Effect of thickness on the degree of conversion of two bulk-fill and one conventional posterior resin-based composites at high irradiance and high temporal resolution.

OBJECTIVES: This study: 1) measures the effect of sample thickness and high irradiance on the depth-dependent time delay before photopolymerization reaction onset; 2) determines if exposure reciprocity exists; 3) measures the conversion rate at four irradiance levels; 4) determines the time, t0, at which the maximum DC rate is reached for two bulk-fill and one conventional posterior resin-based composites (RBCs). METHODS: Tetric PowerFill IVA shade (Ivoclar Vivadent) and Aura bulk-fill ultra universal restorative (SDI), and one conventional posterior resin-based composite (RBC), Heliomolar A3 (Ivoclar Vivadent), that were either 0.2 mm, 2 mm, or 4 mm thick were photocured using a modified Bluephase G4 (Ivoclar Vivadent) light-curing unit (LCU) that delivered a single emission band (wavelength centered at 449 nm). The same radiant exposure of 24 J/cm2 was delivered at irradiances ranging from 0.5 to 3 W/cm2 by adjusting the exposure time. PowerFill was also photocured for 3 s or 6 s using a Bluephase PowerCure LCU (Ivoclar Vivadent) on the 3 s mode setting. The degree of conversion (DC) was measured in real-time at a high temporal resolution at 30 °C using Attenuated Total Reflection (ATR) FTIR spectroscopy with a sampling rate of 13 DC data points per second. The DC data were analyzed using a phenomenological autocatalytic model. The RBC viscosity was measured at 21 °C and 30 °C. Light transmission through the RBC samples at 22 °C was monitored with time to calculate the extinction coefficients of the RBCs. RESULTS: The time delay before photopolymerization started increased as the RBC thickness increased and the irradiance decreased. An autocatalytic model described the DC data. The time t0 was less than 77 ms for the 0.2 mm thick samples of PowerFill irradiated using the highest irradiance of 3 W/cm2. Among the three RBCs for each sample thickness and irradiance level, the PowerFill had the smallest time t0. There was a time delay of 0.59 s and 1.25 s before the DC started to increase at the bottom of 4 mm thick samples for the PowerFill and Aura, respectively, when an irradiance of 1 W/cm2 was delivered. The time delay increased to 3.65 s for the Aura when an irradiance of 0.5 W/cm2 was delivered. The extinction coefficients near 449 nm were 0.78 mm-1, 0.76 mm-1, and 1.55 mm-1 during the first 2 s after the start of photocuring of PowerFill, Aura, and Heliomolar, respectively. Only PowerFill followed exposure reciprocity. At T = 30 °C, the viscosity was 3400, 17000, and 5200 Paˑs for PowerFill, Aura, and Heliomolar, respectively. SIGNIFICANCE: The time delay between when photopolymerization starts at the top and bottom of 2- or 4-mm thick RBC restorations may affect the structural integrity of the bond between the tooth and the bottom of the restoration. Only PowerFill followed exposure reciprocity between irradiance levels of 0.5 to 3 W/cm2. Exposure reciprocity did not occur for Aura or Heliomolar, neither of which are optimized for short light exposure or high irradiance conditions.

Synthesis, characterization and evaluation of azobenzene nanogels for their antibacterial properties in adhesive dentistry.

The presence of cariogenic bacteria within the prepared tooth cavity at the adhesive resin-dentin interface is detrimental to the long-term stability and function of composite restorations. Here, we report the synthesis and incorporation of methacrylated azobenzene nanogels within bisphenol A-glycidyl methacrylate/hydroxyethyl methacrylate/ethanol (B/H/E) adhesive resins and evaluate their ability to reduce the bacterial invasion of cariogenic Streptococcus mutans biofilms while preserving the mechanical strength and structural integrity of the critical interfacial connection between the restoration and the tooth. The azobenzene nanogel, with a hydrodynamic radius of < 2 nm and a molecular weight of 12,000 Da, was polymerized within B/H/E adhesive formulations at concentrations of 0.5 wt.%, 1.5 wt.%, and 2.5 wt.%. While the double-bond conversion, cytocompatibility, water solubility, and sorption of the adhesive networks were comparable, azobenzene nanogel networks showed improved hydrophobicity with a ≥ 25° increase in water contact angle. The polymerized adhesive surfaces formulated with azobenzene nanogels showed a 66% reduction in bacterial biofilms relative to the control while maintaining the mechanical properties and micro-tensile bond strength of the adhesive networks. The increased hydrophobicity and antibacterial activity are promising indicators that azobenzene nanogel additives have the potential to increase the durability and longevity of adhesive resins.

