Curing network topology

Obviously, there are many subtle differences in the structure, morphology, or network topology between radiation cured and sulfur cured elastomers, but their physical properties may be nearly equal, provided that precautions are taken to avoid the occurrence of chain scissions. A comparison of radiation cross-linked and sulfur cured natural rubber (gum and carbon-black-reinforced compounds) is in Table 5.4. ... 

Thus, the analysis of the reactivity ratios of the primary and secondary amino groups indicates that for conventional curing agents this cannot be regarded as a serious factor affecting the network topology. 

To be able to relate mechanical properties to the composition of rubbery compounds and curing chemistry it is essential to understand the network topology of the resultant cured materials. 

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. 

For the mechanical behaviour of glassy networks, the relations (if it exists) between network topological parameters, such as crosslink density, cure conversion, concentration of monocycles and connectivity of chemical crosslinks are important. 

The authors of Ref. [9] used the cross-linked epoxy polymers based on diglycidyl ether of bisphenol A (ED-22) of anhydride curing (EP-2) [11]. Two series of EP-2 are used - one of them was cured at atmospheric pressure (EP-2-200) and another at hydrostatic pressure that allowed to obtain 10 samples of EP-2, differing by cross-linked networks topology [11]. 

Computer simulations were performed taking into account the component ratio P, the relative reactivity of primary and secondary amines, and different probabilities of monocycle formation. The simulations showed the structural features of networks (both topology and defects) at all stages of the cure process. Two examples of network structure received from computer simulation are shown in Fig. 2. Here... 

This demonstrates the importance of the understanding of the network formation which determines both the chemical (topological) and physical structure of the resulting matrices. In this contribution, the special features of the network build-up from N,N-diglycidylamines will be reviewed. Also, attention will be paid to diffusion control of curing caused by the... 

For some highly crosslinked thermosetting polymers with rigid backbones, topological restrictions limit the maximum attainable conversion. For the reaction between epoxidized novolacs and cresol-based novolacs, a maximum conversion, Xmax= 0.80 was reported (Hale et al., 1991). A similar value of x ,ax was reported for the cure of an epoxidized novolac with 4,4 -diaminodiphenylsulfone (DDS) (Oyanguren and WilKams, 1993a). In these cases, partially reacted networks may be modeled as a random solution of reacted functionalities with a concentration equal to x/x ax... 

Crosslinking polyurethanes in the presence of unsaturated polyester-styrene mixtures produce unique topologically interpenetrating networks. The cured mixtures exhibited one glass transition tem-... 

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