Radiation Crosslinking Research Articles

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 50–500kGy 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 100eV 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.

Effect of Gamma Irradiation Crosslinking on PA6 Fibers

Gamma irradiation crosslinking of nylon6 fibers with and without the presence of triallyl cyanurate(TAC) was studied, XRD、DSC、IR and TG were used to analyze the effects of irradiation crosslinking on structure and properties of PA6 fibers. The melt-drip performance of PA6 fibers was improved after irradiation crosslinking. The breaking strength increased first then decreased as a function of absorbed dose, while elongation at break decreased. The melting temperature and crystallinity decreased with increasing the dose. Besides, the onset temperature of degradation fell slightly and the amount of non-volatile residue at 600oC increased as the irradiation dosage increased.

Multi-walled carbon nanotubes acting as free radical scavengers in gamma-irradiated ultrahigh molecular weight polyethylene composites

Multi-walled carbon nanotubes (MWCNTs) were incorporated in ultrahigh molecular weight polyethylene (UHMWPE), which is a polymer used in industrial and orthopedic applications. The composites were prepared by ball milling and thermo-compression processes at concentrations up to 3wt.% and subsequently gamma irradiated at 90kGy. Electrical conductivity measurements showed a low percolation threshold of 0.5wt.%. Electron spin resonance detection of the radiation-induced radicals proved the radical scavenger behavior of MWCNTs: when the nanotube concentration increased, the number of radicals generated by the gamma irradiation process decreased. Allyl radicals seem to be the radicals most affected by the presence of nanotubes in this polymeric matrix. Fourier transformed infrared spectroscopy measurements and an accelerated ageing protocol were performed to ascertain the influence of the irradiation on the oxidation index. The results pointed to the positive contribution of the MWCNTs in increasing the oxidative stability of the composite compared to pure UHMWPE. Crosslinking density induced by gamma irradiation was obtained by swelling measurements. The findings showed that, despite the radical scavenger performance, MWCNTs are capable of maintaining the efficiency of the crosslinking density, unlike the other antioxidants, which inhibit radiation crosslinking.

Sustained-release of flutamide from radiation-crosslinked poly(4-acryloyl morpholine-acrylic acid) hydrogels

The present work reports the effect of radiation cross-linking on aqueous solution of 4-acryloyl morpholine (ACMO)/acrylic acid (AA) in the presence of flutamide (FLT), an anticancer drug. Use of a γ-irradiation process within 5 kGy provided an insoluble hydrogel. Ultraviolet-visible absorption, fluorescence spectroscopy, and Fourier transform infrared spectroscopy were employed to study the complex formation of FLT with ACMO and AA. The ability of the prepared ACMO/AA-based hydrogels to be used as drug carriers for anticancer-specific drug delivery system was estimated using FLT as a model drug. In vitro drug release studies in a pH 7 buffer solution show that 20%–30% of the FLT was continuously released from the FLT-incorporated ACMO/AA hydrogel within 1 day, followed by a gradual release upon 7 days under in vitro non-degradation hydrogel conditions. The recognition affinity of the ACMO/AA/FLT hydrogel is increased when synthesized in a high-concentration template solution. Open image in new window

Long-term safety and efficacy follow-up of prophylactic higher fluence collagen cross-linking in high myopic laser-assisted in situ keratomileusis

The purpose of this study was to evaluate the safety and efficacy of ultraviolet A irradiation cross-linking on completion for cases of high myopic laser-assisted in situ keratomileusis (LASIK). Forty-three consecutive LASIK cases treated with femtosecond laser flap and the WaveLight excimer platform were evaluated perioperatively for uncorrected visual acuity, best corrected spectacle visual acuity, refraction, keratometry, topography, total and flap pachymetry, corneal optical coherence tomography, and endothelial cell count. All eyes at the completion of LASIK had cross-linking through the repositioned flap, with higher fluence (10 mW/cm(2)) ultraviolet light of an average 370 μm wavelength and 10 mW/cm(2) fluence applied for 3 minutes following an earlier single instillation of 0.1% riboflavin within the flap interface. Mean follow-up duration was 3.5 (range 1.0-4.5) years. Mean uncorrected visual acuity changed from 0.2 to 1.2, best corrected spectacle visual acuity from 1.1 to 1.2, spherical equivalent from -7.5 diopters (D) to -0.2 D, keratometry from 44.5 D to 38 D, flap pachymetry from 105 μm to, total pachymetry from 525 to 405, and endothelial cell count from 2750 to 2800. None of the cases developed signs of ectasia or significant regression during follow-up. Prophylactic collagen cross-linking for high-risk LASIK cases appears to be a safe and effective adjunctive treatment for refractive regression and potential ectasia. This application may be viewed as prophylactic customization of the biomechanical behavior of corneal collagen.

