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Multilayered thiol-ene network films with two and three different components were fabricated by spin coating and photopolymerization. The distinctive glass transition temperatures of each layer component were observed at corresponding glass transition regions of each bulk sample. Sub-Tg aging of 10-, 21-, and 32-layered thiol-ene films was investigated in terms of enthalpy relaxation. Enthalpy relaxation of each layer component occurred independently and presented the characteristic time and temperature dependency. Overlapped unsymmetrical bell-shaped enthalpy relaxation distribution having peak maximum at Tg-10°C of each layer component was observed, resulting in broad distribution of enthalpy relaxation over wide temperature range. In addition, enthalpy relaxation of each layer component in the multilayered thiol-ene films was significantly accelerated comparing to that of bulk thiol-ene samples. Dynamic mechanical thermal properties of multilayered thiol-ene films also showed two and three separated glass transition temperature. However, for 32-layered thiol-ene film consisting of three different layer components, glass transition and damping region are overlapped and the width is extended more than 100°C. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

A detailed evaluation of the kinetics of the thiol-Michael reaction between hexanethiol and hexyl acrylate is described. It is shown that primary amines are more effective catalysts than either secondary or tertiary amines with, for example, quantitative conversion being achieved within 500 s in the case of hexylamine with an apparent rate constant of 53.4 mol L−1 s−1 at a catalyst loading of 0.057 mol %. Certain tertiary phosphines, and especially tri-n-propylphosphine and dimethylphenylphosphine, are shown to be even more effective species even at concentrations 2 orders of magnitude lower than employed for hexylamine and performed in solution with quantitative conversions reached within ca. 100 s for both species and apparent rate constants of 1810 and 431 mol L−1 s−1, respectively. The nature of the thiol is also demonstrated to be an important consideration with mercaptoglycolate and mercaptopropionate esters being significantly more reactive than hexanethiol with reactivity mirroring the pKa of the ...

Non-extractable photoinitiators could be advantageous for use in biological, electronic, and food packaging applications. Therefore, thioxanthone and benzophenone derivatives were synthesized from 1,6-hexane dithiol and chlorinated benzophenone or thioxanthone. The efficiency of thiol-functionalized photoinitiators in combination with amine co-initiators was compared to benzophenone and isopropylthioxanthone with amine co-initiators, and the cleavage of photoinitiator 2,2-dimethoxy-1,2-diphenylethan-1-one in acrylic resins. The reaction kinetics were analyzed using photo-differential scanning calorimetry and real-time FTIR. Coating physical properties were evaluated by pendulum and pencil hardness, steel–wool scratch and mandrel bend tests. The non-extractable photoinitiators had higher absorbance than their benzophenone or isopropylthioxanthone counterparts due to the sulfide substitution on the phenyl ring, and the free thiol groups reacted with the acrylate by either an amine catalyzed Michael addition or a free-radical chain process. The combination of thiol-functionalized photoinitiators with secondary amines provides an efficient photoinitiator system that is locked into the photopolymerized network and cannot be extracted with typical solvents.

Multifunctional alkynes (2, 3, or 4 ynes per monomer) were prepared utilizing the nucleophile-catalyzed thio−Michael addition reaction from commercially available multifunctional thiols (2, 3, or 4 thiols) and propargyl acrylate. Real-time FTIR (RTIR) and NMR spectroscopies indicate that the conjugate addition under these conditions proceeds to high conversions within seconds using the nucleophilic catalyst dimethylphenylphosphine, in the absence of solvent, at ambient temperature, and with no side products. A family of polymer networks was prepared by the photoinitiated thiol−yne reaction employing a 2:1 ratio of thiol to alkyne, which resulted in uniformly cross-linked materials of systematically increasing cross-link density. Photopolymerization kinetic profiles indicate that the thiol−yne reaction proceeded rapidly to high conversion with conversions decreasing with increasing functionality of the thiol and/or alkyne groups. Differential scanning calorimetry (DSC) and dynamic mechanical thermal analys...

Thiol−isocyanate−ene ternary networks with systematic variations (100/100/0, 100/80/20, 100/60/40, 100/40/60, 100/20/80, and 100/0/100) were prepared by sequential and simultaneous thiol−ene and thiol−isocyanate click reactions. The thiol−isocyanate coupling reaction was triggered thermally or photolytically to control the sequence with the thiol−ene photopolymerization. Triethyl amine (TEA) and 2,2-dimethoxy-2-phenyl acetophenone (DMPA) were used for the sequential thermally induced thiol−isocyanate coupling and photochemically initiated thiol−ene reaction, respectively. A thermally stable photolatent base catalyst (tributylamine·tetraphenylborate salt, TBA·HBPh4) capable of in situ generation of tributylamine by UV light was used with isopropylthioxanthone (ITX) for the simultaneous thiol−isocyanate/thiol−ene curing systems. The kinetics of the hybrid networks investigated using real-time IR indicate that both thiol−isocyanate and thiol−ene reactions were quantitatively rapid and efficient (>90% of conv...

