Crystallization behavior and morphology of PE-g-LCP copolymers

Abstract

This study presents DSC and optical microscopy investigations on copolymers of semiflexible liquid crystalline polymer SBH 112 grafted to functionalized low molecular mass polyethylene (PEox) obtained by melt polycondensation or reactive blending procedures. The crystallization behavior of the PE-g-SBH copolymers has been studied under non-isothermal measurement conditions carried out at different cooling rates. The crystallization temperature (Tcr) of the PE component of the copolymers decreases steadily upon increasing the concentration of the SBH grafts. It was found that the copolymers prepared by reactive blending crystallize at slightly higher Tcr than those prepared by polycondensation and with a higher rate, confirmed by the determination of the crystallization rate coefficients (CRC). The results have been interpreted by the fact that the PE crystallizable segments and SBH grafts of the copolymers obtained by reactive blending are longer than those of the copolymers prepared by polycondensation. The overall nonisothermal crystallization kinetics has been studied by the Harnisch and Muschik equation. The results show that the mechanism of the crystallization of the PE phase changes only when the SBH content overruns ca.50%, due to the decrease of both nucleation and crystal growth rates. The morphology of the copolymers crystallized nonisothermally from melt has been examined by polarization microscopy. Fairly homogeneous morphology with tiny PE spherulites is observed for PE-g-SBH copolymers prepared by polycondensation with SBH as the minor phase. No sign of the dispersed LCP domains can be recognized. On the contrary, the morphology of the copolymers prepared by reactive blending is distinctly biphasic. The allegedly longer PE segments crystallize into tiny spherulites too, but the LC domains formed by the long SBH branches present in this type of copolymers appear clearly in the micrographs at room temperature. It is concluded that the copolymers prepared by reactive blending would be more effective as compatibilizers for PE/SBH blends than those prepared by polycondensation.