Gamma-radiolysis of polysalicylide

Polysalicylide, a condensation product of acetylsalicylic acid, was irradiated with ⁶⁰Co-γ-rays. Viscosity, end-group analysis, and ultraviolet (UV) spectral measurement technique, and electrical conductivity were used to study the changes that occurred during γ-radiolysis. It was observed that polysalicylide undergoes chain scission caused by weak links. The enthalpy of fusion is 16.0 and 13.4KJ/mol^?1 repeat units of polysalicylide in air and N₂, respectively. The indirect effective band gap decreased and electrical conductivity increased after irradiation.     

γ-Irradiation of polystyrene emulsions

Aqueous polystyrene emulsions were subjected to γ-irradiation at 30°C and 0.6 Mrad/hr dose rate. Analogy with water-soluble systems suggests that such conditions might suppress chain scission and favor crosslinking. The (extrapolated) infinite-dose gel content and gel-permeation analysis of the polymer in the pre-gel region indicate that the extent of chain scission was negligible. The G (crosslink) value obtained from the dose for incipient gelation and molecular weight variations in the pre-gel region is 0.051. Charlesby-Pinner plots were linear, but linearity cannot be construed as indicating that chain scission has produced an effective random molecular weight distribution. Our results are consistent with the conclusions that crosslinking events are random and directly proportional to dose. The probability that a given crosslink is effective in increasing the gel content decreases with dose past the incipient gel point, however, because of cyclization on preformed gel. The crosslink density/dose ratio is a decreasing function of dose in this range. Attempts to predict dose–gel relations with assumptions of various initial molecular weight distributions were unsuccessful, possibly because of the neglected influence of cyclization on the measured gel content.     

Degradation and stability of polyaniline on exposure to electron beam irradiation (structure-property relationship)

Polyaniline (PAni) was prepared by electrochemical polymerization and subjected to different doses of electron beam (EB) irradiation. The effect of EB irradiation causes both chain scission and cross-linking process in PAni, which depends on irradiation dose. The degree of chain scission and cross-linking in PAni by EB irradiation is characterized through XRD, TGA, DSC, solubility, EPR and electrical properties measurement. The results reveal that with increase in EB irradiation dose from 0 to 150 kGy DC and AC conductivity and dielectric constant are found to increase mainly due to the chain scission or further doping in PAni. Due to irradiation there is change in the structure of PAni, such as decrease in the d-spacing, inter-chain separation, thermal stability and T_g but increase in the percent crystallinity and solubility. With further increase in the EB irradiation dose from 150 kGy onwards the DC and AC conductivity and dielectric constant are decreased due to the cross-link formation or dedoping in PAni, which causes the decrease in percentage of crystallinity and solubility and increase in d-spacing, inter-chain separation, thermal stability and T_g of PAni.     

Radiation-induced crosslinking of polyacrylonitrile fibers and the subsequent regulative effect on the preoxidation process

To investigate the radiation effect on polyacrylonitrile (PAN) fibers as well as on the preoxidation process, PAN fibers were irradiated by γ-rays at room temperature at 50–500 kGy in vacuum and then were thermally oxidized in air. Gel fraction determination indicated that γ irradiation led to the predominant crosslinking of PAN fibers, with G values (the number of event per 100 eV absorbed) of G(X)=0.28 and G(S)=0.16 for chain crosslinking and scission, respectively. It was found that irradiation caused a slight change in the crystal structure and tensile strength at low dose. Radiation led to a reduction of the onset temperature of cyclization reaction and moderated the exothermic behavior. The density of the PAN fibers after thermal oxidation was used to evaluate the preoxidation extent. It was proven that radiation could significantly accelerate the preoxidation process and consequently shortened the preoxidation time. Radiation crosslinking may have potential application in the production of PAN-based carbon fibers.     

γ-radiolysis of ketone polymers. III. Copolymers of ethylene and carbon monoxide

The γ-radiolysis of copolymers of ethylene and carbon monoxide (CO-PE) containing 0–9 wt-% of CO has been studied in the solid phase under vacuum with respect to hydrogen production, scission, and crosslinking. In this regard, the introduction of carbonyl groups into polyethylene increased both the scission and crosslinking efficiency. However, a theoretical statistical treatment for random scission and crosslinking, which was used to calculate G(s) and G(x) yields, predicted gelation with irradiation dose; this was confirmed experimentally. The increased G(x) values are attributed to the localization of absorbed energy in the carbonyl group arising from preferential trapping of thermalized secondary electrons. A stoichiometric balance is shown to be consistent with the reduction of one carbonyl group to a secondary alcohol per excess crosslink observed. The G(H₂) yield is reduced by the presence of ketone groups to a level which is in close agreement with the yield obtained for the model ketone 12-tricosanone. This quenching of H₂ production is explained by intramolecular electronic energy transfer along the chain to the carbonyl group with concomitant intermolecular charge transfer between neighboring molecules.     

