Suppression of the Fragility-Confinement Effect via Low Molecular Weight Cyclic or Ring Polymer Topology

We used differential scanning calorimetry and spectroscopic ellipsometry to measure the molecular weight (MW) dependence of bulk fragility (m_bulk) and spectroscopic ellipsometry to measure the thickness dependences of the glass transition temperature (T_g) and fragility (m) in supported thin films of low MW cyclic or ring polymer. The effects of confinement on T_g and m of thin polymer films are important in a range of advanced technology applications, including nanoimprinting. It has previously been shown that nanoconfined films of high MW linear polystyrene (PS) exhibit major T_g- and m-confinement effects whereas films of low MW cyclic PS (c-PS) show at most a very weak T_g-confinement effect. In the absence of chain ends, c-PS exhibits very weak T_g,bulk- and m_bulk-MW dependences compared to linear PS. Despite low MW c-PS having m_bulk values similar to that of high MW linear PS, we found that low MW c-PS films show a very weak m-confinement effect because of a weak free-surface effect; e.g., m for a 27 nm thick film of 3.4 kg/mol c-PS is the same as m_bulk within error. Overall, these results support a strong correlation between the susceptibility of fragility perturbation and the susceptibility of T_g perturbation caused by MW reduction, chain topology, and/or confinement.