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.     

Effect of gamma radiation on fluoropolymers

A series of fluoropolymer films were synthesized by reacting several bisphenol monomers with 1,3‐bis(1,1,1,3,3,3‐hexafluoro‐2‐pentafluorophenyl methoxy‐2‐propyl)benzene via a nucleophilic aromatic substitution to form polyethers. The bisphenols used included two diphenol‐substituted spirodilactams (SDL; aliphatic and aromatic), biphenol, bisphenol A, bisphenol AF, bisphenol O, and bisphenol F to form seven different polymers. Polymers were irradiated by a Gamma beam 651‐PT at a dose rate of 10.5 kGy/h; the absorbed dose in each film was varied between 300 and 1000 kGy. The effect on the chemical structure upon radiation was studied by DSC, TGA, FTIR‐ATR, and NMR after and before irradiation. Thermal analysis showed a lessened thermal stability and a lower T_g after irradiation. Further, irradiation caused a decrease in molecular weight as a result of cleavage of sp³ bonds. These data allowed calculation of the radiochemical yield scission (G_s) for each of these polymers. The SDL aromatic system proved the most radiation‐resistant. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1617–1626, 2009     

FTIR study of gamma-irradiated cis-1,4-polyisoprene

The effect of γ-irradiation on the structure and oxidation of cis-1,4-polyisoprene is investigated by ATR-FTIR technique. This method provides the valuable insight into the type of oxidation products produced and the extent and nature of intramolecular cyclization and chain scission reactions. The formation of ketones, alcohols and/or ethers, and hydroperoxides is apparent already at small doses of γ-radiation and it increases with the exposure time significantly. At the highest dose of 309 kGy a decrease in the intensity of C=O stretching mode of ketones (1717 cm⁻¹) was observed while the overall area of the band remained the same as in the case of 188 kGy dose. The shoulders observed at 1740 cm⁻¹ and 1772 cm⁻¹ could be assigned to C=O stretching frequency of esters and five-membered-ring lactones, respectively. Higher doses of γ-radiation also cause the formation of two relatively strong bands in the region of conjugated double bonds. These could origin from the aromatic products or cycloenes with one double bond formed by cyclization and chain scission processes.     

Mechanism of degradation induced embrittlement in polyethylene

The thermal oxidation of polyethylene films in air at 80 °C and 90 °C has been studied by tensile testing, IR spectrophotometry and molar mass determination from rheometric measurements. In the conditions under study, the polymer predominantly undergoes chain scission and embrittles suddenly when the weight average molar mass reaches a critical value (90 kg mol⁻¹), far before significant damage of the entanglement network (M_e = 1.9 kg mol⁻¹) in the amorphous phase.The following embrittlement mechanism is proposed: chain scission in the amorphous phase induces chemicrystallization. The thickness of the interlamellar amorphous layer (l_a) decreases until a critical value of the order of 6-7 nm, below which plasticity cannot be activated and the polymer behaves in a brittle manner, as previously shown for virgin polyethylene. Using (l_a, M_W) maps, it is possible to explain the differences observed in the embrittlement behaviour of semi-crystalline polymers predominantly undergoing chain scission.     

Radiation-Induced Cross-linking of a Tetrafluoroethylene-Propylene Copolymer

Radiation induced cross-linking of tetrafluoroethylene-propylene copolymer with γ-rays from a ⁶⁰Co source and with electron beam from a Van de Graaff accelerator was investigated.

The soluble fraction of the cross-linked copolymer in tetrahydrofuran was measured to determine the G value of the cross-linking and the probability ratio between cross-linking and chain scission. It was found that the tetrafluoroethylene-propylene copolymer can be cross-linked as well as polyethylene (i.e., G = 3). Moreover, it was also found that the copolymer could be cross-linked without chain scission under the absence of oxygen.

The cross-linking occurs mainly by the abstraction of hydrogen fluoride from polymer chains. This was determined from the analysis of gases evolved from the copolymer by irradiation by means of fluorometric and mass spec-trometric measurements.     

γ Radiolysis of poly(4,4′-isopropylidene diphenylene sebacate)

Poly-(4,4′-isopropylidene diphenylene sebacate) (PIDPS), a condensation product of bisphenol-A and sebacic acid, was irradiated with ⁶⁰Co γ rays. Viscosity, end-group analysis, and IR spectral measurement techniques were used to study the chemical changes occurring during γ radiolysis. It is observed that PIDPS undergoes random chain scission owing to weak links which may be present or be incorporated by the oxygen from air. The G value of random chain scission is estimated to be 9, whereas the enthalpy of fusion is found to be 6.2 kcal/mol repeat unit of PIDPS.     

