Polymer Degradation and Stability (v.91, #2)

Calendar (I-II).

IR laser ablation of poly(vinyl chloride): Formation of monomer and deposition of nanofibres of chlorinated polyhydrocarbon by Jadranka Blazevska-Gilev; Jaroslav Kupčík; Jan Šubrt; Zdeněk Bastl; Vladimír Vorlíček; Anna Galíková; Diana Spaseska; Josef Pola (213-220).
IR laser-induced ablation of poly(vinyl chloride) was examined under different irradiation conditions and its volatile and solid products were characterized by mass, infrared, UV and X-ray photoelectron spectroscopy, electron microscopy and thermogravimetry. It is demonstrated that the major component among the volatile products is monomeric vinyl chloride and that the process causes deposition of Cl-containing polymeric films. The proportion between the volatile and solid products as well as the nature of the deposited films at different laser fluences have been examined. We show that the deposited films incorporate less Cl atoms than poly(vinyl chloride) and that they initially contain conjugated C=C bonds and incorporate nano-sized fibre and necklace features. The process represents the first example of thermal degradation of poly(vinyl chloride) into monomer and makes it possible to fabricate crosslinked Cl-containing intractable polymer films.
Keywords: Poly(vinyl chloride); Laser ablation; Degradation; Laser-induced polymer films;

Recovery of polyols from flexible polyurethane foam by “split-phase” glycolysis: Glycol influence by Carolina Molero; Antonio de Lucas; Juan F. Rodríguez (221-228).
Scrap of flexible polyurethane foams from slabstock manufacturing comprises about 10% of the total production, leading to not only an environmental problem but also an economic one. The general purpose of polyurethane chemical recycling is to recover a valuable constituent, the polyol. Among the processes suitable, glycolysis, and specially glycolysis in two phases seems to be the method that allows better quality products. In this study glycolysis of flexible polyurethane foams in “split phase” was conducted with different glycols, in order to study their activity and select a system to obtain the highest quality recovered polyol. Reaction kinetics and the products obtained were investigated. Times required to reach complete conversion, chemical properties of the polyol phase and its purity depended on the glycol employed. Diethylene glycol proved to be the most suitable glycol to obtain a high purity in the polyol phase.
Keywords: Polyol; Polyurethane; Recovery; Glycolysis;

Stabilization effect of lignin in natural rubber by A. Gregorová; B. Košíková; R. Moravčík (229-233).
A series of carbon black filled natural rubbers containing lignin was tested from the view point of their thermo-oxidative aging. Lignin is biopolymer that belongs to the main components of wood. Mechanical properties and crosslink density of lignin stabilized vulcanisates were measured before and after thermo-oxidative aging for 24, 72, 168, 240 and 408 h at 80 °C. The results were compared with those from NR vulcanisates stabilized with the commercial rubber antioxidant N-phenyl-N-isopropyl-p-phenylene diamine (IPPD). The results obtained show that lignin exerts a stabilizing effect in carbon black filled natural rubber. Its effect is comparable with that of conventional synthetic antioxidant. Moreover, the addition of lignin increased the stabilizing effect of IPPD.
Keywords: Lignin; Natural rubber; Thermo-oxidative aging; Crosslink density;

Preparation and combustion properties of flame retarded polypropylene–polyamide-6 alloys by Yong Tang; Yuan Hu; Lei Song; Ruowen Zong; Zhou Gui; Weicheng Fan (234-241).
In this work, polypropylene (PP) and polyamide-6 (PA6) alloys were prepared by blending PP and PA6 or nano-PA6 while using a compatibiliser such as maleic anhydride-g-polypropylene (MAPP). Their physical structures were characterized by Hitachi X650 scanning electron microscope (SEM) and transmission electron microscopy (TEM). An intumescent flame retardant (IFR) was added to the PP/PA6 alloys and their flammability behaviour was evaluated using cone calorimeter and real time Fourier transform infrared (FTIR). The probable mechanisms are discussed.
Keywords: Polypropylene; Polyamide-6; Alloys; Clay; Flame retardants;

