In situ analysis of elastic properties and structural relaxation of two polymeric materials*

We report on the elastic property reconstitution in two types of polymer materials subject to strain: a two-component epoxy resin DGEBA/DETA, which has a highly cross-linked network, and polyvinylidene fluoride (PVDF), a chain-like polymer. Our setup for materials characterization combines a miniature tensile tester placed into the optical path of a Brillouin light scattering (BLS) system, to probe the elastic properties of a sample while it is uniaxially strained. BLS yields the adiabatic moduli of the materials, while their isothermal moduli are derived from the measured stress-strain curves.
Upon straining each of these materials, their moduli drop. When maintained at constant strain, the materials undergo gradual stress relaxation during which the load necessary to maintain this elongation decreases. During stress relaxation, both the adiabatic and isothermal moduli reconstitute to the values characterizing the unstrained and stress-free materials, albeit at significantly different rates. With our analysis we attempt to correlate the relaxational modulus with the entropic aspects of the structural changes associated with elastic deformations of these materials.