Kinetics studies of hybrid structure formation by controlled photopolymerization

The Preparation of Acryloxyl Group Functionalized Siloxane Polymers and the Study of Their Ultra Violet Curing Properties

Polysiloxane with multiple acryloxyl groups at the terminal site of the polymer chain was synthesized by the condensation reaction between hydroxyl-terminated polysiloxane and acryloyl chloride and used to improve the cross-linking density of UV-curable silicone materials initiated from dual acryloxy-terminated symmetric polysiloxane or single acryloxy-terminated asymmetric polysiloxane with the mixture of Irgacure 1173 and Irgacure 184 at a mass ratio of 1:1 as the photoinitiator. The effects of factors such as initiator composition, UV irradiation time, structure, and molecular weight of linear dual acryloxy-terminated or single acryloxy-terminated asymmetric siloxane oligomers on the gelation yield, thermal properties, water absorption, and water contact angle of UV-cured film were investigated. The synthesized cross-linking density modifier can be copolymerized with acryloxy-functionalized linear polysiloxanes under the action of a photoinitiator to increase the cross-link density of UV-cured products effectively. Both linear dual acryloxy-terminated or single acryloxy-terminated asymmetric siloxane oligomers can be copolymerized with cross-link density modifiers within 20 s of UV irradiation. The gelation yields of the UV-cured products obtained from the dual acryloxy-terminated siloxane oligomers were greater than 85%, and their surface water contact angles increased from 72.8° to 95.9° as the molecular weight of the oligomers increased. The gelation yields of UV-cured products obtained from single acryloxy-terminated asymmetric siloxane oligomers were less than 80%, and their thermal stabilities were inferior to those obtained from the dual acryloxy-terminated siloxane oligomers. However, the water contact angles of UV-cured products obtained from these single acryloxy-terminated asymmetric siloxane oligomers were all greater than 90°.

A Review on Modeling Cure Kinetics and Mechanisms of Photopolymerization

Photopolymerizations, in which the initiation of a chemical-physical reaction occurs by the exposure of photosensitive monomers to a high-intensity light source, have become a well-accepted technology for manufacturing polymers. Providing significant advantages over thermal-initiated polymerizations, including fast and controllable reaction rates, as well as spatial and temporal control over the formation of material, this technology has found a large variety of industrial applications. The reaction mechanisms and kinetics are quite complex as the system moves quickly from a liquid monomer mixture to a solid polymer. Therefore, the study of curing kinetics is of utmost importance for industrial applications, providing both the understanding of the process development and the improvement of the quality of parts manufactured via photopolymerization. Consequently, this review aims at presenting the materials and curing chemistry of such ultrafast crosslinking polymerization reactions as well as the research efforts on theoretical models to reproduce cure kinetics and mechanisms for free-radical and cationic photopolymerizations including diffusion-controlled phenomena and oxygen inhibition reactions in free-radical systems.

3D printing optics with hybrid material

Three-dimensional (3D) printing technology has evolved tremendously in recent years, but due to stringent requirements on surface finish and limited material selection for optical performance purposes, 3D printing optics is still lagging behind. This paper reports on a quantitative study on the printing process of optical lenses using acrylic and cationic combined commercial hybrid material. By utilizing its unique curing property with digital light process technology, we demonstrate the concept of continuous printing in the top-down light projection setup. Also, an equal thickness and equal arc combo method has been proposed and evaluated to further help smooth the surface finish. Millimeter-level spherical lenses have been successfully fabricated, and their optical performance has also been discussed.

Moving Towards a Finer Way of Light-Cured Resin-Based Restorative Dental Materials: Recent Advances in Photoinitiating Systems Based on Iodonium Salts

The photoinduced polymerization of monomers is currently an essential tool in various industries. The photopolymerization process plays an increasingly important role in biomedical applications. It is especially used in the production of dental composites. It also exhibits unique properties, such as a short time of polymerization of composites (up to a few seconds), low energy consumption, and spatial resolution (polymerization only in irradiated areas). This paper describes a short overview of the history and classification of different typical monomers and photoinitiating systems such as bimolecular photoinitiator system containing camphorquinone and aromatic amine, 1-phenyl-1,2-propanedione, phosphine derivatives, germanium derivatives, hexaarylbiimidazole derivatives, silane-based derivatives and thioxanthone derivatives used in the production of dental composites with their limitations and disadvantages. Moreover, this article represents the challenges faced when using the latest inventions in the field of dental materials, with a particular focus on photoinitiating systems based on iodonium salts. The beneficial properties of dental composites cured using initiation systems based on iodonium salts have been demonstrated.

