Photopolymerization Of Monomers Research Articles

Understanding multivinyl monomer photopolymerization kinetics through modeling and GPC investigation of degradable networks

Multivinyl monomers that react to form highly crosslinked, biodegradable networks are being developed as scaffolds for tissue engineering and vehicles for drug delivery; however, this work demonstrates their usefulness in characterizing better the complexities of the kinetics and structural evolution during crosslinking photopolymerization. The molecular weight distributions (MWDs) of the degradation products of networks formed through the free radical photopolymerization of multivinyl monomers validate a novel kinetic model to test hypotheses as to the important kinetic mechanisms during crosslinking. The kinetic model, in conjunction with the experimental results for the degradable network, provides insight into the fundamental termination mechanisms (i.e. chain length dependent termination (CLDT), chain transfer to either a unimolecular species or polymer, and the accumulation of persistent radicals) that control the MWD of the backbone kinetic chains throughout the polymerization. Specifically, the importance of CLDT during autoacceleration and the impact of light intensity on the MWD of the backbone kinetic chains are presented.

A Modeling Investigation of Chain Length Dependent Termination during Multivinyl Free Radical Chain Photopolymerizations: Accounting for the Gel

A mathematical model of multivinyl monomer free radical photopolymerizations that takes into account chain length dependent termination (CLDT) and gel formation (an infinite network) was utilized to obtain a more accurate picture of network formation and multivinyl monomer photopolymerization kinetics. The model predicts an increased probability that a radical chain is attached to the gel with increasing chain length and double bond conversion. Accounting for CLDT delays radical incorporation into the gel and induces gel formation at earlier double bond conversions. Increasing the initiation rate also delays gel formation. When the model is expanded to account for the probability that a highly mobile sol radical reacts to become a nearly immobile gel radical, the importance of CLDT prior to reaction diffusion-controlled termination is increased. This result is attributed to bimolecular termination occurring between a short (<10-mer) sol radical and a long gel radical (short−long radical termination). The transition from a CLDT important regime to reaction diffusion-controlled termination is characterized by a transition from primarily short−long to primarily long−long (gel−gel) radical termination.

Thermal Properties and Photo-Polymerization of Diepoxy Monomers with Mesogenic Group

ABSTRACT We synthesized two diepoxy monomers containing a mesogenic group to investigate thermal conductivity of the resulting epoxy resins prepared by photo-polymerization of the monomers. One diepoxy monomer shows an enantiotropic nematic phase, whereas the other displayed enantiotropic smectic A and nematic phases. The epoxy resins were prepared by cationic photo-polymerization of the diepoxy monomers in the liquid-crystalline (LC) state. The structures of LC phases for each diepoxy monomer were fixed in the thin films of epoxy resins. Thermal conductivity of the LC epoxy resins was larger than that of the conventional epoxy resins. The epoxy resin prepared by photo-polymerization of the monomer in the smectic A phase displayed larger thermal conductivity than that in the nematic phase.

The impact of water on photopolymerization kinetics of methacrylate/vinyl ether hybrid systems

AbstractReal‐time FT‐near infrared (NIR) spectroscopy was used to monitor individual monomer photopolymerization kinetics within the hybrid methacrylate/vinyl ether system composed of 2‐hydroxyethyl methacrylate and tri(ethylene glycol) methyl vinyl ether. Photoinitiation conditions have been manipulated to provide staged polymer formation of the selected hybrid systems. Significant differences were observed in the effect of water on hybrid systems with different polymerization sequence. The radical/cationic dual initiator system drastically decreased the effect of water on the vinyl ether polymerization. Even with significant water present, both the methacrylate and vinyl ether monomers can be effectively polymerized under certain initiation conditions. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Synthesis of Activated Monomers for Cationic Photopolymerization

A series of novel cationically photopolymerizable epoxide and vinyl ether monomers bearing substituted benzyl ether groups were prepared. Monomers with one methoxy, methylenedioxy, and three methoxy groups on the aromatic ring of the benzyl ether group were synthesized by straightforward methods. Kinetic studies using real‐time infrared spectroscopy of the cationic photopolymerizations of these novel monomers showed that those monomers with one and two electron‐donating substituents exhibit elevated rates of photopolymerization. A mechanism has been proposed involving a redox interaction between the monomer and the photoinitiator to account for the observed results. Photoinduced chain decomposition of the photoinitiator gives rise to photoamplification effect that generates a large number of carbocation species that initiate polymerization.

