Rubbing-induced molecular alignment and its relaxation in polystyrene (PS)
thin films are studied with optical birefringence. A novel relaxation of th
e alignment is observed that is distinctly different from the known relaxat
ion processes of PS. First, it is not the Kohlrausch-Williams-Watts type bu
t instead is characterized by two single exponentials plus a temperature-de
pendent constant. At temperatures several degrees or more below the glass-t
ransition temperature (T-g), the relaxation time falls between that of the
a and beta relaxations. Second, the decay time constants are the same withi
n 40% for PS with weight-average molecular weights (M-w's) of 13,700-550,00
0 Da at temperatures well below the sample T-g 's, indicating that the mole
cular relaxations involved are mostly local within the entanglement distanc
e. Nonetheless, the temperature at which the rubbing-induced molecular alig
nment disappears (TO) exhibits a strong M-w dependence and closely approxim
ates the T-g of the sample. Furthermore, To depends notably on the thicknes
s of the polymer in much the same way as previously found for the T-g of su
pported PS films. This suggests that the a process becomes dominant near T-
g. Preliminary spectroscopic studies in the mid-infrared range show a signi
ficant 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 al
ong the rubbing direction, which indicates that the relaxation is connected
with the reorientation of this C-C bond. We exclude the observed relaxatio
n, as predominantly a near-surface one, because detailed studies on the eff
ects of rubbing conditions on the degree of molecular alignment indicate th
at the alignment is not local to the polymer-air surface. (C) 2001 John Wil
ey & Sons, Inc.