Dual crosslinking of carboxylated nitrile butadiene rubber latex employing the thiol‐ene photoreaction

At present, the most common used crosslinking process for carboxylated nitrile butadiene rubber (XNBR) latex is an accelerated sulfur curing system with zinc oxide. To avoid allergenic reactions related to residual accelerator levels in dipped XNBR latex articles such as medical gloves, a dual curing process has been developed combining thermal and photochemical crosslinking reactions. The two‐step procedure involves the formation of covalent and ionic bonds to ensure good mechanical properties of the final products. The photochemical thiol‐ene reaction is used to generate covalent crosslinks between the remaining C?C double bonds of the butadiene units whereas the carboxylic moieties are conventionally cured with divalent metal oxides (ZnO) under elevated temperature (formation of ionic crosslinks). The photochemical curing step is carried out both in the latex phase using a falling film photoreactor (prevulcanization) as well as in the solid phase by UV irradiation of dried XNBR films (postvulcanization). The mechanical properties and crosslink densities of the cured XNBR films are determined and the influence of selected curing parameters is assessed. The results give evidence that a combined approach of thermal prevulcanization and photochemical postvulcanization makes the production of latex articles (e.g., gloves) with tailored properties and good skin compatibility feasible. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Application of thiol‐ene photopolymerization for injectable intraocular lenses: A preliminary study

The thiol‐ene photopolymerization, which could be achieved with less ultraviolet (UV) intensity and photoinitiator content was first introduced into the application of injectable intraocular lenses (IOLs), thus the damages to the eye from the UV radiation and photoinitiator toxicity were greatly reduced. The bulk samples were obtained from the mixture of poly(ethylene glycol) diacrylate (PEGDA) and pentaerythritol tetrakis(3‐mercaptopropionate) (PTMP) by the thiol‐ene photopolymerization. It was found that PEGDA‐PTMP bulk samples had the similar transparency and the hydrophilicity as the ones from PEGDA by homophotopolymerization. Meanwhile, when PTMP was added into PEGDA, the elastic modulus of the bulk samples was decreased and their flexibility was improved. The experimental results indicated that the thiol‐ene photopolymerization was one potential method to obtain the ideal injectable IOLs, although many works have to be done in the future study, such as selecting proper monomers and testing the biocompatibility of polymerized samples. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Photocrosslinking of functionalized rubbers. X. Butadiene–acrylonitrile copolymers

The photocrosslinking of polyacrylonitrile‐block‐polybutadiene‐block‐polyacrylonitrile (ABA) was shown to proceed within seconds at ambient temperature upon UV exposure in the presence of an acylphosphine oxide photoinitiator. The curing process was followed by infrared spectroscopy, insolubilization, and hardness measurements. Complete insolubilization could not be achieved with the neat ABA rubber because of the poor reactivity of the 2‐butene double bond and the low vinyl content of the polybutadiene chain. The addition of multifunctional acrylate monomers (20 wt %) causes a substantial increase of both the reaction rate and the crosslink density of the polymer, which becomes completely insoluble in toluene in less than 1 s upon UV irradiation. An even greater effect was observed by using small amounts (1 wt %) of a trifunctional thiol crosslinker. Both the thiol and the photoinitiator concentrations were shown to affect the kinetics of the thiol–ene polymerization and the polymer network crosslink density. A direct relationship was found to exist between the swelling degree of the UV‐cured rubber and the interchain molecular weight of the network. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2204–2216, 2001     

Thiol‐ene photofunctionalization of 1,4‐polymyrcene

1,4‐Polymyrcene was synthesized by anionic polymerization of β‐myrcene and was subjected to photochemical functionalization with various thiols (i.e. methyl thioglycolate, methyl 3‐mercaptopropionate, butyl 3‐mercaptopropionate, ethyl 2‐mercaptopropionate and 2‐methyl‐2‐propanethiol) using benzophenone/UV light as the radical source. The yield of thiol addition to the trisubstituted double bonds of 1,4‐polymyrcene decreased in the order 1° thiol (ca 95%) > 2° thiol (ca 80%) > 3° thiol (<5%), due to the reversibility of the thiol‐ene reaction. Remarkably, thiol addition to the side‐chain double bonds was 8 ? 10 times (1° thiol) or 24 times (2° thiol) faster than to the main‐chain double bonds, which can be explained by the different accessibility of the double bonds and steric hindrance. Despite the use of a 10‐fold excess of thiol with respect to myrcene units, the thiol‐ene addition was accompanied by chain coupling reactions, which in the extreme case of 3° thiol (or in the absence of thiol) resulted in the formation of insoluble crosslinked material. As an example, a methyl‐thioglycolate‐functionalized 1,4‐polymyrcene was saponified/crosslinked to give submicron polyelectrolyte particles in dilute alkaline solution. © 2018 Society of Chemical Industry     