Nach Sharpless’ visionarer Charakterisierung verschiedener idealisierter Reaktionen als Klick-Reaktionen hat man innerhalb der Materialwissenschaften und Synthesechemie grose Anstrengungen unternommen, um solche Reaktionen zu finden und zu implementieren. In diesem Aufsatz diskutieren wir die radikalvermittelte Thiol-En-Reaktion, die uber all die Merkmale einer Klick-Reaktion verfugt: hohe Effizienz, einfache Durchfuhrung, keine Nebenprodukte, hohe Reaktionsgeschwindigkeiten, hohe Ausbeuten. Daruber hinaus besteht die Moglichkeit, die Thiol-En-Reaktion photoinitiiert auszufuhren, was insbesondere fur Photopolymerisationen zur Synthese extrem einheitlicher Polymernetzwerke genutzt wird. Der Reaktionsmechanismus wird nach dem aktuellen Kenntnisstand erlautert, und zentrale Anwendungen der Thiol-En-Reaktion in der Material- und Molekulsynthese, der Biofunktionalisierung, der Polymersynthese und der Oberflachen- und Polymermodifizierung werden zusammengefasst.

Methacrylate monomers have been widely used in medical and dental applications such as bone cements, dental fillings, bioadhesives, and hydrogels. One major problem of these monomers resides in their low rates of polymerization leading to leaching problems that cause irritation of the surrounding tissues and even cell death. Here we describe the synthesis and polymerization of new mono-methacrylates containing multifunctional pyrrolidinone moieties. Such monomers possess stronger inherent hydrogen bonding potential susceptible to increase their reactivity and polymerization rates. These monomers were shown to homopolymerize rapidly leading to crosslinked polymers. The corresponding rates of polymerization were found to be comparable to difunctional methacrylates such as hexanediol dimethacrylate. Intermolecular hydrogen bonding interactions involving the pyrrolidinone unit leads these monomers to behave as “pseudo” difunctional monomers upon homopolymerization.

The controlled reversible addition–fragmentation chain transfer (RAFT) homopolymerization of an unprotected isocyanate-containing monomer, in this case 2-(acryloyloxy)ethylisocyanate (AOI), is reported. The polymerization conditions were optimized in terms of the choice of RAFT chain transfer agent (CTA), polymerization temperature and the reaction medium. Direct RAFT polymerization of AOI requires a neutral CTA, and relatively low reaction temperature to yield AOI homopolymers with low polydispersities. Efficient side-chain functionalization of poly(2-(acryloyloxy)ethylisocyanate) (PAOI) homopolymers was achieved via reaction with model amine, thiol and alcohol compounds yielding urea, thiourethane and urethane derivatives, respectively. Reactions with amines and thiols (in the presence of base) were rapid, quantitative and efficient. However, the reaction with alcohols catalyzed by dibutyltin dilaurate (DBTDL) was relatively slow but proceeded to completion. Selective reaction pathways for the addition of difunctional ethanolamine and mercaptoethanol were also investigated.

Gold nanoparticles and multi-functional acrylate (TMPTA) were incorporated into a photopolymerized thiol-ene (TMPMP–APE) network as a physical and chemical approach to intentionally control sub-Tg aging. The degree of the restriction effect was evaluated by differential cooling rate measurements allowing the quantification of the apparent activation energy for enthalpy relaxation (Δh*) upon sub-Tg aging. Incorporation of gold nanoparticles (0.01 to 1 wt%) into the TMPMP–APE network increased Tg and decreased ΔCp at Tg due to molecular mobility restrictions. The extent of enthalpy relaxation and apparent activation energy for enthalpy relaxation (Δh*) clearly indicated the significant restrictive effect of the gold nanoparticles on the molecular mobility in the thiol-ene network. A TMPMP–APE–TMPTA ternary system was investigated in order to correlate Δh* and network uniformity as a chemical approach. TMPTA, being capable of homopolymerization as well as TMPMP–TMPTA copolymerization, was incorporated into a TMPMP–APE network structure, thereby decreasing the network uniformity and significantly affecting the sub-Tg aging. The extent of enthalpy relaxation decreased and the distribution was drastically broadened as a function of TMPTA content due to molecular mobility restrictions, which were also quantified by measuring values for the apparent enthalpy relaxation activation energy (Δh*).

Objectives Several mechanisms have been purported to describe how mouthguards protect the orofacial complex against injury. As the properties needed for these mechanisms to be effective are temperature and frequency dependent, the specific aim of this study was to provide a comprehensive thermal characterization of commercial mouthguard materials. Methods Five commercially representative thermoplastic mouthguard materials (Essix™ Resin, Erkoflex™, Proform™-regular, Proform™-laminate, and Polyshok™) were tested. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) techniques were implemented to measure thermal transitions and mechanical properties. Measurements were conducted three times per sample. One-way ANOVA and one-sample t-tests were used to test for differences between commercial products on selected mean thermal property values. Results The DSC measurements indicated no differences between commercial materials for mean glass transition (p = 0.053), onset melt (p = 0.973), or peak melt (p = 0.436) temperatures. Likewise, DMA measurements revealed no differences between commercial materials for the mean glass transition (p = 0.093), storage modulus (p = 0.257), or loss modulus (p = 0.172) properties, respectively. The one-sample t-tests revealed that glass transition temperatures were different from intra-oral temperature (p Significance Commercialized mouthguard materials are sensitive to repetitive heating and cooling cycles, prolonged thermal treatment, and have glass transitions well below their end-use intra-oral temperature. As such, these materials are functioning as elastomers and not optimal mechanical damping materials. Dental clinicians, healthcare practitioners, or end-users should be aware that these materials are at best problematic with respect to this protective mechanism.