Photo-polymerization kinetics of a dental resin at a high temporal resolution.

OBJECTIVES: This study: 1) aims to measure with high temporal resolution the intrinsic rate of the degree of conversion (DC) of a dental resin-based composite (RBC) photo-cured at two irradiances; 2) aims to determine the transition time at which the DC rate is maximum; 3) used two different irradiances to measure the shift in transition time; 4) aims to compare transition times measured using DC and shrinkage strain. METHODS: Samples (n = 20) 1 mm thick by 10 mm diameter of Filtek One bulk-fill restorative A2 shade (3M Oral Care) were photocured for 20 s with a single emission peak (wavelength centered at 455 nm) light-emitting-diode-based light-curing unit at irradiance levels of 890 mW/cm2 and 209 mW/cm2, and initial sample temperature of T = 23 °C. The DC was measured in real-time using Attenuated Total Reflection (ATR) FTIR spectroscopy with a sampling rate of 13 DC data points per second. The data were analyzed within a phenomenological autocatalytic model. In addition, the axial shrinkage strain was measured using 3 samples of the RBC with the same outer dimensions and under similar experimental conditions using the bonded disk method and an interferometric technique. RESULTS: For the 890 mW/cm2 and 209 mW/cm2 irradiance levels, the DC with time was found to agree with the model enabling the determination of transition times of 0.66 ± 0.05 s and 2.3 ± 0.2 s, and the DC at these times of 5.5 ± 0.2% and 6.4 ± 0.2%. The maximum linear strain rate at 0.76 ± 0.01 s and 1.98 ± 0.02 s for the 890 mW/cm2 and 209 mW/cm2 irradiance levels, respectively, are within two standard deviations of the corresponding transition times. SIGNIFICANCE: At an irradiance level much greater than 1000 mW/cm2, the photo-polymerization kinetics of a dental RBC may be too fast to be measured accurately using ATR-FTIR spectroscopy. A viable alternative to monitor the kinetics is through the measurements of the axial shrinkage strain employing the bonded disk method and an interferometric technique.

Relocation and reinforcement of the adhesive/composite interface with spontaneous amine-peroxide interfacial polymerization.

OBJECTIVES: This study demonstrates a spontaneous redox polymerization process located at the adhesive-composite interface that precedes photocure of the composite with the intent to improve bonding. METHODS: An aromatic amine and benzoyl peroxide redox initiator system was partitioned between BAPO-photoinitiated BisGMA/HEMA adhesive and BisGMA/TEGDMA resin-composites. The composite was placed on the photocured adhesive layer with a brief delay before photopolymerization of the composite layer. Micro-tensile bond strength between the adhesive and composite was assessed in comparison with the non-redox active control materials. RESULTS: The presence of amine or peroxide in these resins without the redox initiation contribution enhanced both the rate and the final conversion of the BAPO-based photopolymerizations. Control formulations using redox-only initiation showed active polymer formation starting at approximately 30 s when physical mixing of the redox components was involved; however, simply by waiting 60 s between composite placement and photocure provided adequate time for passive interfacial diffusion of benzoyl peroxide from the pre-cured adhesive into the overlaid aromatic amine-containing composite such that a sufficient degree of redox initiated interfacial polymerization occurred prior to the composite photocure. The result was a significant increase in the adhesive to composite micro-tensile bond strength with the failure site moved away from the mainly interfacial failure noted for the control. SIGNIFICANCE: The stress-free autonomous pre-conversion of a redox-initiated thin film of composite that then provides a compositionally homogeneous interface for composite photopolymerization offers a means to enhance at least short-term bond strength between the adhesive and composite phases during restorative placement.

Determining Michael Acceptor Reactivity from Kinetic, Mechanistic, and Computational Analysis for the Base-catalyzed Thiol-Michael Reaction.

