Rheological relaxation time

Figure 12.10 Evolution of rheological relaxation time and DLS medium mode with Ci2(EO)4/Ci6SE = P1695 ratio, R. 90% water (adapted from ref [9] with permission from ACS...

According to Cates s model the rheological relaxation time T is now a combination of the average lifetime of the micelles and the reptation time. For the limiting case of rapid breaking micelles one obtains ... 

The parameter is made temperature dependent according to the rheological relaxation time shift factor Uj-... 

The rheological parameter variations with blend compositions are shown in Figs. 4,5 and 6. The variation of relaxation time (Fig. 4) seems to show an increasing pattern up to 40% of NBR, then there is a sharp decrease until 60% of NBR, and finally there is a further increase of relaxation time in the preblends. Preheating of blends, however, results in a longer relaxation time than in the preblends and a maximum at 50 50 ratio at all shear rates. 

Earlier studies [14,15] clearly reveal that there is a reaction between two polymers and that the extent of reaction depends on the blend ratio. As 50 50 ratio has been found to the optimum (from rheological and infrared studies) ratio for interchain crosslinking, the higher heat of reaction for the NBR-rich blend may be attributed to the cyclization of NBR at higher temperatures. There is an inflection point at 50 50 ratio where maximum interchain crosslinking is expected. Higher viscosity, relaxation time, and stored elastic energy are observed in the preheated blends. A maximum 50-60% of Hypalon in NBR is supposed to be an optimum ratio so far as processibility is concerned. 

The rheological parameters with blend compositions are shown in Figs. 10-12. The relaxation time increases with the increase in NBR content up to a 50% level for both the preblends and preheated blends. Comparing the preblends and preheated blends, we see that... 

Rheological parameters, such as relaxation time, shear modulus, and stored elastic energy, are determined from the extrudate swell and stress-strain data as previously described. Representative examples of the variation of these parameters with blend ratios for both blends are shown in Figs. 16-18. Figure 16 shows that relaxation time for both preblends without heating and... 

The plot of the rheological parameters (relaxation time, shear modulus, and stored elastic energy) are shown in Figs. 22-24. The relaxation time increases as the ACM content is increased to attain a maximum at 60 40 = ACM XNBR blend ratio for the preblends. For lower shear rate the rise is sharp and after 60 40 blend ratio, // remains almost constant, whereas for the higher shear rate region the rise is not sharp and after 60 40 blend ratio ty decreases as ACM percent increased in the blend. In the case of the preheated blends the /y increases up to 50 50 blend ratio and then decreases with the addition of ACM in the blend. The preheating increases the ty in both shear rate regions. 

The plot of the rheological parameters (relaxation time, /r shear modulus, G and stored elastic energy, W ) are given in Figs. 28-30. The relaxation time of both preblends and preheated blends remains almost constant up to 50 50 blend ratio and then shoots up drastically at both shear rates. Up to 50 50 blend ratio it is observed that the relaxation time is more at lower shear rate. Preheating of blends lowers the values. 

To is known as the Vogel-Fulcher temperature and is located about 30 K below Tg. r is the asymptotic value of the relaxation time of the correlator 4> for T—>oo. Also the rheological shift factors a (T) mentioned above approximately follow such temperature dependences [34] ... 

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