Polymorphism of isotactic polybutene-1 as revealed by microindentation hardness. Part II: correlations to microstructure

The influence of polymorphism on the micromechanical properties of isotactic polybutene-1 (iPBu-1) has been investigated by means of the microhardness technique. Hardness data, H, of form I (hexagonal) are shown to be notably larger than those of form II (tetragonal). The H values of both polymorphic forms are shown to depend on the molecular weight, Mw, and the crystallization temperature, Tc. The hardness behaviour with Mw and Tc has been correlated to the changes of degree of crystallinity and nanostructure as derived from small angle X-ray scattering. The hardness values for iPBu-1 infinitely thick crystals (Hc∞), of forms I and II, have been calculated for the first time. Hc∞ of form I is shown to be notably larger than that of form II. This result is a consequence of the denser packing of the hexagonal crystal modification, and accounts for the large difference in H values found for forms I and II. Finally, the variation of the mechanical b-parameter, proportional to the surface-free energy of the crystals, with molecular weight is discussed.