Crystalline and supermolecular structures in linear polyethylene irradiated with fast electrons

A report is given of structural investigations on high molecular weight, highly linear polyethylene crosslinked by irradiation with fast electrons in both the molten and the solid state. For the melt-irradiated samples it could be concluded from X-ray measurements that insertion of lattice distortions of the first kind occurs on crystallization. The density of these distortions shows a distinct maximum for an irradiation dose of 25 Mrad. It is assumed, that for lower doses the material crystallizes according to a folding procedure, for higher doses according to a micellar one. For solid state irradiated samples crystalline structure and morphology are not influenced measurably by the crosslinking. The recrystallization behaviour is independent on the dose for the resulting inhomogeneous network.presented in part at the Frühjahrstagung of the Deutsche Physikalische Gesellschaft, Fachausschuß Physik der Hochpolymeren, Marburg (1981).     

Modelling molecular weight changes induced in polydimethylsiloxane by gamma and electron beam irradiation

A commercial linear polydimethylsiloxane (PDMS) was subject to gamma irradiation under vacuum and in air, as well as to accelerated electron beam radiolysis (EB). All irradiation treatments were done at room temperature. The molecular weight changes induced by the radiation processes have been investigated using size exclusion chromatography (SEC) with refraction index (RI) and multi angle laser light scattering (MALLS) detectors to obtain the number and weight average molecular weights of the irradiated samples.

The analysis of the data indicates that crosslinking reactions predominated over scission reactions in all cases. Gamma irradiation under vacuum was the most efficient process within the analyzed dose range, reaching the gel point earlier. Irradiation in the presence of oxygen induces oxidative effects, both in gamma and EB irradiations. A previously developed mathematical model of the irradiation process that accounts for simultaneous scission and crosslinking and allows for both H and Y crosslinks fitted well the measured molecular weight data.     

Effect of electron radiation and triallyl isocyanurate on the average molecular weight and crosslinking of poly(ε‐caprolactone)

Investigation of the effect of electron radiation on the flow rate and average molecular weight of poly(ε‐caprolactone) (PCL) as well as on formation of the gel fraction of this polymer being irradiated in the presence of triallyl isocyanurate (TAIC) was the aim of the present paper. It was found that PCL macromolecules upon the electron radiation underwent both degradation and linking, because of which the polymer molecular weight increased. The processes associated with elongation of the polymer chains prevailed over the degradation ones. It was also found that PCL irradiated in the presence of TAIC underwent crosslinking resulting in formation of a significant amount of the gel fraction. The largest amount of this fraction was created upon the radiation with the dose of 60 kGy, which was confirmed by the results of determination of the swelling index. Changes in properties of PCL, occurring because of the electron radiation, are important for controlling viscosity of polymeric materials during processing of these materials. Copyright © 2015 John Wiley & Sons, Ltd.     

Crosslinking of poly(-caprolactone) by radiation technique and its biodegradability

Poly(-caprolactone), PCL, (melting temperature, 60°C) was gamma-irradiated in the solid state at 30–55°C, the molten state, and the supercooled state (irradiation at 45–55°C after melting, 80°C) under vacuum to improve its heat resistance. Irradiation of PCL in the supercooled state led to the highest gel content and this polymer has high heat resistance. On the other hand, relatively smaller doses such as 15 and 30 kGy were effective to improve processability of PCL by formation of branch structure during irradiation.     

RADIATION CROSSLINKING OF BIODEGRADABLE POLY(BUTYLENE SUCCINATE) AT HIGH TEMPERATURE

Poly(butylene succinate), (PBS) with different molecular weight was γ-irradiated at different temperatures and various doses. PBS with high molecular weight and smaller peak area of crystal melting gave the highest gel content at the same temperatures and dose. A two-step irradiation (irradiation in molten state after irradiation at room temperature) gave the highest gel content in different conditions. This is due to the formation of network structure by pre-irradiation at room temperature that leads to less degradation. PBS prepared by two step irradiation was effective for improvement of heat stability because of high gel content formation. Unirradiated PBS sheets broke immediately at 110°, while the irradiated sample (gel fraction, 50%) by a two step-method, did not break even up to 200 minutes at 130°C. The PBS sheets are biodegradable even after crosslinking.     

