Temporal relaxation of rubbed polystyrene ͑PS͒ thin films ͑50 nm thick͒ was studied by probing the decay of the optical anisotropy in the sample. Our results show that the relaxation process is characterized by two single-exponential decays plus a temperature-dependent constant. Both relaxations are orders of magnitude faster than the main chain ͑␣͒ relaxation at temperatures well below (Ͼ10°C)T g , with activation energies 3.0 kcal/mol and 5.1 kcal/mol, respectively, either of which are much smaller than that of the ␣ relaxation ͑ϳ50 kcal/mol͒. The decay time constants are found to be independent of the sample molecular weights M w ͑ϭ13.7 K to 550 K Daltons, M w /M n р1.1͒ at these temperatures. This shows the local nature of the relaxation modes of rubbed PS. DOI: 10.1103/PhysRevE.63.061603 PACS number͑s͒: 68.15.ϩe, 61.20.Lc, 68.60.Ϫp Rubbing with piled fabrics such as velour and velvet creates surface scratches ͓1͔ and molecule alignments ͓2,3͔ in polymer thin films. After rubbing, the molecular alignment decreases at elevated temperatures as the polymer relaxes. The understanding of such relaxation dynamics and mechanism is of practical importance. For example, rubbed polyimide films are widely used in the liquid-crystal display industry. Reduction in molecular alignments of rubbed polymers due to relaxations may lead to failure of the device since its operation relies heavily on the integrity of the molecular alignment, yet little is known about the relaxation dynamics of rubbed polymer films. In addition to the recent findings on the different dynamical behaviors of polymer thin films compared to the bulk ͓4-8͔, the subject matter is very interesting fundamentally as well. Liu et al. used nearedge x-ray absorption fine structure ͑NEXAFS͒ to study the absorption anisotropy of rubbed thin polystyrene ͑PS͒ films on Si covered with native oxide, and found that after annealing at an elevated temperature, some amount of anisotropy disappeared in the initial 30 min while the rest remained unaffected with subsequent annealing at the same temperature ͓3͔. However, when the glass transition temperature T g of the polymer was reached, all of the remaining anisotropy disappeared over a short time. Recently Schwab et al. ͓8͔ studied the relaxation of rubbed PS films on glass substrates by measuring the optical anisotropy of the rubbed films as the sample was heated at a constant rate of 1°C/min. The authors suggested that the relaxation of the polymer ͑i.e., disappearance of the anisotropy͒ followed the structural relaxation dynamics of polymer backbones related to the glass transition, and could be described by the KohlrauschWilliams-Watts ͑KWW͒ stretched exponential function ͓7,9͔:where ␤ is about 0.38 for PS. The authors further modeled by a simple Arrhenius relationship below the T g ͓8,10͔:Here, ⌬E is the activation energy and R the gas constant. Using the parameters found in Ref.͓8͔ based on this model ͑i.e., (T g )ϭ5 s, ⌬Eϭ205 kJ/mol, Rϭ8.315 J/mol, T g ϭ97°C͒, the decay time constant at 20°C below T g is ϳ2...

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 (T g ), the relaxation time falls between that of the ␣ and ␤ relaxations. Second, the decay time constants are the same within 40% for PS with weight-average molecular weights (M w 's) of 13,700 -550,000 Da at temperatures well below the sample T g '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 (T 0 ) exhibits a strong M w dependence and closely approximates the T g of the sample. Furthermore, T 0 depends notably on the thickness of the polymer in much the same way as previously found for the T g of supported PS films. This suggests that the ␣ process becomes dominant near T g . Preliminary spectroscopic studies in the mid-infrared range show a significant degree of bending of the phenyl ring toward the sample surface, with the COC bond connecting the phenyl ring and the main chain tends to lie along the rubbing direction, which indicates that the relaxation is connected with the reorientation of this COC 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.

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