Polymer crosslinking, formation

The second step is the condensation reaction between the methylolphe-nols with the elimination of water and the formation of the polymer. Crosslinking occurs hy a reaction between the methylol groups and results in the formation of ether bridges. It occurs also by the reaction of the methylol groups and the aromatic ring, which forms methylene bridges. The formed polymer is a three-dimensional network thermoset ... 

This is one of the most universal techniques for obtaining hydrogels from water-soluble polymers. Crosslinked PEO, PVA, PAAm, PAAc and its salts, as well as some polymer blends were obtained by this method. Although all polymers mentioned above have their own specific features, in most cases the gelation doses do not exceed 1-2 Mrad, i.e. they are substantially lower than for the same polymer in bulk. This is due to the fact that in aqueous media crosslinking occurs indirectly, namely because of the OH radical formation and their attack on the macromolecules. There exists a developed theory of these processes [73],... 

Chain compositional heterogeneity is of particular relevance to functional copolymers which find widespread use in the coalings and adhesives industries.13,240,246 In these applications, the functional copolymer and a crosslinking agent are applied together and are cured to form a network polymer. The functional copolymers are based on functional monomers with reactive groups (e.g. OH), it is desirable that all copolymer molecules have a functionality of at least two. Nonfunctional polymer will not be incorporated and could plasticize the network or be exuded from the polymer. Monofunctional polymers are not involved in crosslink formation and will produce dangling ends. 

Figure 2.18 Crosslink formation in chain growth polymers by the incorporation of dienes with monomers...

Many polymers solidify into a semi-crystalline morphology. Their crystallization process, driven by thermodynamic forces, is hindered due to entanglements of the macromolecules, and the crystallization kinetics is restricted by the polymer s molecular diffusion. Therefore, crystalline lamellae and amorphous regions coexist in semi-crystalline polymers. The formation of crystals during the crystallization process results in a decrease of molecular mobility, since the crystalline regions act as crosslinks which connect the molecules into a sample spanning network. 

The neutral polymer radicals which are produced also often undergo further reactions, which can result in chemical changes in the polymer. These reactions may include crosslinking or scission of polymer chains, formation of small molecule products, changes in the stereochemistry of the polymer chains, changes in the crystallinity of the polymer or a variety of other chemical and physical processes. 

Although XPS has provided a wealth of information on the surface chemistry of treated polymer surfaces, several possible processes are beyond the capabilities of the technique. Some of these are crosslinking, formation of double bonds, reorientation of surface molecules and the formation of... 

When primary aliphatic isocyanates that show the lowest reactivity compared to secondary or aromatic isocyanates are used in combination with hydrophilic (pre-) polymers, crosslinking may be performed in aqueous solution without the use of additional crosslinkers. At neutral pH, hydrolysis of isocyanates to carbaminic acid with subsequent decarboxylation yields amines. These amines react much more rapidly than water with isocyanates, resulting in crosslinking if the functionality per macromolecule is more than two [43], This crosslinking reaction can be quenched by adjustment of the pH value. At pH values above 10, carbamate formation is faster than decarboxylation, whereas at pH values below 3 an almost quantitative protonation of the formed amino groups results in the formation of ammonium. In both cases, chemical crosslinking is prevented. 

Lead stabilisers have been used in a variety of PVC as well as other polymers for many years. In some halogenated polymers, such as chlorinated PE (CPE), chlorosulphonated polyethylene (CSM), polychloroprene (CR) and epichlorohydrin (ECO), dibasic lead phthalate and dibasic lead phosphite are used to scavenge HC1 arising from crosslinking as well as from degradation. In some of these cases, the metal may participate in crosslink formation. With lead-based stabilisers, the result is typically a product with greater water and chemical resistance than if a light metal, with more soluble halide salts, were used instead. In other cases, lead stabilisers may be used solely for function in metal oxide... 

