Hisatsugu Kashiwabara

Free radicals trapped in polyethylene matrix: 2. Decay in single crystals and diffusion

Abstract The kinetics of the decay of main chain free radicals trapped in polymer matrix in vacuum was investigated. The decay behaviour of alkyl radicals trapped in the lamellar surface of the crystallites of solution grown polyethylene should be different from that of the same radicals trapped in the inner part of the crystallite, and this difference was clearly observed. The analysis of the data of the decay reaction was made, based on three-dimensional diffusion-controlled reaction theory, and reasonable interpretation of the data was made. Alkyl radicals were also observed in an irradiated urea—polyethylene complex and the decay rate of these radicals were very slow in spite of the large mobility. According to these occurrences, it was concluded that the mechanism of three-dimensional diffusion of the free radicals across the chain may occur in the inner part of the crystallites via hydrogen abstraction reactions.

Free radicals trapped in polyethylene matrix: 1. Location of the alkyl radicals in irradiated polyethylene

Abstract Two types of alkyl radicals were found to be trapped in irradiated crystals grown from polyethylene solution. One of them corresponds to the broad sextet pattern of the e.s.r. spectrum and the other corresponds to the sharp sextet pattern. The free radicals attributed to the broad sextet began to disappear at a lower temperature than the temperature at which the free radicals attributed to the sharp sextet disappeared. When butadiene molecules were brought into contact with the specimen, the decay of the free radicals corresponding to the broad sextet was accelerated. When the specimen was subjected to fuming nitric acid treatment, no broad sextet was observed. The mat of the crystals, was aligned so that the c-axes of its crystallites were perpendicular to its surface. The broad sextet showed no anisotropy when the angle between the direction of applied magnetic field and that of the c-axis of the crystallite was varied. On the other hand, the sharp component of the spectrum showed apparent anisotropy. Consequently, it can be concluded that the broad component comes from the free radicals trapped in the lamellar surface and the sharp component is attributed to the free radicals trapped in the inner part of the crystallite. Hence, the locations of these two types of free radicals have been clarified with much more certainty than before.

E.s.r. studies of peroxy radicals in polyethylene: 1. Temperature dependence of spectra and molecular motion of radical sites

Abstract The temperature dependence of the e.s.r. spectrum of the peroxy radicals in polyethylene was successfully observed. Two kinds of radical site were found in the amorphous region of polyethylene, one bearing the radicals corresponding to the usual e.s.r. spectrum of the amorphous pattern as for the usual polymer peroxy radicals, and the other presenting the singlet-like pattern which has never actually been observed. Computer simulation of these spectra was carried out in order to determine the anisotropic g-values as well as the ratios of the amounts of the radicals located at respective radical sites. The temperature dependence of these values made it possible to discuss the difference of the radical sites and the molecular motion of these radical sites. The motion around the chain axis was found to be much more rapid than that around the CO bond axis in the peroxy raidcals.

Relation between diffusion controlled decay of radicals and α-relaxation in polyethylene and polyoxymethylene

Abstract Diffusion controlled process theory was applied to the analysis of the data concerning decay reactions of free radicals in irradiated polyethylene and polyoxymethylene in detail. Diffusion constants of free radical sites were estimated from the data of the decay reactions and their time constants were calculated by using Stokes-Einstein relation. These time constants were plotted against the inverse temperature in the relaxation maps, which were arranged by Wada. In the case of polyethylene, the time constants of decay reactions of alkyl radicals trapped in the lamellar surface and the inner part of crystallites were in good agreement with the relaxation times of α′- and α-processes, respectively. However, it was found that the time constants of main chain radicals trapped in polyoxymethylene were closely related to α-relaxation processes. The activation energies of the decay reactions of free radicals were in good agreement with those of the corresponding relaxation processes in both cases. From these results, it was concluded that the decay reactions of the free radicals reflect the molecular motions associated with the respective relaxation processes. Concerning the decay reaction of free radicals in the inner crystallite of polyethylene, effects of lamellar thickness and crystal surface were also studied by using solution grown crystals, which were crystallized at various temperatures, and the fuming nitric acid treated materials. It was found that the diffusion constant of free radical sites and its activation energy were strongly dependent on the lamellar thickness and half-life of free radicals was a function of the amount of the fold surface; i.e., the reaction rate was much depressed as the lamellar thickness was increased and the amount of the fold surface was decreased.

