Maximum Rate Of Polymerization Research Articles

Effect of preheating on mechanical properties of a resin-based composite containing elastomeric urethane monomer

This study investigated the effect of preheating an elastomeric urethane monomer (Exothane-24) experimental resin composite on its physicochemical properties. Two resin matrices were formulated: (a) 50 wt% Bisphenol-glycidyl methacrylate (Bis-GMA) and 50 wt% triethylene glycol dimethacrylate (TEGDMA); and (b) 20 wt% Exothane-24, 40 wt% Bis-GMA and 40 wt% TEGDMA. A photoinitiator system (0.25 wt% camphorquinone and 0.50 wt% ethyl-4-dimethylamino benzoate) and 65 wt% of the inorganic filler (20 wt% 0.05 μm silica and 80 wt% 0.7 μm BaBSiO2 glass) were added to both matrices. These formulations were then assigned to four groups: Exothane-24 (E); Exothane-24 plus preheating (EH); no Exothane-24 (NE); and no Exothane-24 plus preheating (NEH). NEH and EH were preheated at 69 °C. The dependent variables were as follows: film thickness (FT); polymerization shrinkage stress (PSS); gap width (GW); maximum rate of polymerization (Rpmax); and degree of conversion (DC). Data were statistically analyzed by two-way ANOVA and Tukey's test (α = 0.05). Preheating reduced FT for both composites. PSS and GW were significantly lower for EH, when compared with E. The DC for EH and NEH and the Rpmax for EH increased significantly. Preheating improved most of the physicochemical properties (FT, PSS, GW, and DC) of the experimental resin composite containing Exothane-24.

Effect of polymerization mode on shrinkage kinetics and degree of conversion of dual-curing bulk-fill resin composites

ObjectivesTo assess the behavior of dual-cure and conventional bulk-fill composite materials on real-time linear shrinkage, shrinkage stress, and degree of conversion.Materials and methodsTwo dual-cure bulk-fill materials (Cention, Ivoclar Vivadent (with ion-releasing properties) and Fill-Up!, Coltene) and two conventional bulk-fill composites (Tetric PowerFill, Ivoclar Vivadent; SDR flow + , Dentsply Sirona) were compared to conventional reference materials (Ceram.x Spectra ST (HV), Dentsply Sirona; X-flow; Dentsply Sirona). Light curing was performed for 20 s, or specimens were left to self-cure only. Linear shrinkage, shrinkage stress, and degree of conversion were measured in real time for 4 h (n = 8 per group), and kinetic parameters were determined for shrinkage stress and degree of conversion. Data were statistically analyzed by ANOVA followed by post hoc tests (α = 0.05). Pearson’s analysis was used for correlating linear shrinkage and shrinkage force.ResultsSignificantly higher linear shrinkage and shrinkage stress were found for the low-viscosity materials compared to the high-viscosity materials. No significant difference in degree of conversion was revealed between the polymerization modes of the dual-cure bulk-fill composite Fill-Up!, but the time to achieve maximum polymerization rate was significantly longer for the self-cure mode. Significant differences in degree of conversion were however found between the polymerization modes of the ion-releasing bulk-fill material Cention, which also exhibited the significantly slowest polymerization rate of all materials when chemically cured.ConclusionsWhile some of the parameters tested were found to be consistent across all materials studied, heterogeneity increased for others.Clinical relevanceWith the introduction of new classes of composite materials, predicting the effects of individual parameters on final clinically relevant properties becomes more difficult.

Blue Laser for Polymerization of Bulk-Fill Composites: Influence on Polymerization Kinetics.

The objective of this study was to compare the polymerization kinetics of bulk-fill resin composites cured with a LED-curing device and a diode laser (449 nm). Three bulk-fill composites were light-cured with constant radiation exposure at 10 J/cm2 by varying radiant exitance and curing time. The following three light-curing protocols were used: (I) 3300 mW/cm2 for 3 s; (II) 2000 mW/cm2 for 5 s; and (III) 1000 mW/cm2 for 10 s. The degree of conversion (DC) was monitored in real time at a data acquisition rate of 2 spectra/s over a 5-min period and again after seven days using Fourier transform infrared spectroscopy. DC amounted to 30.9-61.7% at 4-mm depth after 5 min. DC values of two sculptable composites were significantly higher with the laser, regardless of the curing protocol used, but not for the flowable composite. The maximum polymerization rate (2.0-22.1%/s) was less affected by the type of curing device for one of the composites, while the other two composites achieved significantly higher values when cured with the laser. Laser curing generally increased the DC and the maximum polymerization rate while it shortened the onset of the maximum reaction rate. New handheld laser devices with adjustable power have the potential to be used as a photopolymerization light source for new generations of bulk-fill composites.

Polymerization and shrinkage kinetics and fracture toughness of bulk-fill resin-composites.

