Publications by authors named "Jeffrey W Stansbury"

76 Publications

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

J Mech Behav Biomed Mater 2021 Dec 7;124:104884. Epub 2021 Oct 7.

Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, B3H 4R2, Canada. Electronic address:

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/cm and 209 mW/cm, 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/cm and 209 mW/cm 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/cm and 209 mW/cm irradiance levels, respectively, are within two standard deviations of the corresponding transition times.

Significance: At an irradiance level much greater than 1000 mW/cm, 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.
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http://dx.doi.org/10.1016/j.jmbbm.2021.104884DOI Listing
December 2021

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

Dent Mater 2021 Oct 6. Epub 2021 Oct 6.

School of Dental Medicine, Craniofacial Biology, Aurora, CO, USA; Chemical and Biological Engineering, Boulder, CO, USA; Materials Science and Engineering, Boulder, CO, USA. Electronic address:

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.
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http://dx.doi.org/10.1016/j.dental.2021.09.017DOI Listing
October 2021

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

Polym Chem 2021 Jul 29;12(25):3619-3628. Epub 2021 May 29.

Department of Chemical and Biological Engineering, University of Colorado Boulder, 596 UCB, Boulder, CO 80309-0596, United States.

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 (k), reverse propagation (k), chain-transfer (k), and overall reaction (k) 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. 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.
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http://dx.doi.org/10.1039/d1py00363aDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8409055PMC
July 2021

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.

Dent Mater 2021 10 26;37(10):1592-1600. Epub 2021 Aug 26.

Department of Chemical and Biological Engineering, University of Colorado Boulder, 596 UCB, Boulder, CO, United States; Materials Science and Engineering Program, University of Colorado Boulder, 596 UCB, Boulder, CO, United States. Electronic address:

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/mvs 1.6 MJ/m) 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/mmvs 38 μg/mm) and much lower water solubility (2.3 μg/mmvs 4.4 μg/mm).

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.
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http://dx.doi.org/10.1016/j.dental.2021.08.010DOI Listing
October 2021

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

Dent Mater 2021 08 5;37(8):1295-1306. Epub 2021 Jun 5.

University of Colorado-Anschutz Medical Campus, Craniofacial Biology Department, Research Complex-I North, 13065 E. 17th Avenue, Aurora, CO 80045 United States of America. Electronic address:

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.
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http://dx.doi.org/10.1016/j.dental.2021.05.001DOI Listing
August 2021

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

Dent Mater 2021 04 8;37(4):578-587. Epub 2021 Feb 8.

Department of Chemical and Biological Engineering, University of Colorado Boulder, CO, United States; Department of Craniofacial Biology, School of Dental Medicine, University of Colorado Anschutz Medical Campus, Aurora, CO, United States. Electronic address:

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.
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http://dx.doi.org/10.1016/j.dental.2021.01.013DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7996127PMC
April 2021

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

Dent Mater 2021 02 19;37(2):336-350. Epub 2021 Jan 19.

Department of Craniofacial Biology, School of Dental Medicine, University of Colorado Anschutz Medical Campus, Aurora, CO, USA.

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.
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http://dx.doi.org/10.1016/j.dental.2020.11.030DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7855936PMC
February 2021

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

J Mech Behav Biomed Mater 2020 08 7;108:103755. Epub 2020 Apr 7.

Department of Craniofacial Biology, School of Dental Medicine, University of Colorado Anschutz Medical Campus, RC1-South, L18-1101, Mail Stop 8120, 12801 E. 17th Avenue, Aurora, CO, 80045, USA; Materials Science and Engineering, University of Colorado, Boulder, CO, 80309, USA. Electronic address:

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.
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http://dx.doi.org/10.1016/j.jmbbm.2020.103755DOI Listing
August 2020

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

Polym Chem 2020 Jan 3;11(1):39-46. Epub 2019 Dec 3.

Department of Chemical and Biological Engineering, University of Colorado Boulder, 3415 Colorado Avenue, Boulder, Colorado 80303, United States.