Radiation-induced degradation and crosslinking of poly(ethylene oxide) in solid state

The effects of g-irradiation on solid poly(ethylene oxide) (PEO) of an initial weight-average molecular weight of 6.3^.10⁵ Da were investigated by gel permeation chromatography and viscometry. The parameters studied were changes in number- and weight-average molecular weight, molecular weight distribution and viscosity of PEO in aqueous solution. Irradiation of poly(ethylene oxide) powder in the presence of oxygen leads to the dominance of chain scission reactions. Their high radiation-chemical yield [G(scission) » 2.5^.10^-6 mol/J] indicates the occurrence of effective chain reactions. Upon irradiation in vacuum, crosslinking and scission occur side-by-side and the changes in molecular weight are less pronounced in the studied dose range (up to 20 kGy). Scission dominates for doses up to ca. 15 kGy, while for higher doses intermolecular crosslinking gains in importance. The competition between these processes seems to depend not only on the applied dose but also to be influenced by the inhomogenity of the material (molecular weight and/or possibly the crystallinity). Parallel occurrence of scission and crosslinking leads to the broadening of the molecular weight distribution.     

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.     

Effect of solvent on radiation grafting and crosslinking of polyethylene

Crosslinking of polyethylene influences its swelling properties. It could be expected that pre-crosslinking of polyethylene influences the rate and yield of grafting as well. This is demonstrated by pre-crosslinking of polyethylene and by its subsequent grafting with styrene after the trapped radicals had been annealed out.In order to obtain more direct information about the influence of swelling agent on polyethylene crosslinking, the elastic modulus of the crosslinked polyethylene was investigated. Stress–strain curves of polyethylene samples irradiated in different environments were recorded in molten state at 165 °C. The results show that irradiation of swollen polyethylene produces fewer effective crosslinks than does irradiation of dry polymer.     

13C-NMR study of crosslinking and long-chain branching in linear polyethylene induced by 60Co gamma ray irradiation at different temperatures

H-type crosslinking and Y-type long-chain branching have been examined for linear polyethylene samples irradiate at different temperatures with ⁶⁰Co gamma rays in vacuum by solution-state ¹³C-NMR spectroscopy. In this study, relatively low molecular weight samples were irradiated with doses less than a gel dose in both solid and molten states. Resonance lines associated with H-links and Y-branches have clearly been observed for each irradiated sample and their radiation yields significantly depend on the irradiation temperature and the morphology of the samples. In particular, the G-value for the production of Y-branches is higher than that for H-links at lower temperatures, while the latter is superior to the former at higher temperatures. This may be due to the decrease in the concentration of the primary radicals, which are formed by main-chain scissions and associated with the production of Y-branches, by the recombination with small fragments at higher temperatures. The radiation yields of methyl branches and double bonds are also briefly discussed.     

Elastic behavior,birefringence, and swelling of amorphous polyethylene networks

Polyethylene networks were prepared by γ-irradiation of linear polyethylene, both molten and crystalline. The elastic and photoelastic properties of the networks were studied at high temperatures, i.e., in the molten state. The equilibrium swelling was also measured in several solvents. Values of the crosslinking efficiency G of γ-radiation, the molecular, weight M_e between entanglements, the optical anisotropy α of the equivalent random link, and the polymer-solvent interaction parameter μ are deduced. Samples prepared by irradiation in the amorphous state showed markedly non-Gaussian elastic behavior. The presence of a large non-Gaussian term in the optical anisotropy is also deduced. The value of α obtained for swollen samples, which showed substantially Gaussian elastic behavior, was 3.9 × 10^?24 cm.³, about one-half of that obtained for dry samples. It corresponds to an equivalent random link of only about 5 CH₂ units, on the basis of Denbigh's values for bond polarizabilities. The samples prepared by irradiation in the crystalline state showed lower values for α, which also depended upon the degree of crosslinking. This is attributed to the nonrandom chain configurations prevailing at the time of crosslinking. The same samples were found to show more nearly Gaussian elastic behavior, which is attributed to the same cause.     

