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

Abstract

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...