Photopolymerization of highly filled dimethacrylate-based composites using Type I or Type II photoinitiators and varying co-monomer ratios

Graphical abstract

Highlights

  • MAPO and CQ-based filled resin composites were mixed using four BisGMA/TegDMA ratios..

  • Polymerization was monitored using both FT-NIRS and DEA and post-cure properties were also determined.

  • Initial ionic viscosity increased with BisGMA content, impacting final conversion.

  • Specific BisGMA contents were shown to be optimal (40 mol% – MAPO or ≤20 mol% – CQ).

  • Free and filler-bound monomer mobility and reactivity likely varied with the initiator.

Abstract

Objectives

The use of a Type I photoinitiator (monoacylphosphine oxide, MAPO) was described as advantageous in a model formulation, as compared to the conventional Type II photoinitiator (Camphorquinone, CQ). The aim of the present work was to study the kinetics of polymerization of various composite mixtures (20–40–60–80 mol%) of bisphenol A glycidyl dimethacrylate/triethylene glycol dimethacrylate (BisGMA/TegDMA) containing either CQ or MAPO, based on real-time measurements and on the characterization of various post-cure characteristics.

Methods

Polymerization kinetics were monitored by Fourier-transform near-infrared spectroscopy (FT-NIRS) and dielectric analysis (DEA). A range of postcure properties was also investigated.

Results

FT-NIRS and DEA proved complementary to follow the fast kinetics observed with both systems. Autodecceleration occurred after ≈1 s irradiation for MAPO-composites and ≈5–10 s for CQ-composites. Conversion decreased with increasing initial viscosity for both photoinitiating systems. However despite shorter light exposure (3 s for MAPO vs 20 s for CQ-composites), MAPO-composites yielded higher conversions for all co-monomer mixtures, except at 20 mol% BisGMA, the less viscous material. MAPO systems were associated with increased amounts of trapped free radicals, improved flexural strength and modulus, and reduced free monomer release for all co-monomer ratios, except at 20 mol% BisGMA.

Significance

This work confirms the major influence of the initiation system both on the conversion and network cross-linking of highly-filled composites, and further highlights the advantages of using MAPO photoinitiating systems in highly-filled dimethacrylate-based composites provided that sufficient BisGMA content (>40 mol%) and adapted light spectrum are used.

Introduction

The polymerization kinetics of dimethacrylate-based resins have been extensively studied, particularly in relation to their application as dental restorative materials . For this application, there is a specific demand for fast curing, highly filled resin-based materials, which are able to withstand the mechanical demand of masticatory load and degradative effects of the aqueous and frequently acidic environment. Glass and silica particles of micron and sub-micron dimensions, surface-modified with methacryl-functional silanes have been used as reinforcing fillers within dimethacrylate resin matrices. Free radical polymerization (FRP) is commonly used in these systems, initiated by light-sensitive molecules, which in most commercially available materials is a combination of camphorquinone (CQ) and a tertiary amine (Type II system). Specific curing mechanisms have been identified, most notably an autoacceleration and autodecelaration, associated with high polymerization rates, along with diffusion controlled termination and free radical entrapment . Such fundamental work on polymerization kinetics was carried out with spectroscopic methods, mid or near-infrared spectroscopy being the most-used, but also with calorimetric and mechanical techniques such as differential scanning calorimetry and dynamic mechanical analysis . Amongst these studies, the effects of light irradiance, resin and photoinitator chemistry have been studied.

Although previous work has provided useful information, many fundamental studies were conducted using unfilled resins, while dental restorative resin-based materials are reinforced with glass filler particulates to improve mechanical and physical material properties, typically up to 50–70 vol% (70–80 wt%) . Few studies are available which report fundamental polymerization kinetics parameters in heavily filled resin composites. In these paste-like materials, light transport to deep layers is restricted due to, and amongst other factors, the presence of fillers and pigments, the absorption of photoinitiator molecules. Further, bulk viscosity is very high and functional group conversion is lower than their resin counterparts .

