Frustrated structures of polycaprolactam and poly(p-benzamide) in their rod–coil–rod triblock copolymers

Abstract

A series of rod–coil–rod triblock copolymers were synthesized by two-step polycondensation with polycaprolactam (PA6) as the flexible block and poly(p-benzamide) (PBA) as the rod. Proton nuclear magnetic resonance (1H NMR), UV–vis spectrophotometry and differential scanning calorimetric (DSC) were first performed to determine the fundamental molecular structure and thermal property of each polymer. Through wide angle X-ray scattering (WAXS), and Fourier transformation infrared spectroscopy (FT-IR) measurements, frustrated structures of both components were studied. For PA6, a quasi-γ mesomorphic order was found in the transition-region nearby the PBA domain which is more favored with the increase of the PBA content owing to stretching from the PBA rod block and different cross-section areas of the rod and coil chains. On the other hand, the mesomorphic order of PA6 segments imposes stretching and constraint on the PBA blocks, which leads to a reduction of order of PBA block when the volume fraction of rod reaches approximately 45%. α crystals of PA6 can form only in the triblock copolymer with low volume fraction of PBA, which exhibit the Brill transition feature during the heating process though this transition ends prematurely as the melting of crystals.