Dielectrical and electro-mechanical behavior of NBR at high electric field strength

The aim of this work is to present a new approach to characterize the dielectric properties of elastomers especially at low frequencies and high electric field strength, close to the operating conditions of dielectric elastomer actuators (DEA). By means of an electro-rheological measuring cell, static (DC) and alternating (AC) high electric field strength were applied to different types of acrylonitrile-butadiene rubbers (NBR), with varying content of polar acrylonitrile groups. Based on fundamental theory of electricity complex permittivity and polarization were calculated from Coulomb stress measurements at varying frequencies. During high voltage measurements it was observed, that dielectric loss current increases, if a certain critical field strength is exceeded. It is shown, that polar NBR rubbers exhibit a critical field strength far below of the non-polar polybutadiene rubber (BR), acting as a reference sample. NBR exhibits extraordinary high permittivity at low frequency, which makes this material as a favorite candidate for DEA. Furthermore, mechanical properties of NBR can be easily altered by application of an alternating electric field. This phenomenon opens new opportunities in a great variety of applications.     

Thermal stability of acrylonitrile/chlorosulphonated polyethylene rubber blend

The properties of chlorosulphonated polyethylene (CSM) rubber, acrylonitrile rubber (NBR) and their blend (50/50 w/w) were studied. Fourier transform infrared (FTIR) studies supported that CSM/NBR rubber blend is self curable, when cross-linking takes place between acrylonitrile groups of NBR and –SO₂Cl groups or in situ generated allyl chloride moieties of CSM. The thermal stability of vulcanizates was analyzed in nitrogen by thermogravimetry. It was found that the initial degradation temperature of elastomer based on CSM rubber is lower than of pure NBR rubber. By adding NBR to CSM rubbers, the degradation temperature of crosslinked material increased, indicating higher thermal stability. The activation energy for the degradation are determined using the Arrhenius equation The activation energies for the rubber blends are higher than for elastomers based on pure rubbers. It was found that the mass loss of the blends at any temperature was between those of the pure rubbers. The differential scanning calorimetry (DSC) was used for the glass transition temperature determination. It is estimated thermodynamic immiscibility of NBR/CSM blend based on noticed two different glass transition temperatures, corresponding to CSM and NBR rubbers.     

Photo-oxidation of copolymers of butadiene and acrylonitrile

The photo-oxidation of two copolymers of butadiene and acrylonitrile (NBR rubbers) with acrylonitrile contents of 21·7 wt% and 41·6 wt% was compared with the photo-oxidation of polybutadiene and polyacrylonitrile samples under the same experimental conditions. The general features of the photo-oxidation of NBR rubbers over the composition range studied reflect the behaviour of the pure homopolymers, polybutadiene and polyacrylonitrile. The main process observed is the photo-oxidation of butadiene units whereas the photo-oxidation of acrylonitrile units is negligible under the same experimental conditions.     

Preparation of poly(methyl methacrylate)–poly(acrylonitrile‐co‐butadiene) core–shell nanoparticles

Poly(methyl methacrylate)–poly(acrylonitrile‐co‐butadiene) (PMMA–NBR) core–shell structured nanoparticles were prepared using a two‐stage semibatch microemulsion polymerization system with PMMA and NBR as the core and shell, respectively. The Gemini surfactant 12‐3‐12 was used as the emulsifier and found to impose a pronounced influence on the formation of core–shell nanoparticles. The spherical morphology of core–shell nanoparticles was observed. It was found that there exists an optimal MMA addition amount, which can result in the minimized size of PMMA–NBR core–shell nanoparticles. The formation mechanism of the core–shell structure and the interaction between the core and shell domains was illustrated. The PMMA–NBR nanosize latex can be used as the substrate for the following direct latex hydrogenation catalyzed by Wilkinson's catalyst to prepare the PMMA–HNBR (hydrogenated NBR) core–shell nanoparticles. The hydrogenation rate is rapid. In the absence of any organic solvent, the PMMA–HNBR nanoparticles with a size of 30.6 nm were obtained within 3 h using 0.9 wt % Wilkinson's catalyst at 130 °C under 1000 psi of H₂. This study provides a new perspective in the chemical modification of NBR and shows promise in the realization of a “green” process for the commercial hydrogenation of unsaturated elastomers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Enhanced dielectric performance of a block copolymer‐polythiophene nanocomposite

