Composition-dependent properties of polyethylene Kaolin composites. III. Thermoelastic behavior of injection molded samples

Injection-molded samples of high-density polyethylene (HDPE) differing in t he orientation pattern (with respect to the melt flow direction) and in fil ler content (untreated and surface-treated kaolin, respectively) were chara cterized by wide-angle X-ray scattering, microhardness, and stretching calo rimetry techniques. The crystallinity of the polymer matrix in filled sampl es shows the same value as that found for the neat polymer regardless, fill er content, and/or filler surface treatment. The thermoelastic behavior of all samples in the strain interval below the apparent yield point epsilon* is quantitatively discussed in terms of classical equations for elastic sol ids. Analysis of thermoelastic parameters of the boundary interphase (BI) r eveal an unusually stiff, highly oriented structure of the matrix polymer w ithin BI. Discrepancies of experimental values for the internal energy incr ement in the inelastic strain interval above epsilon* between unfilled and filled samples is explained in terms of the filler debonding process. The l atter process is discussed in light of the formation of a polymer-free fill er surface and of the concomitant inelastic deformation (plastic flow) of a polymer matrix in the interstitial space between filler particles. (C) 199 9 John Wiley & Sons, Inc.