Vitamin E can be used to hinder scissioning in radiation cross‐linked UHMWPE during high‐temperature melting

High‐temperature melting (HTM) of ultrahigh molecular weight polyethylene (UHMWPE) was shown to improve its elongation and toughness. This was believed to be due to increased scissioning and increased diffusion of polymer chains. It was hypothesized here that the toughness of previously radiation cross‐linked UHMWPEs could also be improved by HTM. To test this hypothesis, the wear resistance, tensile mechanical properties, and Izod impact strength of radiation cross‐linked virgin (no additive) and antioxidant‐blended (with vitamin E) UHMWPEs were tested. The results suggested that although the impact strength of cross‐linked UHMWPEs could be improved significantly by HTM, the wear resistance was decreased. Thus, this procedure can be optimized to be especially suited in high‐stress applications, such as total knee replacements with lessened wear concerns. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42735.     

High temperature melted,radiation cross-linked,vitamin E stabilized oxidation resistant UHMWPE with low wear and high impact strength

In a previous communication we showed improvement in the wear resistance and toughness of cross-linked ultrahigh molecular weight polyethylene (UHMWPE) for total joint implants by radiation cross-linking after high temperature melting (HTM). In this study, we hypothesized that introduction of vitamin E into UHMWPE before high temperature melting could improve the oxidative stability of these UHMWPEs with low wear and high toughness. Vitamin E was blended with UHMWPE powder at concentrations of 0.1 and 0.2 wt% and consolidated, followed by melting at 300 and 320 °C for 5 h, and subsequent irradiation with electron beam to 150 kGy. These vitamin E/UHMWPE blends showed improved tensile and impact toughness and good wear resistance in comparison with the radiation cross-linked vitamin E/UHMWPE blends. Aggressive accelerated aging with or without pro-oxidant lipids showed that vitamin E-blended, high temperature melted and subsequently irradiated UHMWPE had good oxidation resistance.     

Homogenization of natural rubber network induced by nanoclay

Vulcanization gives birth to the nonuniformity of rubber network, the identification of which is the basis of improvement of performance of rubber products. We established a visco‐hyperelastic constitutive equation to reveal the quantitative distribution of the inhomogeneous network phases according to a three‐phase model. The cross‐linked network was assumed to be composed of the cross‐linking cluster, the low network chain density domain and the fluid‐like mass. The incorporation of clay with high specific surface area induced the effective uniformity of network structure by decreasing the content of the cross‐linking cluster and increasing that of the low network chain density domain. These structural variations were responsible for excellent mechanical properties and strong strain‐induced crystallization ability probed by the in situ synchrotron wide‐angle X‐ray diffraction. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40324.     

Homogenization in supercritical carbon dioxide enhances the diffusion of vitamin E in ultrahigh‐molecular‐weight polyethylene

Vitamin E stabilization of radiation‐crosslinked ultrahigh‐molecular‐weight polyethylene (UHMWPE) joint implants was successfully introduced to improve long‐term oxidation resistance. Current clinically available vitamin E stabilized UHMWPE implants were prepared by the postirradiation diffusion of vitamin E into 100‐kGy‐irradiated UHMWPE by a two‐step process, which included doping in pure vitamin E at an elevated temperature below the melting point followed by an annealing step at an elevated temperature in inert gas to homogenize the antioxidant throughout components of desired thickness. We hypothesized that the diffusion of vitamin E could be enhanced with supercritical carbon dioxide (SC‐CO₂) during homogenization without an increase in the surface vitamin E concentration, which would thus result in faster homogenization. Our hypothesis tested positive; crosslinked UHMWPE doped with vitamin E at 120°C and homogenized in SC‐CO₂ at 10–12 MPa had a greater penetration of vitamin E than those homogenized in inert gas. We attributed the faster diffusion of vitamin E in irradiated UHMWPE in SC‐CO₂ to the dissolution of vitamin E in the supercritical fluid and a rate of diffusion that was closer to that of the supercritical fluid in the polymer. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Oxidation resistance and abrasive wear resistance of vitamin E stabilized radiation crosslinked ultra‐high molecular weight polyethylene