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 (ϵ) and toughness (T) attained in bulk were not realized for 3D printed components (ϵ ~ 790%, T ~ 102 MJ m vs. ϵ < 5%, T < 0.5 MJ m). 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 (T). 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 (ϵ ~ 790%, T ~ 95 MJ m). 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.
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http://dx.doi.org/10.1039/C9PY01452GDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6941418PMC
January 2020

Vinyl sulfonamide based thermosetting composites via thiol-Michael polymerization.

Dent Mater 2020 02 30;36(2):249-256. Epub 2019 Nov 30.

Department of Chemical and Biological Engineering, University of Colorado Boulder, Boulder, CO, United States. Electronic address:

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.
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http://dx.doi.org/10.1016/j.dental.2019.11.012DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7012731PMC
February 2020

Dynamic Covalent Chemistry at Interfaces: Development of Tougher, Healable Composites through Stress Relaxation at the Resin-Silica Nanoparticles Interface.

Adv Mater Interfaces 2018 Sep 3;5(18). Epub 2018 Jul 3.

Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309-0596, USA.

The interfacial region in composites that incorporate filler materials of dramatically different modulus relative to the resin phase acts as a stress concentrator and becomes a primary locus for composite failure. A novel adaptive interface (AI) platform formed by coupling moieties capable of dynamic covalent chemistry (DCC) is introduced to the resin-filler interface to promote stress relaxation. Specifically, silica nanoparticles (SNP) are functionalized with a silane capable of addition fragmentation chain transfer (AFT), a process by which DCC-active bonds are reversibly exchanged upon light exposure and concomitant radical generation, and copolymerized with a thiol-ene resin. At a fixed SNP loading of 25 wt%, the toughness (2.3 MJ m) is more than doubled and polymerization shrinkage stress (0.4 MPa) is cut in half in the AI composite relative to otherwise identical composites that possess a passive interface (PI) with similar silane structure, but without the AFT moiety. In situ activation of the AI during mechanical loading results in 70% stress relaxation and three times higher fracture toughness than the PI control. When interfacial DCC was combined with resin-based DCC, the toughness was improved by 10 times relative to the composite without DCC in either the resin or at the resin-filler interface.
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http://dx.doi.org/10.1002/admi.201800511DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6521971PMC
September 2018

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

Soft Matter 2019 May;15(18):3740-3750

Department of Chemical and Biological Engineering, University of Colorado Boulder, UCB 596, Colorado 80309, USA.

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.
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http://dx.doi.org/10.1039/c8sm02181cDOI Listing
May 2019

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

Dent Mater 2019 06 16;35(6):928-936. Epub 2019 Apr 16.

Craniofacial Biology Department, University of Colorado, Aurora, CO, United States; Chemical and Biological Engineering, University of Colorado, Boulder, CO, United States. Electronic address:

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.
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http://dx.doi.org/10.1016/j.dental.2019.03.007DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6530473PMC
June 2019

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

J Am Chem Soc 2019 04 8;141(15):6279-6291. Epub 2019 Apr 8.

National Renewable Energy Laboratory, Golden , Colorado 80401 , United States.

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 S2 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 R = 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.
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http://dx.doi.org/10.1021/jacs.8b13679DOI Listing
April 2019

Photopolymerization kinetics of methyl methacrylate with reactive and inert nanogels.

J Mech Behav Biomed Mater 2018 09 2;85:218-224. Epub 2018 Jun 2.

Department of Craniofacial Biology, School of Dental Medicine, University of Colorado Anschutz Medical Campus, Aurora, USA; Materials Science and Engineering, University of Colorado Boulder, Boulder, USA. Electronic address:

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 (T) 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.
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http://dx.doi.org/10.1016/j.jmbbm.2018.06.003DOI Listing
September 2018

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

Polym Chem 2018 Jan 29;9(1):121-130. Epub 2017 Nov 29.

Department of Chemical and Biological Engineering, University of Colorado Boulder, Boulder, CO.

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., T of 115 °C and a rubbery modulus of 1.0 MPa for the CuAAC foam, T 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/m. In contrast, the CuAAC foam exhibited pronounced ductile behavior in the glassy state with three times higher toughness of 850 MJ/m after the first cycle of compression to 80% strain. Additionally, when the CuAAC foam was heated above T 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.
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http://dx.doi.org/10.1039/c7py01121kDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5736377PMC
January 2018

Application of an Addition-Fragmentation-Chain Transfer Monomer in Di(meth)acrylate Network Formation to Reduce Polymerization Shrinkage Stress.

