Relaxation in cross-linked

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... 

The earliest published analysis of stress relaxation in cross-linked rubbers was performed by Chasset and Thirion [43], They established the following empirical equation ... 

Differing from the previous studies (5-7) where the parameters Goo, m, loay have been treated as constants, we find that they depend on cross-link density which is consistent with the measurements of Dickie and Ferry (4). Figure 5 shows the dependence of the viscoelastic relaxation on cross-link density. The solid curves are calculated from Equations 17, 19 and 20 by using a value of xq = 2.5 x 102 hrs at T = 25°C. Figure 5 resembles the corresponding figure in ref. 5. 

Clearly, the basic imaging scheme can be extended to include relaxation-time contrast for discrimination of variations in cross-link density and strain, and the ID MRI-MOUSE (magnetic resonance imaging MOUSE) can be extended with further gradient coils to permit imaging in three dimensions. Numerous applications of the MRI-MOUSE can be envisioned in soft matter analysis, in particular in those areas, where imaging with conventional equipment has proven to be successful, and where smaller, less expensive, and mobile devices are in need. 

The equilibrium 20 is important not only in the synthesis of linear polysiloxanes but also in their applications. The effects of water vapor on inducing chain cleavage at high temperature are not only reduced molecular weights but also a dramatic increase in the rates of chemically induced stress relaxation at 250 °C in cross-linked poly(dimethylsiloxane) networks under load (70). Slow hydrolytic bond cleavage in cross-linked networks is seen even in studies of stress relaxation in air at room temperature, and appreciable rates of stress relaxation in the loaded networks are measured at temperatures as low as 70 (7i). The stress relaxation is greatly accelerated... 

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. 

Water-based permanent gels are not structurally different from other permanent gel systems. They acquire special character mainly when they are ionized and the ionic strength of the medium is sufficiently low. Suitable copolymers in aqueous solution can form a vast variety of reversible gel systems because of the moderately strong, primary interactions possible within the aqueous environment. Structural organization (mainly cooperative effects) are then used, to a limited extent, to create sufficiently long-lived bonds that form gels of adequately long relaxation and cross-link turnover times. 

Korb el al. proposed a model for dynamics of water molecules at protein interfaces, characterized by the occurrence of variable-strength water binding sites. They used extreme-value statistics of rare events, which led to a Pareto distribution of the reorientational correlation times and a power law in the Larmor frequency for spin-lattice relaxation in D2O at low magnetic fields. The method was applied to the analysis of multiple-field relaxation measurements on D2O in cross-linked protein systems (see section 3.4). The reorientational dynamics of interfacial water molecules next to surfaces of varying hydrophobicity was investigated by Stirnemann and co-workers. Making use of MD simulations and analytical models, they were able to explain non-monotonous variation of water reorientational dynamics with surface hydrophobicity. In a similar study, Laage and Thompson modelled reorientation dynamics of water confined in hydrophilic and hydrophobic nanopores. 

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). 

N.m.r. relaxation times have been measured for water protons in cross-linked hen egg-white lysozyme crystals below the freezing point as a function of the mole fraction of protons in the water phase. Data were analysed using a cross relaxation model that eliminated the necessity of postulating long residence times for water molecules in the domain of the protein. 

Mechanochemistry initiated and/or terminated in cross-linked polymers presents a special challenge because crucial characterization techniques require studies of polymers in solution. An evaluation for the extent of reaction can be made, however, by several semiquantitative means such as swelling in solvent [49] and stress relaxation tests [50, 51]. 

Specinfo, from Chemical Concepts, is a factual database information system for spectroscopic data with more than 660000 digital spectra of 150000 associated structures [24], The database covers nuclear magnetic resonance spectra ( H-, C-, N-, O-, F-, P-NMR), infrared spectra (IR), and mass spectra (MS). In addition, experimental conditions (instrument, solvent, temperature), coupling constants, relaxation time, and bibliographic data are included. The data is cross-linked to CAS Registry, Beilstein, and NUMERIGUIDE. 

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