Radiation effects on styrene–butadiene–ethylene–propylene diene monomer‐multiple walled carbon nanotube nanocomposites: vulcanization and characterization

Varying compositions of styrene–butadiene rubber (SBR) and ethylene–propylene diene monomer (EPDM) 50:50 blend containing multiple walled carbon nanotube (MWNT) as nanoparticulate filler (0.5–5%) were prepared and their efficacy for radiation vulcanization was analyzed by gel‐content, Charlesby‐Pinner parameter, and crosslinking density measurements. Radiation sensitivity of the nanocomposites increased with increase in the MWNT fraction and radiation dose in the dose range studied. The elastic modulus, tensile strength increased with the radiation dose, while elongation at break exhibited downward trend. The extent of reinforcement as assessed using Kraus equation suggested high reinforcement of blend on MWNT addition. The reinforcing mechanism of nanocomposites was studied by various micromechanics models which predicted higher modulus than the experimentally observed results, indicating agglomeration in the nanocomposites. The thermal stability of the composites increased with increase in MWNT loading has been attributed to the antioxidancy induced by nanotubes and higher crosslinking extent of the nanocomposites. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermally amendable tailor‐made functional polymer by RAFT polymerization and “click reaction”

This investigation reports the preparation and characterization of thermally amendable functional polymer bearing furfuryl functionality via reversible‐addition fragmentation and chain transfer (RAFT) polymerization and Diels‐Alder (DA) reaction. In this case, furfuryl methacrylate (FMA) was polymerized using 4‐cyano‐4‐[(dodecylsulfanylthiocarbonyl)sulfanyl] pentanoic acid as RAFT reagent and 4,4′‐azobis(4‐cyanovaleric acid) as thermal initiator. ¹H NMR, ¹³C NMR, and matrix‐assisted laser desorption ionization time‐of‐flight mass spectrometry analysis showed that furfuryl group in poly(furfuryl methacrylate) (PFMA) was not affected during RAFT polymerization and the tailor‐made polymer had RAFT end group. The DA reaction was successfully carried out between the reactive furfuryl functionality of PFMA and different bismaleimides. The thermoreversible property of these DA polymers was characterized by FT‐IR and DSC analysis. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3365–3374     

The reaction of furfuryl alcohol resins with hexamethylenetetramine: A 13C and 15N high-resolution solid-state NMR study

The reactions between furfuryl alcohol (FA)/poly(furfuryl alcohol) (PFA) and hexamethylenetetramine (HMTA) have been studied by ¹³C and ¹⁵N high-resolution solid-state NMR techniques. Highly crosslinked polymer networks similar to those obtained from the thermal curing of FA resins under acidic condition are formed. Possible reaction pathways are postulated on the basis of changes of chemical structures during the curing; α-substituted furfurylamines are shown to be the initial intermediates. Their further reactions with FA/PFA, together with thermal decomposition, produce methylene linkages between furan rings, resulting in chain extension and crosslinking, which occurs at both the methylene linkages and the 3- or 4-C of furan rings. Various side-products such as amines, imines, amides, imides, and nitriles are also formed during the reactions, and some of these can remain in the resins up to 205°C. The crosslinked network can be heterogeneous with different structures on nanometer scales when higher oligomers of FA resins react with HMTA. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2233–2243, 1997     

Homo‐ and diblock copolymers of poly(furfuryl glycidyl ether) by living anionic polymerization: Toward reversibly core‐crosslinked micelles

We report the synthesis and characterization of well‐defined homo‐ and diblock copolymers containing poly(furfuryl glycidyl ether) (PFGE) via living anionic ring‐opening polymerization using different initiators. The obtained materials were characterized by SEC, MALDI‐TOF MS, and ¹H NMR spectroscopy and molar masses of up to 9400 g/mol were obtained for PFGE homopolymers. If the amphiphilic diblock copolymer PEG‐block‐PFGE was dissolved in water, micelles with a PFGE core and a PEG corona were formed. Hereby, the hydrophobic PFGE core domains were used for the incorporation of a suitable bismaleimide and heating to 60 °C induced the crosslinking of the micellar core via Diels‐Alder chemistry. This process was further shown to be reversible. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

