Cross-linked polymer network

Polymers with the mechanical and chemical properties we have discussed in this section are called elastomers. In the next couple of sections we shall examine the thermodynamic basis for elasticity and then apply these ideas to cross-linked polymer networks. 

Hyperbranched polyurethanes are constmcted using phenol-blocked trifunctional monomers in combination with 4-methylbenzyl alcohol for end capping (11). Polyurethane interpenetrating polymer networks (IPNs) are mixtures of two cross-linked polymer networks, prepared by latex blending, sequential polymerization, or simultaneous polymerization. IPNs have improved mechanical properties, as weU as thermal stabiHties, compared to the single cross-linked polymers. In pseudo-IPNs, only one of the involved polymers is cross-linked. Numerous polymers are involved in the formation of polyurethane-derived IPNs (12). 

So far, we have not introduced a specific model of the polymer network chains. This problem can be rigorously solved for cross-linked polymer networks consisting of phantom chains [13], or even in the more general case of filled networks where the chains interact, additionally, with spherical hard filler particles [15]. 

PJ Flory, R Rehner Jr. Statistical mechanics of cross-linked polymer networks. J ChemPhys 11 521-525, 1943. 

Hydrogels are 3D cross-linked polymer networks. They can withstand acid conditions and release the entrapped drug molecules. Purdue University researchers have used a poly[methacrylic acid-g-poly(ethylene glycol)] hydrogel to encapsulate insulin, which could be released by pH trigger. 

Figure 5.15 Examples of linker cross-linked polymer networks exhibiting liquid crystalline polymer characteristics.

Figure 5.20 Examples of metal cross-linked polymer networks.

Kokufuta, Zhang and Tanaka developed a gel system that undergoes reversible swelling and collapsing changes in response to saccharides, sodium salt of dextran sulfate (DSS) and a-methyl-D-mannopyranoside (MP) [126]. The gel consists of a covalently cross-linked polymer network of W-isopropylacrylamide into which concanavalin A (ConA) is immobilized. As shown in Fig. 31, at a certain temperature the gel swells five times when DSS ions bind to ConA due to the excess ionic pressure created by DSS. The replacement of the DSS by non-ionic MP brings about collapse of the gel. The transition can be repeated with excellent reproducibility. 

P. J. Flory and J. Rehner, Statistical Mechanics of Cross-Linked Polymer Networks I. Rubberlike Elasticity, J. Chem. Phys., 11, 512-520 (1943). 

Chemical gels are covalently cross-linked polymer networks, featuring very high viscosity and well-defined pore structure. Polyacrylamide is the most widely used chemical gel material, usually cross-linked with N,N-methylene-bisacryl-amide (BIS). The pore size of the gel is determined by the relative concentration of monomer and cross-linker used during polymerization (%T, total monomer concentration and %C, cross-linker concentration as a percent of the total monomer and cross-linker concentration [34]). Highly cross-linked ( 5%C) poly-... 

Capillary SDS-gel electrophoresis is a rapid automated separation and characterization technique for protein molecules and is contemplated as a modern instrumental approach to sodium dodecylsulfate-polyacrylamide slab-gel electrophoresis (SDS-PAGE). Size separation of SDS-protein complexes can be readily attained in coated capillaries filled with cross-linked gels or non-cross-linked polymer networks. Figure 9 depicts one of the early applications of the technique for the analysis of a standard protein test mixture ranging in size from 14.2 to 205 kDa. 

Induced porosity in cross-linked polymer networks Mean field theory and simulations... 

Figure 3.6 Schematic of a cross-linked polymer network. (Reprinted from Pierre-Gilles de Gennes, Scaling Concepts in Polymer Physics. Copyright 1979, by Cornell University. Used by permission of the publisher, Cornell University Press.)...

Entrapment, also called inclusion, occlusion, and lattice entrapment, involves the formation of a highly cross-linked polymer network in the presence of an enzyme, so that the enzyme is trapped in interstitial spaces. Smaller species, such as substrates and products, freely diffuse through the polymer network, while the large... 

The reactive epoxies form a tight, cross-linked polymer network and are characterized by toughness, good adhesion, corrosive-chemical resistance, and good dielectric properties. 

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