Organic thermal stabilizers for rigid poly(vinyl chloride). Part XI: Anthraquinone derivatives by Magdy W. Sabaa; Emad H. Oraby; Abir S. Abdel Naby; Riham R. Mohamed (242-254).
Anthraquinone and 1-aminoanthraquinone derivatives have been examined as thermal stabilizers or co-stabilizers for rigid PVC in air, at 180 °C. Their high stabilizing efficiency is detected by their high induction period values (Ts) when compared with some of the common reference stabilizers used industrially such as dibasic lead carbonate, calcium–zinc soap and octyl tin mercaptide. Blending these organic stabilizers with some of the reference stabilizers in different ratios had synergistic effect on both the induction period and the dehydrochlorination rate.A probable mechanism for the stabilizing mode of these derivatives has been proposed. The stabilizing efficiency is attributed partially to the stabilizers' ability to intervene in the radical chain degradation process of PVC and to the replacement of the labile chlorine atoms on PVC chains by a relatively more stable moiety of the organic stabilizer.
Keywords: Poly(vinyl chloride); Induction period (Ts); Degradation; Thermal stability; Extent of discoloration; Blends;

Thermal degradation and fire resistance of unsaturated polyester, modified acrylic resins and their composites with natural fibres by Liliana B. Manfredi; Exequiel S. Rodríguez; Maria Wladyka-Przybylak; Analía Vázquez (255-261).
The thermal degradation and fire resistance of different natural fibre composites were studied. Unsaturated polyester (UP) and modified acrylic resins (Modar) were used as matrix composites. The smoke emission of the materials was also analysed, as well as, the performance against the fire of the biocomposites and glass reinforced composites was compared. Thermal degradation indicated that the Modar matrix composites were more resistant to temperature than the composites with UP matrix. Flax fibre, due to their low lignin content, exhibit the best thermal resistance among the natural fibres studied.From the results obtained about the thermal and fire resistance of the composites it is possible to conclude that the flax fibre seems to be the most adequate to be used, due to the long time to ignition and the long period prior to reach the flashover. On the other hand, the jute fibre composites showed a short duration but a quick growing fire with the lowest smoke emission. The low smoke is an important advantage, which reduces one of the main hazards of fire.
Keywords: Unsaturated polyester; Natural fibres; Thermal degradation; Fire resistance; Smoke emission;

A study of 40 para-substituted anilines is presented. These serve as model structures of amine type antioxidants. Molecules and their radical structures were studied using the AM1 and PM3 quantum chemical methods in order to calculate the N–H bond dissociation enthalpies (BDEs) and ionisation potentials (IPs) which are among the most important characteristics of antioxidants. Calculated BDEs were compared with available experimental values and the results of DFT calculations to ascertain the suitability of AM1 and PM3 methods for amine BDEs calculation. The results show that both methods reproduce experimental BDEs and DFT data satisfactorily. Comparison with experimental data shows that AM1 and PM3 methods overestimate the IP values. The results also indicate that dependences of N–H bond BDEs and IPs on Hammett constants of the substituents are linear.
Keywords: Bond dissociation enthalpy; BDE; Ionisation potential; AM1 quantum chemical method; PM3 quantum chemical method; Amine; Aniline;

