Highest crosslinking

The apparent crosslink densities for the vulcanized samples are shown in Fig. 21. SPTh shows the highest crosslink density all other crosslink densities are significantly lower and show only minor differences. 

With neat hexamethylene diamine (HMDA), each amine group leads to a crosslink and, consequently, DGEBA/HMDA networks have the highest crosslink density. 

We have shown that for an epoxy system cured at elevated temperature, the highest crosslink density is obtained at mix ratios (expressed in equivalents) H/R > 1.0. At smaller mix ratios a strong dependence of Tg on composition and degree of cure is observed. 

In this paper we address the area of the structure-property relations of epoxy matrices, with specific emphasis on amine-cured epoxides. In attempts to correlate the structure-property relations of amine-cured epoxides, there have been a number of studies on the mechanical properties of these glasses as a function of epoxide amine ratios and the chemical structure of the constituent epoxide and amine monomers.(1-15) Generally, there is no direct correlation between the chemistry of the epoxide and amine monomers and the mechanical properties of epoxies with the exception that as the distance between crosslinks becomes shorter, these glasses become more brittle.(1,10,15) Also, for a specific amine-cured epoxide system, the Tg is always highest for the fully reacted, highest crosslink density glass.(3,6,9,12,... 

They evaluated this order and thus the crosslink densities were determined and showed that the highest crosslink density possessed by the DCP membrane results in the lowest gas permeability and the lowest crosslink density possessed by the EV membrane exhibited the highest gas permeability. In fact as the number of crosslinks per unit volume of the polymer molecules increases, it becomes very difficult for the gas molecules to pass through the tightly cross-linked system. Table 24.6 shows the permeability (P) and crosslink density (v) values for a blend of 50/50 SBR/NR. 

It has been found that tertiary amines are catalysts that not only affect the rate of the anhydride-epoxide reaction, but its course as well [82,83]. Dearborn and coworkers [84], for example, report that the highest crosslinking in amine-catalyzed anhydride-epoxide resin cures was obtained at a 1 1 anhydride expoxide ratio. This implies that the amine largely or wholly suppresses the undesirable side reaction of epoxide polymerization. This method of curing is potentially a powerful tool in polymer chemistry, since in its broadest sense it enables one to prepare a linear polyester containing at regular intervals along the chain various substituents available for further reactions. 

Figure 23.9 Behavior of human dermal fibroblasts after 5-days growth on coUagen-coated silicone substrates where substrate A is control formulation, B is the medium crosslink density formulation, and C is the highest crosslink density formulation. Magnification is at lOX.

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