Amorphous Phase Modulus and Micro-Macro Scale Relationship in Polyethylene via in Situ SAXS and WAXS

The small strain mechanical behavior of bulk polyethylene was investigated at the local scale of the lamella stackings by means of combined in situ SAXS and WAXS at different testing temperatures. Three different thermal treatments on four materials afforded studying a wide range of crystallinities (X-c) and microstructures. The local strain in tensile direction of the amorphous phase in equatorial region of the spherulites was determined via SAXS. The amorphous to macroscopic strain ratio proved to be fairly constant in the preyield strain domain for every materials. This ratio also proved to be strongly dependent on X-c. The local tensile stress on the amorphous phase in equatorial region was assessed from the strain on the crystals as measured by WAXS, using theoretical values of the elastic constants. The apparent tensile modulus of the amorphous phase, Ma, was shown to reach a maximum value of 300 MPa at RT for X-c = 50% and exhibited a monotonic drop with increasing both X-c and temperature. Evidence was given of the major role of the density of molecular stress transmitters on the amorphous phase stiffness over that of structural confinement. Comparison between Ma and macroscopic modulus revealed a significant modification of the mechanical coupling of the crystalline lamellae in relation to X-c that was assigned to an increasing lamella percolation throughout the spherulites with increasing X-c.