Entanglement density, macromolecular orientation, and their effect on elastic strain recovery of polyolefin films

Abstract

For amorphous polymers, restoring forces are generated by the progressive orientation of the macromolecular chains in the stretching direction leading to a decrease in the system entropy. Orienting the chains in the future stretching direction thus reduces the entropy variation induced by the stretching and limits the entropic restoring force magnitude. Entropic restoring forces created during stretching have been correlated to the number of junction points by previous studies. Reducing the entanglement density (i.e., the number of junction points) is supposed to limit the entropic restoring force magnitude. In this study, the influence of blend ratio of low molecular weight wax and orientation level on the mechanical properties of the thin films, especially the elastic recovery, were evaluated. Elastic energy strain recovery was calculated from hysteresis curve obtained during 60% loading (stretch) and unloading (recovery) cycle and compared to rheological and orientation measurement. It has been shown that a decrease in entanglement density can minimize elastic recovery, Nevertheless, a compromise must be found, in order to limit the permanent deformation caused by chain flow. Macromolecular orientation is also a way to adjust the film mechanical properties. A LDPE 3 × 3 biaxial orientation leads to a 25% reduction in transversal direction elastic recovery (compared to MDO cast film) without altering machine direction mechanical behavior. However, for ethylene vinyl acetate, the uniaxial macromolecular orientation seems to impact the film behavior in the transverse direction by causing a smaller inter-atom distance, favoring a higher bond strength. The latter acts as transient physical nodes, increasing entropic restoring forces.