Solvent swelling cross-linked polymer

Similarly, polymers dissolve when a solvent penetrates the mass and replaces the interchain secondary bonds with chain-solvent secondary bonds, separating the individual chains. This cannot happen when the chains are held together by primary covalent cross-links. Thus, linear and branched polymers dissolve in appropriate solvents, whereas cross-linked polymers are insoluble, although they may be swelled considerably by absorbed solvent. 

These highly amorphous elastomers have relatively low Tt values (—73 C) and tend to crystallize when stretched. The cold flow of these thermoplastic polymers is reduced when they are crosslinked (vulcanized) with a small amount (2%) of sulfur. Since these polymers of isoprene have a solubility parameter of 8.0 H, they are resistant to polar solvents but are soluble in many aliphatic and aromatic hydrocarbon solvents. The cross-linked derivatives swell but do not dissolve in these solvents. 

To be useful as a catalyst support, the gelular polymers must be swellable, the propensity of which can be controlled by the degree of cross-linking. Polymers can be tailor-made to swell in a particular solvent by appropriate selection of monomer and cross-linking agent. 

FIG. 7.1 Equilibrium swelling as a function of the solubility parameter of the solvent for linear and cross-linked polymer. The cross-link density of 2 is larger than that of 1. 

Other relationships between % and an observable physical property such as osmotic pressure [20, 43], freezing point depression of polymer [20, 52] or solvent [20, 53], and gas liquid chromatography [46-54], were established in like fashion. The relationship determined for swelling of cross-linked polymer to thermodynamic equilibrium in excess liquid has particular significance for the subject of this review. It is given here in the form of the Flory-Rehner equation. 

An alternative approach to anchoring complexes is to embed them in a polymer. The complex is incorporated in the cross-linked polymer using a solvent that produces the maximum swelling. The solvent is evaporated and the surface washed free of excess complex. A second solvent that causes minimum swelling is nsed for reactions, so that the catalyst is effectively caged in the polymer. ... 

They have been combined with FV models and they have been applied to semicrystalline polymer-solvent systems. The results are satisfactory but they are not predictive the and / parameters should be estimated from experimental data. However, the swelling of cross-linked polymers can be estimated with such equations. In one of the very few works reported on the prediction of SLLE for polymer solutions, the Entropic-P/ and UNIFAC models have been compared for semicrystalline polymer-solvent systems. These two models are shown to yield similar results for SLLE. 

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