Evaluation of biocompatible monomers as substitutes for TEGDMA in resin-based dental composites

Accepted Manuscript Evaluation of biocompatible monomers as substitutes for TEGDMA in resin-based dental composites

Alma A. Pérez-Mondragón, Carlos E. Cuevas-Suárez, Oscar R. Suárez Castillo, J. Abraham González-López, Ana M. HerreraGonzález PII: DOI: Reference:

S0928-4931(17)34852-X doi:10.1016/j.msec.2018.07.059 MSC 8771

To appear in:

Materials Science & Engineering C

Received date: Revised date: Accepted date:

16 December 2017 9 July 2018 22 July 2018

Please cite this article as: Alma A. Pérez-Mondragón, Carlos E. Cuevas-Suárez, Oscar R. Suárez Castillo, J. Abraham González-López, Ana M. Herrera-González , Evaluation of biocompatible monomers as substitutes for TEGDMA in resin-based dental composites. Msc (2018), doi:10.1016/j.msec.2018.07.059

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ACCEPTED MANUSCRIPT EVALUATION OF BIOCOMPATIBLE MONOMERS AS SUBSTITUTES FOR TEGDMA IN RESIN-BASED DENTAL COMPOSITES Alma A. Pérez-Mondragóna, Carlos E. Cuevas-Suárezb, Oscar R. Suárez Castilloc, J. Abraham González-Lópezd, Ana M. Herrera-Gonzálezd*

Doctorado en Ciencias de los Materiales, Universidad Autónoma del Estado de

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a

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Hidalgo, Carretera Pachuca-Tulancingo Km. 4.5 Colonia Carboneras, Mineral de la

b

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Reforma Hidalgo. C.P. 42184. México.

Área Académica de Odontología. Instituto de Ciencias de la Salud. Universidad

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Autónoma del Estado de Hidalgo. Circuito ex–Hacienda la Concepción Km. 1.5. San

c

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Agustín Tlaxiaca, Hidalgo. C.P. 42160. México.

Área Académica de Química. Instituto de Ciencias Básicas e Ingeniería. Universidad

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Autónoma del Estado de Hidalgo, Carretera Pachuca-Tulancingo Km. 4.5 Colonia

d

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Carboneras, Mineral de la Reforma Hidalgo. C.P. 42184. México Laboratorio de Polímeros, Instituto de Ciencias Básicas e Ingeniería. Universidad

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Autónoma del Estado de Hidalgo, Carretera Pachuca-Tulancingo Km. 4.5 Colonia

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Carboneras, Mineral de la Reforma Hidalgo. C.P. 42184. México.

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ACCEPTED MANUSCRIPT ABSTRACT This

works

reports

the

synthesis

and

characterization

of

diallyl(5-

(hydroxymethyl)-1,3-phenylene) dicarbonate (HMFBA) and 5-(hydroxymethyl)1,3-phenylene bis(2-methylacrylate) (HMFBM) monomers and its evaluation as Bis-GMA eluents in the formulation of composite resins for dental use. The

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experimental materials formulated with HMFBA and HMFBM monomers

with

Bis-GMA/TEGDMA.

Regarding

volumetric

contraction

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formulated

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presented flexural strength values similar to those of the control group

percentage, the values obtained of experimental materials with HMFBA was

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1.88% and for HMFBM was 4.15%, both lower than control resin (4.68%). In the case of double bond conversion, the resin formulated with HMFBA monomer

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exhibited a greater degree of conversion (87%). Besides, the DMA analyses proved that the values for Tg guarantee a good mechanical performance at

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body temperature. The new resins formulated with HMFBA and HMFBM

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monomers exhibit a cellular viability close to 100%, which indicates the absence

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of cytotoxicity towards fibroblastic cells.