Thio-acrylate networks were prepared using two methods. The first method involved the in-situ photopolymerization of a multifunctional thiol mixed with a multifunctional acrylate in the presence of a photoinitiator (photo-cure only), while the second method utilized an extremely efficient thio-acrylate Michael reaction followed by the photopolymerization of unreacted acrylate functional groups (amine catalysis/photo-cure). The thio-acrylate Michael reaction was catalyzed by a primary amine that promoted a rapid 1-to-1 Michael addition reaction of thiol to acrylate. Kinetic analysis via real-time infrared (RTIR) spectroscopy verified the 1-to-1 addition, the rates of the thio-acrylate Michael reaction and the total incorporation of the thiol into the networks at various concentrations when the amine catalyst was used. Differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) data show very narrow glass transition temperatures for the networks prepared when the amine catalysis/photo-cure sequence was used. The two step sequential process can be used to target network films that can be tailored to have high energy damping properties at a given temperature, e.g., room temperature. Finally, in all cases, whether the photo-cure only or the amine catalysis/photo-cure process was used, the glass transition temperature increased with the initial acrylate feed concentration.

Oxygen transport characteristics, i.e., permeability, diffusivity, and solubility, have been determined for a variety of photopolymerized thiol−ene networks. Despite the abundance of thiol−ene research and the importance of such information, it is until now absent from the literature. One model network, which showed high oxygen barrier properties, was selected for further modification. Covalent bonding of functional groups to the network was enabled by an interesting approach: thio-Michael addition of various functionalized acrylates to a tetrathiol monomer. Cyano, hydroxyl, amide, and alkyl functional groups were explored. These new modified thiols were subsequently copolymerized with an isocyanurate-based multifunctional ene. This modification technique enabled a study on how different functional groups embedded in a uniform network affect Tg and oxygen barrier properties. All studied networks exhibited oxygen diffusivity and permeability which both spanned nearly 3 orders of magnitude. Correlation of b...

The highly uniform and dense network structure of photopolymerized thiol−enes was chemically modified, and the enthalpy relaxation of the networks was measured. n-Alkyl acrylate and hydroxyl acrylate groups were incorporated into thiol−ene networks using a phosphine-catalyzed Michael addition reaction. The effect of flexible alkyl side chains and hydrogen bonding on sub-Tg relaxation was evaluated without sacrificing network uniformity. Overall both the rate and extent of enthalpy relaxation decreased as a function of the flexible n-alkyl chain length, while hydrogen bonding resulted in enhanced enthalpy relaxation. A trithiol−triene−triacrylate ternary system was investigated by correlating enthalpy relaxation and network uniformity. A multifunctional acrylate (TMPTA), being capable of homopolymerization as well as thiol−acrylate copolymerization, was incorporated into a thiol−ene network structure, thereby decreasing the network uniformity and significantly affecting the enthalpy relaxation behavior. In...

N,N-Diethylacrylamide (DEAm) was homopolymerized by reversible addition−fragmentation chain transfer (RAFT) radical polymerization yielding a homopolymer with a calculated degree of polymerization of 30 (PDEAm30), as determined by 1H NMR spectroscopic end-group analysis, and a polydispersity index (Mw/Mn) of 1.10. Aminolysis of the dithioester end groups followed by treatment with tris(carboxyethyl)phosphine hydrochloride yielded the corresponding macromolecular secondary thiol (PDEAm30-SH). Reaction of PDEAm30-SH with a range of commercially available isocyanates, catalyzed by NEt3, gave the corresponding thiocarbamate end-functional polymers in essentially quantitative yield as determined by a combination of 1H NMR spectroscopy and UV−vis spectrophotometry. These reactions were shown to be rapid as evidenced by the real-time kinetics for the reaction between PDEAm30-SH and hexyl isocyanate in the presence of NEt3. Additionally, these facile reactions were shown to proceed without any apparent detrimenta...

Sequential thiol-ene/thiol-ene and thiol-ene/thiol-yne reactions have been used as a facile and quantitative method for modifying end-groups on an N-isopropylacrylamide (NIPAm) homopolymer. A well-defined precursor of polyNIPAm (PNIPAm) was prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization in DMF at 70 °C using the 1-cyano-1-methylethyl dithiobenzoate/2,2′-azobis(2-methylpropionitrile) chain transfer agent/initiator combination yielding a homopolymer with an absolute molecular weight of 5880 and polydispersity index of 1.18. The dithiobenzoate end-groups were modified in a one-pot process via primary amine cleavage followed by phosphine-mediated nucleophilic thiol-ene click reactions with either allyl methacrylate or propargyl acrylate yielding ene and yne terminal PNIPAm homopolymers quantitatively. The ene and yne groups were then modified, quantitatively as determined by 1H NMR spectroscopy, via radical thiol-ene and radical thiol-yne reactions with three representative commercially available thiols yielding the mono and bis end functional NIPAm homopolymers. This is the first time such sequential thiol-ene/thiol-ene and thiol-ene/thiol-yne reactions have been used in polymer synthesis/end-group modification. The lower critical solution temperatures (LCST) were then determined for all PNIPAm homopolymers using a combination of optical measurements and dynamic light scattering. It is shown that the LCST varies depending on the chemical nature of the end-groups with measured values lying in the range 26–35 °C. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3544–3557, 2009

