Solid state structure and melting behavior of interdiffused polyethylenes in microlayers

Gradient structures, produced by interdiffusion in microlayers of a high de nsity polyethylene/linear low density polyethylene polymer pair that cocrys tallizes isomorphically, were studied experimentally. Microlayers were take n into the melt for a period of time, and the compositional gradient was fi xed by crystallization upon quenching. High specific interfacial area of mi crolayers offset the low diffusion mobility of polymeric chains so that the microlayer in the melt approached compositional homogeneity on a laborator y time scale. Taking advantage of the systematic change of the melting temp erature with the blend composition, the compositional gradient was visualiz ed by progressively melting the microlayer with increasing temperature. Thi s made it possible to monitor the kinetics of interdiffusion without using a chemical label. The compositional profiles were analyzed with a diffusion model formulated for a polydisperse system. Diffusion coefficients for lig htly branched and linear polyethylene chains, which correlated well with th e data of previous studies, were obtained. It was found that the interlayer boundaries remained stationary during a characteristic time of interdiffus ion of the component main fractions, and moved at long times as high molecu lar weight fractions became involved in interdiffusion. The moving boundary phenomenon was investigated with optical and atomic force microscopy and t he development of crystalline morphology in the microscopic compositional g radient was described. (C) 1999 Elsevier Science Ltd. All rights reserved.