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,T-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,T-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, xi(CRR), in bulk polymer systems at T-g reveal that PS MW has no significant effect on xi(CRR) unless PS is oligomeric or nearly oligomeric. However, changes to repeat unit structure and diluent addition affect xi(CRR) values, but not in a manner that yields an obvious correlation with the T-g-nanoconfinement effect.