Influence of semicrystalline homopolymer addition on the morphology of semicrystalline diblock copolymers

We have studied blends of polyethylene "homopolymer" (hydrogenated polybutadiene) with diblock copolymers of polyethylene-poly(ethylene-alt-propylene), E/EP, varying both homopolymer loading and the block copolymer and homopolymer molecular weights. The pure block copolymers form homogeneous melts which are miscible with the homopolymer. The two components readily cocrystallize, even at homopolymer loadings as high as 75 wt % and even for a homopolymer which has nearly three times the molecular weight of the E block in the diblock. Small-angle X-ray scattering (SAXS) shows a single domain structure up to at least 50 wt % homopolymer addition, a result confirmed by transmission electron microscopy (TEM). Surprisingly, the Bragg spacing observed by SAXS, which reflects the sum of E and EP lamellar thicknesses, decreases continuously and substantially as homopolymer is added. The strong chain stretching in the EP block is progressively relaxed as E homopolymer is added; the reduction in Bragg spacing is only weakly opposed by the enthalpic resistance to formation of additional internal interface between E and EP domains, due to the small interaction parameter between the two blocks.