Elastically inactive cycles

In this contribution, we report equilibrium modulus and sol fraction measurements on diepoxidet-monoepoxide-diamine networks and polyoxypropylene triol-diisocyanate networks and a comparison with calculated values. A practically zero (epoxides) or low (polyurethanes) Mooney-Rivlin constant C and a low and accounted for wastage of bonds in elastically inactive cycles are the advantages of the systems. Plots of reduced modulus against the gel fraction have been used, because they have been found to minimize the effect of EIC, incompleteness of the reaction, or possible errors in analytical characteristics (16-20). A full account of the work on epoxy and polyurethane networks including the statistical derivation of various structural parameters will be published separately elsewhere. 

In the foregoing considerations, formation of elastically inactive cycles and their effect have not been considered. For epoxy networks, the formation of EIC was very low due to the stiffness of units and could not been detected experimentally the gel point conversion did not depend on dilution in the range 0-60% solvent therefore, the wastage of bonds in EIC was neglected. For polyurethanes, the extent of cyclization was determined from the dependence on dilution of the critical molar ratio [OH] /[NCO] necessary for gelation (25) and this value was used for the statistical calculation of the fraction of EIC and its effect on Ve as described in (16). The calculation has shown that the fraction of bonds wasted in EIC was 2-2.5% and 1.5-2% for network from LHT-240 and LG-56 triols, respectively. 

Diluent added during crosslinking has two main effects it Increases the population of elastically Inactive cycles and it weakens the interchain constraints. Studies of poly(oxypropylene) triol-diisocyanate networks in the presence of diluent have shown that the effect of diluent on the equilibrium modulus is much stronger than would correspond to the effect of cycles (Figure 10) (32) which again corroborates the concept of permanent interchain constraints. 

The description of a network structure is based on such parameters as chemical crosslink density and functionality, average chain length between crosslinks and length distribution of these chains, concentration of elastically active chains and structural defects like unreacted ends and elastically inactive cycles. However, many properties of a network depend not only on the above-mentioned characteristics but also on the order of the chemical crosslink connection — the network topology. So, the complete description of a network structure should include all these parameters. It is difficult to measure many of these characteristics experimentally and we must have an appropriate theory which could describe all these structural parameters on the basis of a physical model of network formation. At present, there are only two types of theoretical approaches which can describe the growth of network structures up to late post-gel stages of cure. One is based on tree-like models as developed by Dusek7 I0-26,1 The other uses computer-simulation of network structure on a lattice this model was developed by Topolkaraev, Berlin, Oshmyan 9,3l) (a review of the theoretical models may be found in Ref.7) and in this volume by Dusek). Both approaches are statistical and correlate well with experiments 6,7 9 10 13,26,31). They differ mainly mathematically. However, each of them emphasizes some different details of a network structure. 

The influence of monocycles on network mechanical behaviour is also interesting. The importance of elastically inactive cycles in the behaviour of rubbery networks is well known 13). Variations of the concentration of monocycles in the considered networks were performed by curing the compounds given in Scheme IV 26). 

The application of Sn anodes is still hampered by their inherent poor cycling stability resulting from the large volume change. Several strategies have been proposed to overcome this problem. One of the effective strategy is to prepare intermetallic compounds (M M), which consist of an inactive phase M and an active phase M . Another useful approach is either to disperse the nanometer-sized tin-based materials into a carbon matrix or to prepare the carbon-encapsulated hollow tin nano-particles. The carbon component has good elasticity to effectively accommodate the strain of... 

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