A polymeric photobase generator containing oxime–urethane groups: Crosslinking reaction and application to negative photoresist

A polymeric photobase generator containing oxime–urethane groups was prepared by copolymerization of methyl methacrylate and methacryloxyethyl benzophenoneoxime urethane, and its photo and thermal crosslinking reaction after irradiation was examined from the measurement of UV and IR absorption spectral changes, insoluble fraction, and molecular weight changes. The photo‐crosslinking reaction of the copolymer film was more efficient when irradiations were carried out with 310 nm UV light in the presence of benzophenone than with 254 nm UV light without the addition of benzophenone. The crosslinking reaction increased after postexposure baking (PEB), and this thermal crosslinking reaction mechanism was studied from the identification of the photolysis products of a model compound, benzophenoneoxime phenylurethane, by a high‐performance liquid chromatography. The results indicate that the thermal crosslinking reaction of the copolymer after PEB is due to the formation of urea‐type chemical bonds. Resist properties of the copolymer were examined from the measurement of normalized thickness and micropattern development. A negative tone image with a resolution of 2 μm was obtained with this copolymer, having a sensitivity (D) of 1200 mJ/cm² and contrast (γ_n) of 1.31, when irradiation was carried out with 310 nm UV light in the presence of benzophenone following chloroform development. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 975–984, 2004     

Crosslinking of styrene–butadiene rubber with 1,2-bis(benzocyclobutenyl)ethane

1,2-Bis(benzocyclobutenyl)ethane (BBCB) was used as a thermally activated crosslinking agent in styrene–butadiene rubber (SBR), both in the raw and carbon black-filled materials. Diels–Alder crosslinking reactions occurred to a significant degree, despite the lack of substituents of opposing electronic effects. The reaction is free of toxic crosslinking promoters and evolved by-products. Various physical properties such as ultimate elongation, tensile strength, and modulus were measured and compared to those properties of a sulfur-cured sample of SBR. Linear relationships were found between the amount of BBCB present and the modulus of the materials, as well as the amount of BBCB and solvent uptake in swell tests. © 1993 John Wiley & Sons, Inc.     

Sulfonated polybenzimidazoles: Proton conduction and acid–base crosslinking

A series of soluble, benzimidazole‐based polymers containing sulfonic acid groups (SuPBI) has been synthesized. SuPBI membranes resist extensive swelling in water but are poor proton conductors. When blended with high ion exchange capacity (IEC) sulfonated poly(ether ether ketone) (SPEEK), a polymer that has high proton conductivity but poor mechanical integrity, ionic crosslinks form reducing the extent of swelling. The effect of sulfonation of PBI on crosslinking in these blends was gauged through comparison with nonsulfonated analogs. Sulfonic acid groups present in SuPBI compensate for acid groups involved in crosslinking, thereby increasing IEC and proton conductivity of the membrane. When water uptake and proton conductivity were compared to the IEC of blends containing either sulfonated or nonsulfonated PBI, no noticeable distinction between PBI types could be made. Comparisons were also made between these blends and pure SPEEK membranes of similar IEC. Blend membranes exhibit slightly lower maximum proton conductivity than pure SPEEK membranes (60 vs. 75 mS cm^?1) but had significantly enhanced dimensional stability upon immersion in water, especially at elevated temperature (80 °C). Elevated temperature measurements in humid environments show increased proton conductivity of the SuPBI membranes when compared with SPEEK‐only membranes of similar IEC (c.f. 55 for the blend vs. 42 mS cm^?1 for SPEEK at 80 °C, 90% relative humidity). © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3640–3650, 2010     

The effect of photocrosslinkable groups on thermal stability of bulk heterojunction solar cells based on donor–acceptor‐conjugated polymers