The effect of gamma radiation on the oxidation and wear resistance of ultra‐high molecular weight polyethylene (UHMWPE) has been extensively studied since these properties are critical for the longevity of UHMWPE components of total joint replacement prostheses. While gamma radiation increases wear resistance of UHMWPE, the free radical generated in the lamellar regions by radiation must be stabilized before oxidative degradation occurs as the polymer ages. Initially, post‐radiation melting conducted to quench free radicals but this treatment also decreases its mechanical properties. Recently, it has been replaced by incorporation of Vitamin E into UHMWPE to combat oxidative degradation. In this study, we assessed wear resistance of Vitamin E stabilized UHMWPE under abrasive wear conditions and oxidation resistance by shelf‐aging irradiated components for 2 years. Equilibrium swelling experiments showed that Vitamin E decreased crosslink density, which affected wear resistance, but oxidation resistance was better preserved with increasing concentration of Vitamin E. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44125.     

TEOS as an improved alternative for chitosan beads cross‐linking: A comparative adsorption study

In this work the use of tetraethoxysilane (TEOS) for cross‐linking of chitosan hydrogel beads was studied at the level of 1 mmol TEOS per gram of chitosan. They were compared with glutaraldehyde and epichlorohydrin cross‐linked beads. The hydrogels were characterized by FTIR, SEM, water content, nitrogen content, and their point of zero charge. The performance of the anionic dye Remazol Black (RB) and the cationic Cd(II) adsorptions was assessed in order to characterize the sorbate–sorbent interaction. Adsorption experimental data were analyzed using two‐ and three‐parameter isotherm models along with the evaluation of mean adsorption energy and standard free energy. The adsorption was observed to be pH dependent. The uptake rate of RB and Cd(II) showed that the three type of beads followed a similar kinetic behavior. For both sorbates the TEOS cross‐linked beads showed the higher maximum adsorption capacity, followed by epichlorohydrin and glutaraldehyde cross‐linked beads. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41005.     

Multi‐point monitoring of cross‐linking reactions

The feasibility of deploying optical fiber sensors to obtain qualitative and quantitative information on the cross‐linking characteristics of thermosetting resin systems has been demonstrated by a number of researchers. This article is concerned with the demonstration of a low‐cost fiber‐optic Fresnel sensor system for monitoring the cross‐linking reactions at multiple locations. Cross‐linking reactions can be influenced by a number of parameters including the stoichiometry of the resin and hardener, and the heat‐transfer rates in and out of the preform as a function of the cross‐section of the preform. In situations where there is a variation in the thickness of the preform or when large components are processed, a facility to monitor the cross‐linking reactions at multiple locations will be valuable because the rate and extent of the cross‐linking can be inferred. In this article, six Fresnel sensors were immersed in individual vials containing an epoxy/amine resin system and processed (cross‐linked) in an air‐circulating oven. One additional vial with a Fresnel sensor immersed in the neat epoxy resin (no hardener) was co‐located in the oven to enable the effect of temperature to be monitored. The feasibility of using the multiplexed Fresnel sensors for cure monitoring was demonstrated successfully. The sensors in the post‐cross‐linked samples were used to study the effects of heating and cooling cycles. The feasibility of detecting the glass transition temperature using the Fresnel sensor is discussed along with the factors that give rise to variability in the output Fresnel signals. © 2014 The Authors. Journal of Applied Polymer Science Published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41088.     

Cross‐Linked Fluorinated Poly(Aryl Ether) (C‐FPAE) Films: Preparation Strategy,Performance Study,and Low Dielectric Applications

The production of low dielectric materials that can be used in high temperature environments is the primary aim of this work. A cross‐linked structure is introduced into fluorinated poly(aryl ether) (named as FPAE) with high molecular weight (M_w, 140 000 g mol^?1) and linear molecular structure using nucleophilic substitution reaction at the ortho‐position of decafluorobiphenyl monomer units in the FPAE molecular chain. The curing temperature and curing time are optimized and the final conditions for the cross‐linking reaction in this study are determined to be 300 °C for 1 h. Moreover, the dielectric constant and dielectric loss of the C‐FPAE film respectively are 2.67 and 0.006 at 1000 Hz when 1 wt% of crosslinking agent is added, and the cross‐linked fluorinated poly(aryl ether) film shows excellent thermal stability (T_d(5%), 495 °C), dimensional stability, hydrophobic properties, and high storage modulus in high temperature environments. Such novel low dielectric material with excellent performances has important application value in the aerospace and the integrated electronics field.     