To determine degree of conversion (DC), maximum polymerization rate (RPmax), polymerization shrinkage (PS), maximum shrinkage rate (PS Rmax) and fracture toughness (KIC) of different types of bulk-fill (BF) composites plus the effect of viscosity reduction techniques. BF specimens were created in 2 mm deep molds: SonicFill 3 (SF3), Viscalor (VC), One Bulk Fill (OBF) and Beautifil Bulk (BBR). SF3 was applied via sonic insertion using a SonicFill handpiece (Kerr Corp. USA). Viscalor was pre-heated in a Caps Warmer in T3 mode (at 68 °C) for 30 s (T3-30 s) and 3 min (T3-3 min), respectively. Specimens were irradiated at zero distance from the upper surface with an Elipar S10 LED unit (3 M ESPE, USA) of mean irradiance 1200 mW/cm2 for 40 s. Real-time polymerization kinetics and DC at 5 min and 24 h post-irradiation (DC5 min and DC24 h) were measured using ATR-FTIR (n = 3). PS was measured up to 1 h on 1 mm thick discs via the bonded-disk technique (n = 3) and PS Rmax obtained by numerical differentiation (n = 3). For fracture toughness, single-edge-notched specimens (32 × 6 ×3 mm) of each BF composite were prepared and measured by three-point bending after 7 d water storage (n = 5). Data were analysed using One-way ANOVA, independent T-tests and Tukey post-hoc tests (p < 0.05). SF3 showed the significantly highest DC5 min, DC24 h and RPmax (p < 0.05), followed by OBF (p < 0.05). Regardless of pre-heating, VC showed comparable conversion kinetics to BBR (p > 0.05). There was no significant difference in PS of these BF composites, except OBF had the highest PS (p > 0.05). However, PS Rmax significantly varied among materials (p = 0.047) and SF3 had the highest PS Rmax. Regarding fracture toughness, BBR had the lowest KIC (p < 0.05), whereas other composites showed similar KIC (p > 0.05). Strong correlations of filler content (wt%)-PS/KIC were found. Different pre-heating times had no significant influences on DC %, RPmax, PS, PS Rmax and KIC of VC (p > 0.05). Different types of bulk-fill composites showed comparable shrinkage. A highly filled BF giomer composite (BBR) had the lowest fracture toughness, whereas others had similar KIC. Pre-heating had no adverse effects on Viscalor properties. Sonication and pre-heating are beneficial techniques to enhance composite flowability without either increasing shrinkage or reducing fracture toughness.

Influence of Initiator Concentration on the Polymerization Course of Methacrylate Bone Cement.

Background: The amount of oxidant (initiator) and reductant (co-initiator) and their ratio have a significant effect on the properties of polymethacrylate bone cement, such as maximum temperature (Tmax), setting time (tset) and compressive strength (σ). The increase in the initiating system concentration causes an increase in the number of generated radicals and a faster polymerization rate, which shortens the setting time. The influence of the redox-initiating composition on the course of polymerization (rate of polymerization and degree of double bond conversion) and the mechanical properties of bone cement will be analyzed. Methods: Bone cements were synthesized by mixing a powder phase composed of two commercially available methacrylate copolymers (Evonic) and a liquid phase containing 2-hydroxyethyl methacrylate (HEMA), methyl methacrylate (MMA), and triethylene glycol dimethacrylate (D3). As an initiating system, the benzoyl peroxide (BPO) as an oxidant (initiator) in combination with a reducing agent (co-initiator), N,N-dimethylaniline (DMA), was used. Samples were prepared with various amounts of peroxide BPO (0.05%, 0.1%, 0.2%, 0.3%, 0.5% and 0.7% by weight) with a constant amount of reducing agent DMA (0.5 wt.%), and various amounts of DMA (0.25%, 0.35% and 0.5% by weight) with a constant amount of BPO (0.3 wt.%). The polymerization kinetics were studied by differential scanning calorimetry (DSC). Doughing time and compressive strength tests were carried out according to the requirements of the ISO 5833:2002 standard. Results: The increase in polymerization rate was due to the increase in the amount of BPO. In addition, the curing time was shortened, as well as the time needed to achieve the maximum polymerization rate. The final conversion of the double bonds in the studied compositions was in the range 74-100%, and the highest value of this parameter was obtained by the system with 0.3 wt.% of BPO. The doughing times for each BPO concentration were in the range of 90-140 s. The best mechanical properties were obtained for the cement following the initiating system concentrations: 0.3 wt.% of BPO and 0.5 wt.% of DMA. Nevertheless, all tested cements met the requirements of the ISO 5833:2002 standard. Conclusions: Based on the conducted polymerization kinetic studies, the best reaction conditions are provided by an initiating system containing 0.3 wt.% of BPO oxidant (initiator) and 0.5 wt.% of DMA reductant (co-initiator). A decrease in the DMA amount caused a decrease in the polymerization rate and the amount of heat released during the reaction. The change in BPO and DMA concentrations in the composition had little effect on the doughing time of the studied bone cement. The cements showed similar doughing times, ranging from 90-225 s, which is comparable to the bone cement available on the market.