Three different types of photocrosslinkable groups into a low band‐gap donor–acceptor‐conjugated polymer, namely poly{benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐ thieno[3,4‐b]thiophene} (PBT), were developed to comparatively investigate the effect of the photocrosslinkable groups on the thermal stability of bulk heterojunction solar cells. Compared with vinyl groups, bromine‐ and azide‐ photocrosslinkable groups are more prompt for photocrosslinking to yield a denser crosslinking network, probably due to the different crosslinking mechanisms and reaction rates. In contrast to the reference device decreasing to less than 10% of its initial efficiency value after 80 h of annealing at 150 °C, a great improvement in the thermal stability of performance of all these crosslinked functional copolymers devices demonstrates that photocrosslinking can effectively improve the thermal stability of the active layer by suppressing [6,6]‐phenyl‐C₆₁‐butyric acid methyl diffusion and phase separation. Furthermore, the solar cells with crosslinked bromine‐ and azide‐functionalized PBT polymers showed very thermally stable photovoltaic device performance by retaining 78 and 66% of their initial device efficiency, respectively, whereas vinyl‐functionalized PBT devices retained only 51% of its initial value after long‐time thermal annealing. This suggests that an appropriate crosslinking network with homogenous active morphology could dramatically enhance the device stability without sacrificing the performance. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4156–4166     

Characterization of ring‐opening polymerization of genipin and pH‐dependent cross‐linking reactions between chitosan and genipin

In this study, a novel chitosan‐based polymeric network was synthesized by crosslinking with a naturally occurring crosslinking agent—genipin. The results showed that the crosslinking reactions were pH‐dependent. Under basic conditions, genipin underwent a ring‐opening polymerization prior to crosslinking with chitosan. The crosslink bridges consisted of polymerized genipin macromers or oligomers (7 ～ 88 monomer units). This ring‐opening polymerization of genipin was initiated by extracting proton from the hydroxyl groups at C‐1 of deoxyloganin aglycone, followed by opening the dihydropyran ring to conduct an aldol condensation. At neutral and acidic conditions, genipin reacted with primary amino groups on chitosan to form heterocyclic amines. The heterocyclic amines were further associated to form crosslinked networks with short chains of dimmer, trimer, and tetramer bridges. An accompanied reaction of nucleophilic substitution of the ester group on genipin by the primary amine group on chitosan would occur in the presence of an acid catalysis. The extent in which chitosan gels crosslinked with genipin was significantly dependent on the crosslinking pH values: 39.9 ± 3.8% at pH 5.0, 96.0 ± 1.9% at pH 7.4, 45.4 ± 1.8% at pH 9.0, and 1.4 ± 1.0% at pH 13.6 (n = 5, p < 0.05). Owing to the different crosslinking extents and different chain lengths of crosslink bridges, the genipin‐crosslinked chitosan gels showed significant difference in their swelling capability and their resistance against enzymatic hydrolysis, depending on the pH conditions for crosslinking. These results indicated a direct relationship between the mode of crosslinking reaction, and the swelling and enzymatic hydrolysis properties of the genipin‐crosslinked chitosan gels. The ring‐opening polymerization of genipin and the pH‐dependent crosslinking reactions may provide a novel way for the preparation and exploitation of chitosan‐based gels for biomedical applications. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1985–2000, 2005     

