We use calorimetry and rheology to investigate reports of extremely fragile polymers and the speculation that D,L-lactic acid should be extremely fragile. The dynamic fragilities of lactic acid, polysulfone, bisphenol-A polycarbonate, and poly(vinyl chloride) were studied. The polymers were used as received and after a wash-precipitation treatment. The current dynamic fragility findings are not in agreement with those reported by C. Evans, H. Deng, W. Jager, J. Torkelson, Macromolecules 2013, 46 (15), 6091-6103 of extremely high fragilities for the mentioned polymers. We also found no sample preparation history effect on the dynamic fragility values. The calorimetric and rheological results for the D,L-lactic acid show dynamic fragility values that are consistent with each other and are not extremely fragile. Calorimetric measurements that use a broad range of cooling rates gave smaller dynamic fragility values than those obtained from a limited range at higher cooling rates. The importance of the results is discussed.

We have used a liquid dewetting method to investigate the glass transition temperature T g of high molecular weight linear, long branched 3-arm star, and short branched 8-arm star polystyrene (PS) in the form of ultrathin films. The results of these dewetting experiments are consistent with prior studies of dewetting of linear PS films and show that, independent of molecular architecture, the glass transition temperature T g reductions with decreasing film thickness, while important below about 20 nm, are weaker than those observed for linear PS supported on a rigid substrate and as well as those observed in freely standing films. The lack of a strong molecular architecture effect on the T greductions is consistent with the T g reductions for the dewetting from a liquid substrate reflects changes in segmental dynamics upon confinement rather than chain effects. This contrasts with changes, including increases seen in dewetting from a rigid substrate, for different molecular architectures reported in the literature.