Dielectric elastomer actuators (DEAs) transform electrical energy into mechanical work. However, despite displaying exceptional features, the low permittivity of elastomers restricts their application. Hence, to overcome this limitation, DEAs are fabricated by dispersing poly(3‐methylthiophene acetate) (P3TMA), a polarizable conducting polymer, into poly[styrene‐b‐(ethylene‐co‐butylene)‐b‐styrene] (SEBS), a thermoplastic elastomer with excellent mechanical properties. Although high‐quality SEBS:P3TMA films are obtained for all compositions (between 0.5 and 20 wt % P3TMA), their thickness and surface roughness increase with the nano‐sized filler content. Moreover, the conducting particles are well integrated into the SEBS network with no evidence of aggregation or significant change in the mechanical properties of the composites. P3TMA, which forms encapsulated conductive domains within the polymeric matrix, improves the dielectric behavior of SEBS:P3TMA by increasing their dielectric constant with low dielectric losses and no current leakage. Thus, indicating the potential future application of these nanocomposites as elastomer actuators or high energy density capacitors. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1896–1905     

Thermal stability and flammability of nanocomposites made of diene rubbers and modified halloysite nanotubes

This paper presents a method of modification of halloysite with the use of aqueous solutions of halogens and an alcoholic solution of boric acid. The effect of modified nanoadditives on the thermal properties, flammability, and fire hazard of peroxide and sulfur vulcanizates of butadiene–acrylonitrile (NBR) and butadiene–styrene (SBR) rubbers was described and assessed. The test results obtained by spectrometric methods, oxygen index, and cone calorimeter were interpreted from the point of view of the chemical structure of the investigated diene elastomers and the particular method of halloysite modification. The analysis of values obtained by the method of cone calorimetry it confirms that most NBR and SBR vulcanizates filled with modified halloysite are more resistant to fire when compared with material without modification. In addition, the modified halloysite are crucial in making self-extinguishing elastomeric materials.     

Methods to ease the release of thin polydimethylsiloxane films from difficult substrates

Silicone elastomers are used as dielectric electroactive polymers for making actuators, generators, sensors, and as artificial muscles in medical applications. Current requirements in the actuator manufacturing put a strict limitation on the thickness of the elastomers, such that a maximum permissible thickness is around 25–50 µm. The relatively small Young's modulus for these elastomers is a requirement for actuation capabilities. However, peeling and release of such films during manufacture processes are very difficult. To ease the release of the films, techniques such as the use of release agents like surfactants and detergents, incorporating resins in the silicone matrix and grafting/adding low surface energy functionalities to the silicone elastomer have been tested. The methods used are required not to interfere with the Young's modulus and the dielectric permittivity in a negative way. Polysorbate‐20, a non‐ionic surfactant, fulfills all requirements and gives the lowest peel forces for the films. Copyright © 2013 John Wiley & Sons, Ltd.     

Melt rheology and morphology of thermoplastic elastomers from polyethylene/nitrile‐rubber blends: The effect of blend ratio,reactive compatibilization,and dynamic vulcanization

A study of the melt‐rheological behavior of thermoplastic elastomers from high‐density polyethylene and acrylonitrile butadiene rubber (NBR) blends was carried out in a capillary rheometer. The effect of the blend ratio and shear rate on the melt viscosity reveals that the viscosity decreases with the shear rate but increases with NBR content. Compatibilization by maleic anhydride modified polyethylene has no significant effect on the blend viscosity, but a finer dispersion of the rubber is obtained, as is evident from scanning electron micrographs. The melt‐elasticity parameters, such as the die swell, principal normal stress difference, recoverable shear strain, and elastic shear modulus of the blends, were also evaluated. The effect of annealing on the morphology of the extrudate reveals that annealing in the extruder barrel results in the coalescence of rubber particles in the case of the incompatible blends, whereas the tendency toward agglomeration is somewhat suppressed in the compatibilized blends. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1104–1122, 2000     

