Electron beam modification and crosslinking: Influence of nitrile and carboxyl contents and level of unsaturation on structure and properties of nitrile rubber

The structural changes of nitrile rubber with varying nitrile contents, hydrogenated nitrile rubber and carboxylated nitrile rubber in the presence and absence of a polyfunctional monomer, namely trimethylolpropane triacrylate, at different doses of electron beam irradiation, were investigated with the help of FTIR spectroscopy (in the attenuated total reflectance mode), dynamic mechanical thermal analysis and sol–gel analysis. Solid-state NMR with gated high power decoupling technique was used to understand the mechanism of crosslinking of the irradiated samples. The allylic radicals generated in the butadiene chains react to form intermolecular crosslinkages. There was a significant decrease in the concentration of olefinic groups for the nitrile rubber on irradiation. This was also affirmed by the increase in the carbon resonances due to C–C linkages from the NMR technique, indicating more crosslinkages. The spectroscopic crosslink densities were determined and the results were compared with the swelling measurements. The variation in the crosslink clustering for rubbers with different acrylonitrile contents was explained using the NMR technique. The increase in crosslinking was also revealed by the increase in the percent gel content and dynamic storage moduli with radiation doses. The lifetime of spurs formed and the critical dose, an important criterion for overlapping of spurs, were determined for both the grafted and the ungrafted nitrile rubbers of different grades and compared using a mathematical model. The ratio of scissioning to crosslinking for nitrile rubber was determined using Charlesby–Pinner equation. The mechanical properties had also been studied for both the modified and the unmodified systems.     

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.     

Localization of Failures in Structural Order of <Emphasis Type="Italic">trans</Emphasis>-1,4-Butadiene Units in Interdefective Regions of a Butadiene-Nitrile Elastomer Matrix

The fractional free volume of chains passing and incorporated into the ordered structures of segments in trans-1,4-configuration in the copolymers of butadiene and acrylonitrile at different content of acrylonitrile units is calculated in order to determine the localization of order disturbances of butadiene trans-1,4-units. Amorphization of the structure occurs in the immediate vicinity of structural defects of acrylonitrilebutadiene rubbers formed by alternating acrylonitrile and trans-1,4-units of butadiene as well as cis-1,4-and 1,2-isomers of butadiene.     

Considering factors on determination of microstructures of SBR vulcanizates using pyrolytic analysis

Properties of styrene-butadiene rubbers (SBRs) are depending on their microstructures (contents of 1,4-unit, 1,2-unit, and styrene), but it is hard to determine the microstructures of SBR vulcanizates. Pyrolytic method such as pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) has been used for microstructures of cured rubbers without pretreatment. Microstructure of SBRs can be estimated using the major pyrolysis products (butadiene, 4-vinylcyclohexene (VCH), and styrene). In this study, considering factors for determination of microstructures of SBR vulcanizates using Py-GC/MS were investigated. The principal considering factors were found to be change of the major pyrolysis products due to radicals formed in carbon backbone and sulfur by dissociation of sulfide crosslinks in SBR vulcanizates. Relative abundances of the major pyrolysis products of raw and cured SBRs were different due to rearrangements of the radicals. Influencing factors on pyrolysis behaviors of SBR vulcanizates were found to be 1,2-unit block, alternating sequence of 1,4- and 1,2-units, styrene-1,4-unit and styrene-1,2-unit sequences, and location of the radicals. Especially, the 1,2-unit block influenced on change of the VCH/butadiene ratio, while the styrene-1,2-unit sequence affected change of the styrene/(butadiene + VCH) one.     

High actuation performance offered by simple diene rubbers

Dielectric elastomers are materials well known for their superior actuation behavior under applied electric field. The simplicity of material fabrication and clear working principle of dielectric elastomer actuators (DEAs) can offer various applications of dielectric elastomers. In this work, we have compared a number of different types of commercially available elastomers in terms of actuation performance. It was found that well‐known commercial rubbers like acrylonitrile‐butadiene rubbers (NBR) can offer higher actuation performance in DEAs than the frequently used dielectric elastomers, such as acrylic rubber and silicone. The acrylonitrile content of the NBR was found to play an important role in the dielectric and consequently actuation properties. More interestingly, we observed that addition of organic oil, such as dioctyl adipate, can greatly enhance the actuation performance. Copyright © 2016 John Wiley & Sons, Ltd.     

