Cold drawing of polymers:: Plasticity and aging

Physical aging and mechanical deformation on polyisobutylmethacrylate (PIBMA) and polymethylmethacrylate (PMMA) have been investigated below but close to the glass transition temperature using calorimetry and elastic neutron scattering. We have studied the effect of physical aging before, during and after the deformation on the mechanical, thermal responses combined to a structural analysis ranging from the radius of gyration to the monomeric scales. On one hand we report that, as expected, plastic strains rejuvenate glassy polymer aged before drawing, and also that no aging effect occurs during the plastic deformation. Moreover, the enthalpic relaxation functions as measured by calorimetry on aged samples, before and after stretching, remain quite similar. On the other hand, we find that stretching a glassy chain induces a strong anisotropy at large scale, around the radius of gyration. This anisotropy disappears gradually at smaller scale down to the monomer structure. However, while the anisotropy is lost at the local scale, the static structure factor is still altered by the plastic strain. Theses changes at the monomer scale may be responsible of the complete loss of thermal history operated before stretching. The cold-drawn sample posses a slightly modified structure leading to a distinct glassy state which enthalpic relaxation remains unaltered. So plastic deformations do not simply rejuvenate a glassy polymer, it induces a 'metamorphosis'. (c) 2006 Elsevier B.V. All rights reserved.