NOTES 257prepared by I h . J. L. Cooney.) Contact angles were observed on sessile drops of about 3 cc. total volume on chemically cleaned, flat surfaces of the polymeric solid, which appcar to be physically compatible with the above assumptions. There is naturally some uncertainty in both observed angles arid in volume fractions of polymer in the substrates. The results are presented in Figure 1, from which it appears that the proposed linear relationship treats the data adequately.Analysis shows that the contact angle on a heterogeneous surface gives the legitimate work of adhesion according to the usual relation W,,,] = yL( 1 + cos e).Thus, the wettability of a heterogeneous surface can be interpreted simply in tcwns of the wettability of the individual phases.J. E. The Decomposition of Benzoyl Peroxide in PolyethyleneSt,udying the crosslinking process of polyethylene (PE) with benzoyl peroxide (BP),' u-c have detcrmined the kinetics and the mechanism of the peroxide decomposition reaction in an inert atmosphere.? BP in PlC decomposes by a radical chain mechanism with a second-order induced dccomposition reaction caused by an interaction of benzoyloxy radicals wit,h benzoyl pcroxidc. Thr termination is effected partly by a recombination of two polymer rndic:tls :i,nd partly hy :I deact,ivatiori between benzoyloxy and macro radicals. The experimentally determinc,d value of the rate constant of spontaneous cleavage is k , (set.-') = 2.9 X lozG exp (50,500/RT] and t,hat, of the induced decomposition is Izi (kg. mole-' sec.-l) = 1.6 X 10" exp {28,500/RT)The relatively high values of the activation energy of spontaneous and induced decomposition of BP in PE we att,ribute similarly as other authors3v4 to a hindered diffusion, which, as the slowest process, influences the rate of the decomposition reaction.?We should like to note, that Haas4 in an analysis of BP decomposition in polystyrene (PS) has reached different conclusions. The author has found a first-order relationship for this reaction and therefore he supposes that the decomposition is not induced by radicals. The obtained BP decomposition activation energy values are considerably lower than ours. This disagreement can be explained by different experimental conditions. While in P E the BP decomposition was investigated in an inert environment, with PS this was not the case. .4s it can be seen from thc results in Table I, the presence of oxygen plays an important role in the dccomposition kinetics of BP in PE. (The experimental methods used for sample preparation and for BP analysis \\-ere described p r e viously.2)The decomposition of El' in PE in the, presence of air is swmingly a monomolecular reaction, as in the case of P S 4 The activation energy of spontaneous cleavage calculated from the results in Table I is 40.8 kcal./mole.
In a study of chemical modification of hydrocarbon polymers an investigation was made of the grafting of methyl methacrylate onto polypropylene and polyethylene. The grafting process was carried out both through preliminary polymer hydroperoxide formation and through chain transfer reactions. Polymer hydroperoxide‐induced grafting proceeds via direct free radical initiation of side chain growth and via hydrogen atom transfer from the hydrocarbon polymer to the polymethyl methacrylate radical. Here the contributions to the chain transfer from the initiator and the methyl methacrylate radical were determined for the separate polymers. Since the grafting was carried out in swollen polymers the effect of viscosity of the medium on the course of the grafting of methyl methacrylate was examined. During the grafting of methyl methacrylate to polyethylene and polypropylene the possibility of crosslinking of these polymers arises. Of particular interest is crosslinking of polypropylene, in view of the fact that the direct reaction induced by ionizing radiation or by hydroperoxide decomposition is connected with certain complications brought about by degradation of the polymer backbone. In the case in question, the formation of a crosslinked structure is due to recombination of the growing methyl methacrylate chains. The crosslinking of the graft polymer is hindered by the chain transfer reaction and by disproportionation during chain termination. In the case of methyl methacrylate, the ratio between recombination and disproportionation is favorable to crosslinkage of the graft polymers, a fact which has been confirmed experimentally. Based on the data concerning the chain transfer reactions and the conditions of formation and decomposition of polymer hydroperoxides a discussion has been presented of the most favorable procedures for grafting methyl methacrylate onto polypropylene and polyethylene and for the formation of crosslinked graft polymers.
The present paper examines the possibility of the utilization of the bimolecular cumyl hydroperoxide redox decomposition, catalyzed by Co11 naphthenate for the polymer crosslinking. The study of the mechanism and the kinetics of this reaction was performed both in a model medium heptane and polymer or polymer solution. In the case of redox initiation, the value of the crosslinking efficiency was found to be about 0.4, and its decrease can be attributed mainly to the formation of ions and, partly, to recombination of peroxy radicals producing dialkyl peroxide. On the basis of data obtained concerning the efficiency and rate of the process, conclusions are made on its applicability, especially for the crosslinking below the melting point of a crystalline polymer, as is the case of the crosslinking of polyethylene films.
Crosslinked polyethylene, owing to changes in structure, is a product with technically important properties. The crosslinking may be brought about with the aid of various free radical sources, such as ultraviolet light, ionizing irradiation of high intensities, and organic peroxides. The kinetics and mechanism of the crosslinking process depend predominantly upon the free radical source employed for crosslinking of polyethylene. When peroxides are used, the kinetics and mechanism depend on both the chemical mechanism of the peroxide decomposition and the reaction of primary radicals with the polyethylene macromolecules. When radiation is used, the excited molecules dissociate after absorption of energy, hydrogen is evolved, and radicals from the polymer form crosslinks. The hydrogen atom eliminated is quite mobile and easily abstracts another hydrogen from the polymer. Hence, during a single primary reaction two polymer radicals or one crosslink and one hydrogen molecule are formed. The number of crosslinks as well as the amount of hydrogen produced are then simply proportional to the absorbed radiation dose. Crosslinking of polyethylene by the peroxide method is a more complicated process, the polymer radicals not forming directly, but by transfer to the polyethylene macromolecules of radicals produced by peroxide decomposition. In the termination reaction, in addition to crosslinkages, products of the reaction between primary and polymer radicals may form. When the peroxide decomposes by a chain mechanism the dependence of the crosslinks upon the initial peroxide concentration becomes more involved. In the induced decomposition, the free radical sources are inefficiently consumed and. owing to the difference in reaction orders of the simultaneous ineffective induced and effective self decomposition, of peroxide, the ratio between the two reactions varies in the course of the process. From a kinetic analysis of the experimental results quantitative relations between the formation of crosslinks in polyethylene and the free radicals produced from the various sources were established. Both the simplest cases of direct production of radicals capable of crosslinkage as well as the cases in which the formation of the primary radicals is determined by the initiation process and rate of radical transfer and the nature of the termination reactions are of major importance have been considered. Finally a still more complicated case, in which crosslinking of polyethylene takes place during chain decomposition of the peroxide, has been treated. With the aid of the derived equations and the experimentally determined rate constants the crosslink concentration was calculated and the results have been compared with experimental values.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.