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.     

Improving the ceramic yield of polycarbosilane by radiation cross‐linking in the presence of multifunctional monomers

An interesting method for radiation cross‐linking polycarbosilane (PCS) at high efficiency and low cost was developed by adding multifunctional monomers (trimethylolpropane trimethacrylate (TMPTMA) or divinylbenzene (DVB)). The effect of multifunctional monomers on the radiation cross‐linking and ceramic yield of PCS was investigated. The results revealed that the addition of a small amount of multifunctional monomer can substantially improve the ceramic yield and reduce the absorbed dose by promoting cross‐linking. The ceramic yields of radiation cross‐linked “PCS/2%DVB (300 kGy)” and “PCS/2%TMPTMA (300 kGy)” were 74 wt% and 78 wt%, respectively, while that of the radiation cross‐linked “PCS (300 kGy)” was only 64 wt%. The presence of double bonds in multifunctional monomers stimulated the combination reactions of free radicals in PCS induced by γ‐ray irradiation and subsequently enhanced the efficiency of cross‐linking. Furthermore, it was found that the incorporation of a small amount of multifunctional monomer hardly affected the chemical composition and crystallization behavior of the final SiC ceramics.     

Effect of cross‐linking time on the thermal and mechanical properties and pervaporation performance of poly(vinyl alcohol) membrane cross‐linked with fumaric acid used for dehydration of isopropanol

Cross‐linked poly (vinyl alcohol) membranes were prepared using fumaric acid as the cross‐linking agent and were used for the pervaporation separation of water/isopropanol mixtures. Cross‐linking process was carried out at 150°C at three different times of 10, 30, and 60 min. The membranes were characterized by different known methods of FT‐IR, TGA, XRD as well as tensile test. The effects of cross‐linking time on the thermal and mechanical properties of the membranes and also their pervaporation performance were investigated. Formation of more ester groups by increasing the cross‐linking time was confirmed by the FT‐IR results. TGA analyses showed that thermal stability of the membranes is improved by prolonging the duration of cross‐linking process. This was due to the formation of more compact structure in the membranes. The XRD results revealed that the crystalline regions of the membranes were relatively diminished with an increase in the cross‐linking time. No specific trend was observed for the variation of tensile strength at break with the cross‐linking time. The PVA membrane cross‐linked for 60 min showed high selectivity of 1492 for water permeation for the feed mixture containing 10 wt % water. The temperature dependency of the permeation flux was investigated using Arrhenius relationship, and the activation energy values were calculated for total permeation (E_p), water (E_pw), and IPA (E_pIPA) fluxes. Lower value of E_pw in comparison with E_pIPA supported excellent dehydration performance of the cross‐linked membranes. Despite large increase in activation energy of water with prolonged cross‐linking time, the selectivity was improved. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2013     

Gamma radiation on poly(ϵ‐caprolactone) in the presence of vinyltrimethoxysilane

The performance and radiation‐induced cross‐linking of polycaprolactone (PCL) in the presence of vinyltrimethoxysilane (VTMS) have been investigated. Radiation‐induced cross‐linking of PCL in the presence of VTMS followed the Charlesby–Pinner equation, and VTMS promoted the radiation‐induced cross‐linking of PCL. As the concentration of VTMS increased, the gelation dose and the ratio of degradation to cross‐linking (p₀/q₀) decreased and the efficiency of radiation‐induced cross‐linking increased. Differential scanning calorimetry analyses showed differences between the first and second scans. Glass‐transition temperature (T_g) and mechanical properties of the polymers increased. Radiation‐induced cross‐linking of PCL in the presence of VTMS was found to retard hydrolytic degradation greatly. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Peroxide induced cross‐linking by reactive melt processing of two biopolyesters: Poly(3‐hydroxybutyrate) and poly(l‐lactic acid) to improve their melting processability

Poly(3‐hydroxybutyrate) (PHB) and poly(l ‐lactic acid) (PLLA) were individually cross‐linked with dicumyl peroxide (DCP) (0.25–1 wt %) by reactive melt processing. The cross‐linked structures of the polymer gel were investigated by nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopies. The size of the polymer crystal spherulites, glass transition temperature (T_g), melting transition temperature (T_m), and crystallinity were all decreased as a result of cross‐linking. Cross‐linking density (ν_e) was shown to increase with DCP concentration. Based on parallel plate rheological study (dynamic and steady shear), elastic and viscous modulus (G″ and G′), complex viscosity (η^*) and steady shear viscosity (η) were all shown to increase with cross‐linking. Cross‐linked PHB and PLLA showed broader molar mass distribution and formation of long chain branching (LCB) as estimated by RheoMWD. Improvements in melt strength offer bioplastic processors improved material properties and processing options, such as foaming and thermoforming, for new applications. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41724.     

