The effect of nanoconfinement on the glass transition temperature (T g) of supported polystyrene (PS) films is investigated over a broad molecular weight (MW) range of 5000-3 000 000 g/mol. Polystyrene MW is shown to have no significant impact on the film thickness dependence of T g - T g,bulk. In contrast, a small modification to the repeat unit structure of PS has a dramatic impact on the T g-nanoconfinement effect. The strength of the thickness dependence of T g is greater for poly(4-methylstyrene) (P4MS) than for PS and yet much greater for poly(4-tert-butylstyrene) (PTBS). The T g reduction for PTBS is 47 K below T g,bulk for a 25 nm thick film, with the onset thickness for confinement effects in PTBS being 300-400 nm. Measurements of the size of cooperatively rearranging regions, ξ CRR, in bulk polymer systems at T g reveal that PS MW has no significant effect on ξ CRR unless PS is oligomeric or nearly oligomeric. However, changes to repeat unit structure and diluent addition affect ξ CRR values, but not in a manner that yields an obvious correlation with the T g-nanoconfinement effect.
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