Poly(ethylene oxide) irradiated in the solid state,melt and aqueous solution—a DSC and WAXD study

Interactions of the aggregate state of poly(ethylene oxide), PEO, and γ-irradiation conditions (total dose, atmosphere) on its thermal and crystalline properties were investigated by DSC and WAXD taking into account sample molecular mass and form. In PEO irradiated in the solid state and in the presence of oxygen, chain scission dominated over concurrent crosslinking up to 200 kGy, particularly in PEO powders, due to a large surface being in contact with air. In solid samples the degree of crystallinity and crystallite size increased with the dose up to 50 kGy, probably not just due to partial crystallization upon degradation of amorphous phase, but to recrystallization of broken tie molecules. The least changes in crystallinity and phase transformation temperatures occurred in solid films. A substantial decrease in crystallinity and transformation temperatures without the initial crystallinity increase was achieved in samples that were amorphous on irradiation, at temperatures above the PEO melting temperature and in aqueous solutions. Radiation crosslinking of the PEO aqueous solution in an inert atmosphere is the most suitable way to obtain a lower degree of crystallinity and phase transformation temperatures while preserving mechanical properties.     

含硫醚可溶性聚芳醚酮的合成及电子束辐照的研究

聚醚醚酮(PEEK)因具有优异的机械性能、耐热性、耐化学腐蚀性等优点而广泛应用于航空航天、电子器件、机械仪表等领域．具有刚性结构的聚醚醚酮有极好的耐溶剂性,不溶于一般的有机溶剂,并且需在较高的温度下进行加工．近20年来,人们不断开发性能优异的聚醚醚酮新材料,在聚合物主链上引入不同的功能基团(如萘环、氮杂萘环等)及侧基功能基团(如苯、三氟甲基等),以提高聚芳醚酮的溶解性,或者通过共聚和共混等方式进一步改善材料的使用性能和加工性能,     

辐照对聚己内酯结构和性能的影响

研究了γ 辐照对不同分子量的聚己内酯 (PCL)的力学性能、热性能、结晶行为的影响 ,在此基础上 ,研究了辐射交联后的PCL的形状记忆行为 .研究结果表明 ,PCL的分子量越大 ,辐射交联所需的凝胶化剂量越低 .溶胶分数S +S1 2 与 1 D的关系很好地符合Charlesby Pinner关系式 ,说明PCL的辐射交联属于无规交联 .剂量对PCL的力学性能影响显著 ,剂量越大 ,抗张强度和断裂伸长率下降越多 ,但分子量较高的PCL的抗张强度受剂量的影响较小 .DMA分析表明 ,聚己内酯辐照交联后的弹性模量和耐热性能显著提高 .交联度较高的PCL表现出高弹态 ,可以拉伸 ,并具有较好的形状记忆行为 .DSC分析表明 ,辐射交联使PCL的结晶度有所增加 ,但也使结晶熔点有所降低     

Modification of perfluorinated polymers by high-energy irradiation

This review surveys about the possibilities for the modification of perfluorinated polymers using high-energy irradiation: degradation, functionalization, branching, and cross-linking. The reaction mechanisms for the different reaction conditions are discussed. Electron irradiation of polytetrafluoroethylene (PTFE) with a very high dose leads to a complete degradation of the macromolecules to low-molecular products. In the presence of oxygen perfluorocarboxylic acids and in an inert atmosphere, mixtures of perfluorinated olefins and paraffins can be obtained. Virgin PTFE is disintegrated by high-energy irradiation in air with a lower dose into a micropowder modified with COOH groups. This powder can be homogeneously incorporated in other polymers. So, the special properties of PTFE can be made effective in these polymers. Micropowders functionalized with COOH groups and polyamides (PA) form by reactive extrusion PTFE-PA blockcopolymers which can be used as slide bearing materials. The copolymers poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) and poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether) (PFA) irradiated in air show a significantly higher degree of COOH functionalization compared with PTFE. Irradiation of molten PTFE in an inert atmosphere leads to formation of different kinds of double bonds, CF₃ side groups, long-chain branches as well as cross-links. Irradiation of PFA in vacuum results in the generation of COF and COOH groups; in molten state also branches and cross-links are formed.The focus of the present paper is on the work that has been carried out at the Institute of Polymer Research Dresden.     

