using a shear step strain at a strain of 0.5% [37]. The relaxation modulus (G(t)) at each temperature was plotted against time.…”

“…The rheological properties of TLCP above its melting point are critical in the processing of this material. Transient behaviors of TLCPs are related to the rigid TLCP molecules and domain structure [37,38]. The transient responses of pure TLCP at various test temperatures, following step strain were measured, as shown in Figure 1.…”

The transient shear rheology of a thermotropic liquid crystalline polymer in the super-cooled state

“…The shear rheology of commercial TLCPs has been well documented under conventional processing conditions above Tm [18][19][20][21][22][23][24][25][26][27][28][29]. Other than the 73/27 HBA/HNA, two other commercial TLCPs have been widely studied.…”