Molecular weight measurements irradiated

Table II. Molecular weight measurements on copolymer films undergoing irradiation. (9)...

A number of studies on photochemistry of the nucleic acid bases in aqueous solutions demonstrated that while uracil undergoes reversible hydration under exposure to UV irradiation, the other bases (thymine, adenine, and guanine) were stable [41,42], However, the sensitivity of dissolved thymine to UV irradiation can be significantly increased if the solution is rapidly frozen [43]. In 1960 the thymine photoproduct was isolated from irradiated frozen aqueous solution of thymine. Elemental analysis, molecular weight measurements, powder X-ray diffraction, NMR and IR spectroscopy confirmed that the most likely photoproduct is a thymine dimer [20]. Similar photoproduct was obtained by hydrolysis of irradiated DNA. Its formation was attributed to reaction between two adjacent thymine groups on the same DNA chain [44], Independently an identical compound was isolated from DNA of UV-irradiated bacteria [45]. 

Thioacetophenone polymerizes spontaneously to a white solid. Conversion to polymer is increased by irradiation with ultraviolet light Highest conversion reported is 74%. Polymerization is also brought about by such anionic initiators as AlEt2Cl, AlEtj, BF3 - OEt, and SnC. Molecular weights measured on tetra-hydrofuran or brazene solutions using a vapor-pressure osmometer were in the 1000-2000 range. These are minimum values, since the polymer decomposes readily. 

The average number of scissions per chain, N, produced by absorption of a given dose can be determined by measuring the initial number average molecular weight (M ) and the number average molecular weight after irradiation (Afn). 

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]. 

Butyl rubber is a copolymer of isobutylene and I -2% isoprene. As a result the polymer chains contain internal double bonds which are expected to participate in cross-linking reactions. However, the overall molecular mass is expected to fall on irradiation due to the predominance of main-chain scission through the isobutylene units. Thus the radiation chemistry of the isoprene units within butyl rubber is accessible to study via solution NMR. In a comprehensive study Hill identified the primary free radical species by electron spin resonance spectroscopy at low temperatures, and the products of their subsequent reaction by C solution-state NMR. A number of new cross-link structures were identified and the mechanisms of cross-linking determined. Initial reaction involves addition of radicals either directly to the isoprene double bonds or to allyl radicals. Further addition of hydrogen atoms results in a mixture of fully-saturated and unsaturated cross-link structures. Cross-links of both H- and Y-type were identified and the yields of products agreed closely with the yields determined from measurement of changes in molecular weight on irradiation. 

Soluble Fractions. The above descriptions of the change in molecular weight on irradiation presuppose the abihty to accurately measure the molecular weight distribution of the polymer after irradiation. A number of authors have pointed out that for polymers for which cross-linking dominates chain scission, measurement of molecular weight is hmited to doses well below 50% of the gel... 

Figure 5.2. Gel permeation chromatogram (GPC) of the liquid silicon precursor for Si film formation for (a) cyclopentasilane (CPS) and (b) UV-irradiated CPS, both of which were diluted with toluene (1 vol.%) before GPC measurements. The UV-irradiation conditions were 405 nm, 100mW/cm2, and a 10-min irradiation for 1cm3 of CPS. The broad peak around Mw = 2600 corresponds to polysilanes of various molecular weights, as a result of the photo-induced polymerization of CPS. [Reproduced with permission from Ref. 10. Copyright 2006 Nature Publishing Group.]...

It was found from the absorption spectrum that 1.1 % of the incident photons were absorbed at 2537 A by a PMMA film of 0.5 ym thickness (Fig. 5). The molecular weight distribution and the average molecular weight of the coated polymer which was irradiated for the least irradiation time required for the dissolution of polymer coating in the developer were measured by gel-permeation chromatography (Fig. 7). 

The radiation G values, G(s) and G(x), of a polymer can be determined by measuring molecular weight changes which occur during irradiation and applying equations 1 and 2 (5.61. 

Methyl Radicals. Yoshida and Ranby (41) have recently shown by ESR measurements that the methyl free radical is produced in polypropylene by irradiation with ultraviolet light at liquid nitrogen temperature but decays at that temperature. The reaction is slow at —196°C., which suggests that all other hydrocarbon free radicals of larger molecular weight than the methyl free radical can be frozen in and immobilized at that temperature. 

Polyisobutylene. The solution viscosity of an irradiated polyisobutylene block was measured in CC14 at 30 °C. to determine the degree of degradation (5). The variation of viscosity-average molecular weight, Mv, with the dose, r, is shown in Figure 3. Nitrous oxide reduced the... 

Like the vinyl ethers, isopropenyl acetate does not readily form homopolymers by free radical initiation. It does participate in the donor-acceptor mode of polymerization with maleic anhydride and the copolymer was made by free radical polymerization (see Table III). When attempts were made to measure molecular weight by GPC using THF as a solvent on STYRAGEL columns, the polymer did not elute. Therefore, viscosity in acetone at 30"C was used as a measure of molecular size changes on irradiation. The changes on exposure... 

---