Optimizing natural rubber blends for examination gloves: Vacuum radiation for mechanical strength and solvent barrier

Natural rubber (NR) is emblematic of sustainability compared to synthetic rubber. However, the tradition of adding sulfur as a vulcanization ingredient results in the release of toxic substances and the potential for health issues. In this study, a feasible strategy was proposed to replace sulfur and discover a safe bulk modification process for NR films. The results have shown that the NR particle size was disintegrated to below 10 nm by gamma irradiation. High tension strength up to 24.45 MPa was observed in the vulcanized NR blend film, which could be elongated up to 800% strain after exposure to an optimum dose of 14 kGy. In comparison to commercial NR latex and nitrile gloves, the vulcanized NR/ SIS films exhibited better chemical resistance ability against hexane, methanol, toluene, and acetone, as revealed by the permeation test. The appearance of amorphous regions and highly oriented NR crystallites was observed through transmission electron microscopy. Findings from this study propose the vacuum radiation strategy that can replace conventional vulcanization methods, resulting in NR films with high mechanical and barrier performance. Furthermore, the emission of toxic substances is reduced by this green process, making it practically useful for potential chemical-resistant examination glove applications.     

3D‐Printing of High‐κ Thiol‐Ene Resins with Spiro‐Orthoesters as Anti‐Shrinkage Additive

Tri(ethylene glycol) divinyl ether and the spiro‐orthoester 2‐((allyloxy)methy)‐1,4,6‐trioxospiro[4.4]nonane can be formulated in different ratios and crosslinked by thiol‐ene reactions. The spiro‐orthoester is used as anti‐shrinkage additive, enabling shrinkage reduction of up to 39%. Addition of a radical photoinitiator for the thiol‐ene reaction and a cationic photoinitiator for the double ring‐opening of the spiro‐orthoester enables dual‐curing for application in 3D‐printing. The formulation free of the spiro‐orthoester shows gelation during the printing process and, correspondingly, low resolution. The formulations containing the spiro‐orthoester exhibit higher resolutions in the range of 50 µm. The resins containing mixtures of tri(ethylene glycol) divinyl ether and the spiro‐orthoester show permittivities as high as 10⁴. The dielectric loss factor of the resins is in the range of 0.5–7.6, and the conductivity in the range of 1.3?10^?11 to 2.0?10^?11 S cm^?1. These high‐κ materials can be 3D‐printed by digital light processing for the next generation of electronic materials.     

High‐speed photocrosslinking of thermoplastic styrene–butadiene elastomers

Photoinitiated thiol/ene polymerization was used to crosslink a triblock styrene/butadiene/styrene (SBS) polymer of low vinyl content (8%). The crosslinking process was followed by infrared spectroscopy (loss of unsaturation), insolubilization, swelling, and hardness measurements. The photogenerated thiyl radicals react with both the vinyl and the 2‐butene double bonds of the copolymer. Concentrations of less than 1 wt % in the trifunctional thiol crosslinker and in the acylphosphine oxide photoinitiator proved to be sufficient to create, within 0.5 s, a permanent chemical network in the elastomeric phase. This UV‐curing technology was successfully applied to crosslink rapidly commercial SBS–Kraton® thermoplastic elastomers. It proved also effective in the case of the much less reactive triblock styrene/isoprene/styrene (SIS) polymer which contains no vinyl double bonds. The thiol/ene polymerization was shown to be a much more efficient process to crosslink SBS and SIS thermoplastic elastomers than was the copolymerization of the rubber double bonds with a diacrylate monomer. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1902–1912, 2000     

Preparation and characterization of hybrid thiol‐ene/epoxy UV–thermal dual‐cured systems

Hybrid thiol‐ene/epoxy coatings were prepared by combining thiol‐ene photo‐curable formulations with epoxy monomers, through a dual UV–thermal curing process. An increase in glass transition temperature and in storage modulus was observed for the hybrid thiol‐ene/epoxy coatings when compared with the pristine thiol‐ene UV‐cured system. Also, the bisphenol A moieties introduced into the hybrid networks during the dual‐curing process induced an increase in thermal stability of the cured materials. It has been demonstrated that the addition of epoxy monomer to the thiol‐ene photo‐curable system is a good strategy to follow in order to improve the final properties of thiol‐ene‐based coatings leading to a wide range of possible applications for the hybrid materials. Copyright © 2010 Society of Chemical Industry     

