Physical aging of hydroxypropyl methylcellulose acetate succinate via enthalpy recovery

Abstract

There are two regimes of physical aging behavior for HPMCAS: near-Tg and well below Tg. The latter regime exhibits significant thermodynamic evolution, despite the assumed kinetic stability.

Physical aging and vitrification in polymers and other glasses: Complex behavior and size effects

“…The first evidence in this sense was reported by Bauwens et al 95 on glassy polycarbonate (PC), who showed that the enthalpy recovered at the plateau in the aging time was a fraction of the totally recoverable enthalpy at the extrapolated liquid line. Subsequent studies, in glassy polymers of different nature, showed the same outcome in PS; [96][97][98][99] styrene-acrylonitrile (SAN) copolymer; 100 PVAc; 84,96,101 poly(methyl methacrylate) (PMMA); 102,103 polymeric methylcellulose acetate succinate (HPMCAS); 104 and different high T g polymers. 105 The outcome of the latter study is shown in Figure 6, where the time evolution of the T f resulting from aging at room temperature is shown for these polymers.…”

“…On further temperature reduction, if the α relaxation with rapidly growing time scale was the exclusive mechanism controlling equilibration, a sharp decrease in the aging rate would be observed. Instead, Greiner and Schwarzl 76 rather observed a smooth decrease of the aging rate with decreasing temperature À an outcome showed by several others 104,[150][151][152] À or, in some cases, even multiple maxima in the aging rate, plastically showing the existence of mechanisms beyond the α relaxation controlling physical aging. In all these works, the aging rate was defined from the time dependent variation of a property at the maximum slope.…”

“…Calorimetric scans obtained on heating up a glass previously aged close to T g exhibit the well-known narrow endothermic overshoot in proximity of the step in the specific heat underlying the glass transition. 127 An example in this sense is presented in Figure 9 (panel A-C) on glassy polymeric HPMCAS aged at or above T g À 20 K. 104 However, once the aging temperature is further decreased, the signature of aging does not anymore result in the typical response close to the glass T g . In contrast, isothermal aging in these conditions results in the appearence of a low-temperature broad endotherm, generally located at temperatures tens of Kelvin larger than the aging temperature.…”

Effect of anisotropic deformation on the differential scanning calorimetry response of polymer glasses

“…9,10 Specifically, it was found that cooling at conventional rate and subsequent aging at a sub-T g temperature T a results in an emergence of an endothermic peak (as compared to the reference), where the location of the peak depends on T a . This dependence can be summarized (neglecting some material dependency) as follows: in case of T a higher than T g À 40 C the endothermic peak is located slightly above T g and in case of T a lower than T g À 40 C the endothermic peak is typically located at T a + 40 C 11 ; however, lower values, specifically in the T a + 30 C to T a + 35 C range 12 and the T a + 20 C to T a + 25 C range 13,14 have been reported for some systems. In contrast, an extremely rapid quenching, known as a hyper-quenching, results in a broad exothermic peak occurring below conventional T g .…”

Length Scale of Molecular Motions Governing Glass Equilibration in Hyperquenched and Slow-Cooled Polystyrene

“…These aspects are captured by models as those based on Narayanaswamy’s material time concept, , specifically approaches based on the Tool–Narayanaswamy–Moynihan (TNM) and the Kovacs–Aklonis–Hutchinson–Ramos (KAHR) models. Approaches based on the free volume holes diffusion model , are also able to provide a description of physical aging, above all its acceleration in geometrically confined glasses. − Common features of the models aiming to describe physical aging is that they rely on the exclusive role of the main α-relaxation, and as a result, they are able to catch the time evolution in proximity of the glass transition temperature ( T g ) but fail to describe aging significantly below T g . − While these works indicate the presence of molecular processes associated with aging beyond the α-relaxation, the underlying mechanisms and the associated length scales for glasses equilibration are yet to be completely explored.…”