Consequence of excess configurational entropy on fragility: the case of a polymer-oligomer blend

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.

Effect of a zirconium tungstate filler on the cure behavior of a cyanate ester resin

In this work, the effect of a negative thermal expansion zirconium tungstate (ZrW2O8) filler on the cure behavior of a unique low-viscosity bisphenol E cyanate ester (BECy) is characterized by differential scanning calorimetry. The cure rate of BECy is found to increase upon the incorporation of ZrW2O8 particles, and the catalytic effect increases with increasing filler loading. The catalytic effect is primarily attributed to the presence and nature of hydroxyl groups on the ZrW2O8 particle surface. In comparison with BECy/ZrW2O8 composites of the same loading, silane-coated ZrW2O8 particles resulted in a higher value of the glass transition temperature (T(g)) and a delay in the onset of the cure. The ability of a phenomenological reaction model to describe the cure behavior of the composites is also examined.