Influence of the initiating mechanism on the cationic photopolymerization of a cycloaliphatic epoxy resin with arylsulfonium salts

The photocuring process of widely used 3,4‐epoxycyclohexylmethyl 3′,4′‐epoxycyclohexane carboxylate has been investigated with differential scanning photocalorimetry and attenuated total reflection/Fourier transform infrared. Mixed salts of triarylsulfonium hexafluoroantimonate have been employed as the photoinitiator. The photocuring of the biscycloaliphatic resins exhibits a complex behavior: the overall heat of reaction (including dynamic thermal postcuring) depends on the photocuring temperature, surprisingly high reaction rates are observed at lower photocuring temperatures, and the range of the glass transition of the fully cured material broadens and shifts to higher temperatures as the photocuring temperature increases. It is assumed that the balance between the initiation step and the propagation step is responsible for the changes in the reaction mechanism that produce the observed experimental results. This balance may depend on the amount of the photoinitiator, the irradiation intensity, and the photocuring temperature. The structure and final properties of the material may therefore depend on the adjustment of these parameters. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 16–25, 2007     

Novel thermosets obtained by UV‐induced cationic copolymerization of DGEBA with an spirobislactone

The photocuring process of the diglycidyl ether of bisphenol A (DGEBA) with the bislactone 1,6‐dioxaspiro[4,4]nonane‐2,7‐dione (s(γ‐BL)) was studied. Triarylsulfonium hexafluoroantimonate was employed as photoinitiator. FTIR/ATR was used to study the evolution of epoxy, lactone, and intermediate spiroorthoester groups to identify the different reactions that take place during the photocuring process. Photo‐DSC and DSC were used to study the thermal evolution of the photocuring process and to assess the T_g of the fully cured material. Thermogravimetric analysis (TGA) was used to determine the thermal stability of the fully cured material. The thermomechanical properties of the materials were investigated using dynamic mechanical‐thermal analysis. Shrinkage undergone during photocuring and gelation was studied with TMA. A strong influence of the photocuring temperature on the photocuring process of the DGEBA‐ s(γ‐BL) system was observed. Differences in the reactivity of the different species were observed with respect to the thermally cured system using ytterbium triflate as cationic thermal initiator. As a consequence, photocured materials exhibited a superior thermal stability and lower flexibility. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5446–5458, 2007     

One‐ and two‐photon activity of cross‐conjugated photoinitiators with bathochromic shift

Intrigued by the good performance of 1,5‐diphenylpenta‐1,4‐diyn‐3‐one ( DPD ) as photoinitiator for radical polymerization we prepared and investigated several donor substituted derivatives. UV‐Vis spectroscopy revealed a gradual red‐shift of λ_max and higher extinction in the order of the donor capability. A methoxy‐substituted derivative ( O‐DPD ) exhibited significant photoinitiation activity in photo‐DSC experiments. Steady state photolysis experiments showed decreased decomposition rates with increasing donor capability. A dimethylamino derivative N‐DPD was even photostable under these conditions. Because of to the D‐π‐A‐π‐D system of these compounds two‐photon induced 3D photopolymerization experiments were performed and N‐DPD showed outstanding performance compared to often applied single photon initiators. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3280–3291, 2007     

Hybrid organic/inorganic epoxy resins prepared by frontal polymerization

The syntheses of hybrid epoxy resins made from different ratios among bisphenol‐A diglycidyl ether, 3‐glycidoxypropyltrimethoxysilane (GPTMS), and diethylenetriamine were successfully performed by using frontal polymerization. Conversions were always almost quantitative, and, because of the use of this alternative convenient technique, materials were prepared in very short times. Samples were characterized by DSC, TGA, IR spectroscopy, and solvent extraction. It was found that those materials containing a relatively high‐Si amount exhibit two different transition temperatures, with the highest one that increases as the content of GPTMS raises. The analogies and the differences with the analogous samples prepared by the classical batch technique are discussed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Curing kinetics of epoxy resins with hyperbranched polyesters as toughening agents