Polymerization Kinetics, Shrinkage Stress, and Bond Strength to Dentin of Conventional and Self-adhesive Resin Cements.

To evaluate the kinetics of polymerization and shrinkage stress of resin cements, as well as their bond strength to dentin after 24-h or one-year water storage. Three conventional resin cements were evaluated: RelyX Ultimate (RUL), Panavia V5 (PNV), and Multilink N (MLN); and three self-adhesive resin cements: RelyX Unicem 2 (RUN), Panavia SA Cement Plus (PSA), and G-CEM LinkAce (GCL). Degree of conversion (DC), maximum polymerization rate (RPmax) and gel time values were obtained using Fourier-transform infrared spectroscopy (FTIR/ATR). Shrinkage stress values were determined with a tensiometer, using a universal testing machine (n=5). Indirect resin composite restorations (Solidex) were fabricated and cemented to the dentin surface using self-adhesive resin cements, or conventional resin cements with self-etching adhesive (n=5). Bonding performance was evaluated with the microtensile bond strength (µTBS) test after 24 h or one year of water storage. MLN exhibited a higher DC (76.7%), whereas the percentage of other materials differed slightly (ranging from 54% to 58.5%). The RPmax and shrinkage stress values differed significantly between the cements. PSA showed the longest gel time. Significantly higher µTBS were observed for conventional resin cements after 24-h and one-year storage; a decrease in µTBS was observed for MLN only. Self-adhesive resin cements may not perform as well as conventional resin cements. Although both categories of cements presented similar polymerization kinetics and shrinkage values, the self-adhesive resin cements showed lower µTBS compared to those of conventional resin cements. Nevertheless, storage time only affected the bonding performance of MLN.

Influence of the Addition of Sialon and Aluminum Nitride Fillers on the Photocuring Process of Polymer Coatings

This article presents the results of a study on polymer coatings containing poly ethoxylated bisphenol A diacrylate (Bis-AEA10) with aluminum silicon nitride oxide (Sialon) and aluminum nitride (AlN). The polymer coatings were obtained by the photopolymerization technique. Investigations were carried out to determine the effect of the AlN and Sialon content on the UV-curing kinetics, as well as on the mechanical (hardness), thermal (Tg, thermal stability), physicochemical (water contact angle), and structural properties of the polymer coatings. Polymerization rates were characterized as functions of double-bond conversion using the photo-Differential Scanning Calorymetry technique (photo-DSC). The results obtained showed that a small addition of sialon filler (3–5 wt.%) to Bis-AEA10 increases the photopolymerization rate of the varnish, while the addition of more Sialon decreases the rate of photopolymerization. However, for the systems containing AlN filler, the maximum polymerization rate was observed for samples containing 10 wt.% filler. In the case of a varnish composition containing AlN, the maximum polymerization rate is characterized by the system containing 10 wt.% of AlN. This shows that the AlN filler has a good influence on the polymerization process. In either case, the final double bond conversion was high (80%–95%). Mechanical tests have shown that introducing the filler into the polymer matrix increases its hardness. The content of Sialon and AlN in the coatings causes an increase (up to 4–5 wt.%) and a decrease (&gt;4–5 wt.%) in the glass transition temperature. The effect of the addition of fillers on the physicochemical properties of the coating surface has also been investigated and characterized by the water contact angle method. The addition of 20 wt.% Sialon and AlN increased the contact angle of the samples by approximately 40% and 31%, respectively, resulting in coatings with hydrophobic surface properties.

Polymerization Kinetics and Development of Polymerization Shrinkage Stress in Rapid High-Intensity Light-Curing.

This study investigated polymerization kinetics, linear shrinkage, and shrinkage stress development for six contemporary composite materials of different viscosities cured using radiant exitances of 1100–2850 mW/cm2. Real-time measurements of degree of conversion, linear shrinkage, and shrinkage stress were performed over 5 min using Fourier-transform infrared spectrometry, a custom-made linometer, and a custom-made stress analyzer, respectively. For most tested variables, the factor “material” had a higher effect size than the factor “curing protocol”. Maximum polymerization rate and maximum shrinkage stress rate were the most affected by changes in curing conditions. In contrast, no significant effects of curing conditions were identified within each material for shrinkage stress values measured at the end of the 5 min observation period. Linear shrinkage and shrinkage stress values measured after 5 min were closely correlated (R = 0.905–0.982). The analysis of polymerization kinetics suggested that the two composites specifically designed for rapid light-curing responded to higher radiant exitances differently than other composites. Polymerization kinetics and shrinkage stress behavior of contemporary restorative composite materials of different viscosities were overall more affected by material type than differences in curing conditions. Subtle differences in polymerization kinetics behavior shown by the two composites specifically designed for rapid high-intensity light-curing did not translate into significant differences in the development of polymerization shrinkage stress.

