Correlated Molecular Motion during Release of Residual Stress in Polymer Glassy Films

Correlated molecular motion during the process of residual stress release in polymer glassy films is studied at the single-molecule level. Using poly(n-butyl methacrylate) (PnBMA) and poly(vinyl acetate) (PVAc) as the model polymers, thin films fabricated by spin-casting without thermal annealing were chosen as samples for investigation. Single-molecule fluorescence defocused microscopy was used to track the rotational motion of the fluorescent probes doped inside the polymer films. Under the activation effect of residual stress at experimental temperatures, the rotational motions of individual probes are discovered to be correlated a few degrees below the glass transition temperature (T_g), by analyzing the cross-correlation function of the rotational trajectories of different probes. Detailed investigations into the dependence on residual stress strength, intermolecular distance, probe-polymer interaction, and molecular orientation have been conducted. The results have revealed that the physical mechanism of the motion correlation is the randomization process from the state with preferred molecular orientation and presumably the polymer chain stretching.