Polymer network imperfect

Another type of network imperfection, resulting from cross-linking of two units not distantly related structurally, is indicated in Fig. 94. Cross-linkages such as B are wasted (except insofar as the loop may be involved in entanglements not otherwise operative). The proportion of these short path cross-linkages should be small ordinarily but could become very large if the cross-linking process were carried out in a dilute solution of the polymer. 

The properties of a polymer network depend not only on the molar masses, functionalities, chain structures, and proportions of reactants used to prepare the network but also on the conditions (concentration and temperature) of preparation. In the Gaussian sense, the perfect network can never be obtained in practice, but, through random or condensation polymerisations(T) of polyfunctional monomers and prepolymers, networks with imperfections which are to some extent quantifiable can be prepared, and the importance of such imperfections on network properties can be ascertained. In this context, the use of well-characterised random polymerisations for network preparation may be contrasted with the more traditional method of cross-linking polymer chains. With the latter, uncertainties can exist with regard to the... 

The viscoelastic responses of two polymer networks are shown in Fig. 7.29. One is a nearly perfect network (circles) made by end-linking linear chains with two reactive ends. The storage modulus for this network (filled circles) is independent of frequency and much larger than the loss modulus (open circles). For comparison, an imperfect network made by linking a mixture of chains with one and two reactive ends is also shown. 

Equation (6-83) assumes that the network is a perfect one in that all chains in the network are effective in giving rise to the elastic stress. Ideally each crosslink connects four network chains, while each such chain is terminated by two crosslinks. However, as illustrated in Figure 6-2, a number of network imperfections are possible. Each linear polymer chain with molecular weight M, even if all crosslinks are "normal," must give rise to two terminal chains that are incapable of supporting stress. Thus the number of effective chains must not include the imperfections due to chain ends. On incorporating this correction, the shear modulus can be written as 18... 

Some PS (from imperfect cross linking of the polymer network II) is also extracted. For example, analysis by U.V. spectroscopy at 262 nm reveals 11.0% of PS in the sol fraction of a 50/50 decrosslinked IPN, which explains the high values obtained when toluene was employed as the extracting fluid. 

Quasi-model polymer networks are polymer networks with an almost perfect stmcture, with the most frequendy occurring imperfection concerning the constancy in the number of chains emanating from the cross-linking nodes. 

Sharaf, M. A., The Effects of Network Imperfections on the Small-Strain Moduli of Polydimethylsiloxane Elastomers Having High Functionality Cross-Links. Int. J. Polym. Mater. 1992,18(3-4), 237-252. 

Rubber failure by application of stress has been studied extensively owing to its overwhelming scientific and practical interest. In particular, two mechanisms have been pul- forward as relevant in delaying rubber failure. The first mechanism is based on viscoelastic energy dissipation, which can be increased by increasing the glass transition of the base polymer, or by other routes such as the use of additives or controlled network imperfection. The second mechanism is... 

The permanent structure required for elastic recovery is obtained by joining N linear chains by chemical cross-linkages. A polymer network may then be characterized by the number // of its junctions, their functionality (or average functionality) and by the number of chain ends. The number v of chains in the network, including those with only one end attached, is given by equation (43). The effective number of chains is less than v owing to imperfections due to free chain ends. 

If R can react with itself or additional components (R contains vinyl, methacryl or epoxy groups, for example), the result of the condensation process is a flexible network of inorganic oxide covalently bonded to organic polymers, namely a hybrid nanocomposite lacking interface imperfections. The properties of this hybrid nanocomposite are intermediate between those of polymers and glasses, and can meet unique requirements. 

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