In prior work we reported results from torsion in stress relaxation experiments in which we measured simultaneously, torque and normal force for two types of amorphous polymers. In one set the materials, poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA), have large sub-vitreous β relaxations, while in the second set, polycarbonate (PC) and polysulfone (PSF), have weak β relaxations. It was found that the prominent β process influences the magnitude of the normal force modulus of PMMA and PEMA while the shear modulus is primarily affected by the α process. In the present work we have extended our study on the effect of the β and α relaxations on the normal force and torque in amorphous polymers by examining the aging behavior of both torque and normal forces of PMMA, PEMA and PC measured at various temperatures between the β and α transitions and at a given deformation (γ=0.04). It is found that for the PMMA the normal force shows aging behavior that is different from the torque response for the PMMA but ages in the same fashion as the torque for both the PEMA and the PC. These results are only partially consistent with the hypothesis that the β process is the cause of the differing behaviors.
- Glassy polymer
- Nonlinear mechanical properties
- Nonlinear viscoelasticity
- Normal force
- Physical aging