Relevance of cage recombination in the plastic deformation of polymers

For a polymer in which permanent rupture of individual molecules is the rate-limiting process for plastic deformation, the kinetics of chain-end diffusion and secondary radical reactions should be compared with the kinetics of caged radical recombination in the calculation of activation parameters for plastic deformation. If mechanisms of cage escape are slower than those for cage recombination, the activation parameters for plastic deformation will differ from those for the initial bond-breaking process. For the case of polyethylene deformed in the vicinity of 250°K, the critical thermally activated event appears to involve scission of the polymer molecule near the site of an abstracted hydrogen atom. For this system the dominant cage-escape mechanism is diffusion, which is faster than either hydrogen abstraction or unzipping to the monomer. However, at low stresses the rate of cage recombination is expected to be higher than the rate of cage escape, so that the activation parameters for deformation should be the sum of those for chain scission and diffusion. The contribution of diffusion (ca. 15 kcal/mole) to the activation energy for deformation (E^*, extrapolated to zero stress conditions) is relatively modest. However, the calculated molar activation volume for deformation V^* increases by almost an order of magnitude, i.e., from ca. 10 to ca. 76 cm³/mole when diffusion is required. Consideration of experimental values of E^* and V^* for high molecular weight polyethylene indicates that, in the regime examined, chain scission plus chain-end diffusion is required to effect plastic deformation.     

Radiation Crosslinking of Chlorinated Poly(Isobutylene-co-Isoprene) with Polyfunctional Monomers

Abstract

It is well known that poly(isobutylene-co-isoprene) (butyl rubber, IIR) is degraded by irradiation. However, we demonstrated that IIR, after chlorination (CIIR) crosslinks by electron beam (EB) irradiation. The gel dose for CIIR was 3.8 kGy. To avoid scission of the main chain, various polyfunctional monomers were added for crosslinking of CIIR at lower doses. It was found that trimethylolpropane trimethacrylate (TMPT) is the most effective accelerator for crosslinking of CIIR at a lower dose. The tensile strength of EB crosslinked by CIIR increases almost linearly with increasing TMPT content.     

Radiation-induced degradation of butyl rubber vulcanized by three different crosslinking systems

Butyl rubber (IIR) is an isobutylene/isoprene copolymer and is provided with good properties including low permeability to gases, good thermal stability and high resistance to oxygen and ozone action, among others. It is well known that the major effect of ionizing radiations on butyl rubber is chain scission accompanied with a significant reduction in molar mass. This work aimed to study the effects of gamma radiation on the properties of butyl rubbers vulcanized by three different curing systems, such as, the ones based on sulfur, sulfur donor and phenolic resin to identify which curing system is the most stable under irradiation. The butyl rubber vulcanized by three different systems was gamma irradiated with doses of 25 kGy, 50 kGy, 100 kGy, 150 kGy and 200 kGy. Irradiated and non-irradiated samples were characterized by the following techniques: tensile, elongation and hardness. It was observed that doses higher than 150 kGy practically destroy the assessed properties for all butyl compounds, irrespective of the vulcanization system used; however compounds cured with phenolic resin showed a decrease in properties proportional to the dose.     

Effects of gamma irradiation on poly(vinyl chloride)/polystyrene blends: Investigation of radiolytic stabilization and miscibility of the mixture

Commercial polystyrene (PS) has been studied as a modifier for commercial poly(vinyl chloride) (PVC) when it was submitted to gamma irradiation. PVC/PS blends were prepared with 100/0, 95/05 and 90/10 compositions. Results for gamma-irradiated (⁶⁰Co) blends are reported and changes in viscosity-average molar mass (M_v) were analyzed. The study showed that the addition of PS into PVC decreased by 73% (95/05) and 79% (90/10) the number of scissions/100 eV in the dose range of 25-100 kGy. Viscosity analyses by the Pan et al. criterion and analyses of FT-IR spectra in the C-Cl vibration region showed negligible intermolecular interactions between the components of PVC/PS blends. However when the films of blends were irradiated to 50 kGy, certain intermolecular interactions were observed by the viscosity method. The addition of PS to PVC and the main scission effect induced by gamma irradiation decreased crosslink density of blends causing changes in the elongation of break and Young's modulus.     

