Polymeric Free Radicals Research Articles

Self-healing and degradable functionalization of EVA hot-melt adhesives based on disulfide bonds and polymer free radical copolymerization

This study aims to provide a new approach for the industrialization of self-healing adhesive or membrane materials. EVA hot melt adhesive has a market share of over 50 %, and it needs to be cured during use. It is a thermosetting resin that is difficult to recycle and reuse. In response to the defects mentioned above, we developed a self-healing and degradable EVA-based hot-melt adhesives (EDS) based on disulfide bonds and polymer free radical copolymerization. Specifically, we used divinyl disulfide as a crosslinking agent. Under the action of a peroxide initiator, disulfide and EVA formed a cross-linked network through free radical copolymerization. In the study, we characterized the Functionality and comprehensive performance of EDS using various testing methods such as FT-IR, DSC, and SEM. The research results indicate that EDS achieves self-healing and degradation by utilizing the rearrangement reaction of disulfide bonds under ultraviolet light and its reducibility under strong alkaline conditions. While maintaining the same electrical insulation performance, the barrier performance, thermal conductivity, aging resistance, and mechanical properties of EDS have all been improved. Meanwhile, we discussed the self-healing mechanism, curing kinetics, degradation mechanism, and aging mechanism of EDS.

Surface modification of iron-zinc ions incorporated calcium phosphate biocomposite flexible films for biomedical applications

In this work, the poly(methyl methacrylate) (PMMA)-Iron (Fe)-Zinc (Zn) ions incorporated hydroxyapatite nanocomposite films were developed by solvent evaporation technique and exposed to nitrogen 7+ ions implantation on different fluences viz., 1 × 1015, 5 × 1015, and 1 × 1016 ions/cm2 are denoted as PFZH115, PFZH515, and PFZH116 respectively. The N7+ ion implantation modified the surface properties but did not affect the bulk properties of the films. The ion beam implantation leads to the polymer chain scission and free radical formation due to the production of localized heating. The bombardment of the low-energy ions removed the oxygen in the composite matrix resulting in the replacement of nitrogen ions. The ion-induced surface diffusion results in the orientation of polarized dipoles and the reduction of wettability. All the samples are hemocompatible. The formation of craters is due to the continuous impingement of the ion beams that paved the way for the leaching of zinc ions from the hydroxyapatite matrix leading to the increased antimicrobial activity of the films. The surface engineering led to the proliferation of fibroblast cells by 10% which made the ion beam implantation a potential way to fabricate novel bone/dental filling materials in the biomedical field.

Free radical scavenging behavior of multidimensional nanomaterials in γ-irradiated epoxy resin and mechanical and thermal performance of γ-irradiated composites

In order to explore the scavenging ability of free radicals in polymers and the derivation mechanism with the different influence of the composition and various structure of nanomaterials, two-dimensional graphene oxide, one-dimensional carbon nanotubes and zero-dimensional carbon black in carbon nanomaterials, and halloysite nanotubes in silicate nanomaterials were selected to carry out a task. The results manifest that the content of free radicals produced by γ-ray irradiation in epoxy resin reduced because of the introduction nanoparticle. Among them, the epoxy resin reinforced by graphene oxide has the least free radical, followed by carbon nanotubes, halloysite nanotubes and carbon black, respectively. The analysis shows that the free radical scavenging capability was significantly different in variety types of nanomaterials. The free radical scavenging ability of carbon nanomaterials is ascribed to its well-organized honeycomb lattice, which is different from the fact that silicate nanomaterials is attributed to its electron acceptor sites. Meanwhile, the stable electron cloud was formed in the carbon nanomaterials, which is more conducive to eliminate the free radicals. The addition of nanomaterials decreases the number of free radicals in the resin, and correspondingly weakens the damage of free radicals for the resin, resulting in improving the thermal stability and mechanical properties of the epoxy resin.