Fast response of the diffraction efficiency in photopolymer prepared by sol–gel process

Abstract New photopolymers were designed and prepared using the functionalized triethoxysilane via sol–gel process. They were based on organic–inorganic hybrid materials containing photopolymerizable monomers, photoinitiator, and photosensitizer that is sensitive to the visible light wavelength. The photopolymerization of the diffused monomer in the polymer binder was studied by virtue of infrared spectroscopy. The holographic gratings were successfully fabricated in these sol–gel processed photopolymer samples by conventional optical interference method. We also investigated the effect of sol–gel processed binder on the rate of diffraction behavior. The dynamic behavior of the sol–gel processed photopolymer (SG-PP) was compared with that of the photopolymer (Cell-PP) with the cellulose ester binder.

Development of New Direct Laser Printing System

A new photo- and heat-sensitive printing system has been developed. Full-colored and high-quality images can be printed quickly and easily by using this system. Images are printed by dry process using only one sheet of sensitive paper. No other materials are needed thus resulting in no waste. The noteworthy features of the newly developed sensitive paper are the chromogenic development using heat-sensitive microcapsules, the spectral sensitization by unique sensitizing dyes and borates, the photopolymerization of monomers functioning as color developers, and the bleaching of sensitizing dyes during fixing process. By laser irradiation, developers polymerized and lose ability to move into the capsules. In the unexposed area, however, developers are able to move into the capsules and change the precursors to dyes.

The effect of fiber orientation on the toughening of short fiber‐reinforced polymers

AbstractThe effect of fiber orientation on the toughening of polymers by short glass fibers generally below their critical length was investigated using specimens with either well‐aligned or randomly oriented fibers. The fibers were aligned by an electric field in a photopolymerizable monomer, which was polymerized while the field was still being applied. These materials were fractured with the aligned fibers in three orientations with respect to the crack plane and propagation direction. Specimens with fibers aligned normal to the fracture plane were the most tough, those with randomly oriented fibers were less tough, and those with fibers aligned within the fracture plane were the least tough. The fracture behaviors compared favorably with predictions based on observed processes accounting for fiber orientation. The processes considered were fiber pull‐out (including snubbing), fiber breakage, fiber–matrix debonding, and localized matrix‐yielding adjacent to fibers bridging the fracture plane. Fibers not quite perpendicular to the fracture plane provided the greatest toughening; these fibers pulled out completely and gave a significant contribution from snubbing. Fibers at higher angles provided less toughening, involving nearly equal contributions from pull‐out, breakage, and debonding. Fibers within the fracture plane provided the least toughening, involving debonding alone. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2740–2751, 2003

Kinetics studies of hybrid structure formation by controlled photopolymerization

Real-time FT-NIR spectroscopy was used to monitor the individual monomer photopolymerization kinetics within the hybrid methacrylate/vinyl ether system composed of 2-phenoxyethyl methacrylate and tri(ethylene glycol) methyl vinyl ether. Photopolymerization processing conditions, such as light intensity, photoinitiator type (both free radical and cationic) and initiator ratios and concentrations, that provide preferential direction of polymer formation based on individual monomer photopolymerization kinetics and overall conversion have been evaluated. Single source UV-light irradiation was employed to produce either single or dual-stage hybrid polymerization, validating the potential of one-step, one-pot methodology for initiating stage-curable polymerizations.

Rigid-particle toughening of glassy polymers

The nature of the toughening of glassy polymers by rigid particles was investigated by using aligned assemblages of spherical glass particles. The particles were aligned by an electric field in a photopolymerizable monomer, which was polymerized while the field was still being applied. These materials were fractured with the aligned particle strings in three orientations with respect to the crack plane and propagation direction. The fracture toughness and fracture energy of these and of random arrangements of particles (formed without the electric field) were all higher than of the matrix alone. The increases were compared with predictions from processes on or in the immediate vicinity of the fracture plane and from those away from the fracture plane. The increases were inconsistent with those predicted by on-fracture plane processes as represented by crack pinning and bowing. But the increases did correlate with the size of the process zones, which could extend more than 100 μm away from the fracture plane. Detailed calculations showed that the increase in fracture energy arose almost completely from off-fracture plane processes of particle-matrix debonding and the accompanying local inelastic deformation of the matrix around the particles.

