Irradiation cross-linked linear polymers

Network polymers, which resemble fisherman s nets. For polymer electrolytes, the knots in the nets are often siloxane or isocyanate units and the rope between the knots is PEO-like material. Many polymer electrolytes have been developed from network polymers, as these have been perceived as an attractive way for avoiding crystallinity problems. This topic has been comprehensively reviewed [24, 25, 30, 35, 36]. Irradiation cross-linked linear polymers. Gentle irradiation of a PEO-LiC104 polymer electrolyte by y-rays from a Co source has been used to provide a crystallization-free material with enhanced room temperature conductivity [37-39]. 

Especially for the synthesis of nanogels (e.g. by intramolecular cross-linking of polymer molecules with high molecular weights), the method of pulse irradiation is useful, see Fig. 10. The polymer is dissolved and the solution, flowing through a quartz irradiation cell (effective volume about 1 ml), is pulse-irradiated with electrons of several MeV (in our experiments 6 MeV) generated by a linear accelerator (ELU-6, Eksma, Russia). Pulse frequency of 5.0 Hz and pulse duration of 3 ps were applied at a flow rate of 5.0 ml/s. 

Butadiene and isoprene have two double bonds, and they polymerize to polymers with one double bond per monomeric unit. Hence, these polymers have a high degree of unsaturation. Natural rubber is a linear cis-polyisoprene from 1,4-addition. The corresponding trans structure is that of gutta-percha. Synthetic polybutadienes and polyisoprenes and their copolymers usually contain numerous short-chain side branches, resulting from 1,2-additions during the polymerization. Polymers and copolymers of butadiene and isoprene as well as copolymers of butadiene with styrene (GR-S or Buna-S) and copolymers of butadiene with acrylonitrile (GR-N, Buna-N or Perbunan) have been found to cross-link under irradiation. 

Cross-linking is a predominant process during irradiation of siloxane polymers. Chain scissions are negligible. ° ° The cross-link density increases linearly with a dose up to 160 Mrad (1,600 kGy). ° At 5.0 MGy (500 Mrad) the G(X) value is 0.5. Free radical scavengers, such as n-butyl and frrf-dode-cyl mercaptan and diethyl disulfide, are the most effective antirads. ° - ° At a concentration of 10%, two-thirds of the cross-links were prevented from forming however, the scission yield was also increased. 

Linear polystyrene can be generated on insoluble polymers by /-irradiation of the latter in a solution of styrene [110]. Polystyrene grafted onto polytetrafluoroethylene [111-116], polyethylene [2,110,117], or polypropylene [15] can be functionalized in the same way as cross-linked polystyrene, and loadings of up to 1.0 mmol/g can be attained. These supports, which are also available as crown-shaped pins (Multipin, 2-3 mm diameter, 8-10 pmol per crown), have been used for the synthesis of peptides [2,110,111,118], oligonucleotides [112-115,117,119], and small molecules [120-122]. 

The principal polyphosphazenes that have been used in hydrogels are those with linear or branched ethyleneoxy side chains, aryloxy groups with carboxylic acid substituents, or mixed-substituent polymers that bear hydrophilic methylamino side groups plus a hydrophobic cosubstituent such as phenoxy or trifluoroethoxy. Cross-linking is usually accomplished by gamma-ray irradiation or, in the case of the carboxylic acid functional species, by treatment with a di- or tri-valent cation. Here, we will consider another example based on MEEP (3.79), a polymer that is well suited to the clean method of radiation cross-linking. 

Prompt emission spectra of films irradiated in either vacuum or air indicated the formation of at least two fluorescing species. These species are denoted as Products II and II. The most prominent emission, that of Product II, was highly structured with maxima at about 405, 435, and 455 nm. Product II can be associated with both the polymer chain and low molar mass fragments it was extractable from the photolyzed film with methanol but not with cyclohexane, and it was present in methylene chloride extracts of cross-linked residues of films that had been extracted previously with methanol. The fluorescence spectra are shown in Figure 6 excitation maxima correspond very well with the weak absorption bands noted in the spectra of the irradiated films. Plots of absorbance increases at 350 nm against fluorescence intensity at 435 nm were linear for both vacuum and air irradiations. Therefore, it is concluded that Product II is a major emissive contributor to the yellowing of poly(sty-rene-aft-methyl methacrylate) films subjected to 254-nm irradiation. 

Simultaneous cross-linking and main-chain cleavage of linear polymers 195 Table 7.3 Predominant effects upon UV irradiation of polymers in the absence of oxygen [27]. 

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