Crosslinked network systems, densely

DSC studies have shown that multifunctional monomers react quickly to form densely crosslinked networks from liquid monomer solutions. However, even a small amount of unreacted monomer can effectively plasticize a crosslinked network, rendering it more pliable. Eor this reason, mechanical analysis was combined with DSC studies to characterize the physical changes occurring in the proposed dimethacrylate system as polymerization proceeds. Static compression tests (Perkin-Elmer, DMA7e) were completed on disks (d = 11.5 mm, t = 1.7) immediately after they were irradiated for varied times. 

At the end of the ft peak, the entropies (Table 10) are smaller in the DMH-MDA95 and 60 systems than in the HMDA network, indicating that cooperativity is likely to have a larger spatial extent when it proceeds from the crosslink points. It thus appears that the secondary diamines lead to an enhancement of the mobility and a development of a cooperativity that is more limited than the one observed in densely crosslinked networks. 

In the HA60/AP19 system (Fig. 100b), the effect of antiplasticiser is intermediate between the behaviours observed in the densely crosslinked network and quasi-linear system, because some residual cooperative motions still occur in the pure network and can be hindered by the presence of the antiplasticiser. 

CROSSLINKED NETWORK MODEL CALCULATIONS Our research has shown that tetrathiol crosslinked norbomene resins form a densely crosslinked, three dimensional network. Recently there has been considerable interest in crosslinked networks from a theoretical and practical point of view (29-31). As part of our study we attempted to analyze the polymer network using the Miller-Macosko formalism as applied by Bauer (32). For the purposes cf this analysis we assumed that the curable formulation was an A2 (ene) B4 (thiol) type system. We also assumed, based on HPLC analysis of EBPA DN and acrylate precursor batches that the norbomene resins was a mixture of oligomers consisting of difunctional olefin (85%) and monofimctional olefin (15%). The thiol crosslinker was assumed to be essentially tetrafunctional. Furthermore, we made the not unreasonable assumption that there would be no thiol-thiol or norbornene-norbornene reactions. In one case, a chain extending... 

Existing theories for crosslinking polymerization cannot account for the observed inhomogeneities formed in more densely crosslinked systems Since most of the applications to be reported here are related to the formation of densely crosslinked networks, and since the question of the kinetics of formation of these networks, has hardly been addressed in the literature, we will discuss a few peculiarities of these systems. At present, it is not possible-to give a complete picture since our knowledge of these systems is still limited. 

Dense polymer systems, such as melts, glasses, and crosslinked networks or rubber are extremely complex materials. Besides the local chemical interactions and density correlations which are common to all disordered hquids and solids, the chain conformations also play an important role. Their influence is twofold. First the intrachain entropy dominates over the positional entropy of the center of mass of the chains. This leads to the well known effect that a weak effective repulsion between different types of chains is sufficient to drive phase segregation. The static and dynamic properties of mixtures of two types of chains is an important and challenging problem, which is reviewed by Binder in Chapter 7 of this volume. Here we consider dense melts of ehains of the same chemical composition. In this case the entropy is at its maximum when the chains have a random walk structure. Since the average end-to-end distance Nfor a random... 

In the case of epoxy networks with a secondary diamine, like DMHMDA, the network architecture is such that flexible aliphatic sequences are present as chain extenders between the crosslink points. In such architectures, the motions of the HPE units can develop towards other HPE sequences (either along the chain or spatially neighbouring) without involving the crosslink points in their cooperativity. Thus, with these systems a different nature of cooperativity exists compared to the other network architectures. The introduction of an antiplasticiser in such a local packing does not affect the cooperativity as much as with the densely crosslinked architecture, for the crosslinks are not so much involved. Once more, it is important to point out that the flexible nature of the aliphatic amines does not matter since the same behaviours are observed for fully aromatic systems with identical architecture [68]. 

Thus, the level of sophistication which one may consider for the application of rubber-like elasticity theory to epoxy networks may depend on the application. For highly crosslinked systems (M < 1,000), a quantitative dependence of the rubbery modulus on network chain length has recently been demonstrated , but the relevance of higher order refinements in elasticity theory is questionable. Less densely crosslinked epoxies, however, are potentially suitable for testing modern elasticity theories because they form via near quantitative stepwise reactions. Detailed investigations of such networks have been reported by Dusek and coworkers in recent studies ... 

The LSCEs represent a new class of macromolecular systems distinguished by macroscopically uniform anisotropy. The concept of LSCE can be furthermore extended to other synthesis routes and to the densely crosslinked systems. Macroscopic uniaxially oriented films can be formed by mechanical force, alignment surface [30], magnetic and electric fields [31], polarized light [32], etc., and then crosslinked to form an anisotropic network (LC network is abbreviated as LCN) if the mixture components contain polymerizable or crosslink-able bifunctional monomers (Figure 9.13). Alternatively, amorphous or liquid crystalline side-group and/or main-chain polymers incorporating additional... 

Anisotropic gels are a mixture of a solid crosslinked LC network and non-reactive low molar mass LCs [33] produced by the in situ polymerization of LC monomers with two polymerizable groups, such as diacrylates, in the presence of low molar mass LCs without reactive units. The resultant solid polymer network incorporates the three-dimensional structure of the original LC mixture. Main aspects around LC networks, especially those involving densely crosslinked solids, that arose from low molar mass bifunctional LC monomers are stated in Chapter 10. Here the description focuses only on some specific and innovative system, especially for the networks formed by the moderate crosslinking of preformed linear polymers. 

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