Photochemical transformation of diketone dopants in polyester matrices: Effect of dopants concentration and polyester structure on changes in molecular characteristics and hydrolysis of the matrices

Photochemical transformations of photoactive diketone derivatives, 1,2-diphenylethane-1,2-dione (benzil, BZ) and 1,4-bis(benzoylcarbonyl)benzene (bisbenzil, BIS), and the effects of their transformations on changes in the molecular characteristics and hydrolytic properties of poly(ε-caprolactone) (PCL) and polylactide (PLA) matrices are reported. Studies were performed in a broad dopant concentration range of 0.2–7.5 wt%. Structural changes were observed by FTIR spectroscopy. FTIR spectroscopy proved the progressive transformation of BZ to benzoyl peroxides and, in the case of PCL, the formation of polar groups. Changes in molecular characteristics were observed in GPC traces. Irradiation of the doped polymer films at λ > 400 nm under an air atmosphere resulted mainly in the degradation of the polymer matrices. Additionally, high dopant concentrations in PCL led to partially crosslinked structures with gel contents up to 15%. The photochemical transformation of dopants had a large impact on the PCL matrices compared to that of the PLA matrices. A subsequent hydrolysis process was investigated by determining the extractable products as well as using GPC. Hydrolysis of irradiated PCL samples showed a significant acceleration of hydrolysis compared to that of nonirradiated samples. Furthermore, the hydrolysis rate gradually increased with increasing dopant concentration. The PCL crosslinking and degradation is adjustable by the type of dopant, concentration of dopant and intensity of light during irradiation of PCL. In contrast, a photochemical pretreatment had no observable effect on the hydrolysis of PLA under the investigated conditions.     

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.     

Influence of electron beam irradiation on physicochemical properties of poly(trimethylene carbonate)

Electron beam (EB) irradiation of poly(trimethylene carbonate) (PTMC), an amorphous, biodegradable polymer used in the field of biomaterials, results in predominant cross-linking and finally in the formation of gel fraction, thus enabling modification of physicochemical properties of this material without significant changes in its chemical structure. PTMC films (M_w: 167-553 kg mol⁻¹) were irradiated with different doses using an electron accelerator. Irradiation with a standard sterilization dose of 25 kGy caused neither significant changes in the chemical composition of the polymer nor significant deterioration of its mechanical properties. Changes in viscosity-, number-, weight-, and z-average molecular weights of PTMC for doses lower than the gelation dose (D_g) as well as gel-sol analysis and swelling tests for doses above D_g indicate domination of cross-linking over degradation. EB irradiation can be considered as an effective tool for increasing the average molecular weight of PTMC and sterilization of PTMC-based biomaterials.     

The influence of irradiation temperature and oxygen on crosslink formation and segment mobility in gamma-irradiated polydimethylsiloxanes

By swelling to equilibrium and esr spin probe technique the changes in crosslinking density and polymer segment mobility in γ-irradiated polydimethylsiloxanes (PDMS) were determined in dependence of irradiation atmosphere and irradiation temperature. In comparison with the polymer samples irradiated in the presence of oxygen, the samples irradiated in vacuum showed in spite of a higher degree of crosslinking a higher mobility of the polymer segments. The discrepancy can be explained taking into account that irradiation in presence of oxygen generates polar side groups: -COOH, -CHO and -OH. Dipolar interactions and hydrogen donor acceptor interactions should contribute to the decreased mobility of polymer segments in presence of these groups.     

融熔态聚乙烯辐射交联效果的溶液高分辨^13C NMR研究（Ⅱ）

研究了聚乙烯在低于凝胶化剂量的~(60)Co γ线辐照后,交联结构的形成及其影响因素。结果“H”、“Y”型交联链的生成不仅与辐照剂量有关,而且还与原试样中所含微量异种结构单元相关,但与试样的分子量无关。还讨论了融熔态聚乙烯的辐射交联机制。     

RADIATION DEGRADATION OF CHITOSAN IN THE PRESENCE OF H2O2

INTRODUCTIONChitosan, poly-β-(1 -?4)-D-glucosamine, can be obtained from chitin by deacetylation with alkali. It is soluble indilute acidic medium due to the presence of amino groups. The use of chitosan in many areas, such as foodprocessing, biochemistry, Pharmaceuticals, medicine, and agriculture has been developed over the pastdecades[1,2].In recent years, it has been reported that many properties of chitosan depend on the molecular weight[3]. Thechitosan oligomers possess better fun…