Contemporary mouthguard materials need to perform consistently over a wide range of possible temperatures (-20 to 40 degrees C). Therefore the specific aim of this study was to characterize commercialized mouthguard materials' properties and investigate the effect of temperature on these properties.Five commercially representative thermoplastic mouthguard materials (Essix Resin, Erkoflex, Proform-regular, Proform-laminate, and Polyshok) were tested. The durometer hardness, water absorption, tear strength, and impact attenuation of the mouthguard materials were measured according to ASTM D2240-05, D570-98 (2005), D624-00, and ASTM D6110-06f (modified) guidelines. Tests were conducted on five separate specimens at both room 23+/-2 degrees C and intra-oral 37+/-2 degrees C temperatures. Independent t-tests (alpha=0.05) were used to test for differences between room and intra-oral temperatures.Material hardness decreased (p0.05) from room to intra-oral temperatures for all mouthguard materials. Water absorption increased (p0.05) from room to intra-oral temperatures for all mouthguard materials. Tear strength decreased (p0.05) from room to intra-oral temperatures for all mouthguard materials. Impact attenuation between room and intra-oral temperatures was different (p0.05) for the Erkoflex, Proform-laminate, and Polyshok material respectfully. However, there was no difference between temperatures for the Essix Resin (p=.058) or Proform-regular (p=.275) materials.Temperature measureably affects the physical and mechanical properties of mouthguard materials. It is particularly noteworthy that none of the commercialized products met current ANSI and SAI standards for impact attenuation.

The photopolymerization of four different types of ene monomers with both primary and secondary multifunctional thiols has been evaluated. To understand the effect of ene monomer structures on polymer properties, a comprehensive investigation of the basic physical, mechanical and optical properties was conducted for the secondary and primary thiol–ene networks. The results indicate that ene structure and functionality can significantly affect the rigidity and the physical and mechanical properties of the thiol–ene networks. The heat capacity in the rubber state correlates with the network crosslink density and flexibility of the ene component. Networks formed from the secondary thiol–ene systems are basically equivalent to those made from primary thiol–enes with respect to physical, mechanical and optical properties. The secondary thiol monomer samples evaluated were found to have exceptional storage stability and relatively low odor.

Highly elastic polythiourethanes were synthesized through sequential thiol reactions involving 1,6-hexanedithiol (HDT), 1,4-butanediol diacrylate (BDDA), and several diisocyanates (ISO). Thiol-terminated prepolymers prepared by the phosphine-catalyzed thiol Michael addition of HDT and BDDA form flexible thioether oligomers which were then incorporated as soft segments into polythiourethane main chains through a triethylamine-catalyzed thiol−isocyanate reaction with HDT and ISO to give polymers with both hard and soft segments. Real-time FTIR, used to investigate the kinetic conversion profiles of both reactions, and NMR showed that both the thiol Michael addition and the thiol−isocyanate reactions are very fast and efficient, having the chemical attributes generally associated with thiol−ene radical click reactions. The effects of the soft and hard segment length, soft/hard segment weight ratio, and chemical structure of the ISO on thermal and dynamic thermal mechanical properties was characterized in ter...

In this study vinyl ester monomers were synthesized by an amine catalyzed Michael addition reaction between a multifunctional thiol and the acrylate double bond of vinyl acrylate. The copolymerization behavior of both methacrylate/vinyl ester and acrylate/vinyl ester systems was studied with near-infrared spectroscopy. In acrylate/vinyl ester systems, the acrylate groups polymerize faster than the vinyl ester groups resulting in an overall conversion of 80% for acrylate double bonds in the acrylate/vinyl ester system relative to only 50% in the bulk acrylate system. In the methacrylate/vinyl ester systems, the difference in reactivity is even more pronounced resulting in two distinguishable polymerization regimes, one dominated by methacrylate polymerization and a second dominated by vinyl ester polymerization. A faster polymerization rate and higher overall conversion of the methacrylate double bonds is thus achieved relative to polymerization of the pure methacrylate system. The methacrylate conversion in the methacrylate/vinyl ester system is near 100% compared to only ~60% in the pure methacrylate system. Utilizing hydrophilic vinyl ester and hydrophobic methacrylate monomers, polymerization-induced phase separation is observed. The phase separated domain size is on the order of ~1 μm under the polymerization conditions. The phase separated domains become larger and more distinct with slower polymerization and correspondingly increased time for diffusion.

