Cross linking in solution

These observations can be qualitatively explained in terms of the constrained-junction theory. If a network is cross-linked in solution and the solvent then removed, the chains collapse in such a way that there is reduced overlap in their configurational domains. It is primarily in this regard, namely reduced chain-junction entangling, that solution-cross-linked samples have simpler topologies, and these diminished constraints give correspondingly simpler elastomeric behavior. 

The most recent experimental work to settle the controversy regarding the factor B is the swelling pressure work of Van de Kraats (105,106). This author used poly( >-nitrophenyl methacrylate) gels, cross-linked in solution by aminolysis with hexamethylene diamine. A typical example of his results is shown in Fig. 14, where it is seen that % is a constant over a range in polymer concentration from 5 to 20%. 

Villanueva A, Canete M, Trigueros C, Rodriguez-Borlado L, Juarranz A (1993) Photodynamic induction of DNA-protein cross-linking in solution by several sensitizers and visible light. Biopolymers 33 239-244... 

There has been some interest in cross-linking in solution, since the network chains in the subsequently dried elastomer are super compressed. These materials can have some unusual properties, including unusually high extensibility.87 89... 

The use of monomers bearing more than two associating groups is a straightforward way to introduce a controlled amount of branches or crosshnks in a supramolecular polymer structure [6,58,121,123-127]. The improvement of the mechanical properties can be spectacular. For instance, trifunctional monomer 17 (Fig. 21) forms highly viscous solutions in chloroform, and is a viscoelastic material in the absence of solvent [124]. The reversibly cross-linked network displays a higher plateau modulus than a comparable covalently cross-linked model. This is explained by the fact that the reversibly cross-hnked network can reach the thermodynamically most stable conformation, whereas the covalent model, which has been cross-linked in solution and then dried, is kinetically trapped. 

Figure 7.1 schematically shows the preparation of networks by cross linking in solution followed by removal of the solvent. Success in obtaining elastomers with fewer entanglements is supported by the observation that such networks come to elastic equilibrium much more rapidly than elastomers cross linked in the dry state. Table 7.1 shows results on PDMS networks cross linked in solution by means of y radiation. - Note the continual decrease in the time required to reach elastic equilibrium, and in the extent of stress relaxation as measured by the ratio of equilibrium to initial values of the reduced stress, [f ], upon decrease in the volume... 

Cross linking in solution to prepare networks of simpler topology. XL is the cross-linking step done in solution. 

Aspects of greatest importance appear to be (i) investigating the effects of cross linking in solution, (ii) studying the effects of swelling on networks cross linked in the bulk (dry) state, (iii) building on the demonstration by... 

Erman, B. Mark, J. E., Interpretation of Stress-Strain Isotherms for Elastomers Cross-Linked in Solution. Macromolecules 1987,20, 2892-2897. 

Xu, P. Mark, J. E., Elasticity Measurements on Bimodal Networks in Elongation and Compression. Networks Cross-linked in Solution and Studied Unswollen, and Networks Cross-linked in the Undiluted State and Studied Swollen. Polymer 1992,33,1843-1848. 

Block copolymers are useful in many applications where a number of different polymers are connected together to yield a material with hybrid properties. For example, thermoplastic elastomers are block copolymers containing a rubbery matrix (polybutadiene or polyisoprene) containing glassy hard domains (often polystyrene). The block copolymer, a kind of polymer alloy, behaves as a rubber at ambient conditions, but can be molded at high temperatures because of the presence of the glassy domains that act as physical cross-links. In solution, attachment of a water-soluble polymer to an insoluble polymer leads to the formation of micelles in amphiphilic block copolymers. The presence of micelles leads to structural and flow characteristics of the polymer in solution, that differ from either parent polymer. 

The synthesis of hydrido-amino phosphazenes can be carried out directly from aminophosphines. Thus, the dialkylamino(amino)phosphine (Me2N)2PNH2 (generated in situ by amminolysis of the corresponding chlorophosphine) underwent a fast kinetically controlled polycondensation process without cross-linking in solution at low temperature to form the low P-hydrido(dimethylamino)polyphosphazene [N P(H)(NMc2)] (186) with absolute = 41.000 (PDI = 1.5) in a planar cis-trans or twisted helical conformation. The proposed mechanism for the polymerization was supported by ab initio calculations on the model (H2N)2PNH2 reported in ref. 1 (see above). 

Figure 2. Effect of cross-linking in solution on molecular deformation. 

Closed loops occur when a chain cross-links to itself. The number of such loops can be estimated (Walsh et a/., 1974 Allen et a/., 1974) but it is difficult to estimate their influence on the elastic properties. The closed loop effect is not negligible when cross-linking in solution. 

Pfansteil and Her demonstrated that as the K2O-P2O5 ratio is reduced from 0.998, the influence of cross-linking in solutions of Kurrol s salt diminishes, if it is not eliminated entirely. The behavior expected for very long linear chains persists after the expected behavior for cross-linkages is gone, as judged from viscosities of aqueous solutions obtained from salts with K2O--P2O5 ratios well below 0.98. 

The cases where the filler causes compression of the chain are relevant to another area of rubberlike elasticity, specifically the preparation of networks by cross linking in solution followed by removal of the solvent [57]. This is shown schematically in Figure 14.13. Such experiments were initially carried out to obtain elastomers that had fewer entanglements and the success of this approach was supported by the observation that such networks came to elastic equilibrium much more rapidly. They also exhibited stress-strain isotherms in elongation that were closer in form to those expected from the simplest molecular-theories of rubberlike elasticity. 

Figure 14.13. Foniiiiig a super-compressed network by cross linking in solution, followed by drying...

Cross-Linking in Solution. This approach (5) did produce elastomers that had fewer entanglements, as indicated by the observation that such networks came to elastic equilibrium much more rapidly than elastomers cross-linked in the dry state. When a network is cross-linked in solution and the... 

---