A combined experimental and computational study of the reactivities of seven commonly used Michael acceptors paired with two thiols within the framework of photobase-catalyzed thiol-Michael reactions is reported. The rate coefficients of the propagation (kP), reverse propagation (k-P), chain-transfer (kCT), and overall reaction (koverall) were experimentally determined and compared with the well-accepted electrophilicity parameters of Mayr and Parr, and DFT-calculated energetics. Both Mayr's and Parr's electrophilicity parameters predict the reactivities of these structurally varying vinyl functional groups well, covering a range of overall reaction rate coefficients from 0.5 to 6.2 s-1. To gain insight into the individual steps, the relative energies have been calculated using DFT for each of the stationary points along this step-growth reaction between ethanethiol and the seven alkenes. The free energies of the individual steps reveal the underlying factors that control the reaction barriers for propagation and chain transfer. Both the propagation and chain transfer steps are under kinetic control. These results serve as a useful guide for Michael acceptor selection to design and predict thiol-Michael-based materials with appropriate kinetic and material properties.

Evaluation of a photo-initiated copper(I)-catalyzed azide-alkyne cycloaddition polymer network with improved water stability and high mechanical performance as an ester-free dental restorative.

OBJECTIVE: The objective is to develop and characterize an ester-free ether-based photo-CuAAC resin with high mechanical performance, low polymerization-induced stress compared with common BisGMA/TEGDMA (70/30) resins, and improved water stability in comparison to previously developed urethane-based photo-CuAAC resins. METHODS: Triphenyl-ethane-centered ether-linked tri-azide monomers were synthesized and co-photopolymerized with ether-linked tri-alkyne monomers under visible light irradiation using a copper(II) pre-catalyst and CQ/EDAB as the initiator. The ether-based CuAAC formulation was investigated for thermo-mechanical properties, polymerization kinetics and shrinkage stress, and flexural properties with respect to a conventional BisGMA/TEGDMA (70/30) dental resin. In addition, both the ether-based CuAAC resin and the urethane-based CuAAC resin were examined for their water stability using the BisGMA/TEGDMA (70/30) resin as a control. RESULTS: The ether-based CuAAC network (AK/AZ-1) exhibited a slightly lower glass-transition temperature compared with the BisGMA/TEGDMA network (108 °C vs 128 °C), but because of its much sharper glass transition, the AK/AZ-1 CuAAC-network maintained storage modulus higher than 1 GPa up to 100 °C. In addition, the ether-based AK/AZ-1 network exhibited reduced shrinkage stress (0.56 MPa vs 1.0 MPa) and much higher flexural toughness (7.6 MJ/m3vs 1.6 MJ/m3) while showing slightly lower flexural modulus and slightly higher flexural strength compared with the BisGMA/TEGDMA network. Moreover, the ether-based AK/AZ-1 CuAAC network displayed comparable water stability in comparison to the BisGMA/TEGDMA network with slightly higher water sorption (46 µg/mm3vs 38 µg/mm3) and much lower water solubility (2.3 µg/mm3vs 4.4 µg/mm3). SIGNIFICANCE: Employing the ether-based hydrophobic CuAAC formulation significantly improved the water stability of the CuAAC network compared with previously developed urethane-based CuAAC networks. Furthermore, compared with the conventionally used BisGMA/TEGDMA formulation, the reduced shrinkage stress, comparable flexural strength/flexural modulus, and the superior flexural toughness of the ether-based CuAAC network make it a promising ester-free alternative to the currently widely-used methacrylate-based dental restoratives.

Suppression of hydrolytic degradation in labile polymer networks via integrated styrenic nanogels.

