Thermal polymerization of thiol–ene network‐forming systems

The thermal polymerization of a tetrafunctional thiol (PETMP) and divinyl ether (TEGDVE) was monitored by temperature‐ramping differential scanning calorimetry (DSC) and the effects of inhibitor type and concentration, oxygen inhibition and initiator type were studied. The incorporation of inhibitors was required to produce a stable system at room temperature. Butylated hydroxytoluene (BHT) inhibited polymerization at low temperatures, but was inefficient at high temperatures and polymerization rates, and hence BHT is an ideal stabilizer. In contrast, a nitroxide inhibitor (NO‐67) was a very effective inhibitor and no polymerization occurred until all of the nitroxide was depleted. The presence of oxygen retarded the onset of polymerization but did not change the final conversion significantly. Polymerization with initiators having higher half‐life temperatures shifted the DSC peak to higher temperature because the rate of initiator decomposition and thus initiation was slower. Rheological investigations of the cure at different temperatures revealed that the gel time decreased significantly with increasing cure temperature, and the calculated apparent activation energy for PETMP/TEGDVE was 54 kJ mol^?1. Dynamical mechanical thermal analysis of the cured material was undertaken and frequency‐superposed results revealed that the glass transition region of PETMP/TEGDVE/azobisisobutyronitrile was much narrower than that of free‐radically cured dimethacrylate, but was similar to that of an epoxy resin cured with an aromatic diamine. This behaviour could be attributed to PETMP/TEGDVE network homogeneity, or to the less constrained crosslinks in the PETMP/TEGDVE network. Copyright © 2007 Society of Chemical Industry     

Photopolymerization kinetics,photorheology and photoplasticity of thiol–ene–allylic sulfide networks

BACKGROUND: Thiol–ene networks are of interest due to their facile photopolymerization and their open network structure. In this work, an allylic disulphide divinyl ether monomer is reacted with tetrathiol and divinyl ether monomers, which allows the network structure to permanently change in shape if stressed while under irradiation. We also study the photo‐differential scanning calorimetry (DSC) kinetics and photorheology during cure and the dynamic mechanical properties after cure. RESULTS: The heat of polymerization is similar for the thiol–ene systems and suggests ca 80% conversion of the vinyl ether groups. An increase in the initiator concentration increases the photocure rate as expected. The activation energy for photopolymerization is 7.6 kJ mol^?1. DSC and rheometry studies show that the polymerization kinetics is slowed by the addition of the allylic disulfide divinyl possibly due to the formation of less reactive radicals. However, as shown by dynamic mechanical thermal analysis, the network structure is not changed very much by addition of this monomer. If radicals are generated by irradiation of a photoinitiator in the network while a stress is being applied, the polymer will permanently deform depending on the fraction of 2‐methylenepropane‐1,3‐di(thioethyl vinyl ether) in the network, due to a bond interchange reaction. CONCLUSION: The rate of thiol–ene reaction is slowed by the addition of the allylic disulfide divinyl ether. Photoplasticity is observed in the networks containing the allylic disulfide groups. Further work is required to optimize the extent of photoplasticity in these systems. Copyright © 2007 Society of Chemical Industry     

Photopolymerization kinetics of hyperbranched acrylated aromatic polyester

The influences of the irradiation temperature, comonomer content, sample thickness, and photoinitiator concentration on the polymerization kinetics of hyperbranched acrylated aromatic polyester (HAAPE) were investigated with photo‐differential scanning calorimetry and IR measurements. The maximum photopolymerization rate increased with the temperature rising up to 110°C but decreased beyond 110°C. An activation energy of 16 kJ mol^?1 for the photopolymerization was obtained below 110°C from an Arrhenius plot, but it was negative beyond 120°C. A remarkable synergistic effect between HAAPE and the comonomer trimethylolpropane triacrylate with a molar fraction of around 0.4 was observed from a photopolymerization kinetic study of the resins. The final unsaturation conversion in an ultraviolet‐cured film decreased with the sample thickness, and this became more remarkable as the photoinitiator concentration increased. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1500–1504, 2003     