The influence of surface structure of low industrial grade seaweed and semi‐refined carrageenan on mechanical and physical properties of natural rubber latex composites

Composites consisting of natural rubber (NR) latex as their matrix and low industrial grade seaweed (LIGS) and its extractive (semirefined carrageenan, SRC‐LIGS) as the filler were prepared via normal prevulcanization process. An analysis regarding the particle size and morphology of seaweed (LIGS and SRC‐LIGS), as well as surface properties and mechanical properties (tensile and tear properties) of NR latex composites, was consequently generated. Furthermore, post‐processing treatment for NR latex composites have also been studied, specifically involving leaching, heat aging, water absorption, and soil burial. The particle size of LIGS and SRC‐LIGS obtained was recorded to be lower than 100 μm. Thus, the results are indicative of SRC‐LIGS's role in improving the thermal properties of NR latex composites. After 8 weeks of soil burial, the incorporation of LIGS and SRC‐LIGS into the NR latex composites has accelerated biodegradation processes, thus highlighting their advantage as biodegradable fillers. These properties have consequently contributed to SRC‐LIGS/NR latex composites as a potential composite for use in biodegradable applications, such as polybag for pottery and plants. J. VINYL ADDIT. TECHNOL., 25:278–286, 2019. © 2018 Society of Plastics Engineers     

Synthesis and properties of ultraviolet‐curable resins via a thio–ene (thiol and allyl) addition reaction

Benzophenone diallyl ester (I) and benzophenone tetraallyl ester (II) based on 3,3′,4,4′‐benzophenone tetracarboxylic dianhydride (BTDA) with allyl alcohol (AAL) were synthesized. Glycidyl methacrylate (GMA) was added to I and formed diallyl diglycidyl methacrylate (III). These BTDA‐based allyl‐containing compounds (II and III) reacted with 1,4‐butanedithiol and 4,4′‐thiol‐bisbenzene‐thiol to produce ultraviolet (UV)‐curable resins via a thio–ene addition reaction. The ester (III) was cured easily when exposed to UV or sunlight radiation without any photoinitiator and only required a lower thermal curing temperature. The diallyl ester (I) and tetraallyl ester (II) required the addition of benzophenone to increase the photosensitivity, which reduced the exposition time. These resins used AAL as a monomer to successfully reduce the oxygen effect of the photocuring. The resin BTDA–2Allyl–2GMA had a glass‐transition temperature of 166°C and a hardness of 6H. The resultant UV‐curable coatings had excellent hardness, chemical resistance, adhesion, and tensile properties. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1878–1885, 2002     

Optical anisotropy of chalcone-based epoxy compound under irradiation of linearly polarized UV light

Photocrosslinkable chalcone-epoxy compound comprising 1,3-bis-(4-hydroxy-phenyl)-propenone was synthesized for studying the optically induced anisotropy of the thin films. Chalcone group was introduced into the main chain unit of the epoxy oligomer. We observed a photodimerization behavior and an optical anisotropy of this material by irradiation of linearly polarized UV light (LPL). With a trace amount of cationic photoinitiator, polymerization of epoxy groups was also conducted at the similar wavelength range used for photodimerization. Optical anisotropy of the thin film was also investigated by using polarized FT-IR and UV absorption spectroscopy either with or without cationic photoinitiator. Two kinds of photochemical reaction were employed to enhance the anisotropy using the chalcone-epoxy compound.     

Photocuring of a thiol‐ene system based on an unsaturated polyester

To produce a photocurable thiol‐ene system, unsaturated polyester was prepared from the condensation reaction of ethylene glycol, diethylene glycol, and fumaric acid. Diallyl groups were introduced into the ends of the unsaturated polyester by a sequential condensation reaction. The coating formulation studied contained an equimolar ratio of thiol and vinyl groups of the prepared unsaturated polyester, including 1 wt % Irgacure 184. The curing behaviors of the unsaturated polyester with multifunctional thiols were investigated using real‐time FTIR spectroscopy. The rates of disappearance of thiol and vinyl groups of the unsaturated polyester were similar, demonstrating that there was little free‐radical homopolymerization of the internal fumaric group or the end‐capped vinyl ether group during the photocuring process and that the thiol‐ene reaction is the dominant process. The kinetics of the model compounds demonstrated that the reaction of the terminal allyl double bond with the thiyl radical is faster than that of the internal fumaric double bond in the UV curing of the unsaturated polyester. The storage stability of the thiol‐ene system based on unsaturated polyester was effectively increased by the addition of N‐PAL. The Raman spectra revealed that the presence of a multifunctional thiol (penta 3‐MP4) in the coating formulation increased the degree of surface curing due to the chain‐transfer ability of the thiyl radical. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 342–350, 2005     