OBJECTIVE: The objective of this study was to demonstrate an approach with potential to increase the life of dental restorative polymers in water, by maintaining their strength and toughness with varied content of inert or reactive styrenic pre-polymeric additives. It was hypothesized that addition of styrene-co-divinylbenzene nanogels to a conventional dimethacrylate resin (e.g. TEGDMA) would reduce its susceptibility towards hydrolytic degradation, while maintaining equivalent mechanical properties. METHODS: Polymerization kinetics and functional group conversions were determined by Fourier transform infrared spectroscopy. Triple-detection gel permeation chromatography was used for nanogel particle characterization. A goniometer was used to measure water contact angle on experimental and control photocured polymers. Hydrolytic degradation and mass loss evaluation was performed after extended water storage of an intentionally hydrolytically degradable polymer. Resin viscosity was determined rheometrically and polymer mechanical properties were evaluated using three-point flexural testing with TEGDMA-nanogel formulations. RESULTS: The polymer network with highest level of nanogel loading (50 wt%) and the highest level of internal nanogel crosslinking (50 mol%) had the lowest degree of equilibrium swelling ratio and mass loss. The flexural modulus and ultimate strength of polymerized TEGDMA and styrenic nanogel-modified TEGDMA were not statistically different (p > 0.05). SIGNIFICANCE: Due to improved shielding throughout the bulk of methacrylate-based polymers, including an example with an intentionally hydrolytically labile network structure, and a dramatic decrease of water uptake while maintaining equivalent mechanical properties, styrenic nanogel additives especially in high loading levels provide an excellent alternative to eliminate the adverse effects of water and presumably salivary fluids.

Photopolymerization shrinkage-stress reduction in polymer-based dental restoratives by surface modification of fillers.

OBJECTIVES: This research explores the use of polymer brushes for surface treatment of fillers used in polymer-based dental restoratives with focus on shrinkage stress reduction. The influence of interfacial reactive groups on shrinkage stress is explored. METHODS: Oligomers of varying lengths and with varying number of reactive groups along the length were synthesized by modifying commercial oligomers. Surface of silica fillers (OX50) was treated with methylaminopropyltrimethoxysilane and this was further reacted with the synthesized oligomers to obtain a series of polymer brushes on the surface. Fillers modified with γ-methacryloxypropyltrimethoxysilane were used as a control. Filler surface treatment was confirmed using diffuse reflectance spectroscopy and thermogravimetric analysis. Fillers were added at 30 wt % to a resin made of BisGMA/TEGDMA and polymerization kinetics, shrinkage stress, volumetric shrinkage, flexural strength and modulus, viscosity were measured. RESULTS: Composites with polymer brush functionalized fillers showed up to a 30 % reduction in shrinkage stress as compared to the control, with no reduction in flexural strength and modulus. Shrinkage stress reduced with increasing length of the polymer brush and increased with increase in number of reactive groups along the length of the polymer brush. SIGNIFICANCE: The interface between inorganic fillers and an organic polymer matrix has been utilized to reduce shrinkage stress in a composite with no compromise in mechanical properties. This study gives insights into the stress development mechanism at the interface.

Visible-Light Photoinitiation of (Meth)acrylate Polymerization with Autonomous Post-conversion.

Conversion plateaus rapidly in radical photopolymerizations (RPPs) following discontinuation of irradiation due to rapid termination of reactive radicals, which restricts the wider use of RPPs in applications that involve nonuniform light access including those with attenuated light transmission or irregular surfaces. Based on our recent report of a radical dark-curing photoinitiator (DCPI) that continues polymerization beyond the cessation of irradiation by enabling latent redox initiation with photo-released amine in the presence of a suitable oxidant, we developed a new DCPI with an absorption spectrum that extends well into the visible range. Our design process involved a series of computational investigations of candidate molecules, including a systematic study of substituents and their position-dependent effects on absorption characteristics, electronic transitions, and the photochemical mechanism and its associated energetics. Our quantum chemical computations identified the target compound 5,7-dimethoxy-6-bromo-3-aroylcoumarin-DMPT/BPh4 and predicted that it would facilitate the dark-curing mechanism by concurrent photo-radical generation and photo-induced release of an efficient redox reductant under visible irradiation. This reductant-tethered chromophore was then synthesized and optically characterized with UV-vis spectroscopy that revealed its strong visible-light absorption with a molar absorptivity of 5710 M-1 cm-1 at 405 nm and 50 M-1 cm-1 at 455 nm. We then demonstrated extensive dark-curing of >35% additional conversion over 25 min following brief activation of the shelf-stable one-part system by irradiation with a 455 nm LED that was ceased at 20% conversion. In contrast, shuttering irradiation of the control formulation at that same point resulted in immediate cessation of conversion, which plateaued at 20%. We determined a remarkable initiator efficiency of 2.82 that results from the additional redox-generated radicals with a 77% photo-reductant generation quantum yield. The combination of superior photo- and dark-curing efficiencies of this new visible DCPI is expected to open new application opportunities in RPP, especially those involving resins that are highly light attenuating, surfaces that possess irregular features that produce uneven irradiance, and production lines where continued dark-curing downstream of the light activation step enhances line efficiencies.

