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Major Impact of Cyclic Chain Topology on the Tg-Confinement Effect of Supported Thin Films of Polystyrene

High purity cyclic PS (c-PS) samples with number-average molecular weight (MW) of 3.4 and 9.1 kg/ mol were synthesized via atom transfer radical polymerization and "click" chemistry with narrow MW distribution. Bulk glass transition temperature (Tg) measured by differential scanning calorimetry exhibited a much weaker MW dependence for c-PS relative to its linear precursor and anionically polymerized linear PS (A-PS). Using ellipsometry and fluorescence spectroscopy, major differences were observed in the Tg confinement effect in c-PS films supported on silicon substrates compared to A-PS. Whereas a large Tg reduction with confinement is commonly observed for A-PS supported on silica, within error, no confinement effect is seen in c-PS/3.4k films on Si/SiOx substrates down to 21 nm thickness. Although the c-PS linking group contains nitrogen and oxygen atoms potentially able to undergo hydrogen bonding, Tg is invariant with confinement for c-PS/3.4k or slightly reduced for c-PS/9.1k regardless of the level of substrate-surface hydroxyl groups. Ellipsometry indicates that the near elimination of the Tg confinement effect in c-PS originates mainly from a very weak perturbation to Tg near the free surface (in comparison to linear PS) rather than a strong perturbation at the polymer-substrate interface. We hypothesize that unlike linear polymers, the packing efficiency of cyclic PS segments, i.e., cyclic PS fragility, is not significantly perturbed by the free surface, which in turn results in at most a very weak Tg perturbation at the free surface and an invariance of average Tg across the film with confinement.

L. Zhang, R. Elupula, S.M. Grayson, J.M. Torkelson, "Major Impact of Cyclic Chain Topology on the Tg-Confinement Effect of Supported Thin Films of Polystyrene," Macromolecules, 49, 257-268 (2016). [Link]