Macromolecules, Vol.50, No.3, 1147-1154, 2017
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;bulk and mbulk MW dependences compared to linear PS. Despite low MW c-PS having m(buik) 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-buik 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.