Investigation into the ultrasonic setting of glass ionomer cements Part I Postulated modalities

Glass ionomer cements (GICs) are formed by the reaction of an ion leachable alumino-silicate glass with an aqueous solution of poly (alkenoic acid). Water is used as the reaction medium [1]. An acid-base reaction occurs, whereby the acid attacks and degrades the glass structure, releasing metal cations which are then chelated by the carboxylate groups and serve to crosslink the polyacid chains. The cement consists of residual glass particles embedded in a hydrogel polysalt matrix [2]. The setting reaction in GICs is a continuous process evident by the increase in compressive strength of the cement with ageing time [3–5]. GICs undergo a two-step setting reaction. During the first step the material is susceptible to water uptake and during the second it is susceptible to dehydration. For example, when GICs are stored in water after an initial set of 15 min, a surface softening occurs, which may be caused by an inhibition of the setting reaction in a superficial layer of the cement [6]. This short-term relationship with water restricts the full potential of GICs for healthcare applications. It is for this reason that resin modified GICs (RMGICs) were developed. These materials, which are conventional GICs to which an organic, photo-polymerizable monomer has been incorporated [7], can be command set by the application of an intense light source. However, RMGICs have recognized drawbacks, related to the presence of both non-polymerized monomer and the resin itself [8, 9]. The authors have already shown that conventional GICs can be command set by ultrasonic excitation [10]. Ultrasound not only imparts an instant set to a GIC, but also imparts superior mechanical properties when compared to its chemically cured counterpart, particularly within the first 24 h after setting. The ultrasonically cured GIC does not require the incorporation of additional chemicals and therefore avoids the drawbacks associated with RMGICs. In the previous study [10], nano-indentation testing was used to show that ultrasonically cured Fuji IX had hardness an order of magnitude greater than its chemically cured counterpart (Table I), while also

DISTINCT DYNAMIC REGIMES IN A MATERIAL SYSTEM WITH OPTICALLY MODULATED DIFFUSIVITY

We report on distinct dynamic regimes of optically induced redistribution of molecules in a reacting material. The physical system is a mixture of photopolymerizable monomer and nematic liquid crystal with space-time modulated diffusivity. An empirical model is developed which provides an excellent description of all the stages of the experimentally observed highly intricate dynamics. This dynamics is governed by spatially inhomogeneous photogeneration of radicals, their diffusion, and the photopolymerization-induced change in the diffusion constant.

Synthesis of epoxy monomers that undergo synergistic photopolymerization by a radical‐induced cationic mechanism

AbstractA series of novel, cycloaliphatic, cationically photopolymerizable epoxide monomers bearing benzyl ether groups were prepared. These monomers display a considerable enhancement in the rate of their cationic ring‐opening polymerizations in comparison with monomers that do not contain such groups. In this article, a synergistic free‐radical mechanism is proposed that accounts for this effect, and supporting evidence is offered for its verification. During UV irradiation of an onium salt cationic photoinitiator, the aryl radicals that are generated abstract labile benzyl hydrogens present in such monomers to generate the corresponding carbon‐centered radicals. Subsequently, these radicals are oxidized to benzyl carbocations by the onium salt via a nonphotochemical chain process. The observed increase in the rate and extent of the cationic ring‐opening polymerization of the epoxide monomers is due to the aforementioned mechanism, which effectively increases the number of reactive cationic species present during polymerization. © 2001 John Wiley &amp; Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3578–3592, 2001

Effect of the monomer and filler systems on the remineralizing potential of bioactive dental composites based on amorphous calcium phosphate