3D printing restorative materials using a stereolithographic technique: a systematic review.

OBJECTIVE: To present through a systematic review a qualitative analysis of studies published on stereolithography-based 3D printing of restorative materials and their clinical applicability. METHODS: The literature search was conducted based on the question: "What is the state-of-the-art of available restorative materials for 3D printing based on stereolithography?" Online search was conducted in three databases (MEDLINE/PubMed, Scopus and Web of Science) with no restriction for year of publication. Data are reported based on PRISMA, including publication details such as authors and their countries, year and journal of publication, and study design. The synthesis is focused on describing the dental restorative materials and properties evaluated, applied methods, 3D printers used and clinical applicability. RESULTS: Studies that fit the inclusion criteria were performed in Asia (21), Europe (16) and USA (10), mostly using polymer-based restorative materials (38) for 3D printing constructs. Stereolithographic-printed ceramic-based restorative structures were evaluated by 9 studies. Many studies reported on dimensional accuracy (14), strength (11) and surface morphology (9) of the printed structures. Antibacterial response, cytotoxicity, internal and marginal fit, fracture and wear resistance, density, viscosity, elastic modulus, hardness, structural shrinkage and reliability, degree of conversion, layer cure depth, fatigue, and color were also evaluated by the included studies. Many of them (11) published a proof of concept as an attempt to demonstrate the clinical feasibility and applicability of the technology to print restorative materials, but only 5 studies actually applied the 3D printed restorative structures in patients, which highlights an increasing interest but limited early-stage translation. SIGNIFICANCE: The fast expansion of stereolithographic-based 3D printing has been impressive and represents a great technological progress with significant disruptive potential. Dentistry has demonstrated an incredible willingness to adapt materials, methods and workflows to this promising digital technology. However, esthetic appearance, wear resistance, wet strength and dimensional accuracy are the main current clinical limitations restricting the progression to functional part production with 3D printing, which may explain the absence of clinical trials and reports on permanent/definitive dental restorative materials and structures.

Photoreactive nanogels as versatile polymer networks with tunable in situ drug release kinetics.

A simple, yet powerful approach to synthesize photoreactive nanogel networks <5 nm that can swell between ~3 and ~200 times their initial radius with control over the size and surface charge via a solution polymerization reaction protocol was demonstrated. Nanogels with hydrodynamic radii from 0.9 nm to 3.2 nm and surface charges from -6.4 mV to -16.5 mV with dramatically different abilities to swell were synthesized by altering the solvent ratio before synthesis. Additionally, the control over the release kinetics of a small molecule over a period of 30 days was demonstrated by the methacrylate functionalization of the nanogels post-synthesis and the subsequent photo-aggregation of the nanogels. Thepotential to control the release of small molecule drugs via the concentration of photoreactive groups and the photo-induced aggregation of the nanogels offers the unique ability to tailor the in situ release kinetics of the delivery network.

Additive manufacture of lightly crosslinked semicrystalline thiol-enes for enhanced mechanical performance.

Photopolymerizable semicrystalline thermoplastics resulting from thiol-ene polymerizations were formed via fast polymerizations and achieved excellent mechanical properties. These materials have been shown to produce materials desirable for additive manufacturing (3D printing), especially for recyclable printing and investment casting. However, while well-resolved prints were previously achieved with the thiol-ene thermoplastics, the remarkable elongation at break (ϵmax) and toughness (T) attained in bulk were not realized for 3D printed components (ϵmax,bulk ~ 790%, Tbulk ~ 102 MJ m-3 vs. ϵmax,print < 5%, Tprint < 0.5 MJ m-3). In this work, small concentrations (5-10 mol%) of a crosslinker were added to the original thiol-ene resin composition without sacrificing crystallization potential to achieve semicrystalline, covalently crosslinked networks with enhanced mechanical properties. Improvements in ductility and overall toughness were observed for printed crosslinked structures, and substantial mechanical augmentation was further demonstrated with post-manufacture thermal conditioning of printed materials above the melting temperature (Tm). In some instances, this thermal conditioning to reset the crystalline component of the crosslinked prints yielded mechanical properties that were comparable or superior to its bulk counterpart (ϵmax ~ 790%, T ~ 95 MJ m-3). These unique photopolymerizations and their corresponding monomer compositions exhibited concurrent polymerization and crystallization along with mechanical properties that were tunable by changes to the monomer composition, photopolymerization conditions, and post-polymerization conditioning. This is the first example of a 3D printed semicrystalline, crosslinked material with thermally tunable mechanical properties that are superior to many commercially-available resins.