Surface-patterned microgel-tethered molecular beacons

We use focused electron beams to create surface-patterned biotinylated poly(ethylene glycol) microgels to which molecular beacon probes are immobilized viabiotin–streptavidin binding. We hypothesize that immobilizing molecular beacon probes to highly hydrated microgels will maintain the high performance typical of untethered molecular beacon probes free in solution. Monte Carlo simulations show that the microgel mesh size resulting from focused radiation crosslinking is highly non-uniform and approaches infinity at the microgel–water interface. This structure is important because the extremely liquid-like environment at the diffuse microgel surface reduces the compromising effects that hard surfaces can exert on molecular beacon secondary conformation, accessibility of nucleic acid targets to the molecular beacons, and the efficiency of energy transfer between the molecular beacon fluorophore and quencher. We assess the performance of microgel-tethered molecular beacons using molecular beacons designed to distinguish between methicillin-sensitive and methicillin-resistant Staphylococcus aureus. We show that each microgel presents approximately 11 800 such molecular beacons and that each molecular beacon on average occupies an area of microgel surface corresponding to an individual streptavidin molecule. The signal-to-background ratio we measure ranges from 40–50, substantially higher than many other surface-tethering approaches. Furthermore, this platform exhibits both low non-specific background and high specific fluorescence when the microgel-tethered molecular beacons are exposed to multiple targets, and it thus promises to lend itself well to high-sensitivity, self-reporting oligonucleotide based assays.

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 200kGy, 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 50kGy, 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.

The John Charnley Award: An Accurate and Sensitive Method to Separate, Display, and Characterize Wear Debris: Part 1: Polyethylene Particles

Numerous studies indicate highly crosslinked polyethylenes reduce the wear debris volume generated by hip arthroplasty acetabular liners. This, in turns, requires new methods to isolate and characterize them. We describe a method for extracting polyethylene wear particles from bovine serum typically used in wear tests and for characterizing their size, distribution, and morphology. Serum proteins were completely digested using an optimized enzymatic digestion method that prevented the loss of the smallest particles and minimized their clumping. Density-gradient ultracentrifugation was designed to remove contaminants and recover the particles without filtration, depositing them directly onto a silicon wafer. This provided uniform distribution of the particles and high contrast against the background, facilitating accurate, automated, morphometric image analysis. The accuracy and precision of the new protocol were assessed by recovering and characterizing particles from wear tests of three types of polyethylene acetabular cups (no crosslinking and 5 Mrads and 7.5 Mrads of gamma irradiation crosslinking). The new method demonstrated important differences in the particle size distributions and morphologic parameters among the three types of polyethylene that could not be detected using prior isolation methods. The new protocol overcomes a number of limitations, such as loss of nanometer-sized particles and artifactual clumping, among others. The analysis of polyethylene wear particles produced in joint simulator wear tests of prosthetic joints is a key tool to identify the wear mechanisms that produce the particles and predict and evaluate their effects on periprosthetic tissues.