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Keywords: biomaterials; composites; photopolymerization; MMA crosslinking

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ACCEPTED MANUSCRIPT 1. Introduction Polymeric materials find many applications in different everyday scopes. Within odontology, acrylic resins based on methyl methacrylate and composite resins based

on

2,2-bis(4-(2-hydroxy-3-methacrylooxypropyl-oxy)phenyl)

propane

(Bis-GMA) and triethylene glycol dimethylacrylate (TEGDMA)[1,2] have

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important applications thus these materials are widely used in the restoration or

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complete substitution of dental organs[3–12]. Compound resins based on Bis-

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GMA/TEGDMA, although possessing excellent mechanical properties, they still have certain drawbacks that limit their use[3,13]. Monomer Bis-GMA has high

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molecular weight and high viscosity, limiting the incorporation of inorganic filler in the formulation of composite resins. In addition to its high molecular weight,

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the high viscosity of Bis-GMA monomer is also caused by intermolecular hydrogen bond interactions, making necessary incorporation of dimethacrylates

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with low molecular weight as diluents[14–17], for example, triethylene glycol

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dimethacrylate (TEGDMA) or aliphatic dimethacrylates based on urethanes such as urethane dimethacrylate (UDMA). By including TEGDMA or UDMA, the

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viscosity of the inorganic matrix decreases, allowing the increment of inorganic filler content in the final material. However, the addition of monomers with low

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molecular weight causes high percentages of volumetric shrinkage and a decrease in the mechanical properties of the material, thus compromising the overall performance of the restorations[10,18–21]. Over the last years, alternative monomers have been developed with the objective

to obtain polymeric materials for dental use with

specific

characteristics, such as lower polymerization shrinkage and higher double bond conversion. The design of new monomers, in this work, was based either on 3

ACCEPTED MANUSCRIPT higher molecular weight molecules or on bulky substituted monomers[11,12,22– 27]. Thus, the objective of this research was to synthesize two new bi-functional monomers, both containing an aromatic group and high molecular weight, when they are compared with the TEGDMA monomer, and to evaluate their

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performance as Bis-GMA diluents in the formulation of composite resins.

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2. Experimental section

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2.1 Materials and instruments

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The materials and solvents used in the synthesis of the monomers as well as in the silanization of SiO2 particles were bought from Sigma-Aldrich (St. Louis, MI,

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USA). The solvents used were distilled using the techniques described in the literature[28]. The RMN spectra were obtained on a 400-MHz Varian 1400

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spectrophotometer (Varian, Inc. Palo Alto, CA, USA) using deuterated

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chloroform as solvent, with tetramethylsilane as internal reference. The FT-IR spectra were obtained using a Frontier spectrophotometer (Perkin Elmer,

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Waltham, MA, USA) by means of attenuated total reflectance (ATR) technique. The photopolymerization of the composite dental resins was made using a Bluephase

16i

(Liechtenstein,

AUSTRALIA)

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Ivoclar-Vivadent

photopolymerization unit equipped with a LED visible light source with an intensity of 1000 mW/cm2. The flexural tests were performed using an Instron 4465 universal mechanical test machine (Instron, Norwood, MA). For the polymerization contraction stress analysis, the Polymerization Stress Tester (Proto-Tech, Portland, OR, USA). equipment was used. For the DMA analysis, a TA Instruments Q800 equipment (KU Leuven, Belgium) with a liquid nitrogenbased system (cantilever type) was used. The working at a frequency of 1Hz 4

ACCEPTED MANUSCRIPT and amplitude of 20 µm. The analysis consisted in programing a method to heat the sample at a rate of 5°C/min with a temperature range between -50 °C and 200 °C. The images of SEM were made by means of electron scanning microscopy using a JSM-IT300 microscope operated at 30 kV (Illinois, United States). The samples were covered with gold before the analysis. Finally, the

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viscosity determinations were made at 25°C in a BROOKFIELD viscometer with

2.2

Synthesis

of

monomer

(5-(hydroxymethyl)-1,3-phenylene)

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dicarbonate (HMFBA)

diallyl

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DV-III equipment (Arizona, United States).

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Compound HMFBA was synthesized by means of a nucleophilic acyl substitution reaction. 0.5 g of 3,5-dihydroxybenzyl alcohol (3.57 mmol)

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dissolved in dry THF were placed in a round bottom, and cooled in an ice bath,

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and 1.2 mL of allyl chloroformate (11.49 mmol) and 1.5 mL (10.97 mmol) of triethylamine were simultaneously added.