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 degradation of polypropylene

Post-irradiation degradation of isotactic polypropylene irradiated by Co⁶⁰-γ-ray has been followed for 12 months. Effects of irradiation doses (10–100 kGy) up on the change of the structure and mechanical properties as well as flowability of this polymer has been studied. Carbonyl index increases with increasing post-irradiation time. The rate of which was much higher for doses above 50 kGy. Tensile strength declines with time and those samples irradiated above 50 kGy become quite brittle, just after irradiation. Results reveal that post-irradiation degradation of polypropylene irradiated by γ-ray occurs via chain scission mechanism.     

Polyterephthalates Bearing Bio-based Moieties

Dimethyl terephthalate was reacted with 5,5′-Isopropylidene-bis(ethyl 2-furoate), 1,4:3,6-dianhydrohexitols and ethan-1,2-diol in order to obtain PET incorporating bio-based moieties. Polycondensation was achieved in two steps: (i) the formation of a hydroxyethyl-terminated oligomer by reaction of starting diester mixture with excess ED and, (ii) a polycondensation step with elimination of ED was used to obtain high molar mass copolyesters. Copolymers of various compositions were synthesized and characterized by ¹H-NMR, SEC, DSC and TGA. The resulting materials are amorphous polymers (T _g = 104–127 °C) with good thermal stability.     

Main‐Chain Scission of a Charged Fifth‐Generation Dendronized Polymer

During divergent synthesis of the next higher‐generation dendronized polymer (DP), the fifth‐generation DP, PG5, with a number‐average degree of polymerization, (i.e., number of monomeric units) P_n, of ca. 500 underwent main‐chain scission. This happened in the step when its peripheral Boc groups were removed by the treatment with trifluoroacetic acid (TFA), and thus a heavily charged polyelectrolyte formed as an intermediate. Atomic Force Mircoscopy (AFM) analysis of the product after drop‐casting onto mica showed a large majority of short deprotected PG5 chains with P_n of ca. 40, as well as some smaller features that by MALDI‐TOF mass spectrometry and ¹H‐NMR spectroscopy were assigned to the hypothetical monomer, deprotected MG5. This behavior is compared to a recently reported main‐chain scission of a closely related PG5 which, however, resulted in significantly longer fragments. While this difference cannot yet be fully explained, questions are formulated which will guide future research.     

Hydrolytic stability of polybenzobisoxazole and polyterephthalamide body armor

Previous work conducted at the National Institute of Standards and Technology (NIST) to investigate the field failures of soft body armor containing the material poly(p -phenylene-2,6-benzobisoxazole), or PBO, revealed that this material was susceptible to hydrolysis, and a mechanism of this hydrolysis was proposed. In this work, viscometric estimations of the molar mass of environmentally conditioned PBO are used to support a previously proposed mechanism of PBO hydrolysis. Results with PBO were compared with poly(p-phenylene terephthalamide), or PPTA, which has been used in body armor applications for more than 30 years. Losses in tensile strength were found to correspond to a reduction in molar mass for PBO. This indicates that chain scission due to complete hydrolysis is occurring in this material. Similar trends were observed for PPTA, but the relationship between molar mass reduction and losses in tensile strength was not as evident for this material. Confocal microscopy, mechanical properties measurements, and molecular spectroscopy are used to further investigate the degradation of both PBO and PPTA.     

Photolysis of poly(o-acetylstyrene) in solution

Dilute solutions of poly(o-acetylstyrene) (POAS) were exposed to long-wave (λ ≥ 300 nm) UV radiation under high vacuum at 25 ± 1°C. Methane and much smaller amounts of ethane were formed, indicating α-cleavage (Norrish Type I). The quantum yield for CH₄ formation (5 × 10^?5 mol einstein^?1) was an order of magnitude lower than that observed for similar studies of POAS films. Molecular weight ( M _n) measurements indicate that chain scission occurs, and this is attributed to β-scission of the macroradicals formed by H-abstraction at the α-C atoms by the carbonyl triplet, and to a lesser extent, by the CH₃ radicals. Quenching by both naphthalene and cyclooctadiene conformed to Stern–Volmer kinetics. The effects on chain scission of a number of additives with varying transfer activities were found to be complex and unexpected. Rates of scission not only increased, even when substances with high transfer activity (e.g., cumene) were present, but also varied with the concentration of additive, being higher at lower additive concentrations. It would appear that solvent quality has a dominant influence (these additives are poor solvents). Tighter coiling of the polymer in the solutions containing poorer solvents results in more segment-segment contacts and with them more photoreduction and chain scission. However, at higher nonsolvent concentrations, diffusive separation of the fragments (and chain scission) becomes more difficult, and the balance is shifted in favor of cyclization (and perhaps also intermolecular crosslinking). The relative high photostability of POAS in solution (cf. thin film) has been interpreted in terms of increased competition from photoisomerization. © 1992 John Wiley & Sons, Inc.     

