Radiation grafting, importance

The dose and dose rate of irradiation are important factors in any radiation grafting system. In the direct method the total dose determines the number of grafting sites, while the dose rate determines the number of grafting sites, and the dose rate determines the length of the grafted branches. The length of the branches is also con-... 

The correct choice of solvents is essential to the success of radiation-induced graft copolymerization. Their influence on radiation grafting has been the subject of many studies. It has been established that solvents play an important role in grafting because of the significance of polymer swelling. The grafting patterns to PP are solvent-dependent, thus, it is essential to examine the role of solvents. It was found that grafting in the alcohol is better than when other polar solvents such as dimethyl-... 

Some important specific features of radiation grafting should be pointed out. In contrast to chemical modification, the final grafted copolymers can be obtained... 

A theory for this acid effect has been developed essentially from the wool and cellulose work (3,4). Recently, in a brief communication, we reported analogous acid enhancement effects in the radiation grafting of monomers such as styrene in methanol to nonpolar synthetic backbone polymers like polypropylene and polyethylene (5). In the present work, detailed studies of this acid enhancement effect are discussed for the radiation grafting of styrene in various solvents to polyethylene. The results are fundamentally important since most of the experiments reported here have been performed in solvents such as the low molecular weight alcohols which, unlike cellulose and wool systems, do not swell polyethylene. 

The parameters which predominantly influence the acid effect in radiation grafting of styrene monomer to polyethylene film are the structure of solvent, the concentration of monomer and the dose rate. Because these three variables are inter-related, it is difficult to predict, a priori, the conditions required to yield an optimum in grafting. In this respect the type of solvent used is particularly important. 

An Important observation recently made concerning acid and salt effects In UV and radiation grafting to polyethylene Is that In Che swelling of polyethylene In Che presence of methanolic solutions of styrene, partitioning of styrene Into polyethylene Is significantly Improved by the Inclusion of mineral acid or lithium salt In the grafting solution. Styrene labelled with tritium was used for these sophisticated experiments which Indicate chat most swelling occurs within Che first few minutes of exposure of backbone polymer... 

The focus of this review will be the effects of so-called ionizing radiation, namely gamma and electron-beam irradiation, on polymers. We will not consider, for example, the effects of UV radiation on materials, although this is an important method in the field of radiation grafting. - ... 

For radiation grafting, the stabilization of radicals in the polymer films is very important. Due to the flexibility of polymer chains, rearrangements and chemical interactions are possible over longer distance, in particular above the glass transition temperature. To lower the probability of radical recombination, exposed polymer substrates can be stored at low temperatures to reduce the chain mobility inside the polymer. Temperatures of -80°C are usually sufficient to stabilize the radicals over weeks to months. 

However, the most important method of producing heavily grafted materials utilizes radiation techniques, usually using either °Co gamma radiation or Van de Graaff-type electron accelerators. According to Chapiro (1962, Chapter 12), there are four major types of radiation grafting ... 

The above theories invoke essentially the physical properties of the grafting system to explain the observed copolymerization phenomenon. Swelling either from the solvent or monomer or both is also an important factor in these reactions. However if the data in Figures 1 and 2 are considered, a further theory would appear to be necessary to explain the solvent properties observed, especially the trend in the alcohol data to n-octanol and also the benzene, pyridine, chloroform and carbon tetrachloride results. Thus, as previously proposed for radiation grafting processes, it is necessary to consider the radiation chemistry of the system and in particular the radiolysis products of the solvent in any complete analysis of the copolymerization process. It has been suggested that a contribution to the mechanism of the acceleration effect of methanol can be due to the radiolytic scavenging properties of styrene and hence the relative numbers of styrene... 

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