Rate of physical aging

The consensus form the membrane literature is that thin glassy polymeric membranes exhibit an increase in the rate of physical aging with decreasing film thickness. The range of polymeric membranes that have been shown to exhibit this anomalous aging response include polysulfone, [42, 43, 125, 126] polyimides, [42, 43, 50, 125, 126] polynorborene [127] and poly(phenylene oxide)s [43, 125, 126]. Figure 3.8 shows the impact of film thickness on the physical aging of polysulfone... 

Baer and coworkers demonstrated that thin glassy films without free interfaces exhibited bulk physical aging behavior. For instance, PSF, when confined between polyolefin layers, showed no dependence of film thickness on the rate of physical aging. Motivated by these studies, Roth and coworkers devised a set of experiments to directly investigate the influence of an adjacent mbbery layer on Tg and physical aging in thin polymeric films [137]. The sample geometry consisted of a thin PS... 

For both PVME and PMMA blended with PHS the rate of physical aging was retarded compared to that of pure PVME or PMMA, due to the intermolecular hydrogen bonding. This is evident in Figure 6.9 where the average relaxation time, (f), for the KWW distribution ... 

The effect of physical aging on the crystallization state and water vapor sorption behavior of amorphous non-solvated trehalose was studied [91]. It was found that annealing the amorphous substance at temperatures below the glass transition temperature caused nucleation in the sample that served to decrease the onset temperature of crystallization upon subsequent heating. Physical aging caused a decrease in the rate and extent of water vapor adsorption at low relative humidities, but water sorption could serve to remove the effects of physical aging due to a volume expansion that took place in conjunction with the adsorption process. 

A unified approach to the glass transition, viscoelastic response and yield behavior of crosslinking systems is presented by extending our statistical mechanical theory of physical aging. We have (1) explained the transition of a WLF dependence to an Arrhenius temperature dependence of the relaxation time in the vicinity of Tg, (2) derived the empirical Nielson equation for Tg, and (3) determined the Chasset and Thirion exponent (m) as a function of cross-link density instead of as a constant reported by others. In addition, the effect of crosslinks on yield stress is analyzed and compared with other kinetic effects — physical aging and strain rate. 

Fig. 13 Stress-strain compression curves of physically aged PMMA obtained at a strain rate of 2 x 10-3 s-1 at various temperatures (From [32])...

From a practical point of view, the main consequence of physical ageing by structural relaxation is embrittlement (decrease in fracture resistance Chapter 12). For the other aspects of mechanical behavior, ageing has either no effect or a favourable effect (increase of relaxation times, leading to a decrease of creep or relaxation rates). This is the reason why, in most thermoset applications, the knowledge of short-term properties is considered to be sufficient for engineering design, as far as fracture and durability are not concerned. 

An important consequence of physical ageing is that, during a creep experiment or during creep in practice, the rate of creep is continuously reduced. Quantitative theories, supported by experimental results, give the picture as presented in Figure 7.10 the dotted lines indicate the creep as extrapolated from observations at higher temperatures the drawn lines are experimentally determined. The differences, in a favourable sense, are enormous ... 

In the vicinity of glass transition, both Eqs. (47) and (48) become Eqs. (42) and (43), respectively. The calculated dependence of the physical aging rate on temperature for polystyrene (PS), poly(vinyl chloride) (PVC), and poly(vinyl acetate) (PVAc) is shown in Fig. 17. There are five parameters (e, p, f xr, 7 ) in Eqs. (23), (2), (15) and (19). We have chosen p = 1/2. ft = 1/30, and xr = 30 min for these linear polymers in our theoretical calculation. The other two parameters r. = h and Tr are listed in Table 1. The calculation reveals that the Struik exponent (p) increases from zero above 7 to a constant below Tg, and then decreases to zero at 200 K below Tg. The three polymers all show a similar type of temperature dependence of physical aging rate, which compares well with the reported observations (see Fig. 15 of Ref. 2). 

The master curves and shift factors of transient and dynamic linear viscoelastic responses are calculated for linear, semi-crystalline, and cross-linked polymers. The transition from a WLF dependence to an Arrhenius temperature dependence of the shift factor in the vicinity of Tg is predicted and is related to the temperature dependence of physical aging rate. 

Guo, J.-H. Robertson, R.E. Amidon, G.L. Influence of physical aging on mechanical properties of polymer free films the prediction of long-term aging effects on the water permeability and dissolution rate of polymer film-coated tablets. Pharm. Res. 1991, 8 (12), 1500-1504. 

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