Effect of gamma irradiation on molecular weight of fluorinated aromatic polyethers

A series of fluoropolymer films was synthesized by reacting 1,3-bis(1,1,1,3,3,3-hexafluoro-2-pentafluorophenylmethoxy-2-propyl)benzene (12F-FBE) with a series of bisphenol monomers via a polycondensation reaction. The biphenols used included a diphenol-substituted spirodilactam, biphenol, bisphenol A, bisphenol AF, bisphenol F and bisphenol O. Polymers films from these new fluoropolyaryl ethers were irradiated by a Gamma Beam 657-PT at a dose rate of 9 kGy/h; the absorbed dose was varied between 30 and 150 kGy. The effect on the chemical structure upon radiation was studied by DSC, TGA, FTIR-ATR, NMR, and GPC, both before and after irradiation. The data obtained allowed the determination of the degradation radiochemical yield (G _s), between 0.24 and 7.43 crosslinking radiochemical yield (G _x), from 0.03 to 1.47 and the ratio of G _s/G _x was between 2.89 and 8.28. There was no apparent physical change, from 30 to 150 kGy; the films continue to be flexible and transparent after irradiation.     

Controlled Radical Polymerization of N-Vinylpyrrolidone by Reversible Addition-Fragmentation Chain Transfer Process

Summary: Poly(N-vinylpyrrolidone) (PNVP) was polymerized by RAFT process using diphenyldithiocarbamate of diethylmalonate (DPCM) as the reversible chain transfer agent in the presence of a small percentage of a conventional radical initiator (AIBN). The molar mass of the polymers synthesized by this method was found to increase with conversion and time. The presence of end group in the polymer chain could be confirmed by ¹H NMR spectra. The molar masses calculated using ¹H NMR spectroscopy and static light scattering (SLS) showed good agreement with the theoretical molar masses. The RAFT compound was fully consumed during the initial stages of the polymerization itself. The controlled nature of these polymers was further confirmed by generating diblock copolymers by sequential addition of monomers such as styrene or n-butyl acrylate (n-BA). PNVP efficiently participated as a macro-RAFT reagent, and cross-over reaction between the two blocks efficiently occurred. The successful diblock copolymer synthesis using PNVP as macro-transfer reagent further confirms the “controlled” nature of such synthetic procedure.     

Radical polymerization of N-vinylpyrrolidone in the presence of syndiotactic poly(methacrylic acid) templates

Radical polymerization of N-vinylpyrrolidone along poly(methacrylic acid) templates of high syndiotatic content was followed dilatometrically in dimethylformamide, which was used as solvent. The effects of template concentration, template molar mass, and temperature on polymerization rate and average molar mass of the formed polyvinylpyrrolidone (PVP) were examined. Template concentrations were varied around the critical concentration for homogeneous segmental distribution, C^*. Below this concentration, template coils can act as separate microreactors wherein growing PVP radicals exhibit maximum rate enhancement, i. e., relative rate ν_R = ν_R,max. In the free solution, blank polymerization occurs, i. e., ν_R = 1. Consequently, ν_R can be approximated by the equation ν_R = ?ν_R,max + (1 ? ?), where ? represents the volume fraction occupied by template coils. The slight increase in UR and PVP molar mass with the template chain length is supposed to be caused by the influence of translational diffusion on the termination step. Over the investigated temperature range of 50–70°C, the activation energy and entropy were almost identical for blank and template polymerization. An expected decrease of ΔE^≠ and ΔS^≠ in template systems is supposed to be compensated by the effects of desolvation of the template macromolecules during the propagation step.     

Preparation and characterization of cyclic poly(oxypropylene)

Poly(propylene glycol) [α-hydro-ω-hydroxypoly(oxypropylene)] of number-average molar mass M̄_n ≈ 2000 g · mol⁻¹ (PPG2000) was cyclised with high conversion (ca. 75%) by reaction with dichloromethane in the presence of powdered KOH. The cyclic product was separated from chain extended polymer by preparative GPC, giving an overall yield of polymer (M̄_n ≈ 2000 g · mol⁻¹, narrow molar mass distribution) in excess of 50%. Characterisation by analytical GPC and ¹³C NMR spectroscopy confirmed cyclisation. DEPT and ¹H-coupled NMR spectra were used to show that the links in cyclic poly(oxypropylene) were 77% single acetal, 12% double acetal and 11% triple acetal (or higher). This complexity probably results from competitive reaction with water introduced with KOH.