Relaxation blends

Therefore, for blends such as Mj, the blend relaxation fimction is given by... 

A more comprehensive study of this system has been carried out by Cowie and Ferguson [1991]. These authors followed the enthalpy and stress relaxation of a series of PMMA/SAN blends with SAN compositions spanning the miscibility window, i.e., from 13.3 to 30 wt% AN. It was found that the blends relaxed faster than either of the components, when aging temperatures were (Tg - T ) = 10°C, but that this was no longer the case at (Tg - T ) = 20°C where blend aging was intermediate to both components. The data were analyzed using both the P-M and C-F approaches and examples of AH(t, T ) vs. log t plots are shown in Figure 14.6. The C-F model... 

Excellent work was reported by Balsamo et al. (2006) on the semicrystalline morphology of PCL/PSMA14 miscible blends as well as on their crystallization kinetics. The authors used a combination of dielectric, calorimetric, and microscopy characterization tools to investigate crystallization features of PCL in miscible PCL/PSMA14 blends over the whole composition range. The results achieved allowed to draw the following conclusions with respect to the miscibility effect on the blend relaxation dynamics and crystallization kinetics of PCL ... 

In dynamic mechanical analysis experiments T for CTA did not shift on addition of PCL but there was marked loss of modulus above -50 °C in the 40 60 and 60 40 blends. Relaxation peaks for T and T for PCL were also observed. 

SHS71 < SHS71/PVME. This is consistent with the finding that the SHS71/ PVME blend exhibited the slowest relaxation while SHS/PMM A blends relaxed faster than the corresponding SHS copolymers close to Tg. 

The Metravib Micromecanalyser is an inverted torsional pendulum, but unlike the torsional pendulums described eadier, it can be operated as a forced-vibration instmment. It is fully computerized and automatically determines G, and tan 5 as a function of temperature at low frequencies (10 1 Hz). Stress relaxation and creep measurements are also possible. The temperature range is —170 to 400°C. The Micromecanalyser probably has been used more for the characterization of glasses and metals than for polymers, but has proved useful for determining glassy-state relaxations and microstmctures of polymer blends (285) and latex films (286). 

Elastomeric Fibers. Elastomeric fibers are polyurethanes combiaed with other nonelastic fibers to produce fabrics with controlled elasticity (see Fibers, elastomeric). Processing chemicals must be carefully selected to protect all fibers present ia the blend. Prior to scouriag, the fabrics are normally steamed to relax uneven tensions placed on the fibers duriag weaving. Scouriag, which is used to remove lubricants and siting, is normally conducted with aqueous solutions of synthetic detergents and tetrasodium pyrophosphate, with aqueous emulsions of perchloroethylene or with mineral spidts and sodium pyrophosphate. 

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. 

Shear modulus also changes with the blending ratio (Fig. 5), such as the relaxation time. Preblends show an... 

Figure 4 Variation of relaxation time (t/ ) with the percent NBR content in NBR-CSPE blend.

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. 

Starting from the assumption that the geometry relaxation after excitation is of primary importance with respect to the luminescence response, we decided to employ a solid polymer matrix to suppress conformational changes of the oligomers. For the measurements, dilute blends with polysulfone as the transparent host matrix were prepared. In Figure 16-13, the PL decay curves for the two cyano compounds in both chloroform and polysulfone are presented, as are the PL spectra of Ooct-OPV5-CN in chloroform and polysulfone [69J. 

The most desirable annealing temperatures for amorphous plastics, certain blends, and block copolymers is just above their glass transition temperature (Tg) where the relaxation of stress and orientation is the most rapid. However, the required temperatures may cause excessive distortion and warping. 

The organoleptic properties of black tea depend to a considerable extent on the astringency resulting from the interaction of caffeine with the oxidized galloyl ester of the flavanols. The aroma components of black tea also constitute a unique flavor profile that blends well with the taste of the nonvolatile materials. The caffeine provides a moderate level of stimulation, which adds further to the appeal of the beverage, although tea has been shown to provide relaxation as well as revival of character.119... 

A more complex but faster and more sensitive approach is polarization modulation (PM) IRLD. For such experiments, a photoelastic modulator is used to modulate the polarization state of the incident radiation at about 100 kHz. The detected signal is the sum of the low-frequency intensity modulation with a high-frequency modulation that depends on the orientation of the sample. After appropriate signal filtering, demodulation, and calibration [41], a dichroic difference spectrum can be directly obtained in a single scan. This improves the time resolution to 400 ms, prevents artifacts due to relaxation between measurements, and improves sensitivity for weakly oriented samples. However, structural information can be lost since individual polarized spectra are not recorded. Pezolet and coworkers have used this approach to study the deformation and relaxation in various homopolymers, copolymers, and polymer blends [15,42,43]. For instance, Figure 7 shows the relaxation curves determined in situ for miscible blends of PS and PVME [42]. The (P2) values were determined... 

Figure 7 Relaxation of orientation measured simultaneously for both components in miscible PS/PVME blends following a rapid deformation (1 m/s) to a draw ratio of 2 at Tg +15°C. The time-resolved dichroic difference spectra were acquired using PM-IRLD. Reproduced with permission from Pellerin et al. [42]. Copyright 2000 American Chemical Society.

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