Cross-linked, network-type polymers

Solution Properties. Zwitterionic polymers show interesting aqueous solution behavior. As a general rule, they are insoluble in pure water due to the formation on intra- and interchain ion contacts resulting in an ionically cross-linked network-type structure. Polyampholytes and polybetaines which are not soluble become soluble upon the addition of low molecular weight electrolytes, such as NaCl (Fig. 51). This dissolution process can best be understood in terms of the low molecular weight electrolyte penetrating the ionically cross-linked network whereupon the ions screen the net attractive interactions between the polymer chains and hence promote solubility. The addition of the salt also results in an-tipolylelectrolyte behavior, ie chain expansion upon the addition of the salt. 

The commonly used resins in the manufacture of decorative and industrial laminates ate thermosetting materials. Thermosets ate polymers that form cross-linked networks during processing. These three-dimensional molecules ate of essentially infinite size. Theoretically, the entire cured piece could be one giant molecule. The types of thermosets commonly used in laminates ate phenoHcs, amino resins (melamines), polyesters, and epoxies. 

The hydroxyfunctional hyperbranched polyesters have been characterized with respect to their mechanical and theological properties, both as thermoplastics and in cross-linked networks. The high number of terminal groups in hyperbranched polymers has a large impact on the properties, and also makes it easy to functionalize the polymers for various applications. One option is to attach reactive groups at chain ends, forming a cross-linkable polymer. Variations in functionality and the type of functional groups will affect both the polymer properties and the final cross-linked material properties. 

Vulcanization of a rubber or an elastomer consists in creating chemical cross-links among the polymer chains in order to form a three-dimensional network. There are various types of cross-linking agents, and the most commonly used are described below. The process of vulcanization with sulfur is the most widely used. It is easy to obtain a broad variety of... 

Hydrogels are another type of polymer structure comprised of a hydrophilic cross-linked network that swells in water. They can exist as homopolymers, copolymers, or multipolymers and are generally biocompatible and have low degradation. Hydrogels can be produced with a wide range of swelling... 

As indicated above, multifunctional monomers, instead of merely forming branches, can link main chains together to such an extent that the molecules are transformed into network type structures, known as cross-linked polymers, as shown in Fig. 3.3. This is especially likely if one of the monomers has a functionality of four, such as divinylbenzene. These cross-linked networks can be two-dimensional, but most often are three-dimensional. 

A polymer is composed of repeating units (i.e., monomers) that are linked together into long chains that can be linear, branched, or cross-linked. If a polymer contains two different types of monomers, it is a copolymer. A linear polymer is a thermoplastic. At elevated temperatures it melts and flows as a liquid. In a cross-linked polymer, the repeat units are actually linked into a three-dimensional network of macroscopic size. It is a thermoset. Once the polymerization is completed, the cross-linked polymer cannot be softened or melted. It is hard, infusible, and insoluble. Hence a thermoset adhesive is the most durable but is also difficult to characterize as compared to a thermoplastic one. 

Matejka et al. synthesized several types of photochromic polymers based on a copolymer of maleic anhydride and styrene with azobenzene moieties both in the side chains and cross-links of the polymer network (Matejka et al., 1981 Matejka and Dusek, 1981 Matejka et al., 1979). The photomechanical effect was enhanced by an increase in the content of photochromic groups, and the photoinduced contraction of the sample amounted to 1% for a polymer with 5.4mol% azobenzene moieties. 

The structures listed in Table 1.5 are divided into three categories short sequences, long sequences, and networks. Within the first category, a sequence of placement of individual CRU is considered, within the second the placement of long sequences of CRU defines the copolymer type, while to the third belong cross-linked networks, cross-linked polymers, and chemical-type interpenetrating polymer networks. The network is a cross-linked system in which macromolecules of polymer A are cross-linked by macromolecules of polymer B (Sperling 1992). The composition can be expressed as, e.g., block-co-poly(butadiene/styrene) (75 25 wt%) or grq/if-co-poly[isoprene/(isoprene acrylonitrile)] (85 15 mol%). 

The cross-linking of each polymer network is simply controlled by the type and concentration of the cross-linker employed in the system. The higher the crosslinker level (especially in polymer network 1), the finer the morphology. 

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