Consequence of Excess Configurational Entropy on Fragility: The Case of a Polymer-Oligomer Blend

Abstract

By taking advantage of the molecular weight dependence of the glass transition of polymers and their ability to form perfectly miscible blends, we propose a way to modify the fragility of a system, from fragile to strong, keeping the same glass properties, i.e., vibrational density of states, mean-square displacement, and local structure. Both slow and fast dynamics are investigated by calorimetry and neutron scattering in an athermal polystyrene-oligomer blend, and compared to those of a pure 17-mer polystyrene considered to be a reference, of the same Tg. Whereas the blend and the pure 17-mer have the same heat capacity in the glass and in the liquid, their fragilities differ strongly. Thus, the difference in fragility is related to an extra configurational entropy created by the mixing process and acting at a scale much larger than the interchain distance, without affecting the fast dynamics and the structure of the glass.

Enhanced Tg-Confinement Effect in Cross-Linked Polystyrene Compared to Its Linear Precursor: Roles of Fragility and Chain Architecture

“…According to Table , bulk fragility values obtained via both DSC and ellipsometry agree well with each other, consistent with the observation for other polymer systems by Lan and Torkelson . For neat PS (high MW), we obtained m = 160 ± 15 by DSC and m = 161 ± 6 by ellipsometry (reported errors are standard deviations from three measurements), consistent with values reported previously. ,,− …”

Dramatic Tunability of the Glass Transition Temperature and Fragility of Low Molecular Weight Polystyrene by Initiator Fragments Located at Chain Ends

“…We refer to values calculated by the Moynihan area matching method in the following text.) We note that we obtained m = 165 ± 12 by DSC for a high MW PS standard with M n = 107 kg/mol, consistent with other studies reporting fragility by DSC. ,,,− We report an m value of 90 ± 6 for the PS/Anionic/3.6k sample, significantly lower than that for high MW PS. The value is within error, in reasonable agreement with those reported by Santangelo et al ( m = 97 for M w = 2.4 kg/mol by dynamic mechanical analysis) and by Sánchez et al ( m = 105 for M w = 4 kg/mol by DSC) for low MW PS standards.…”

Extremely fragile glass-formers? Calorimetric and rheological determinations

“…However, for example with their PSF, which shows a fragility change from m = 124 for the as‐received material to m = 286 for the wash‐precipitated material, the glass transition temperature itself changed by only 1 °C, which is not consistent with the amounts of plasticizer that need to be present in the polymer to create a more than doubling of the fragility. For example, Dalle‐Ferrer et al mixed a narrow PDI polystyrene of 92 kg/mole with a 0.735 kg/mole narrow PDI material in a ratio of 38% oligomer to 62% polymer. The fragility of the pure polymer was m = 180 while that of the mixture was m = 85.…”