Also, fundamental studies have focused primarily on detailing polymerization mechanisms in reactions using low light irradiance , (∼50 mW/cm 2 ) in order to monitor polymerization under slower reaction rates. However, in highly filled resin composites, much higher light intensities are required to polymerize in depth within acceptable curing times. Clinically relevant irradiances range from 500 to 3000 mW/cm 2 , in general around 1000 mW/cm 2 used to light cure a ∼2 mm thickness increment of conventional resin composite in ∼20 s. Higher rates would be expected under such settings , affecting the polymer structure due to decreased chain length and increased amount of cross-links .

Furthermore, several photoinitiators (UV or visible, Type I or II) were used in the above-mentioned fundamental studies, without formal comparisons of their impact on polymerization kinetics. The Type II initiation system previously mentioned (CQ) absorbs in the visible range (400–500 nm; λ max = 470 nm), while Type I molecules such as acylphosphine oxides absorb in the UV–vis region ( λ max ∼380 nm) with a much higher absorption efficiency and radical yield . Most interestingly for dental materials, their use in model dental resin-based composites led to shorter curing times (3–10 s , compared to 20 s with CQ). Several studies by our group focused on a model composite to compare the use of either a Type I (phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, MAPO) or a Type II (CQ) photoinitiating system . The Type I-based composite presented faster kinetics under short irradiation times (1–3 s) and improved functional group conversion, reduced monomer elution and improved mechanical properties.

However the effect of such a potentially useful photoinitiation system within resins of varying viscosity is unknown. Further, determining the influence of kinetics on the formation of a cross-linked polymer network is also important to determine the effect of network structure on resulting mechanical properties.

Consequently, the aim here was to study the kinetics of polymerization of a series of BisGMA/TegDMA formulations, using either MAPO or CQ-based resins filled to 75 wt%. The study of polymerization kinetics will be the first objective of the present work and achieved by use of complementary methods adapted to appropriately follow fast polymerizations, i.e. near-IR spectrometry and dielectric analysis (DEA). These methods will be discussed with regard to their suitability to follow ultra-fast polymerizations. The second objective will be to further characterize the resulting polymer network of the cured materials by measuring elastic modulus and flexural strength by three-point bending, free monomer elution by reverse phase high pressure liquid chromatography (HPLC) and trapped free radicals by electron paramagnetic resonance (EPR).

Materials and methods

Materials

Bisphenol A glycerolate dimethacrylate (BisGMA, Sigma–Aldrich), triethylene glycol dimethacrylate (TegDMA, Sigma–Aldrich, 95%), initiator Camphorquinone (CQ, Sigma–Aldrich, 97%) with its co-initiator 2-(Dimethylamino) ethyl methacrylate (DMAEMA, Sigma–Aldrich, 98%) and initiator phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (MAPO, BASF) were used as received. Micro barium glass (G018-186/K6, d 50 = 3 ± 1 μm, Schott AG, Landshut Germany) and nano silica (AEROSIL R7200, 12 nm, Evonik Industries, Germany), both methacryl-silane treated particles were also used as received.

Materials preparation

Four different BisGMA/TegDMA fractions were selected: 20/80, 40/60, 60/40 and 80/20 mol%. Two series were prepared by either using CQ/DMAEMA or MAPO in equimolar concentrations (0.20/0.80 wt% or 0.42 wt%, respectively), yielding a total of 8 different resin mixtures. In a final step, the micro and nano particles were introduced in amounts of 65 and 10 wt%, respectively. The resin composites were homogenized using a dual asymmetric centrifuge at 3500 rpm (Speed mixer, FlackTek, USA).

Methods – polymerization characteristics

Near-infrared spectra and ionic viscosities were recorded in real-time during irradiation and polymerization using Fourier-transform near-infrared spectroscopy (FT-NIRS) and dielectric analysis (DEA). In both methods, 2 mm-thick material layers were cured and analyzed. Such a thickness corresponds to the maximum thickness recommended in clinical settings, which is a major experimental difference compared to fundamental studies. In these, thin films were used to avoid issues with light transmission or variations in conversion, conditions suitable to extract intrinsic kinetic parameters. However the polymerization of dental resin composites occurs in non-ideal situations, in which high filler contents and thick layers (>1 mm) should be expected. Therefore, the present work intended to evaluate polymerization in a practical manner and in settings as close as possible to what would occur clinically.

