Impacts of polystyrene molecular weight and modification to the repeat unit structure on the glass transition-nanoconfinement effect and the cooperativity length scale

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.