We apply picosecond acoustic techniques to determine the longitudinal wave speed of submicron polymer films in the through-thickness dimension. The time-of-flight of an elastic wave propagating through the polymer film is measured using an optical pump/probe technique. The longitudinal wave speed in poly(methyl methacrylate) films of thickness ranging from 20 to 130 nm has been determined. A pronounced increase in longitudinal wave speed is observed for films of thickness less than 40 nm.
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Lee et al. (1996) studied this question.
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