Examination of the influence of cooperative segmental dynamics on the glass transition and coefficient of thermal expansion in thin films probed using poly(n-alkyl methacrylate)s

The thermal behavior of thin films of a homologous series of poly(n-alkyl methacrylate)s supported on silicon substrates was probed using spectroscopic ellipsometry. Deviations from bulk behavior for the glass transition temperature (T_g) and coefficient of thermal expansion (CTE) were observed for films thinner than approximately 60 nm, consistent with most observations for confinement effects in polymer films. However, the extent of the decrease of CTE and the deviation in T_g are significantly influenced by the alkyl chain length. As the alkyl chain length is increased from methyl to n-octyl, the deviation from bulk behavior is significantly suppressed. This behavior is similar to that observed by Torkelson and coworkers (Physical Review Letters 2004; 92: 095702) for polystyrene films as small molecule diluents were added; this behavior was attributed to the decrease in size of the cooperative segmental dynamics, ξ_i(CRR), with addition of solvent. ξ_i(CRR) decreases as the alkyl chain length is increased; this is consistent with the hypothesis for the relationship between ξ_i(CRR) and confinement effects in thin polymer films. However, a two order of magnitude difference between the thickness of nanoconfinement onset and ξ_i(CRR) for poly(n-octyl methacrylate) results in uncertainty for ξ_i(CRR) as the origin of the nanoconfinement effect.