Polym Chem 2017 Aug 21;8(30):4339-4351. Epub 2017 Jun 21.

Department of Chemical and Biological Engineering, University of Colorado Boulder.

A new addition-fragmentation chain transfer (AFT) capable moiety was incorporated into a dimethacrylate monomer that participated readily in network formation by copolymerizing with multifunctional methacrylates or acrylates. The process of AFT occurred simultaneously with photopolymerization of the AFT monomer (AFM) and other (meth)acrylate monomers leading to polymer stress relaxation via network reconfiguration. At low loading levels of the AFM, a significant reduction in shrinkage stress, especially for acrylate monomers, was observed with nominal effects on conversion. At higher loading levels of the AFM, the photopolymerization reaction kinetics and final double bond conversion were significantly lowered along with a delay in the gel-point conversion. Electron paramagnetic resonance studies during polymerization revealed the presence of a distinct radical species that was present in proportional quantities to the AFM content in the system. The lifetime and the character of the persistent radicals were altered due to the presence of the distinctive radical, in turn affecting the polymerization kinetics. With polymerization conducted at higher irradiance, the differential conversion between the control resin and samples with moderate AFM content was minimal, especially for the methacrylate-based formulations.
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http://dx.doi.org/10.1039/C7PY00702GDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5665588PMC
August 2017

Water-soluble clickable nucleic acid (CNA) polymer synthesis by functionalizing the pendant hydroxyl.

Chem Commun (Camb) 2017 Sep;53(73):10156-10159

Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309-0596, USA.

Synthetic biomacromolecules that mimic natural polymeric structures are of significant interest. For most applications of these materials, however, aqueous solubility is a necessity. Here, we present the synthesis of an intrinsically water soluble single stranded DNA analog formed by the synthesis of a Clickable Nucleic Acid (CNA). These molecules are formed with pendant hydroxyl groups present on the main polymer backbone, and subsequent modification of those hydroxyls with sulfonate moieties further enhances the hydrophilicity of these molecules.
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http://dx.doi.org/10.1039/c7cc05542kDOI Listing
September 2017

Thiol-functionalized nanogels as reactive plasticizers for crosslinked polymer networks.

J Mech Behav Biomed Mater 2017 10 27;74:296-303. Epub 2017 Apr 27.

Department of Craniofacial Biology University of Colorado-School of Dental Medicine, Aurora, CO 80045, USA. Electronic address:

Significant efforts have been expended to mitigate plasticizer migration from crosslinked methacrylic and poly(vinyl chloride) polymer networks by synthesizing reactive plasticizers that can blend homogenously within the networks to reduce polymer property change, acute toxicity and downstream environmental effects of plasticizer migration with limited and varying amount of success. We hypothesized that appropriate thiol-functionalized nanogels synthesized using the same monomers as the parent network to generate highly compact, crosslinked structures will form thermally stable, homogenous networks and perform as optimal reactive plasticizers. Nanogels were synthesized via a thiol-Michael addition solution polymerization and incorporated at different mass ratios within a polyethylene glycol 400 urethane dimethacrylic monomer to form photo-crosslinked networks. While maintaining the inherent hydrolytic stability, thermal stability and biocompatibility of the parent matrix at ~99% acrylic group conversion, the PEG400 urethane dimethacrylic -nanogel networks retained optical clarity with >90% visible light transmission at 20wt% nanogel concentration within the matrix. The addition of the nanogels also enhanced the elongation of the parent matrix by up to 320%, while a 37°C reduction in glass transition temperature (∆T) and ≥50% reduction in modulus was observed. A 52% reduction in the shrinkage stress of the material was also noted. The results indicate that the application of thiol-functionalized nanogels as plasticizers to alter the bulk properties of the parent matrix while mitigating plasticizer migration by covalently crosslinking the nanogels within the polymer matrix provides a simple yet efficient technique to generate network-specific plasticizers with the ability to alter targeted properties within polymers.
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http://dx.doi.org/10.1016/j.jmbbm.2017.04.023DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5582010PMC
October 2017

Kinetics and mechanics of photo-polymerized triazole-containing thermosetting composites via the copper(I)-catalyzed azide-alkyne cycloaddition.