A series of photoinitiated reactions involving radical chain addition of dithiols across the triple bonds of dialkynes results in quantitative loss of all of the thiol and alkyne groups in 1:1 molar functional group neat mixtures. The reactions proceed rapidly to give uniform networks with relatively narrow differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) glass transition ranges. The glass transitions of the network films are directly related to the network structure and range from below 0 °C to about 40 °C as determined by DMA. The thiol-alkyne based hydrocarbon networks with sulfide linking groups have refractive index values which extend as high as 1.66. There is a linear relationship between the density and the refractive index, both of which increase linearly with weight percent sulfur.

A series of newly developed UV-curable polyurethane coatings were prepared by blending multifunctional thiol- and ene-terminated polyurethane aqueous dispersions. The composition, structure, solution stability and mechanical properties of the title coatings were characterized in detail by FT-IR, photo-DSC and DMA measurements. It was found that the resulting polyurethane coatings showed good solution stability and high photopolymerization activity even after a long time (i.e. 1 month). The incorporation of a waterborne polyurethane chain into the both multifunctional thiols and ene monomers promoted their solution stability and avoided any reaction between thiols and ene groups as a result of their high reacting activity in non-aqueous systems. UV-cured films prepared by this method were found to exhibit excellent physical properties with improvements over what can be attained directly with current UV-curable urethane–acrylate based systems. This method allows for the preparation of high performance UV-curable polyurethane aqueous coatings based on thiol-ene chemistry systems.

The hydrogen bonding behavior of a homologous series of small molecule and polymeric urethanes, thiourethanes, and dithiourethanes was investigated in solution, melt, and solid states. The relative hydrogen bonding strengths in both small molecule and polymer systems were evaluated, and the results were compared to theoretical calculations of hydrogen bonding strength. The results for NMR and FTIR analysis of the small molecule models indicated that the NH protons on the carbamate and thiocarbamates have greater hydrogen bonding strengths than the NH protons of the dithiocarbamate. The polyurethane and polythiourethanes were found to have approximately equivalent physical and mechanical properties as a result of a similar extent of hydrogen bonding, whereas the polydithiourethane, due to a lower degree of hydrogen bonding, has reduced thermal and mechanical transition temperatures as well as lower hardness values. The polythiourethane and polydithiourethane networks exhibit narrower glass transitions comp...

Thermal frontal polymerization is a process in which a localized reactionpropagates through an unstirred system by the coupling of the thermal diffusion andthe Arrhenius kinetics of an exothermic polymerization. A trithiol was found to affectthe front velocity and the time for inducing a front upon exposure to UV light fortrimethylolpropane triacrylate polymerization fronts with either kaolin or calciumcarbonate filler present. The addition of trithiol and filler both decreased the frontvelocity. VV C 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 8091–8096, 2008 Keywords: addition polymerization; frontal polymerization; kinetics (polym.);radical polymerization INTRODUCTION Frontal polymerization is a process in which thereaction propagates directionally through thereaction vessel. To date three main types of fron-tal polymerizations have been reported: thermalfrontal polymerization (TFP), which uses anexternal energy source to initiate the front 1–4 ;photofrontal polymerization in which the localizedreaction is driven by an external UV sourceapplied continuously

Thiol oligomers were copolymerized with a triallyl ether by a photoinduced polymerization process. These oligomeric thiol-ene systems comprise the same duced polymerization process. These oligomeric thiol-ene systems comprise the same components as a photopolymerized thiol-ene-acrylate ternary system, yet the photopolymerized networks have much lower glass transition temperatures. An investigation into the effect of oligomeric thiol design on network formation was conducted by analyzing the reaction kinetics and thermal/mechanical properties of the thiol-ene networks. Real-time FTIR analysis shows that total conversion is >90% for all thiols investigated. Photo-DSC analysis shows that the maximum exotherm rate is roughly equivalent for all of the thiols when the equivalent weight of the thiol is taken into account. As would be expected, the glass transition temperature and tensile strength increase with thiol functionality and lower thiol equivalent weight for thiols with functionality from 2 to 4. Films made using the oligomeric thiols have essentially the same glass transition temperatures and tensile modulus values regardless of thiol design. These results distinguish the method for generation of networks consisting of an initial Michael reaction of thiols and acrylates followed by a photoinitiated copolymerization with a multifunctional ene from the traditional photolysis of the corresponding thiol-ene-acrylate ternary systems with no Michael reaction.

Physical aging behavior of photopolymerized thiol−ene networks was investigated by measuring the extent of enthalpy relaxation in terms of network density and molecular structure. The homogeneous network structure of the thiol−enes, having narrow glass transition temperature ranges, showed characteristic temperature and time dependency relationships for enthalpy relaxation. All thiol−ene films annealed at different temperatures (Ta) for 1 h according to the isochronal method showed maximum enthalpy relaxation peaks at approximately Tg − 10 °C by DSC. The extent of enthalpy relaxation as a function of annealing time (ta) was obtained by the isothermal aging method. Correlations between the extent of enthalpy relaxation and the heat capacity difference at Tg were made and related to thiol−ene chemical group rigidity and network linking density. Pendulum hardness values for a selected thiol−ene film showed a clear change in hardness upon aging, indicating sub-Tg mechanical relaxation, consistent with the rel...

