Polymer cross-linking, hydrogen-bonded

When polymer molecules are completely dissolved in solution, the solution behaves as a liquid. Liquids, for example, continuously deform in response to a continuously applied stress. Polymers can also form gels, which do not behave as liquids. The individual polymer chains in a gel form a continuous network within the liquid phase. The network is maintained by chemical or physical interactions between polymer ehains these interactions may involve covalent cross-links, hydrogen bonds, or physical entanglement of the molecules. The network restricts the response of the gel to an applied stress. As a result, the mechanieal properties of a gel are qualitatively different from the mechanical properties of a fluid. 

The structure of repeat units of individual polymers constituting a blend and the nature of interactions between polymers in a blend are the factors that influence solubility characteristics of a blend. Thus, soluMlity is affected by cross-linking, hydrogen bonding, formation of donor-acceptor complexes, dipole-dipole interactions, ion-dipole interactions, ion-ion interactions, and segmental interactions. 

Furthermore, crystalline polymers do obey the rules even at room temperature in so far as swelling behaviour is concerned. This again is a demonstration that crystalline regions serve as physical cross-links. Some crystalline polymers with strong hydrogen bonding groups can be made to dissolve at room temperature. But in these cases a very specific interaction between polymer and solvent must occur. For example, cellulose is soluble in 70% sulphuric acid and in aqueous ammonium thiocyanate nylon 6.6 is soluble in phenol and in a 15% calcium chloride solution in methanol. 

Hydrogen bonding stabilizes some protein molecules in helical forms, and disulfide cross-links stabilize some protein molecules in globular forms. We shall consider helical structures in Sec. 1.11 and shall learn more about ellipsoidal globular proteins in the chapters concerned with the solution properties of polymers, especially Chap. 9. Both secondary and tertiary levels of structure are also influenced by the distribution of polar and nonpolar amino acid molecules relative to the aqueous environment of the protein molecules. Nonpolar amino acids are designated in Table 1.3. 

The use of TAG as a curing agent continues to grow for polyolefins and olefin copolymer plastics and mbbers. Examples include polyethylene (109), chlorosulfonated polyethylene (110), polypropylene (111), ethylene—vinyl acetate (112), ethylene—propylene copolymer (113), acrylonitrile copolymers (114), and methylstyrene polymers (115). In ethylene—propylene copolymer mbber compositions. TAG has been used for injection molding of fenders (116). Unsaturated elastomers, such as EPDM, cross link with TAG by hydrogen abstraction and addition to double bonds in the presence of peroxyketal catalysts (117) (see Elastol rs, synthetic). 

It may also be argued that plasticised PVC may be considered as a thermoplastic elastomer, with the polymer being fugitively cross-linked by hydrogen bonding via the plasticiser molecules. These materials were, however, dealt with extensively in Chapter 12 and will not be considered further here. The ionomers are also sometimes considered as thermoplastic elastomers but the commercial materials are considered in this book as thermoplastics. It should, however, be kept in mind that ionic cross-linking can, and has, been used to fugitively crosslink elastomeric materials. 

Cross-linking or curing, i.e., forming covalent, hydrogen, or other bonds between polymer molecules, is a technique used very widely to alter polymer properties. The hrst commercial method of... 

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