Radiation polymerization of alkyl methacrylates in polymer–monomer compositions

This work summarizes the results of a study of the mechanism of polymerization in monomer-polymer compositions initiated by ionizing radiation. Attention was mainly paid to the investigation of the polymerization directly in the γ-radiation field. The next monomers were chosen for monomer-polymer compositions: heptyl acrylate (HA), heptyl methacrylate (HMA) and some other acrylic and methacrylic monomers. The polymer components are: chlorinated polyethylene (CPE) (molecular mass M_n ≅ 1 × 10⁵), chlorinated paraffins (CP) (molecular mass M_r ≅ 450), butadiene-nitrile rubber BN-1 (molecular mass M_w ≅ 3 × 10⁵) and BN-2 (molecular mass M_w ≅ 2500). Compositions that are crosslinked (BN-1+HMA, BN-2+HMA, CPE+HA) and noncrosslinked under γ-radiation were specially selected. The dynamic of radiation polymerization in the system studied was measured by calorimetric technique. A calorimeter was situated directly in the zone of γ-ray₆₀ Co. The dynamic of polymerization was followed by the heat release rate recorded by the calorimeter. Viscosity of the systems was measured on a rotational viscosimeter REOTEST-2 at room temperature. The investigations carried out have allowed us to establish the kinetic regularities of the radiation polymerization processes in the crosslinking and noncrosslinking compositions under irradiation based on HMA and some polymers. A comparison of the main characteristic of these processes was also made. The employment of the crosslinking and noncrosslinking polymers in the compositions significantly changes the radiation polymerization dynamics and allows the construction of the composites at the molecular level to be changed. The results obtained may serve as a basis for choosing the optimal system for producing-radiation-cured monomer-polymer compositions.     

Double‐crosslinked reversible redox‐responsive hydrogels based on disulfide–thiol interchange

We present novel redox‐responsive hydrogels based on poly(N‐isopropylacrylamide) or poly(acrylamide), consisting of a reversible disulfide crosslinking agent N,N′‐bis(acryloyl)cystamine and a permanent crosslinking agent N,N′‐methylenebisacrylamide for microfluidic applications. The mechanism of swelling/deswelling behavior starts with the cleavage and reformation of disulfide bonds, leading to a change of crosslinking density and crosslinking points. Raman and ultraviolet‐visible spectroscopy confirm that conversion efficiency of thiol–disulfide interchange up to 99%. Rheological analysis reveals that the E modulus of hydrogel is dependent on the crosslinking density and can be repeatedly manipulated between high‐ and low‐stiffness states over at least 5 cycles without significant decrease. Kinetic studies showed that the mechanical strength of the gels changes as the redox reaction proceeds. This process is much faster than the autonomous diffusion in the hydrogel. Moreover, cooperative diffusion coefficient (D_coop) indicates that the swelling process of the hydrogel is affected by the reduction reaction. Finally, this reversibly switchable redox behavior of bulky hydrogel could be proven in microstructured hydrogel dots through short‐term photopatterning process. These hydrogel dots on glass substrates also showed the desired short response time on cyclic swelling and shrinking processes known from downsized hydrogel shapes. Such stimuli‐responsive hydrogels with redox‐sensitive crosslinkers open a new pathway in exchanging analytes for sensing and separating in microfluidics applications. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2590–2601     

Isobutylene‐rich macromonomers: Dynamics and yields of peroxide‐initiated crosslinking

New isobutylene‐rich elastomers bearing multiple pendant styrenic, acrylic, maleimidic, vinylic, and allylic functional groups have been prepared and examined in the context of peroxide‐initiated crosslinking. Halide displacement from brominated poly(isobutylene‐co‐isoprene) (BIIR) by the requisite carboxylate nucleophiles in homogeneous toluene solutions provide the desired esters in quantitative yield without complications from dehydrohalogenation or premature crosslinking. Heating the resulting macromonomers with dicumyl peroxide to 160 °C under solvent‐free conditions gives thermoset derivatives, with reaction rates and yields depending markedly on functional group structure. In general, high cure extents can only be achieved using highly reactive pendant functional groups, owing to the competitive balance between crosslinking through C?C oligomerization, and degradation through β‐scission of backbone macroradical intermediates. Independent control of crosslinking rates and cure extents is gained through the use of nitroxyl radical traps bearing acrylate functionality. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 123–132     

Molecularly templated reaction for forming poly(dimethyl siloxane)–graphene oxide composite elastomers

