Nanoscale mechanical and thermal properties of a multilayer polyethylene film with varying densities

Various atomic force microscopy methods for the imaging and measurement of the modulus of polyethylene (PE) layers in a five-layer multilayer film are described. The film includes linear low-density polyethylene (LLDPE) products with densities ranging from 0.940 to 0.912 gm/cm3. Conventional TappingMode AFM phase imaging can reveal the layers but is sensitive to measurement conditions. PeakForce Tapping AFM using quantitative nanomechanics (QNM) mode can resolve all five layers based on at least three different signals – stiffness mapping, penetration, and energy dissipation. In particular, the difference is statistically significant for the modulus and penetration data. This is remarkable considering that the method is sensitive to density differences of only 0.004 density units. Quasistatic AFM indentation was also done using the same probe and peak load. Here we see the effect of time dependence on the mechanical properties, where the slower interaction times yield estimated moduli much closer to the long-time moduli measured in bulk tensile experiments, compared to the value obtained at much higher frequency using PeakForce Tapping AFM. Nano-thermal analysis of the same five-layer film using heated tip technology can also discriminate the layers statistically, based on softening temperature, which shows excellent correlation with bulk properties.