Effect of prevulcanization on the rheological behavior of natural rubber/styrene butadiene rubber latex blends

The effect of prevulcanization on the rheological behavior of natural rubber (NR), styrene butadiene rubber (SBR) latices, and their blends was studied with special reference to shear rate, blend ratio, vulcanizing systems, prevulcanization time, and accelerator systems. The NR latex showed a sharp increase in viscosity with increase in prevulcanization time due to high extent of crosslinking. However, SBR latex showed marginal effect on viscosity with prevulcanization time due to its low dry rubber content and low degree of unsaturation. Blends showed variations in viscosity according to the change in composition. The use of a single accelerator was found to have marked influence on the viscosity of the blends compared with a combination of accelerators. Swelling experiments were carried out in order to determine the crosslink density of the blends. The viscosity changes have been correlated with the crosslinking density of the latices and their blends. © 1996 John Wiley & Sons, Inc.     

Fabrication of polymer‐dispersed liquid crystals with low driving voltage based on the thiol‐ene click reaction

Polymer‐dispersed liquid crystals (PDLCs ) with a well‐defined polymer matrix were successfully fabricated by the thiol‐ene click reaction based on poly(ethylene glycol) diacrylate (PEGDA ) and trimethylolpropanetris‐(3‐mercaptopropionate) (TMTP ). UV ?visible spectrophotometry, Fourier transform IR spectroscopy, SEM and polarized optical microscopy were employed to explore the PDLC films obtained. Electro‐optical properties were studied with a UV ?visible spectrophotometer. It was found that the PDLC films with optimal thiol content fabricated by the thiol‐ene click reaction showed high transmittance, low driving voltage and a low memory effect. It was concluded that the driving voltage change of PDLCs with different thiol concentrations was caused by the polymerization rate and the structure of the polymer matrix. © 2017 Society of Chemical Industry     

Natural rubber–styrene butadiene rubber latex blends: Time-dependent rheological behavior and film formation

This article focuses mainly on the effect of maturation time on the rheological behavior of unvulcanized natural rubber (NR)–styrene butadiene rubber (SBR) latex blends. Viscosity shows a composition-dependent behavior with maturation time. It was found that there is a marginal decrease in viscosity for all the systems with maturation time except for the 70/30 NR–SBR blend. In this blend, there is a sharp decrease in viscosity with maturation time. This is associated with the exchange of stabilizers with one another until an equilibrium is reached; that is, all the particles of the blend are stabilized with random mixture of stabilizers. The structural build up observed in 70/30 NR–SBR blend was found to be diminished as the maturation time increases. At equilibrium, there is no further exchange of stabilizers. The behavior of this blend has been explained with the help of a schematic model. The effects of blend ratio and surface active agents on the viscosity were also studied. In addition, the time-dependent flow behavior of prevulcanized latex blends was evaluated as a function of vulcanizing systems and prevulcanization time. There is a regular increase in viscosity with prevulcanization time. However, after 3 h, the viscosity of almost all blends levels off, indicating that the curing reaction is complete within this time. Finally, the morphological changes occurred during film formation of the blends were studied using scanning electron microscopy. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1473–1483, 1998     

Dual‐Cure Coatings: Spiroorthoesters as Volume‐Controlling Additives in Thiol–Ene Reactions

Most thiol–ene systems exhibit shrinkage during cross‐linking, potentially resulting in micro‐cracks and delamination. Oligocyclic monomers like spiroorthoesters (SOEs), on the contrary, show expansion during the ring‐opening polymerization. In this communication, a photocurable thiol–ene system composed of a trifunctional thiol, a bisfunctional allyl‐bisphenol A compound, and an SOE compound bearing one olefin function shows expansion in the range from ?3.07 to +1.70 vol% if the SOE content is increased from 0–30 wt%. Network formation can be accomplished under visible light if a radical as well as a cationic photoinitiator (dual‐cure mechanism) and a sensitizer are used. The elasticity of the cured resin increases upon the addition of the SOE; correspondingly, the glass‐transition temperature shows a (minor) decrease from 16 to 3 °C. A tailor‐made combination of the allyl‐bisphenol A compound (90 wt%) and the SOE (10 wt%) yields networks that are volume‐neutral during curing.     

