Nucleation and morphology of polyethylene under shear flow

Effect of the shear flow (γ̇ = 0.5 ∼ 5 s^-1) on the nucleation and on the morphology of polyethylene (PE) during crystallization from the melt was studied by means of polarizing optical microscopy and small angle X-ray scattering. In order to analyze the results by the nucleation theory, we observed the effect of shear flow on the equilibrium melting temperature (T_m⁰). The T_m⁰ under shear flow and that under quiescent state are almost the same. T_m⁰ = 140.2-140.3°C. Therefore, the shear flow does not affect the T_m⁰. The "heterogeneous" primary nucleation rate (I) and the induction onset time of nucleation (τ_onset) of the isolated crystals under shear flow are also almost the same as those under quiescent state. The heterogeneous primary nucleation means that isolated nuclei are sporadically generating from the melt with the aid of heterogeneities such as nucleating agents. After the generation of primary nuclei, the "shish" were generated independent of the primary nucleation. We found that the shish were formed by the chain elongation caused by the velocity difference between polymer chains and dust particles, etc. After that, we observed that the "kebabs" were formed on the shish. It was found that the nucleation from the melt after melting of the "shish-kebabs" (at melt annealing temperature T_max = 160°C for 5 min) was accelerated compared with the ordinary melt crystallization after melting of folded chain crystals. This indicates that the "solid memory effect" of the former solid is significant. This is because of the low entanglement density within the melt of shish kebabs.