2.1

Control, light-cured UDMA (group LC)

The monomer was mixed with 100 ppm hydroquinone monomethyl ether (HQME, Fluka, France), 0.5% (weight) camphoroquinone (CQ, Aldrich, Germany), and 1% (weight) 4, N , N -trimethylanilin (TMA, Aldrich, Germany) in a planetary mixer (Thinky AR 250, Tokyo, Japan). The mix was left at room temperature for 1 day before being cast into (2 × 4 × 20) mm silicone molds. Light-curing was performed three times (ones in the middle of the sample and once on each extremity of the sample) for 40 s each, with a LED curing unit (Radii, SDI, Victoria, Australia) operated at a power density of 1200 mW/cm ² (measured with a curing radiometer, Dentsply Caulk, Milford, USA).

2.2

Control, thermo-cured UDMA (group TC)

The monomer was mixed with initiator, 0.5% benzoyl peroxide (BPO; Sigma Aldrich, Steinheim, Germany); 100 g was placed inside a flexible silicone tube (25 mm internal diameter) and was thermally cured at 90 °C, at ambient pressure (0.1 MPa), in a furnace (Memmert, Schwabach, Germany), for 4 h.

2.3

Experimental, HP cured UDMA (group 90I300)

The composition (monomer with 0.5% BPO) and curing temperature (90 °C) for this group was the same as that of the TC control group, the difference being the high pressure during curing. The monomer-initiator mix (100 g) was placed inside a flexible silicone tube (25 mm internal diameter) and cured at 90 °C under high pressure (300 MPa), in a custom-built autoclave, for 4 h.

2.4

Experimental, HT/HP cured UDMA (group 190I300)

The composition (monomer with 0.5% BPO) for this group was the same as that of the TC control group, the difference being the high temperature and high pressure during curing. This group differed from 90I300 group by the high temperature during curing. The monomer-initiator mix (100 g) was placed inside a flexible silicone tube (25 mm internal diameter) and cured at 190 °C under high pressure (300 MPa), in a custom-built autoclave, for 1 h.

2.5

Experimental, no initiator HT/HP cured UDMA (group 190NI300)

This group differed from the experimental 190I300 group by the absence of an initiator. The monomer (100 g) was placed inside a flexible silicone tube (25 mm internal diameter) and cured at 190 °C under high pressure (300 MPa), in a custom-built autoclave, for 1 h.

2.5.1

Thermo-polymerization under high pressure

A differential scanning calorimeter (DSC 823, Mettler Toledo, Greifensee, Switzerland) was used to determine the thermo-polymerization temperature of UDMA without initiator at atmospheric pressure (0.1 MPa). Since the determined polymerization temperatures were in the (160–190) °C range, it was decided to perform all the polymerization reactions at 190 °C. Thereafter, approximately 100 g of UDMA, with or without BPO, was placed inside a flexible silicone tube (25 mm internal diameter, 1 mm thickness), which was then introduced into a custom-built autoclave with pressure and temperature control (LabVIEW version 8.6, National Instruments, USA). A thermocouple was placed in the proximity of the sample to enable accurate monitoring and, via feed-back, control of the temperature. In the first stage, the pressure within the autoclave was increased to 300 MPa at a rate of 1 MPa/s at ambient temperature. In the second stage, the temperature was increased to 90 °C or to 190 °C, at a rate of 2 °C/min. The sample was then maintained at 300 MPa and 90 °C/190 °C for 60 min before being cooled off and the pressure released at a temperature inferior to 60 °C.

2.5.2

Flexural strength

One part of each polymer block was cut with an Isomet saw (Buehler) under water irrigation, into 30 rectangular bars (2 × 4 × 20) mm. Each bar was polished under water irrigation up to a final 4000 grit silicon carbide (SiC) paper and its dimensions were measured with a digital caliper (Mitutoyo Co., Kawasaki, Japan) before being tested. Flexural strength ( σ _f ) was determined by loading the samples in a three point bending device (with a 16 mm span) at a cross-head speed of 1 mm/min, using a computer controlled (NexyGen ^® , Lloyd, UK) Lloyd LRX (Lloyd, UK) universal testing machine. Flexural strength (in MPa) was calculated using the formula:

where F is the load (in N) at fracture, L the specimen span (in mm), h the specimen width (in mm), and c the specimen height (in mm).

2.5.3

Fracture toughness

One part of each polymer block was cut, with an Isomet saw under water irrigation, into eight rectangular bars (8 × 8 × 15) mm, which were then wet ground on 1200 grit SiC to obtain (6 × 6 × 6 × 12) mm equilateral triangular prisms. Fracture toughness ( K _IC ) was determined using the notchless triangular prism (NTP) specimen K _IC test. The prisms were secured into one half of the specimen holder and a sharp scalpel was used to create a small (<0.1 mm-deep) defect along the loading edge before securing the second half of the specimen holder. The specimens were loaded in tension, using a computer controlled (Bluehill 2, Instron) universal testing machine (Instron model 4301, Instron Canada Inc.), at a crosshead speed of 0.01 mm/min until crack arrest or fracture. The maximum load (in N) recorded before crack arrest or complete failure ( P _max ) was used to calculate K _IC in MPa.m ^1/2 using the following equation, proposed by Barker and adopted by ASTM standard E1304:

where Y ^* _min is the minimum dimensionless stress intensity coefficient (28 for NTP samples ) D the specimen holder diameter (12 mm), and W the specimen holder length (10.4 mm).