Thiol–norbornene photoclick hydrogels for tissue engineering applications

Thiol–norbornene (thiol–ene) photoclick hydrogels have emerged as a diverse material system for tissue engineering applications. These hydrogels are crosslinked through light‐mediated orthogonal reactions between multifunctional norbornene‐modified macromers [e.g., poly(ethylene glycol) (PEG), hyaluronic acid, gelatin] and sulfhydryl‐containing linkers (e.g., dithiothreitol, PEG–dithiol, biscysteine peptides) with a low concentration of photoinitiator. The gelation of thiol–norbornene hydrogels can be initiated by long‐wave UV light or visible light without an additional coinitiator or comonomer. The crosslinking and degradation behaviors of thiol–norbornene hydrogels are controlled through material selections, whereas the biophysical and biochemical properties of the gels are easily and independently tuned because of the orthogonal reactivity between norbornene and the thiol moieties. Uniquely, the crosslinking of step‐growth thiol–norbornene hydrogels is not oxygen‐inhibited; therefore, gelation is much faster and highly cytocompatible compared with chain‐growth polymerized hydrogels with similar gelation conditions. These hydrogels have been prepared as tunable substrates for two‐dimensional cell cultures as microgels and bulk gels for affinity‐based or protease‐sensitive drug delivery, and as scaffolds for three‐dimensional cell encapsulation. Reports from different laboratories have demonstrated the broad utility of thiol–norbornene hydrogels in tissue engineering and regenerative medicine applications, including valvular and vascular tissue engineering, liver and pancreas‐related tissue engineering, neural regeneration, musculoskeletal (bone and cartilage) tissue regeneration, stem cell culture and differentiation, and cancer cell biology. This article provides an up‐to‐date overview on thiol–norbornene hydrogel crosslinking and degradation mechanisms, tunable material properties, and the use of thiol–norbornene hydrogels in drug‐delivery and tissue engineering applications. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41563.     

Effect of temperature on sulfur prevulcanization of natural rubber latex

Sulfur prevulcanization of natural rubber latex was conducted at 60, 70, 80, and 90°C for different periods. The extent of crosslinking was assessed. Tensile properties, water absorption, leaching, and stress-relaxation characteristics of the films were also evaluated. The volume fraction of rubber (Vr), which is a measure of crosslink density of the films, showed a maximum when prevulcanization was conducted at 80°C for 2 h or at 90°C for 1 h. At lower temperatures, the rate of reaction was slow. At each temperature, tensile strength and elongation at break decreased when the prevulcanization time increased, whereas the modulus increased up to a maximum crosslinking and thereafter decreased. Water absorption and leaching were more rapid in prevulcanized film than in postvulcanized film. The rate of stress relaxation slightly increased as the extent of prevulcanization increased. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 1913–1920, 1997     

纳米氧化锌对天然胶乳膜干燥及硫化性能影响研究

采用失重法和溶胀法研究了普通氧化锌和纳米氧化锌活化的天然胶乳膜的干燥动力学,以及干燥过程中胶乳膜交联密度变化.结果表明:天然胶乳厚胶膜在成膜后仍含有大量水分并且去除困难.纳米氧化锌活化胶膜的干燥速率明显高于普通氧化锌胶膜.干燥温度对干燥过程影响显著,干燥时间随干燥温度升高明显降低.随着干燥的进行,2种氧化锌活化的胶乳膜的交联密度均迅速上升,在相同条件下,纳米氧化锌活化胶乳膜交联密度均比普通氧化锌的大.纳米氧化锌对天然胶乳膜的干燥及硫化过程均有促进作用.     

Surface Modification of Polycarbonate Urethane with Zwitterionic Polynorbornene via Thiol‐ene Click‐Reaction to Facilitate Cell Growth and Proliferation

Herein, we grafted the zwitterionic polynorbornene onto polycarbonate urethane (PCU) film surface by a convenient route of thiol‐ene click‐chemistry. The PCU film surface was first treated with hexamethylene‐1,6‐diisocynate and subsequently with two different thiol agents (l ‐cysteine and β‐marcaptoethanol) in the presence of di‐n‐butyltin dilaurate (DBTDL) to immobilize sulfhydryl groups onto the surface. Here, DBTDL acted as selective catalyst for the reaction between surface‐tethered isocyanates and amine/hydroxyl groups in thiol agents over that of free thiol groups. In the next step, zwitterionic polynorbornene (poly(NSulfoZI)) having functionalizable double bonds was grafted onto these surfaces by photo‐initiated thiol‐ene click‐reaction. The modified surfaces were characterized by water contact angle and XPS analysis. Moreover, the cytocompatibility of these surfaces was investigated by model endothelial cells, EA.hy926, for 1, 3, and 7 d culture times, which showed enhanced cell adhesion and growth. Therefore, the poly(NSulfoZI) functionalized PCU surface using l‐cysteine as thiol agent could be a good candidate for tissue engineering material application.