15th International Conference on Muon Spin Rotation, Relaxation and Resonance

Glasses occupy more volume than required for molecular close packing. The distribution of this ``free volume'' is related to other key properties such as dynamic heterogeneity (stretched exponential relaxation). As a glass ages, it equilibrates by thermally activated structural relaxation producing permanent densification with slowed relaxation times. Mechanical deformation can significantly alter glassy structure and relaxation, leading to apparent over-aging or rejuvenation via irreversible plastic shear flow that explores microscopic configurations that are otherwise inaccessible$^1$.

Nanoimprint$^2$ is a technique that deforms thin polymer films by indentation of a patterned die for lithographic patterning and measuring mechanical properties. Few techniques are capable of studying local properties of polymer films, however the spin-lattice relaxation of implanted $^8$Li$^+$ is sensitive to the molecular dynamics in the glassy state, including modification by processing parameters$^3$

We report initial results on a 300 nm thick atactic polystyrene film plastically modified by nanoimprint stamping$^4$ using a 1 mm ultra-smooth spherical die. While the \elip\ beam can easily be stopped in the film, the beamspot is $\sim 2$ mm in diameter, so a large array of imprints was produced over an area $\sim 3$ mm$^2$, leaving an inelastic strain of a few tenths of a percent over an areal fraction $\sim 20$%.

To ensure the beam overlapped the imprinted area, a new method was developed. Using scintillation from an Al$_2$O$_3$ crystal, the beamspot image was fit with a Gaussian profile. Partially automation allowed the overlap to be maximized in real time. We find a small but significant change in the bulk of the film (away from the surface), compared to an unimprinted control, the relaxation is slower and more inhomogeneous (lower stretching exponent).

$^1$ McKenna, JPCM15, S737 (2003)
$^2$ Traub, Ann.Rev.Chem.Bio.Eng. 7, 583 (2016).
$^3$ McKenzie, SoftMatter14, 7324 (2018).
$^4$ Cross, Rev.Sci.Inst.79, 013904 (2008).