Network structure heterogeneity

Fig. 1 Top. Chemically crosslinked OR gels with different BIS contents (a-l) Cbis= 1 mol% and (a-2) Cbis = 5 mol%. (b) Clay-dispersed OR gel withCBis = 1 mol% and Ccuy = 5 mol%. Bottom. Representations of networks of chemically crosslinked OR gels (c-1) heterogeneous network structure of an OR gel (Cbis = 5 mol%), (c-2) uniform network structure of an OR gel (Cbis = 1 mol%), and (c-3) rupture of crosslinked chains in a unidirectional extension of an OR gel...

Difficulties arise in characterising commercial branched and network structures in this way because of their heterogeneity. In these cases the R/Si ratio (or specifically the CH3/Si ratio in methylsilicones) is a useful parameter. On this basis the R/Si ratios of four types are given in Figure 29.1. 

The degradation of the matrix in a moist environment strongly dominates the material response properties under temperature, humidity, and stress fatigue tests. The intrinsic moisture sensitivity of the epoxy matrices arises directly from the resin chemical structure, such as the presence of hydrophilic polar and hydrogen grouping, as well as from microscopic defects of the network structure, such as heterogeneous crosslinking densities. 

STRUCTURALLY HETEROGENEOUS SYSTEMS NETWORK STRUCTURES AND DYNAMICS OF HYDROGELS... 

Depending on the conditions of synthesis, copolymerization of divinyl/vinyl-monomers in the presence of an inert solvent leads to the formation of expanded (preswollen) or heterogeneous (porous) structures [54,99,100]. If the solvent remains in the network (gel) phase throughout the copolymerization, expanded networks are formed. If the solvent separates from the network phase the network becomes heterogeneous. According to Dusek et al., heterogeneities may appear in poor solvents due to the polymer-solvent incompatibility (x-induced syneresis), while in good solvents due to an increase in crosslink density (v-induced syneresis) [99]. 

This is a theoretical study on the entanglement architecture and mechanical properties of an ideal two-component interpenetrating polymer network (IPN) composed of flexible chains (Fig. la). In this system molecular interaction between different polymer species is accomplished by the simultaneous or sequential polymerization of the polymeric precursors [1 ]. Chains which are thermodynamically incompatible are permanently interlocked in a composite network due to the presence of chemical crosslinks. The network structure is thus reinforced by chain entanglements trapped between permanent junctions [2,3]. It is evident that, entanglements between identical chains lie further apart in an IPN than in a one-component network (Fig. lb) and entanglements associating heterogeneous polymers are formed in between homopolymer junctions. In the present study the density of the various interchain associations in the composite network is evaluated as a function of the properties of the pure network components. This information is used to estimate the equilibrium rubber elasticity modulus of the IPN. 

Swollen networks usually show a negligible C2 term. If they do exhibit a C2 term, there is reason to believe that the network structure is inhomogeneous, or even heterogeneous due to microsyneresis (see Chapter II, Sections 3 and 4). These observations suggest that the explanation for C2 might be coupled with a certain structuring in the network beyond that implied by the ideal network picture. 

The critical network structural parameters that control the mechanical performance of epoxies are macroscopic heterogeneities in crosslink density and the network topography on the molecular level. 

In the case under consideration different physical structures were realized due to the formation of the polymer network in the surface layers the filler surface, as usually happens in filled systems. As is known79, this induces considerable changes in the structure of the material. It is also possible that in these conditions a more defective network structure is formed. These results show that even the purely physical factors influencing the formation of the polymer network in the interface lead to such changes in the relaxation behavior and fractional free-volume that they cannot be described within the framework of the concept of the iso-free-volume state. It is of great importance that such a model has been devised for a polymer system that is heterogeneous yet chemically identical. 

Network structures have been quantitatively determined by means of real-time XH NMR T2 relaxation experiments for several polymers [174-178]. The effect of the curing conditions on sol and gel fractions and the spatial heterogeneity of the network structure has been studied for polyethylene [174], polyacrylamide [175], PDMS [176], BR [177], epoxy resins [178] and EPDM [179]. 

Network structure analysis is discussed in Chapters 7, 8,10 and 13. These chapters deal with the characterisation of the structure of chemical and physical networks, rubber-filler physical network, network defects and its heterogeneity using NMR relaxation techniques and NMR imaging. 

In the toluene-modified network the heterogeneity of the structure can be increased in two independent ways by a decrease in volume fraction of monomers (F, ) or by an increase in DVB content of the monomer solution. Thus, a toluene-modified network prepared with 15% DVB content (15% tol resin) has an expanded network structure and the network obtained by using 27 % DVB content (27 % tol resin) has a structure which is intermediate between an expanded and a macroporous structure. The network prepared with 34% DVB content (34% tol resin) and 55% DVB content (55 % tol resin) contain an appreciable portion of their porosity in the form of macropores... 

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