This study explores the molecularly templated reaction of pyrene‐terminated telechelic poly(dimethyl siloxane) (PDMS) with graphene oxide (GO) to produce composite elastomers. These materials undergo chemical crosslinking between secondary amides near PDMS chain ends and epoxies on the surface of GO as confirmed by infrared spectroscopy, rheology, gel content, and mechanical property measurements. The incorporation of pyrene end groups introduces π–π interactions with GO surfaces that enhance the reaction efficacy of the nearby secondary amide groups. As a comparison, methoxy‐terminated telechelic PDMS containing the same secondary amides near the chain ends did not exhibit appreciable crosslinking with GO. Depending on the concentration of the amide groups, the pyrene‐terminated PDMS/GO elastomer can be highly crosslinked (e.g., up to 96 wt % gel) but highly extensible (e.g., extensional strains of more than 200%). This general strategy could be implemented using other amide containing polymers to produce a wide range of high‐performance thermosets and elastomers. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1406–1413     

Amphiphilic sorbents based on polysiloxanes crosslinked by an N,N′‐heterocycle

New amphiphilic sorbents have been synthesized by crosslinking of the chloromethyl side‐functionalized polysiloxanes with piperazine. Three structures, differing by the crosslinking degree, have been synthesized as potential sensitive materials for humidity sensors able to work at high humidity or as metal sorbents. Correlations between structure and morphology of the polymeric matrix, as well as their swelling capacity in solvents with different polarities, are discussed. Copyright © 2006 John Wiley & Sons, Ltd.     

Maleation of poly(3,4‐epoxy‐1‐butene) for accelerated crosslinking in the presence of a redox catalyst

Accelerated crosslinking of novel poly(3,4‐epoxy‐1‐butene) (3,4‐PEPB) oligomers in the presence of a cobalt‐based redox catalyst was investigated. Previous studies using model compounds, 3,4‐dimethoxy‐1‐butene and 1,4‐dimethoxy‐2‐butene, suggested that maleation of hydroxyl‐terminated 3,4‐PEPB oligomers would result in more rapid crosslinking in thin films. Novel maleated oligomers offered a unique combination of both electron‐rich and electron‐poor olefinic sites, and quantitative maleation significantly increased the crosslinking rate of 3,4‐PEPB. Efficient copolymerization between terminal maleate groups and olefinic groups in the repeating unit was proposed to account for accelerated crosslinking rates. Furthermore, the addition of novel reactive diluents, such as maleic acid mono‐ethyl ester, also effectively improved the 3,4‐PEPB crosslinking rate. Sol fraction measurements as a function of coating thickness revealed that the crosslinking rate versus oxygen diffusion was less significant for the maleated oligomers because of the presence of reactive electron‐poor olefins. Sol fractions were constant for catalyst concentrations greater than 0.25–0.50 wt % (as compared with oligomer feed). This observation suggested that a redox process was not a dominant factor in determining crosslinking rates at various experimental conditions. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2789–2798, 2002     

A tailor‐made polymethacrylate bearing a reactive diene in reversible diels–alder reaction

A tailor‐made polymethacrylate bearing a pendant furfuryl group was prepared by atom transfer radical polymerization (ATRP), an important method of recent advances in controlled radical polymerization. It was otherwise difficult to prepare via conventional radical polymerization, because of several side reactions involving the reactive diene functionality of the furfuryl group. Successful Diels–Alder (DA) chemistry was carried out using this reactive furfuryl group of the tailor‐made polymer as diene and a bismaleimide as a dienophile. Interestingly, the resultant material was observed to be thermoreversible as evidenced by FT‐IR and DSC studies. This example of application of a tailor‐made polymer having controlled molecular architecture and with reactive diene functionality in DA chemistry will open new possibilities to prepare newer tailor‐made reversible materials. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4441–4449, 2007     

Thermal Breakdown by the Retro Diels–Alder Reaction of Crosslinking in Poly[styrene‐co‐(furfuryl methacrylate)]

Crosslinked poly[styrene‐co‐(furfuryl methacrylate)] has been produced by the Diels–Alder (D‐A) reaction between the furan ring (within the copolymer) and the maleimide (MI) group (within the other reactant, bismaleimide). The retro D‐A reaction was followed by the analysis of MI groups produced at different times at five constant temperatures. The process was shown to follow first‐order kinetics, and the rate constants were determined. The findings are believed to be the first to provide quantitative information on the breakdown by a retro D‐A reaction of crosslinking in a polymer system. The D‐A and retro D‐A processes constitute a thermoreversible gelling system with respect to the formation and breakdown of crosslinks.

