Synergistic properties of poly(ethylene 2,6-naphthalate) fibers blended with novel thermotropic liquid crystalline copolyesters

• Main Author: Lo, Verna Charlene
• Language: ENG
• Published: ScholarWorks@UMass Amherst 1996
• Subjects: Materials science|Plastics
• Online Access: https://scholarworks.umass.edu/dissertations/AAI9638992

This dissertation discusses the formulation, processing, and properties of blends of semi-flexible thermotropic liquid crystalline polymer (TLCP) copolyesters within a matrix of poly(ethylene 2,6-naphthalate) (PEN). The TLCPs, both main-chain flexible and side-chain flexible, are categorized into three main classes: segmented block, alternating, and random (statistical). The TLCPs and the matrix were characterized in terms of their thermal, mechanical, and rheological properties. Several new structure-property relationships for the pure PEN fiber are established with strong implications for further processing studies. Fibers containing between 0% to 20% by weight of TLCP were melt-extruded into monofilaments with initial diameters from 100-120 $\mu$m. As-spun fibers were subjected to a two-stage post-treatment processing to maximize their mechanical performance. The TLCPs were found to serve several roles within the matrix when blended in concentrations from 0.25% to 20% by weight. Thermally, they were able to plasticize and nucleate crystallinity within the matrix. A semi-flexible phenylene-based TLCP was able to plasticize the matrix by lowering the glass transition temperature by as much as 14 degrees. A semi-flexible naphthalene-based TLCP, when blended in at 1% concentration, was able to nearly double the amount of PEN re-crystallized from the melt (from 20% to 40% crystallinity). Mechanically, all hot-drawn polyblends of main-chain flexible TLCP gave rise to Young's modulus values between 29-31 GPa. Thermal data and morphology data indicate that even ideal levels of in-situ reinforcement are insufficient to account for the observed mechanical property enhancements. Evidence from thermal studies shows clearly that the TLCPs can modify the matrix in which they are blended. Striking matrix modification effects are found to occur, unexpectedly, both in the presence and in the absence of the nematic texture within the polyblend upon TLCP addition. There is also a relationship between the amount of PEN re-crystallized from the melt of the polyblends to the final mechanical properties of the fibers. Both thermal and mechanical properties of the final fibers are highly process-dependent. To this end, instrumentation techniques for new processing methods were investigated and developed.