The effects of hyperbranched polyesters on the cure kinetics of diglycidyl ether of bisphenol A (DGEBA) in the presence of m‐phenylene diamine were investigated with nonisothermal differential scanning calorimetry. The results showed that the addition of hyperbranched polyesters enhanced the cure reaction of DGEBA with m‐phenylene diamine, and this resulted in a reduction of the peak temperature of the curing curve and the activation energy because of the low viscosity and large number of terminal hydroxyl groups. However, when linear poly(ethylene glycol) was added, the activation energy of the blends also slightly decreased, whereas the peak temperature of the curing curve increased. The curing kinetics of the blends were calculated by the isoconversional method of Málek. The two‐parameter autocatalytic model (i.e., the ?esták–Berggren equation) was found to be the most adequate for describing the cure kinetics of the studied systems. The obtained nonisothermal differential scanning calorimetry curves showed results in agreement with those theoretically calculated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2649–2656, 2004     

Frontal cationic curing of epoxy resins

We studied the frontal curing of trimethylolpropane triglycidyl ether (TMPTGE) using two BF₃‐amine initiators and two fillers, kaolin and fumed silica. In the case of kaolin, the range of concentrations allowing for frontal polymerization to propagate was dependent on its heat absorption effect whereas in the case of silica it was a consequence of the rheological features of this additive. However, for both systems the velocity and front temperature show the same trends; in all cases front velocities were on the order of 1 cm/min with front temperatures about 200 °C. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2000–2005, 2010     

Radical induced cationic frontal polymerization as a versatile tool for epoxy curing and composite production

Radical induced cationic frontal polymerization (RICFP) is an extremely powerful and elegant alternative curing technique that allows cationic bulk curing of epoxy resins with very little energy consumption, as well as curing in compartments that are not readily accessible. We recently introduced a bisphenol‐A diglycidylether (BADGE) based system that allows the bubble‐free photocuring of this widely used epoxy resin. In this article, we describe the high storage stability and possibilities to influence the curing speed via the initiator concentrations of different formulations. These properties allow the adjustment of the frontal polymerization to ones need. We also show that the (thermo)mechanical and electrical properties of frontal cured epoxy polymers compares favorably with those of state of the art material. Finally, different strategies to overcome the challenges on producing epoxy resin based mica composites via RICFP are presented. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3751–3759     

Studies on curing kinetics of a novel combined liquid crystalline epoxy containing tetramethylbiphenyl and aromatic ester‐type mesogenic group with diaminodiphenylsulfone

The curing kinetics of a novel liquid crystalline epoxy resin with combining biphenyl and aromatic ester‐type mesogenic unit, diglycidyl ether of 4,4′‐bis(4‐hydroxybenzoyloxy)‐3,3′,5,5′‐tetramethyl biphenyl (DGE‐BHBTMBP), and the curing agent diaminodiphenylsulfone (DDS) was studied using the advanced isoconvensional method (AICM). DGE‐BHBTMBP/DDS curing system was investigated the curing behavior by means of differential scanning calorimetry (DSC) during isothermal and nonisothermal processes. Only one exothermal peak appeared in isothermal DSC curves. A variation of the effective activation energy with the extent of conversion was obtained by AICM. Three different curing stages were confirmed. In the initial curing stage, the value of E_a is dramatically decreased from ～90 to ～20 kJ/mol in the conversion region 0–0.2 for the formation of LC phase. In the middle stage, the value of E_a keeps about ～80 kJ/mol for cooperative effect of reaction mechanism and diffusion control. In the final stage, a significant increase of E_a from 84 to 136 kJ/mol could be caused by the mobility of longer polymer chains. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3922–3928, 2007     

Carboxylic acid‐containing benzoxazines as efficient catalysts in the thermal polymerization of benzoxazines

