Interdiffusion of Linear and Branched Polyethylene in Microlayers Studied via Melting Behavior

Kinetics of interdiffusion of a miscible polymer pair, high density polyethylene (HDPE) and linear low density polyethylene (LLDPE), was studied experimentally in order to characterize the conditions required to construct gradient morphologies from microlayers. Microlayers were taken into the melt for a period of time, and the compositional gradient was fixed by crystallization upon quenching. High specific interfacial area of microlayers offset the low diffusion mobility of polymeric chains so that the microlayer system in the melt approached compositional homogeneity on a laboratory time scale. This specific pair of polymers with broad-molecular-weight distribution formed isomorphic blends upon crystallization from the melt. Thermal analysis of the quenched microlayered pair indicated systematic changes in the melting behavior with the composition gradient, which made it possible to quantify the progress of interdiffusion without chemical labeling. The thermograms were analyzed by applying a diffusion model formulated especially for a polydisperse system. The analysis revealed the role of different fractions and allowed us to extract the diffusion coefficients for elementary chains in the developing melt blend from the net kinetics of interdiffusion. It was confirmed that the molecular-weight dependence of the polyethylene chain diffusion coefficient follows reptation theory. The magnitudes of the diffusion coefficients and activation energy were found to correlate well with data of previous studies on monodisperse polyethylene species. An effect of the heterogeneous chain microstructure of LLDPE, in contrast to the homogeneous branch distribution of its monodisperse analogues, was revealed.