Radiolysis Of Poly Research Articles

Gas production in the radiolysis of Poly(dimethysiloxanes)

A variety of small poly(dimethyl siloxanes) were irradiated with γ-rays followed by the determination of the production of methane and molecular hydrogen and characterization of spectroscopic changes in the medium. The yields of methane was found to be about twice that of molecular hydrogen indicating that breakage of the C-Si bond occurs at a frequency comparable to the breakage of the C-H bond. Both yields slowly decrease with increasing molecular weight of the medium. The presence of oxygen decreases the yield of both gases suggesting radical precursors to methane and molecular hydrogen, presumably the methyl radical and H atom, respectively. Temperature gravimetric analysis and UV–visible spectroscopy both suggest the formation of higher molecular weight compounds with radiolysis, which agrees with bond loss and formation observed in infrared spectroscopy.

Gas Production in the Radiolysis of Poly(vinyl chloride)

The yields of H2 and Cl- were determined in the radiolysis of deaerated, aerated, and water mixtures of poly(vinyl chloride) (PVC) powders with gamma-rays and 5 MeV He ions. H2 yields with gamma-rays are low at about 0.25 molecule/100 eV and they double with He ion radiolysis indicating a second order formation process. The production of H2 in the gamma-radiolysis of water-PVC mixtures is much greater than expected from the weight fraction of the components and is due to acidification of the aqueous phase by the evolution of HCl from the polymer. Cl- yields in the gamma-radiolysis of PVC with number average weights of 22,000, 47,000, and 99,000 Daltons are 19.6, 33.8, and 32.5 atoms/100 eV. Cl- continuously evolves from the polymer for days following radiolysis. The extremely large yields suggest that a chain process involving radicals stabilized on the polymeric chain are responsible. Reflectance UV/vis and infrared spectroscopy show subtle changes in the PVC with radiolysis while UV/vis absorption spectra clearly indicate the formation of polyenes with 1 to 11 units. Cl- formation is probably initiated by Cl radical production followed by an electron rearrangement mechanism along the PVC chain to produce more Cl- and polyenes.

Comparative Study of the Radiolysis of Poly(uridylic acid) and Poly(methyluridylic acid) in Nitrous Oxide-Saturated Aqueous Solutions

A comparative study of the radiolysis of poly(uridylic acid) and poly(methyluridylic acid) in aqueous solutions saturated with N2O was performed. The radiation-chemical yields of formation of terminal phosphate groups, intact bases, 5-hydroxy-5,6-dihydrouracil, and 5-hydroxy-5,6-dihydro-3-methyluracil and the radiation-chemical yields of decomposition of these two polynucleotides were measured. Methylation was found to inhibit almost completely the formation of terminal phosphate groups and intact bases. It was suggested that the uracil-N(3)-yl radical can be a precursor of strand breaks in poly(uridylic acid).

Some aspects of the radiolysis of poly(methacrylic acid) in oxygen-free aqueous solution

Abstract Hydroxyl radicals generated pulse radiolytically in dilute N 2 O-saturated aqueous solutions of poly(methacrylic acid) (2.5×10 −3 –1×10 −2 mol dm −3 , MW=2.8×10 5 Da; p K a ≈7) react with this polymer with rate constants of 3.1×10 7 dm 3 mol −1 s −1 (protonated form) and 1.2×10 8 dm 3 mol −1 s −1 (deprotonated form). The primary and secondary carbon-centred radicals thus generated are characterized by an UV spectrum displaying a shoulder at around 320 nm. Changes in the UV spectrum in the 10 ms time range have been attributed to an H-transfer from a methylene group to a primary radical site. The secondary radicals undergo chain scission ( k =1.8 s −1 at pH 7–9) as has been followed by pulse radiolysis with conductometric detection. The ensuing end-chain radicals undergo efficient depolymerization resulting in the release of monomer.