Alkyl thiols and alkenes (enes) polymerize via an extremely rapid step-growth, free-radical chain process, uninhibited by air, to give high-density networks with excellent mechanical and physical properties. These thiol−ene coatings are potentially useful for a wide variety of coatings, adhesives, and optical applications. In this work, a series of nanogold-containing UV-cured, thiol−ene coatings were prepared from trimethylolpropane tris(3-mercaptopropionate) (trithiol) and pentaerythritol allyl ether (triene) monomers using a unique procedure which facilitates precomplexation of the gold−thiol prior to photocuring. Irgacure 651 (1 wt %) was used as a photoinitiator, and nanogold was incorporated at 0−1 wt %, average ∼10 nm size particles by TEM. Physical and mechanical properties were characterized using bulk tack analysis and other standard techniques: DSC, TGA, pencil hardness, and gel fractions. In general, films were found to be low absorbing in the visible range and highly uniform and to contain we...

Several aspects of the photochemistry and photophysics of four main chain liquid crystalline polyesters with a rigid trans-stilbene 4,4'-dicarboxylate mesogen as chromophore and flexible spacer groups are reported. The three polymers with the longest 'spacer' groups are liquid crystalline at room temperature, two have smectic phases. Chromophore aggregation has a dramatic effect on the photophysics and photochemistry of these polymers. Each of the polymers in poor solvents or as films has greatly perturbed UV-Vis absorption and fluorescence spectra due to aggregation of the stilbene chromophore. These effects are more pronounced upon annealing above the glass transition temperature, T(g), and in the mesophase. Film fluorescence is excitation wavelength dependent and is suppressed at elevated temperatures. The stilbene 'environment' in both films and solution is clearly heterogeneous and energy transfer processes relatively slow. The dominant photochemical reaction upon direct excitation above 300 nm is 2 + 2 photocycloaddition rendering polymer films insoluble. No significant trans-to-cis photoisomerization can be detected upon initial irradiation of the polymer films. There is evidence for the formation of aldehyde and carboxylate functionality upon irradiation in the presence of air. Loss of the aggregate UV-Vis absorption and fluorescence occurs during irradiation. Difference UV-Vis spectra of irradiated films suggest preferential initial consumption of dimeric aggregates. Loss of stilbene UV-Vis absorption upon irradiation above 300 nm can be partly photoreversed upon subsequent 254 nm irradiation. The rate of stilbene chromophore loss from films increased significantly above Tg and in the smectic phase above room temperature.

The kinetics and thermal/physical properties of the trithiol-TAE (triallyl ether) system were measured with respect to increasing polyoligomeric silsesquioxane (POSS) concentrations in order to understand how the presence of POSS nanoparticles affects network formation at low loadings. Vinyl POSS monomer (vPOSS-Bu4) with both vinyl and carboxylate pendant groups was synthesized via a thermally initiated, free-radical reaction to improve the compatibility of the inorganic particles with the trithiol and triallyl ether comomoners. Chemically modified vPOSS-Bu4 particles were incorporated into the trithiol-TAE polymer networks by a thiol-ene free-radical photopolymerization at molar concentrations of 0, 1, and 5 ene mol%. The polymerization rates were analyzed using real-time FTIR and photo-DSC. The polymerization rates showed no significant changes with increasing vPOSS-Bu4 concentration. Thermal analyses of the films by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) demonstrated that thermal stability improves without affecting T g as the POSS concentration increased. Additionally, scratch resistance increased and flame spread decreased markedly with increasing POSS concentration for concentrations up to 5 mol% vPOSS-Bu4.

Polymerizable ionic liquids were synthesized from the neutralization reac- tion between tributylmethylammonium hydroxide and methacrylic or acrylic acid, and their photopolymerization kinetics have been determined. The acrylate monomer polymerization rate exhibited a profound dependence on the water content as follows: the monomer viscosity was a strong function of the water content, increasing sub- stantially as the water content increased from less than 5 to 30%. For the tributyl- methylammonium acrylate with less than 5% water, the viscosity was 2000 times greater than that of butyl acrylate. The high viscosity is proposed to reduce both the propagation and termination steps. Both monomers exhibited an increase in rate with temperature. V

The adhesive properties, as measured by bulk tack and peel strength analysis, were found to decrease in polystyrene-block-polybutadiene-block-polystyrene (SBS) and polystyrene-block-polyisoprene-block-polystyrene (SIS) PSA films containing common singlet oxygen generators, acridine, rose bengal, and C60 fullerene, when irradiated with a tungsten halogen light in air. The addition of the singlet oxygen quencher, β-carotene, to the C60 fullerene samples was found to significantly deter the rate of adhesive loss in the fullerene-SBS and -SIS PSA nanocomposites. The presence of oxygen was essential to the mechanism of adhesive loss and, in combination with the effects of singlet oxygen generators and a singlet oxygen scavenger, strongly supports a singlet-oxygen mediated process. FTIR investigations of fullerene-SBS and -SIS systems suggest the initial formation of peroxides which, upon further irradiation, lead to the generation of carbonyl-containing compounds of a ketonic type after crosslinking. Rates of SBS and SIS C-H abstraction were comparable and found to decrease when the high-pressure, mercury xenon irradiation source was filtered to allow only light of λ > 390 nm. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

