Cross linking and gel formation

By contrast with PMMA, poly(methyl acrylate), PMA, and several other aliphatic polyacrylates were found by Shultz and Bovey (240) to imdergo cross-linking and gel formation on irradiation with 1-MeV electron beams. They reported G(S) = 0.15 and G(X) = 0.52 for PMA. Graham (241,242) reported that the phenyl, benzyl, and 2-phenyl ethyl acrylate polymers also undergo cross-linking on y radiolysis imder vacuum. There was evidence of side-chain scission also, and according to Fox and co-workers (243,244) the major volatile products were similar to those observed for PMMA. 

Thermo-oxidative degradation of PET is a complex phenomenon that gives formation of many species. Cross-linking and gel formation could occur. 

Low-density polyethylene (LDPE) is extensively used for the manufacture of films. During processing, which is carried out at temperatures of approximately 200°C, cross-linking, and thus formation of gel, can occur through oxidation if the polymer is not stabilized. Such gel particles are visible in the film as agglomerates, known as fish eyes or arrow heads. The processing stabilizers used in LDPE consist of systems commonly used for polypropylene, namely, combinations of a phosphite or phosphonite and a long-term heat stabilizer (hindered phenol) in overall concentrations up to 0.1%. Concentrations seldom exceed 0.1%, since the compatibility of any additive in LDPE is considerably lower than in any other polyolefins. 

Additional hydrolysis to promote polymerisation and cross-linking leading to a three-dimensional matrix and gel formation. 

In conclusion, the self-condensation of 2-furaldehyde promoted by heat occurs with the formation of di- and trifurylic intermediates. The functionality of the growing chain increases after each oligomerization step until gelation and precipitation of the resin occurs. Thus, the process is non-linear from the onset since the condensation product 4 possesses three sites for further attack, namely the free C-5 position and the two formyl groups. It is interestering to note that while the polycondensation of 2-furfuryl alcohol is essentially linear and cross-linking is due to side reactions, the thermal resinification of 2-furaldehyde is intrinsically non-linear and gel formation occurs at earlier conversions. 

In the foregoing consideration the effect of intramolecular cross-linking on network formation and the gel point was not taken into account. In reality intermolecular crosslinking is always accompanied by closed loop formation. Every new bond between segments of the same (branched) polymer chain is an intramolecular crosslink if it does not increase its molecular weight. In a network a crosslink is intermolecular... 

Viscosity changes in poly-D,L-alanine are difficult to interpret because of gel formation. In this case, the variation of the gel point (defined as the dose at which a 0.5% solution formed a stable gel) with pH was studied, and again it was found that the maximum degree of cross-linking (lowest gel point) was in the region of pH 4 (Figure 6). 

In contrast to agarose, polyacrylamide is a synthetic polymer, and gel formation depends on chemical cross-linking. The bifunctional acrylamide derivative, N,N -methylene-6is-acrylamide is included in the polymerisation mixture to insert cross-links between chains of... 

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