The use of crosslinking promoters in the ?-radiolysis of poly(tetrafluoroethylene-co-perfluoromethylvinyl ether).I

Incorporation of 1 wt % of triallyl isocyanurate (TAIC) significantly enhanced the radiation crosslinking of the perfluoroelastomer, poly(tetrafluoroethylene-co-perfluoromethylvinyl ether) (TFE/PMVE). The dose for gelation was lowered by 70% with the presence of TAIC. The additive also improved the tensile properties of TFE/ PMVE both before and after crosslinking by irradiation. Higher radical yields were obtained with the presence of TAIC at 77 K, indicating the crosslinking promoter was acting as a radical trap. ESR studies showed that radiolysis of TAIC and subsequent photobleaching cleaved an allyl branch from the ring structure. Upon thermal annealing, an allyl radical on the TAIC molecule was observed. (C) 1999 John Wiley & Sons, Inc.

Radiolysis of poly(acrylic acid) in aqueous solution

Poly(acrylic acid), PAA, reacts with OH-radicals yielding -CHCH(CO 2H)- (β-radicals) and -CH 2C(CO 2H)- (α-radicals) in a ratio of approximately 2:1. This estimate is based on pulse radiolysis data where the absorption spectrum of the PAA-radicals was compared with the spectra of α-radicals from model systems. The β-radicals convert slowly into α-radicals ( k = 0.7 s −1 at pH 10). This process has also been observed by ESR. At PAA-concentrations of 10 −2 mol dm −3 chain scission dominates over other competing reactions except at low pH. The rate of chain scission was followed by pulse conductometry and in the pH range 7–9 k = 4 × 10 −2s −1 was observed. Oxygen reacts with PAA-radicals with k = 3.1 × 10 8 dm 3 mol −1 s −1 at pH 3.5 and k = 1.0 × 10 8 dm 3 mol −1 s −1 at pH 10. The corresponding peroxyl radicals undergo slow intramolecular H-transfer yielding a UV-absorbing product whose properties are that of 1,3-diketones.

Radiolysis of poly(phenyl methacrylate)

Powdered poly (phenyl methacrylate), synthesized by free-radical bulk polymerization, has been irradiated to doses, between 200 kGy and 600 kGy, of cobalt-60 γ-rays under an argon atmosphere. A G (scission) value of 1·8 has been calculated from the changes in number-average molecular weight as determined by gel permeation chromatography. Energies of activation for the decomposition of irradiated polymers have been calculated from the results of thermogravimetric experiments.

An ESR study of free radicals formed in irradiated poly(alpha-methyl styrene) by gamma-radiation

The radiolysis of poly(α-methyl styrene) with γ-radiation has been studied at 77 and 300 K by ESR spectroscopy. The radical intermediates formed at these temperatures have been identified by utilization of the techniques of photobleaching, thermal annealing and simulation. The radical yields were found to be 0.105±0.005 at 77 K and 0.05±0.003 at 298 K.

The kinetics of polymer degradation in solution—XIV. Radiolysis of poly(dimethyl itaconate)

Poly(dimethyl itaconate) (PDMI), underwent main-chain scission upon irradiation with 60Co-γ-rays or 16 MeV electrons in dilute 1,4-dioxane or acetone solution; crosslinking was not detected. In dilute 1,4-dioxane solution, a pronounced indirect radiation effect was found with G(S) p (related to the energy absorbed directly by the polymer) decreasing gradually from 4.8 to 2.3 scissions/100 eV for c PDMI increasing from 0.038 to 0.38 base mol/l. Pulse radiolysis experiments in conjunction with time-resolved light scattering measurements indicated at least two different intermediates, the decay of which resulted in main-chain scission. On the basis of results from quenching experiments with N 2O, the existence of a short-lived intermediate ( τ ⪡ 50 μsec), probably an electronically highly excited state resulting from the combination of electrons with positive ions, is suggested. Long-lived intermediates ( τ ≈ 3 msec) are attributed to lateral macroradicals formed by bond cleavage in the side groups. The decomposition of these radicals via main-chain scission was suppressed by O 2 and C 2H 5SH.

