Coupling effects of spinodal decomposition and crystallization on mechanical properties of polyolefin blends

The influences of preferentially occurred liquid-liquid phase separation (LLPS) and following crystallization processes on the mechanical properties of statistical copolymer blends of poly(ethylene-co-hexene) (PEH) and poly(ethylene-co-butene) (PEB) have been investigated in detail through tensile deformation tests with a relatively high extension rate to avoid the effect of interfacial properties of the blends. Crystallinity and lamellar thickness of the samples are estimated by using the wide-angle X-ray diffraction and small-angle X-ray scattering techniques, respectively. The tensile modulus and yield stress are found to increase with LLPS time up to 6 h, but decrease afterwards, under the conditions of temperature of 120 °C and isothermal crystallization time of 10 min. It is considered that the instantaneous tensile properties are substantially largely affected by the much perfect lamellar structures formed during crystallization with a long time prior LLPS step. This finding is further experimentally substantiated by the scanning electron microscope observation. Whereas the strain-hardening modulus described by a simple neo-Hookean relation increases with LLPS time and reaches a plateau after 6 h, which can be accounted for by the cooperation effect between amorphous entanglement density, insensitive to LLPS time, and crystallinity redistribution. The similarity of the results observed on the blends experiencing the spinodal decomposition (SD) process supports that the redistribution of crystallizable components contributes to the tensile stress increase, which is primarily controlled by the development of LLPS process. This simple relationship gives us a new insight of what controls the mechanical properties of the phase separated polymer blends and of how we might be able to predict the mechanical properties of as yet unmixed polymer pairs.