E.s.r. studies on oxidation processes in irradiated polyethylene: 1. Diffusion of oxygen into amorphous parts at low temperatures

Abstract Oxidation processes in irradiated polyethylene were studied by e.s.r. By utilization of the characteristic feature of power saturation of the e.s.r. spectrum of peroxide radicals, quantitative measurement of the process of the reaction of oxygen with allylic radicals trapped in the amorphous part of the polyethylene were made and the data were analysed based on the diffusion-controlled process theory. Diffusion equations were solved by computer simulation method. Diffusion of the oxygen into the amorphous part of the polyethylene is discussed and diffusion constants at low temperature are estimated. Related values such as the activation energy of the diffusion process and the solubility constant, are also estimated.

E.s.r. studies on oxidation processes in irradiated polyethylene: 2. Diffusion of oxygen into crystalline regions

Abstract The oxidation process in irradiated polyethylene was investigated by means of the e.s.r. method, and diffusion of oxygen molecules into the crystalline region of polyethylene was studied in detail. Computer simulation was carried out in order to determine various parameters of the process including diffusion constant. In the course of the simulation, the crystallite was assumed to have a plate-shaped form and the diffusion constant was considered to be larger at the region near the surface of the crystallite, Df, and to be smaller at the inner region of the crystallite, Ds. Making these assumptions in the simulation gave a much better result than the assumption based on a sphere-shaped crystallite. D f D s was found to be about 2 for thicker crystallite and it was 4 ∼ 5 for thinner crystallite. Diffusion constants at various temperatures were determined, and the order of magnitude of diffusion constants in the crystalline region was found to be 10−16 cm2/sec at about 320K both for thicker and thinner crystallites. The activation energy of diffusion of oxygen in the crystallites was found to be 32 kcal/mol.

E.s.r. studies of peroxy radicals in polyethylene: 2. Peroxy radicals in urea — polyethylene complexes

Abstract Temperature dependent e.s.r. spectra of peroxy radicals in urea—polyethylene complex materials (UPEC) were discussed. Peroxy radicals in UPEC were much more stable than in normal polyethylene. Change of anisotropic g-values of the radicals with temperature were investigated and it was found that the g1-axis, which was assigned to be parallel to the chain axis, seemed to be almost unchanged throughout the temperature range in the present study, but the g2- and g3-axes changed drastically. Using these results, it was concluded that the main motion of the radical sites was rotation around the chain axis and this motion was found to be much faster than in normal polyethylene, which we have already studied and which we reported recently. This is a reflection of the properties of the materials studied, i.e. interaction between molecular chains in UPEC must be much weaker than in normal polyethylene because of the presence of the wall of urea molecules in the former.

Ionic products from the mechanical fracture of solid polypropylene

Abstract When polypropylene was fractured with tetracyanoethylene (TCNE) in the dark in vacuo at 77 K, the TCNE anion radical, TCNE — was detected (using e.s.r. methods). The TCNE — is produced by the attraction of an electron by TCNE from an anion arising from the mechanical fracture of solid polypropylene. This is presumably by ‘heterogeneous scission’ of main-chain carbon—carbon bonds in the polymer. Free radicals of the chain-end type arising from the mechanical fracture of solid polypropylene are also produced by ‘homogeneous scission’. At least 37% of scission occurs by the heterogeneous process.

Hindered oscillation and e.s.r. spectra of alkyl free radicals trapped in irradiated polyethylene crystals

Structures of alkyl radicals trapped in solution grown crystals of polyethylene and urea-polyethylene complex were studied in detail. A twisted configuration of methylene group with angle of 4.6° from the regular trans-trans configuration was found for the rigid state configuration of the alkyl radical at 77K. Temperature dependences of hyperfine splitting widths due to the β-protons were observed, and analysed in terms of the hindered oscillation of the β-methylene group. The value of the potential barrier height of hindered oscillation was found to be 900 kcal/mol for polyethylene crystal and 600 kcal/mol for urea-polyethylene complex; indicating that the radical site in urea-polyethylene complex is more mobile than that in polyethylene crystal.

E.s.r. studies on oxidation processes in irradiated polyethylene: 3. Direct detection of the reaction ROO· + R'H → ROOH + R'· , in urea—polyethylene complexes

Abstract Conversion of peroxy radicals into alkyl radicals ROO· + R′H → ROOH + R′ has been directly observed by the e.s.r. method in the case of polyethylene in a urea—polyethylene complex system. Abstraction of hydrogen atoms was found to be an intramolecular reaction in this case, of activation energy 24.6 kcal/mol. Evidence confirming this was provided by i.r. spectrometry.