Many authors elucidated functionalization of polymers containing reactive oxirane moieties. Epoxidized NR, BR, IR and/or the respective model hydrocarbons, poly (butadiene-co-isoprene, various epoxy resins, poly (2,3-epoxypro-pyl methacrylate) and its copolymers or grafted systems were mostly exploited. Stabilizers based on epoxidized unsaturated rubbers are of the top interest. The mechanism of the functionalization process was studied in details by means of 3,4-epoxy-4-methylheptane and 1,2-epoxy-3-ethyl-2-methylpentane as model compounds [289]. The ring opening of the asymmetric oxirane is regiospecific. Aliphatic primary amines attack the least substituted carbon atom and can be involved in crosslink formation. Aromatic primary and secondary amines are less reactive than aliphatic ones because of their lower basicity the attack on the least substituted carbon atom is however preferred too. 

This is an interesting comparison with regard to control of polymer crosslinking by biuret- or allophanate-forming side reactions. For example, 1,5-naphthalene diisocyanate showed the greatest tendency toward alloph-anate formation, and hexamethylene diisocyanate the least. 

Our earlier structural studies (15) have shown that in the case of plasma-polymerized HMCTSN and HMCTSO other crosslinking reactions may take place. Ultraviolet radiation emitted by the plasma may cause a hemolytic cleavage of Si-C and C-H bonds in SiCH groups, fo-lowed by crosslinking in the polymer via formation of methylene and ethylene linkages between silicon atoms. 

The stress-strain behavior of thermosets (glassy polymers crosslinked beyond the gel point) is not as well-understood as that of elastomers. Much data were analyzed, in preparing the previous edition of this book, for properties such as the density, coefficient of thermal expansion, and elastic moduli of thermosets [20,21,153-162]. However, most trends which may exist in these data were obscured by the manner in which the effects of crosslinking and of compositional variation were superimposed during network formation in different studies, by... 

The rather non-conventional approach to radiation chemistry of polymers leads to conclusions which indicate that the role of multi-ionization spurs in radiation chemistry of polymers cannot be neglected. In spite of low participation of these spurs in radiolysis of low Z materials (ca 20% of total deposited energy), these spurs can explain formation of two basic, different types of crosslinks. Formation of low molecular weight products of radiolysis is also explained, as well as other phenomena. Application of spurs philosophy to polymers is also advantageous in explanation of energy transfer from single ionization spurs and lack of transfer from multi-ionization spurs. 

The main chemical changes induced by ionizing radiation are (a) main-chain scission, (b) crosslinking formation, (c) volatile products formation, (d) formation and decay of unsaturation, and (e) cyclization. The yields strongly depend on the chemical structure of the polymer. 

A quite different interpretation of polypropene radiolysis has been given recently by Veselovskii et al. [321]. The crosslinking of isotactic polypropene is strongly inhibited in the presence of 2-methylbutene-l which contains a vinylidene double bond, whereas inhibition is far less important in the presence of other olefins. This indicates, according to the authors, that most of the intermolecular bonds produced by irradiation of isotactic polypropene are formed by reactions of the vinylidene-type double bonds formed by scission of the main chain. Since the inhibiting effect of hydroquinone, anthracene and stannous chloride on the quantity of gel in isotactic polypropene is small, network formation would not involve free radicals, but reaction of the excited vinylidene double bonds with the polymer chain. No gel is, however, found even after a 150 Mrad dose in atactic amorphous polypropene irradiated at room temperature in the presence of the radical acceptors. In this case, radicals play the main role in the crosslink formation. 

A 30 G splitting triplet superimposed on a large singlet is also observed after irradiation of the polymer at 77°K [344]. The contribution of the triplet was greatly enhanced by the presence of electron acceptors [344]. It thus seems clear that the additives markedly affect the mechanism of radiolysis by interaction with the cation radical. Additives with a hi i electron affinity inhibit cation and electron recombination and thus increase the yield of a-pyridyl radicals which are involved in crosslink formation. Additives that have a lower ionization potential than pyridine (9.28 eV) would interact with the polymer radical-cation according to... 

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