The hydrolytic degradation of poly(bisphenol A)carbonate (PC) has been characterized by various liquid chromatography techniques. Size exclusion chromatography (SEC) showed a significant decrease in molecular mass as a result of hydrolytic degradation, while ‘liquid chromatography at critical conditions’ (LC-CC) was very successful for observing differences in functionality due to degradation, i.e. the formation of OH end-groups. To characterize and identify the observed differences semi on-line coupling of liquid chromatography to matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and Fourier transform infrared spectroscopy (FT-IR) has proved to be very useful.Comprehensive two-dimensional liquid chromatography (2D-LC) was also applied to study the hydrolytic degradation of poly(bisphenol A)carbonate. LC-CC × SEC showed that the formation of poly(bisphenol A)carbonate with OH end-groups occurred over the whole molecular mass range. This information could not be obtained with the separate liquid chromatographic techniques, thereby illustrating the usefulness of 2D-LC.The main degradation processes of poly(bisphenol A)carbonate under the applied hydrolysis conditions turned out to be disappearance of cyclic PC oligomers, chain scission of PC and (subsequent) formation of OH end-groups. FT-IR, SEC and LC-CC have been used to follow the hydrolytic degradation with time. LC-CC proved to be the most promising technique to detect the degradation of poly(bisphenol A)carbonate at an early stage.
Keywords: Poly(bisphenol A)carbonate; Two-dimensional liquid chromatography; Degradation; SEC; LC-CC; LC × SEC; MALDI-TOF-MS; FT-IR; Hyphenated techniques; Hydrolysis;

Chemical modification of sugarcane bagasse was achieved by esterification with phthalic anhydride in pyridine after ultrasound irradiation. The parameters including ultrasonic irradiation time 0–40 min, phthalic anhydride dosage 2.5–7.5 g, reaction time 30–150 min, and reaction temperature 80–115 °C were optimised and the extent of phthalation was measured by weight percent gain. It was found that degradation of the cell wall polymers occurred at higher temperature (>90 °C) under the experimental conditions used.FT-IR and CP/MAS 13C NMR spectroscopy studies also clearly produced evidence for phthalation reaction. The phthalated bagasse preparations showed decreased thermal stability after chemical modification.
Keywords: Sugarcane bagasse; Phthalic anhydride; Chemical modification; Phthalation; Ultrasound;

Flammability of styrenic polymer clay nanocomposites based on a methyl methacrylate oligomerically-modified clay by Xiaoxia Zheng; David D. Jiang; Dongyan Wang; Charles A. Wilkie (289-297).
Nanocomposites of polystyrene, high impact polystyrene, acrylonitrile–butadiene–styrene terpolymer, polypropylene, and polyethylene were prepared using a methyl methacrylate oligomerically-modified clay by melt blending and the thermal stability and fire retardancy were studied. These nanocomposites were characterized by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis and cone calorimetry. The results show a mixed morphology, depending on the polymer.
Keywords: Fire retardancy; Nanocomposites; Oligomerically-modified;

An oligomerically-modified clay was made using a surfactant which is the ammonium salt of an oligomer. The newly modified clay contains 37.5% inorganic clay and 62.5% oligomer. Polyethylene and polypropylene nanocomposites were made by melt blending the polymer with the oligomerically-modified clay in a Brabender mixer at various clay loadings. The structure of the nanocomposites was characterized by X-ray diffraction and transmission electron microscopy. Mechanical testing showed that the polyethylene nanocomposites had an enhanced Young's modulus and slightly decreased elongation, while the changes for polypropylene nanocomposites are small compared with the virgin polymers. The thermal stability and flame properties were evaluated using thermogravimetric analysis and cone calorimetry, respectively. The plasticising effect of the oligomer was suppressed because of the increased inorganic content. The maximum reduction in peak heat release rate is about 40%.
Keywords: Fire retardancy; Nanocomposites; Oligomerically modified;

The photodegradation behaviour of the collagen and poly(ethylene glycol) PEG blends has been studied by Fourier transform infrared spectroscopy (FTIR), UV–Vis spectroscopy and viscometry. Surface properties before and after UV irradiation were observed using optical microscope.Collagen and PEG were immiscible and the films obtained from the mixture were fragile with poor mechanical properties. The photochemical stability of the collagen and PEG blend was different from that of the single components. In general collagen/PEG blends are less stable under UV irradiation than pure collagen. The influence of PEG on the photochemical stability of collagen depends on its concentration in the blend. Microscope photographs showed that the surface characteristics of collagen and collagen/PEG blends in film form are not drastically altered after UV irradiation.
Keywords: Collagen; Poly(ethylene glycol); UV irradiation; Biomaterials; Blends;