After standing for one hour, the

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reaction mixture was warmed to 60 °C under constant stirring, until a heterogeneous mixture containing a white solid by-product and an orange liquid

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was obtained. The solid was filtered off, and the liquid phase was washed with distilled water. The organic phase was dried, and the volatiles were evaporated. Monomer HMFBA was purified by means of column chromatography using silica as the stationary phase and a mixture of dichloromethane and ethyl acetate in an 80/20 (V/V) ration as the mobile phase. The yield was 80%. FTIR/ATR (cm-1): 3396 (OH), 1764 (C=O), 1639 (C=C), 1224 (C-O). 1H NMR (400MHz, CDCl3): 7.12 (2H, d, J= 1.68 Hz, H2), 7.03 (1H, t, J= 2.08 Hz, 5

ACCEPTED MANUSCRIPT H3), 6.01 (2H, ddt, J= 5.88, 10.44, 17.16 Hz, H5), 5.46 (2H, dd, Jtrans= 1.32, 17.16 Hz, H6), 5.36 (2H, dd, Jcis= 1.12, 10.4 Hz, H7), 4.75 (4H, dt, J= 1.12, 5.89 Hz, H4), 4.69 (2H, d, J= 5.24 Hz, H1).

13

C NMR (100 MHz, CDCl3): 152.3

(C=O), 150.9 (-CH-O-), 143.4 (-C-CH2-), 130.4 (-CH=CH2), 119.2 (CH=CH2),

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68.8 (-O-CH2-CH), 63.6 (-C- CH2-OH). 2.3 Synthesis of monomer 5-(hydroxymethyl)-1,3-phenylene bis(2-methacrylate)

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(HMFBM)

Compound HMFBM was also synthesized through a nucleophilic acyl

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substitution reaction. In a round bottom to a solution of 0.5 g (3.57 mmol) of 3,5dihydrobenzyl alcohol in 70 mL of ethanol was added, drop by drop, a mixture

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of 0.87 mL (8.92 mmol) of methacryloyl chloride and 1.1 mL (7.89 mmol) of triethylamine. After stirring for one hour, the mixture was heated to 60 °C, and

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stirred for three hours. The reaction mixture was cooled to room temperature

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and the ethanol was evaporated under reduced pressure. The residue was diluted with 25 mL of dichloromethane, and washed with distilled water. The

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organic phase was dried, and the solvent was evaporated and the residue was purified by column chromatography using silica as the stationary phase and a

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mixture of dichloromethane and ethyl acetate in an 90/10 (V/V) ration as the mobile phase. The yield was 98%. FTIR/ATR (cm-1): 3508 (OH), 1734 (C=O), 1602 (C=C), 1137 (C-O). 1H NMR (400MHz, CDCl3): 7.02 (2H, d, J= 0.72 Hz, H2), 6.87 (1H, t, J= 0.36 Hz, H3), 6.32 (2H, dc, Jtrans =0.96, 1.36, 2.36 Hz, H5), 5.74 (2H, q, Jcis= 1.52, 3.04 Hz, H6), 4.67 (2H, s, H1), 2.03 (6H, dd, J=0.96, 0.52, H4).

13

C NMR (100MHz,

6

ACCEPTED MANUSCRIPT CDCl3):165.5 (C=O), 151.3 (-CH-O-), 143.6 (-C-CH2-), 135.5 (-C=CH2), 127.6

MA

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(C=CH2), 117.0 (C-Car), 114.4 (Car-C), 69.2 (-C- CH2-OH), 18.3 (-O-C-CH3).

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Figure 1. Synthesis of monomers HMFBA and HMFBM

2.4 Preparation of the composite resins

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Two experimental composite resins were formulated using as the organic matrix a combination of Bis-GMA/HMFBA or Bis-GMA/HMFBM with mass ratio of

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70/30. A control resin was formulated using a Bis-GMA/TEGDMA system in the same weight ration. For all groups, a binary photopolymerization system comprising camphorquinone (CQ) and 4-diethylaminobenzoate (E4DMAB) in a concentration of 2% and 4% by weight (%W), respectively was used [29–31]. For the formulation of polymeric based composites, a 55/10 mixture of silanized fillers was added to the organic matrix. The size of micrometric and nanometric filers was 14 µm and 20-30 nm, respectively.

