Confinement effects on glass transition temperature, transition breadth, and linear expansivity: An ultraslow X-ray reflectivity study on supported ultrathin polystyrene films

摘要

X-ray reflectivity measurements of the glass transition in thin polystyrene films supported on Si substrates were performed at slow cooling rates ranging from 0.62 to 0.01 ?C/min. At a cooling rate of 0.14 ?C/min, a depression in the glass transition temperature T_g was clearly observed with decreasing thickness. However, at a cooling rate of 0.62 ?C/min, only a slight decrease in T_g for a 12-nm-thick film was observed, while at an ultraslow cooling rate of 0.01 ?C/min, a significant reduction in the T_g of ultrathin films (12 and 6 nm) was observed. As the thickness decreased, a broadening in the width of the glass transition, w, was found at higher cooling rates (0.62 ?C/min and 0.14 ?C/min), while narrowing of w was observed at ultraslow cooling rates of 0.01 ?C/min and 0.04 ?C/min. A narrow distribution of relaxation time in the ultrathin films indicates that most segments are able to relax under the ultraslow cooling process, thus showing an inherent reduction in the T_g of the confined thin polymer films.