The origin of the β transition and its influence on physical ageing

Antiplasticization of Polymer Materials: Structural Aspects and Effects on Mechanical and Diffusion-Controlled Properties

“…One notes that the rate of change of both functions decreases with ageing in advancing towards a "mature" (stable) state and brings about an upward shift in the glass transition temperature. The trendlines for the specific volume bear a similarity with the experimental data reported in [59], showing a reduction in the gradient of the change in the specific volume at temperatures below 50 • C after physical ageing. In this latter case the trendlines for the aged and unaged samples allude to a likely intersection at lower temperatures, possibly about −50 • C in correspondence to a more realistic temperature for the β transition of polystyrene [60], in concordance with the values for other glassy polymers.…”

“…In this latter case the trendlines for the aged and unaged samples allude to a likely intersection at lower temperatures, possibly about −50 °C in correspondence to a more realistic temperature for the β transition of polystyrene [60], in concordance with the values for other glassy polymers. It should also be noted that the graphs representing the variation of specific volume with temperature in [59] indicate that the Tg of the sample decreased The representative lines shown in Figure 2a indicate that densification by physical ageing arises purely from the reduction in the free volumes of polymer chains from the "juvenile" (unaged) state until the glass reaches a "mature" (minimum free energy) state, characterized by its equilibrium Tg (also known as Tα). The trace for a "mature" glass indicates that the change in the specific volume with a temperature below the Tg of the polymer was expected to take place at the same rate as that of the volume occupied by the polymer chains in relaxations-free state, while the constant gap was to allow for a likely temperature independent compressibility.…”

“…In this latter case the trendlines for the aged and unaged samples allude to a likely intersection at lower temperatures, possibly about −50 • C in correspondence to a more realistic temperature for the β transition of polystyrene [60], in concordance with the values for other glassy polymers. It should also be noted that the graphs representing the variation of specific volume with temperature in [59] indicate that the Tg of the sample decreased with increasing ageing time. Since this is physically impossible the discrepancy may have to be attributed to experimental errors in the dilatometry measurements, bearing in mind that an increase in Tg has also been observed in related experimental studies by Yavari et al [61].…”

“…The trace for a "mature" glass indicates that the change in the specific volume with a temperature below the Tg of the polymer was expected to take place at the same rate as that of the volume occupied by the polymer chains in relaxations-free state, while the constant gap was to allow for a likely temperature independent compressibility. The descriptive lines for Figure 2a were derived from the changes in modulus with temperature within the glassy state outlined in the work of Mascia et al [9,12], shown in the inset, and also from a report by Lee [58], as well as from an article by Lee and McGarry [59] on atactic polystyrene. The graphs in Figure 2a illustrate the connection between changes in the specific volume (ψsp) and shear modulus (G) with temperature for a glassy polymer undergoing physical ageing through the relationship between relaxation time and free volume in Equation (11) (i.e., d(logG)/dT is proportional to −dψsp/dT).…”

Physical aging in polymers and polymer composites: An analysis and method for time‐aging time superposition

“…Extrapolation of such results to temperatures far below Tg should be made with cautlon. as other phenomena such as secondary p transltlon can come Into play (2). However, the typlcal physical aging analysis descrlbed hereln nonetheless provides quite valuable Information for polymeric behavlor below Tg.…”