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ACCEPTED MANUSCRIPT 2.5 Flexural strength and flexural modulus

The flexural strength of the composite materials was evaluated using the criteria established in section 7.11 of the ISO-4049[32] international standard. The flexural modulus was evaluated in accordance to ANSI/ADA No. 27

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specification. Bar-shaped specimens (2x2x25 mm) were prepared by filling the uncured composite material into a stainless-steel mold placed on a glass slide

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covered by a polyester strip. A second strip and a glass slide were used to

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cover the mold and the materials were irradiated by 10 seconds with an intensity of 460 mW/mm2 three times per side. Once the specimens were

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obtained, their dimensions were measured with an accuracy of 0.01 mm using

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digital calipers. The specimens (n=3) were kept in distilled water at 37 °C for 24 hours. The three-point bending test was performed using a load cell of 1 kN with

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at a cross-head rate of 1 mm/minute. The data were obtained using program

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Series IX. The flexural strength and the flexural modulus were calculated using

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the following formulas:

𝜎=

3𝐹𝑙 … … … … (1) 2𝑏ℎ2

𝐹1 𝑙 3 𝐸= … … … … . (2) 4𝑏ℎ3 𝑑

Where ()flexural strength (MPa), (F) force at the moment of fracture (N), (l) distance between the supports (mm), (E) flexural modulus (MPA), (F 1) Force recorded where the deformation stops being directly proportional to the force registered in the graph (N), (d) deflection of the specimen (mm).

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ACCEPTED MANUSCRIPT 2.6 Characterization by Dynamic Mechanical Analysis (DMA) The formulated resins were analyzed by means of DMA. The specimens were made in a plastic mold of 1 x 5 x 30 mm. The specimens were irradiated for 30 seconds on both sides. Once polymerized, the DMA test was made using a frequency of 1 Hz, 20 amplitudes, and 30 °C while measuring the mechanical

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bending property.

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2.7 Polymerization kinetics

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The degree of double bond conversion and the rate of polymerization was

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determined by means of FT-IR spectroscopy [33]. Over the window of the diamond cell of the equipment, a small composite resin sample (~5µg) was

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placed. Before irradiating the sample, an infrared spectrum was recorded. Later, the composite resin sample was irradiated for 30 seconds and another infrared

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spectrum was recorded. The experiment was repeated three times for each of

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the formulated groups. In each spectrum, the height of the absorption band corresponding to the C=C aliphatic bond at 1638 cm-1 as well as the height of

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the absorption band corresponding to the C=C aromatic bond at 1609 cm-1 were measured. This last absorption band is used as an internal reference

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because the concentration of aromatic double bonds did not change during the polymerization reaction, and, thus, the percentage of absorbance of this band before and after the polymerization reaction remains constant. The percentage of double bond conversion was determined according to the following equation: 𝐴 (𝐴1638 ) 𝑝𝑜𝑙𝑦𝑚𝑒𝑟

𝐷𝑒𝑔𝑟𝑒𝑒 𝑜𝑓 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 (%) = 100 ( 1609 ) … … … … (3) 𝐴 (𝐴1638 ) 𝑚𝑜𝑛𝑜𝑚𝑒𝑟 1609

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ACCEPTED MANUSCRIPT Where: A1638 maximum height of the band at 1638 cm-1, A1609 maximum height of the band at 1609 cm-1.

2.8 Measurement of polymerization shrinkage Polymerization shrinkage was obtained using Archimedes’ principle[34]. The

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measurements were made using an analytical balance coupled to a density

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determination kit. Small spheres of composite resins from each of the

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formulated groups were weighted. Unpolymerized resin spheres were weighted in air and in a solvent as quickly as possible to avoid the solvent to diffuse

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through the sample. Sphere-shaped specimens were polymerized and its

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weight was recorded in air and in solvent.

