title: Glass transition in thin supported polystyrene films probed by temperature-modulated ellipsometry in vacuum.
abstract: Glass transition in thin (1-200 nm thick) spin-cast polystyrene films on silicon surfaces is probed by ellipsometry in a controlled vacuum environment. A temperature-modulated modification of the method is used alongside a traditional linear temperature scan. A clear glass transition is detected in films with thicknesses as low as 1-2 nm. The glass transition temperature (T(g)) shows no substantial dependence on thickness for coatings greater than 20 nm. Thinner films demonstrate moderate T(g) depression achieving 18 K for thicknesses 4-7 nm. Less than 4 nm thick samples are excluded from the T(g) comparison due to significant thickness nonuniformity (surface roughness). The transition in 10-20 nm thick films demonstrates excessive broadening. For some samples, the broadened transition is clearly resolved into two separate transitions. The thickness dependence of the glass transition can be well described by a simple 2-layer model. It is also shown that T(g) depression in 5 nm thick films is not sensitive to a wide range of experimental factors including molecular weight characteristics of the polymer, specifications of solvent used for spin casting, substrate composition, and pretreatment of the substrate surface.
authors.affiliation: Department of Chemical and Biological Engineering and Center for Nanotechnology, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA. efremov@wisc.edu
authors.forename: Mikhail Yu
authors.initials: MY
authors.lastname: Efremov
authors.forename: Anna V
authors.initials: AV
authors.lastname: Kiyanova
authors.forename: Julie
authors.initials: J
authors.lastname: Last
authors.forename: Shauheen S
authors.initials: SS
authors.lastname: Soofi
authors.forename: Christopher
authors.initials: C
authors.lastname: Thode
authors.forename: Paul F
authors.initials: PF
authors.lastname: Nealey
mesh_terms: D005898:Glass / Q000737:chemistry*; D009584:Nitrogen / Q000737:chemistry; D010084:Oxidation-Reduction; D044367:Phase Transition; D011137:Polystyrenes / Q000737:chemistry*; D013499:Surface Properties; D013696:Temperature; D014618:Vacuum
chemical_list: D011137:Polystyrenes; D009584:Nitrogen