Excited Triplet States of Organic Molecules and Reactive Free Radicals in Polymers

Excited Triplet States of Organic Molecules and Reactive Free Radicals in Polymers

Polytriphenylamine Derivative and Carbon Nanotubes as Cathode Materials for High-Performance Polymer-Based Batteries

Composites of polytriphenylamine (PTPA), its novel derivative poly(4-carbamoyl-N,N-diphenylaniline-2,2,5,5-tetramethyl-pyrrolin-1-oxyl) (PTPA-PO), and multiwalled carbon nanotubes (CNTs) were synthesized by in situ polymerization. The characterization results showed that the CNTs were homogeneously distributed in the polymer matrix and formed a cross-linked conductive network. The electrical properties of PTPA/CNT composites were better than those of traditional acetylene black as conductive agents. Electrochemical tests showed that the initial specific discharge capacity of the PTPA/CNT composites was 107.6 mAh g–1 (theoretical capacity of PTPA is 109 mAh g–1). Furthermore, further research to increase the specific capacity demonstrated that the as-synthesized polytriphenylamine derivative, PTPA-PO, with a CNT cathode presented two well-defined plateaus and an enhanced discharge capacity of 139.3 mAh g–1. Additionally, the PTPA-PO/CNT electrode showed superior cycling performance and remained above 90% of the initial capacity after 100 cycles. The enhanced electrochemical performance of PTPA-PO was due to its combination of the conducting polymer PTPA and free radical active site pendant PO, which increased its electrochemical reaction rate, and this composite is a promising material for high-performance polymer-based organic batteries.

Morphology, interfacial interaction, and thermal degradation of polycarbonate/MCM-41 (nano)composites

ABSTRACTThis article reports on the morphology, interfacial interaction, thermal stability, and thermal degradation kinetics of polycarbonate (PC)/mesoporous silica (MCM-41) composites with various MCM-41 contents, prepared by melt compounding. The composites with low filler loadings (<0.3 wt%) maintained their transparency because of the well dispersed MCM-41 particles, but at higher filler loadings the composites lost their transparency due to the presence of agglomerates. The presence of agglomerates decreased the thermal stability of PC due to the reduced effectiveness of the particles to immobilize the polymer chains, free radicals, and volatile degradation products.

Reactivity of Benzophenone Ketyl Free Radicals in an Elongated Elastomer Film.

The effect of polymer film elongation leading to the thinning of the film up to three times on the decay of transients was studied. Kinetics of benzophenone (B) triplet state (3)B* and ketyl free radical BH• in soft rubber poly(ethylene-co-butylene) (abbreviated as E) was investigated by nanosecond laser flash photolysis. We monitored decay kinetics of the triplet state of (3)B* and of BH• decay in the polymer cage and decay of BH• in the polymer bulk. The fast exponential decay of (3)B*(lifetime τT ≈ 200 ns) is accompanied by hydrogen atom abstraction from E with the formation of BH• and a polymer free radical R•. The decay of BH• in the polymer cage occurs during τc ≈ 1 μs. Cage recombination, in turn, was followed by a cross-termination of BH• in the polymer bulk (τb ≈ 100 μs under our conditions) and is characterized by a rate constant kb ≈ 10(8) M(-1) s(-1). We studied changes of rates of transients decay upon elongation (thinning) of E. Decay of (3)B* is practically independent of elongation of the film. Recombination of BH• in the solvent bulk occurs with a two times lower kb than in a nonelongated E. The decrease in kb is ascribed mainly to a lower fractional polymer free volume Vf in elongated E compared with that in nonelongated E. Dependencies of log10 kb versus [Formula: see text], where [Formula: see text] is the thickness of the film, turned out to be linear with a negative slope. At the same [Formula: see text] recombination proceeds slower in the elongated elastomer compared with the nonelongated elastomer. Cage effect increases twice as well due to a lower rate of radicals escape from the polymer cage in the elongated film. We observed relatively large effects of external magnetic field (B = 0.2T) on the kinetics of cage recombination and recombination in the polymer bulk. Magnetic field effect on recombination rates in the cage and in the solvent bulk does not depend on elongation.