To determine the density of the polymerized and unpolymerized spheres, the

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following formula was used:

𝑚𝑤𝑎𝑡𝑒𝑟 (𝜌 − 𝜌𝑎𝑖𝑟 ) + 𝜌𝑎𝑖𝑟 … … … … (4) 𝑚𝑎𝑖𝑟 − 𝑚𝑤𝑎𝑡𝑒𝑟 𝑤𝑎𝑡𝑒𝑟

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𝜌=

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(𝜌) density of the material, (𝑚𝑤𝑎𝑡𝑒𝑟 ) weight in grams of the sample in water, (𝑚𝑎𝑖𝑟 ) weight in grams of the sample in air, (𝜌𝑤𝑎𝑡𝑒𝑟 ) density of the water at the

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temperature of the measurement, (𝜌𝑎𝑖𝑟 ) density of the air (0.0012g/cm). Based on the results, the following equation was applied to determine the volumetric shrinkage: 1 1 1 ∆𝑉 = ( − ) 100% … … … … (5) 𝜌15 𝑚𝑖𝑛 𝜌𝑠𝑝 𝜌𝑠𝑝 (∆𝑉) Percentage of change in volume (contraction), (𝜌15 𝑚𝑖𝑛 ) density of the polymerized samples, (𝜌𝑠𝑝 ) density of the unpolymerized samples. 10

ACCEPTED MANUSCRIPT 2.9 Polymerization stress For this analysis, a Proto-Tech® Polymerization Stress Tester equipment was used. The equipment is designed to observe the stress produced by the dental restoration material when it is polymerized in a mold in real time. The equipment is made of two presses on which an 18 mm long acrylic tube is placed. A

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sample of the material which was previously formulated with the new monomers

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was placed in one of the acrylic tubes, and later, it was photopolymerized in the

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upper and lower parts of the acrylic tubes for 200 seconds.

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2.10 Cytotoxicity assay

The cytotoxicity test was done according to ISO 10993-5 (2009). Mouse

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fibroblast cells (L929) were cultivated at a density of 2x104 cells in 96-well plates containing DMEM (Dulbeccos´s Modified Eagle Medium culture) medium,

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10% L-glutamine, 10% fetal bovine serum (FBS), penicillin (100 U/ml), and

5% CO2 for 24 h.

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streptomycin (100 U/ml). The cells were incubated at 37 ºC under 95% air and

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The relation of cell viability was evaluated by means of the WST-1

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colorimetric assay. Disc-shaped specimens (5x1 mm) were prepared for each of the formulated groups. These specimens were placed in 24-well plates with 1 ml of DMEM, and they were stored at 37 ºC with at pH of 7.2. After 24 h, 200 μl of DMEM were transferred from each well to a 96-well plate which contained the previously cultivated cells. The plate was incubated (37ºC, 5% CO 2) for a period of 24 h. After this period, the medium was aspirated, and a WST-1 solution was applied. The results were read in a spectrophotometer with a wavelength of 450

11

ACCEPTED MANUSCRIPT nm, where the absorbance values were considered as an indicator of cell viability.

2.11 Statistical analysis

The statistical analysis was made using the IBM SPSS Statistics 23 (Armonk,

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NY. USA) statistical package. The values were analyzed to verify the normality

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and homogeneity of the variance. A one-way ANOVA followed by a

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complementary Tukey test was made to evaluate each of the dependent variables. The level of significance for all the test was set to p<0.05.

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3.1 Characterization

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3. Results and discussion

Figure 1 shows the synthesis and chemical structure of monomers HMFBA and

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HMFBM. The chemical structures of both monomers were confirmed by 1H- and 13

C-RMN techniques as well as FT-IR spectroscopy. The main evidences for

the formation of monomers HMFBA and HMFBM by FT-IR are the two

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absorption bands caused by the vibrational elongation mode belonging to the

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ester group: the absorption band at 1764 cm-1 belonging to the carbonyl group and the band at 1224 cm-1 caused by the elongation of the C-O bond. Also, the absorption bands at 1643 and 1639 cm -1 correspond to the allylic and methacrylic terminal double bonds (C=C) respectively, while the absorption band at 3396 cm-1 corresponds to the vibrational elongation mode of the OH group, which proves that an OH group remained unreacted. The evidence for the formation of monomer HMFBA by 1H-NMR is the presence of the signals that correspond to the protons of the double bonds. The two doublets of 12