Effect of thickness of CAD/CAM materials on light transmission and resin cement polymerization using a blue light-emitting diode light-curing unit.

Evaluate the effect of thickness of high-translucency (HT) CAD/CAM materials on irradiance and beam profile from a blue light-emitting diode light-curing unit (LCU) and on the degree of conversion (DC) and maximum polymerization rate (Rpmax ) of a light-cured resin cement (LCC). The direct output from the LCU, the light transmission and irradiance ratio (IR) through one conventional composite and nine HT CAD/CAM materials (0.5, 1.0, 1.5, or 2.0-mm thick; n=5) were measured with a integrating sphere coupled to a spectrometer. The light beam was assessed with a beam profiler camera. The DC at 600 s and the Rpmax of one LCC was determined using a Fourier transform infrared spectrometer (n=5). Data were analyzed by ANOVA followed by Tukey's tests, and Dunnett's test was also used for irradiance data (α=0.05). A significant decrease in irradiance through all materials occurred as thickness increased. Thin CAD/CAM materials improved light homogeneity, which decreased with the increase in thickness. The DC of the LCC directly exposed to light was the same as when exposed to 45%, 25%, 15%, or 5% IRs. Rpmax decreased with the decrease in IR. Although the HT CAD/CAM materials reduced the irradiance from the LCU, minor effects were observed in the LCC's DC. Despite the light attenuation of blue light through different CAD/CAM materials that were up to 2-mm thick, the degree of conversion of one brand of light-cured resin cement was clinically acceptable when the LCU was used for 30 s.

81 - Improvement of methacrylate-based polymer networks by introduction of catechol moieties

Dental adhesives mainly consist of methacrylate-based monomers as well as solvents and initiators. The bonding mechanism is based on the penetration of adhesive resins into the demineralized dentin surface and formation of interlocking with exposed collagen fibrils. However, phase separation into hydrophilic/hydrophobic domains leads to water sorption and deterioration of mechanical properties. Furthermore, the esterases and matrix metalloproteinases (MMPs) present in oral environment contribute to methacrylate hydrolysis and collagen degradation, respectively. All these factors accelerate decay along the adhesive interface and cause sub-optimal longevity of the resin filling. A bio-inspired moiety, catechol, has been found to form strong bidentate hydrogen bonds with a variety of surfaces. The purpose of the present study was to design a novel bio-inspired adhesive with catechol moieties and evaluate whether catechol can reduce the heterogeneity of methacrylate-based network structure, decrease water sorption, inhibit the enzyme activities, and improve the longevity of adhesive materials. Catechol methacrylate was synthesized by Steglich esterification which is described in the previous literature. The control adhesive formulations include urethane dimethacrylate (UDMA) as crosslinker (50 wt%), 2-hydroxyethyl methacrylate (HEMA) as monomer (50 wt%), and additional 0.5 wt% 2,2-Dimethoxy-2-phenylacetophenone (DMPA) as photoinitiator. In the novel catechol-contained adhesive, 25 wt% of the HEMA was replaced by catechol methacrylate, with the other components unchanged. To evaluate the performance of these two formulations, dynamic mechanical analysis (DMA), Fourier-transform infrared spectroscopy (FT-IR), water sorption and enzyme (esterases) degradation were conducted to characterize the performance. The novel catechol-contained adhesive showed similar photopolymerization kinetics (final conversion and maximum polymerization rate) to the control one, although multiple works showed that catechol exerts an inhibitory effect on the radial photopolymerization. The water sorption of the novel adhesive, however, was nearly 50% lower than the control adhesive. Moreover, the catechol-contained adhesive showed a higher resistance of the esterase hydrolysis, with less weight loss and lower release of 2-hydroxyethyl methacrylate (HEMA) and methacrylic acid (MMA). The addition of catechol to adhesive formulations reduces the water sorption and improves the resistance of enzyme degradation of methacrylate. These might contribute to the formation of extra hydrogen bonding of catechol in the low-density domains and the inhibitory behaviors of catechol to esterases. Introducing catechol into dental adhesive formulations provides a promising way to enhance the long-term performance of dental adhesive materials. Cytotoxicity and micro-tensile bonding strength tests are essential for the future works.