Studies on the nature of multiple glass transitions in low acrylonitrile,butadiene–acrylonitrile rubbers

A study of the occurrence of multiple glass transitions in acrylonitrile–butadiene rubbers (NBR) has been made. Copolymerization theory was used to predict the change in comonomer composition with conversion for comonomer ratios both above and below the calculated azeotropic composition of 64% butadiene/36% acrylonitrile by weight. The results of these calculations suggested that multiple glass transitions, which occur only in NBR of less than 36% acrylonitrile, were due to an incompatibility of copolymer species of divergent comonomer compositions. This was shown by differential thermal analysis to be the case for various experimental polymers of known comonomer composition. A series of NBR's was prepared by incremental addition of acrylonitrile monomer during polymerization, and the resultant glass transition temperatures were evaluated. Results obtained showed that experimental samples which had single glass transitions also had a much narrower spread of comonomer species than the corresponding rubber polymerized with the use of full initial charge of both monomers. The data indicate that NBR's having a single glass transition, regardless of acrylonitrile content, may be prepared by incremental addition of acrylonitrile monomer during polymerization. Existing copolymerization theory appears to be adequate for predicting incremental monomer addition schedules suitable for the polymerization of NBR's having a single glass transition.     

Effect of chitosan derivatives as rubber antioxidants for increasing durability

A series of chitosan derivatives, namely polydiethylamino-ethylmethacrylate-chitosan-graft-copolymer (chitosan-g-DEAEMA), polycarboxy-chitosan-graft copolymer (chitosan-g-COOH), polyvinyl alcohol chitosan-graft-copolymer (chitosan-g-VOH), and carboxymethyl-chitosan (CM-chitosan), were synthesized and investigated as antioxidants for natural rubber (NR) and acrylonitrile butadiene rubber (NBR) mixes and vulcanizates to increase their durability. The rheometric characteristics of the rubber mixes were determined using an oscillating disc rheometer. The physico-mechanical properties of the rubber vulcanized were measured before and after exposure to thermal oxidative aging. It was found that the CM-chitosan had an accelerating effect on the curing process of NR and NBR. Also, the investigated polymers enhanced the properties of rubbers (NR and NBR) especially after ageing up to 7 days compared with commercial antioxidants, such as phenyl ß-naphthylamine (PßN) and N-isopropel-Nphenyl-p-phenylene diamine (IPPD) which are used in the rubber industry. After ageing, the retained values of tensile strength, modulus at 100 % strain, and elongation at break were improved. The optimum concentration of the investigated compounds used to give good properties was found to be 1–2 parts per 100 of rubber (phr). In addition, these prepared polymers showed a decrease in the equilibrium swelling of rubber in toluene which is the proper solvent and consequently increases the crosslink density for rubbers.     

Improving the mechanical properties of short pineapple leaf fiber reinforced natural rubber by blending with acrylonitrile butadiene rubber

This work proposes a simple method for improving the rubber to filler stress transfer in short pineapple leaf fiber-reinforced natural rubber (NR). This was achieved by replacing some of the non-polar NR by polar acrylonitrile butadiene rubber (NBR). The amount replaced was varied from 0% to 20% by weight. The mixing sequence was designed so that the fiber would be coated with polar NBR before being dispersed in the NR matrix. A comparison system in which the mixing was carried out in a single step was also examined. Despite the fact that the two rubbers are immiscible, it was found that significant improvement of the stress transfer in the low strain region can be obtained. The mixing sequence affected the mechanical properties of the resulting composites. It is concluded that frictional stress transfer between the immiscible rubbers contributes more to the total stress transfer than does the frictional stress transfer between non-polar NR and polar cellulose fiber.     

