Synthesis and properties of lyotropic poly(amide-block-aramid) copolymers

Abstract

This thesis describes the synthesis and properties of liquid crystalline block copolymers comprised of alternating rigid and flexible blocks for the preparation of self-reinforcing materials. The incentive for this work was the expectation that the rigid segments would phase separate on a microscopic scale, and would form nematic domains, which will cause an enhanced orientation of the flexible segments leading to coil stretching. As a result the coils in the copolymer will be stiffer compared to unstretched coils, and the mechanical properties will be better than expected from the pure flexible polymer, therefore the term "self-reinforcement". For this purpose a block copolymer was synthesised that is comprised of alternating rigid poly(p-phenylene terephthalamide) (PPTA) (commercially known as Twaron or Kevlar) blocks and flexible coil polyamide 6,6 (commercially known as nylon 6,6) blocks. It is shown that if the aramid content in the copolymers is at least 50 mol% the polymers are able to show a lyotropic liquid crystalline phase in sulphuric acid. From these liquid crystalline solutions fibers have been spun that show promising mechanical properties. Due the combination of the rigid blocks providing stiffness and flexible blocks providing elasticity these block copolymers may be considered in high energy absorption applications.