Onium salts improve the kinetics of photopolymerization of acrylate activated with visible light

The aim was study the influence of onium salts on the kinetics of photopolymerization in the visible light region. Trimethylolpropane triacrylate TMPTA was selected as a monomer, and activated by 1,3-bis(phenylamino)squaraine (SQ) used as a photosensitizer in addition to tetramethylammonium n-butyltriphenylborate (B2). The iodonium salt [A–I–B]⁺X⁻ functioned as a second radical initiator, bearing a different substitution pattern for the cation. The ternary system was formulated with different concentrations of both borate and diphenyliodonium salts. Differential scanning calorimetry was used to investigate the polymerization reaction over the photoactivation time carried out at 300 nm < λ < 500 nm irradiation. When the squaraine dye/borate salt was used as photoinitiator, a slow polymerization reaction was observed and a lower monomer conversion. The addition of a third component (onium salt) increased the polymerization rate and conversion. Ternary photoinitiator systems showed improvement in the polymerization rate of triacrylate leading to high conversion in a short photoactivation time. The photoinitiating ability of bi- and tri-component photoinitiators acting in the UV-Vis region for initiation polymerization of triacrylate was compared with those of some commercially used photoinitiating systems. It was also found, that, the parallel electron transfer from an excited state of the sensitizer to [A–I–B]⁺X⁻, and an electron transfer from a ground state of R(Ph)₃B⁻N(CH₃)₄⁺ to an excited state of the sensitizer results in two types of initiating radical.

The chemical mechanisms were investigated by steady state photolysis and nanosecond laser flash photolysis experiments. A mechanism for initiating polymerization using both onium salts is proposed here.

Hybrid free-radical and cationic photo-polymerization of bio-based monomers derived from seed oils – control of competitive processes by experimental design

Experimental design helps define optimal compositions for a 3-component photo-initiating system for IPNs synthesized from mixed epoxidized and acrylated triglycerides.

New bimolecular photoinitiating systems based on terphenyl derivatives as highly efficient photosensitizers for 3D printing application

New 2-(diethylamino)-4,6-diphenyl-benzene-1,3-dicarbonitrile derivatives were proposed as photoinitiators in conjunction with iodonium salts for: (i) the cationic polymerization, (ii) the free-radical polymerization, and (iii) the synthesis of IPNs.

Predicting the crystal habit of photoinitiator XBPO and elucidating the solvent effect on crystal faces

Elaborating the solvent effect on crystal faces using an affinity degree model at the molecular level.

Comparison of the Physical and Mechanical Properties of Resin Matrix with Two Photoinitiator Systems in Dental Adhesives

This study evaluated the physical and mechanical properties of resin matrices in dental adhesives with two photoinitiator systems. Resin matrix specimens were made with five different kinds of photoinitiators. Neat resin consisted of 60% 2,2-bis[4-2(2-hydroxy-3-methacryloxypropoxy)phenyl]propane (Bis-GMA) and 40% hydroxyethyl methacrylate (HEMA) by weight, along with camphorquinone (CQ, 1 mol %) and additional components (1 mol % each) as follows: Group 1, 2-(dimethylamino)ethyl methacrylate (DMAEMA); Group 2, ethyl-4-(dimethylamino) benzoate (EDMAB); Group 3, diphenyliodonium hexafluorphosphate (DPIHFP); Group 4, DMAEMA+DPIHFP; Group 5, EDMAB+DPIHFP. The degree of conversion (DC), flexural strength, flexural modulus, microhardness, and ultimate tensile strength were tested. The contribution of each photoinitiator to the DC in a selected group was analyzed with contour plots. One-way ANOVA and Tukey tests (p < 0.05) were used for statistical analyses. The DC of Groups 2, 4, and 5 was similar. The flexural strength was similar in all groups, but flexural modulus was significantly different. Group 3 had the lowest values for all physical and mechanical properties. Among all methods, the microhardness test revealed the greatest degree of difference among the five specimens. CQ, EDMAB, and DPIHFP were the most effective photoinitiators and CQ was the most influential factor for the DC rate.

Visible-Light Initiated Free-Radical/Cationic Ring-Opening Hybrid Photopolymerization of Methacrylate/Epoxy: Polymerization Kinetics, Crosslinking Structure, and Dynamic Mechanical Properties

The effects of polymerization kinetics and chemical miscibility on the crosslinking structure and mechanical properties of polymers cured by visible-light initiated free-radical/cationic ring-opening hybrid photopolymerization are determined. A three-component initiator system is used and the monomer system contains methacrylates and epoxides. The photopolymerization kinetics is monitored in situ by Fourier transform infrared-attenuated total reflectance. The crosslinking structure is studied by modulated differential scanning calorimetry and dynamic mechanical analysis. X-ray microcomputed tomography is used to evaluate microphase separation. The mechanical properties of polymers formed by hybrid formed by free-radical polymerization. These investigations mark the first time that the benefits of the chain transfer reaction between epoxy and hydroxyl groups of methacrylate, on the crosslinking network and microphase separation during hybrid visible-light initiated photopolymerization, have been determined.

