Elastomer relaxation, chemical

Solid-state NMR magnetisation relaxation experiments provide a good method for the analysis of network structures. In the past two decades considerable progress has been made in the field of elastomer characterisation using transverse or spin-spin (T2) relaxation data [36-42]. The principle of the use of such relaxation experiments is based on the high sensitivity of the relaxation process to chain dynamics involving large spatial-scale chain motion in elastomers at temperatures well above the Tg and in swollen networks. Since chain motion is closely coupled to elastomer structure, chemical information can also be obtained in this way. 

In addition to the physical stress relaxation effect, chemical changes to the elastomer (e.g., by oxidation) can also increase relaxation rate. We then have... 

An important feature of filled elastomers is the stress softening whereby an elastomer exhibits lower tensile properties at extensions less than those previously applied. As a result of this effect, a hysteresis loop on the stress-strain curve is observed. This effect is irreversible it is not connected with relaxation processes but the internal structure changes during stress softening. The reinforcement results from the polymer-filler interaction which include both physical and chemical bonds. Thus, deforma-tional properties and strength of filled rubbers are closely connected with the polymer-particle interactions and the ability of these bonds to become reformed under stress. 

These results clearly indicate that the multi-frequency dynamic analysis method allows us to estimate the contribution of different relaxation mechanisms during curing of elastomers, and the changes in chemical and physical networks densities can be studied separately. 

From a fit of Equation (10) to spatially resolved relaxation curves, images of the parameters A, B, T2, q M2 have been obtained [3- - 32]. Here A/(A + B) can be interpreted as the concentration of cross-links and B/(A + B) as the concentration of dangling chains. In addition to A/(A + B) also q M2 is related to the cross-link density in this model. In practice also T2 has been found to depend on cross-link density and subsequently strain, an effect which has been exploited in calibration of the image in Figure 7.6. Interestingly, carbon-black as an active filler has little effect on the relaxation times, but silicate filler has. Consequently the chemical cross-link density of carbon-black filled elastomers can be determined by NMR. The apparent insensitivity of NMR to the interaction of the network chains with carbon black filler particles is explained with paramagnetic impurities of carbon black, which lead to rapid relaxation of the NMR signal in the vicinity of the filler particles. 

Solid state NMR offers several advantages for the investigation of filled rubbers since molecular properties of elastomer chains can be measured selectively by NMR e>q)eriments. The method is very sensitive to the molecular scale heterogeneity in a sample. The network structure which is composed of chemical, physical and topological junctions can also be andyzed by NMR relaxation experiments [11,12,14,15],... 

The elastic properties of rubbers are primarily governed by the density of netw ork junctions and their ability to fluctuate [35]. Therefore, knowledge of the network structure composed of chemical, adsorption and topological junctions in filled elastomers as well as their relative weight is of a great interest. The H T2 NMR relaxation experiment is a well established method for the quantitative determination of the network structure in the elastomer matrix outside the adsorption layer [14, 36]. The method is especially attractive for the analysis of the network structure in filled elastomers since filler particles are "invisible" in this experiment due to the low fraction of protons at the Aerosil surface as compared with those in the host matrix. 

For materials applications, the chemical shifts of methanol and ethylene glycol can be monitored in the liquid state to follow temperature [Hawl]. The most sensitive ehemical shift is the Co resonance of aqueous Co(CN)e with a sensitivity of 0.05 K at 7 T and 0.2 K at 2T [Dorl]. Furthermore, dibromomethane dissolved in a liquid crystal is a temperature sensitive NMR compound [Hed 1 ], and known phase-transition temperatures can be exploited to calibrate the temperature control unit [Hawl J. In temperature imaging of fluids, temperature can be determined from the temperature dependence of the selfdiffusion coefficient but convective motion may arise in temperature gradients [Hedl]. In the solid state, the longitudinal relaxation time of quadrupolar nuclei like Br is a temperature sensitive parameter [Suil, Sui2]. In elastomers, both T2 and Ti depend on temperature (Fig. 7.1.13). In filled SBR, T2 is the more sensitive parameter with a temperature coefficient of about 30 xs/K [Haul]. 

Figure 5-16. Physical stress relaxation at 25 °C in a crosslinked network made by a zinc oxide cure of a halobutyl rubber. The elastomer has been soaked in oil to speed the relaxation. The line is the fit to the data using the Chasset-Thirion equation with E =0.018 MPa, t= 1.04 s and m = 0.31. The nature of the polymer and the cure makes for a chemically stable structure otherwise, the plot would begin to curve downward at long time.

To reduce the cost, these elastomers have been diluted with some PBT homopolymer. Because of the chemical similarity between the hard segment of the copoly(ether-ester) elastomers and the PBT, they form fairly compatible blends. When the hard segment content in the copoly(ether-ester) is > 80 wt%, it was found to be completely miscible with PBT, showing a single T, amorphous phase and co-crystallization of the PBT segments of the elastomer with PBT homopolymer. As the hard segment content was lowered to < 60%, the blend exhibited incomplete miscibility, with two Tg s for two amorphous phases and also two separate crystalline phases. [Runt et al., 1989]. Nevertheless, a partial miscibility was indicated due to changes in the T observed in DSC and dielectric relaxation spectra. The partial miscibility and low interfacial tension between the phases makes the blend very compatible. 

The book covers aspects from the morphology to mechanical aspects focused on the elasticity and inelasticity of amorphous to crystalline polyurethane elastomers, in relation to their sensitivity to chemical and physical structure. In such polymers, resilience of the material is an important attribute. In many applications they are in commercial competition with other, relatively soft, elastomeric materials. The choice of material for any given application then hinges on a spectrum of key properties offered by relatively soft polymers—stiffness and strain recovery characterizing their elasticity, but also inelastic effects such as hysteresis and stress relaxation. In these respects the mechanical properties of polyurethane elastomers are similar to those of other elastomers. 

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