Ageing PVAC

Fig. 11 Calculated change in loss tangent of PVAc as a function of aging time r, and loading time 1 h (dotted curves), 15 h (solid curves)) [32]...

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). 

Fig. 19. Calculated dependence of yield stress on aging time and temperature of PVAc in the vicinity of Ts [28]...

Figure 3.5 shows the temperature variation of H for the four above mentioned polymers, namely PMMA, PVAc, PET and PEEK. In the case of the two amorphous polymers (PMMA and PVAc) H decreases with T and the Tg value can he clearly identified with a hend in the 7/ vs T plot. However, the two semicrystalline materials quenched into the amorphous state (PET and PEEK) show an apparent maximum just before the glass transition takes place. Also, in the case of PET tested immediately after quenching it is observed that H increases with T above room temperature. These phenomena will be discussed in Section 3.3 in the light of the physical ageing undergone by the above polymers. It is seen that H follows an exponential decrease as a function of T given by (see eq. (2.5))... 

Table 14.18. Physical aging data for PVAc derived using stress relaxation [Jamieson (1990)]...

Other workers have exploited the S-S theory to successfully correlate mechanical aging and volume aging data (e.g., [Higuchi et al., 1996] for polycarbonate). Ferguson and Cowie [2008] have attempted the same correlation for PVAc, and the results of this for two aging temperatures are shown in Figure 9.5. 

Figure 8 Pull-out strength (A) and loss of tensile strength during accelerated ageing (B) for rovings coated with ternary PVAc/PEGS/cement mixtures in comparison to the as-received glass.

The third type of volume recovery experiment, termed the memory or crossover experiment, demonstrates that the relaxation behavior is dependent on the prior thermal history of the material. In this experiment, the material is aged partially into equilibrium at temperature Ti and then heated to a temperature Ta where the volume initially lies on the equilibrium line. Because the volume departure from equilibrium at Ta is initially zero, no evolution of volume might be expected. However, the observation is that the volume increases away from equilibrium and then decreases back to equilibrium. The thermal history involved in the memory experiment is depicted schematically in Figure 4. The experimental results of Kovacs for poly(vinyl acetate) (PVAc) (9) are shown in Figure 5 for several Ti, in addition to the down-jump response. The memory experiment is attributable to a distribution of relaxation times for example, if at T the fastest relaxing regions reached equilibrium density by the time the jump... 

Both ESR and fluorescence spectroscopy give an indirect measure of motion in polymers as they make use of either spin label or probe methods. In the case of ESR, nitroxyl radicals dispersed (spin probe) in a polymer matrix or covalently bonded to the polymer chains (spin label) are employed to probe the local environment. Therefore, ESR spectra provide information on molecular motion and microstructure of polymer matrices. Similarly, fluorescent probes are sensitive to the glass structure. This is because photon emission increases when non-radiative processes are hindered by lack of mobUity of the probe. Interestingly, studies on poly (vinyl acetate) (PVAc) have shown that changes in the fluorescence intensities with aging time and temperature follow closely those observed by volumetric relaxation [85]. 

Interaction contribution (aging time) of PVF2-PVAC blends heat treated at 75°C... 

Blends of NBR with Polyvinyl Acetate PVAC) and its Copolymers Stollfuss investigated blends of NBR with polyvinyl acetate (PVAC) and its copolymers. The object was to obtain better low-temperature flexibility, compression set behaviour, and ageing resistance than that possible with NBR/PVC blends. Although the solubility parameters are similar in the two cases and the formation of a homogeneous phase was expected, only two-phase systems were found, except in the case of a vinyl acetate/maleic acid ester copolymer. All the mechanical properties, and also the resistance to ozone, were inferior to those of the NBR/PVC blends. Only the resistance to ageing was improved. 

Research on the physical aging of glassy polymers began with the discovery by Kovacs [1] that the volume of the polyvinyl acetate (PVAC) system slowly decreased (densified) with time, when rapidly quenched from above its glass transition temperature ). This research area... 

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