Stress Relaxation via Covalent Dynamic Bonds in Nanogel-Containing Thiol-Ene Resins.

Functional nanogels are attractive additives for use in polymer composites. In this study, nanogels with internal allyl sulfide moieties throughout their network structure were prepared via a thiol-Michael addition reaction. The excess thiol-functionalized nanogels were less than 60 nm as discrete particles but act as room-temperature liquids in the bulk state. The reactive nanogels can be dispersed in and swollen by a thiol-ene matrix resin, which upon photopolymerization yields dramatically decreased levels of polymerization shrinkage stress. Furthermore, the postcured nanogel-modified polymers effectively relaxed applied stresses as well as enhanced toughness during exposure to a UV light source that activated the addition-fragmentation as a means for dynamic bond exchange. These nanogels provide a generic approach to introduce adaptable network performance that significantly improves a number of key properties of glassy cross-linked polymer.

Computational and Experimental Evaluation of Peroxide Oxidants for Amine-Peroxide Redox Polymerization.

Amine-peroxide redox polymerization (APRP) is the prevalent method for producing radical-based polymers in the many industrial and medical applications where light or heat activation is impractical. We recently developed a detailed description of the APRP initiation process through a combined computational and experimental effort to show that APRP proceeds through SN2 attack by the amine on the peroxide, followed by the rate-determining homolysis of the resulting intermediate. Using this new mechanistic understanding, a variety of peroxides were computationally predicted to initiate APRP with fast kinetics. In particular, the rate of APRP initiation can be improved by radical and anion stabilization through increased π-electron conjugation or by increasing the electrophilicity of the peroxy bond through the addition of electron-withdrawing groups. On the other hand, the addition of electron-donating groups lowered the initiation rate. These design principles enabled the computational prediction of several new peroxides that exhibited improved initiation rates over the commonly used benzoyl peroxide. For example, the addition of nitro groups (NO2) to the para positions of benzoyl peroxide resulted in a theoretical radical generation rate of 1.9 × 10-9 s-1, which is ~150 times faster than the 1.3 × 10-11 s-1 radical generation rate observed with unsubstituted benzoyl peroxide. These accelerated kinetics enabled the development of a redox-based direct-writing process that exploited the extremely rapid reactivity of an optimized redox pair with a custom inkjet printer, capable of printing custom shapes from polymerizing resins without heat or light. Furthermore, the application of more rapid APRP kinetics could enable the acceleration of existing industrial processes, make new industrial manufacturing methods possible, and improve APRP compatibility with biomedical applications through reduced initiator concentrations that still produce rapid polymerization rates.

Vinyl sulfonamide based thermosetting composites via thiol-Michael polymerization.

OBJECTIVE: To assess the performance of thiol Michael photocurable composites based on ester-free thiols and vinyl sulfonamides of varying monomer structures and varied filler loadings and to contrast the properties of the prototype composites with conventional BisGMA-TEGDMA methacrylate composite. METHODS: Synthetic divinyl sulfonamides and ester-free tetrafunctional thiol monomers were utilized for thiol-Michael composite development with the incorporation of thiolated microfiller. Polymerization kinetics was investigated using FTIR spectroscopy. Resin viscosities were assessed with rheometry. Water uptake properties were assessed according to standardized methods. Thermomechanical properties were analyzed by dynamic mechanical analysis. Flexural modulus/strength and flexural toughness were measured on a universal testing machine in three-point bending testing mode. RESULTS: The vinyl sulfonamide-based thiol-Michael resin formulation demonstrated a wide range of viscosities with a significant increase in the functional group conversion when compared to the BisGMA-TEGDMA system. The two different types of vinyl sulfonamide under investigation demonstrated significant differences towards the water sorption. Tertiary vinyl sulfonamide did not undergo visible swelling whereas the secondary vinyl sulfonamide composite swelled extensively in water. With the introduction of rigid monomer into the polymer matrix the glass transition temperature increased and so increased the toughness. Glassy thiol-Michael composites were obtained by ambient curing. SIGNIFICANCE: Employing the newly developed step-growth thiol-Michael resins in dental composites will provide structural uniformity, improved stability and lower water sorption.