Physicochemical Effects of Niobic Acid Addition Into Dental Adhesives

The incorporation of metallic oxides in dental adhesives has been a strategy to confer improved radiopacity and physicochemical properties for polymers. Tailoring the structure of these fillers could contribute to their application in therapeutic strategies for dental restorations. The aim of this study was to evaluate the incorporation of niobic acid into experimental dental adhesives, and compare these adhesives to niobium pentoxide containing adhesives. A control group without Nb2O5·n H2O or Nb2O5 was also used for comparison. Niobium-based particles have been used as a feasible approach, mainly because of their bioactivity. In this study, hydrated niobium pentoxide, also called niobic acid (Nb2O5·n H2O), was incorporated into an experimental dental adhesive as a potential catalyst for monomer conversion. A base resin for dental adhesive was formulated with methacrylate monomers and photoinitiators. Two types of oxides were tested as filler for this adhesive: Nb2O5·n H2O or niobium pentoxide (Nb2O5). Both fillers were added separately into the experimental adhesive at 0, 2.5, 5, and 10 wt.%. One group without Nb2O5·n H2O or Nb2O5 (0 wt.% of filler addition) was used as a control group. The formulated materials were analyzed for radiopacity according to the ISO 4049 and used FTIR analysis to assess the degree of conversion (DC) and the maximum polymerization rate (RPmax). Mechanical properties were analyzed by ultimate tensile strength (UTS) in a testing machine. Softening in solvent was conducted by measuring Knoop microhardness before and after immersion of samples in ethanol. Normality of data was assessed with Shapiro-Wilk, and comparisons between factors were conducted with two-way ANOVA and Tukey at 5% of significance. Both fillers, Nb2O5 or Nb2O5·n H2O, increased the radiopacity of dental adhesives in comparison to the unfilled adhesive (p &amp;lt; 0.05). There were no differences among groups for the ultimate tensile strength (p &amp;gt; 0.05), and all groups containing Nb2O5 or Nb2O5·n H2O improved the resistance against softening in solvent (p &amp;lt; 0.05). The groups with 5 and 10 wt% addition of Nb2O5 showed decreased DC compared to the control group (p &amp;lt; 0.05), while the addition of Nb2O5·nH2O up to 10 wt% did not alter the DC (p &amp;gt; 0.05). The polymerization rate did not change among groups (p &amp;gt; 0.05). In conclusion, Nb2O5·n H2O is a promising filler to be incorporated into dental adhesives providing proper mechanical properties, improved resistance against solvents, and increased radiopacity, without changing the DC.

Incorporation of Apigenin and tt-Farnesol into dental composites to modulate the Streptococcus mutans virulence

ObjectivesThe aim of this in vitro study was to incorporate two anti-caries agents, Apigenin and tt-Farnesol, to resin composite and resin cement to reduce the virulence of Streptococcus mutans around dental restorations. MethodsApigenin (Api, 5 mM) and tt-Farnesol (Far, 5 mM) were added alone, together, and combined with fluoride (F). Biofilm of S. mutans was grown on composite discs, and the dry-weight, bacterial viability, and the polysaccharides (alkali-soluble, intracellular and water-soluble) were quantified. CLSM images of the S. mutans biofilm were obtained after three years of water-storage. The effect of the additions on the physicochemical properties and the composite colorimetric parameters were also analyzed. ResultsThe additions did not affect bacterial viability. Api alone and combined with Far or combined with Far and F decreased the bacterial dry-weight, alkali-soluble and intracellular polysaccharides. After three years, the composites containing the additions presented a greater EPS matrix on the top of biofilm. Statistical difference was obtained for the degree of conversion; however, the maximum polymerization rate and curing kinetics were unaffected by the additions. No difference was observed for the water-soluble polysaccharides, flexural strength, and elastic modulus. Api increased the yellowness of the composites. SignificanceApi, alone and combined, reduced the expression of virulence of S. mutans without jeopardizing the physicochemical properties of the composites.

Evaluation of physico-mechanical properties and filler particles characterization of conventional, bulk-fill, and bioactive resin-based composites

ObjectiveThis study evaluated physical and mechanical properties and characterized the filler particles of seven composites. Materials and methodsFiltek Supreme (FS, 3M Oral Care), Forma (FO, Ultradent), Charisma Diamond (CD, Kulzer), Spectra Smart (SS, Dentsply), Filtek Bulk Fill (FB, 3M Oral Care), Tetric N-Ceram Bulk Fill (TB, Ivoclar), and Cention N (Ivoclar) in self- (CNSC) or dual-curing (CNDC) were evaluated. Fillers size, shape, and content were analyzed by scanning electron microscopy (SEM) and X-ray dispersive energy spectroscopy (EDX). Disk-shaped specimens (n = 5) were prepared for sorption (SP) and solubility (SL). Flexural strength and elastic modulus were tested at 24 h and 12 months (n = 10). Degree of conversion (DC%) and maximum rate of polymerization (Rpmax) were evaluated using micro-Raman spectroscopy. SP and SL results were submitted to Kruskal-Wallis one-way ANOVA and Dunn's pairwise test (α = 0.05). Mechanical properties were analyzed by 2-way ANOVA and Tukey's test (α = 0.05). DC% of CNSC and CNDC was compared by independent t-test (α = 0.05). Rpmax results were analyzed by 1-way ANOVA and Tukey's test (α = 0.05). ResultsThe composites differed regarding filler size, shape, and content. CD and CNSC showed lower SP than FS. SS had lower SL than CNSC and CNDC. CNDC presented higher DC% than CNSC. CD, TB, and CNDC showed the highest Rpmax. TB, CNSC, and CNDC showed the lowest 24-h flexural strengths. Mechanical properties of CD did not decrease, while FO, TB, and CNSC showed a significant reduction after storage. ConclusionsMonomer composition and fillers characteristics greatly influenced the physico-mechanical properties of the tested composites.

