Rubbing-induced molecular alignment and its relaxation in polystyrene thin films

Abstract

Rubbing-induced molecular alignment and its relaxation in polystyrene (PS) thin films are studied with optical birefringence. A novel relaxation of the alignment is observed that is distinctly different from the known relaxation processes of PS. First, it is not the Kohlrausch-Williams-Watts type but instead is characterized by two single exponentials plus a temperature-dependent constant. At temperatures several degrees or more below the glass-transition temperature (Tg), the relaxation time falls between that of the α and β relaxations. Second, the decay time constants are the same within 40% for PS with weight-average molecular weights (Mw's) of 13, 700-550, 000 Da at temperatures well below the sample Tg's, indicating that the molecular relaxations involved are mostly local within the entanglement distance. Nonetheless, the temperature at which the rubbing-induced molecular alignment disappears (To) exhibits a strong Mw dependence and closely approximates the Tg of the sample. Furthermore, To depends notably on the thickness of the polymer in much the same way as previously found for the Tg of supposed PS films. This suggests that the α process becomes dominant near Tg. Preliminary spectroscopic studies in the mid-infrared range show a significant degree of bending of the phenyl ring toward the sample surface, with the C-C bond connecting the phenyl ring and the main chain tends to lie along the robbing direction, which indicates that the relaxation is connected with the reorientation of this C-C bond. We exclude the observed relaxation, as predominantly a near-surface one, because detailed studies on the effects of rubbing conditions on the degree of molecular alignment indicate that the alignment is not local to the polymer-air surface. © 2001 John Wiley & Sons, Inc. J. Polym. Sci. Part B: Polym. Phys.