Influence of chain entanglements and melt memory effect on the crystallization behavior of polyphenylene sulfide

Polyphenylene sulfide (PPS) has a prospective application in high-frequency communication due to its excellent dielectric properties. However, owing to the strong melt memory effect, the crystalline structure of PPS is very sensitive to the processing conditions and difficult to be controlled. To date, the melt memory effect of polymer cannot be detected directly and quantitatively. In this work, four PPS samples were successively prepared via polymerization, extrusion and dissolution treatment. The Fourier infrared transform spectroscopy (FTIR), high-temperature gel permeation chromatography (HT-GPC) and dynamic rheological tests disclose that these four PPS samples have analogous molecular structure and only differ in entanglement density. The molecular weight between entanglements (Me) decreases gradually from 1188 to 354 g/mol. Furthermore, differential scanning calorimetry (DSC) and polarized optical microscopy (POM) measurements were carried out to study the effect of entanglement density on the crystallization kinetics of PPS. It is found that entanglement density is a key indicator to characterize the melt memory effect of PPS semi-quantitatively, and the residual local ordered structures in PPS melt is proportional to Me. In addition, the crystallization kinetics of PPS is determined by the cooperation of melt memory effect (nucleation) and entanglement density (growth). Accordingly, the relationship of "chain entanglements" → "melt memory effect" → "crystallization behavior" of PPS is established, constructing a roadmap for the precise control of crystalline structures of PPS products during practical processing.