Synthesis, Thermal Properties, and Radiolysis of Poly(Cyclohexyl α-Chloroacrylate)

Abstract Cyclohexyl α-chloroacrylate (CCA) was polymerized by radical anionic and γ-radiation initiation. The anionic polymerization of cyclohexyl α-chloroacrylate gave moderately isotactic polymer in toluene and syndiotactic-rich polymer in THF. Poly(cyclohexyl α-chloroacrylate) (PCCA) was found to undergo two-stage weight loss in thermogravimetric analysis, and the first-stage weight loss was attributed to the lactonization reaction. PCCA degraded under γ-radiation, and the radiation yields of crosslinking and scission, G x and G s, were 0.6 and 3.8, respectively.

Radiolysis of poly(U) in oxygenated solution.

Aqueous N2O/O2-saturated solutions of poly(U) were irradiated at 0 degrees C and the release of unaltered uracil determined. Immediately after irradiation G(uracil release) was 1.5 which increased to a value of 5.3 +/- 0.3 upon heating to 95 degrees C. Thereby all of the organic hydroperoxides (G = 6.8 +/- 0.7) and some of the hydrogen peroxide (G = 1.7 +/- 0.2) was destroyed leaving G(peroxidic material; mainly hydrogen peroxide) = 1.0 +/- 0.7. G(chromophore loss) = 8-11 was measured immediately after irradiation, but no increase was observed upon heating. Addition of iodide destroyed the hydroperoxides and caused immediate base release to rise to G = 4 and further heating brought the value to that observed in the absence of iodide. In contrast, on reducing the hydroperoxides with NaBH4, immediate uracil release rose to only G = 2.8 and no further increase was observed on heating. A major product (G = 2.7) is carbon dioxide. There are also osazone-forming compounds produced (G = 2.7), all of which are originally bound to poly(U). Heating in acid solutions, as is required for this test, releases glycoladehyde-derived osazone (G = 0.8) and further unidentified low molecular weight material (G = 0.9). It is concluded that the primary radicals which cause these lesions are the base OH adduct radicals. In the presence of oxygen these are converted into the corresponding peroxyl radicals which abstract an H atom from the sugar moiety. In the course of this reaction base-hydroperoxides are formed. However, such base hydroperoxides cannot be the only organic hydroperoxides, but some (G congruent to 2.5) sugar-hydroperoxides must be formed as indicated by the increase in base release by the addition of iodide. It is speculated that a sugar-hydroperoxide located at C(3') is reduced by iodide to a carbonyl function at C(3'), a lesion that releases the base, while reduction with NaBH4 reduces it to an alcohol function at C(3') thus preventing base release.

On the kinetics of polymer degradation in solution—XII. Radiolysis of poly(2,2,2-trichloroethyl methacrylate)

The title compound (PTCMA) was irradiated in O 2-free dioxane solution by 60Co-γ-rays or by 100 nsec-pulses of 16 MeV electrons. At concentrations below ca 0.04 base mol/l, main-chain scission occurred as was concluded from the decrease of the light scattering intensity (LSI). G( S) = 3.7 ± 0.4 was independent of the polymer concentration and equal to G( S) observed with solid PTCMA. This result indicates that in dilute solution main-chain scission is induced by the direct action of radiation on the polymer. The free radical [presumably C(CH 3)(CH 2)] giving rise to main-chain scission has a lifetime of 2.6 msec. It reacts with ethane thiol (k = 6.6 × 10 4M −1sec −1). At concentrations above 0.04 base mol/l, the polymer crosslinked (gel formation, increase of the LSI according to 2nd order kinetics). It is assumed that crosslinking is due to the combination of radicals of the type CH 2C(CH 3)(COOCH 2CCl 2) formed via dissociative electron capture processes involving mainly electrons in spurs that otherwise recombine with parent ions. This conclusion was inferred from the finding that [( τ 1/2) 1] −1 ∞ ( c polymer) 2[( τ 1/2) 1): 1st half-life of LSI increase after the pulse, c polymer : polymer concentration].