A study of roles of different valence of copper oxide interaction with electron beam irradiation in polyethylene composite

The aim of this research was to investigate the interaction of electron beam irradiation on the different valence of copper (I) and copper (II) oxides (Cu2O and CuO) added low-density polyethylene (LDPE) composites. The results showed the increasing of Cu2O loading level in replacing the CuO has significantly reduced the gel content (or degree of cross-linking networks) in LDPE matrix. This is due to the poorer effect of Cu2O in inducing the polymeric free radicals. Meanwhile, the application of low irradiation dosage (≤100 kGy) has significantly increased the crystallite size for crystallite peak (110) of all LDPE composites. However, further increment in irradiation dosages from 100 to 300 kGy has gradually reduced the crystallite size of deflection peak (110). The tensile strength of all LDPE composites was gradually decreased with increasing of Cu2O loading level due to agglomeration of Cu2O and CuO particles in LDPE matrix. In addition, the increasing of irradiation dosages on all Cu2O /CuO added LDPE composites has gradually increased the tensile strength by inducing the formation of the cross-linking networks in LDPE matrix. Nevertheless, the increasing of irradiation dosage has gradually decreased the elongation at break of all Cu2O /CuO added LDPE composites. This is due to the higher degree of cross-linking networks in LDPE matrix could restrict the mobility of LDPE macromolecular chains when subjected to straining stress.

Kinetics of benzophenone ketyl free radicals recombination in a polymer: reactivity in the polymer cage vs. reactivity in the polymer bulk.

The decay kinetics of intermediates produced under photolysis of benzophenone (B) dissolved in soft rubber poly(ethylene-co-butylene) films (abbreviated as E) was studied by ns laser flash photolysis in the temperature range of 263-313 K. We monitored decay kinetics of the triplet state of (3)B* and of benzophenone ketyl free radical BH˙. The fast exponential decay of (3)B* (life-time τT ≈ 200 ns) is accompanied by hydrogen atom abstraction from E with a formation of BH˙ and a polymer free radical R˙. Decay of (3)B* was followed by decay of BH˙ in the polymer cage (geminate recombination) with τc ≈ 1 μs. Cage recombination in turn was followed by a decay of BH˙ in the polymer bulk (τb ≈ 100 μs). Fortunately, all three processes are separated in time. Both cage and bulk reactions were decelerated by the application of magnetic field (MF) of 0.2 T by approximately 20%. Geminate recombination was fit to the first-order kinetic law, and recombination in the solvent bulk fits well to the second-order law. Both geminate recombination and recombination in the solvent bulk are predominantly a reaction between BH˙ and R˙. It was assumed that the reaction radius ρ12 and a mutual diffusion coefficient D12 of BH˙ and R˙ are the same for the cage and bulk recombination, respectively. This led to an estimation of ρ12 = 3.3 nm and D12 = 1 × 10(-7) cm(2) s(-1). These values are discussed. We obtained activation energy, Eact, equal to 6 kcal mol(-1) and 7 kcal mol(-1) for cage decay and for recombination in the polymer bulk, respectively. These Eact coincide with each other within experimental error of their determination (±0.5 kcal mol(-1)). This indicates the same diffusion character in the cage and in the polymer bulk. It was demonstrated that an exponential model of cage effect sufficiently describes the obtained experimental data in rubber.

Reprint of: Photostability of wool fabrics coated with pure and modified TiO2 colloids

The surface of wool fabrics was coated with TiO2 and TiO2-based nanocomposite colloids and the impact of this coating on the photostability of wool was investigated. TiO2 along with TiO2/Metal and TiO2/Metal/SiO2 sols were synthesized through a low-temperature sol–gel method and applied to fabrics. Composite colloids were synthesized through integrating the silica and three noble metals of silver (Ag), gold (Au) and platinum (Pt) into the synthesis process of sols. Four different molar ratios of Metal to TiO2 (0.01%, 0.1%, 0.5% and 1%) were used to elucidate the role of metal type and amount on the obtained features. Photostability and UV protection features of fabrics were evaluated through measuring the photo-induced chemiluminescence (PICL), photoyellowing rate and ultraviolet protection factor (UPF) of fabrics. PICL and photoyellowing tests were carried out under UVA and UVC light sources, respectively. PICL profiles demonstrated that the presence of pure and modified TiO2 nanoparticles on fabrics reduced the intensity of PICL peak indicating a lower amount of polymer free radicals in coated wool, compared to that of pristine fabric. Moreover, a higher PICL peak intensity as well as photoyellowing rate was observed on fabrics coated with modified colloids in comparison with pure TiO2. The surface morphology of fabrics was further characterized using FESEM images.

