Physical cross-links types

B. Type 3 physical cross-linked gels, agarose... 

There are three types of cross-linking one is chemical and two physical. Physical cross-linking results from formation of crystalline regions within polymer structures and from chain entanglement. Cross-linked materials have good dimensional memory. Chemically cross-linked materials do not dissolve and do not melt. 

There are certain advantages and disadvantages to this type of material compared to conventional elastomers. The advantages concern the existence of reversible physical cross-links. When a TPE is heated, the cross-links disappear if the temperature increases beyond the Tg of the hard phase or above the of the hard domain if it is crystalline. In contrast, conventional elastomers display thermostable structure. The physical nature of... 

The discussion above has been limited to amorphous polymers. However, if the polymer is semicrystalline, the dotted line in Figure 1.19 is followed. Since the crystalline regions in the polymer matrix tend to behave as a filler phase and also as a type of physical cross-link between the chains, the height of the plateau (i.e., the modulus) will be governed by the degree of crystallinity]... 

Used by the presence of positive counter-ions such as Na or K. The ionic groups tend to form clusters and, if ionic groups are attached to a small number of the chains of both types of polymer in a blend, mixed clusters can be formed and act as physical cross-links. 

The autohesion effect is especially good, therefore, when weak crystallization occurs on applying pressure or during annealing, as, for example, with natural rubber or with l,5-trans-poly(pentenamers) (physical cross-linking). On the other hand, if the crystallization is too strong, the deformability of the adhesive is too small (see Section 7.4.2). If adherent and adhesive are chemically different, then in the EfE type this leads to interdiffusion and thence to heteroadhesion. Of course, marked interdiffusion is only possible when the different macromolecules are compatible with one another, and the strength of the autoadhesion or heteroadhesion depends on both diffusion and adsorption. 

Block copolymers of the type (sty) -(bu) -(sty) form thermoplastic elastomers, i.e., physically, and therefore reversibly, cross-linked products (see Section 5.5.4). They are produced by anionic polymerization, since the physical cross-linking properties can only be achieved by long, molecularly homogeneous blocks, and thus only with specific ratios of m/n. 

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