Polymer chains cross linking

It is somewhat difficult conceptually to explain the recoverable high elasticity of these materials in terms of flexible polymer chains cross-linked into an open network structure as commonly envisaged for conventionally vulcanised rubbers. It is probably better to consider the deformation behaviour on a macro, rather than molecular, scale. One such model would envisage a three-dimensional mesh of polypropylene with elastomeric domains embedded within. On application of a stress both the open network of the hard phase and the elastomeric domains will be capable of deformation. On release of the stress, the cross-linked rubbery domains will try to recover their original shape and hence result in recovery from deformation of the blended object. 

Polymer networks can be formed by chemical reactions between polymer chains (cross-linking) or by using trifunctional comonomers during the polymerisation. If such a network is dissolved in a second monomer and this second monomer is again polymerized into a second network, one obtains a structure in which both polymers are intertwined. These polymer chains only have very local mobility. In cases where both polymers are partially or completely immiscible the L1/L2 phase-separation is reduced to a very small scale. The properties of such an IPN are completely different from the uncross-linked polymer blend [15]. 

Polymer radicals may recombine upon bimolecular termination. When this cross-linking takes place between two polymer molecules the molecular yield increases. When there are more than one radical on the polymer chain, cross-linking between these two sites will form only a loop (Fig. 9.1), that is, no increase in the molecular weight ensues despite the fact that cross-linking has occurred. In competition to recombination, radicals may disproportionate, when at least... 

Chemical changes frequently accompany the shifting of the polymer chains. Cross-links are broken under the combined influence of thermal stress and the induced shear forces and are often reformed in a manner so as to accommodate the newly formed alignment of the polymer chains. When cross-links break and reform to an appreciable extent, polymers will exhibit very poor recovery. 

Hydrocarbon free radicals [such as those, for example, from reactions (34-14) or (34-12)] can also recombine with each other causing cross-linking of the polymer chains. Cross-linking can also occur by recombination of two oxyradicals ... 

It is noteworthy that the same results were found by Chompff and Duiser using a different approach [70]. They proposed a decoupling procedure which, upon being applied to polymer chains cross-linked into a network,... 

If dispersions are vigourously probe sonicated until only small unilamellar liposomes remain, no polymerisation occurs on irradiation. ) Most diacetylene polymers are highly crystalline an insoluble in all solvents. However, with one exception, phospholipi polymers will dissolve in chloroform or chloroform-alcohol mixture The exception is the polymer formed when same chain lipid is subjected to prolonged irradiation. These polymers are probably cross-linked. A polymer chain cross-linking reaction is only possible when the monomer contains more than one diacetylene group. 5)... 

Polymer chain entanglement has a significant effect on the processing of polymer fibre. According to rubber elasticity theory, the maximum stretch ratio of polymer fibre (A ax) and the number of statistical links of the polymer chain cross-linking points are in the relationship shown in Equation... 

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