Bimodal networks example

For chains having fewer than 50 bonds, such as the short chains in a bimodal network, for example, the distribution departs markedly from the Gaussian limit. 

Bulky crosslinks or side-groups in the network chains, e.g., dendritic wedges [73], may also influence molecular mobility and viscoelastic properties of polymer networks. For example, UV curing of difunctional acrylates results in the formation of zip-like network junctions, which may be regarded as extreme cases of bimodal networks [52], Results obtained with the NMR T2 relaxation method agree well with those of mechanical tests... 

For chains having fewer than 50 bonds, such as the short chains in a bimodal network for example, the distribution departs markedly from the Gaussian limit. Among various representations of w r) for short chains are the Hermite series (Flory and Yoon (1974, 1974)), the Fixman-Alben distribution, and Monte Carlo simulations (Erman and Mark, 1988). The Fixman-Alben distribution is given by... 

Studies of bimodal networks have focused elongation because of the simplicity of this type of deformation. Examples of other deformations, equibiaxial extension (compression), shear, and torsion, are discussed later. 

Measurements of the mechanical and optical properties of bimodal networks as a function of temperature and degree of swelling were used to test further the conclusion cited in section 7.3.2.4 that crystallization was not the origin of the improved properties. - 244,246 p j, example,... 

Stress-strain isotherms have also been calculated with this approach. Examples are unimodal networks of polyethylene and POMS, " polymeric sulfur and seleniirm, short n-alkane chains, natural rubber, several polyoxides, and elastin, and bimodal networks of PDMS. It is possible to include excluded volume effects, in such simulations. In the case of the partially helical polymer polyoxymethylene, the simulations were used to resolve the overall distributions into contributions from imbroken rods, once-broken rods, twice-broken rods, and so on. It was also shown how applying stresses to the ends of chains of this typ>e can be used to bias the distributions in the direction of increased helical content and increased average end-to-end distances. In this sense, imposition of a stress has the same effect on the helix-coil equilibriirm as a decrease in temperature. ... 

In the case of such noncrystalUzable, unfilled elastomers, the mechanism for network rupture has been elucidated to a great extent by studies of model networks similar to those already described. For example, values of the modulus of bimodal networks formed by end-linking mixtures of very short and relatively long chains as illustrated in Fig. 6.4 were used to test the weakest-link theory [7] in which rupture was thought to be initiated by the shortest chains (because of their very limited extensibility). It was observed that increasing the number of very short chains did not significantly decrease the ultimate properties. The reason [85] is the very nonaffine... 

The advantage of the swelling method is that it is not limited by the crosslinking reactions of each phase so any interference from these will be limited. A good representative example is the synthesis developed by Hamurcu and Baysal [75]. They synthesized a bimodal PDMS (15 000 gmoD1)/ PDMS (75 000 g mol ) IPN with the same condensation curing system. First, the 75 000 g mol 1 PDMS network was formed from the corresponding a, tw-dihydroxypolydimethylsiloxane and tetraethylorthosilicate catalyzed by stannous 2-ethylhexanoate. It was then swollen in a 15 000 g mol 1 a,a>-dihydroxy-poly(dimethylsiloxane) monomer. The second monomer was then crosslinked via the same condensation cure. The sequential full IPN structure... 

Data on NMR-speetra recorded during homopolymerization of bifunctional (diallyl) monomers (for example, DIAP, DEGBAC) have been published. It is found [26] that the H-NMR-speetrum of the system (in the case of DAIP homopolymerization, initiator [BP] = 3.81%, Treact = 80°C) is bimodal, in which the presence of a wide component in the spectrum is confirmed by formation of a network fraction, and a narrow component is confirmed by a quasi-linear structure. Intensity of the narrow component decreases during polymerization, but that of the wide one increases. Comparison of the correlation of wide and narrow components at different stages of polymerization allows determination of the ratio of fiactions in samples. 

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