Effects of copolymer composition on the formation of ionic species,hydrogen evolution,and free-radical reaction in γ-irradiated styrene-butadiene random and block copolymers

Block and random copolymers of butadiene and styrene as well as polybutadiene and polystyrene homopolymers have been investigated with respect to formation of trapped electrons, contribution of ionic species to crosslinking, and hydrogen gas evolution due to γ radiation. The decay kinetics of the disubstituted benzyl radical has also been studied. The yields of electron trapping G(e^?) are measured. The G(e^?) increase linearly with increased polystyrene content in block polymers, while in random copolymer a deviation from a linear relation is observed. The contribution of ionic reactions to crosslinking is about 25–35% of the total crosslinking yield. Hydrogen production in block copolymers is approximately a linear function of the weight-fraction additivity of the yield of hydrogen formation in polystyrene and polybutadiene homopolymers. Energy transfer from butadiene units to styrene units in random copolymers resulted in a deviation from such an additivity relation. The decay of the disubstituted benzyl free radical in block copolymers is a second-order reaction. In random copolymer, the decay is best interpreted in terms of equation based on a second-order decay mechanism of a fraction of the free radicals decaying in the presence of other nondecaying free radicals.     

Crosslinker free thermally induced crosslinking of hydrogenated nitrile butadiene rubber

Hydrogenated nitrile butadiene rubbers (HNBRs) are hydrogenated copolymers of acrylonitrile and butadiene, which are often used in composites or polymer blends. These copolymers are usually cured with peroxides or vulcanized with sulfur to reinforce their mechanical resistance and improve their chemical stability. However, using such crosslinking reagents can be problematic for high value systems where residual H₂O₂ or S can be detrimental for the application. To address this limitation, we studied the thermally induced crosslinking of HNBR at high temperature (240 °C) with oxygen. To understand the impact of conditions (temperature, time, and atmosphere) on the chemical structure and the mechanical properties of HNBR, different thermal treatments were investigated. We show that HNBR forms a ladder structure during treatment in the presence of O₂ which result in a reinforcement of the elastomer. Tensile tests and DSC show both alkene and nitrile moieties are involved in the reaction, leading to a mechanical resistance comparable with a HNBR crosslinked with peroxides or sulfur. These findings will help achieving a better control on the crosslinking to provide HNBRs with desired properties. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1825–1833     

Experimental branching results for diene polymers by use of gel-permeation chromatography

Branching analyses in styrene–butadiene rubbers and polybutadiene rubbers have revealed large differences in branching between rubbers polymerized in different ways. The functionalities of several star-branched solution-polymerized styrene–butadiene rubbers were calculated and compared to their expected structures. Emulsion-polymerized polybutadiene rubber and a series of solution-polymerized polybutadienes made with different catalysts had different degrees of random branching, and evidence is presented indicating that the different available catalyst systems provide some latitude in making rubbers of different branching contents. Random branching analyses on a series of emulsion-polymerized styrene–butadiene rubbers revealed the dependency of branching on molecular weight and molecular weight distribution. The influence of polymerization temperature on the branching of emulsion-polymerized styrene–butadiene rubber was also studied.     

Acrylonitrile Copolymerizations. IV. Butadiene Copolymerization

The kinetics of the acrylonitrile–butadiene radical copolymerization, carried out in solution at 60°C, have been followed using gas chromatographic analysis. Remote units effects are observed only on the butadiene-ended radicals but they seem to involve a quite long sequence of butadiene units. The following values of the reactivity ratios are proposed: r_A = 0.067, r_AB = 0.70, r_ABB = 0.66, rABB = ^0.17,rBi → 001 to 0-06 for large values of L The results are discussed in terms of either polarity effects, or differences in reactivities between 1,4- or 1,2-butadiene radicals, or finally of charge transfer complexes between the monomers.     