AbstractAmorphous calcium phosphate (ACP), because of its high solubility and its rapid transformation to hydroxyapatite (HAP) in aqueous environments, has been utilized as the filler phase in the preparation of bioactive polymer‐based composites that have remineralization potential. The goal of this study was to evaluate the effects of chemical structure and compositional variations of seven photopolymerizable matrix monomer systems on the long‐term release of calcium and phosphate ions, i.e., their remineralizing potential, when used with three types of ACP filler systems: unhybridized ACP (u‐ACP), silica‐hybridized ACP (Si‐ACP), and zirconia‐hybridized ACP (Zr‐ACP) composites. The monomer systems evaluated were: 2,2‐bis[p‐(2′‐hydroxy‐3′‐methacryloxypropoxy)phenyl]‐propane (Bis‐GMA)/triethyleneglycol dimethacrylate (TEGDMA) [BT resin], Bis‐GMA/TEGDMA/­2‐hydroxyethyl methacrylate (HEMA) [BTH resin],­Bis‐GMA/TEGDMA/HEMA/zirconyl methacrylate (ZrMA) [BTHZ resin], TEGDMA/pyromellitic glycerol dimethacrylate (PMGDMA) [TP resin], a urethane dimethacrylate, [U], and two HEMA‐modified U resins with the mass fraction of 6.6% and 13.2% HEMA, ([U66H] and [U132H], respectively). All the visible light polymerizable composites consisted of 60% resin and 40% ACP filler on a mass fraction basis. It was shown that ion release from the immersed composites was affected by both the chemical structure and composition of the monomer system as well as the type of filler system. Whereas BT, U and U66H resins formulated with u‐ACP gave low to moderate levels of ion release, the resins with elevated amounts of HEMA (BTH, BTHZ and U132H) gave high and sustainable release of mineral ions. The remineralizing capacity of BT, U, U66H and U132H composites generally increased when hybridized fillers were employed instead of u‐ACP. Utilization of hybrid ACPs had, however, no effect on ion release from BTHZ composites. The TP composites initially gave high ion release, but it was not sustainable because of significant Ca‐binding by the matrix due to the high content of carboxylic acid groups derived from the PMGDMA comonomer. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Design and synthesis of highly reactive photopolymerizable epoxy monomers

AbstractThe synthesis of a series of novel cationically photopolymerizable epoxide monomers bearing benzyl, allyl, and propargyl acetal and ether groups that can stabilize free radicals was carried out. These monomers display enhanced reactivity in cationic photopolymerization in the presence of certain onium salt photoinitiators. Specifically, this article describes schemes for the synthesis of cycloaliphatic epoxy monomers bearing free‐radical stabilizing groups. During UV irradiation of an onium salt cationic photoinitiator, the aryl radicals that are generated abstract labile protons present in such monomers to generate the corresponding carbon‐centered radicals. Subsequently, these radicals can interact with the onium salt by a redox mechanism to induce the decomposition of these salts. The overall result is that additional cationic species are generated by this mechanism that increase the rate and extent of the cationic ring‐opening polymerization of the epoxide monomer. An investigation of the photopolymerizations of the monomers prepared during this work was carried out using Fourier transform real‐time infrared spectroscopy, and conclusions were drawn with respect to the relationship between their structures and reactivity. © 2001 John Wiley &amp; Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2385–2395, 2001

Crystallization of poly(ethylene oxide) in a photopolymerizable monomer under an electric field

The miscibility behavior and crystallization of poly(ethylene oxide) (PEO) from two non-polar solvents when subjected to a high electric field were studied. The two solvents, both photopolymerizable, were urethane dimethacrylate (UDMA) and 1,6-hexanediol dimethacrylate (HDDMA). The electric field was found to decrease miscibility in at least HDDMA, causing liquid–liquid phase separation at temperatures above the PEO melting temperature and well above the dissolution temperature. Cooling the solutions below 45°C under the field caused the irregular spherulites forming in HDDMA (a low-viscosity solvent) to align in groups and, above 0.2kV/mm, to elongate. Lamellae within the spherulites tended to align with their planes in the field direction. Only branched lamellae formed in UDMA (a high-viscosity solvent), with the longest lamellae in the field direction and the few branches at large angles to these. For both solvents, crystallization was retarded slightly by the field, as was the total amount of crystallinity in HDDMA, and possibly also in UDMA. PEO crystallinity in UDMA was much less than that in HDDMA, with or without the field, presumably because of viscosity.