FT-NIRS was performed in transmission mode (Nicolet6700, Thermo Scientific, Hemel Hemstead, UK) on disk shaped specimens contained in white Teflon molds (12 × 2 mm). The system rested on a glass slide and was positioned so that the laser beam passing between two optic fibers was aligned with the center of the mold. The light tip was placed beneath and in contact with the glass slide, vertically aligned with the center of the mold. The degree of double-bond to single-bond conversion (DC) was followed through the variations of peak height located at 6164 cm −1 , attributed to vinyl CH 2 groups . Absorbance data was collected for 120 s, at a sampling rate of 0.33 s −1 , with the first 10 s used prior to irradiation to establish initial absorbance ( A initial ). DC was calculated according to Eq. (1) , with A ( t ) the absorbance at a given time

DC(t)=1A(t)Ainitial
D C ( t ) = 1 − A ( t ) A i n i t i a l

DC 110 s was determined as the mean conversion value of the last 10 s of monitoring ( Fig. 1 ). Conversion rates were determined as the first time derivative of DC. DEA was carried out using a previously described setup . Briefly, a dielectric analyzer (DEA 231, Netzsch Gerätebau) was associated with a comb sensor (Mini-IDEX, 6 × 4 mm, electrode distance: 100 μm, Netzsch Gerätebau). As specified above, uncured material was placed in a 2 mm-thick layer atop the electrode, which was previously sprayed with technical grade silicone oil. The thickness of the composite layer was controlled by pressing a glass slide (1 mm thick) on top, with two glass slides placed on either sides of the electrode. A 1 kHz frequency was selected, corresponding to a sampling rate of 0.05 s −1 . Monitoring was performed for 120 s with the first 10 s used as signal baseline to determine initial viscosity ( ηion0
η 0 i o n
). Different parameters were determined as detailed in Fig. 1 .

Fig. 1
Conversion and ionic viscosity as a function of monitoring time for MAPO 20/80 followed by FT-NIRS and DEA, respectively. The data was independently measured and show one sample only. The calculated variables are highlighted in red. Depending on the method and due to varying sampling rates, the determination of rates of signal change could not always be carried out (for example in a).

Irradiation conditions

Light irradiation was carried out with an AURA light engine (Lumencor Inc., USA), providing 2 discrete and independent spectral outputs, at 385–410 and 420–460 nm. The output adaptor was a 1 m quartz optic fiber (5 mm diameter). The incident irradiance was calibrated and set at 1000 mW/cm 2 for both spectral outputs, using a thermosensor (S310C, Thorlabs, UK) connected to a PC via a USB converter (USB 100, Thorslabs). Further details are provided in the supporting information (S1). MAPO and CQ-composites were irradiated for 3 or 20 s respectively, based on previous works . The corresponding radiant exposures, defined as the product of irradiance and irradiation time was hence 3 J/cm 2 and 20 J/cm 2 .

Methods – post-cure material characterization

Using the previously detailed irradiation parameters, different samples were prepared for the analysis of post-cure properties, i.e. trapped free radical concentration, un-reacted monomers release and the flexural modulus and flexural strength – rectangular-shaped bars of 5 × 2 × 2 mm, disk-shaped specimens of 5 × 2 mm and bars of 25 × 2 × 2 mm, respectively were cured in white Teflon molds for the different analyses. To the difference of samples used for kinetics evaluation, top and bottom surfaces of the bars were covered using polyester film during irradiation. The 25 × 2 × 2 bars were irradiated in five successive and non-overlapping cycles (light tip was 5 mm).

Trapped free radical concentration was determined by means of electron paramagnetic resonance (EPR) using a Miniscope MS200 EPR spectrometer (Magnettech, Germany), 1 h post irradiation. As previously described, the height of the fourth and fifth peak of typical nine-peak spectra were measured, corresponding to the concentration of trapped propagating and allylic radicals, respectively . The following settings were used: 0.5 mW, microwave power; 336.7 mT, magnetic center field and 19.8 mT, field sweep; 0.1 mT, modulation amplitude. All measurements were carried out at ambient temperature (22 ± 2 °C) and in triplicates.