First‐order rate plot for the retro D‐A breakdown of crosslinked poly(ST‐co‐FM).     

Crosslinking effects on hybrid organic–inorganic proton conducting membranes based on sulfonated polystyrene and polysiloxane

New hybrid semi‐interpenetrating proton‐conducting membranes were obtained using sulfonated polystyrene (SPS) and inorganic–organic polysiloxane phases with the aim of improving the mechanical and thermal characteristics of the pristine polymer and to study the effects of crosslinking in the latter phase in several of their properties, mainly proton conductivity. Siloxane phases were prepared using poly(dimethylsiloxane) (PDMS) and PDMS with tetraethoxysilane (TEOS) or phenyltrimethoxysilane (PTMS) as crosslinking agents. To study the crosslinking effect, membranes were prepared with different TEOS:PDMS and PTMS:PDMS mole ratios. The films obtained were characterized by FTIR, ²⁹Si‐HPDEC MAS‐NMR, ¹³C‐CP‐MAS NMR, elemental and thermal analyses. Certain properties, such as water uptake (WU), ion exchange capacity (IEC) and the state of the water, were determined. The proton conductivity was measured at different temperatures (30°C and 80°C) and relative humidities (50–95%). The water content of the hybrid membranes declined significantly, compared with the SPS membranes, depending on the nature and amount of siloxane phase added. Nonetheless, the conductivity values remained relatively high (>100 mS cm^?1 at 80°C and 95% RH) when compared to Nafion®117 presumably because of the formation of well developed proton channels, which makes them potentially promising as proton exchange membranes for fuel cells. These membranes proved to be thermally stable up to 350°C. Scanning electron microscopy (SEM) and scanning electrochemical microscopy (SECM) were used to characterize the hybrid membranes microstructures; the latter provided contrast for the conductive domains. Copyright © 2015 John Wiley & Sons, Ltd.     

High Solid Acrylic–Melamine Latexes with Tunable Crosslinking Capability

Miniemulsion polymerization is employed to produce high solid content (50%) acrylic/melamine latexes with varied crosslinking capability, for their potential application as waterborne crosslinkable coatings. This synthesis strategy allows the efficient incorporation of a hydrophobic crosslinker, iso‐butylated melamine (iBMF), into polymer particles, and to obtain latexes with different iBMF concentrations and acrylic polymer with varied content of hydroxylic groups (OH–). The involved crosslinking mechanisms and the physical transformations during film thermosetting of acrylic/iBMF nanocomposite are exhaustively investigated by combining thermal, spectroscopic, and thermo‐mechanical analyses. The influence of reactive groups concentration (iBMF and OH– content in the acrylic polymer) on the rate of curing, the crosslinking degree, and consequently onto the sensitive properties of cured films are discussed here.     

Crosslinking of polyimides via spirodilactone unit in polymer backbone

A new approach for the crosslinking of polyimides via the lactamization of spirodilactone unit in polyimide backbone was studied by two means: model reaction and the comparison of the properties of the polyimide precursors to those of the crosslinking polymers. Polyimides 4 and 5 were soluble in N,N′dimethylacetamide (DMAc), dimethylformamide (DMF), dimethylsulfoxide (DMSO), N′-methylpyrrolidone (NMP), and other common organic solvents, whereas their corresponding crosslinking polymers were insoluble in these solvents. The glass transition temperatures for polyimide 5 and its crosslinking polymer were 262°C and 291°C, whereas those for polyimide 4 and its crosslinking polymer were 265°C and 360°C. The weight-loss rate of the crosslinking polymers was apparently slower than that of the precursors when the temperature was > 400°C. The 10% weight-loss temperature for the polyimides 4 and 5 was < 500°C, whereas that for the crosslinking polymers was close to or above 600°C. The results indicate that this type of crosslinking polymer has good thermal properties. The temperature for the formation of lactam was above 180°C. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3680–3686, 1999