Mechanical properties,surface chemistry,and barrier characteristics of electron beam irradiated‐annealed LDPE/PA6/LDPE multi‐layer films at N2

The effects of electron beam irradiation in the nitrogen environment, on chain scission, crosslinking, crystallinity, mechanical performance, and barrier properties of LDPE/PA6/LDPE multi‐layer films were studied. The evaluation of radiation‐induced crosslinking effect by the gel content measurement and Charlesby–Pinner plot suggested more of crosslinking over chain scission, in all the layers, which was more pronounced in polyethylene phase. The FTIR analysis results showed good agreement with those observed by the gel content measurements. It is believed that the crosslinking reaction had occurred through the C? N bonds in polyamide‐6, and vinyl group in polyethylene layers. The evaluation of radiation effect on the crystallinity and crosslinking of films by FTIR technique showed that by increasing the applied doses, the crystallinity in all the layers was decreased and the crosslinking was increased. The differential scanning calorimetry of irradiated samples revealed that due to the crosslinking reaction, the crystallinity was decreased by the applied dose. The tensile strength of the films was increased and the percent elongation at break was decreased, by increasing the applied doses. This study was also indicated that the radiation‐induced crosslinking effect on the tensile properties was dominantly observed up to 50 kGy. The surface free energy analysis of the films using the contact angle measurement and geometric mean equation indicated that the surface polarity was decreased by increasing the absorbed doses. It was found that due to the decline in the surface polarity and the simultaneously formation of crosslinked network in these films, both water vapor transmission rate and oxygen permeability were significantly decreased. Copyright © 2009 John Wiley & Sons, Ltd.     

γ-radiation-induced crosslinking of polystyrene

The γ-radiation-induced crosslinking of polystyrene was studied at 30–100°C in vacuo with a dose rate of 6.35 × 10⁵ rad/hr. The amount of hydrogen formation increased with increasing irradiation time, and the rate of the formation decreased with the time. The results were well described by the zero-order formation kinetics with respect to the hydrogen concentration combined with the first-order disappearance. The apparent rate constant for the formation of hydrogen increased somewhat with rising irradiation temperature, and the one for the disappearance was little affected by the temperature. The gel fraction increased with the time by the irradiation beyond the critical time for incipient gel formation, and the rate of gel formation decreased with the time. The gel formation was retarded by rising irradiation temperature, and only a little gel fraction was observed at 100°C. The G values for the crosslinking and main-chain scission were obtained from the gel data by using the Charlesby–Pinner equation. On the basis of these results, the mechanism of the γ-radiation-induced crosslinking of polystyrene was discussed.     

Temperature effects on radiation induced phenomena in polytetrafluoroetylene (PTFE)-Change of G-value

Temperature dependencies on the radiation induced phenomena and G-value of polytetrafluoroethylene (PTFE) have been studied in a temperature range 77–653 K. It is well known that main chain scission occurs very effectively below the melting temperature of PTFE (600 K). We have found in our experiments that G-value of chain scission increases significantly with increasing irradiation temperature, until 600 K. In addition to that, we have realized that crosslinking occurs by irradiation in the molten state at 613 K (Tabata, 1992; Oshima et al., 1995; Tabata et al., 1996). In the molten state, G-value of crosslinking was found to be 0.35 (number of crosslinking/100 eV absorption), as a lower limit, and the apparent G-value of chain scission is drastically reduced. Above 633 K, radiation induced crosslinking mainly occurs, however parallel thermal depolymerization or decomposition takes place to some extent.     

结晶聚合物的辐照效应——Ⅰ.尼龙-1010的聚集态结构对其辐射化学反应的影响

本文用DSC,WAXD,ESR和介电谱以及凝胶分数测定等技术,研究Nylon-1010的聚集态对γ-辐射化学反应以及辐照后热处理结果的影响。结果表明,辐照Nylon-1010的交联与裂解反应主要发生在非晶区或结晶表面,进一步证明了非晶区也是辐射后交联和后裂解主要反应区。     

Effect of gamma irradiation on density and thermal expansion changes of uniaxially oriented linear low-density polyethylene

The density and thermal expansion behaviour of linear low density polyethylene, over the temperature range from −95° to +10°C, is measured. The influence of drawing on density and thermal expansion behaviour is related to the chain scission and crosslinking induced by the γ irradiation.     

