Molecular Motion in Free-Standing Thin Films of Poly(methyl methacrylate), Poly(4-tert-butylstyrene), Poly(α-methylstyrene), and Poly(2-vinylpyridine)

• Published in: Macromolecules 2011-09, Vol.44 (17), p.7034-7042
• Main Authors: Paeng, Keewook, Ediger, M. D
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Structure, morphology and analysis
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma201266r

A photobleaching technique has been used to measure the reorientation of dilute fluorescent probes in free-standing polymer films from 7 nm to 20 μm thick. Similar to our previous report on free-standing polystyrene films, two subsets of probes with distinct dynamics were observed in free-standing poly(methyl methacrylate) films. The more mobile subset of probes is consistently interpreted in terms of a surface layer, and the thickness of the mobile surface layer is shown to increase with temperature. Poly(methyl methacrylate) has a mobile surface layer thickness of 4 nm at the bulk T g. Similar measurements on free-standing poly(2-vinylpyridine) and poly(4-tert-butylstyrene) films indicate mobile surface layer thicknesses from 7 to 10 nm at their bulk T g values, similar to the 7 nm value previously reported for polystyrene. In contrast to the other four polymers, free-standing poly(α-methylstyrene) films show no evidence of a mobile surface layer. We interpret these results in terms of contributions from a bulk correlation length and specific surface effects.