Photo-responsive liposomes composed of spiropyran-containing triazole-phosphatidylcholine: investigation of merocyanine-stacking effects on liposome-fiber assembly-transition.

A spiropyran-containing triazole-phosphatidylcholine (SPTPC) was synthesized through a copper-catalyzed azide alkyne cyclo-addition (CuAAC) reaction. In water, SPTPCs self-assembled and a spontaneous spiropyran-to-merocyanine (SP-to-MC) isomerization occurred, resulting in coexistence of liposomes and fibers, and switching from the spiropyran (SP) to the merocyanine (MC) isomeric structure induced a reversible transition between these molecular assemblies. Study of the self-assembly of SPTPCs and photo-induced liposome-fiber assembly-transition revealed that the presence of MC enabled additional inter-membrane interaction during self-assembly and that the MC-stacking effect was the driving force for the assembly-transition. Exposure to UV light induced switching from SP to MC, where the planar structure of MC and the confinement of MC led to enhanced MC-stacking. The effect of MC-stacking was both advantageous and disadvantageous: MC-stacking perturbed the hydrophobic phase in the bilayer membrane and facilitated the liposome-to-fiber transition, otherwise the MC-stacking retarded switching of MC to SP, and caused an incomplete recovery of MC to SP during fiber-to-liposome recovery, thus a fatigue of SP was induced by MC-stacking during the liposome-to-fiber transition cycle. To decrease the intermolecular interactions and suppress MC-stacking, photo-inert triazole-phosphatidylcholine (TPC) was incorporated to prepare two-component TPC/SPTPC-liposomes, which exhibited better recovery kinetics. The photo-adaptive behavior of TPC/SPTPC-liposomes confirmed the disturbance of bilayer membranes by inter-membrane MC-stacking and the formation of MCTPC-enriched phases in the bilayer membrane.

Modification of filler surface treatment of composite resins using alternative silanes and functional nanogels.

OBJECTIVES: This study probes how modifiedapproaches for filler surface treatment in dental composites based on alternative silanes and functional nanogel additives affects physicochemical properties of these materials with a focus on polymerization stress development. METHODS: Nanogels were synthesized from isobornyl methacrylate, ethoxylated bisphenol-A dimethacrylate and isocyanatoethyl methacrylate followed by partial further reaction with 2-hydroxyethyl methacrylate to provide both isocyanate and methacrylate functionalization. A barium glass filler (˜1 µm particle size) was treated with either γ-methacryloxypropyltrimethoxysilane (MPS), N-methylaminopropyltrimethoxy (MAP) or N-allylaminopropyltrimethoxy (AAP) silanes. The reactive nanogels were then covalently attached to the aminosilane-treated fillers. Surface treatment was characterized by thermogravimetric analysis (TGA) and diffuse reflectance infrared spectroscopy (DR-IR). Composites were formulated with 60 wt% of the various functionalized fillers and the materials were evaluated for polymerization kinetics, polymerization stress (PS), volumetric shrinkage, mechanical properties and photorheology. Data were evaluated by one-way ANOVA and Tukey's test at 5% significance level. RESULTS: Filler surface treatments were confirmed by TGA and DR-IR analyses. Nanogel-functionalized fillers significantly reduced PS up to 20%, while the degree of conversion and elastic modulus were not compromised. Similar storage modulus development during polymerization was observed among materials by photorheology although the rate of polymerization was significantly increased for nanogel-based treatments. A significant decrease in flexural strength was observed for amino functional silane groups; however, there was no statistical difference in strength for the MPS control group compared with the nanogel-modified composites. SIGNIFICANCE: Filler surface treatment modified with a reactive nanogel enables significant PS reduction, without compromise to degree of conversion or mechanical properties of dental composites.

Rational Design of Efficient Amine Reductant Initiators for Amine-Peroxide Redox Polymerization.