ACCEPTED MANUSCRIPT doublets signal at 5.46 ppm (Jtrans= 17.16 Hz) integrates for two protons (H6) in a trans position in relation to H5, the doublet of doublets signal at 5.36 ppm (Jcis= 10.4 Hz) integrates for two protons (H7) in a cis position in relation to H5. For monomer HMFBM, the doublet quadruples signal at 6.32 ppm (Jtrans =0.96, 1.36, 2.36 Hz) is attributed to the two protons (H5) in a trans position in relation

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to the methyl in the methacrylic group. Also, the quintuple signal at 5.74 ppm

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(Jcis= 1.52, 3.04 Hz) was assigned to the two protons (H6) in a cis position in relation to the methyl in the methacrylic group (Figure 2). Further corroboration

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was acquired from 13C NMR and FT-IR spectrum (Supplementary data).

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Figure 2. H NMR of monomers HMFBA and HMFBM

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ACCEPTED MANUSCRIPT The viscosity of the monomers TEGDMA, HMFBA and HMFBM were evaluated (Table 1), and the monomer with higher viscosity was the TEGDMA. However, despite the difference in viscosities, all formulations allowed the incorporation of the same amount of inorganic filler. Table 1. Molecular weight and viscosity of the monomers used in this study.

Molecular weight (g/mol)

Viscosity (mPa.s)

TEGDMA

286.32

72.04

HMFBA

276.29

51.66

HMFBM

308.29

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Monomer or Mixture monomer

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SC

68.85

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3.2 Flexural strength and elastic modulus

The values for the flexural strength and elastic modulus are shown in Table 2.

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The resins formulated with the experimental monomers HMFBA and HMFMB

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exhibit a flexural strength similar to the control resin (p>0.05). In the case of monomer HMFBA, the presence of double allylic bonds, which are well-known for their low reactivity, it was not an impediment for obtaining adequate

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mechanical properties, which proves its potential application in composite resins

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for dental use. In the case of monomer HMFBM, as the methacrylate group is present in its chemical structure, the mechanical behavior was similar to the control group, as expected.

For both experimental groups, a slight decrease in the flexural strength could be attributed to a decrease in the mobility in the organic matrix due to the presence of bulky aromatic groups. This functional group, due to its size, decrease the mobility of monomer system during polymerization and therefore, a low cross-

14

ACCEPTED MANUSCRIPT linking density is obtained, reducing the mechanical properties of the corresponding polymer network. On the other hand, as aromatics groups are not present in the structure of the TEGDMA monomer, a better cross-linked polymeric structure is expected, leading to a higher ability to resist flexural

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forces [15].

Regarding the values obtained for the elastic modulus, both formulations using

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experimental monomers HMFBA and HMFBM exhibit smaller values than the

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control resin, and these differences were statistically significant (p<0.05). One of the main problems that the materials having an elevated elastic modulus

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have is that, because of their rigidity they tend to generate large tensions during

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the polymerization process. Which, along with the polymerization contraction that these materials suffer, may encourage the apparition of gaps between the

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tooth and the restoration, in turn, causing problems such as bacterial filtration,

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hypersensitivity, and formation of secondary caries. On the other hand, the dental resins with a smaller elastic modulus present better distribution of the stress by polymerization contraction, thus allowing the resins to adapt better to

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the dental cavity. This is confirmed by the data obtained in the polymerization

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stress test (Table 2).

Table 2. Values for flexural strength, elastic modulus, Tg, Polymerization shrinkage and Polymerization stress for the resins formulated with monomers HMFBA and HMFBA and the control [Mean ± SD]. Flexural Elastic Elastic modulus Tg Polymerization Polymerization Formulation strength modulus by (MPa) (ºC) shrinkage (%) stress (N) (MPa) DMA (MPa) Bis-GMA/TEGDMA

60.54 ± 10.46a

6358.20 ± 577.47a

6435

105.4

4.68 ± 1.74ab

17.6 ± 1.67a

Bis-GMA/HMFBA

59.28 ± 5.88a

4550.67 ± 1046.09b

5896

95.1

1.88 ± 0.71a

7.4 ± 1.34c

Bis-GMA/HMFBM

49.49 ± 5.60a

4349.17 ± 625.49b

5277

99.4

4.15 ± 1.37b

12.4 ± 1.52b

Same superscript ab letters in each column indicates that there are no statistically significant differences.

