Cross-linking covalent

Two other important commercial uses of initiators are in polymer cross-linking and polymer degradation. In a cross-linking reaction, atom abstraction, usually a hydrogen abstraction, occurs, followed by termination by coupling of two polymer radicals to form a covalent cross-link ... 

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. 

Fig. 6.2. How Young s modulus increases witl) increasing density of covalent cross-links in polymers, including rubbers above tbe glass temperature. Below To, be modulus of rubbers increases markedly because tbe Van der Waals bonds take hold. Above Tq they melt, and the modulus drops.

Many of the most floppy polymers have half-melted in this way at room temperature. The temperature at which this happens is called the glass temperature, Tq, for the polymer. Some polymers, which have no cross-links, melt completely at temperatures above T, becoming viscous liquids. Others, containing cross-links, become leathery (like PVC) or rubbery (as polystyrene butadiene does). Some typical values for Tg are polymethylmethacrylate (PMMA, or perspex), 100°C polystyrene (PS), 90°C polyethylene (low-density form), -20°C natural rubber, -40°C. To summarise, above Tc. the polymer is leathery, rubbery or molten below, it is a true solid with a modulus of at least 2GNm . This behaviour is shown in Fig. 6.2 which also shows how the stiffness of polymers increases as the covalent cross-link density increases, towards the value for diamond (which is simply a polymer with 100% of its bonds cross-linked. Fig. 4.7). Stiff polymers, then, are possible the stiffest now available have moduli comparable with that of aluminium. 

The conventionally covalently cross-linked rubbers and plastics cannot dissolve without chemical change. They will, however, swell in solvents of similar solubility parameter, the degree of swelling decreasing with increase in cross-link density. The solution properties of the thermoelastomers which are two-phase materials are much more complex, depending on whether or not the rubber phase and the resin domains are dissolved by the solvent. 

With a typical of 25 000-30000 the molecular size is low compared wjth most conventional covalently cross-linked elastomers. With such rubbers values of about 100000 are desirable so that the effects of a significant amount of non-load-bearing chain ends do not occur. Such a problem does not arise in block copolymers terminated by hard segments. 

The thermoplastic rubbers have properties similar to those of the cast polyurethane rubbers but, because of the absence of covalent cross-links, have rather higher values for compression set, a common problem with thermoplastic rubbers. Their main uses are for seals, bushes, convoluted bellows and bearings. 

Handel, S.E., Hendry, K.A.K., Sheterline, P. (1990). Microinjection of covalently cross-linked actin oligomers causes disruption of existing actin filament architecmre in PtK2 cells. J. Cell Sci, 97, 325-333. 

Other clues to the self-association of recombinant resilin in solution, and thus a degree of defined stmcture, include the propensity of the monomer proteins to covalently cross-link very rapidly through dityrosine side chains using a mthenium-based photochemical method [29]. Proteins which do not naturally self-associate do not form biomaterials when exposed to the Ru(ll)-based photochemical procedure (Elvin, C.E. and Brownlee, A.G., personal communication). Furthermore, Kodadek and colleagues showed that only intimately associated proteins are cross-linked via this zero-A photochemistry procedure [45]. 

Andersen, S.O., Covalent cross-links in a structural protein, resilin, Acta Physiol. Scand. Suppl., 263, 1-81, 1966. 

Elasticity measurements can serve as a measure of the degree of interconnection in gels. Covalently cross-linked networks can be distinguished from physically cross-linked networks by the use of a technique termed mechanical spectroscopy [333]. Compression of gels has also been used to assess the physical structure [28,168,303]. 

Collagen triple helices are stabilized by hydrogen bonds between residues in dijferent polypeptide chains. The hydroxyl groups of hydroxyprolyl residues also participate in interchain hydrogen bonding. Additional stability is provided by covalent cross-links formed between modified lysyl residues both within and between polypeptide chains. 

The pectin network revisited.--The importance of the interconnections of the pectic polysaccharides to the integrity of the pectin network has been highlighted by the recent discovery that RG-II is present in primary walls as a mixture of monomers and dimers [54]. The dimers are covalently cross-linked by borate diesters [55,56]. If single molecules of homogalacturonan are covalently attached to both RG-I and RG-II, the covalently cross-linked RG-n dimers would explain how the network of the three types of pectic polysaccharides is covalently connected and covalently cross-linked. 

The two previous sections covered gels whose structure is formed through covalent cross-links these gels are basically gigantic molecules of macroscopic di-... 

Simple physical entanglements can be sufficient to produce a structurally stable gel if the polymer has a sufficiently great molecular weight and if the polymer is of only modest hydrophilicity. In this case, the polymer will swell in water without dissolving, even in the absence of covalent cross-links. Poly(2-hydroxyethyl methacrylate) (PHEMA) is a prominent example of this type of hydrogel when uncross-linked, it will dissolve in 1,2-propanediol but only swell in water. 

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