The release of un-reacted monomers was determined by means of reverse phase high pressure liquid chromatography (HPLC, Agilent 1100 series) equipped with a UV detector and a C18 column (140 × 4.6 mm, 3 μm particle diameter, BDS Hypersil, Thermo Scientific). Briefly, the 2 mm thick disks were each placed in 1 mL of 75/25 vol% ethanol/distilled water solution and left in the dark for one week at 37 °C ( n = 3). Quantification was carried out for BisGMA and TegDMA, based on established calibration curves for the two monomers prior to testing (range 10–500 μg/mL). A gradient method was used, starting from a 60/40% H 2 O/acetonitrile mixture and dropping to 0/100% over 19 min. Both reagents were of analytical grade.

Flexural modulus and strength were determined on bars busing a 3-point bending platform (LRX plus, Lloyd Instruments) with a 20 mm span between supports and a 0.75 mm/min crosshead speed ( n = 5). The cured bars were stored for 1 week in 75/25 vol% ethanol/distilled water, at 37 °C prior to reflect the conditions for the release study. Mechanical properties were determined using standard formulas of beam theory.

Statistical analysis

Regressions and their determination coefficients ( R 2 ) were determined using Excel (Microsoft) and Spearman correlations coefficients were established using JMP 11 (SAS Institute Inc.). When p -values were higher than 0.01, values were specified. Comparison of the photoinitiator effect at a given co-monomer ratio was carried out using one-way ANOVA and student’s t -test test at a 0.05 significance level.

Materials and methods

Materials

Bisphenol A glycerolate dimethacrylate (BisGMA, Sigma–Aldrich), triethylene glycol dimethacrylate (TegDMA, Sigma–Aldrich, 95%), initiator Camphorquinone (CQ, Sigma–Aldrich, 97%) with its co-initiator 2-(Dimethylamino) ethyl methacrylate (DMAEMA, Sigma–Aldrich, 98%) and initiator phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (MAPO, BASF) were used as received. Micro barium glass (G018-186/K6, d 50 = 3 ± 1 μm, Schott AG, Landshut Germany) and nano silica (AEROSIL R7200, 12 nm, Evonik Industries, Germany), both methacryl-silane treated particles were also used as received.

Materials preparation

Four different BisGMA/TegDMA fractions were selected: 20/80, 40/60, 60/40 and 80/20 mol%. Two series were prepared by either using CQ/DMAEMA or MAPO in equimolar concentrations (0.20/0.80 wt% or 0.42 wt%, respectively), yielding a total of 8 different resin mixtures. In a final step, the micro and nano particles were introduced in amounts of 65 and 10 wt%, respectively. The resin composites were homogenized using a dual asymmetric centrifuge at 3500 rpm (Speed mixer, FlackTek, USA).

Methods – polymerization characteristics

Near-infrared spectra and ionic viscosities were recorded in real-time during irradiation and polymerization using Fourier-transform near-infrared spectroscopy (FT-NIRS) and dielectric analysis (DEA). In both methods, 2 mm-thick material layers were cured and analyzed. Such a thickness corresponds to the maximum thickness recommended in clinical settings, which is a major experimental difference compared to fundamental studies. In these, thin films were used to avoid issues with light transmission or variations in conversion, conditions suitable to extract intrinsic kinetic parameters. However the polymerization of dental resin composites occurs in non-ideal situations, in which high filler contents and thick layers (>1 mm) should be expected. Therefore, the present work intended to evaluate polymerization in a practical manner and in settings as close as possible to what would occur clinically.

FT-NIRS was performed in transmission mode (Nicolet6700, Thermo Scientific, Hemel Hemstead, UK) on disk shaped specimens contained in white Teflon molds (12 × 2 mm). The system rested on a glass slide and was positioned so that the laser beam passing between two optic fibers was aligned with the center of the mold. The light tip was placed beneath and in contact with the glass slide, vertically aligned with the center of the mold. The degree of double-bond to single-bond conversion (DC) was followed through the variations of peak height located at 6164 cm −1 , attributed to vinyl CH 2 groups . Absorbance data was collected for 120 s, at a sampling rate of 0.33 s −1 , with the first 10 s used prior to irradiation to establish initial absorbance ( A initial ). DC was calculated according to Eq. (1) , with A ( t ) the absorbance at a given time

D C ( t ) = 1 − A ( t ) A i n i t i a l
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Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Photopolymerization of highly filled dimethacrylate-based composites using Type I or Type II photoinitiators and varying co-monomer ratios
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