Microhardness studies of chain-extended PE: III. Correlation with yield stress and elastic modulus

The correlation between microhardness, H, yield stress, Y, and Young's modulus, E, has been explored on various chain-extended polyethylene (PE) samples and compared to chain-folded PE. Mechanical properties have been derived from tensile and compressive experiments. The tensile yield stress, Y-t, is shown to correlate with hardness following H similar to 3Y(t) while H similar to E-t/10 (E-t is Young's modulus derived from tensile experiments). On the other hand, the compressive yield stress, Y-c, is shown to correlate to H following the mechanical models of elastoplastic indentation. Hence, H/Y-c increases with decreasing elastic strain tending towards H similar to 3Y(c) for a fully plastic deformation. The atmospheric pressure compression-moulded samples show the lowest H/Y-c ratios as a consequence of the largest fraction of compliant phase. Moreover, the H/E-c ratio (E-c being the elastic modulus under compressive force) is shown to diminish with the decrease of Y-c/E-c. Chain extended PE provides the lowest values for the H/E-c ratio from all the PE samples investigated. (C) 2000 Elsevier Science Ltd. All rights reserved.