Using thermogravimetric analysis to determine polymer thermal stability: Relevance of changes in onset temperature of mass loss

the field of polymer composites and nanocomposites, where composite samples were characterised, thermogravimetric analysis (TGA) data are almost always presented as part of a whole range of analysis methods. Most of the time the shifting of the TGA mass loss steps to higher temperatures is interpreted as being indicative of delays in the thermal degradation process and hence improvements in the thermal stability of the polymer(s) brought about by the presence of the filler (nano)particles. The question one should ask oneself: Is it possible to measure the onset of degradation of a polymer or composite by using TGA, or does one only measure the onset of mass loss? Of course one only measures the onset of mass loss, which means that anything that may delay the transport of volatile degradation products out of the composite will cause a delay in the evaporation of these products, and the onset of mass loss will only be seen after a longer time or at a higher temperature, depending on whether the analysis was performed as function of time or temperature. This means that the (nano) filler probably had no effect on the thermal stability of the polymer matrix, but that it either contributed to the formation of a carbonaceous char layer or strongly interacted with the volatile degradation products, both of which would retard the diffusion of these degradation products out of the polymer. This would give rise to the mass loss step starting at higher temperatures (after longer periods of time) giving the impression of an apparent increase in thermal stability. On the other hand, one cannot neglect the possibility that the (nano)filler can retard the actual degradation process though strong interaction with the polymer chains and/or free radical chains, which can restrict the chain mobility and slow down the degradation propagation reactions. Whatever the reason for the delayed onset of mass loss, one has to delve deeper to find the real reason and not just assume that it is the result of the (nano)filler improving the thermal stability of the polymer. It is also important to realise that, although the onset of degradation is not necessarily delayed in the presence of (nano)filler particles, any delay in the volatilization of the degradation products (which is measurable with TGA) is an important factor to keep in mind regarding the flammability of the polymer.

Photostability of wool fabrics coated with pure and modified TiO2 colloids

The surface of wool fabrics was coated with TiO2 and TiO2-based nanocomposite colloids and the impact of this coating on the photostability of wool was investigated. TiO2 along with TiO2/Metal and TiO2/Metal/SiO2 sols were synthesized through a low-temperature sol–gel method and applied to fabrics. Composite colloids were synthesized through integrating the silica and three noble metals of silver (Ag), gold (Au) and platinum (Pt) into the synthesis process of sols. Four different molar ratios of Metal to TiO2 (0.01%, 0.1%, 0.5% and 1%) were used to elucidate the role of metal type and amount on the obtained features. Photostability and UV protection features of fabrics were evaluated through measuring the photo-induced chemiluminescence (PICL), photoyellowing rate and ultraviolet protection factor (UPF) of fabrics. PICL and photoyellowing tests were carried out under UVA and UVC light sources, respectively. PICL profiles demonstrated that the presence of pure and modified TiO2 nanoparticles on fabrics reduced the intensity of PICL peak indicating a lower amount of polymer free radicals in coated wool, compared to that of pristine fabric. Moreover, a higher PICL peak intensity as well as photoyellowing rate was observed on fabrics coated with modified colloids in comparison with pure TiO2. The surface morphology of fabrics was further characterized using FESEM images.