Generic roll‐to‐roll compatible method for insolubilizing and stabilizing conjugated active layers based on low energy electron irradiation

Irradiation of organic multilayer films is demonstrated as a powerful method to improve several properties of polymer thin films and devices derived from them. The chemical cross‐linking that is the direct result of the irradiation with ～100 keV electrons is fast and has a penetration power compatible with thin plastic foils of one to two hundreds of microns typical of devices explored in organic electronics. We demonstrate here that active layers and complete devices can be subjected to electron irradiation‐induced cross‐linking thus facilitating multilayer solvent processing and morphological stability. The method is fast, generic, contactless, and fully compatible with high‐speed roll‐to‐roll processing of i.e. polymer solar cells at web speeds in excess of 60 m min^?1. We employ fully printed, flexible, and foil‐based indium‐tin‐oxide free polymer solar cells in this study to demonstrate the technique. We also demonstrate that polymer solar cells are exceptionally stable towards ionizing radiation and find that doses as high as 100 kGy can be used before any significant decrease in performance is observed. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40795. Together with Mokarian‐Tabari et al., J. Appl. Polym. Sci. (2014) 131 , 40798, doi: 10.1002/app.40798 , this article is part of a Special Issue on Polymers for Microelectronics. The remaining articles appear in J. Appl. Polym. Sci. (2014) volume 131 , issue 24. This note was added on 1st July 2014.     

Preparation and properties of novel boric acid modified poly(aryl ether sulfone) membranes

In this article, boric acid‐modified poly(aryl ether sulfone) (PES‐B) membranes were prepared by solution blending, solution casting, evaporation, and programmed temperature curing method for the first time. The chemical modification of poly(aryl ether sulfone) was accomplished by the curing of poly(aryl ether sulfone) with pendent phenyl hydroxide (PES‐OH) under the function of boric acid. The reaction mechanisms and the effects of boric acid content were thoroughly investigated. Processing conditions and structures of PES‐B were determined by FT‐IR spectra. It has been found that the B(OH)₃/PES composite membranes were completely cured after treatment at 300°C for 4 h. When boric acid content was over 4.7%, PES‐B membranes presented phase separation and full cross‐linking structures. DSC measurements demonstrated that the addition of boric acid had influenced the glass transition temperature of PES‐B, which provided the proof of the appearance of cross‐linking network structure. TGA results confirmed that the thermal stability of the prepared PES‐B composites was improved. Furthermore, the cross‐linked composite membranes exhibited excellent mechanical property and solvent resistance. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40860.     

Genipin‐cross‐linked chitosan‐based hydrogels: Reaction kinetics and structure‐related characteristics

The main aim of this work is the synthesis and characterization of cross‐linked chitosan systems. Chitosan hydrogels can be prepared by physical or chemical cross‐linking of polymer chains. Chemical cross‐linking, leading to the creation of hydrogel networks possessing improved mechanical properties and chemical stability, can be achieved using either synthetic agents or natural‐based agents. In this work, the cross‐linker Genipin, a naturally derived compound, was selected because of the lower acute toxicity compared to many other commonly used synthetic cross‐linking reagents. In particular, the chemical stabilization of chitosan through genipin cross‐linking molecules was performed and characterized by calorimetric analyses (differential scanning calorimetry), swelling measurements in different pHs, and ionic strength. The reaction kinetics was carried out by means of rheological measurements, and both the activation energy (E_a) and the reaction order (m) were calculated. The hydrogel analyses were carried out at different concentrations of genipin (GN1 and GN2). The results were used to evaluate the possibility to use the chemical cross‐linked chitosan–genipin hydrogel for biomedical applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42256.     

