THE CRYSTALLIZATION OF BLENDS OF DIFFERENT TYPES OF POLYETHYLENE - THE ROLE OF CRYSTALLIZATION CONDITIONS

Cocrystallization in blends of linear and branched polyethylenes has b een studied under both isothermal and slow-cooling crystallization con ditions. Before the more common, polydisperse-type polyethylenes were studied and the results analysed, model systems were investigated in d etail. The components used in the model binary blends were molecular w eight fractions of linear polyethylene, hydrogenated poly(butadiene) a s a model for the ethylene-1-alkene copolymers, and a three-arm star h ydrogenated poly(butadiene) as a model for the long chain branched pol yethylenes. It was found that a key factor in governing the extent of cocrystallization in these blends is the closeness of crystallization rates of each of the components. The extent of the cocrystallization t hus diminishes with increasing concentration of the linear component i n the blend. It is found that copolymer composition and molecular stru cture also have a strong influence on cocrystallization. The amount of cocrystallization is favoured at the lowest isothermal crystallizatio n temperatures and is maximized under quenching conditions. The genera l features that influence cocrystallization, which have evolved from t his study, are discussed. (C) 1998 Elsevier Science Ltd. All rights re served.