8Li Spin Relaxation as a Probe of the Modification of Molecular Dynamics by Inelastic Deformation of Glassy Polystyrene

Abstract

The molecular dynamics of inelastic strain in glasses remains poorly understood, in contrast to the plasticity of crystalline materials that is well-characterized by measurements of dislocation activity. We report initial results on a 300 nm thick atactic polystyrene film undergoing plastic strain in its glassy state. This physical modification was applied by nanoimprint stamping with a 1 mm ultra-smooth spherical die to induce a stress exceeding mechanical yield (0.8% residual strain). Using 8Li implanted-ion βNMR, we monitor the spin-lattice relaxation to infer depth-resolved rates of molecular dynamics. We find a significant change in the bulk molecular dynamics of the imprinted film (away from the surface) compared to an identically prepared control film. The relaxation is ∼ 20% slower in the film left densified by imprinting. We expect this relaxation to be coupled to the motion of the phenyl side rings; wherein slower dynamics due to densification is reasonable, as tighter packing should increase the energy barrier to molecular motion. In addition, we see an increase in the apparent thickness of a nanometric mobile surface layer, but this may be an artefact of surface roughening caused by imprinting.