Outside-the-(cavity-prep)-box thinking

Direct placement restorative materials must interface with tooth structures that are often compromised by caries or trauma. The material must seal the interface while providing sufficient strength and wear resistance to assure function of the tooth for, ideally, the lifetime of the patient. Needed are direct restorative materials that are less technique-sensitive than current resin-based composite systems while having improved properties. The ideal material could be successfully used in areas of the world with limited infrastructure. Advances in our understanding of the interface between the restoration adhesive system and the stages of carious dentin can be used to promote remineralization. Application of fracture mechanics to adhesion at the tooth-restoration interface can provide insights for improvement. Research in polymer systems suggests alternatives to current composite resin matrix systems to overcome technique sensitivity, while advances in nano- and mesoparticle reinforcement and alignment in composite systems can increase material strength, toughness, and wear resistance, foreshadowing dental application.

Recent advances and developments in composite dental restorative materials

Composite dental restorations represent a unique class of biomaterials with severe restrictions on biocompatibility, curing behavior, esthetics, and ultimate material properties. These materials are presently limited by shrinkage and polymerization-induced shrinkage stress, limited toughness, the presence of unreacted monomer that remains following the polymerization, and several other factors. Fortunately, these materials have been the focus of a great deal of research in recent years with the goal of improving restoration performance by changing the initiation system, monomers, and fillers and their coupling agents, and by developing novel polymerization strategies. Here, we review the general characteristics of the polymerization reaction and recent approaches that have been taken to improve composite restorative performance.

Hybrid polymerization of vinyl and hetero-ring groups of glycidyl methacrylate resulting in thermoresponsive hyperbranched polymers displaying a wide range of lower critical solution temperatures

Hybrid polymerization of glycidyl methacrylate (GMA) with potassium hydride (KH) and various oligo(ethylene glycol)s as the initiating system, in which both vinyl polymerization and ring-opening polymerization occur simultaneously, generates hyperbranched poly(ether-ester)s. The reaction process has been followed by an in situ nuclear magnetic resonance technique. The experimental results indicate that both the vinyl and epoxy groups of GMA undergo polymerization, with the reactivity of the latter being much higher than that of the former. Interestingly, the resulting hyperbranched polymers exhibit a sharp phase transition in water at the lower critical solution temperature (LCST). Significantly, the LCST values can be accurately controlled from 0 to 100 degrees C by changing the hydrophilic/hydrophobic balance of GMA and various oligo(ethylene glycol)s or by modification of the precursor polymer through acetylation. This novel stimuli-responsive hyperbranched polymer is a promising candidate for a new generation of commercially viable thermoresponsive polymers following on from the widely used poly(N-isopropylacrylamide) (PNIPAM).

(Meth)Acrylate Vinyl Ester Hybrid Polymerizations

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.

Onium salt improves the polymerization kinetics in an experimental dental adhesive resin

The aim of this study was to evaluate the influence of an onium salt in the polymerization kinetics of a dental adhesive model resin. A monomer mixture, based on Bis-GMA, TEGDMA and HEMA, was used as a model dental adhesive resin, which was polymerized using camphorquinone (CQ) as a photo-initiator in addition to either binary or ternary photo-initiator systems. The binary systems were formulated with different concentrations of diphenyliodonium hexafluorphosphate or ethyl 4-dimethylaminobenzoate in relation to the monomer and 1mol% of CQ. The ternary system was a blend of 1mol% of CQ, 2mol% of ethyl 4-dimethylamino benzoate and 0.25, 0.5, 1, 2 or 4mol% of onium salt. Real time Fourier Transform infrared spectroscopy was used to investigate the polymerization reaction over the photo-activation time. When CQ was used as photo-initiator, a slow polymerization reaction was observed and a lower monomer conversion. The addition of a second component (onium salt or amine) increases the polymerization rate and conversion independent on the co-initiator concentration. Ternary photo-initiator system showed an improvement on the polymerization rate of the dental adhesive model resin leading to high conversion in short photo-activation time. Also, a mechanism for initiating polymerization using an amine or onium salt as a co-initiator component is proposed.

Evaluation and Control of Thiol-ene/Thiol-epoxy Hybrid Networks

The development of thiol-ene/thiol-epoxy hybrid networks offers the advantage of tailorable polymerization kinetics while producing a highly crosslinked, high T(g) polymer that has significantly reduced shrinkage stress. Stoichiometric mixtures of pentaerythritol tetra(3-mercaptopropionate) (PETMP)/triallyl-1,3,5-triazine-2,4,6-trione (TATATO) (thiol-ene, mixture 1) and PETMP/bisphenol a diglycidyl ether (BADGE) (thiol-epoxy, mixture 2) were prepared and hybrid mixtures of 75/25, 50/50, 25/75, and 10/90 w/w of mixtures 1 and 2 were polymerized using a combination of both radical and anionic initiation. The light exposure timing and the relative initiation conditions of the two types were used to control the order and relative rates of the radical and anionic polymerizations. The 50/50 w/w thiol-ene/thiol-epoxy hybrid material exhibited a final stress of only 0.2 MPa, which is 90 % lower than the stress developed in a control dimethacrylate resin. Kinetic analysis indicates composition affects network development in thiol-ene/thiol-epoxy hybrid networks and produces materials with robust mechanical properties.