Viscoelastic effects in cutting of elastomers by a sharp object

The cutting behavior of elastomers by a sharp object was investigated using various elastomers such as acrylonitrile–butadiene rubber (NBR), styrene–butadiene rubber (SBR), and natural rubber (NR). The effects of crosslinking density, cutting rate, and temperature on the cutting energy of elastomers were investigated. The cutting behavior of swollen elastomers was also investigated. It was found that the cutting energy increased as the molecular weight between crosslinks increased. It was also found that the cutting energies of various elastomers did not yield a single line. Moreover, even in the threshold condition of cutting process, the cutting energy was much higher than the threshold fracture energy. These results suggest that the cutting behavior cannot be explained by only a C C bond rupture process, but it includes other energy dissipation processes. The curves for cutting energies obtained at different cutting rates and temperatures were well superimposed on a single master curve when they were shifted using the WLF (Williams, Landel, and Ferry) equation. Therefore, it is supposed that the cutting of elastomers by a sharp object includes viscoelastic energy dissipation process and is the viscoelastic behavior. It was also found that the variation of cutting energy over a considerable range of effective rates was smaller than that of the tear energy. It is attributed to the fact that the change of the crack tip diameter, i.e., roughening or reduction, was restricted by the diameter of razor blade. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1283–1291, 1998     

Impact modification of PA-6 and PBT by epoxy-functionalized rubbers

Glycidylmethacrylate (GMA) grafted apolar (ethylene/propylene/diene - EPDM) and polar (acrylonitrile/butadiene - NBR) rubbers were melt blended with polyamide-6 (PA-6) and polybutylene terephthalate (PBT). The toughness of the blends containing 5, 10 and 50 wt.% epoxy functionalized rubbers was assessed by various methods (notched Charpy, perforation impact) as a function of temperature (T=23 and −40°C). The notched Charpy tests served to deduce the fracture toughness (K_c) and energy (G_c) data. It was established that EPDM-g-GMA is a slightly better impact modifier than is NBR-g-GMA albeit the latter polar rubber is more compatible with both matrices than the less polar EPDM-g-GMA. This finding was traced to the difference in the glass transition temperature (T_g of NBR is higher than that of EPDM) and to the dispersion of the epoxy functionalized rubbers.     

Effect of the spatial network structure and cross-link density of diene rubbers on their thermal stability and fire hazard

The paper presents the results of investigating the effect of the density and spatial network structure of diene rubbers (NBR and SBR) on their thermal properties and fire hazard. The rubbers were either conventionally cross-linked by means of sulfur and organic peroxide or nonconventionally with the use of iodoform (CH₃I). Based on thermo-kinetic analysis, the destruction activation energy of the elastomers investigated and their vulcanizates was determined under air and inert gas. The analysis of particular stages of their thermal decomposition was also presented. During the combustion of the elastomeric materials obtained, it has been found that their fire hazard depends not only on the elastomer chemical structure but also on the method of its cross-linking.     

Synthesizing a new dielectric elastomer exhibiting large actuation strain and suppressed electromechanical instability without prestretching

Prestrain provides high actuation performance in dielectric elastomers (DEs) but increases the bulk, mass, and fatigue of the resulting actuators. Based on our experiments on prestrain‐locked interpenetrating polymer films and the model developed by Zhao and Suo, materials with a certain stress–strain relationship should be capable of high strain without prestrain by suppressing electromechanical instability (EMI). Here, we report the synthesis of an acrylic elastomer capable of achieving high actuation performance without prestrain. DE films were directly fabricated by ultraviolet curing of precursors comprising a mixture of acrylate comonomers. Varying the amount of crosslinker comonomer in the precursor allowed us to tune the stress–strain relationship and completely suppress EMI while maintaining high strain performance. Addition of plasticizing agents increased strain sensitivity. The result is a new DE, synthesized from scratch, capable of high actuation strain (>100%), high energy density (>1 J g^?1), and good temperature and frequency response without requiring prestretching. The material can be fabricated using conventional coating techniques and the process can allow for high volume throughput of stacked DE actuators. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

