Weak Shear-Induced Slowdown in Crystallization of Less-Entangled Poly(ε-caprolactone)

Self-nucleation (SN) and strong shear methods were employed to obtain less-entangled melts in poly(ε-caprolactone) and its blends with poly(styrene-co-acrylonitrile). The effect of shear flow on the isothermal and nonisothermal crystallization behavior of the less-entangled melts was studied by rheology and shear-Raman spectroscopy. Weak shear with a Rouse Weissenberg number WiR < 1 causes a lower crystallization temperature (Tc) and a longer half-crystallization time (t1/2) for both SN and strongly sheared melts under nonisothermal and isothermal conditions, respectively. This indicates that the crystallization of the less-entangled melts is decelerated by weak shear, implying that the melt memory effect from the crystallization is weakened. This phenomenon is ascribed to the accelerated chain re-entanglement under a weak shear flow, as verified by the rheological responses under the interrupted shear flow, stress relaxation, and oscillatory time sweep. It is believed that shear flow increases the free energy of polymer chains, which overcomes the free energy barriers for re-entanglement, activates the reptation, and speeds up the chain re-entanglements.

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