Reaction model describing antioxidant depletion in polyethylene–clay nanocomposites under thermal aging

Antioxidants are typically added to polyethylene to extend its durability, and recently clay nanoparticles have been blended into polyethylene to improve mechanical properties. However, the clay nanoparticles also accelerate the rate of antioxidant depletion in polyethylene. This paper uses a mathematical model to describe the underlying mechanisms of antioxidant (hindered-phenol) depletion and to predict experimentally-measured antioxidant profiles in polyethylene–clay nanocomposites. The mathematical model uses a reaction kinetic scheme that includes free radical initiation and propagation reactions, antioxidant stabilization reactions and free radical termination reactions. In the model, alkyl free radicals oxidize rapidly. The role of antioxidants is to stabilize the oxidized free radicals to hydroperoxides, and interrupt propagation reactions. However, in nanocomposites, continuous depletion of antioxidant is caused by the clay acting as a catalyst to decompose hydroperoxides and regenerate alkyl free radicals. This cyclic hydroperoxide generation and decomposition leads to much faster antioxidant depletion in polyethylene nanocomposites. Phenoxyl radicals of antioxidants generated by stabilization reactions contribute to terminate polymeric free radicals and limit their accumulation. Predictions of antioxidant depletion are compared to experimental results for accelerated aging of polyethylene and nanocomposite samples.

Post-irradiation effects in polyethylenes irradiated under various atmospheres

If a large amount of polymer free radicals remain trapped after irradiation of polymers, the post-irradiation effects may result in a significant alteration of physical properties during long-term shelf storage and use. In the case of polyethylenes (PEs) some failures are attributed to the post-irradiation oxidative degradation initiated by the reaction of residual free radicals (mainly trapped in crystal phase) with oxygen. Oxidation products such as carbonyl groups act as deep traps and introduce changes in carrier mobility and significant deterioration in the PEs electrical insulating properties. The post-irradiation behaviour of three different PEs, low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) was studied; previously, the post-irradiation behaviour of the PEs was investigated after the irradiation in air (Suljovrujic, 2010). In this paper, in order to investigate the influence of different irradiation media on the post-irradiation behaviour, the samples were irradiated in air and nitrogen gas, to an absorbed dose of 300kGy. The annealing treatment of irradiated PEs, which can substantially reduce the concentration of free radicals, is used in this study, too. Dielectric relaxation behaviour is related to the difference in the initial structure of PEs (such as branching, crystallinity etc.), to the changes induced by irradiation in different media and to the post-irradiation changes induced by storage of the samples in air. Electron spin resonance (ESR), differential scanning calorimetry (DSC), infra-red (IR) spectroscopy and gel measurements were used to determine the changes in the free radical concentration, crystal fraction, oxidation and degree of network formation, respectively.

Hydroxyl radical release from dental resins: Electron paramagnetic resonance evidence

It is well known that polymeric free radicals remain trapped inside dental resins for a long time after photopolymerization. Moreover, although these high molecular mass compounds have very limited mobility, there is evidence to suggest that they disappear progressively over time. The purpose of this study was to provide new experimental data to help understand this phenomenon. To determine whether low molecular mass free radicals are released by dental composites stored in hydrophilic media, we used electron paramagnetic resonance spectroscopy to perform spin-trapping experiments on experimental and commercial samples stored in ethanol. Under these conditions, ethoxy radicals were produced. Further experiments demonstrated that (1) hydroxyl radicals were released from the methacrylated resin and (2) they reacted with ethanol molecules to produce "secondary" ethoxy free radicals. In addition to the well-known monomer toxicity of methacrylated resins, we may have identified a new source of concern for these biomaterials.

The effect of dyes on photo-induced chemiluminescence emission from polymers

Weak photo-induced chemiluminescence (PICL) emission is observed when polymers are exposed to UVA or visible light. The presence of dyes can either increase PICL intensity via Type I photosensitisation which generates polymer free radicals, or reduce it via photo-protection. PICL studies on the eight Blue Wool Standards (BWSs) that are used commercially as lightfastness standards show higher PICL intensity from the least photostable BWSs that use triphenylmethane dyes and lower intensity from more photostable BWSs using UVA and visible wavelengths. The relative PICL intensities do not correlate in a stepwise manner with lightfastness ratings of the BWSs. However dye/polymer combinations that emit high levels of PICL relative to the undyed material are unlikely to have acceptable lightfastness. The xanthene dyes fluorescein and eosin Y are more strongly photosensitising than triphenylmethane dyes on wool and both produce higher PICL emission than undyed wool when irradiated with visible light.

