T. Schuman et al., INTERDIFFUSION OF LINEAR AND BRANCHED POLYETHYLENE IN MICROLAYERS STUDIED VIA MELTING BEHAVIOR, Macromolecules, 31(14), 1998, pp. 4551-4561
Kinetics of interdiffusion of a miscible polymer pair, high density po
lyethylene (HDPE) and linear low density polyethylene (LLDPE), was stu
died experimentally in order to characterize the conditions required t
o construct gradient morphologies from microlayers. Microlayers were t
aken into the melt for a period of time, and the compositional gradien
t was fixed by crystallization upon quenching. High specific interfaci
al area of microlayers offset the low diffusion mobility of polymeric
chains so that the microlayer system in the melt approached compositio
nal homogeneity on a laboratory time scale. This specific pair of poly
mers 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 fr
actions and allowed us to extract the diffusion coefficients for eleme
ntary chains in the developing melt blend from the net kinetics of int
erdiffusion. 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 we
re found to correlate well with data of previous studies on monodisper
se polyethylene species. An effect of the heterogeneous chain microstr
ucture of LLDPE, in contrast to the homogeneous branch distribution of
its monodisperse analogues, was revealed.