The photopolymerization kinetics of mixtures containing a trithiol and a trivinyl ether (in different molar ratios) with a cationic photoinitiator were investigated by real-time infrared and real-time rheology. Using this combination of real-time methods to follow both chemical conversion and rheological property development, a clear picture of physical property development during the complete polymerization process is obtained. This represents the first example of a thiol−ene radical/ene cationic two-step hybrid photopolymerization process in which thiol copolymerizes with vinyl ether functional groups in a rapid radical step growth process followed by vinyl ether cationic homopolymerization. The sequential thiol−vinyl ether copolymerization and the vinyl ether cationic polymerization result in cross-linked networks with thermal and mechanical properties that are combinations of each system.

A complete characterization of the electron density distribution of divinyl fumarate and its effect on various properties has been performed by using a combination of UV−vis spectroscopy, cyclic voltammetry, theoretical calculations, and a diagnostic Michael addition reaction involving an aliphatic thiol and the fumarate carbon−carbon double bond. The results show that the presence of the conjugation between the two vinyl ester double bonds and the fumarate carbon−carbon double bond significantly changes the electron density in both; that is, the vinyl ester double bonds of divinyl fumarate are more electron rich and the fumarate double bonds are more electron poor compared to nonconjugated analogues. This electron density distribution greatly influences the copolymerization behavior of divinyl fumarate. Divinyl fumarate also acts as both a monomer and photoinitiator in the photopolymerization of 1,6-hexanediol diacrylate. Because of the larger electron density deficiency of the fumarate group on divinyl ...

Model ternary thiol−ene/acrylate photopolymerization involving acrylate homopolymerization and copolymerization of thiol−ene and thiol−acrylate monomers were monitored by real-time FTIR. In all ternary mixtures, including those prepared with different acrylate concentrations, acrylate conversion was 100%. However, thiol−ene conversions were found to be controlled by their initial concentrations. The influence of acrylate monomer chemical structure on the thermophysical properties of ternary thiol−ene/acrylate systems was studied with DMA, DSC, and the absorbance of a nondestructive impact energy. The addition of acrylate to the thiol−ene system increased the rubbery modulus while the tan δmax shifted to higher temperatures. Densely cross-linked, heterogeneous matrix formation was observed with the broadening of tan δ peaks at high acrylate concentrations. The high impact absorption of these ternary thermoset photopolymers was correlated with the dynamic mechanical damping ability of the networks. Acrylate...

Photocurable, ternary-component mixtures of a 1:1 molar multifunctional thiol–ene (trithiol and triallyl ether) blend and a 16-functional acrylate based monomer have been photopolymerized, and the final film properties of the ternary crosslinked networks have been measured. The photopolymerization kinetics, morphology, and mechanical and physical properties of the films have been investigated with real-time infrared, atomic force microscopy, and dynamic mechanical analysis. The photopolymerization process is a combination of acrylate homopolymerization and copolymerizations of thiol with allyl ether and acrylate functionalities. The tan δ peaks of the photopolymerized ternary systems are relatively narrow and tunable over a large temperature range. The morphology is characterized by a distinct phase-separated nanostructure. The photocured thiol–ene/acrylate ternary systems can be made to exhibit good mechanical properties with enhanced energy absorption at room temperature by the appropriate selection of each component concentration. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 822–829, 2007.

Thiourethane-based thiol-ene (TUTE) films were prepared from diisocyanates, tetrafunctional thiols and trienes. The incorporation of thiourethane linkages into the thiol-ene networks results in TUTE films with high glass transition temperatures. Increases of Tg were achieved by aging at room temperature and annealing the UV cured films at 85 °C. The aged/annealed film with thiol prepared from isophorone diisocyanate and cured with a 10,080-mJ/cm2 radiant exposure had the highest DMA-based glass transition temperature (108 °C) and a tan δ peak with a full width at half maximum (FWHM) of 22 °C, indicating a very uniform matrix structure. All of the initially prepared TUTE films exhibited good physical and mechanical properties based on pencil hardness, pendulum hardness, impact, and bending tests. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5103–5111, 2007

Hydrophobic polymer films, having an aqueous phase distributed throughout the matrix, were formed by polymerizing a solution of dodecyl acrylate and 1,6 hexanediol diacrylate containing nanometer-sized drops of water stabilized by sodium bis(2-ethylhexyl) sulfosuccinate (AOT). Photopolymerization-induced aggregation of the water drops and/or phase separation occurred, as the initially clear solutions became opaque films. The polymerized films became clear, as the relative humidity was reduced. Clear films still contained 20–50% of the initial water. The transition from opaque to clear films was reversible provided that the film did not become completely dry and form cracks. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Polymerizable ionic liquids were synthesized from the neutralization reaction between trialkylamines (tributylamine, trihexylamine, trioctylamine, 2-(dimethyl-amino)ethyl acrylate, and 2-(dimethylamino)ethyl methacrylate) and acid-containing (meth)acrylates to study the kinetics of their photopolymerization. The ionic liquids formed from acrylic acid and methacrylic acid with trialkylamines showed low or moderate increases in rate compared to their monofunctional analogues. The ionic liquids formed from (meth)acrylic acid and a tertiary amine with a pendant acrylate exhibited rates typical of traditional di(meth)acrylates, although the (meth)acrylates were not covalently attached to the same molecule. All of the phosphate-based ionic liquids exhibited very rapid polymerization rates and strong dependence on the initial viscosity. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3009–3021, 2007