Differential Scanning Calorimetry Cure Studies of Tetra-N-glycidyldiaminodiphenylmethane Epoxy Resins. Part 1 - Reaction with 4,4′-Diaminodiphenylsulphone

Rates and extents of cure are determined using differential scanning calorimetry in the isothermal mode over the temperature range 170–220°C, and from d.s.c. scans at various heating rates. The isothermal data are consistent with an autocatalytic mechanism at conversions up to about 20–30%. Data from d.s.c. scans fit a simple kinetic model which indicates that the apparent activation energy (E) for cure increases with increasing conversion, consistent with an increasing degree of diffusion control. At low levels of conversion the isothermal and dynamic data both provide estimates for E of about 70 kJ mol^?1. The heat of cure is about 105 kJ mol^?1 epoxide, and is constant over a wide range of amine concentration. This indicates that any parallel or competing processes which occur must have the same heat of reaction.     

Curing kinetics of UV‐initiated cationic photopolymerization of divinyl ether photosensitized by thioxanthone

Photodifferential scanning calorimetry was used to investigate the photocuring kinetics of UV‐initiated cationic photopolymerizations of 1,4‐cyclohexane dimethanol divinyl ether (CHVE) monomer with and without a photosensitizer, 2,4‐diethylthioxanthone (DETX), in the presence of a diaryliodonium‐salt photoinitiator. Two kinetics parameters, the rate constant (k) and the order of the initiation reaction (m), were determined for the CHVE system with different amounts of added DETX photosensitizer (0–1 wt %) and at different isothermal temperatures (25–55°C) using an autocatalytic kinetics model. The photosensitized CHVE system exhibited much higher k and m values than did the nonphotosensitized system, which was attributable to the effects of photosensitization. Furthermore, the values of k and m for both CHVE systems increased significantly with increasing isothermal temperature because of a thermal contribution toward increasing the mobility of active species. The addition of DETX lowered the activation energy for the UV‐curable vinyl ether system. The collision factor for the system with DETX was higher than that obtained for the system without DETX, indicating that the reactivity of the former was greater than that of the latter because of the photosensitization effect. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1345–1351, 2005     

Photoreactivity of vinyl ether/oxirane‐based resin systems

We evaluated the reactivity in homopolymerization and as comonomers in dioxirane/polyol visible‐light curable systems the following vinyl ethers (VEs): mono‐, di‐, and tri(ethylene glycol) divinyl ethers (EGDVE, DEGDVE, and TEGDVE, respectively); 1,6‐hexanediol divinyl ether (HDDVE); cyclohexanedimethanol divinyl ether (CHDMDVE); glycidyl vinyl ether (GVE); 4‐(1‐propenyloxymethyl)‐1,3‐dioxolan‐2‐one (POMDO); and 1,4‐butanediol vinyl ether (BDVE). The dioxirane/polyols (80/20) were Cyracure UVR 6105 or ERL 4206 dioxiranes with polytetrahydrofuran [PTHF; number‐average molecular weight (M_n) = 250]. Reactivities were evaluated by photodifferential scanning calorimetry with visible light. For VE homopolymerization, the relative reactivity ranking (based on exotherm peak maximum time) was TEGDVE > EGDVE > DEGDVE > HDDVE > CHDMDVE > BDVE > GVE ? POMDO. For VEs in UVR 6105/PTHF, the ranking was GVE > TEGDVE > CHDMDVE > BDVE > EGDVE ? DEGDVE = HDDVE = POMDO. In ERL 4206/PTHF, the ranking was: GVE > TEGDVE > BDVE > DEGDVE ? HDDVE > EDGDVE ? CHDMDVE = POMDO. The incorporation of an electron‐donating reaction promoter, ethyl‐4‐dimethylaminobenzoate, generally shortened induction times and exotherm peak maximum times and increased rate constants for homopolymerizations and ternary polymerizations. Experimentally determined polymerization reactivities were compared with previously reported semiempirical quantum mechanical calculations of activation energies and heats of reaction. The results of laboratory and computational studies for selected compounds were in general agreement. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 314–326, 2002     