The autocatalytic thermal polymerization behavior of three benzoxazine monomers containing carboxylic acid functionalities is reported. Several mixtures of these carboxylic monomers and 3‐phenyl‐3,4‐dihydro‐2H‐1,3‐benzoxazine were prepared and their thermal polymerization behavior was analyzed by differential scanning calorimetry. The acid character of these reactive monomers increases the concentration of oxonium species, thus catalyzing the benzoxazine ring opening reaction. In this way the polymerization temperature decreased by as much as 100 °C in some cases. The existence of decarboxylation processes at high temperatures has been established by FTIR‐ATR and related to the increase in thermal stability observed by TGA in some cases. A relationship between the presence of carboxylic groups in the resulting materials and their flame retardancy behavior has also been established. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6091–6101, 2008     

Facile synthesis of self‐healing gel via magnetocaloric bottom‐ignited frontal polymerization

A series of the self‐healing gels facilely fabricated by VI (N‐vinyl imidazole) and MAH‐β‐CD (β‐cyclodextrin grafted vinyl carboxylic acid groups) via bottom‐ignited frontal polymerization (BIFP) initiated by magnetocaloric effect. Once ignited the bottom phase, the heat upward propagates to generate the “front” in the upper phase. Then, no further energy is added to maintain the reaction and the whole polymerization process experiences within minutes. In this system, the dependence of frontal velocity and temperature, along with morphology, swelling capacity, mechanical property, and self‐healing efficiency, on the preparation parameters is investigated. Interestingly, the gels show good swelling capacity in the organic solvent, comparatively almost no absorption in water. Moreover, the as‐prepared gels exhibit excellent auto‐healing properties without any external stimuli at ambient temperature. The healed sample possesses 97% recovery of its tensile strength after 8 h healing time, which relies largely on the host–guest interaction between VI and MAH‐β‐CD. The results demonstrate that FP can be utilized as an efficient and energy‐saving method to synthesize self‐healing supramolecular gels. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2585–2593     

Surface modification of UV‐cured epoxy resins by click chemistry

A novel method for surface modification of UV‐cured epoxy network was described. Photoinitiated cationic copolymerization of a bisepoxide, namely 3,4‐epoxy cyclohexylmethyl 3,4‐epoxycyclohexanecarboxylate (EEC) with epibromohydrine (EBH) by using a cationic photoinitiator, [4‐(2‐methylpropyl)phenyl]4‐methylphenyl‐iodonium hexafluorophosphate, in propylene carbonate solution was studied. The real‐time Fourier transform infrared spectroscopic, gel content determination and thermal characterization studies revealed that both EEC and EBH monomers take part in the polymerization and epoxy network possessing bromomethyl functional groups was obtained. The bromine functions of the cured product formed on the glass surface were converted to azide functionalities with sodium azide. Independently prepared alkyne functional poly(ethylene glycol) (PEG) was subsequently anchored to azide‐modified epoxy surface by a “click” reaction. Surface modification of the network through incorporation of hydrophilic PEG chain was evidenced by contact angle measurements. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2862–2868, 2010     

Novel,environmentally friendly crosslinking system of an epoxy using an amino acid: Tryptophan‐cured diglycidyl ether of bisphenol A epoxy

Tryptophan, an amino acid, has been used as a novel, environmentally friendly curing agent instead of toxic curing agents to crosslink the diglycidyl ether of bisphenol A (DGEBA) epoxy resin. The curing reaction of tryptophan/DGEBA mixtures of different ratios and the effect of the imidazole catalyst on the reaction have been evaluated. The optimum reaction ratio of DGEBA to tryptophan has been determined to be 3:1 with 1 wt % catalyst, and the curing mechanism of the novel reaction system has been studied and elucidated. In situ Fourier transform infrared spectra indicate that with the extraction of a hydrogen from NH₃⁺ in zwitterions from tryptophan, the formed nucleophilic primary amine and carboxylate anions of the tryptophan can readily participate in the ring‐opening reaction with epoxy. The secondary amine, formed from the primary amine, can further participate in the ring‐opening reaction with epoxy and form the crosslinked network. The crosslinked structure exhibits a reasonably high glass‐transition temperature and thermal stability. A catalyst‐initiated chain reaction mechanism is proposed for the curing reaction of the epoxy with zwitterion amino acid hardeners. The replacement of toxic curing agents with this novel, environmentally friendly curing agent is an important step toward a next‐generation green electronics industry. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 181–190, 2007     