Effect of gamma irradiation on linear low density polyethylene/magnesium hydroxide/sepiolite composite

Radiation crosslinking is generally used to improve the thermo-mechanical properties of the composites. A study has been carried out to investigate the effect of gamma radiation on the thermo-mechanical properties of linear low density polyethylene containing magnesium hydroxide (MH) and sepiolite (SP) as non-halogenated flame retardant additives. The developed composites are irradiated at different doses upto maximum of 150 kGy. Infrared spectra of the irradiated composites reveal the reduction in the intensity of O–H band with increase in the absorbed doses, thus indicates a distinct structural change in MH at higher doses. The thermogravimetric analysis results of unirradiated and composites irradiated at low doses (≤75 kGy) show two steps weight loss, which is changed to single step at higher doses with lower thermal stability. The melting temperature ( T m) and crystallization temperature ( T c) of irradiated composites are lowered with irradiation whereas Vicat softening temperature (VST) is increased. The increasing trend in gel content with increase in the absorbed dose confirms the presence of crosslinked network. The mechanical properties, results show significant improvement in the modulus of irradiated composites. The results also confirm that MH gradually loses its OH functionality with irradiation.

Preparation and characterization of SPEEK membranes crosslinked by electron beam irradiation

Crosslinked sulfonated poly(ether ether ketone) (SPEEK) membranes were prepared by the radiation crosslinking of SPEEK with various crosslinker contents. The prepared membranes were subjected to a comparative study of proton exchange membranes (PEM) for fuel cell applications. The crosslinked SPEEK membranes were characterized by 1H nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR), dynamic mechanical analysis (DMA), and Small angle X-ray scattering (SAXS). 1H NMR and FTIR-ATR confirmed the sulfonation and crosslinking of the membranes. DMA indicated that the ionic modulus and cluster T g increased with decreasing crosslinker content. This suggests that the amount of clustering in the crosslinked membrane could be reduced by increasing the crosslinker content. SAXS data showed that the crosslinker did not affect the ionic domain morphology in the membranes. The proton conductivity of all the membranes was > 10−2 S/cm. The overall findings suggest that the crosslinked SPEEK membranes offer the possibility for improving the performance of proton exchange membrane fuel cell (PEMFC), provided membranes with thermal and hydration stability.

High Density Polyethylene Membrane Filled with Alumina Prepared by a Gamma Ray Irradiation

High density polyethylene (HDPE) membrane filled with alumina particles was prepared by a wet process for a Li-ion secondary battery. Soybean oil and dibutyl phthalate (DBP) were premixed as the co-diluents. Gamma ray irradiation was used for crosslinking of HDPE. The HDPE membrane filled with alumina particles had excellent mechanical property and thermal stability due to the alumina particles and irradiation crosslinking. The tensile strength of the membrane increased with an increased amount of alumina up to 15 wt%. The thermal shrinkage of the membrane decreased with an increased amount of alumina up to 15 wt%. The electrochemical stability of the irradiated membrane after extraction was improved with irradiation dose up to 50 kGy.

Wettability and Water Absorption of Irradiation Cross-Linked UHMWPE

Ultra-high molecular weight polyethylene (UHMWPE) was prepared by thermal compression molding method and was irradiated by gamma rays at dose of 100 kGy in air. The wettability and water absorption of non-irradiated and irradiation crosslinked UHMWPE were investigated. It was shown that irradiation crosslinking increased the wettability of UHMWPE. The water absorption rate of UHMWPE was extremely low (<0.02%). After irradiation crosslinking, the water absorption rate of UHMWPE increased.

Performance comparison of modified poly(vinyl alcohol) based membranes in alkaline fuel cells

There are several problems which are holding back the use of fuel cells. The utilization of fuel cells depends on the start-up costs which are very high due to the use of expensive materials for their construction. In that respect, we describe a cost-effective alkaline fuel cell (AFC) that uses solid, polymer based, membrane instead of conventionaly used, highly concentrated, corrosive, liquid alkaline electrolyte. This approach to AFC is potentially the basis of a simple, low-cost system, that can solve one of the problems of the highly-efficient and environment-friendly AFC. The focus of this paper are low cost composite alkaline membranes, based on poly(vinyl alcohol) (PVA). The PVA matrix is made by solution cast method and gamma irradiation crosslinking. Three different types of membranes are obtained in this manner – plain PVA membrane, PVA membrane cross-linked using gamma irradiation (γ-PVA) and composite PVA membrane doped with Mo (PVA-Mo). These membranes are immersed in the alkaline solution and investigated as anion exchange membranes. The performance of the solid alkaline fuel cells (SAFCs) containing these PVA membranes has been studied under hydrogen and oxygen gas flow on the Pt/C catalyst. Both, γ-PVA and PVA-Mo membranes are modified to absorb larger amounts of alkaline solution than the PVA membrane, thus greatly improving the performance of the SAFC, in terms of output power. This is clearly indicated in the polarisation curves. The electrochemical impedance spectroscopy measurements during the SAFC operation were also performed to give better insight in the effect observed. Investigation presented in this paper clearly indicates that solid alkaline PVA membranes can be used for the construction of the SAFCs.

