Solid-state modification of isotactic polypropylene (iPP) via grafting of styrene. II. Morphology and melt processing

Grafting of vinyl monomers onto isotactic polypropylene (iPP) in the solid state represents a convenient route to chemically modify iPP and, consequently, its properties. Solid-state modification can be carried out on iPP powder directly from the polymerization reactor. The modified powder is then processed in the melt, usually with the addition of fillers and/or additives, to obtain the final product. In this work we have studied the effect of melt processing on the morphology of solid-state polymerized PP/polystyrene (PS) blends, i.e., of a iPP powder previously modified in the solid-state with styrene (St) and optionally in the presence of divinylbenzene (DVB). A series of samples containing different amounts of PS and displaying different grafting efficiencies were investigated before and after processing in the melt. Transmission electron microscopy, scanning electron microscopy, and solid-state NMR were used to investigate the morphology on different length scales. It was shown that PS coalescence during processing can be hindered, thereby stabilizing the initially polymerized iPP/PS blends morphology. Indeed, reducing the PS amount in the blend or increasing the grafting efficiency resulted in less coalescence of the PS domains. Crosslinking of the PS phase during the solid-state polymerization resulted also in a very fine but heterogeneous morphology. (c) 2005 Wiley Periodicals, Inc.