Visco-Elastic Property Reconstitution in Chain-Like Polymers

We investigate the elastic deformation and structural reorganization process in a chain-like polymer, e.g., polyvinylidene fluoride (PVDF) during straining and stress relaxation. Using a unique experimental setup in which a miniature tensile tester is placed into the optical path of a Brillouin light scattering (BLS) system, we simultaneously measure the adiabatic and isothermal elastic moduli of the polymer as a function of the strain history. Upon straining, elastic moduli of the polymer drop instantaneously, while during stress relaxation at constant strain both the static and adiabatic moduli reconstitute, tending towards the unstrained values. While such behavior is expected for the former, it is not for the latter. Although a detailed analysis allows us to identify a spectrum of relaxation rates, general visco-elastic theory (e.g., Maxwell-Wiechert model) is not adequate to describe the observed behavior, not even that of the static moduli. An improved description of material’s visco-elastic response requires taking the structural rearrangements during straining and relaxation into account, and that the relaxation mechanisms, as well as the inherent elastic properties of the strained and unstrained polymer are indeed different.