Novel aspects of microindentation hardness in very low crystallinity ethylene-1-octene copolymers: A model for deformation

The microindentation hardness, H (critical stress required to mechanically deform a material), of a series of homogeneous ethylene-1-octene copolymers with high comonomer content and low crystallinities (α<0.4) has been determined. The H values obtained for the series of ethylene–octene copolymers are found to be notably smaller than those of linear and commercial short-chain branched polyethylene. The microhardness of an ethylene based material having a nearly zero crystallinity value has been measured for the first time. Results are discussed using a description of hardness in the light of the crystal characteristics (surface free energy and dimensions) and the energy required for plastic deformation. In high crystallinity materials, lamellar shearing and crystal fracture mechanisms prevail. In contrast, resistance to deformation for low crystallinity ethylene-1-octene copolymers (α=0.074–0.22), showing well-developed indentations, involves the following deformation modes: (i) bond rotation of the molecules within the amorphous phase, (ii) elastic compression and bending of the nanocrystals and (iii) slippage of the dispersed nanocrystals through the amorphous matrix. The extent of the latter deformation mechanism should be modulated by the viscosity of the amorphous phase. According to these deformation mechanisms, the average dimensions of the nanocrystals after deformation remain practically unaffected. Within this context, it is shown that the dissipated energy during deformation, as derived from the mechanical parameter, b, decreases with increasing short-chain branching content.