A thermal and rheological investigation during the complex cure of a two‐component thermoset polyurethane

The complex cure kinetics of the reaction between oligomeric diphenylmethane diisocyanate (PMDI) and glycerol was characterized through thermal and rheological techniques. Isoconversional analysis of Differential scanning calorimetry (DSC) data resulted in the activation energy varying with conversion. Isothermal analysis gave activation energies ranging from 5 kJ/mol to 33.7 kJ/ mol, whereas nonisothermal data gave values for the activation energy ranging from 49.5 to 55 kJ/mol. Incomplete cure was evident in isothermal DSC, becoming diffusion controlled in the final stages of cure. DMA analysis on the cured material gave a glass transition temperature of 104 ± 3°C, which was evidence for vitrification of the curing system. The primary and secondary hydroxyl group reactivity was dependant on the isothermal cure temperature. Rheological studies of viscosity increase and tan δ changes with time revealed a complex cure process, with primary and secondary hydroxyl reactivity also showing dependence on isothermal cure temperatures, reflecting similar results obtained from isothermal DSC studies. The independence of tan δ on frequency was used to determine the point where the polymer formed an infinite network and was no longer able to flow, providing an overall activation energy attained at the gel point of 77.4 ± 4.4 kJ/mol. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Photoinitiated cationic polymerization of urethane vinyl ether crystalline monomers

Photoinitiated cationic polymerization of crystalline monomers based on urethane vinyl ether has been investigated by means of differential photocalorimetry (DPC). The crystalline monomers in the melt state polymerized rapidly by exposure to UV light in the presence of a cationic photoinitiator such as an iodonium salt, sulfonium salt or iron arene salt. The kinetics of the cationic photopolymerization process were studied by following k_p[M⁺] as a function of conversion. High conversions, of around 90 %, were obtained for most of the systems investigated. The efficiency of the cationic photoinitiators in initiating the polymerization of the vinyl ether monomers was in the order: iodonium salt > iron arene salt > sulfonium salt. Monomers modified with different saturated alcohols had different activities in photopolymerization, although they all carry the same functional group, i.e. vinyl ether. Copyright © 2005 Society of Chemical Industry     

Synthesis, characterization and photopolymerization of vinyl ether and acrylate functionalized hybrid oligo-caprolactone

Linear vinyl ether-(oligo-caprolactone)-acrylate (VPCLA), combining fast free radical and complete cationic photopolymerizable groups, was synthesized, functionalized, and photopolymerized to produce polycaprolactone (PCL) network. Fourier Transform Infrared (FTIR) spectra confirmed that the C = C peaks from both vinyl ether and acrylate end groups were consumed after photopolymerization. Kinetics parameters obtained from differential scanning photo-calorimetry (DPC) analysis showed that photopolymerization of VPCLA at early stage was accelerated as the time needed to reach peak maximum was shortened, and the induction time was significantly shortened compared to monofunctional vinyl ether-(oligo-caprolactone) (VPCL). The activation energy (E_a) was calculated to be 14 kJ/mol, assuming second-order autocatalytic model was followed. Rate of polymerization of the hybrid oligomers was doubled in dual photoinitiators system, which contained both cationic and radical photoinitiators. Furthermore, the conversion was greatly improved at the presence of divinyl ether/hydroxybutyl vinyl ether in 1:1 ratio.     