密封橡胶材料的压力-体积-温度关系的研究

制备了3种密封胶材料——丁苯橡胶、丁腈橡胶及丁腈/氯化丁基混合橡胶材料,利用压力-体积-温度(P-V-T)高压膨胀计对样品进行测试,得到了比容Vsp随着温度T和压力P变化的关系;采用Tait方程研究了P-V-T性能,并计算出Tait方程中相应的参数.结果表明,Tait方程计算所得的P-V-T理论关系与实验结果一致,具有较好的规律性;b3、b4均为常数,即B为常数;Tait方程计算所得值与实验值的平均残差均小于0.02,吻合度较好;因此Tait方程可以较好地描述3种密封橡胶体系的P-V-T关系,并且为定量研究密封橡胶材料的P-V-T关系提供理论基础.同时,根据Tait方程所得的参数,计算了3种橡胶材料的热膨胀系数α和等温压缩系数β,研究了3种橡胶材料的α和β随着压力、温度的变化规律,结果表明3种材料中丁苯橡胶的尺寸稳定性最好,而丁腈/氯化丁基复合橡胶的尺寸稳定性较差并且其α随着压力的变化有较大的改变.准确地描述了3种密封橡胶材料的α和β随压力、温度的变化.     

Shape memory effect exhibited by smectic-C liquid crystalline elastomers

It was long expected and recently shown that main-chain liquid crystalline elastomers (MC-LCEs) may serve as high performance soft actuators due to a coupling of their intrinsic characteristics of high, yet labile, ordering and network strain. Here, we present the synthesis of new siloxane-based smectic MC-LCEs. These new materials exhibit a unique thermomechanical behavior known as the shape memory effect, which has never been observed before in such LCEs. To achieve targeted transition temperatures required for facile actuation at low temperatures, specifically temperatures ranging from 15 to 65 degrees C, we have designed and prepared such elastomers incorporating two distinct mesogenic groups, termed 5H and 5tB, coupled with hydride-terminated poly(dimethylsiloxane) spacers.     

Study of dielectric parameters of liquid crystal elastomer

Liquid crystal elastomers exhibit several interesting phenomena like elastic response, shape memory effect and mechanical actuation triggered by external stimuli and have got much importance among all types of actuated materials. In the present study, we synthesised nematic liquid crystal elastomers (NLCEs) by using Finkelmann procedure. The chemical structure of elastomers is confirmed by Fourier transform infrared spectroscopy. The dielectric response of NLCEs and the monomer used in its synthesis has been studied in the frequency range 20 Hz to 1 MHz. The effect of bias on dielectric response of NLCE and its comparison with its liquid crystal analogue is also reported.     

Thermal properties of diene elastomers

The paper presents the results of investigating the effect of the macromolecule chemical structure and the spatial network structure of butadiene (BR), butadiene-styrene (SBR) and butadiene-acrylonitrile (NBR) rubbers on their thermal properties. The rubbers were cross-linked by the conventional method by means of dicumyl peroxide or sulfur as well as by the non-conventional way using iodoform (CH₃I). The rubber and their vulcanizates were assessed by the derivatographic method (under air) and by means of differential scanning calorimetry (DSC) under inert gas. The thermal cross-linking degree of the polydienes and the efficiency of the cross-linking processes, dependent on both the chemical structure of elastomer macromolecules and their spatial network structure were determined. The cross-linking of elastomers with iodoform changes the thermal properties of polymers, significantly increasing their glass transition temperature during both sample heating and cooling, which results from the increase in mutual interaction of macromolecules connected with their modification with iodine compounds.     

An investigation of radiation-induced structural changes in nitrile rubber

The mechanism of radiation-induced structural changes in nitrile rubber with different acrylonitrile contents were investigated by ESR, NMR, and FTIR. To investigate new structures solid-state NMR methods had to be used due to crosslinking of the irradiated rubbers, and higher probe temperatures were used to obtain better resolution. The radicals generated on the acrylonitrile groups were found to abstract hydrogen from the adjacent butadiene units resulting in the formation of allylic radicals. These allylic radicals reacted to form intermolecular crosslinks and cyclisation. Cyclisation of the butadiene units were found to occur in the initial stages of the irradiation. Radiation yields of radicals increased with acrylonitrile content from 1.42, 1.58, to 2.42 for 18, 30, and 45% acrylonitrile rubbers. The radiation yields for intermolecular crosslinking were higher in rubbers with higher acrylonitrile contents, giving G values of 17.8, 21.3, and 24.5 for 18, 30, and 45% acrylonitrile rubbers, respectively. However, the crosslink clustering was found to be less in the rubbers with a higher acrylonitrile content. © 1996 John Wiley & Sons, Inc.