Benzothiazole sulfide compatibilized polypropylene/halloysite nanotubes composites

Clay-philic benzothiazole sulfide, capable of donating electrons, is grafted onto polypropylene (PP) backbones when N-cyclohexyl-2-benzothiazole sulfonamide (CBS), a commonly used accelerator in the tire industry, is included in the processing of PP/halloysite nanotubes (HNTs) composites. CBS decomposes at elevated temperature and yields benzothiazole sulfide radicals, which react with the PP polymeric free radicals generated during the processing of the composites. On the other hand, the benzothiazole group of CBS is reactive to HNTs via electron transferring. The compatibilization between HNTs and PP is thus realized via interfacial grafting and electron transferring mechanism. The interfacial interactions in the compatibilized systems were fully characterized. Compared with the control sample, the dispersion of HNTs and the interfacial bonding are enhanced substantially in the compatibilized composites. The significantly improved mechanical properties and thermal properties of benzothiazole sulfide compatibilized PP/HNTs composites are correlated to the enhanced interfacial property. The present work demonstrates a novel interfacial design via interfacial grafting/electron transferring for the compatibilization of PP/clay composites.

A novel method to immobilize collagen on polypropylene film as substrate for hepatocyte culture

To improve the biocompatibility of polypropylene membrane with hepatocytes, film was firstly allowed to generate peroxide groups by ammonium peroxydisulfate (APS) thermal decomposition and was then carboxylated by free radical grafting polymerization using acrylic acid as the monomer and ferrous(II) ions as redox initiator. Collagen was then covalently immobilized through amide bonds between the residue amine groups and carboxylic ones in the presence of water-soluble carbodiimide. The film was characterized by Fourier transformed infrared spectroscopy in attenuated total reflection mode (ATR–FTIR), X-photoelectron spectroscopy (XPS) and confocal laser scanning microscopy (CLSM), etc. The experimental results show that there exist hydroperoxide groups after APS aqueous thermal decomposition, which are subsequently converted into the polymeric free radicals initiating the vinyl monomers to subject grafting polymerization. The maximum collagen immobilization content is about 10.5 μg/cm 2 by ninhydrin assay. Hepatocytes cultured on collagen immobilized film show stable phenotype and normal liver-specific functions more than 20 days as observed by scanning electronic microscopy (SEM) and biochemical parameters determination.

Radiation resistant polymer–carbon nanotube nanocomposite thin films

We have studied degradation of poly(methyl methacrylate)–CNT nanocomposite thin films, using a UV-ozone and an e-beam radiation as a function of CNT concentration. We have shown that the addition of CNT fillers can have a dramatic reinforcement effect on the nature of degradation by both high-energy radiations, where polymer-free radicals are mainly responsible for the proliferation of degradation. In addition, CNT networks can effectively disperse the radiation. The saturation in the reinforcement effect was observed when a concentration of CNT was approximately 0.5 wt.%. This concentration was interpreted in terms of a critical concentration for percolation of the CNT network, and the result was consistent with the sheet resistivity measurement for which physical contact between CNT fibers was evident.

Visualization of distance distribution from pulsed double electron-electron resonance data

Double electron-electron resonance (DEER), also known as pulsed electron-electron double resonance (PELDOR), is a time-domain electron paramagnetic resonance method that can measure the weak dipole-dipole interactions between unpaired electrons. DEER has been applied to discrete pairs of free radicals in biological macromolecules and to clusters containing small numbers of free radicals in polymers and irradiated materials. The goal of such work is to determine the distance or distribution of distances between radicals, which is an underdetermined problem. That is, the spectrum of dipolar interactions can be readily calculated for any distribution of free radicals, but there are many, quite different distributions of radicals that could produce the same experimental dipolar spectrum. This paper describes two methods that are useful for approximating the distance distributions for the large subset of cases in which the mutual orientations of the free radicals are uncorrelated and the width of the distribution is more than a few percent of its mean. The first method relies on a coordinate transformation and is parameter-free, while the second is based on iterative least-squares with Tikhonov regularization. Both methods are useful in DEER studies of spin-labeled biomolecules containing more than two labels.