Crystallization behavior of polyamide 6 confined in a strip-like rubbery phase

Abstract

A purified xLDPE/LDPE-g-PB-g-PA6/PA6 blend with polyamide 6 (PA6) confined in a rubbery polybutadiene (PB) phase is prepared via extracting the unreacted LDPE and PA6 from a blend of PB-g-MAH functionalized LDPE (LDPE-g-PB-g-MAH) and PA6 50/50 wt/wt by using selective solvents of LDPE and PA6, respectively. The term xLDPE represents a partially crosslinked LDPE phase that cannot be dissolved by its selective solvent of toluene. The structure and crystallization behavior of PA6 in a xLDPE/LDPE-g-PB-g-PA6/PA6 blend containing 14% of PA6 by weight are studied using transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). Strip-like rather than droplet-in-matrix morphologies are found. Strip-like PA6 domains are located inside the strip-like rubbery PB phase, where LDPE is the matrix. The thickness of the strips composed of both the PB and PA6 phases are about 50–70 nm, in which the PA6 strips are ∼20 nm thick. The slow nucleation rate of the confined PA6 results in the decrease of both its melting temperature and crystallinity when it is subjected to multiple heating–cooling scans at the rate of 10 °C min⁻¹. The crystallinity obtained by the DSC test decreases to zero after 5 heating–cooling cycles. This is because the crystallization rate of the PA6 in the purified xLDPE/LDPE-g-PB-g-PA6/PA6 blend is too slow to be detected when it is cooled from the melt with a cooling rate of 10 °C min⁻¹. Crystallization still can take place when the temperature is higher than the glass transition temperature of PA6. The crystallinity of the PA6 will increase with an increase in annealing time under suitable annealing temperatures. The crystallization of LDPE has no effect on the crystallization of the PA6 phase because of the rubbery PB layer located between them. This work will be a guide for researchers to produce polymer blends with not only a controllable phase structure, but also with a suitable crystallinity.