Dent Mater 2017 06 28;33(6):621-629. Epub 2017 Mar 28.

Department of Chemical and Biological Engineering, University of Colorado Boulder, 596 UCB, Boulder, CO, United States. Electronic address:

Objective: Several features necessary for polymer composite materials in practical applications such as dental restorative materials were investigated in photo-curable CuAAC (copper(I)-catalyzed azide-alkyne cycloaddition) thermosetting resin-based composites with varying filler loadings and compared to a conventional BisGMA/TEGDMA based composite.

Methods: Tri-functional alkyne and di-functional azide monomers were synthesized for CuAAC resins and incorporated with alkyne-functionalized glass microfillers for CuAAC composites. Polymerization kinetics, in situ temperature change, and shrinkage stress were monitored simultaneously with a tensometer coupled with FTIR spectroscopy and a data-logging thermocouple. The glass transition temperature was analyzed by dynamic mechanical analysis. Flexural modulus/strength and flexural toughness were characterized in three-point bending on a universal testing machine.

Results: The photo-CuAAC polymerization of composites containing between 0 and 60wt% microfiller achieved ∼99% conversion with a dramatic reduction in the maximum heat of reaction (∼20°C decrease) for the 60wt% filled CuAAC composites as compared with the unfilled CuAAC resin. CuAAC composites with 60wt% microfiller generated more than twice lower shrinkage stress of 0.43±0.01MPa, equivalent flexural modulus of 6.1±0.7GPa, equivalent flexural strength of 107±9MPa, and more than 10 times higher energy absorption of 10±1MJm when strained to 11% relative to BisGMA-based composites at equivalent filler loadings.

Significance: Mechanically robust and highly tough, photo-polymerized CuAAC composites with reduced shrinkage stress and a modest reaction exotherm were generated and resulted in essentially complete conversion.
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http://dx.doi.org/10.1016/j.dental.2017.03.010DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5450904PMC
June 2017

Influence of nanogel additive hydrophilicity on dental adhesive mechanical performance and dentin bonding.

Dent Mater 2016 11 21;32(11):1406-1413. Epub 2016 Sep 21.

Department of Craniofacial Biology, University of Colorado-School of Dental Medicine, 12800 E 19th Ave./RC1-North-Rm. 2104, Mail Stop 8310, Aurora, United States. Electronic address:

Objective: To assess the influence of hydrophilicity of reactive nanogels on the mechanical performance of dental adhesives and microtensile bond strength (μTBS) to dentin after 24h or 3 months of aging.

Methods: A series of three nanogels were synthesized: NG1-IBMA/UDMA; NG2-HEMA/BisGMA; NG3-HEMA/TE-EGDMA. The nanogels were dispersed in solvent, HEMA or BisGMA/HEMA. The degree of conversion (DC) of the materials was measured and the flexural modulus of these polymers was evaluated in dry or wet conditions. For μTBS analysis, a model adhesive was used without nanogel (control) or with the incorporation of nanogels. μTBS was evaluated after storage in distilled water for 24h or 3 months. The analysis of the fracture was performed after μTBS testing. Data were analyzed using ANOVA and Tukey's test (α=0.05).

Results: Water significantly increased the modulus of NG1 and NG2 dispersed in solvent, while significantly decreased the stiffness of NG3. All polymers dispersed in HEMA and BisGMA/HEMA had significantly lower modulus when stored in water. NG2 showed the highest DC in solvent and BisGMA/HEMA. In HEMA, NG1 and NG3 produced the highest DC. After three months, NG2 showed the best μTBS. The μTBS of NG2-containing adhesive resin significantly increased after 3 months, while storage had no effect in the control group, NG1 and NG3.

Significance: The more hydrophobic IBMA/UDMA nanogel showed higher bulk material mechanical property results, but the best dentin bond strength values, and notably strength values that improved upon storage, were obtained with the amphiphilic nanogel based on BisGMA/HEMA.
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http://dx.doi.org/10.1016/j.dental.2016.09.016DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5506551PMC
November 2016

Reduced shrinkage stress via photo-initiated copper(I)-catalyzed cycloaddition polymerizations of azide-alkyne resins.

