The intrinsic isotherm, asymmetry of approach, and memory effect experiments are performed for a high fictive temperature polystyrene glass in enthalpy space using nanocalorimetry. Using aging at times as short as 0.01 s and relatively high aging temperatures allows the complete evolution of all three of the signatures of structural recovery to be obtained for the first time in enthalpy space. The results from down jump experiments are compared to those obtained at lower temperatures using conventional differential scanning calorimetry (DSC) with respect to the time required to reach equilibrium, the apparent activation energy, and the Tool−Narayanaswamy−Moynihan (TNM) model parameters, and these results are independent of the measurement method being used; on the other hand, the physical aging rate is higher for the high fictive temperature glass.

Trimerization of a mixture containing a mono-and difunctional cyanate ester is investigated under the nanoporous confinement of silanized hydrophobic controlled pore glass using differential scanning calorimetry. The trimerization reaction of the nanoconfined monomer mixture is accelerated relative to the bulk by as much as 12 times in 8 nm pores, but this acceleration is less than half that observed for nanoconfinement of the individual monomers. The absolute reaction rate of the monomer mixture lies between those of the individual species, being slower than the monocyanate ester and faster than the dicyanate ester. The results are consistent with the hypothesis that the reaction acceleration is due to monomer ordering or layering at the pore surface, leading to a local concentration of reactive groups higher than in the bulk. In addition to the influence of nanoconfinement on trimerization kinetics, the molecular weight and glass transition temperature (T g ) of the polycyanurate formed in the nanopores are investigated. The molecular weight decreases approximately 20% for synthesis in the smallest 8 nm pores relative to the bulk value of 5200 g/mol. Upon extraction from the pores, the polymer T g is 5−9 K higher than in the bulk. However, in the 8 nm diameter pores, a T g depression of 44 K is observed relative to the value of the material after extraction from the pores. This depression lies between the values previously observed for the products of the individual cyanate esters which formed a low molecular weight trimer and a cross-linked polymer network. A secondary T g , associated with a less mobile layer at the pore wall, is 26−40 K above the primary value. The implication is that the origin of confinement effects on reactivity and T g differ, with changes in reactivity in this system arising from surface layering or ordering and T g depressions arising from intrinsic size effects. ■ INTRODUCTIONNanoconfinement is known to influence material properties, including changes in reactivity. Previous work in our laboratory has focused on changes in reactivity in nanopores for the free radical polymerization of methyl methacrylate and the trimerization reaction of cyanate esters. In the case of methyl methacrylate, the higher reactivity under nanoconfinement was explained primarily by a decrease in diffusivity of nanoconfined chains which results in a decrease in the rate of termination and earlier autoacceleration. 1,2 On the other hand, the origin of the acceleration in the trimerization reaction is not as clear. Acceleration of the reaction rate is observed for both a monofunctional monomer that forms a low molecular weight trimer and for a difunctional monomer that forms a crosslinked polycyanurate network; the reaction acceleration is similar for the two monomers although it does depend slightly on reaction conditions. 3−7 The acceleration may be attributable to layering of functional groups near the pore wall, as suggested by simulations of Malvaldi et al. 8 where such ordering led to an increase in the ...

The composition dependence of the glass transition temperature (T g ) in mixtures remains an important unsolved problem. Here, it is revisited using three model systems: a series of oligomeric and polymeric cyanurates, blends of oligomeric and polymeric α-methyl styrene, and molecular mixtures of itraconazole and posaconazole. We evaluate several entropy-based models to determine the theoretical T g as a function of molecular composition and compare the results against the experimental data. The assumption that the configurational entropy is invariant at the T g is tested, where the change in configurational entropy is assumed to be given by the integral of ΔC p dlnT, where ΔC p is the temperature-dependent change in the heat capacity at T g . We find that, although the temperature-dependent heat capacities in both liquid and glassy states are nearly independent of composition for several of the systems studied (i.e., they are nearly ideal mixtures), the composition dependence of T g is not well described by simply adding the changes in the massweighted configurational entropy of the components on going from the T g in the pure state to that of the blend. The implication is that either configurational entropy is not invariant at T g or that it cannot be obtained from the integral of ΔC p dlnT.