Reaction kinetics of epoxy resin modified with reactive and nonreactive thermoplastic copolymers

An epoxy resin system based on a triglycidyl p‐amino phenol (MY0510) was crosslinked using stoichiometric amounts of 4,4′‐diaminodiphenyl sulfone. The epoxy was modified with random copolymers, polyethersulfone‐poly(ether‐ethersulfone) (PES:PEES), with either amine or chlorine end groups, at 10 and 20 wt %. The reaction kinetics for both unmodified and modified epoxy systems were studied using differential scanning calorimetry in isothermal and dynamic conditions. The results show that the degree of conversion in thermoplastic‐modified epoxies at any reaction time is smaller compared with the unmodified resin. Gel point (GP) determination was done from rheological measurements. The modified system containing 20% of the PES:PEES additive showed considerable increase in the GP. The reaction rate shows the characteristic of an autocatalytic reaction where the product acts as catalyst. The activation energy, E_a calculated from the isothermal reaction depends on the extent of conversion and increases with increasing PES:PEES content. For unmodified epoxy system, the average E_a is 67.8 ± 4.1 kJ mol^?1 but for systems modified with 20 wt % of amine and chlorine PES:PEES, the value increased to 74.1 ± 3.3 and 77.9 ± 4.4 kJ mol^?1, respectively. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Enhanced cationic photocuring of epoxides with styrene oxide as a reactive diluent

Styrene oxide [StO] has been investigated as a reactive diluent for cationic photocurable formulations to provide key requirements such as low viscosity and negligible odor, in addition to high photoreactivity. The formulations were composed of commercial cyclic ethers including epoxides and oxetanes in StO using diaryliodonium salt as a photoinitiator, and they gave enhanced photocuring by analyzing with differential photocalorimetry. In particular, the cationic formulations composed of StO, bisphenol A diglycidyl ether, (3-glycidoxypropyl)trimethoxysilane and others rendered improved mechanical and thermal properties when their photocured coatings were examined. The gel content and pencil hardness values of photocured films were determined to evaluate the formulations. For the mechanistic explanation of the enhanced photoreactivity, the polymer-bound benzyl cations formed by a chain process in the polymer chains comprising StO moieties were postulated to serve as additional sites for the initiation of cationic photopolymerization of StO and cyclic ether monomers.     

Synthesis and thermal cure of diphenyl ethers terminated with acetylene and phenylacetylene

Two kinds of novel compounds, diphenylacetylene diphenyl ether (DPADPE) and diacetylene diphenyl ether (DADPE), were prepared and polymerized under heating. Raman, DSC and ¹³C CP/MAS NMR analyses were used for studying the polymerization reaction. DPADPE and DADPE have melting points at 190 and 79 °C, with exothermic peaks of the DSC curves at 375 and 215 °C for curing, respectively. Raman and ¹³C CP/MAS NMR spectra show that DPADPE could be cured at a temperature higher than 300 °C and DADPE at a lower temperature of higher than 150 °C. The kinetic parameters for the thermal crosslinking reactions were obtained by the Ozawa method and the results show that the apparent activation energy is 152 kJ mol^?1 for DPADPE and 109 kJ mol^?1 for DADPE. An ene–yne Straus product appears in the cured DADPE, whereas this product has not been identified in the cured DPADPE. The cured DPADPE and DADPE demonstrate good thermal and thermo‐oxidative stability. Copyright © 2006 Society of Chemical Industry     

