Memory Effect in Flow-Induced Heterogeneous Isotactic Polypropylene Melts

Flow-induced polymer melt can develop heterogeneity involving both quasi-ordered structures, namely, nucleation precursors (NPs) and partially disentangled chains (PDCs). These states can retain a memory that affects the subsequent crystallization process. In this work, the interplay between the NP and the PDC in isotactic polypropylene (i-PP) melt was investigated by rheological measurements, and its contribution to the crystallization behavior was assessed. After the application of a steady shear flow, which causes partial disentanglement, the characteristic re-entanglement time follows an Arrhenius temperature dependence with a much higher activation energy at temperatures below the equilibrium melting point, suggesting a cooperative motion of chain segments that are aligned and clustered by shear. With decreasing temperature, the unexpectedly long re-entanglement time, abnormally high activation energy, and increased entanglement density indicate that the NPs can provide extra physical entanglements that increase the melt viscosity and storage modulus, thereby postponing the re-entanglement process of the PDCs to the equilibrium melt state. Moreover, the origin of the flow-memory effect on crystallization was successfully investigated, revealing a synergistic interplay between the NPs and PDCs. The crystallization kinetics becomes slower, and the crystalline morphology changes from a shish-kebab structure to a spherulite with the increase of shear temperature and holding time. The relative shear efficiency, a measure for the flow-induced acceleration of crystallization kinetics, shows a double-decay trend, suggesting the sequential relaxation of the NP and the PDC. Finally, by changing the shear rate, shear strain, and molecular weight, the entanglement density can be efficiently adjusted, while the activation energy of the precursor cluster relaxation is basically unchanged.