Stress relaxation of natural rubber during irradiation

The site of chain scission and crosslinking in vulcanized natural rubber irradiated with 4 MeV electrons has been determined by analysis of stress relaxation data. Sulfur and peroxide vulcanizates of different crosslink densities were prepared and the crosslink densities determined from stress-strain measurements. Stress relaxation was measured during irradiation using modified commercial relaxometers. The specimens were maintained in an atmosphere of nitrogen to minimize oxidative side effects. Scission is deduced to take place in the vicinity of crosslinks, since the rate of continuous stress relaxation is independent of crosslink density. Scission may be associated both with crosslinks initially present and with those subsequently introduced by irradiation. Crosslinking by radiation is largely a random process. However, there is a crosslinking reaction dependent to a slight extent on crosslink density as well as a small contribution from random scission reactions. G values for the random reactions are given.     

Polymer morphology and random chain scission of nylon 66

Nylon 66 films with varying spherulite size but almost constant percentages of crystallinity were prepared (melt method). These films were degraded by NO₂ over a range of temperatures from 35 to 65°C. Random chain scission took place except in the initial stages at low temperatures at which some crosslinking occurred. Observation of the films with the extent of degradation under polarized light revealed that dark bands developed around and also inside spherulite boundaries that became wider with the extent of degradation. This indicates that amorphous material is formed during random chain scission; the spherulites remained practically intact, however. The experimental chain scission rate constants did not change essentially with spherulite diameter until small diameters were reached, at which time the rate constants increased noticeably. Degradation can be accounted for by chain scission in amorphous and interfacial regions; in the latter the rate constants increased with this area and in addition main chain links were weaker in fold regions, due to strain energy, than normal ones in amorphous regions. The energy of activation for chain scission was compatible with a predominantly diffusion-controlled process.     

Photodegradation of ethylene–octene copolymers with different octene contents

The influence of the octene content on the photodegradation behaviour of ethylene–octene copolymers (EOCs) was revealed by investigating the photooxidation of low density polyethylene (LDPE) and EOCs with different octene contents through a series of characterisation methods. LDPE was very sensitive to ultraviolet light and the photostabilities of EOCs decreased with increasing octene concentration. The photodegradation of all samples produced hydroxyl, carbonyl and vinyl groups. The ease of chain crosslinking and scission was increased as the octene content rose. Crosslinking predominated in late irradiation period of LDPE while chain scission was dominant in that of EOCs. Annealing and chain scission promoted the secondary crystallisation of the crystallisable chain segments. Chain scission enhanced the crystallisation ability of the irradiated EOCs while it decreased that of the weathered LDPE. The photostabilities of crystals could be ranked as follows: the chain-folded lamellar crystals > the bundled crystals > the fringed micellar crystals. The thermal stabilities and mechanical properties of samples decreased with increasing irradiation time and the decreasing extent was correlated with the comonomer content.     

The effect of the composition on the properties of γ-irradiated ethylenvinyl alcohol copolymers

Stress-strain and dynamic-mechanical tests were performed on two ethylene-vinyl alcohol copolymers (Eval E and Eval F, with about 45% and 25% of ethylene respectively) irradiated in air. The experimental results, in agreement with the solubility and calorimetric results previously reported [1], show that both crosslinking and chain scission are present, and that a decrease in the degree of crystallinity with the irradiation dose occurs.     

Photo-oxidation of polymers—III: Molecular weight changes in the photolysis and photo-oxidation of polystyrene

Changes of molecular weight distribution resulting from vacuum photolysis and photo-oxidation of polystyrene have been determined by gel permeation chromatography. On irradiation at 253.7 nm, crosslinking is predominant in vacuo; in the presence of oxygen, crosslinking and main chain scission occur simultaneously because of light absorption by two or more different chromophores. Main chain scission is more important than crosslinking in the photolysis and photo-oxidation of polystyrene containing cumene hydroperoxide irradiated at 313 nm or containing benzophenone irradiated in the range 320–420 nm. In this last case, main chain scission and crosslinking are both strongly inhibited if the sample contains napthalene which acts as quencher of the excited triplet state of benzophenone. Only moderate inhibition is observed in the presence of 2,6-di-t-butyl-4 methylphenol.     