Photoinitiators with functional groups. VII. Covalently bonded camphorquinone—amines

Camphorquinone (CQ), a widely used photoinitiator (PI) in dental applications, was covalently bonded to aromatic amines to enhance the rate of electron and proton transfer effect due to the close vicinity of the diketone and the amine group. 10‐bromocamphorquinone and 10‐bromomethylcamphorquinone were selected as suitable precursors for esterification with the carboxyl group containing aromatic amines based on 4‐dimethylaminobenzoic acid. Properties of the new photoinitiating systems were investigated by UV spectroscopy and differential scanning photocalorimetry in lauryl acrylate. Compared to physical mixtures, in all cases similar or even better performance was obtained. Surprisingly, 10‐acetyl derivatives 7 – 9 and 18 especially, were found to be highly reactive. Compared to CQ/ethyl 4‐dimethylaminobenzoate, the rate of photopolymerization was increased by a factor of up to 2. Intramolecular reaction was confirmed by photo‐differential scanning calorimetry experiments with varying PI concentrations. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4948–4963, 2004     

Effect of new nonionic curing agent on curing kinetics and mechanical properties of epoxy resin

The current research work presents a novel nonionic curing agent (AEDA) synthesized by utilizing ethylene glycol diglycidyl ether (EGDE), 3,4-dimethoxyaniline (DI), and triethylenetetramine (TETA). Infrared spectroscopy and nuclear magnetic resonance spectroscopy were used to characterize the structure of AEDA curing agent. Non-isothermal scanning calorimetry was used to determine the activation energy and curing conditions of epoxy resin in the curing process. An impact testing machine, a tensile testing machine and a scanning electron microscope (SEM) were used to analyze the impact strength, tensile strength, bending strength, and micromorphology of the AEDA/E-51 system with different mass ratios. The results show that AEDA is an effective high-temperature curing agent. For the AEDA/E-51 system with the optimal mass ratio of 10:100, the best curing temperature is 92.15°C, and the post-curing temperature is 135.65°C. Furthermore, the apparent activation energy (E_a) of 1670 J/mol, the pre-exponential factor (A) of 3.7 × 10^?4, and the reaction series (n) value of 0.76 are obtained for the AEDA/E-51 system. The impact strength of AEDA/E-51 epoxy resin polymer is 7.82 kJ/m², tensile strength is 14.2 MPa, and bending strength is 18.92 MPa. The micromorphological results of the AEDA/E-51 system are consistent with the results of DSC test and mechanical properties test. Hence, this study provides theoretical support for the practical applications of AEDA as curing agent.     

Kinetics studies on the accelerated curing of liquid crystalline epoxy resin/multiwalled carbon nanotube nanocomposites

A new class of nanocomposite has been fabricated from liquid crystalline (LC) epoxy resin of 4,4′‐bis(2,3‐epoxypropoxy) biphenyl (BP), 4,4′‐diamino‐diphenyl sulfone (DDS), and multiwalled carbon nanotubes (CNTs). The surface of the CNTs was functionalized by LC epoxy resin (ef‐CNT). The ef‐CNT can be blended well with the BP that is further cured with an equivalent of DDS to form nanocomposite. We have studied the curing kinetics of this nanocomposite using isothermal and nonisothermal differential scanning calorimetry (DSC). The dependence of the conversion on time can fit into the autocatalytic model before the vitrification, and then it becomes diffusion control process. The reaction rate increases and the activation energy decreases with increasing concentration of the ef‐CNT. At 10 wt % of ef‐CNT, the activation energy of nanocomposite curing is lowered by about 20% when compared with the neat BP/DDS resin. If the ef‐CNT was replaced by thermal‐insulating TiO₂ nanorods on the same weight basis, the decrease of activation energy was not observed. The result indicates the accelerating effect on the nanocomposite was raised from the high‐thermal conductivity of CNT and aligned LC epoxy resin. However, at ef‐CNT concentration higher than 2 wt %, the accelerating effect of ef‐CNTs also antedates the vitrification and turns the reaction to diffusion control driven. As the molecular motions are limited, the degree of cure is lowered. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Photoinitiating monomers based on di‐ and triacryloylated hydroxylamine derivatives

