Rubbing-induced molecular alignment and its relaxation in polystyrene thinfilms

Citation
Oc. Tsang et al., Rubbing-induced molecular alignment and its relaxation in polystyrene thinfilms, J POL SC PP, 39(22), 2001, pp. 2906-2914
Citations number
26
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Organic Chemistry/Polymer Science
Journal title
JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS
ISSN journal
0887-6266 → ACNP
Volume
39
Issue
22
Year of publication
2001
Pages
2906 - 2914
Database
ISI
SICI code
0887-6266(20011115)39:22<2906:RMAAIR>2.0.ZU;2-W
Abstract
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.