Irradiation of poly(vinyl alcohol) fibers in the presence of chloroform and carbon tetrachloride

Radiolysis of poly(vinyl alcohol) fibers (PVA) in the presence of chloroform and carbon tetrachloride was investigated. Decrease in intrinsic viscosity was observed at lower dosages (up to 2.3 megarads); and above this, an increase was noted. The blank samples irradiated under similar conditions showed a continuous decrease in intrinsic viscosity. A discoloration in the samples irradiated in the presence of CCl4 and CHCl3 was also observed. It is attributed to double bond formation in the backbone. A marginal decrease in the tensile strength of the irradiated fibers was observed. However, the surface characteristics of the fibers did not change on irradiation. The thermogravimetric analysis revealed a better heat resistance in irradiated fibers.

Effect of molecular weight on the fracture surface energy of poly(methyl methacrylate) in cleavage

By the radiolysis of poly(methylmethacrylate) (PMMA), the fracture surface energy (γ) was determined at room temperature as a function of viscosity average molecular weight (¯Mv). Using a modified parallel cleavage technique, results showed that γ decreased more than two orders of magnitude with decreasing molecular weight. In the high molecular weight region (¯Mv≳105), γ (∼1×105 erg cm−2) was relatively insensitive to polymer chain length; whereas for 2.5×104≲¯Mv≲ 1×105, γ was strongly dependent on molecular weight. A linear regression analysis in the range ¯Mv=2 to 2.25×103 indicated that a truly glassy “Griffith” material was approached for which γ ≃ 750 erg cm−2. The results confirm the sigmoidal dependence of γ on molecular weight tested in notched tension. The apparent independence which variations in crack velocity have on γ with decreasing ¯Mv is shown and explained in terms of the increasingly brittle character of PMMA. Problems associated with the measurement and interpretation of experimental data are considered, particularly with respect to the lower ¯Mv regions.

Mechanism of the radiolysis of poly(methyl methacrylate) at 77K effect of ethanethiol on post‐irradiation reactions

Mechanism of the radiolysis of poly(methyl methacrylate) at 77K effect of ethanethiol on post‐irradiation reactions

Identification of free radicals produced by the radiolysis of poly(methyl methacrylate) and poly(methyl acrylate) at 77°K

AbstractThe evolution of the ESR spectra of poly(methyl methacrylate) (PMM) and poly(methyl acrylate) (PMA) gamma irradiated at liquid nitrogen temperature has been studied as a function of temperature between 100°K and 320°K. In both cases, six radicals have been identified which are already present at 100°K. Some of these are a direct consequence of high energy irradiation while others are due to subsequent hydrogen transfer reactions. The results are discussed on the basis of a mechanism previously proposed for the radioysis of PMM.

On the mechanism of radiation-induced crosslinking of poly(vinyl chloride) in the presence of some polyfunctional monomers

Some features of low-temperature radiolysis of poly (vinyl chloride) and the effect of added polyfunctional monomers such as triallyl cyanurate have been studied using the technique of infrared spectrophotometry. It has been found that the radiation yield of HCl formed in the radiolysis of poly(vinyl chloride) alone at 100°K is equal to 2.5. Additives form a complex with radiolytically produced HCl. The structure of this complex corresponds to on oxonium compound. The radiation yield of the complex reaches 50–80 per 100 eV of energy absorbed by additives. At the same time, the addition of triallyl cyanurate in poly (vinyl chloride) decreases the radiation yield of HCl associated with polymeric matrix by 2–3 times. The reaction of the allyl groups of the additive with the polymer free radicals already occurs at low temperatures. On heating, the irraditated at low temperature samples, the complex of additive with HCl decomposes, the remaining allyl groups disappear, and triazine rings are partly transformed to the isocyanurate form. These results are interpreted in terms of a mechanism which postulates the localization of the dehydrochlorination process of poly(vinyl chloride) in the vicinity of the additive molecules and the promotion of crosslinking efficiency by the formation of complex of additive with HCl.

The configuration, conformation and degradation of polyvinyl chloride

Abstract