Crosslinking matrices

Many highly crosslinked polymers have been synthesised and reported since the emergence of Bakelite as a synthetic moulding material [6, 7]. However, to keep the subject matter of this chapter within reasonable bounds, we have restricted our discussions of filled polymers to examples of the more common crosslinked matrices, unsaturated polyesters, epoxies, methacrylics and phenolics. The resins are cured by different mechanisms ... 

The thermal stability, also, depends on the types of side groups present. Amino side groups usually condense at high temperatures to yield crosslinked matrices. 

The second type of polymer, unsaturated polyanhydrides of the structure [-(OOC-CH=CH-CO)x-(OOC-R-CO)y-]n, have the advantage of being able to undergo a secondary polymerization of the double bonds to create a crosslinked matrix. This is important for polymers requiring great strength, for instance. These polymers were prepared from the corresponding diacids polymerized either by... 

The size and shape of polymer chains joined in a crosslinked matrix can be measured in a small angle neutron scattering (SANS) experiment. This is a-chieved by labelling a small fraction of the prepolymer with deuterium to contrast strongly with the ordinary hydrogenous substance. The deformation of the polymer chains upon swelling or stretching of the network can also be determined and the results compared with predictions from the theory of rubber elasticity. 

Macroporous epoxies prepared via the CIPS technique represent an alternative for determining the volume fraction, c ), very accurately from the densities of the porous material, and the neat, fully crosslinked matrix, p, according to... 

Dimethacrylate monomers were polymerized by free radical chain reactions to yield crosslinked networks which have dental applications. These networks may resemble ones formed by stepwise polymerization reactions, in having a microstructure in which crosslinked particles are embedded in a much more lightly crosslinked matrix. Consistently, polydimethacrylates were found to have very low values of Tg by reference to changes in modulus of elasticity determined by dynamic mechanical analysis. 

Also, mechanical data on the influence of low volume fractions (0.03-0.05) of rigid filler particles provide evidence of a localized plastic deformation which would not seem understandable by reference to a uniformly crosslinked network. A non-uniformly crosslinked matrix might also be invoked to account for insensitivity of the rate of diffusion of water on the apparent degree of crosslinking. However, an observed increase in the uptake of water with apparent degree of crosslinking remains unexplained. 

The diffusion coefficient for water was found to be rather insensitive to the proportion of the dimethacrylate crosslinker, especially in sorption (Fig. 7). This insensitivity might be rationalized in a number of ways. One of these is consistent with the view that these systems have a lightly crosslinked matrix which does little to Impede the diffusion of water molecules. 

Because of its chemical inertness, no direct way of curing poly(thiocarbonyl fluoride) has been found. However, creep has been reduced and strength at elevated temperatures improved by milling into the polymer a free-radical generator, such as dicumyl peroxide or azobisisobutyronitrile, and a free-radical acceptor, such as N,N -m-phenylenebismaleimide or triacryloylhexahydro-s-triazine, and curing with heat and pressure (65). A better method is to mill in divinylbenzene and a small amount of benzoyl peroxide and cure with heat and pressure (66). The divinylbenzene forms a crosslinked matrix that mechanically traps poly(thio-carbonyl fluoride) molecules. Since the elastomer is in effect filled with poly(di-vinyl benzene), the final composition is less resilient than untreated poly(thio-carbonyl fluoride). 

In the second stage a crosslinking agent, e.g., l,4,4a,5,8,8a-hexa-hydro-l,4,5,8-dimethanonaphthalene is added, which forms the crosslinked matrix. A polymer is formed where the linear oligomer chains are fixed at the crosslinked matrix as pendent groups. 

In contrast, diallylamino-substituted dyes copolymerized poorly with MMA, despite the reported polymerizability of other aliphatic (24-25) and aromatic (26) diallylamines. The concentration of dye-bearing repeat units m the polymer was far below the monomer feed concentrations. The possibility that these diallyl dye-monomers (or some unidentified impurity in them) acted as inhibitors of polymerization can be ruled out because the dye was found to be present uniformly in all molecular weight fractions and the molecular weights of the copolymers were again nearly identical to control samples of PMMA. Therefore, only a small amount of the diallyl amino substituted chromophores can be covalently incorporated into the crosslinked matrix because of the apparently unfavorable reactivity ratio. The balance of the chromophore remains simply dissolved in the mixture. 

To increase the density of NLO-active chromophores in the crosslinked matrix, it was desirable to completely omit the inactive binder 7 and make the NLO dyes themselves photocrosslinkable. Therefore, we synthesized and studied chromophores substituted with two acrylate or methacrylate groups (27). 

Unsaturated polyester resins are produced by a crosslinking reaction in solution of a prepolymer dissolved in styrene initiated with peroxides. This curing reaction renders an insoluble and infusible crosslinked matrix with high rigidity, but fragile. This matrix is usually reinforced with glass fibers or fillers of micrometric size. Since the mechanical properties of such materials depend strongly on... 

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