Amine-peroxide redox polymerization (APRP) has been highly prevalent in industrial and medical applications since the 1950s, yet the initiation mechanism of this radical polymerization process is poorly understood so that innovations in the field are largely empirically driven and incremental. Through a combination of computational prediction and experimental analysis, we elucidate the mechanism of this important redox reaction between amines and benzoyl peroxide for the ambient production of initiating radicals. Our calculations show that APRP proceeds through SN2 attack by the amine on the peroxide but that homolysis of the resulting intermediate is the rate-determining step. We demonstrate a correlation between the computationally predicted initiating rate and the experimentally measured polymerization rate with an R2 = 0.80. The new mechanistic understanding was then applied to computationally predict amine reductant initiators with faster initiating kinetics. This led to our discovery of N-(4-methoxyphenyl)pyrrolidine (MPP) as amine reductant, which we confirmed significantly outperforms current state-of-the-art tertiary aromatic amines by ∼20-fold, making it the most efficient amine-peroxide redox initiator to date. The application of amines with superior kinetics such as MPP in APRP could greatly accelerate existing industrial processes, facilitate new industrial manufacturing methods, and improve biocompatibility in biomedical applications conducted with reduced initiator concentrations yet higher overall efficiency.

Photopolymerization kinetics of methyl methacrylate with reactive and inert nanogels.

The enhanced in situ photopolymerization kinetics of methyl methacrylate (MMA) to poly(methyl methacrylate) (PMMA) through the incorporation of both inert and reactive nanogel (NG) fillers under ambient conditions has been demonstrated. In addition to the polymerization kinetics, the physical and chemical properties of the prepolymeric NG were also utilized to tune the thermoplasticity and mechanical properties of the PMMA polymer network. The protocol followed in this study imparts superior MMA photopolymerization kinetics (≥ 60% double-bond conversion within 15 min for > 35 wt% nanogel loadings and ≥ 95% double-bond conversion in < 60 min for all NG concentrations) when compared with traditional polymerization mechanisms. PMMA remained a glassy material following the incorporation of both inert and reactive NG as demonstrated by the glass transition temperature (Tg) of the ultimate networks. Network linearity is uncompromised following incorporation of inert NG additives, thereby preserving the thermoplasticity of the PMMA network. As the non-functionalized, inert NG content increases, the maintenance of thermoplasticity occurs at the expense of mechanical properties (10× reduction of maximum strength at 25 wt% loading). These effects are less pronounced when reactive nanogels are employed (no significant reduction of maximum strength at 25 wt% loading with minimal crosslinking). The incorporation of NGs enable high chemical tunability within linear polymer networks. Given the wide range of monomers available for the synthesis of NGs, the methodology detailed in this study offers a scheme for the optimization of linear networks for specific targeted applications, hitherto deemed unrealistic under established polymerization protocols.

Fully recoverable rigid shape memory foam based on copper-catalyzed azide-alkyne cycloaddition (CuAAC) using a salt leaching technique.

This study is the first to employ the use of the copper-catalyzed azide-alkyne cycloaddition (CuAAC) polymerization to form a tough and stiff, porous material from a well-defined network possessing a high glass transition temperature. The effect of the network linkages formed as a product of the CuAAC reaction, i.e., the triazoles, on the mechanical behavior at high strain was evaluated by comparing the CuAAC foam to an epoxy-amine-based foam, which consisted of monomers with similar backbone structures and mechanical properties (i.e., Tg of 115 °C and a rubbery modulus of 1.0 MPa for the CuAAC foam, Tg of 125 °C and a rubbery modulus of 1.2 MPa for the epoxy-amine foam). When each foam was compressed uniformly to 80% strain at ambient temperature, the epoxy-amine foam was severely damaged after only reaching 70% strain in the first compression cycle with a toughness of 300 MJ/m3. In contrast, the CuAAC foam exhibited pronounced ductile behavior in the glassy state with three times higher toughness of 850 MJ/m3 after the first cycle of compression to 80% strain. Additionally, when the CuAAC foam was heated above Tg after each of five compression cycles to 80% strain at ambient temperature, the foam completely recovered its original shape while exhibiting a gradual decrease in mechanical performance over the multiple compression cycles. The foam demonstrated almost complete shape fixity and recovery ratios even through five successive cycles, indicative of "reversible plasticity", making it highly desirable as a glassy shape memory foams.