The influence of hematite nano-crystals on the thermal stability of polystyrene by Milena Marinović-Cincović; Zoran V. Šaponjić; Vladimir Djoković; Slobodan K. Milonjić; Jovan M. Nedeljković (313-316).
A synthetic procedure based on thermal hydrolysis of iron(III) chloride solutions for the preparation of hematite (α-Fe2O3) sol consisting of nano-crystals (NCs) is described. The α-Fe2O3 NCs were characterized by transmission electron microscopy and X-ray diffraction measurements. Incorporation of α-Fe2O3 NCs into polystyrene (PS) was based on the transfer of α-Fe2O3 NCs from the aqueous phase to the organic solvent. A significant shift in the glass transition temperature of PS by 17 °C towards higher temperatures was observed after incorporation of α-Fe2O3 NCs. Also, the thermal stability of PS was improved by about 100 °C in the presence of 3.6 wt% of α-Fe2O3 NCs.
Keywords: Hematite; Polystyrene; Nanocomposite; Thermal stability;

Polyyne solutions in n-hexane or other aliphatic hydrocarbons having C6H2, C8H2 and C10H2 as main components were found to be unstable at relatively high concentration (≈10−2  M) and to separate in few hours a brown precipitate. This phenomenon does not occur in more dilute solutions. The precipitate recovered from the mentioned solutions was analysed by FT-IR spectroscopy and thermogravimetry and found to be constituted by polyyne chains crosslinked and oxidized by oxygen resembling in some way the product obtained by photolysing acetylene in water solution and in presence of air. Polyynes react very quickly with ozone producing as main product polymeric ozonides which are insoluble in hydrocarbons and which have been studied with FT-IR spectroscopy. The oxidized nature of the chains highly crosslinked by ozonides, peroxide and oxygen bridges has been confirmed. The oxidized polyyne fraction remaining in solution after ozonolysis was studied by electronic absorption spectroscopy and by liquid chromatography. Extensive chain breaking was found as expected although some degree of unsaturation was preserved even in the final oxidized products.
Keywords: Polyynes; Oligoynes; Polyacetylenes; Stability; Oxidation; Ozonolysis; Polymeric ozonides; Spectroscopy;

The gas in contact with polyethylene has considerable impact on its oxidation. The rate of oxidation product formation is mostly larger with oxygen blanketing than in air. Similarly, the rate in air is larger than that under nitrogen blanketing. Moreover, the relative effect of the surrounding gas is depending heavily on the particular oxidation product considered. The effect on the alcohol concentration on passing from air to pure oxygen is the same as that on the hydroperoxide concentration. It is only under pure nitrogen that alcohol formation is relatively more affected than hydroperoxide formation. The overall carbonyl groups as well as the ketones show the expected ranking, i.e. faster rate in pure oxygen than in air and faster rate in air than under pure nitrogen. However, carboxylic acids are formed much faster in oxygen than in air. For the acids the results in air and under nitrogen are significantly closer in the initial stages of processing than the results obtained under pure oxygen. This is different for γ-lactones for which formation is faster in oxygen than in air where it is faster than under nitrogen. With trans-vinylene groups the situation is opposite to that observed for carboxylic acids: the rate of formation is close for the experiments performed under air and under oxygen and significantly faster than under nitrogen. The results for hydroperoxides, alcohols and ketones are easily interpreted taking into account the kinetics developed in previous work. Fitting the data to the heterogeneous kinetics shows the effect of the oxygen concentration on this kinetics. It is especially unexpected with respect to its impact on the initiation rate. It is discussed taking into account various possibilities. The only one that is compatible with all the data envisages chain initiation resulting from interaction of oxygen with strained polymer molecules.
Keywords: Polyethylene; Thermal oxidation; Processing; Oxidation products; Hydroperoxides; Alcohols; Ketones; Trans-vinylene groups; Mechanisms; Kinetics;