Three component photoinitiator systems containing N-substituted maleimide/ketocoumarin/tertiary amine have been used for the visible light photopolymerization of acrylate and thiol-ene monomers. Thin-film calorimetry studies were conducted. The polymerization exotherms of these systems with the blue (470 nm) and cyan (505 nm) LED light sources show that the multicomponent initiator package is an effective system for visible light polymerization of acrylate and thiol-ene monomers. Exotherms of a visible light initiator combination of camphorquinone/amine were recorded for comparison purposes.

The relationship between the polymerization rate of various acrylates and methacrylates containing acid groups and their ability to hydrogen bond has been investigated by real-time infrared spectroscopy. The polymerization rate dependence on temperature of the acid-based acrylates and methacrylates is reduced compared to traditional (meth)acrylates with no groups capable of hydrogen bonding. For two of the acid containing monomers, polymerization rates actually decreased with an increase in temperature.

The physical and mechanical properties of several thiol–ene based polymers and their mixtures with the liquid crystal, E7, were characterized to probe their relationship with the liquid crystal film electro-optic performance properties. Kinetic data suggests that high conversion is achieved for each thiol–ene combination. Pre-polymerization phase diagrams indicate that each thiol–ene/E7 mixture phase separates well below room temperature, and thus prior to polymerization at room temperature all are in a single phase. Holographic polymer dispersed liquid crystals (HPDLC) were fabricated for several thiol–ene and E7 mixtures, and electro-optical parameters characterized to probe the relationship between the thiol–ene network properties and the electro-optic performance of the HPDLCs. The photocured matrices exhibited glass transitions and tan δ peak maxima that ranged from temperatures below 0 °C to well above room temperature. There is a clear correlation between the physical nature of the matrix and the electro-optic switching parameters with H-PDLC films fabricated from trithiol-pentaerythritol triallylether, all of which exhibit glass transition temperatures below 0 °C, having the fastest switching times and lowest switching voltages at room temperature. Also, in each case higher liquid crystalline concentration resulted in lower switching voltages.

A thin-film calorimeter (TFC) was used to provide quantitative characterization of photopolymerizable systems. Photopolymerization exotherms measured using the TFC are compared with real-time infrared (RTIR) spectroscopic results. The TFC successfully monitored the in-situ polymerization kinetics of thin/thick films because of its increased sensitivity/resolution over traditional calorimetric instrumentation. Reproducible polymerization exotherms of trimethylolpropane triacrylate were measured on films as thin as 163 nm. Flexibility in the sample cell construction enables the characterization of both volatile reactants (closed cell) and the effect of oxygen inhibition (open cell). Several molecular systems were evaluated. First, exotherms for the reaction of a series of volatile hexene monomers with monofunctional thiol illustrate the effect of ene location on alkene reactivity. Second, vinyl acrylate, a volatile difunctional monomer with a unique polymerization mechanism, was shown to homopolymerize sign...

Poly(ethylene terephthalate) (PET) copolymers containing 4,4′-, 3,5-, and 2,4-benzophenone dicarboxylate chromophores have been synthesized by transesterification of PET with benzophenone 4,4′-dicarboxylic acid (4,4′-BDA), dimethyl benzophenone 4,4′-dicarboxylate (4,4′-BDE), dimethyl benzophenone 3,5-dicarboxylate (3,5-BDE) and dimethyl benzophenone 2,4-dicarboxylate (2,4-BDE). The benzophenone segments in the backbone induce photocrosslinking upon UV irradiation in the solid state most probably by a hydrogen atom abstraction mechanism. The crosslinking rate depends upon the concentration and the structure of chromophores as evidenced by gel content measurements. The photocrosslinking efficiency of 4,4′-benzophenone dicarboxylate containing polymers is higher than for 2,4- or 3,5-benzophenone dicarboxylate containing polymers. Photocrosslinked PET copolymers show increased glass transition temperatures and broadening of melting transitions.

Photoinduced frontal polymerization was performed on a variety of multifunctional (meth)acrylates, and evaluation of the effect of monomer structure on the ability to sustain a traveling front was made. Photo-DSC was used to measure the photopolymerization rate of each monomer at 25 °C. The results were correlated with their ability to initiate and sustain a traveling front polymerization. The start time and the front velocity of each (meth)acrylate that could initiate/sustain a front photolytically were measured. The results clearly demonstrate that the molecular weight per double bond is directly related to the front velocity. Trimethylolpropane triacrylate (TMPTA), which exhibited the fastest front velocity, was evaluated in depth to determine the effect of varying the photoinitiator and thermal initiator on the frontal polymerization rate. When a multifunctional thiol was added to TMPTA, the start time was dramatically reduced and the velocity increased for low thiol concentrations. However, the veloc...