Effect of the addition of functionalized TiO2 nanotubes and nanoparticles on properties of experimental resin composites

ObjectiveTo evaluate the influence of the addition of functionalized and non-functionalized TiO2 nanostructures on properties of a resin composite. MethodsTiO2 nanostructures were synthesized and functionalized, using 3-(aminopropyl)triethoxysilane (APTMS) and 3-(trimethoxysilyl)propyl methacrylate (TSMPM). Characterizations were performed with XRD, EDS, TEM, and TGA. Resin composites containing Bis-GMA/TEGDMA, CQ, DABE, and barium-aluminum silicate glass were produced according to TiO2 nanostructure (nanotube or nanoparticle), concentration (0.3 or 0.9 wt%), and functionalization (APTMS or TSMPM). The resin composite without nanostructures was used as control. The amount of fillers was kept constant at 78.3 wt% for all materials. The degree of conversion (DC - at 0 h and 24 h), maximum polymerization rate (Rpmax), and Knoop microhardness (KHN before and after ethanol softening) were evaluated. Data were analyzed with two-way ANOVA with repeated measures and Tukey's HSD (α = 0.05). ResultsTGA results demonstrated that functionalizations were effective for both nanostructures. For DC, resin composites, time and interaction effect were significant (p < 0.001). Higher DC was found for 0.3-wt%-functionalized-nanotubes at 24 h. For nanoparticles, only 0.9-wt%-non-functionalized and 0.3-wt%-APTMS-functionalized showed DC similar to the control and all other groups showed higher DC (p < 0.05). Rpmax was higher for 0.3-wt%-APTMS-nanotubes, which corresponded to higher DC after 24 h. The lowest Rpmax occurred for 0.9-wt%-TSMPM-nanotubes, which showed smaller DC at 0 h. For KHN, resin composites, ethanol softening and interaction effect were significant (p < 0.001). KHN decreased after ethanol softening all groups, except for 0.3-wt%-TSMPM-nanotubes, 0.9-wt%-TSMPM-nanotubes, and 0.3-wt%-non-functionalized-nanoparticles. ConclusionThe resin with 0.3-wt%-TSMPM-nanotubes showed higher DC after 24 h, while being the most stable material after the ethanol softening. SignificanceThe addition of functionalized TiO2 nanostructures in resin-based materials may improve the properties of the material.

In vitro performance of 2-step, total etch adhesives modified by thiourethane additives

ObjectivesThio-urethane oligomeric additives have been shown to improve the mechanical properties of dental composites and resin cements. To try to harness those same properties in dental adhesives, in this study, these oligomers (TU) were added to the matrix and/or as filler functionalization of experimental adhesives, and the effects on conversion and mechanical properties were analyzed. MethodsBisGMA and HEMA (60/40 wt%) were used as the monomer matrix, made polymerizable by the addition of 0.2 wt% 2,2-dimethoxy-2- phenylacetophenone. 2,6-di-tert-butyl-4-methylphenol was added at 0.5 wt% as the inhibitor. This material was used as the unfilled control (BH). TU oligomers were added at 20 wt % to the matrix (BH+20%TU, unfilled) and/or used as filler functionalization (TF, 10 wt%). Fillers functionalized with methacrylate (MF, 10 wt%) served as the control. The experimental adhesives groups containing fillers were: BH+10%MF; BH+10%TF; BH+20%TU+10%MF; BH+20%TU+10%TF. Flexural properties were tested in three-point bending (wet and dry). Polymerization kinetics was followed in real-time in near-IR. Water Sorption/Solubility (WS/SL, ISO 4049) and Viscosity (rotational rheometry) were also evaluated. For Microtensile bond strength 40 vol% ethanol was added to adhesives, which was applied onto sound dentin from third human molars. The data were analyzed with one-way ANOVA and Tukey post-hoc test, and test t for the comparison between storage time of the microtensile bond strength test (alpha = 0.05). ResultsThere was no significant difference between groups when yield strength (YS) and flexural modulus (FM) were evaluated in dry conditions. After water storage, all the groups containing TU in the matrix showed statistically lower YS/FM values. This was true in spite of the statistically higher conversion for those same groups. The maximum rate of polymerization (Rpmax) was higher for BH+10%TF and no significant difference was found for the groups BH and BH+10% MF. The lowest Rpmax values were found for BH+20%TU+10%TF and BH+20%TU. BH+20%TU+10%TF showed the highest viscosity values followed by BH+20%TU+10%MF and BH+20%TU, with statistically significant difference between them. For the microtensile bond strength test at 24h (p = 0.13) and 6 months (p = 0.11) and WS/SL (p > 0.05), no significant difference was found among groups. The storage time (24 h and 6 months) did not affect the microtensile bond strength results. ConclusionIn spite of improving the conversion, the addition of TU in the matrix reduced the mechanical properties of the adhesives tested after water storage. This did not affect the bond strength at 24 h or 6 months.