The purpose of our study was to design a new class of acrylate‐based monomers with an UV‐cleavable heteroatom bond, offering the possibility to initiate radical polymerization upon irradiation with UV‐light. A method to derive the double bond conversion from the ATR‐IR spectra of the monomers and the cured polymers was employed, that enabled us to calculate the theoretical polymerization heats of the new monomers. Their photopolymerization properties were determined by Photo Differential Scanning Calorimetry. Surprisingly, some of these new compounds exhibited high photoinitiation activity, comparable to well‐established Type II photoinitiator systems like benzophenone/triethanolamine. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 392–403, 2009     

Polymer hydrogels of 2‐hydroxyethyl acrylate and acrylic acid obtained by frontal polymerization

In this work, we report on the synthesis and characterization of homopolymers and copolymers of acrylic acid and 2‐hydroxyethyl acrylate prepared by the use of the frontal polymerization (FP) technique. Tetraethyleneglycoldiacrylate was used as a crosslinker and benzoyl peroxide as an initiator. The maximum temperatures reached by the front were in the range between 214 °C and 296 °C. Besides, front velocities ranged between 3.9 and 10.8 cm/min, the latter being one of the highest values reported so far in the FP literature. Differential scanning calorimetry was used to estimate the conversion degree, which was always comprised between 90% and 96%, and to determine the glass transition temperatures, which were found to be dependent on the composition, with values ranging from 13 °C to 168 °C. Moreover, the obtained materials were allowed to swell in aqueous solutions at various pH. The samples exhibit a moderate increase of the swelling ratio percentage (SR%) at pH ≈ 5–6, and a sudden and larger SR% increase at pH ≈ 12–13 depending on the composition, thus indicating the obtainment of pH‐responsive polymer hydrogels. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Photopolymerization of biocompatible phosphorus‐containing vinyl esters and vinyl carbamates

Phosphorus‐containing vinyl esters and vinyl carbamates were synthesized as new biocompatible and degradable photopolymers. Reactivity of the monomers with one, two, and three polymerizable double bonds was evaluated by photo‐differential scanning calorimetry. With respect to their potential application in the biomedical field, studies on cytotoxicity, mechanical stability, and hydrolytic erosion behavior of the poly(vinyl alcohol)‐based derivatives were performed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2916–2924, 2010     

Rapid synthesis of poly(HPA‐co‐VeoVa 10) amphiphilic gels toward removal of toxic solvents via plasma‐ignited frontal polymerization

Atmospheric air plasma was created and applied in the frontal polymerization (FP) of fabricating poly(HPA‐co‐VeoVa 10) amphiphilic gels (HPA = 2‐hydroxypropyl acrylate, VeoVa 10 = vinyl versatate) with enhanced physicochemical properties. In plasma‐ignited frontal polymerization (PIFP), once ignited by air plasma, no further energy or treatment was required for the following polymerization. In this system, the comparison between PIFP and thermal frontal polymerization (TFP) was conducted and observed that PIFP and TFP differ considerably in terms of swelling capacity, morphology and component distribution of the products. This finding is of great importance that the simultaneous generation of active radicals in the initial stage can spread throughout reactant and anchor on the synthetic polymer with the assistance of FP. More interestingly, the PIFP‐synthesized copolymer possesses remarkable selective absorption towards organic solvents, which can be facilely manipulated by varying the weight ratios of HPA/VeoVa 10. Obviously, these polymer products could serve as an “organic solvent scavenger” in the field of industrial wastewater treatment. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011