The kinetics of the thermal degradation of polymethylphenylsiloxane containing methacryloyl groups (PMPS-M) were investigated by thermogravimetric analysis (TGA). Thermal degradation of PMPS-M had two different processes: “unzipping degradation” and “rearrangement degradation”. The corresponding kinetic parameters of the two degradation stages were determined by using Friedman and Flynn–Wall–Ozawa methods, respectively. Coats–Redfern and Phadnis–Deshpande methods were also used to discuss the probable degradation mechanisms of the two different stages. The results showed that the activation energy obtained from Friedman method was in good agreement with the value obtained using Flynn–Wall–Ozawa method. The solid-state decomposition mechanism followed by the first degradation stage of PMPS-M was a decelerated D4 type (three-dimensional diffusion controlled reaction). However, as for the second degradation stage of PMPS-M, its solid-state decomposition mechanism corresponded to a sigmoidal A3 type, a nucleation and growth mechanism.
Keywords: Polymethylphenylsiloxane; Methacryloyl groups; Thermal degradation; Kinetic analysis; Thermogravimetric analysis;

Thermal yellowing sensitised by pre-photo-oxidation of non-deacidified paper by Dominique Fromageot; Nicolas Pichon; Olivier Peyron; Jacques Lemaire (347-357).
Characterization of oxidation products responsible for yellowing of paper through absorption and through emission is presented using independently UV–visible absorption spectrophotometry and microspectrofluorimetry to survey the thermal oxidation at 100 °C and the photo-oxidation at λ  ≥ 340 nm and at 60 °C of the three types of paper containing low amount of ligneous residues (SA papers) or not (WH and C papers). It was shown that due to secondary oxidation yellowing of product is favoured in the dark. It was shown that pre-photo-oxidation largely increased the yellowing in the dark. The effect of pre-photo-oxidation on yellowing through absorption and on yellowing through emission was shown to be different.
Keywords: Paper; Yellowing; Photo-oxidation; Thermo-oxidation;

An oligomerically-modified clay has been used to fabricate nanocomposites with styrenic polymers, such as polystyrene, high-impacted polystyrene, poly(styrene-co-acrylonitrile) and acrylonitrile–butadiene–styrene by melt blending. The clay dispersion was evaluated by X-ray diffraction and bright field transmission electron microscopy. All of the nanocomposites have a mixed delaminated/intercalated structure. The fire properties of nanocomposites were evaluated by cone calorimetry a nd the mechanical properties were also evaluated.
Keywords: Nanocomposites; Fire retardancy; Oligomerically-modified clays; Styrenics;

Biodegradable aliphatic polyesters. Part I. Properties and biodegradation of poly(butylene succinate-co-butylene adipate) by V. Tserki; P. Matzinos; E. Pavlidou; D. Vachliotis; C. Panayiotou (367-376).
A series of aliphatic homopolyesters and copolyesters was prepared from 1,4 butanediol and dimethylesters of succinic and adipic acids through a two-step process of transesterification and polycondensation. The synthesized polyesters were characterized by means of nuclear magnetic resonance spectroscopy (NMR), gel permeation chromatography (GPC), viscosity measurements, differential scanning calorimetry (DSC), X-ray diffraction (XRD), and mechanical property measurements. The homopolymer poly(butylene succinate) exhibited the highest tensile strength, which decreased with increasing adipate unit content, passed through a minimum at copolyester composition close to equimolarity and then increased towards the value of poly(butylene adipate). It is interesting to note that in contrast to tensile strength, the elongation at break increased for adipate unit content of 20–40 mol%. The biodegradation of the polymers was investigated by soil burial and enzymatic hydrolysis using three enzymes, Candida cylindracea lipase, Rhizopus delemar lipase, and Pseudomonas fluorescens cholesterol esterase. It appears that the key factor affecting material degradation was its crystallinity.
Keywords: Thermal properties; Crystallinity; Mechanical properties; Biodegradation;

