Abstract
Tensile ductility of uniaxially thermoformed sheets of polymethyl methacrylate (PMMA) was analyzed in their longitudinal and transverse directions in connection with their residual birefringence and deformation recovery at reheating. The intensity of the residual birefringence increased almost linearly with the increase of extension ratio. The deformation recovery at the reheating of the thermoformed sheets indicated that molecular rearrangement induced by the uniaxial stretching in the rubbery state was accompanied by little storage of internal strain energy and thus of highly entropic elongation of polymer chains. As expected from these results, specimens cut from the thermoformed sheet in its transverse direction were less ductile than those from an unstretched sheet. Yet, specimens cut in the longitudinal direction were much more ductile in comparison with the unstretched sheet despite the development of tensile stress during the rapid cooling process in the thermoforming. Data of mechanical work consumed in the longitudinal and transverse specimens during their tensile test enabled us to propose a recommendable range of residual strain remaining in the thermoformed PMMA sheet with respect to their tensile ductility.
