Irradiation of polymer solutions

A very different nucleation scheme by Grieser and co-workers employs ultrasonic irradiation of salt solutions to create H- and OH- radicals in solution [73]. These radicals proceed to nucleate growth of quantum-sized (Q-state) particles of cadmium sqlfide. Similar initiation has been used for polymer latices [74]. 

Table 2. Synthesis conditions and equilibrium swelling degrees of PVME gels made by -/-irradiation of aqueous solutions containing 30 wt % PVME and 15 wt % ferric oxide. Reprinted from Polymer (1991) 33 990 by permission of the publishers, Butterworth Heinemann [46]...

Poly(vinyl methyl ether), PVME, is a thermo-sensitive polymer. The aqueous solution has a Lower Critical Solution Temperature (LCST) of 37 °C. Therefore, PVME is soluble in water below its LCST, but insoluble above its LCST. When an aqueous solution of PVME is irradiated with y-rays the solution becomes PVME hydrogel [18, 19]. The gel shows thermo-sensitivity similar to the solution, and swells below 37 °C and shrinks above this temperature. It is important to form a fine porous gel structure to obtain quick response gels. There are two methods for the purpose. One is a method using micro-phase separation by heating. The other is a method using micro-phase separation by blending of polymer solutions. 

Thus, macroradicals have been obtained by stretching fibers (20), deforming plastics by compression (37), ball mill grinding (11), freezing and grinding of polymer solutions (10), ultrasonic irradiation (I), mastication (19), dispersion in a microblender (25), and other mechanical techniques (36). Many reviews on the formation of macroradicals by degradative processes have also been published (5, 12,13,16, 33). 

Fig. 1 Irradiation setup for pulse radiolysis consisting of a solution reservoir, a peristaltic pump, and a quartz irradiation cell. Prior and during irradiation, the polymer solution is continuously saturated with argon...

The effect of irradiation of polymer membranes was studied by several research groups [1-7]. All studies showed that polymeric membranes exhibit limited resistance to ionizing radiation, however the threshold values of doses are sufficiently high to use the membranes for low and medium radioactive solutions treatment for a period of time. Usually the practical lifetime for most membranes is of the order of 4-5 years that is sufficient to avoid deterioration of hltration and separation abilities. Ceramic membranes are expected to be more resistant to y, (3, and a emissions, but systematic studies have not been presented. 

The irradiation of aqueous solutions of carbohydrates has the same effect as it does on alcohols. The hydroxyl groups are attacked to yield carbonyl compounds. Under anoxic conditions, dimer products and, ultimately, polymers are also formed. The primary alcohol... 

Bray JC, Merrill EW. Poly(vinyl alcohol) hydrogels. Formation by electron beam irradiation of aqueous solutions and subsequent crystallization. J Appl Polym Sci 1973 17 3779-3794. 

Figure 3 is the result of pulse radiolysis experiment about the reaction of hydrated electron with polymer chains(0 or 30 mM carboxymethyl chitosan solution with 0.3 M terf-butanol under Ar saturation), and shows the decay of the absorbance as a function of time. This absorbance was measured at wavelength 720 nm, which is the absorption peak of hydrated electron. As seen in Figure 3, the absorbance increases immediately after the irradiation, and attenuates afterwards. This means that hydrated electron is generated immediately after irradiation and diminishes gradually by some reactions of hydrated electron. Compared the absorbance decay of polymer solution with the decay of solution without polymer, the decay of polymer solution is faster than without polymer, so it is obvious that hydrated electron reacts with polymer chains. The decay curve can be fitted by pseudo first-order decay. The pseudo first-order decay is shown by equation (8). From estimating the slope of the pseudo first-order decay rate of the absorbance at 720 nm against polymer concentration, the rate constant of the reaction of hydrated electron with polymer chains can be calculated Figure 4). The rate constants of the reaction of hydrated electron with CM-chitin and CM-chitosan was determined as l.lxlO7 and MxlO M V1]. These values are almost the same with the value of carboxymethyl cellulose(2< ).

SCN or (1)2, which has a strong absorbtion peak at wavelength 472 nm and 380 nm [reaction (10)(11) and (12)(13)]. Irradiating the polymer solution under KSCN or KI existence, OH radical reacts with both polymer chains and a competitor scavenger. Reaction (15) shows the equation between the change of the absorbance of (SCn or (l)2" and the ratio of both reaction rates. A means... 

Poly(vinyl alcohol), or PVAL, is the polymer that was used for this purpose. It is a neutral, partially water-soluble polymer (43,44) that is generally considered to be biocompatible and nontoxic (45-47). Its structure is shown in Figure 3B. PVAL has been used as a matrix for entrapment of several enzymes in gels formed by gamma irradiation of aqueous solutions of polymer/enzyme mixtures (45,48-50). In some of our experiments, allyl methacrylate (AM) was added to the polymer to facilitate cross-linking at lower doses. 

Irradiation is one of the best methods to prepare polymer hydrogels while it avoids the use of potentially toxic additives. Moreover, sterilization for biomedical applications could be performed in the same time. Irradiation of polymers in aqueous solution provokes a radiolysis of water, leading to the formation of reactive radicals, as OH radicals. The radicals could react with macromolecules and transfer the radical center to chains. 

Polymers were deposited onto the quartz crystal (0.8 cm diameter) by casting from chloroform solution. The area coated with the polymer film was usually 0.13 cm2. The quartz crystal was placed in the middle of the sealed glass vessel which had a quartz window for UV irradiation. 2M KNO 3 aqueous solution was placed at the bottom of the vessel to control its humidity (RH=95%) at 25 "C. Irradiation of polymer films on the quartz crystal through the quartz window of the vessel was carried out with 254-nm light using a 5-W low-pressure Hg lamp (Toshiba LP-llB). The intensity of the incident light determined with a chemical actinometer (potassium ferrioxalate) (10) was 0.1 mJ/cm2 - sec at 254 nm. 

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