Irradiated polymers, gelation

One important exception to this rule is vinylketone polymers in which the Norrish type II reaction is responsible for the decrease of the molecular weight of irradiated polymers (see section 4). In contrast to polymethylmethacrylate, polymethylacrylate becomes insoluble on irradiation with 253.7 nm in vacuo [82]. In air, no visible insoluble material is formed and an apparent quantum yield of chain scission of 1.3 x 10-2 has been determined by viscosity measurements [82]. However, a qualitative comparison of sedimentation patterns of initial and irradiated samples indicates that crosslinking also occurs in the presence of air even if gelation is retarded by oxygen. This makes the above-mentioned value meaningless. Photolysis of thin polymethylacrylate films at 253.7 nm in vacuo has also been studied by measuring the insoluble fraction as a function of dose as described in section 2. Quantum yields of 1.9 x 10-3 have been estimated for both the chain scission and the crosslinking processes [83]. 

Richtering W, Fuchs T, Burchard W (1998) Dynamics during thermoreversible gelation of the polysaccharide schizophyllan. Ber Bunsenges Phys Chem 102 1660-1664 Rosiak JM (1998) Gel/sol analysis of irradiated polymers. Radial Phys Chem 51 13-17 Riihe J et al (2004) Polyelectrolyte bmshes. Adv Polym Sci 165 79-150... 

Thus having proved the necessity of the presence of the carbonyl group we tried next to demonstrate the crucial role the pendant methyl groups play in gelation. We went about this task by irradiating bisphenol-A ether ketone polymer under an identical experimental set up used for tetramethyl bisphenol-A ether ketone polymer gelation. 

For all three diallyl phthalate isomers, gelation occurs at nearly the same conversion DAP prepolymer contains fewer reactive allyl groups than the other isomeric prepolymers (36). More double bonds are lost by cyclisation in DAP polymerisation, but this does not affect gelation. The heat-distortion temperature of cross-linked DAP polymer is influenced by the initiator chosen and its concentration (37). Heat resistance is increased by electron beam irradiation. 

Membrane filters are usually made by casting a polymer solution on a surface and then gelling the liquid film slowly by exposing it to humid air. The size of the pores in the membranes can be varied by altering the composition of the casting solution or the gelation condition. Another common technique is to irradiate a thin polymeric film in a field of a-particles and then chemically etch the film to produce well-defined pores. 

An advantage of radiochemical induced cross-linking is the possibility to combine polymers with different properties. For that purpose, the polymers were separately dissolved in water. The solutions were mixed and irradiated. (Gottlieb et al. 2005) describes the synthesis of temperature-sensitive hydrogel blends of PVME (as thermo-sensitive polymer) and the radiation-cross-linkable polymer PVP (a polymer that is applied in pharmaceutics). The experiments show that the gelation dose of the blend is between the gelation doses of the two pure polymers. 

As discussed below solution-state NMR methods cannot be successfully applied to materials irradiated to above their gelation dose. This situation naturally does not arise for polymers for which chain scission is the dominant reaction on irradiation. The two systems of this type which have been most extensively studied are the poly(methacrylate)s and poly(isobutylene). 

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