Pre-heating time and exposure duration: Effects on post-irradiation properties of a thermo-viscous resin-composite

ObjectiveTo evaluate the effects of pre-heating time and exposure duration on the degree of conversion (DC), maximum rate of polymerization (RPmax), polymerization shrinkage strain (PS) and surface micro-hardness (VHN) of Viscalor. MethodsViscalor syringes were pre-heated using a Caps Warmer (VOCO, Germany) in T3 mode (at 68°C) for 30s (T3-30s) and 3min (T3-3min) and then the composite paste was extruded into appropriately sized molds. Light irradiation was applied at zero distance from the upper surface with a LED-LCU of mean irradiance 1200mW/cm2 for either 20 or 40s. The real-time polymerization kinetics and DC at 5min and 24h post-irradiation (DC5min and DC24h) were measured using ATR-FTIR (n=3). PS was obtained with the bonded-disk technique (n=3). Top and bottom Vickers micro-hardness (VHNtop and VHNbottom) were measured at 5min post-irradiation and after 24h dry storage (n=5). Data were analysed using one-way ANOVA, two-way ANOVA, independent t-test and Tukey post hoc tests (p<0.05). ResultsPolymerization kinetic curves of Viscalor from 0 to 15min were similar for different pre-heating times and exposure durations. Pre-heated Viscalor (T3-30s and T3-3min) with 40s exposure had greater VHNtop and VHNbottom than for Viscalor (no heat) (p<0.05). Exposure duration did not significantly affect DC, RPmax and PS (p>0.05). After 24h storage, DC and VHN increased. Pre-heating did not increase the DC24h, relative to no pre-heating (p>0.05). Two-way ANOVA showed that there was no significant interaction between pre-heating time and exposure duration (p>0.05). SignificanceIncreasing irradiation time from 20 to 40s did not affect DC, RPmax or PS, but increased VHNtop. Composite pre-heating had no adverse effect through any premature polymerization. For Viscalor, 3min pre-heating and 20s irradiation were sufficient to provide adequate hardness, without increasing PS or compromising polymerization kinetics.

Bridging a mesoscopic inhomogeneity to macroscopic performance of amorphous materials in the framework of the phase field modeling

One of the big challenges in materials science is to bridge microscopic or mesoscopic properties to macroscopic performance such as fracture toughness. This is particularly interesting for the amorphous materials such as epoxy resins because their micro/meso structures are difficult to characterize so that any information connecting different scales would be extremely useful. At the process level the polymerization rate, which influences considerably the performance of materials, can be changed experimentally. However, it is known that the maximum toughness does not always appear at the maximum polymerization rate, which suggests that some differences in the micro/meso-scopic structure affect the macroscopic property behind. The goal of this article is to present a framework to bridge between a mesoscopic observation of X-ray CT images and the macroscopic criterion of fracture toughness computed via phase field modeling. First we map the X-ray images with different polymerization rates into several categories using different methods: one is singular value decomposition (SVD) and the other is persistent homology. Secondly we compute a crack propagation of each sample and evaluate a scalar value called the effective toughness (ET) via J-integral, which is one of the good candidates indicating a toughness of materials. It turns out that ET reflects the performance of each sample and consistent with the experimental results.

Effect of the photoinitiator system on the polymerization of secondary methacrylamides of systematically varied structure for dental adhesive applications

