Possible crosslinking mechanism

Glutaraldehyde is the most popular b/s-aldchydc homobifunctional crosslinker in use today. Flowever, a glance at glutaraldehyde s structure is not indicative of the complexity of its possible reaction mechanisms. Reactions with proteins and other amine-containing molecules would be expected to proceed through the formation of Schiff bases. Subsequent reduction with sodium cyanoborohydride or another suitable reductant would yield stable secondary amine... 

Gonsideiing the reactions in formulas (1) and (2) and the foregoing discussion of the possible reaction mechanisms, it seems likely that the photoinitiated crosslinking is a second order reaction, i.e. the rate should be a linear function of the UV intensity in square. Assuming that the rate of crosslinking (which is difficult to measure) is proportional to the rate of gel formation (which is easy to measure), the data in Fig. 6 support thds conclusion at low UV intensities, i.e. at... 

Under gamma or electron beam irradiation of NR, several kinds of active species (such as radicals, ion etc) are liberated either inside of NR particles or in the water phase. Further reaction between the active species will easily takes place with the end result a crosslinked NR. The possible reaction mechanism is as following ... 

The relative amount of the PPF block also affects the mechanical properties of the crosslinked P(PF-co-EG). PPF is the only portion of the copolymer that can form covalent bonds for crosslinking, so more PPF block result in more possible crosslinks, yielding a stronger material [75]. Additionally the hydrophobic PPF moieties can interact with each other, forming secondary interactions that further... 

Another possible termination mechanism is unimolecular trapping of radicals which has been demonstrated [9, 10] using electron spin resonance spectroscopy (ESR). Anseth and Bowman [9] have shown that bimolecular termination through reaction diffusion is generally the dominant mechanism even in highly crosslinked methacrylates. 

While general conclusions appear to be premature, it appears that the crosslink sites rearrange themselves during deformation to achieve their lowest free-energy states thus the chains deform less than the affine mechanisms predicts. A modified end-pulling mechanism is also possible. A possible molecular mechanism, which results in minimal changes in R, is illustrated in Rgure 9.26 (107). The debate over the exact molecular mechanism of deformation is sure to continue. 

In this research, the linear polymer and the crosslinked polymer were synthesized from the difunctional N-cyanourea monomer. N-Cyano-N -phenyl urea was used as a model compound to study the possible polymerization mechanisms. 

To study this possibility, a thin film (ca. 200 nm) of Co-PFS on Si substrate was exposed to near UV radiation (k = 350-400 nm, 450 W) for 5 min. The exposed film was developed in THF before characterization. Co-PFS was found to be a negative-tone photoresist. This appears to be consistent with photo-initiated crosslinking mechanism of acetylenes in the presence of metal carbonyls. However, it is also possible to have crossUnking in Co-PFS as a result of decarbonylation of the Co-cluster. The thickness of the film before and after UV treatment was determined by eUip-sometry. A 200-nm-thick film of Co-PFS had a thickness of ca. 170 nm after expo-sme to UV radiation and solvent development. The decrease in thickness is probably a reflection of the decreased volmne of the polymer upon crosslinking. 

There are two possible crossUnking mechanisms which occur in the presence of those peroxycarbamates (Schemes 6 and 7) [30]. One is different from the usual mechanism of diamine crosslinking, while the second one is the classic one where the diamine adds onto the polymeric backbone by nucleophilic Michael addition. Indeed, those peroxycarbamates can undergo a thermal decomposition creating radicals (Scheme 6), and so the crosslinking mechanism can be a nucleophilic addition. 

Numerous possibilities exist for the manufacture of polymers with latent coreactive sites °l The introduction of built in functional groups onto the surface of the polyurethane particle and proprietary reactive groups copolymerised into the backbone of the acrylic and buried deep into the core of the acrylic phase are the usual techniques for crosslinkable copolymer type systems. The crosslinking mechanism is not fully understood, but it is assumed to be made possible via interdiffusion of polymer chains across the former boundaries of adjacent particles. Evidence for the crosslinking can be found in increased solvent resistance, softening temperature and the appearance of a single Tg.. 

Adechanical stahility. ChemisoriDtion to tire surface, intennolecular interactions and crosslinking between adjacent compounds—if possible—all contribute to tire resulting stability of tire monolayer film. Lateral force microscopy investigations revealed tliat tire mechanical stability towards lateral forces on tire nanometre scale is likely to be detennined by tire defect density and tire domain size on a nano- to micrometre scale [163, 1731. 

Scheme 6 Possible mechanism of crosslinking between PVC, ENR, and XNBR.

The formation mechanism of structure of the crosslinked copolymer in the presence of solvents described on the basis of the Flory-Huggins theory of polymer solutions has been considered by Dusek [1,2]. In accordance with the proposed thermodynamic model [3], the main factors affecting phase separation in the course of heterophase crosslinking polymerization are the thermodynamic quality of the solvent determined by Huggins constant x for the polymer-solvent system and the quantity of the crosslinking agent introduced (polyvinyl comonomers). The theory makes it possible to determine the critical degree of copolymerization at which phase separation takes place. The study of this phenomenon is complex also because the comonomers act as diluents. 

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