Chemiluminescence spectral analysis of styrene-butadiene and acrylonitrile-butadiene rubbers using a multichannel Fourier-transform chemiluminescence spectrometer

The chemiluminescence spectra of styrene-butadiene and acrylonitrile-butadiene rubbers isothermally heated in the range between room temperature and 200 °C were measured with a multichannel Fourier-transform chemiluminescence spectrometer. The observed spectra were analyzed by a least-squares fit using a Gaussian function to determine the peak intensity and the peak wavelength of emission bands. It was found that the peak wavelength of acrylonitrile-butadiene rubbers was slightly longer than that of styrene-butadiene rubbers due to the influence of the nitrile groups. The peak intensity of chemiluminescence from styrene-butadiene rubbers decreased with the styrene content, while that from hydrogenated acrylonitrile-butadiene rubbers increased with the iodine value. From these experimental results, it is concluded that the content of double bonds in the butadiene units in rubbers is measurable by chemiluminescence spectral analysis.     

Evolution of the crosslink structure in the elastomers NR and SBR

An experimental positron annihilation lifetime spectroscopy (PALS), differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS) study of the effect of the advance of the crosslinking reaction on the free volume in a copolymer of styrene–butadiene and natural rubbers was carried out. The crosslink density developed in SBR specimens with different sulfur contents and cure temperatures was studied. SAXS technique was applied to study the process of crosslinking in NR as a function of the cure temperature. Finally, a study of different SBR/NR blends is presented using PALS and DSC.     

Radiation crosslinking and scission of poly(vinyl methyl ether) in aqueous solution

For a wide range of poly(vinyl methyl ether) (PMVE) concentrations (1–16 g dm⁻³), in anoxic conditions, polymer-derived radicals recombine in two major pathways: (i) crosslinking and (ii) disproportionation. Both these processes proceed inter- and intramolecularly. The radiation-chemical yields and kinetics of crosslinking have been studied by pulse radiolysis with light scattering intensity detection (LSI). In the absence of oxygen, G-values of intermolecular crosslinking were determined on the basis of LSI changes versus applied dose and compared with the results obtained previously for γ-irradiated samples. It has been found that the first half-life time of intermolecular crosslinking decreases with increasing dose per pulse. Addition of small amounts of macroradical scavenger (cysteamine hydrochloride) decreases, drastically, the increase of LSI signal. On increasing the PVME concentration, intermolecular crosslinking becomes more efficient.

In the presence of oxygen, for diluted PVME solution (0.1 g dm⁻³), decrease of LSI signal consisting of the kinetic of a first-order reaction was observed. The rate constant of LSI decrease was found to be 1.1×10³ s⁻¹ and it was attributed to the main-chain scission.     

Ketonization of a nitrile-butadiene rubber by nitrous oxide: Comparison with the ketonization of other type diene rubbers

Noncatalytic ketonization of a nitrile-butadiene rubber (21 mol% acrylonitrile units) by nitrous oxide was shown to yield polymeric products functionalized with carbonyl (mainly ketone) groups. The reaction was conducted in a benzene solvent at 180-230° and pressure of 3-6 MPa. An assumed ketonization mechanism includes a 1,3-dipolar cycloaddition of N₂O to CC bonds in butadiene units.According to the NMR and GPC data, the main route of the reaction (ca. 85%) proceeds without cleavage of the CC bonds and yields ketone groups in the polymer backbone. The minor route (ca. 15%) includes the cleavage of CC bonds resulting in fragmentation of the macromolecules that leads to a decrease in their molecular weight. The nitrile (-CN) groups remain untouched. The resulting product is a bifunctional low-molecular rubber containing, in addition to originally present nitrile groups, a regulated amount of new ketone groups randomly distributed along the polymer backbone.The results for nitrile-butadiene rubber are compared with the earlier studied ketonization of butadiene and isoprene rubbers. The molecular structure of monomeric units was shown to be an important characteristic of a parent rubber affecting the reaction rate, degree of fragmentation, and consistency (rubber-like, plastic, or liquid) of the resulting material.     