ObjectiveThe aim of this study was to investigate the influence of the photoinitiator system on the polymerization kinetics of methacrylamide-based monomers as alternatives to methacrylates in adhesives dental-based materials. MethodsIn total, 16 groups were tested. Monofunctional monomers (2-hydroxyethyl methacrylate) – HEMA; (2-hydroxy-1-ethyl methacrylate) −2EMATE, (2-hydroxyethyl methacrylamide) – HEMAM; and (N-(1-hydroxybutan-2-yl) methacrylamide) −2EM; were combined with bifunctional monomers containing the same polymerizing moieties as the monofunctional counterparts (HEMA-BDI; 2EMATE-BDI; HEMAM-BDI; and 2EM-BDI) at 50/50 M ratios. BHT was used as inhibitor (0.1 wt%) and the photoinitiators used were: CQ + EDMAB (0.2/0.8), BAPO (0.2), IVOCERIN (0.2), and DMPA (0.2), in wt%. The polymerization kinetics were monitored using Near-IR spectroscopy (∼6165 cm−1) in real-time while the specimens were photoactivated with a mercury arc lamp (Acticure 2; 320−500 nm, 300 mW/cm2) for 5 min, and maximum rate of polymerization (Rpmax, in %.sˉ̄1), degree of conversion at Rpmax (DC@Rpmax, in %), and the final degree of conversion (Final DC, in %) were calculated (n = 3). Initial viscosity was measured with an oscillating rheometer (n = 3). Data were analyzed using Two-way ANOVA for the polymerization kinetics and one–way ANOVA for the viscosity. Multiple comparisons were made using the Tukey’s test (∝ = 0.05). ResultsThere was statistically significant interaction between monomer and photoinitiator (p < 0.001). For the methacrylates groups, the highest Rpmax was observed for HEMA + DMPA and 2EMATE + BAPO. For methacrylamides groups, the highest Rpmax were observed for HEMAM and 2EM, both with DMPA. Final DC was higher for the methacrylate groups, in comparison with methacrylamide groups, independent of the photoinitiators. However, for the methacrylamide groups, the association with BAPO led to the lowest values of DC. In terms of DC@Rpmax, methacrylate-based systems showed significantly higher values than methacrylamide formulations. DMPA and Ivocerin led to higher values than CQ/EDMAB and BAPO in methacrylamide-based compounds. BAPO systems showed de lowest values for both HEMA and HEMAM formulations. For the viscosity (Pa.s), only 2EM had higher values (1.60 ± 0.15) in comparison with all monomers. In conclusion, polymerization kinetics was affected by the photoinitiators for both monomers. Viscosity was significantly increased with the use of secondary methacrylamide. Significancethis work demonstrated the feasibility of using newly-synthesized methacrylamide monomers in conjunction with a series of initiator systems already used in commercial materials.

A detailed description of methyl methacrylate free radical polymerization using different heating policies under isothermal and non-isothermal conditions: a kinetic Monte Carlo simulation

ABSTRACT A robust kinetic Monte Carlo (KMC) algorithm was precisely used to investigate and monitor the bulk free radical polymerization of methyl methacrylate (MMA) employing isothermal (ITPs) and non-isothermal (NTPs) heating policies. While temperature was fixed at 50°C (ITP50), 60°C (ITP60), and 70°C (ITP70) in isothermal policies, the non-isothermal policies are based on either a step increase in temperature profile from 50°C to 70°C at 120 or 60 min (NTP1 or NTP2, respectively), or a step decrease from 70°C to 50°C at 20 min (NTP3). The simulation data were in good agreement with the experimental data. It is revealed that the lower the average temperature is, the lower the rate of gel effect becomes. Furthermore, the initial gelation points, maximum gelation points, and glassy region interval are characterized. As a consequence of rising temperature, the initial glassy region shifts to a higher conversion. The maximum polymerization rate was achieved in a conversion range of 55-59%. It was found that temperature reduces average molecular weights and dispersity and narrowed molecular weight distribution (MWD); polymerization time was dropped by 85% too. Finally, NTP1 or NTP2 policies provide a narrower MWD at a higher average molecular weight.

A comparative kinetic study of ethylene polymerization mediated by iron, cobalt and chromium catalysts bearing the same N,N,N-bis(imino)trihydroquinoline

The iron(II), cobalt(II) and chromium(III) chlorides, [2-{(2,4,6-Me3C6H2)NCMe}-8-{N(2,4,6-Me3C6H2)}C9H8N]MCln (n = 2, M = Fe LFeCl2, Co LCoCl2; n = 3, M = Cr LCrCl3), each bearing the same N,N,N-bis(imino)trihydroquinoline chelating ligand, have been employed as precatalysts for ethylene polymerization with modified methylaluminoxane (MMAO) as the co-catalyst. The kinetic profiles for these homogeneous polymerizations are reported in addition to the properties of the resultant polymers under comparable reaction conditions. All the experimental data indicate that the active metal center plays a key role on the catalytic performances of the complexes, especially the polymerization activity, thermal stability and lifetime of the active species. Under optimized conditions the iron catalyst displays the highest rate of polymerization but displays this for only a short period, while the chromium catalyst shows a lower maximum polymerization rate but sustains its performance over a longer period and at a higher temperature. In terms of the polymer properties, all three metal catalysts afford highly linear polymers with the metal center influencing the molecular weight and type of end group. Specifically, the cobalt and chromium catalysts produce narrowly dispersed low molecular weight polymers incorporating vinyl end groups, while the iron catalyst affords polymers of higher molecular weight displaying broad molecular weight distributions, with both fully saturated and unsaturated chain ends.