Mixed Equilibrium and Nonequilibrium Effects Influence Surface Mobility in Polymer Glasses

We used experimental techniques like angle-resolved X-ray photoelectron spectroscopy, sum-frequency generation vibrational spectroscopy, and contact angle measurements, along with molecular dynamics simulations, to study the glass transition temperature (T_g) of polymer glass near the free surface. Our experiments showed a linear T_g gradient with depth (z), while simulations revealed a double exponential z-dependence. Typically, T_g is determined in simulations by the system's relaxation time at equilibrium, whereas these experiments measure T_g by observing molecular mobility during heating from a sluggish, non-equilibrium glassy state. By reducing thermal annealing time in simulations, we found a decrease in relaxation time and a modified linear T_g gradient near the free surface. We also studied the T_g's dependence on heating/cooling rates for polymer films of varying thickness. Our experimental results showed significant variations in T_g with heating/cooling rates below the bulk T_g, which matched simulation results under non-equilibrium conditions. We attribute these findings to a reduction in mass density in the system's inner region during non-equilibrium cooling and recent research suggesting a decrease in local T_g when polymers are adjacent to softer materials.