Poly(butylene succinate-co-butylene adipate) was obtained from 1,4-butanediol and dimethyl esters of succinic and adipic acids through a two step process of transesterification and polycondensation. High molecular weight polyesters were synthesized using hexamethylene diisocyanate as chain extender. The effect of chain extension reaction time and chain extender content on polyester molecular weight, thermal and mechanical properties, was investigated. Polyesters were characterized by means of nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC), viscosity measurements, differential scanning calorimetry (DSC), X-ray diffraction (XRD) and mechanical property measurements. Chain extension reaction had as a result the significant increase of polyester molecular weight leading to increased tensile strength. Polyester crystallinity, as calculated from XRD and DSC analysis, and melting temperature decreased upon chain extension, while glass transition temperature increased. Polyester biodegradation was investigated by soil burial and enzymatic hydrolysis using the enzyme Pseudomonas fluorescens cholesterol esterase. It appears that biodegradation was affected by polyester crystallinity, rather than by its molecular weight.
Keywords: Thermal properties; Crystallinity; Mechanical properties; Biodegradation;

Thermal and mechanical behaviour of flexible poly(vinyl chloride) mixed with some saturated polyesters by Soheir Y. Tawfik; Jeannette N. Asaad; Magdy W. Sabaa (385-392).
Four saturated polyesters poly(hexamethylene adipate), poly(ethylene adipate), poly(hexamethylene terephthalate) and poly(ethylene terephthalate) were prepared. The resulting materials were characterized by IR and 1H NMR, end group analysis and gel permeation chromatography. The effect of blending these polyesters (5 and 10%) with poly(vinyl chloride) (PVC) in the melt was investigated in terms of changes in the thermal behaviour of PVC by studying the weight loss after 50 min at 180 °C, colour changes of the blend before and after aging for one week at 90 °C, the variation in glass transition temperature and the initial decomposition temperature. The results gave proof for the stabilizing role played by the investigated polyesters against the thermal degradation of PVC. The best results are obtained when PVC is mixed with 5% aliphatic polyesters rather than with aromatic ones. This is well illustrated not only from the increase in the initial decomposition temperature (IDT), but also from the decrease of % weight loss and from the lower extent of discolouration of PVC, which is a demand for the application of the polymer. It was also found that blending PVC with 5% of the four investigated polyesters before and after aging for one week at 90 °C gave better mechanical properties even than that of the unaged PVC blank.
Keywords: Poly(vinyl chloride); Saturated polyesters; Thermal behaviour; Aging; Dehydrochlorination process; Mechanical properties; Colour change;

Investigations of the reasons for fungal durability of heat-treated beech wood by Mohammed Hakkou; Mathieu Pétrissans; Philippe Gérardin; André Zoulalian (393-397).
It is generally accepted that thermal treatment of wood by mild pyrolysis (retification or torrefaction) improves its durability to fungal degradation. However, this property has recently been questioned in the literature and definitely needs further investigation. The increase in durability conferred by thermal treatment is generally explained by four hypotheses: the low affinity of heat-treated wood to water; the generation of toxic compounds during heating; the chemical modification of the main wood polymers and the degradation of hemicelluloses. This study was undertaken to understand the reasons for durability of heat-treated beech wood. In order to confirm or not the above mentioned hypotheses, the durability of heat-treated beech wood towards Coriolus versicolor was evaluated according to different parameters like mass loss, wettability or chemical composition. The heat treatment was carried out in a temperature range of 20–280 °C under inert atmosphere for 10 different temperatures. The results show clearly an important correlation between the temperature of treatment and the fungal durability. At the same time, there was insufficient evidence to support the hypothesis of improved decay resistance due to generation of fungicidal compounds or due to the hydrophobic character of wood. Finally, the most plausible hypothesis to explain improvement of wood durability concerns its chemical modifications. Indeed, degradation of hemicellulose associated with other chemical modifications appearing during treatment could be the origin of improved durability. There is a good correlation between decay resistance and mass loss measurements which are directly correlated to hemicellulose degradation.
Keywords: Coriolus versicolor; Durability; Heat treatment; Polymer degradation; Wood;