FREE RADICAL ELIMINATION IN IRRADIATED UHMWPE THROUGH CRYSTAL MOBILITY IN PHASE TRANSITION TO THE HEXAGONAL PHASE

Ultra-high molecular weight polyethylene (UHMWPE) is radiation cross-linked to decrease wear in total joint applications. Irradiation decreases the strength of UHMWPE and introduces residual free radicals, which can cause oxidation in the long-term. We advanced a method eliminating the free radicals without a reduction in strength. UHMWPE exhibits a hexagonal phase at high pressure and temperature, where chain mobility in the crystalline phase is increased, leading to the formation of extended chain crystals. We hypothesized that the increased chain mobility during transformation from the orthorhombic to hexagonal phase could be used to eliminate the residual free radicals in irradiated UHMWPE. We eliminated the free radicals in 25-, 65- and 100-kGy irradiated UHMWPEs and these materials did not show oxidation after accelerated aging. The ultimate tensile strength and work-to-failure of 25 and 65-kGy irradiated UHMWPEs were improved significantly while that of 100-kGy irradiated UHMWPE was lower compared to irradiated UHMWPE melted at ambient pressure.     

New breathable films using a thermoplastic cross‐linked starch as a porogen

Breathable films, which find in variety of product applications, are conventionally made using mineral porogens such as calcium carbonate (CaCO₃). This article addresses a novel biodegradable and highly breathable film without inorganic porogens. Unexpectedly, a thermoplastic cross‐linked natural polymer (corn starch) was used successfully to create tortuous passages for film breathability. This concept was demonstrated using two types of thermoplastic cross‐linked corn starches as porogens and contrasted to control samples: native corn and chemically cross‐linked starches, respectively. The films discussed had increased breathability and mechanical properties relative to the control samples. The film morphology reveals that filler was irregular when thermoplastic starch or CaCO₃ was used. The difference in filler from chemically modified cross‐linked starch and thermoplastic cross‐linked starch was observable as well. It is believed that spherical particles provided by thermoplastic cross‐linked starch helps film debonding and porosity during the film stretch processes. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41016.     

Preparation and chemical reactions of rigid cross‐linked poly(vinyl chloride) foams modified by epoxy compounds

A novel method to prepare semi‐interpenetrating polymer network rigid cross‐linked poly(vinyl chloride) (c‐PVC) foams with improved shear toughness in the absence of anhydride components is reported. The cross‐linked network structure in the c‐PVC foams was composed of polyurea network modified by epoxy structure. The cellular morphology was characterized by scanning electron microscopy. Tensile, compressive, and shear properties of the foams were studied. The obtained c‐PVC foams showed high shear properties compared with the comparative samples with the same density and cellular morphology. Possible reactions during the preparation of c‐PVC foams were studied by means of Fourier transform infrared spectrometry and nuclear magnetic resonance measurements through the model experiments. The results showed that allophanate structure resulting from the reaction between isocyanate compounds and epoxy compounds formed in the molding step, which was included into the final cross‐linked network in the cross‐linking step. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40567.     

Control of the interfacial properties of ultrahigh‐molecular‐weight polyethylene/magnesium hybrid composites through use of atmospheric plasma treatment

This work aims to explore the use of lightweight magnesium alloys as an advanced composite material in conjunction with lightweight thermoplastic materials, such as ultrahigh‐molecular‐weight polyethylene (UHMWPE). High throughput, environmentally friendly, atmospheric plasma treatment methods were used to control the interfacial properties and improve the adhesion behavior of metallic/UHMWPE composites. Helium‐oxygen dielectric barrier discharges were used, and the plasma‐activated UHMWPE surfaces were characterized through analytical and mechanical characterization methods. Oxygen content on the treated polymer surfaces increased 18.1–36.0%. A reduction in silicon content combined with characterization through microscopy reveal a preference for the attack of the matrix over the polyethylene fibers. Wetting angles for the treated samples decreased as much as 53.7%. Treated UHMWPE/Mg hybrid samples exhibited lap shear strengths up to 113.7% greater than the control. Both the plasma‐induced surface functionalization with oxygen‐containing polar groups (carboxyl, carbonyl, and hydroxyl groups) and the preferential mild etching of the polymer matrix over the fibers lead to the improvement in adhesion. POLYM. COMPOS., 2012. © 2011 Society of Plastics Engineers