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ACCEPTED MANUSCRIPT Representatives SEM images of fractured specimens were obtained to determine the distribution of filler in the composite SEM images of the specimens, which showed good homogeneity between inorganic filler and

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organic matrix (Figure 3).

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Figure 3. Representative SEM images of control (A), experimental Bis-GMA/HMFBM (B) and experimental Bis-GMA/HMFBA (C) polymerized composites.

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3.3 Flexural tests by Dynamic Mechanical Analysis (DMA)

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Table 2 shows the values for the elastic modulus and the glass transition temperature (Tg) of the composites determined by DMA. The values for the

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elastic modulus obtained by DMA confirm those obtained using the three-point flexural strength test. Figure 4 shows that the Tg of Bis-GMA/TEGDMA resin is

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105.4 °C, which confirms what is reported in the references[35]. For the resins formulated with monomers HMBFA and HMFBM, their T g are 95.1 and 99.4 °C, respectively. According to the glass transition temperatures obtained, both the experimental composite resins and the control resin are above room temperature which guarantees a good performance at body temperature.

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ACCEPTED MANUSCRIPT

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Figure 4. First derivative of the elastic modulus used to obtain T g

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3.4 Polymerization kinetics

The resin formulated with monomer HMFBA exhibits better double bond

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conversion rates than the Bis-GMA/TEGDMA control resin (p<0.05, Table 2). This is caused by the flexibility of the structure of monomer HMFBA and its high

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miscibility with Bis-GMA, which generate a better diffusion of the radicals during

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the polymerization reaction, favoring the obtaining of copolymers with a greater

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degree of crosslinking and greater conversion of double bonds.

On the other hand, it is known that the methacrylic monomers are extremely

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reactive, but, because of the steric impediment and less flexibility caused by the aromatic and methyl groups in the monomer, the percentage of double bond conversion decreases. This situation may be potentiated by the presence of functional groups with low mobility within the chemical structure of the monomer. In the case of HMFBM, which possesses an aromatic ring within its chemical structure, its mobility may have been reduced when compared to monomer TEGDMA, and, as such, lower values for degree of double bond conversion than those observed for the control group were presented. However, 17

ACCEPTED MANUSCRIPT these differences were not statistically significant (p=0.290). This data could be confirmed with the rates of polymerization values, which are analyzed below.

The analysis of the polymerization kinetics of copolymer Bis-GMA/TEGDMA proves that this groups presented the highest Rpmax (4.87 %·s-1), which

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suggests that this material vitrifies faster, avoiding significant changes in the double bond conversion percentage after the first seconds of the reaction.

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Besides, during vitrification, it is possible to favor the polymerization termination

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by disproportionation reaction, and thus, encourage the existence of polymeric chains with terminal double bonds, which is confirmed by the values of double

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bond conversion (Table 3). Similarly, the copolymers based on Bis-

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GMA/HMFBM contain two monomers with methacrylic groups which, when polymerized, rapidly increase their viscosity, and, as such, their vitrification.

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However, contrasting with the control group, the presence of an eluent

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monomer with a rigid central structure causes important etheric inhibition, which drastically reduces the rate of polymerization (Figure 5).

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Finally, in the copolymers based on Bis-GMA/HMFBA, it is observed that the rate of polymerization is less than the two previous copolymers. This may be

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explained by the self-inhibiting phenomenon which is characteristic to the allylic monomers. This characteristic allows for the polymerization to occur without reaching vitrification quickly, which encourages the termination by combination, thus decreasing the percentage of terminal double bonds (Table 3).

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Figure 5. Rate of polymerization of the resins formulated with monomers HMFBA and HMFBM and the control resin.

