Novel aspects of the microstructure of poly(ethylene oxide) as revealed by microhardness: Influence of chain ends

The microhardness (H) of a series of melt crystallized samples of poly(ethylene oxide) (PEO) was investigated as a function of molecular weight and crystallization temperature (T-c). It is shown that for a given molecular weight, H and the crystal hardness (H-c) increase with T-c, following a thermodynamic approach which takes into account the dependence of H-c upon crystal thickness (l(c)). A combined analysis of H-data and DSC results reveals that, both the surface free energy (sigma(e)) and the hardness-derived parameter b of the samples are unaffected by T-c. The microhardness of the PEO samples was markedly affected by molecular weight. Furthermore, it is shown that the molecular weight variations produce changes in the surface free energy due to variations in the number of chain ends on the crystal surface, giving rise to a parallel variation in the b parameter. As a consequence, the melting temperature and the crystal hardness exhibit a similar dependence on crystal thickness.