Chemical structure origins of the normal force response in polymeric glasses
Simultaneous normal force and torque measurements were made as a function of deformation and temperature in a series of polymeric glasses with different chemical structures. A large normal force with a fast relaxation rate was found in two n-alkyl methacrylates, PMMA and PEMA. Examination of the change in volume during torsion of PMMA samples cannot explain the large normal force response observed. When the measurements are expanded to polycarbonate and polysulfone, no difference is found between torque and normal force behaviors. The results show that the normal force response in PMMA and PEMA is influenced by their large sub-vitreous p relaxation which has been attributed to side chain motions. In this work the data are analyzed in the context of a time dependent modification of Rivlin's finite elasticity model which allows us to extract the first and second derivatives of the strain energy density function, W1 and W2. Very interestingly, polycarbonate and polysulfone exhibit a behavior closed to neo- Hookean (W2 = 0) while for PMMA and PEMA W2 decreases with time. The microstructural origins of these behaviors are thoroughly examined.