Effect of PP-g-LCP compatibilizer on the structure and crystallization of blends of isotactic polypropylene (PP) with a semiflexible liquid crystalline polymer (LCP)

Blends of i-polypropylene and a semiflexible liquid crystalline polymer (iPP/LCP 90/10 and 80/20 w/w) were compatibilized with 2.5, 5 or 10 wt.-% PP-g-LCP copolymers. The crystal structure, crystallization behaviour and morphology of the compatibilized blends have been investigated by differential scanning calorimetry (DSC), wide angle X-ray scattering (WAXS) and optical microscopy. It is shown that the nucleation rate and crystallization rate of iPP strongly increased in the compatibilized blends in comparison to the uncompatibilized ones. In compatibilized blends, while iPP spherulites get smaller and crystallinity degree increases, the crystal growth mechanism of iPP remains unchanged. An assumption has been made about the mechanism of the compatibilization, using graft copolymers constructed of segments identical to the blended polymers: the PP segments of PP-g-LCP copolymers, if long enough, cocrystallize with bulk iPP, compatibilizing partially the two components; the LCP grafts of the copolymers cocrystallize with the bulk LCP or enter the amorphous phase of the blends. The miscibility of each part of the copolymer compatibilizers with the corresponding bulk component of the blend leads to a reduction of the interfacial tension and to a strong reduction of the dimensions of dispersed LCP phase. The compatibilized iPP/LCP blends display improved crystallization kinetics, enhanced degree of crystallinity and improved interphase adhesion. Consequently, an improvement of the mechanical properties and rheological characteristics should be expected for these blends.