Extrudates liquid crystalline polymers

Miscibility or compatibility provided by the compatibilizer or TLCP itself can affect the dimensional stability of in situ composites. The feature of ultra-high modulus and low viscosity melt of a nematic liquid crystalline polymer is suitable to induce greater dimensional stability in the composites. For drawn amorphous polymers, if the formed articles are exposed to sufficiently high temperatures, the extended chains are retracted by the entropic driving force of the stretched backbone, similar to the contraction of the stretched rubber network [61,62]. The presence of filler in the extruded articles significantly reduces the total extent of recoil. This can be attributed to the orientation of the fibers in the direction of drawing, which may act as a constraint for a certain amount of polymeric material surrounding them. 

Blends of polypropylene (PP) and liquid crystalline polymer (LCP) processed without melting the LCP were compared with conventional melt processed blends. In a first stage, PP was blended with 20 wt% of LCP in a twin-screw extruder with the take-up speed varied to achieve blends with different LCP fiber dimensions. In the second stage, these blends were processed both below and above the Tm of the LCP by extrusion and injection molding. 

Liquid crystalline polymers exhibit anisotropy in extruded and molded articles as a result of preferential orientation of LCP domains or individual chains. Reference (21 highlights some of the molecular structural features of LCP s that account for their fundamental anisotropy. These include the large aspect ratio of the individual polymer chains and their tendency to form aligned, highly crystalline domains. 

To date, reports of investigations on the gas transport properties of main chain liquid crystalline polymers appear to have been limited to the work conducted in our laboratory. Chiou and Paul (4.) have briefly described the transport parameters of an extruded film of an LCP having a similar structure to the commercial product Vectra. This copolyester belongs to the family of napthylene thermotropic polymers (NTP s) commercialized by Hoechst-Celanese Corp. whose synthesis and properties have been described previously (iLS.). Transient permeation experiments were conducted with a series of gases. The effective... 

The morphology of the fracture surfaces of both the extruded and injection-molded specimens was studied via scanning electron microscopy (SEM). Although there were many morphologies across the fracture surfaces, highly elongated liquid crystalline polymer domains parallel to the flow direction were observed for both the extruded and molded specimens. 

Gopakumar et al. [10] reported the in situ compatibilization of poly(phenylene sulfide) (PPS)/wholly aromatic thermotropic liquid crystalline polymer (TLCP) Vectra A950 blends by reactive extrusion. The authors prepared the in situ compatibilized PPS/TLCP blends in a twin-screw extruder by reactive blending of PPS and TLCP in the presence of dicarboxyl-terminated poly(phenylene sulfide) (DCTPPS). Block copolymer was formed during reactive blending, by transesterification reaction between carboxyl... 

For what concerns the mechanical performances of liquid-crystalline polymers, many papers were published on the characterization of the LCP processed under different conditions. The major feature in determining the final properties of the extrudate or of the drawn fibers appears to be the conformation in the alligned state. It appears clearly that well oriented polymers exhibit higher values for the elastic... 

Heino, M. T. and Seppala, J. V., Extruded blends of a thermotropic liquid crystalline polymer with polyethylene terephthalate, polypropylene, and polyphenylene sulfide, J. Appl. Polym. Sci. 44 2185 (1992). 

Blizard, K, G., Federici, C., Federico, O., Chapoy, L. L., The morphology of extruded blends containing a thermotropic liquid crystalline polymer, Polym. Eng. Sci. 30 1442 (1990). 

Liquid crystalline polymers [326,329-336] have found application as high modulus fibers and films with unique properties due to the formation of ordered lyotropic solutions or thermotropic melts which transform easily into highly oriented, extended chain structures in the soUd state. Thermotropic polymers are melt process-able and thick extrudates and molded parts are formed with high strength characteristics as in fiber reinforced thermoplastics. 

Elongational behavior is induced in the entrance of the spinning hole and in the transition region from backhole to actual capillary. In practice hardly any permanent orientation is built up in this way, however, because molecular relaxation is rapid. Spinning hole profiles are smoothened only to prevent the formation of vortices which would lead to extrudate distortion. Promoting orientation already in the spinning holes is not common for melt spinning. It could be beneficial for the orientation of melt-spun liquid-crystalline polymers, however, for example in the production of carbon fiber from pitch. 

For lyotropic liquid crystalline polymers, either wet-spinning or dry-jet-wet spinning are employed to produce fibers. A lyotropic state of a polymer solution is formed at a specified concentration and temperatnre in a nonvolatile solvent. The anisotropic polymer solution is extruded into a non-solvent bath, where fiber coagulation and solvent extraction take place. For thermotropic liquid crystalline polymers, a melt spuming method is applied. 

Rubin, L.S., Blizard, K.G., Haghighat, R.R., Lusignea, R.W., Extruded Thermoplastic, Liquid Crystalline, Polymers and Blends Thereof Having a Planar Morphology Foster-MUler, Inc., US Patent 5,981,007,11/9/1999. 

Because lyotropic liquid-crystalline polymers cannot be extruded, injection molded, or blown into films, other polymers that can be melt processed have been developed. These thermotropic liquid-crystalline polymers convert to a mesophase when the sohd polymer is heated to a temperature above the crystalline melting point. Thus, these polymers show three thermal transitions. In increasing order of temperature, these are glass transition temperature, erystalline melting... 

The melt-spinning process used to convert mesophase pitch into fiber form is similar to that employed for many thermoplastic polymers. Normally, an extruder melts the pitch and pumps it into the spin pack. Typically, the molten pitch is filtered before being extruded through a multi-holed spinnerette. The pitch is subjected to high extensional and shear stresses as it approaches and flows through the spinnerette capillaries. The associated torques tend to orient the liquid crystalline pitch in a regular transverse pattern. Upon emerging from the... 

Applications. The polyamides have important applications. The very high degree of polymer orientation that is achieved when liquid crystalline solutions are extruded imparts exceptionally high strengths and moduli to polyamide libers and lilms. DuPont markets such polymers, e g. Kevlar, and Monsanto has a similar product, e.g.. X-500. which consists of polyamide and hydrazide-lype polymers. Liquid-crystal polymers arc also used in olccirnnpnc displays. 

It was, however, observed that such systems under appropriate conditions of concentration, solvent, molecular weight, temperature, etc. form a liquid crystalline solution. Perhaps a little digression is in order here to say a few words about liquid crystals. A liquid crystal has a structure intermediate between a three-dimensionally ordered crystal and a disordered isotropic liquid. There are two main classes of liquid crystals lyotropic and thermotropic. Lyotropic liquid crystals are obtained from low viscosity polymer solutions in a critical concentration range while thermotropic liquid crystals are obtained from polymer melts where a low viscosity phase forms over a certain temperature range. Aromatic polyamides and aramid type fibers are lyotropic liquid crystal polymers. These polymers have a melting point that is high and close to their decomposition temperature. One must therefore spin these from a solution in an appropriate solvent such as sulfuric acid. Aromatic polyesters, on the other hand, are thermotropic liquid crystal polymers. These can be injection molded, extruded or melt spun. 

Liquid crystals, due to the presence of the ordered domains, are optically anisotropic, i.e. birefringent. This can be easily verified by observing the liquid crystal, e.g. aramid and sulfuric acid solution, at rest between crossed polarizers. The parallel arrays of polymer chains in liquid crystalline state become even more ordered when these solutions are subjected to shear as, for example, in extruding through a spinneret hole. It is this inherent property of liquid crystal... 

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