Crosslinking determination

The sol-gel chemistry has also been used to prepare inorganic inhomogeneities in an organic matrix. Silane end-capped macrodiols can be used. Hydrolysis and condensation of alkoxy silane groups lead to inorganic hard clusters (Fig. 7.6b). Intramolecular reactions and the miscibility of the soft-segment chains with the relatively polar crosslinks determine the size distribution of the clusters (nanofillers). 

The decay of physical crosslinks determined by the DSC measurements occurs at a higher temperature than determined by the stress relaxation method. 

The copolymerization of styrene with divinylbenzene (DVB) is necessary in order to obtain the required stability of the resin. Upon adding divinylbenzene to styrene, the two functional groups of DVB crosslink two polystyrene chains with each other. Part of the resulting skeleton is depicted schematically in Fig. 3-3. The percentage of DVB in the resin is indicated as percent crosslinking . The degree of crosslinking determines the porosity of the resin, which is another characteristic for the classification of PS/ DVB resins. 

There is an optimum level of crosslinking with typical values to be around 70-95 mol %. Conformational flexibiUty of the crossUnker has to be balanced with the overall level of crosslinks. The chemical nature of the crosslinker determines the useful solvent range for a MIP. The size of the crosslinker ideally matches the dimensions of the template, to avoid conformational frustrations within the polymer matrix as a result of a structural misfit, which can lead to a loss in capacity and/or selectivity. 

It is clear that the smaller 0, the greater the second term in Equation (11.41) and the greater L. The maximmn value is attained for tetrahedral angles (0, = 70.5 ) [94]. The distance between the crosslinks determined in this way (we shall call it conformational length is obvionsly equal to ... 

As described above, NMR can be used to measure chemical crosslink concentrations however, the spectra are unaffected by physical entanglements. Since these entanglements contribute to the elastic response of cured rubber (see Chapter 4), the difference between crosslink determinations from NMR and modulus measurements can be used to quantify the entanglement concentration. Most applications of this method have been directed toward assessing the role of filler-polymer interaction on the stiffness of elastomers [95]. 

Another very important aspect for the synthesis of LC elastomers is the functionality of the crosslink. determines the number of chains that meet at a junction. The local topology of the crosslink determines the properties of the... 

The method of crosslinking determines to some extent the requirements with respect to polymer-polymer interdiffusion. 

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