Dent Mater 2016 11 11;32(11):1332-1342. Epub 2016 Aug 11.

Department of Chemical and Biological Engineering, University of Colorado Boulder, 596 UCB, Boulder, CO, United States; Materials Science and Engineering Program, University of Colorado Boulder, 596 UCB, Boulder, CO, United States. Electronic address:

Objectives: Polymerization shrinkage stress and factors involved in the stress development such as volumetric shrinkage and modulus were investigated in photo-CuAAC (photo-initiated copper(I)-catalyzed azide-alkyne cycloaddition) polymerization and compared with conventional BisGMA-based methacrylate polymerization for their use as alternative dental resins.

Methods: Tri-functional alkyne and di-functional azide monomers were synthesized for photo-CuAAC polymerization. Conversion kinetics, stress development and polymerization shrinkage were determined with FTIR spectroscopy, tensometery, and with a linometer, respectively, for CuAAC and BisGMA-based monomer mixtures using a camphorquinone/amine visible light photoinitiator system. Thermo-mechanical properties for the cured polymer matrices were characterized by dynamic mechanical analysis and in three-point bending on a universal testing machine. Polymerization kinetics, polymerization shrinkage stress, dynamic volumetric shrinkage, glass transition temperature (T), flexural modulus, flexural strength, and flexural toughness were compared between the two different resin systems.

Results: A glassy CuAAC polymer (T=62°C) exhibited 15-25% lower flexural modulus of 2.5±0.2GPa and flexural strength of 117±8MPa compared to BisGMA-based polymer (T=160°C) but showed considerably higher energy absorption around 7.1MJ×m without fracture when strained to 11% via three-point bend compared to the flexural toughness of 2.7MJ×m obtained from BisGMA-based polymer. In contrast to BisGMA-based polymers at 75% functional group conversion, the CuAAC polymerization developed approximately three times lower shrinkage stress with the potential to achieve quantitative conversion under ambient temperature photocuring conditions. Moreover, relatively equivalent dynamic volumetric shrinkage of around 6-7% was observed via both CuAAC and dimethacrylate polymerization, suggesting that the low shrinkage stress of CuAAC polymerization was due to delayed gelation along with slower rate of polymerization and the formation of a more compliant network structure.

Significance: CuAAC crosslinked networks possessed high toughness and low polymerization shrinkage stress with quantitative conversion, which eliminated obstacles associated with BisGMA-based dental resins including limited conversion, unreacted extractable moieties, brittle failure, and high shrinkage stress.
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5075250PMC
http://dx.doi.org/10.1016/j.dental.2016.07.014DOI Listing
November 2016

Smart Antibacterial Surface Made by Photopolymerization.

ACS Appl Mater Interfaces 2016 Oct 7;8(41):28047-28054. Epub 2016 Oct 7.

School of Dental Medicine, University of Colorado , Denver, Colorado 80045, United States.

On the basis of the use of photopolymerization technology, a facile and reliable method for in situ preparation of silver nanoparticles (AgNPs) within PNIPAAm functional surfaces is presented as a means to achieve nonfouling, antibacterial films. The surface properties were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), water contact angle, and thermogravimetric analysis (TGA). The antibacterial and release properties of the surfaces were tested against E. coli: at 37 °C (above the LCST of PNIPAAm), the functional films facilitated the attachment of bacteria, which were then killed by the AgNPs. Changing temperature to 4 °C (below the LCST), swollen PNIPAAm chains led the release of dead bacteria. The results showed that AgNPs/PNIPAAm hybrid surfaces offer a "smart" antibacterial capability in response to the change of environmental temperature.
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http://dx.doi.org/10.1021/acsami.6b09343DOI Listing
October 2016

Combined, Independent Small Molecule Release and Shape Memory via Nanogel-Coated Thiourethane Polymer Networks.

Polym Chem 2016 Jan 25;7(4):816-825. Epub 2015 Nov 25.

Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309; Department of Craniofacial Biology, School of Dental Medicine, Anschutz Medical Campus, Aurora, Colorado, 80045.

