Structure of polyamide 6 and poly (p-benzamide) in their rod-coil-rod triblock copolymers investigated with in situ wide angle X-ray diffraction

A series of rod-coil-rod triblock copolymers containing polycaprolactam (PA6) as the coil block and poly (p-benzamide) (PBA) as the rod block were synthesized by a two-step polycondensation reaction. Proton nuclear magnetic resonance (1H NMR), UV–vis spectrophotometry (UVS) and differential scanning calorimetry (DSC) were performed to determine the fundamental molecular structure and thermal property of copolymers. UV–vis spectrophotometry results revealed that the content of PBA homopolymer increased with the block length (number of monomer) of PBA and reached a plateau value ranging from 22 to 40 monomers of PBA. The wide angle X-ray diffraction (WAXD) measurements indicated that the crystallization of PA6 blocks was strongly suppressed due to the stretching from rods after annealed at temperature above the melting point of PA6. Only a few imperfect crystals of PA6 existed in the samples with low volume fraction of PBA. Moreover, the variation of PA6 block length hardly affected the crystallinity of PBA, which was dominantly controlled by the block length of PBA as diffusion was the control step for PBA crystallization at the annealing temperature.