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3.5 Polymerization shrinkage

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Table 2 shows the values for polymerization shrinkage of the evaluated materials. The percentage of polymerization shrinkage varied between

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5.45±0.88 % for the material formulated with monomer HMFBM and 1.88±0.71

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% for the material formulated with monomer HMFBA (p<0.05). However, it is important to point out that none of the materials presented statistically

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significant differences compared to the control. The volumetric contraction caused by the polymerization reaction is one of the major drawbacks of the

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polymeric materials used in dental restorations because this phenomenon is associated with several problems. One of these is the production of internal tensions which may cause microcracks or microbubbles, thus weakening properties of the polymer formed[36,37]. The composite resins formulated with monomers HMFBA and HMFBM showed a 40% and 12% reduction, respectively, in the polymerization shrinkage when compared to the control material. These differences could be explained due to a two-fold mechanism, on the one hand, it has been reported that the polymerization shrinkage of low 19

ACCEPTED MANUSCRIPT molecular weight monomers is greater than those of monomers with high molecular weight. This phenomenon is caused because the quantity of double bonds is reduced with the increase of the molecular weight of the monomer. In the case of HMFBA, this monomer has a greater molecular weight than TEGDMA. On the other hand, the presence of rigid and bulky groups may lead

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to a less density in the polymer network after polymerization and consequently,

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smaller volume shrinkage, [38] which is the case of HMFBM monomer.

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Meanwhile, TEGDMA contains in its structure an aliphatic segment bounded with oxygen atoms which increase the mobility of the chains and favors the

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contraction of the resin[37].

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3.6 Polymerization stress

The values for stress obtained by the volumetric contraction of the polymeric

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matrix varied from 17.6 N for the control group to 7.4 N for the material

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formulated with HMFBA (Table 2). The highest value obtained by the control resin proves that the volumetric contraction of the formulation based on Bis-

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GMA/TEGDMA generates more stress by polymerization compared with the materials formulated with the monomers HMFBA and HMFBM. As composite

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restorations adhere directly to the walls of the dental cavity, they generate tensions by polymerization contraction. This contraction tension is directly related to the marginal integrity of the restorations, which may compromise the longevity of the restorations. The tension by polymerization shrinkage is a phenomenon that depends on several intrinsic characteristics of to the material, such as elastic modulus and the rate of polymerization.

As it was previously shown, the control group formulated with Bis20

ACCEPTED MANUSCRIPT GMA/TEGDMA presented the highest values for polymerization rate and elastic modulus. These two characteristics are the possible reasons why this group exhibited the highest polymerization stress forces. In the case of the resins formulated with monomers Bis-GMA/HMFBA, which present a low elastic modulus and a faster rate of polymerization, the tension forces generated within

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its structure can be dissipated. Finally, for the material formulated with the

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HMFBM monomer, it showed a volumetric contraction close to the control

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group. However, the presence of a reduced elastic modulus allowed it to adequately dissipate the stress generated throughout the material, achieving

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lower values for polymerization stress.

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Table 3. Polymerization kinetics of composites

% Degree of conversion

Rpmax (%s-1)

Control

63.16 (5.94)b

4.87

87.08 (5.53)a

1.77

55.02 (6.39)b

2.48

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HMFBA

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Formulation

HMFBM

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Same superscriptab letters in each column indicates that there are no statistically significant differences.

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3.7 Cytotoxicity test

Figure 6 shows the viability of L929 cells that were cultured with the extraction medium from different resin systems formulated with Bis-GMA/TEGDMA, BisGMA/HMFBA, and Bis-GMA/HMFBM. The experimental system formulated with monomers HMFBA and HMFBM showed a cell viability of approximately 100%, which indicates that both monomers are not cytotoxic to fibroblastic cells.

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Figure 6. Cell viability and growth of L929 fibroblasts.

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Funding sources

This research did not receive any specific grant from funding agencies in the

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Competing interests

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public, commercial, or not-for-profit sectors.

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Authors have no competing interests to declare.

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ACCEPTED MANUSCRIPT Highlights 

Novel monomers (HMFBA and HMFBM) were synthesized and used as BisGMA eluents.



Materials formulated with HMFBA and HMFBM presented good mechanical properties.



HMFBA showed reduced volumetric shrinkage and reduced

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HMFBA and HMFBM showed good in vitro biocompatibility.

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

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polymerization stress.

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