The relative effects of radiation crosslinking and type of counterface on the wear resistance of ultrahigh-molecular-weight polyethylene

The lifetime of total joint replacement prostheses utilizing ultrahigh-molecular-weight polyethylene (UHMWPE) components has historically been determined by their wear resistance. It has been discovered that radiation crosslinking of UHMWPE can substantially increase its wear resistance. However, it is also well recognized that there is a radiation-dose-dependent decrease in several important mechanical properties of UHMWPE, such as fracture toughness and resistance to fatigue crack propagation. In this study, the effect of radiation crosslinking (followed by remelting) on the morphology, tensile properties and wear resistance of UHMWPE was investigated. Wear tests were conducted against both the commonly used cobalt–chromium counterface polished to implant grade smoothness as well as a smoother ceramic (alumina) counterface. The results showed that 50 kGy dose radiation crosslinking increased the wear resistance of UHMWPE against the cobalt–chromium counterface 7-fold, but the coupling of remelted, crosslinked UHMWPE against the smoother alumina counterface led to a 20-fold increase in wear resistance. This study shows that the use of an alumina counterface would circumvent the need to use a high radiation dose in crosslinking UHMWPE, associated with poor mechanical properties, without compromising wear resistance.

Knee Simulator Wear of Vitamin E Stabilized Irradiated Ultrahigh Molecular Weight Polyethylene

Wear and damage of ultrahigh molecular weight polyethylene (UHMWPE) tibial inserts used in total knee arthroplasty are accelerated by oxidation. Radiation crosslinking reduces wear but produces residual free radicals adversely affecting stability. One alternative to stabilize radiation-crosslinked UHMWPE is to infuse the material with vitamin E (vit E). We investigated the properties of 100-kGy e-beam–irradiated UHMWPE that was subsequently doped with vitamin E in comparison with conventional UHMWPE. Both polymers were sterilized with gamma irradiation in vacuum packaging. Vitamin E–doped UHMWPE showed lower wear before and after aging (2.4 ± 0.5 and 2.5 ± 0.8 mg/million cycle, respectively, vs 26.9 ± 3.5 and 40.8 ± 3.0 mg/million cycle for conventional UHMWPE). Conventional UHMWPE showed oxidation after accelerated aging, and its mechanical properties were adversely affected, whereas vit E–doped UHMWPE showed no oxidation or changes in its mechanical properties. Vitamin E stabilization of radiation-crosslinked UHMWPE resulted in low wear and high oxidation resistance; it is an alternative load-bearing material for total knee applications.

LDPE/EVA/PCR/MWNT nanocomposites: Radiation crosslinking and physicomechanical characteristics

AbstractNanocomposites of low‐density polyethylene (LDPE)/ethyl vinyl acetate (EVA)/poly(chloroprene) rubber (PCR) blends and multiple walled carbon nanotubes (MWNT) were prepared in different compositions by melt mixing. The nanocomposites were subjected to different radiation doses and efficacy of radiation crosslinking and physic‐mechanical characteristics were analyzed in detail. Gel content and crosslinking density increased with dose and with the MWNT fraction in the nanocomposites. The elastic modulus of the nanocomposite increased, while elongation at break exhibited downward trend with radiation dose. Micromechanical modeling of elastic modulus indicated presence of agglomeration in the matrix. Bulk density of the nanocomposites increased with the addition of MWNT while the melt flow index of the nanocomposites decreased sharply. DSC, XRD and TGA investigations revealed the peculiarity of the MWNT led nucleation in LDPE/EVA/PCR/MWNT nanocomposites. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers

