Cross-linked elastomers relaxation

Urry DW, Trapane TL, McMichens RB et al (1986) N-15 nmr relaxation study of inverse temperature transitions in elastin pol)q)entapeptide and its cross-linked elastomer. Biopolymers 25 5209-5228... 

NMR parameter images can be translated to material property images by calibration or relationships known from theory. For example, cross-link density can be linked to the transverse relaxation decay [101-103] and the longitudinal relaxation decay in the rotating frame [104, 105]. Relaxation of transverse magnetization in cross-linked elastomers is nonexponential (Fig. 

Sofia P, Fiilber C, Demco DE, Bliimlich B, Spiess FTW (1996) Effect of residual dipolar interactions on the NMR relaxation in cross-linked elastomers. Macromolecules 29 6222-6230 Suzuki M, Hirasa O (1993) An approach to artificial muscle using polymer gels formed by microphase separation. Adv Polym Sci 110 241-261 Tanaka T, Fillmore DJ (1979) Kinetics of swelling of gels. J Chem Phys 70 1214—1218 Tanaka T, Hooker LO, Benedek GB (1973) Spectrum of light scattered from a viscoelastic gel. J Chem Phys 59 5151-5159... 

Introduction of small amounts of ionic groups in hydrocarbon polymers exerts a profound effect on their mechanical properties. These ionic groups neutralized with suitable metal ions act as physical cross-hnks within the polymer matrix. The ionic associations can be thermally relaxed to permit sufficient melt flow at the processing temperature. The ion containing polymer thus behaves as a thermoplastic elastomer having the unique ability to act as cross-linked elastomer at ambient temperature, and to melt and flow at elevated temperatures like thermoplastics [50-53]. 

D.W. Urry, T.L. Trapane, R.B. McMichens, M. Iqbal, R.D. Harris, and K.U. Prasad, Nitrogen-15 NMR relaxation study of inverse temperature transitions in elastin polypentapeptide and its Cross-Linked Elastomer. Biopolymers 25, S209-S228, 1986. 

A quantitative analysis of the shape of the decay curve is not always straightforward due to the complex origin of the relaxation function itself (12,81,87-89) and the structural heterogeneity of the long-chain molecules. Nevertheless, several examples of the detection of structural heterogeneity by T2 experiments have been published, for example the analysis of the gel/sol content in cured (90,91) and filled elastomers (85,86), the estimation of the fraction of chain-end blocks in linear and network elastomers (91,92), and the determination of a distribution function for the molecular mass of network chains in cross-linked elastomers (93,94). 

One type of block polymer is known as thermoplastic elastomers. They consist of a number of rubber blocks tied together by hard crystalline or glassy blocks. These materials can be processed in injection molding and extrusion equipment since the crystalline blocks melt or the glassy ones soften at high temperatures. However, at lower temperatures, such as at room temperature, the hard blocks behave very much as cross-links to reduce creep and stress relaxation. Thermoplastic elastomers have creep behavior between that of very lightly cross-linked rubbers and highly cross-... 

Recently, Genzer and coworkers [85] presented an interesting new approach for the preparation of stable silane-based SAM systems. As a substrate, cross-linked polydimethylsiloxane (PDMS) was oxidized by UV/ozone treatment to yield a thin sihcon dioxide surface. The surface was then treated with fluorinated alkyltrichlo-rosilanes from the gas phase while being mechanically stretched by a certain length Ax. After modification, the elastomer was allowed to relax resulting in a mecdianically assembled monolayer (MAM) at the surface (Fig. 9.8). 

A good elastomer should not undergo plastic flow in either the stretched or relaxed state, and when stretched should have a memory of its relaxed state. These conditions are best achieved with natural rubber (ds-poIy-2-methyl-1,3-butadiene, ds-polyisoprene Section 13-4) by curing (vulcanizing) with sulfur. Natural rubber is tacky and undergoes plastic flow rather readily, but when it is heated with 1-8% by weight of elemental sulfur in the presence of an accelerator, sulfur cross-links are introduced between the chains. These cross-links reduce plastic flow and provide a reference framework for the stretched polymer to return to when it is allowed to relax. Too much sulfur completely destroys the elastic properties and produces hard rubber of the kind used in cases for storage batteries. 

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. 

In elastomer samples with macroscopic segmental orientation, the residual dipolar couplings are oriented as well, so that also the transverse relaxation decay depends on orientation. Therefore, the relaxation rate 1/T2 of a strained rubber band exhibits an orientation dependence, which is characteristic of the orientational distribution function of the residual dipolar interactions in the network. For perfect order the orientation dependence is determined by the square of the second Legendre polynomial [14]. Nearly perfect molecular order has been observed in porcine tendon by the orientation dependence of 1/T2 [77]. It can be concluded, that the NMR-MOUSE appears suitable to discriminate effects of macroscopic molecular order from effects of temperature and cross-link density by the orientation dependence of T2. 

Plasticized polyvinyl chloride can be regarded as the first thermoplastic elastomer, as it is used in an uncross-linked form. Because of the lack of cross-linking, this material exhibits high rates of creep and stress relaxation. As with other thermoplastic elastomers, these disadvantages worsen as the temperature is... 

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