Epoxy Resin−Photopolymer Composites for Volume Holography

Efficient materials for recording volume holograms are described that could potentially find application in archival data storage. These materials are prepared by mixing photopolymerizable vinyl monomers with a liquid epoxy resin and an amine hardener. A solid matrix is formed in situ as the epoxy cures at room temperature. The unreacted vinyl monomers are subsequently photopolymerized during hologram recording. A key feature of these materials is the separation of the epoxy and vinyl polymerizations. This separation allows for a large index contrast to be developed in holograms when components are optimized. The standard material described in this work consists of a low index matrix (n ≅ 1.46), comprised of diethylenetriamine and 1,4-butanediol diglycidyl ether, and a high index photopolymer mixture (n ≅ 1.60) of N-vinylcarbazole and N-vinyl-2-pyrrolidinone. This material is functional in thick formats (several millimeters), which enables narrow angular bandwidth and high diffraction efficiency. A dynami...

Disordered mesoporous silicates formed by templation of a liquid crystal (L3)

ABSTRACTFor a wide range of technological applications the need for optically transparent, monolithic, mesoporous silicates is readily apparent. Potential areas of utility include filtration, catalysis, and optoelectronics among many others. This laboratory has previously reported on the synthesis of such materials thatare formed through the addition of tetramethoxysilane to a liquid crystal solution of hexanol, cetylpyridinium chloride, and 0.2 M hydrochloric acid, and our investigation into the properties of these materials is a continuing process. We have achieved defect and fracture free material of suitable size (0.5 cm × 3 cm diameter disks) via supercritical drying of the silicate under ethanol or CO2. The dried materials are remarkably similar to ordinary glass in strength, texture, and clarity. They possess pore volumes of ca. 1.0 cm3/g, with BET surface areas &gt;1000 m2/g. We can re-infiltrate thedried monolith with hydroxyethylacrylate, a photo-polymerizable monomer, to create an inorganic/organic nanocomposite. There is fracturing upon re-infiltration, but preliminary tests show that the polymerization proceeds despite the mechanical failure. These findings suggest many possible applications for these unique nanocomposites.

Freeform Fabrication of Functional Silicon Nitride Components by Direct Photo Shaping

AbstractThis paper describes a new multilayer solid freeform fabrication process, “Direct Photo Shaping” (DPS), where visible digital light projection is used as a maskless tool to build images on photocurable ceramic dispersions (ceramic powders in photopolymerizable liquid monomers) by flood exposure. For each layer, the projected image is changed according to the CAD data describing the object being built and solidification takes place by photocuring the exposed areas. Multiple layers are dispensed and photocured to fabricate the object of interest. A final rinse with a suitable solvent allows the removal of any uncured ceramic dispersion. The porous free formed “green” ceramic object can then be fired and sintered into a highly dense ceramic part. Digital Light Processing™ technology (developed by Texas Instruments) enables SRI International to project digital, high resolution, high brightness, high contrast visible light to photocure and form components with a good degree of accuracy. This paper describes the Direct Photo Shaping process and its advantages, and how DPS is being applied to the fabrication of ceramic (Honeywell AS800) gas turbine components for military and commercial applications. ASS00 test specimens with flexural strength in excess of 800MPa were fabricated by DPS. A first-stage AS800 turbine vane for the Rolls-Royce Allison Model 501-K industrial gas turbine was fabricated by DPS and tested in a gas-burner test rig at 1204°C. Initial tests show that ceramic samples with optimized surface finish (comparable to that achieved by ceramic bisque machining) can be fabricated by applying a pixel anti-aliasing filter along the boundaries of the sample projected slice images.

The synthesis and cationic photopolymerization of monomers based on dicyclopentadiene

Several new epoxide monomers based on dicyclopentadiene (DCPD) were prepared using straightforward reaction chemistry. Those monomer-bearing groups in addition to the epoxy moiety, which can stabilize free radicals, display a pronounced acceleration of the rate of cationic ring-opening polymerization in the presence of diaryliodonium salt photoinitiators. Mechanistic studies conducted with the aid of model compounds have shown that the apparent rate acceleration is due to the free radical chain-induced decomposition of the photoinitiator. One of the chain carriers in this reaction involves a monomer-derived free radical. Also prepared was dicyclopentadiene monomer (V) bearing polymerizable epoxide and 1-propenyl ether groups in the same molecule. The functional groups in V appear to undergo independent vinyl and epoxide ring-opening polymerization.