Synthesis and photopolymerization of norbornyl epoxidized linseed oil

Norbornyl epoxidized linseed oil was synthesized via Diels-Alder reaction of cyclopentadiene with linseed oil at high pressure (∼200 psi) and high temperature (240 °C), followed by an epoxidation using hydrogen peroxide with a quaternary ammonium tetrakis(diperoxotungsto) phosphate(3−) epoxidation catalyst. The products were characterized using ¹H and ¹³C NMR, FT-IR, and electrospray ionization mass spectroscopy. Photo-induced curing kinetics of norbornyl epoxidized linseed oil coatings was investigated using real-time FT-IR spectroscopy with a fiber optic UV-curing system. The norbornyl epoxidized linseed oil was formulated with three different divinyl ether reactive diluent. The effect of divinyl ether concentration and types of divinyl ether on the curing reaction was investigated. It was found that the curing rate of norbornyl epoxidized linseed oil was lower than that of cycloaliphatic epoxide, but higher than epoxidized linseed oil. The incorporation of divinyl ethers increased the curing rate and overall conversion of the epoxide groups. Of the three divinyl ethers used, coating with triethyleneglycol divinyl ether showed the highest curing rate and coating with cyclohexane dimethanol divinyl ether showed the lowest curing rate.     

Kinetic studies of a UV-curable powder coating using photo-DSC, real-time FTIR and rheology

The curing kinetics of UV-curable powder coatings based on commercial unsaturated polyesters were monitored using photo-DSC, Real-Time FTIR-ATR and a modified rheometer equipped with a UV source. The effect of physical and chemical factors on curing such as type of photoinitiator, photoinitiator concentration, temperature and atmosphere of curing were evaluated. Coatings containing amounts of photoinitiator from 0.5 to 10 wt% were cured at different temperatures in less than 10 s reaching conversions approximately of 60%. The increase of the temperature of curing reduces the final conversion and also the rate of polymerization due to the chain transfer process and depolymerization that dominates the photopolymerization at high temperatures. The reactivity of the photoinitiators was similar for all the studied photoinitiators apart from benzophenone that was found to be the slowest initiator.     

Acrylic resins resisting oxygen inhibition during free‐radical photocuring. I. Formulation attributes

A resin system was found to be resistant to the formation of an oxygen‐inhibited layer when cured in air via conventional free‐radical photopolymerization. The resins, containing multifunctional acrylates and a high concentration of a photoinitiator, were applied as thin film coatings and photocured with either visible light (400–500 nm) or UV light (254 nm). Fourier transform infrared spectroscopy with an attenuated total reflection attachment and pencil hardness were used to assess the surface double‐bond conversion and the surface hardness of the coatings cured in air and without air, respectively. The surfaces of many tested resins could produce similar conversions under both curing conditions. Optimally formulated resins had a high conversion and hardness even when the irradiance was as low as 50 mW/cm² for the visible light and 4 mW/cm² for the UV light. The requirements for possessing such a unique curing property are presented. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Thermal decomposition of poly(oxytetramethylene) glycol

Studies of thermal decomposition on poly(oxytetramethylene) glycol have been conducted by pyrolysis gas chromatography–mass spectrometry, infrared spectroscopy, and thermogravimetric anaylysis (TGA). The major volatile decomposition products are suggested to be a series of molecules made up by the repetition of oxytetramethylene with formyl and/or methyl ends. Absorption peaks, associated with formyl appear in infrared spectrum of a sample preheated at 523°K lower than the onset temperature obtained from the TGA curve. The isothermal TGA curves fit well to the Shimha rate equation for the random decomposition of polymers. The proper activation energies obtained from the thermally controlled and the isothermal TGA data are approximately 60–70 kJ mol⁻¹ and lower than those for other polymers in ordinary thermal decomposition. These data suggest that the major reaction in the thermal decomposition of poly(oxytetramethylene) glycol is an ether cleavage. Two pathways, a radical scission accompanied by β‐hydrogen transfer and a nonradical reaction through a four‐membered ring transition state, are proposed and discussed for the ether cleavage. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1538–1544, 2000     

