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Unusual Expansion and Contraction in Ultrathin Glassy Polycarbonate Films



Christopher L. Soles, Jack F. Douglas, Ronald L. Jones, Wen-Li Wu


The thermal expansion behavior and glass transitions of poly(styrene) (PS) and poly(methyl methacrylate) (PMMA) are extensively studied in the thin film geometry. Both PS and PMMA are similar in that they possess very low entanglement densities. In this work the thermal expansion behavior of poly(carbonate) (PC) is studied as a function of film thickness using specular X-ray reflectivity. Unlike PS and PMMA, PC exhibits one of the largest polymer entanglement densities. It is found that PC, like PS, forms more favorable interactions with the hydrophobic hydrogen passivated surface of silicon as opposed to the hydrophilic native oxide (which PMMA favors). The apparent glass transition temperature of PC does not appear to be altered by confinement on the hydrogen passivated substrate while depressions as great as 30 C to 40 C are encountered in films thinner than 100 on the native oxide substrate. A critical length scale of 100 for confinement-induced deviations is approximately congruous with both the root mean square end-to-end length of a single PC chain and the average segmental length between entanglements. For the sub-100 thick PC films, a thermal history dependent negative coefficient of thermal expansion is sometimes observed upon cooling below 90 C. While the molecular origins of this thermal expansion upon cooling are not understood, conditions which favor such behavior include less favorable interactions with the substrate and a lower molecular weight of the PC. It is surmised that the negative coefficient of thermal expansion is indicative of the polymers 'reluctance' to be confined by the thin film geometry.
No. 8


glass transition, negative coefficient of thermal expansio, polycarbonate, polymer thin film, x-ray reflectivity


Soles, C. , Douglas, J. , Jones, R. and Wu, W. (2004), Unusual Expansion and Contraction in Ultrathin Glassy Polycarbonate Films, Macromolecules, [online], (Accessed April 18, 2024)
Created April 1, 2004, Updated February 19, 2017