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.     

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.     

Fractional free volumes in structural defects of butadiene–nitrile elastomers of various polarities and rotational mobilities of nitroxyl radicals

The correlation between the magnitudes of the fast components of rotational mobility of the nitroxyl radicals (2,2,6,6-tetramethylpiperidine-1-yl)oxyl and (4-benzoyloxy-2,2,6,6-tetramethylpiperidine-1-yl)oxyl measured via EPR spectroscopy, the fractional free volumes in the structural defects of butadiene–nitrile rubbers and the related vulcanized rubbers, in which the nitroxyl radicals are sorbed, and a variation in this correlation with an increase in temperature are established.     

Modification of epoxy resins with liquid polysulfide and carboxylated butadiene rubbers

In this work, we present the results of studying the effect of liquid carboxylated butadiene and polysulfide rubbers on the physicomechanical and adhesive properties of epoxy adhesive compositions with curing agents of different chemical nature—diethylene triamine and polyoxypropylene triamine. It is found that carboxylated butadiene rubbers are more effective modifiers than polysulfide rubbers because they yield a higher complex of properties. It is also shown that the change in the chemical nature of a curing agent has no significant effect on the properties of epoxy-rubber adhesive compositions, which allows one to control their key technological parameters without fear of losing strength characteristics.     

Barrier property of clay/Acrylonitrile‐butadiene copolymer nanocomposite

The resistance to air permeation was investigated for ­an intercalated clay/acrylonitrile‐butadiene copolymer ­nanocomposite. The nanocomposite is prepared by melt mixing the organo‐treated montmorillonite into a rubber matrix, together with peroxide curative, and crosslinked by conventional compression molding for typical rubbers. In the case of intercalated nanocomposite, the air permeability decreases considerably with increasing clay content, and the decreasing trend agrees reasonably with the Neilson's tortuous model. No considerable improvement is found when the pure montmorillonite is added. Copyright © 2002 John Wiley & Sons, Ltd.     

Syntheses and reactions of azo-group containing networks

Azo monomers of the general structure have been terpolymerized with styrene and butadiene (I) to form azo-group containing rubber (SBR). Copolymerization of butadiene and acrylonitrile with 3 yields azo group containing nitrile rubber (II). Networks are formed, if the copolymerization of butadiene and acrylonitrile is carried out in the presence of divinylbenzene (DVB) (III) or in the presence of DVB and 3 (IV). Thermolysis or photolysis of the polymeric azo initiators (I, II) yields networks without addition of a low molar-mass crosslinking agent. Heating or irradiation of the azo-containing pre-network (IV) increases the network density. Heating a mixture of (II) or (IV) with natural rubber yields polymer blends which are stabilized by covalent links between the different polymers. The network characterization was carried out by swelling experiments. The incorporation of the azo monomers into the azo copolymers is almost ideal under “cold rubber” conditions, meaning that the composition of the products does not change with conversion.     

Direct Catalytic Hydrogenation of an Acrylonitrile‐Butadiene Rubber Latex Using Wilkinson's Catalyst

Summary: Hydrogenated acrylonitrile‐butadiene rubber is a high‐performance elastomer that has found important applications in the automobile industry and petroleum fields. A new catalytic process has been investigated for producing hydrogenated acrylonitrile‐butadiene rubber by direct hydrogenation of the rubber in latex form using RhCl(PPh₃)₃ as catalyst. The reactions were achieved without the addition of any organic solvents. This is the first time that the direct catalytic hydrogenation of an acrylonitrile‐butadiene rubber latex has been successfully realized in the absence of any organic solvent with a conversion of higher than 95% without cross‐linking of the polymer.

Effect of RhCl(PPh₃)₃/acrylonitrile‐butadiene rubber ratio on hydrogenation.