Drug releasing shape memory polymers (SMPs) were prepared from poly(thiourethane) networks that were coated with drug loaded nanogels through a UV initiated, surface mediated crosslinking reaction. Multifunctional thiol and isocyanate monomers were crosslinked through a step-growth mechanism to produce polymers with a homogeneous network structure that exhibited a sharp glass transition with 97% strain recovery and 96% shape fixity. Incorporating a small stoichiometric excess of thiol groups left pendant functionality for a surface coating reaction. Nanogels with diameter of approximately 10 nm bearing allyl and methacrylate groups were prepared separately via solution free radical polymerization. Coatings with thickness of 10-30 μm were formed via dip-coating and subsequent UV-initiated thiol-ene crosslinking between the SMP surface and the nanogel, and through inter-nanogel methacrylate homopolymerization. No significant change in mechanical properties or shape memory behavior was observed after the coating process, indicating that functional coatings can be integrated into an SMP without altering its original performance. Drug bioactivity was confirmed via culturing of human mesenchymal stem cells with SMPs coated with dexamethasone-loaded nanogels. This article offers a new strategy to independently tune multiple functions on a single polymeric device, and has broad application toward implantable, minimally invasive medical devices such as vascular stents and ocular shunts, where local drug release can greatly prolong device function.
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http://dx.doi.org/10.1039/C5PY01464FDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4822555PMC
January 2016

A Biosynthetic Scaffold that Facilitates Chondrocyte-Mediated Degradation and Promotes Articular Cartilage Extracellular Matrix Deposition.

Regen Eng Transl Med 2015 Dec;1(1-4):11-21

Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado; BioFrontiers Institute, University of Colorado, Boulder, Colorado; Howard Hughes Medical Institute, University of Colorado, Boulder, Colorado.

Articular cartilage remains a significant clinical challenge to repair because of its limited self-healing capacity. Interest has grown in the delivery of autologous chondrocytes to cartilage defects, and combining cell-based therapies with scaffolds that capture aspects of native tissue and allow cell-mediated remodeling could improve outcomes. Currently, scaffold-based therapies with encapsulated chondrocytes permit matrix production; however, resorption of the scaffold often does not match the rate of matrix production by chondrocytes, which can limit functional tissue regeneration. Here, we designed a hybrid biosynthetic system consisting of poly (ethylene glycol) (PEG) endcapped with thiols and crosslinked by norbornene-functionalized gelatin via a thiol-ene photopolymerization. The protein crosslinker was selected to facilitate chondrocyte-mediated scaffold remodeling and matrix deposition. Gelatin was functionalized with norbornene to varying degrees (~4-17 norbornenes/gelatin), and the shear modulus of the resulting hydrogels was characterized (<0.1-0.5 kPa). Degradation of the crosslinked PEG-gelatin hydrogels by chondrocyte-secreted enzymes was confirmed by gel permeation chromatography. Finally, chondrocytes encapsulated in these biosynthetic scaffolds showed significantly increased glycosaminoglycan deposition over just 14 days of culture, while maintaining high levels of viability and producing a distributed matrix. These results indicate the potential of a hybrid PEG-gelatin hydrogel to permit chondrocyte-mediated remodeling and promote articular cartilage matrix production. Tunable scaffolds that can easily permit chondrocyte-mediated remodeling may be useful in designing treatment options for cartilage tissue engineering applications.
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http://dx.doi.org/10.1007/s40883-015-0002-3DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4758520PMC
December 2015

Controlled nanogel and macrogel structures from self-assembly of a stimuli-responsive amphiphilic block copolymer.

RSC Adv 2016 4;6(69):64791-64798. Epub 2016 Jul 4.

Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309, United States.