2.5.4

Hardness

Fractured σ _f specimens were used for micro-hardness determinations. The specimens were surface coated with a thin (∼10 nm) gold layer, in a sputter-coater (SC500, Bio-Rad, UK), in order to improve reading. Surface micro-hardness was measured by means of a Vickers indenter (MH3, Metkon, Bursa, Turkey), with 10 N load and 20 s dwell time. Thirty determinations on 10 specimens were made for each material.

2.5.5

Density

Fractured K _IC specimens were used for density determinations. The density of samples was determined based on Archimedes’ principle using a XS205 (Mettler Toledo, Greifensee, Switzerland) balance. They were weighed in air and in deionized water and the density (in g/cm ³ ) was then calculated using the following formula:

where ρ is the density of the sample (in g/cm ³ ), A the mass (in g) of the sample in air, B the mass (in g) of the sample in deionized water, ρ _w the density (in g/cm ³ ) of deionized water determined from the measurement of water temperature, and ρ _a the density of air (0.0012 g/cm ³ ). Thirty determinations on five specimens were made for each material.

2.5.6

Analysis of monomer release with high-performance liquid chromatography (HPLC)

From the tested 3 point bending specimens, 18 half-bars of each polymer were randomly selected, their surface area and weight were measured and recorded, and they were placed into three vials (six half-bars per vial). Each vial was then filled with 10 mL of 75% (volume) ethanol and 25% (volume) water, prepared from HPLC-grade ethanol and HPLC-grade water (Fisher Scientific, Bishop Meadow Road, UK). The vials were sealed and stored in an oven (Memmert, Schwabach, Germany) at 37 °C. After 1 d, 7 d, 14 d, and 28 d storage, three 20 μL aliquots of each vial were removed with a micro-syringe for HPLC analysis.

The HPLC analysis was conducted using an Agilent 1260 Infinity Quaternary LC (Agilent Technologies, Waldbronn, Germany), equipped with a quaternary pump (model G1311B) and a UV diode array detector (model G4212B). The column used was Poroshell 120 EC-C18 (Agilent Poroshell, USA) with an internal diameter of 4.6 mm, length of 50 mm, and a filler-particle size of 2.7 μm. The solvent was HPLC-grade 65% acetonitrile (Fisher Scientific, Bishop Meadow Road, UK) in HPLC water, used in isocratic conditions with a flow rate of 1 μL/min . The elution was performed at room temperature and monitored in the whole UV range. For quantification, the 210 nm spectra, where UDMA exhibits significant absorption, were used. Identification of the analyte, UDMA, was made based on the retention time of the UDMA peaks registered for the standard solutions (1.327 min).

For quantitative analysis, a calibration curve was constructed using seven 10 mL solutions of UDMA [(10 ⁻³ ; 10 ⁻⁴ ; 10 ⁻⁵ ; 5 × 10 ⁻⁶ ; 10 ⁻⁶ ; 5 × 10 ⁻⁷ ; and 10 ⁻⁷ ) mol/L] which were obtained from the stock solution sequential dilution. The 1 × 10 ⁻³ mol/L UDMA stock solution was prepared by dissolving the appropriate amount of UDMA in 75% (volume) ethanol and 25% (volume) water. These solutions were stored at ambient temperature. Linearity of the calibration curve, based on the quantitative determination of UDMA in the seven solutions, was assessed by linear regression analysis. The relationship obtained for the linear peak area ( A ) − concentration ( c ) for the analyte (UDMA) was: A = 2.03494 × 10 ⁷ × c , with a correlation coefficient R ² = 0.99168 and σ = 16.17527 (where A = peak area and c = concentration).

The limit of detection (LOD) and the limit of quantification (LOQ) were determined using calibration in the low concentration region [(1 × 10 ⁻⁷ , 5 × 10 ⁻⁶ , 1 × 10 ⁻⁶ , 5 × 10 ⁻⁵ , 1 × 10 ⁻⁵ ) mol/L] . Their values were calculated from the calibration curve according to the formula:

where σ = standard residual deviation of intercept and S = the slope.

The accuracy of the procedure was checked using the standard addition method. One real sample was spiked with appropriate volumes of a standard solution of UDMA and satisfactory results for the recovery, ranging from 102.48% to 105.72%, confirmed that the method was accurate and appropriate for quantitative analysis.

2.5.7

Statistical analysis

The results were analyzed by one way ANOVA followed, if warranted, by Scheffé multiple means analysis, all performed at 0.05 level of significance.

Weibull statistics parameters were calculated for σ _f results using the Weibull statistics option in Excel ^® (Microsoft,USA). The description of the Weibull distribution is given by :

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