Blends of an engineering thermoplastic, poly(ethylene terephthalate) (PET), and two liquid crystalline polymers (LCPs) viz., copolyesters of PET and parahydrox‐ybenzoic acid (PHB) in 40/60 mole percent (LCP60) and in 20/80 mole percent (LCP80) were prepared. A blend of LCP60 and LCP80 in 50/50 weight percent (LCP60‐80) was blended with PET. Both flat films and rods were extruded and their properties examined. The morphology of the films investigated using Scanning Electron Microscopy (SEM) revealed that the LCP phase remained as dispersed droplets in the PET matrix. In spite of the lack of fibrillation in these films, the mechanical properties were enhanced to some extent with a maximum at 10 weight percent of the LCP phase. However, in the case of the rods thin fibrils of the LCP phase of the order of 1 μm in diameter were observed provided the composition of the LCP was 20 weight percent or greater. This success In achieving fibrillation is through to be due to the extensional flow fields present at the entrance of the capillary die and the fact that a short L/D ratio die was used. Differential Scanning Calorimetry (DSC) thermograms of the extruded films indicated that the LCP phase may act as a nucleating agent for the crystallization of PET. Rheology of the blends revealed that the complex viscosity of the blends is not much different from that of pure PET. This is attributed to the partial miscibility of the two components. Based on the DSC results and residence times in the extruder, it is concluded that no significant transesterification reactions appear to have: taken place in the blends. The rheology is studied further with respect to the cooling behavior of the pure components and factors important to the fibrillation of the LCP phase and the formation of in‐situ reinforced composites are discussed.

The supercooling behavior of three thermotropic liquid crystalline polymers (TLCP's), 60 mole% p‐hydroxybenzoic acid and 40 mole% poly(ethyleneterephthalate), 60/40 PHB/PET, 80 mole% p‐hydroxybenzoic acid and 20 mole% poly(ethyleneterephthalate), 80/20 PHB/PET and a copolyester of 73 mole% p‐hydroxybenzoic acid and 27 mole% of 2‐hydroxy‐6‐naphthoic acid, Vectra A900, was studied by means of both rheological and thermal methods. 60/40 PHB/PET and Vectra A900 exhibited a degree of supercooling as high as 80°C while there was only 20°C of supercooling for 80/20 PHB/PET. The degree of supercooling for the blends of 60/40 PHB/PET and 80/20 PHB/PET also decreased with the increase in 80/20 PHB/PET content. The increase in G′ as the temperature decreased was more gradual for 60/40 PHB/PET and Vectra A900 than that for 80/20 PHB/PET, which was beneficial from a processing point of view. The solidification of the LCP melt was attributed to both the crystallization and the freezing of the mesophase. The degree of crystallinity for all three TLCP's was very small, less than 5%, as measured by the DSC. It was found that both 60/40 PHB/PET and Vectra A900 showed an induction period of a few minutes during solidification while 80/20 PHB/PET solidified continuously at the test temperature. Therefore, 60/40 PHB/PET and Vectra A900 seem to be more suitable for use in processes such as blow molding and film blowing than 80/20 PHB/PET.

This paper is concerned with the effect of thermal history on the rheology of three liquid crystal copolymers. The polymers were heated to temperatures above their melting points and then cooled down. During the cooling cycle, the dynamic mechanical properties were monitored. It was found that these properties remained unchanged at temperatures as low as 30°C below the normal melting points for two copolyesters of 60 mole percent p‐hydroxybenzoic acid (PHB) and 40 mole percent poly(ethylene terephthalate) (PET) (60‐40 PHB/PET) and 80 mole percent PHB and 20 mole percent PET (80‐20 PHB/PET). For a copolymer of PHB and 2,6‐dihydroxynaphthaic acid, there was a gradual increase in the properties with decreasing temperature. However, at a critical temperature all three systems exhibit a sudden increase In dynamic mechanical properties. The kinetics of solidification were also monitored, and it was found that at temperatures well below the melting point, it took 4 to 6 min before the solidification process impeded flow. Extrusion studies on these materials were also carried out, and it was observed that in this supercooled state, the polymers exhibited significant die swell. The samples also exhibited a fibrous texture and were more oriented than when extruded isothermally above the melting point.

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...

porous media placed in the entrance of capillaries were found to reduce the pressure drop across the capillaries (t>P,) by a factor of two or three for polystyrene. The reduction in t>P, was found to be a function of the distance of the porous media from the capillary entrance, the type of porous media, the length of the capillary, and the rheological properties of the polymer melt. No significant reduction in t>P, was observed for a polymer melt such as polyethyleneterephthalare (PET) which is nearly devoid of memory. The apparent shear rate for the onset of melt fracture was extended by a factor of three when polystyrene passed through the porous media before entering the capillary. No significant difference in die sweIJ values was observed with the use of porous media in the entrance of the capillaries. The mechanism which accounts for these phenomena is believed to be associated with the break up of the entanglement network in the porous medium which lemporarily changes the rheological properties of the polymer melt.