RAFT polymerization was utilized to prepare an amphiphilic block copolymer containing both hydrophilic and hydrophobic segments. The self-assembly behavior of the block copolymer into nano-scale particulate structures was studied in both water and polar organic solvents. Uniform micelle assemblies were stabilized by reaction within the hydrophobic core, which contained pendant azide groups, through copper-catalyzed azide-alkyne cycloaddition (CuAAC) click chemistry with a dialkyne crosslinker. The reaction preceded efficiently with negligible residual azide functionality and resulted in core-shell nanogel structures that were analyzed by a variety of techniques including light scattering, electron microscopy and the ability to take up hydrophobic molecules. Both thermo- and pH-responsive character of the nanogels and the linear polymers from which they were made were studied through cloud point testing at different pH levels. It was found that these nanogel dispersions in water exhibited the highest cloud point temperatures indicating a highly stable nanogel structure. The solvent-dispersed nanogels were used as building blocks to form extended polymer networks through the inter- as well as intra-particle reaction between hydroxyl groups within the hydrophilic domain of the nanogel shell by crosslinking with a diisocyanate. It was found that as little as 10 wt% nanogel dispersions in solvent reached the percolation threshold to yield highly porous macroscopic networks; while 50 wt% concentrations achieved densely packed and interdigitated nanogels to afford relatively homogeneous structures.
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http://dx.doi.org/10.1039/c6ra03933bDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5507065PMC
July 2016

3D printing with polymers: Challenges among expanding options and opportunities.

Dent Mater 2016 Jan 20;32(1):54-64. Epub 2015 Oct 20.

Colorado Photopolymer Solutions, 1880 South Flatirons Ct, Suite F, Boulder, CO 80301, USA. Electronic address:

Objectives: Additive manufacturing, which is more colloquially referred to as 3D printing, is quickly approaching mainstream adoption as a highly flexible processing technique that can be applied to plastic, metal, ceramic, concrete and other building materials. However, taking advantage of the tremendous versatility associated with in situ photopolymerization as well as the ability to select from a variety of preformed processible polymers, 3D printing predominantly targets the production of polymeric parts and models. The goal of this review is to connect the various additive manufacturing techniques with the monomeric and polymeric materials they use while highlighting emerging material-based developments.

Methods: Modern additive manufacturing technology was introduced approximately three decades ago but this review compiles recent peer-reviewed literature reports to demonstrate the evolution underway with respect to the various building techniques that differ significantly in approach as well as the new variations in polymer-based materials being employed.

Results: Recent growth of 3D printing has been dramatic and the ability of the various platform technologies to expand from rapid production prototypic models to the greater volume of readily customizable production of working parts is critical for continued high growth rates. This transition to working part production is highly dependent on adapting materials that deliver not only the requisite design accuracy but also the physical and mechanical properties necessary for the application.

Significance: With the weighty distinction of being called the next industrial revolution, 3D printing technologies is already altering many industrial and academic operations including changing models for future healthcare delivery in medicine and dentistry.
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http://dx.doi.org/10.1016/j.dental.2015.09.018DOI Listing
January 2016

Ester-free thiol-ene dental restoratives--Part A: Resin development.

Dent Mater 2015 Nov 7;31(11):1255-62. Epub 2015 Sep 7.

Department of Chemical and Biological Engineering, University of Colorado, 3415 Colorado Ave, JSC Biotech Building, Boulder, CO 80309, USA. Electronic address:

Objectives: To detail the development of ester-free thiol-ene dental resins with enhanced mechanical performance, limited potential for water uptake/leachables/degradation and low polymerization shrinkage stress.

Methods: Thiol-terminated oligomers were prepared via a thiol-Michael reaction and a bulky tetra-allyl monomer containing urethane linkages was synthesized. The experimental oligomers and/or monomers were photopolymerized using visible light activation. Several thiol-ene formulations were investigated and their performance ranked by comparisons of the thermo-mechanical properties, polymerization shrinkage stress, water sorption/solubility, and reactivity with respect to a control comprising a conventional BisGMA/TEGDMA dental resin.

Results: The ester-free thiol-ene formulations had significantly lower viscosities, water sorption and solubility than the BisGMA/TEGDMA control. Depending on the resin, the limiting functional conversions were equivalent to or greater than that of BisGMA/TEGDMA. At comparable conversions, lower shrinkage stress values were achieved by the thiol-ene systems. The polymerization shrinkage stress was dramatically reduced when the tetra-allyl monomer was used as the ene in ester-free thiol-ene mixtures. Although exhibiting lower Young's modulus, flexural strength, and glass transition temperatures, the toughness values associated with thiol-ene resins were greater than that of the BisGMA/TEGDMA control. In addition, the thiol-ene polymerization resulted in highly uniform polymer networks as indicated by the narrow tan delta peak widths.