Thermal cracking of coal model diaryl ethers in aromatic solvent

Thermal cracking of nine diaryl ethers in a hydrogen donor solvent or 1-methylnaphthalene was studied kinetically. The rate of conversion of the diaryl ethers was first order with respect to the substrate concentration and increased with increase in size of the aryl structure. The relative rate constant of aryloxygen bond cleavage calculated on the basis of first order reaction has indicated that the ease of cracking depends strongly on the aromatic structure and the position of substitution. The conversion rate of 2, 2′-dinaphthyl ether was remarkably enhanced in the presence of hydrogen donor solvent, for example by a factor of ten in the presence of 9, 10-dihydroanthracene. The activation energy of thermal conversion of 2, 2′-dinaphthyl ether was 214 kJ/mole in methylnaphthalene, 151 kJ/mole in tetralin and 88 kJ/mole in dihydroanthracene. The enhancing effect of the hydrogen donor was considered due to hydrogen transfer to the aromatic nucleus of the diaryl ether from the hydrogen donor and successive fast decomposition of hydrogenated ethers.     

Kinetics of phase formation in jarosite glass-ceramic

The crystallisation behaviour of an iron-rich glass was studied by means of density measurements, Differential Thermal Analysis and X-ray Diffraction. The results highlighted that there is no typical nucleation process in which the number of nuclei is influenced by the thermal treatment and influence the crystallisation process. The kinetics of phase formation was investigated in isothermal and non-isothermal conditions; the Avrami parameter value, 1·5, is indicative of a three dimensional diffusion controlled growth of the pyroxene crystals on a fixed number of nuclei. The activation energy of crystal growth, obtained by the isothermal and non isothermal methods, resulted in being 377kJ mol⁻¹ and 298 kJ mol⁻¹, respectively. Considering the different temperature range, these values are in fair agreement and consistent with values reported by other authors. Using TEM-EDS analysis the high immiscibility of the initial glass was highlighted; the two liquid phases, one richer in Fe and the other in Si and Ca, promote the formation of magnetite and pyroxene during the thermal treatment, respectively.     

Kinetics of polymerization of dimer fatty acids with ethylenediamine

A generally applicable stoichiometric and kinetic model was developed for the polymerization of dimer fatty acids with ethylenediamine. The rate equations were second‐order before 90% conversion and were used between 405 and 475 K. The parameters of the rate equations were determined with nonlinear regression analysis. A comparison of the model predictions and the experimental data showed that the approach was useful in predicting the polymerization kinetics. The equilibrium constant changed from 3.175 to 7.311. The frequency factor and activation energy for the forward rate constant before 90% conversion were 2,716,894 kg mol^?1 min^?1 and 66.7 kJ mol^?1, respectively. The equilibrium constant was independent of the temperature at frequency factor and activation energy values of 74.4 and 9.7 kJ mol^?1, respectively. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2504–2513, 2004     

Thermal degradation behavior and kinetics of porous polymer based on high functionality components

Two kinds of porous polymer were prepared based on high functionality components. Thermogravimetric analysis (TGA) is used to compare the thermal degradation behavior and kinetics of these two materials. The thermogravimetric tests of rigid polyurethane foam (H-RPUF) and polyurea aerogel (H-PUA) were carried out in nitrogen atmosphere at different heating rates. The thermal degradation characteristics of the porous polymer were obtained. The apparent activation energy (E_a) of thermal degradation of the porous polymer was investigated by model-free methods. The results showed that the thermal degradation temperature and ash content of H-RPUF were higher than those of H-PUA, and the volatile content was lower. With the rise of heating rate, thermal hysteresis effect of the two porous polymer was relatively high, while the release amount of volatiles was unchanged. For the Kissinger method, E_a of H-PUA and H-RPUF was 212.8 kJ mol⁻¹ and 157.4 kJ mol⁻¹, respectively. According to Starink method, the average activation energy of H-PUA and H-RPUF was 220.2 kJ mol⁻¹ and 107.2 kJ mol⁻¹, respectively. Obtained by Flynn-Wall-Ozawa model, the average activation energy of H-PUA and H-RPUF was 219.0 kJ mol⁻¹ and 111.5 kJ mol⁻¹, respectively. The data obtained from the three models all show that E_a of thermal degradation of H-PUA is higher than that of H-RPUF, and it is less likely to decompose.