Proton exchange membranes based on semi-interpenetrating polymer networks of Nafion ® and poly(vinylidene fluoride) via radiation crosslinking

A new method to prepare proton exchange membranes based on semi-interpenetrating polymer networks (semi-IPN) of Nafion ® and poly(vinylidene fluoride) (PVDF) via radiation crosslinking was proposed. The tensile strength, degree of crosslinking, water uptake, and swelling ratio of the composite membranes were studied. Compared to the recast Nafion ® 212 membrane, the composite membranes show much better mechanical properties and improved dimensional stability. The tensile strength of the composite membranes ranges from 34.3 MPa to 53.4 MPa, which is higher than that of the recast Nafion ® 212 membrane (23.9 MPa). The dimensional stability of the composite membranes also increases with increasing PVDF content in the membranes. The composite membranes show considerable proton conductivity even at 100–120 °C. The membrane containing 40% PVDF shows the highest proton conductivity of 3.37 × 10 −2 S/cm at 115 °C. These properties make them a great potential in polymer electrolyte membrane fuel cells (PEMFC).

The elimination of free radicals in irradiated UHMWPEs with and without vitamin E stabilization by annealing under pressure

Radiation crosslinking of ultrahigh molecular weight polyethylene (UHMWPE) has been used to decrease the wear of joint implant bearing surfaces. While radiation crosslinking has been successful in decreasing femoral head penetration into UHMWPE acetabular liners in vivo, postirradiation thermal treatment of the polymer is required to ensure the oxidative stability of joint implants in the long term. Two types of thermal treatment have been used: (i) annealing below the melting point preserves the mechanical properties but the residual free radicals trapped in the crystalline regions are not completely eliminated, leading to oxidation in the long-term and (ii) annealing above the melting point (melting) eliminates the free radicals but leads to a decrease in mechanical properties through loss of crystallinity during the melting process. In this study, we hypothesized that free radicals could be reduced by annealing below the melting point under pressure effectively without melting due to the elevation of the melting point. By avoiding the complete melting of UHMWPE, mechanical properties would be preserved. Our hypothesis tested positive in that we found the radiation-induced free radicals to be markedly reduced (below the detection limit of state-of-the-art electron spin resonance) by thermal annealing under pressure in radiation crosslinked virgin UHMWPE and UHMWPE/vitamin-E blends without loss of mechanical properties.

Radiation crosslinking polymerization of sterculia polysaccharide–PVA–PVP for making hydrogel wound dressings

The present study deals with the modification of sterculia gum by PVA–PVP through radiation crosslinking, to develop the hydrogels meant for the delivery of antimicrobial agent to the wounds. The hydrogels were characterized by SEM, FTIR, TGA and swelling studies. For the evaluation of swelling and drug release mechanism, the swelling kinetics and in vitro release dynamics of model drug from this matrix have been studied respectively in the solution of different pHs and simulated wound fluid. After 24h swelling per gram of the hydrogel has taken (17.03±0.19)g of simulated wound fluid and has released (0.230±0.01)mg of drug in the simulated fluid. The release of drug in simulated fluids occurred through non-Fickian diffusion mechanism.

Wear resistant UHMWPE with high toughness by high temperature melting and subsequent radiation cross-linking

The goal of this study was to create wear resistant ultra high molecular weight polyethylene (UHMWPE) with improved strength and toughness. It was previously demonstrated that high temperature melting (HTM) of UHMWPE at 280–320 °C improved its toughness without detrimentally affecting its wear resistance. We hypothesized that radiation cross-linking after high temperature melting could further improve the wear resistance of UHMWPE, and the loss in toughness by radiation cross-linking could be compensated by the improved toughness achieved by the high temperature melting prior to irradiation. In this work, we demonstrated that irradiation after HTM generated UHMWPE with improved toughness compared to the irradiated UHMWPEs without HTM, partly due to the low cross-link density of irradiated HTM UHMWPE. At a given cross-link density, irradiated HTM UHMWPEs showed higher wear resistance than irradiated UHMWPE. Therefore, successive HTM and radiation cross-linking strategy is promising to create UHMWPE materials with low wear and improved mechanical properties for total joint implants.