Significance: Employing the developed thiol-ene resins in dental composites will reduce shrinkage stress and moisture absorption and form tougher materials. Furthermore, their low viscosities are expected to enable higher loadings of functionalized micro/nano-scale filler particles relevant for practical dental systems.
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http://dx.doi.org/10.1016/j.dental.2015.08.148DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5033514PMC
November 2015

Ester-free thiol-ene dental restoratives--Part B: Composite development.

Dent Mater 2015 Nov 8;31(11):1263-70. Epub 2015 Sep 8.

Department of Chemical and Biological Engineering, University of Colorado, 3415 Colorado Ave, JSC Biotech Building, Boulder, CO 80309, USA. Electronic address:

Objectives: To assess the performance of thiol-ene dental composites based on selected ester-free thiol-ene formulations. Further, to point out the benefits/drawback of having a hydrolytically stable thiol-ene matrix within a glass filled composite.

Methods: Composite samples containing 50-65wt% of functionalized glass microparticles were prepared and photopolymerized in the presence of a suitable visible light photoinitiator. Shrinkage stress measurements were conducted as a function of the irradiation time. Degrees of conversion were measured by FT-IR analysis by comparing the double bond signals before and after photopolymerization. Mechanical tests were carried out on specimens after curing as well as after extended aging in water. Dynamic mechanical analysis was employed to track the changes in storage modulus near body temperature. The properties of the thiol-ene composites were compared with those of the BisGMA/TEGDMA control.

Results: Depending on the resin type, similar or higher conversions were achieved in thiol-ene composites when compared to the dimethacrylate controls. At comparable conversions, lower shrinkage stress values were achieved. Although exhibiting lower initial elastic moduli, the thiol-ene composites' flexural strengths were found to be comparable with the controls. Contrary to the control, the mechanical properties of the ester-free thiol-ene composites were shown to be unaffected by extensive aging in water and at least equaled that of the control after aging in water for just five weeks.

Significance: Employing non-degradable step-growth networks as organic matrices in dental composites will provide structurally uniform, tough materials with extended service time.
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http://dx.doi.org/10.1016/j.dental.2015.08.147DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4617615PMC
November 2015

Modification of linear prepolymers to tailor heterogeneous network formation through photo-initiated Polymerization-Induced Phase Separation.

Polymer (Guildf) 2015 Jul;70:8-18

Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309, USA. ; Department of Craniofacial Biology, School of Dental Medicine, University of Colorado, Aurora, CO 80045, USA.

Polymerization-induced phase separation (PIPS) was studied in ambient photopolymerizations of triethylene glycol dimethacrylate (TEGDMA) modified by poly(methyl methacrylate) (PMMA). The molecular weight of PMMA and the rate of network formation (through incident UV-irradiation) were varied to influence both the promotion of phase separation through increases in overall free energy, as well as the extent to which phase development occurs during polymerization through diffusion prior to network gelation. The overall free energy of the polymerizing system increases with PMMA molecular weight, such that PIPS is promoted thermodynamically at low loading levels (5 wt%) of a higher molecular weight PMMA (120 kDa), while a higher loading level (20 wt%) is needed to induce PIPS with lower PMMA molecular weight (11 kDa), and phase separation was not promoted at any loading level tested of the lowest molecular weight PMMA (1 kDa). Due to these differences in overall free energy, systems modified by PMMA (11 kDa) underwent phase separation via Nucleation and Growth, and systems modified by PMMA (120 kDa), followed the Spinodal Decomposition mechanism. Despite differences in phase structure, all materials form a continuous phase rich in TEGDMA homopolymer. At high irradiation intensity (I=20mW/cm), the rate of network formation prohibited significant phase separation, even when thermodynamically preferred. A staged curing approach, which utilizes low intensity irradiation (I=300µW/cm) for the first ~50% of reaction to allow phase separation via diffusion, followed by a high intensity flood-cure to achieve a high degree of conversion, was employed to form phase-separated networks with reduced polymerization stress yet equivalent final conversion and modulus.
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http://dx.doi.org/10.1016/j.polymer.2015.06.002DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4503221PMC
July 2015
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