Effect of oxygen pressure on the oxidation kinetics of unstabilised polypropylene by E. Richaud; F. Farcas; P. Bartoloméo; B. Fayolle; L. Audouin; J. Verdu (398-405).
The thermal oxidation of unstabilised polypropylene films at 80 °C and various oxygen pressures ranging from 0.02 MPa to 5 MPa has been studied by FTIR spectrophotometry (carbonyl growth). The induction time decreases and the maximum oxidation rate increases quasi-hyperbolically when the oxygen pressure increases. The asymptotic behaviour (corresponding to the regime of oxygen excess) is not reached at the highest pressure under study. A kinetic model derived from a classical mechanistic scheme but free of simplifying hypotheses, has been used to simulate the observed behaviour and to determine the elementary rate constants. It is shown that a good simulation of kinetic curves of carbonyl build-up in the whole pressure interval under study can be obtained with a set of physically reasonable rate constant values. The “inverse problem” cannot be, however, totally solved because certain constants are interdependent so that some rate constant values have to be arbitrarily chosen or taken from the literature.
Keywords: Polypropylene; Thermal oxidation; Oxygen pressure effect; Kinetic modelling;

Gamma radiation induced effects on silica and on silica–polymer interfacial interactions in filled polysiloxane rubber by Mogon Patel; Paul R. Morrell; Julian J. Murphy; Anthony Skinner; Robert S. Maxwell (406-413).
We report in our studies to assess the impact of gamma radiation on silica and on the silica–polymer interface in filled polysiloxane rubber. Electron spin resonance (ESR) and solid-state nuclear magnetic resonance (NMR) studies have been performed on samples exposed to gamma radiation. In an effort to probe directly the effect of gamma radiation on the silica surface, we employed 1H and 29Si NMR. Our ESR studies show trapped paramagnetic species (positive holes and/or trapped electrons) within the host silica matrix for all samples exposed to gamma radiation. A sample of pure cab-o-sil irradiated to a dose of 50 kGy also shows an ESR signal. Our studies on real-time aged samples (derived from field trials) also show ESR signatures indicative of silica based trapped paramagnetic species. The growth of trapped paramagnetic species as a function of gamma dose was investigated. This shows that the build up of trapped species occurs rapidly at low gamma dose before reaching saturation at about 20–30 kGy. Radiation induced band gap excitation is the likely process leading to the creation of these paramagnetic species which may be trapped in regions of local charge deficit within the silica matrix. Species that are not trapped may take part in silica surface reactions leading to changes in filler–polymer interfacial interactions. NMR studies combined with ammonia modified swell studies have shown increased polymer segmental chain mobility (softening) at low gamma dose indicative of a possible reduction in filler–polymer interfacial interactions. For those samples exposed to high gamma dose, our ammonia modified swell studies suggest increased polymer–filler interactions presumably through silica–polymer crosslinking effects. Our 1H and 29Si NMR studies on irradiated silica suggest that the silica surface is sensitive to gamma radiation. Our observations are important as they highlight the need to better control the quality (size, purity, etc.) of the silica constituent in filled polymer components used in gamma radiation environments.
Keywords: Filled polysiloxane rubber; Paramagnetic species; Gamma radiation; Silica–polymer interactions;