Polymers that crosslink

The presence of the unsaturated substituent along this polyester backbone gives this polymer crosslinking possibilities through a secondary reaction of the double bond. These polymers are used in paints, varnishes, and lacquers, where the ultimate cross-linked product results from the oxidation of the double bond as the coating cures. A cross-linked polyester could also result from reaction (5.J) without the unsaturated carboxylic acid, but the latter would produce a gel in which the entire reaction mass solidified and is not as well suited to coatings applications as the polymer that crosslinks upon drying. ... 

As mentioned above, it is the polymer that crosslinks during the radiation cure of hot melts. Various polymer systems have been developed and several of these are now commercial. A brief description of each is summarized below. 

For crystalline polymers that crosslink, a decrease in crystallinity is observed by X-ray diffraction at high irradiation doses and is accompanied by a decrease in specific volume the melting behaviour of these polymers is thus strongly affected by irradiation. 

This section concerns the polymers that crosslink in the solid state. Their behaviour in solution depends on the solvent and concentration used crosslinking, cyclization and degradation may occur. 

Currently, industry uses many polymers that crosslink mostly by 2n + 2n type dimerizations with formations of cyclobutane rings. This is like the crosslinking reaction of the cinnamate group. Such materials are used in photoresists, photolithography, and in many ultraviolet light... 

Polymers that Crosslink by Dimerization of Nitrenes and by Other Combinations of Free-Radicals to Form Covalent... 

Polymers that crosslink in the solid state can also be used in solution. Particularly in water, the hydrogen abstraction created by reactive POH radical due to the decomposed water indirectly yields polymer radicals [12], The rate of crosslinking in solution is greater than in solids due to a high mobility of the generated polymer free radicals. 

Polymers that crosslink Polymers that degrade... 

In these cases, the polymer remains processible in the gelled state, because it is in the form of discrete PSA particles dispersed in the reaction medium. However, once the particles are dried, redispersion may be difficult if strong interactions develop between the particle surfaces. Polymerization of the acrylic PSA directly on the substrate, as in the case of UV polymerization, can also yield a covalently crosslinked polymer that does not require any further coating steps [71]. 

One-part moisture condensation cure. The one-part condensation cure system is a room-temperature vulcanizing (RTV) system that is based on a reactive PDMS polymer that undergoes hydrolysis on contact of air moisture, followed by condensation to yield a crosslinked elastomer. The most common systems [3,12,14,33] are based on the reactions shown in Scheme 5. 

Unfortunately, reliable data on the equilibrium swelling and long-term stability of hydrogels obtained by this method are not available. It should be noted that crosslinking of polymers with metal ions is one of the few reactions which can be carried out directly in the soil medium, which is efficiently used, e.g., in oil recovery [65, 66]. 

Table 2,1 provides some information on the two polymers that are supposed to be free of chemical crosslinks. Phenoxy PKHJ is a product of Union Carbide Corporation. Plates were produced by injection moulding after drying of the granulate. On the other hand, polymer E was prepared by casting as were the other experimental polymers but without adding PGCBA to the mixture. 

Another class of silicon-containing polymers that have great potential to be extremely useful precursor materials are poly(chlorocarbosilanes).14f 46 Poly (chlorocarbosilanes) are not useful without modification because of the rapid hydrolysis of Si—Cl bonds, forming HC1 and an insoluble crosslinked polymer network. However, nucleophilic substitution of these Si—Cl bonds with various reagents produces materials widi a broad range of properties that are determined by the nature of the nucleophile used.47 Poly(chlorocarbosilanes) can be easily synthesized by ADMET (Fig. 8.18) without any detrimental side reactions, since the Si—Cl bond is inert to both catalysts 12 and 14. Early studies produced a polymer with Mn = 3000.14f... 

A hydrogel is formed by a water-soluble polymer that has been lightly crosslinked. Hydrogels swell as they absorb water but they do not dissolve. The volume expansion is limited by the degree of crosslinking. The minimum number of crosslinks needed to form a three-dimensional matrix is approximately 1.5 crosslinks per chain, and this yields the maximum expansion possible without separation of the chains into a true solution. Thus, a hydrogel may be more than 95% water and, in that sense, has much in common with living soft tissues. 

SEC analysis shows that some samples have a blmodal MWD. At this time it is not possible to tell whether the bimodallty is an artifact of the polymerization mechanism or, perhaps, a consequence of partial hydrolysis of the polymer i.e., the high MW shoulder in Figure 5 may be due to the formation of aggregates through Intermolecular dipolar interactions of P-OH side groups or to polymer molecules crosslinked by P-O-P bonds. 

Additives that assist the creation of a fracture include viscosifiers, such as polymers and crosslinking agents temperature stabilizers pH control agents and fluid loss control materials. Formation damage is reduced by such additives as gel breakers, biocides, surfactants, clay stabilizers, and gases. 

This equation is not particularly useful in practice, smce it is difficult to quantify the relationship between concentration and activity. The Elory-Huggins theory does not work well with the crosslinked semicrystalline polymers that comprise an important class of pervaporation membranes. Neel (in Noble and Stern, op. cit., pp. 169-176) reviews modifications of the Stefan-Maxwell approach and other equations of state appropriate for the process. 

Although it has been suggested that crosslinking in the presence of MAH involves coupling of appended MAH radicals with other appended MAH radicals or with polymer radicals (7) the former is improbable due to the tendency for disproportionation rather than coupling between radicals derived from strong electron acceptor monomers such as MAH. 

Rubbery amorphous polymers do not hold their shape well unless they are permanently crosslinked. If automobile tires were not crosslinked, they would be a soft sticky mess that would flow under the weight of the car. For this reason, we rarely encounter rubbery amorphous polymers that are not crosslinked. 

We use gel content analysis to determine the weight fraction of a crosslinked polymer that is bound into an insoluble network. We immerse a stainless steel mesh basket containing a known weight of the crosslinked polymer in a suitable solvent (which may be heated to facilitate dissolution). If necessary, we can slice or grind the sample to increase its surface area. After 24 hours or more, we remove the basket from the solvent and dry it to constant weight. We calculate the gel content from Eq. 5.6. 

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