Effects of electron beam irradiation on poly(butylene adipate)diol

Effects of electron beam (EB) irradiation on poly(butylene adipate)diol (PBAD) were studied by means of GPC, DSC, and X-ray diffractometry. Below 5 Mrad, chain scission predominantly occurs, while above 10 Mrad, crosslinking and chain scission take place in parallel. Structure of EB-irradiated PBAD is mainly characterized by the main reactions, degradation and crosslinking. Crystallinity of PBAD increased by EB irradiation. This phenomenon was explained by reorganization due to high molecular mobility of EB-irradiated PBAD. But, increment of crystallinity decreased with increasing dose because of formation of crosslinking, excessive degradation and thermal effect of EB. As the result, the crystallinity of EB-irradiated PBAD with a high dose becomes lower than that of original PBAD by thermal treatment.     

The role of air in the radiolytic degradation of polycarbonate

Bisphenol A polycarbonate (PC) was irradiated with Co-60 γ-rays at room temperature in presence of air or nitrogen. Changes were followed by IR and NMR spectroscopy as well as GPC, X-ray diffraction, melting temperature, DTA, and isothermal TG. The changes observed include chain scission, chain branching/cross-links, decrease in thermal stability, crystallinity and scission of the product. However, there are some differences in samples irradiated in presence of air as opposed to those irradiated in nitrogen, but many changes are similar. Air accelerates the radiolytic degradation of PC. The radiochemical yield of chain scission, G(s), increased to 0.20 from 0.13; the chain branching/cross-links are less numerous as compared to those after irradiation in nitrogen the thermal stability decreases along with the temperature of that rate of maximum mass loss, crystallinity and the melting temperature. The chemical products in both air and nitrogen irradiations are almost identical. The formation of these products could be explained by multiple pathways-free radical formation and by ring as well as side chain attack. The studies suggest that Fries' rearrangement is not an important pathway during the radiolytic degradation as compared with photodegradation.     

Irradiation of chemically crosslinked ultrahigh molecular weight polyethylene

Acetabular cups for artificial hip joints were prepared by compression molding of ultrahigh molecular weight polyethylene in the presence of peroxide. Peroxide crosslinking led to a decrease in the degree of crystallinity, peak melting temperature, and recrystallization temperature, as well as decreased crystal perfection and size. Peroxide crosslinked cups were sterilized with gamma rays at room temperature in air atmosphere to an average dose of 3.4 Mrad. Irradiation produced further crosslinking in amorphous regions plus extensive chain scission of taut tie molecules and led to increased crystallinity and crystal perfection. A significant increase in carbonyl concentration was determined for irradiated specimens. In general, peroxide crosslinking reduces the effect of irradiation on the crosslinked network, because chemical crosslinking stabilizes chain fragments resulting from radiolytic scission and suppresses recrystallization of broken chains from amorphous regions. Wear rates were much lower for chemically crosslinked cups, which showed about one-fifth of the wear of control cups for the period from 0.5 to 1.0 million cycles. © 1996 John Wiley & Sons, Inc.     

Radiation-induced degradation of polycarbonate: Electron spin resonance and molecular weight measurements

Elementary processes in the γ-irradiated degradation of bisphenol A polycarbonate (PC) have been studied by electron spin resonance (ESR) and molecular weight measurements. γ-Irradiation of PC films at ?196°C gave an ESR spectrum attributable to trapped electrons, phenyl and phenoxy type radicals. An ionic species produced in PC was observed by optical absorption measurements. The viscosity average molecular weight, $\text{Mv}$, decreased with increasing irradiation dose both in vacuum and in air. A linear relationship between ($\text{1}\text{Mn}\text{(t)}\text{?}\text{1}\text{(}\text{Mn}\text{(0))}$ and irradiation dose was obtained, where $\text{Mn}$(t) is the number average molecular weight at irradiation time t. Random chain scission processes are suggested for γ-ray induced degradation of PC films. The ratio of the apparent rate constant of chain scission of PC in air to that in vacuum, k_s(air)/k_s(vac), was estimated to be 2·4. A mechanism for γ-ray induced degradation of PC is proposed on the basis of the experimental results.     

Evidence of crosslinking and chain scission in the degradation of poly(tert-butyl crotonate) by γ-irradiation

The effect of γ-irradiation on poly(tert-butyl crotonate) has been investigated by velocity sedimentation and solubility studies. Evidence of significant crosslinking as well as of chain scission has been obtained, the estimates of G(X) and G(S) being 0.66 and 0.59, respectively.