Highlights
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Six series of alternative urethane-dimethacrylate monomers were tested.
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Monomers were tested to verify their potential use in dental restorative matrix.
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Six varied diisocyanates were used for the UDMA synthesis.
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Monomethacrylates containing form one to four oxyethylene units were used.
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HEMA/IPDI appeared to be the most promising alternative monomer.
Abstract
Objective
The aim was accomplished by a comparative analysis of the physicochemical properties of urethane-dimethacrylate (UDMA) monomers and their homopolymers with regard to the properties of basic dimethacrylates used presently in dentistry. The homologous series of UDMA were obtained from four oligoethylene glycols monomethacrylates (HEMA, DEGMMA, TEGMMA and TTEGMMA) and six diisocyanates (HMDI, TMDI, IPDI, CHMDI, TDI and MDI).
Methods
Photopolymerization was light-initiated with the camphorquinone/tertiary amine system. Monomers were tested for viscosity and density. Flexural strength, flexural modulus, hardness, water sorption and polymerization shrinkage of the polymers were studied. The glass transition temperature and the degree of conversion were also discussed.
Results
HEMA/IPDI appeared to be the most promising alternative monomer. The monomer exhibited a lower viscosity and achieved higher degree of conversion, the polymer had lower water sorption as well as higher modulus, glass temperature and hardness than Bis-GMA. The polymer of DEGMMA/CHMDI exhibited lower polymerization shrinkage, lower water sorption and higher hardness, however it exhibited lower modulus when compared to HEMA/TMDI. The remaining monomers obtained from HEMA were solids. Monomers with longer TEGMMA and TTEGMMA units polymerized to rubbery networks with high water sorption. The viscosity of all studied UDMA monomers was too high to be used as reactive diluents.
Significance
The systematic, comparative analysis of the homologous UDMA monomers and corresponding homopolymers along with their physico-mechanical properties are essential for optimizing the design process of new components desirable in dental formulations. Some of the studied UDMA monomers may be simple and effective alternative dimethacrylate comonomers.
1
Introduction
Restorative dental composites undergo hardening due to the polymerization of multifunctional monomers that produce a rigid and heavily cross-linked polymer matrix surrounding the inert filler particles. The most commonly used monomer is the highly viscous 2,2-bis-[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]-propane) (Bis-GMA). Bis-GMA is usually accompanied by triethylene glycol dimethacrylate (TEGDMA), acting as a low viscosity reactive diluent to achieve high filler loading. The stiff molecular structure and hydroxyl groups of Bis-GMA ensure low cure shrinkage, high polymer modulus and desirable adhesion to tooth enamel . They can increase the resin’s viscosity, residual unsaturation in the polymer and its water uptake. On the other hand, TEGDMA has been shown to increase matrix water sorption and its polymerization shrinkage . Alternative dental formulations contain urethane-dimethacrylate monomer–1,6-bis-(methacryloyloxy-2-ethoxycarbonylamino)-2,4,4-trimethylhexane, commonly abbreviated to UDMA, but for the purpose of this work it has been abbreviated to HEMA/TMDI. The advantage of HEMA/TMDI is its lower viscosity, when compared to Bis-GMA. Moreover, urethane linkage can form strong hydrogen bonds and thus improve both durability of the composite’s matrix as well as bonding to the tooth structure .
Current aims in dental research include identifying new dimethacrylates of moderately low viscosities , producing polymers with low polymerization contraction , a high degree of conversion , good mechanical properties and low water sorption . Though innovative ormocer and silorane composite systems have been developed, materials based on Bis-GMA, HEMA/TMDI and TEGDMA are still mostly applied in the dental practice. The adequate properties of HEMA/TMDI, in combination with the low price of its production, may determine the work undertaken leading towards the preparation of new monomers of this kind .
In fact, urethane-dimethacrylates (UDMA) are identified by a wide family of monomers. Their chemical structures can easily be tailored through an appropriate choice of the core and wing segments, resulting in diversity of monomers and corresponding polymers with a wide range of chemical and physico-mechanical properties.
In the present paper, several UDMA monomers, being the structural analogues of HEMA/TMDI, and their homopolymers were characterized. The monomer cores derived from six commercially available diisocyanates (DI): aliphatic – HMDI and TMDI, cycloaliphatic – IPDI and CHMDI, aromatic – TDI and MDI. The wing structures originated from oligoethylene glycols monomethacrylates (OEGMMA), which have from one to four oxyethylene units in the oligooxyethylene chains ( Scheme 1 ) . The monomers were tested for their viscosity and density. Corresponding homopolymers were characterized by the degree of conversion, glass temperature, polymerization shrinkage, water sorption and selected mechanical properties (flexural modulus, flexural strength and hardness).
The aim of this work was to show, how various homologous series of the UDMA structures may influence the monomer/polymer properties in terms of their application in dentistry as Bis-GMA, HEMA/TMDI or TEGDMA substitutes. Understanding the basic individual properties of UDMA monomers and their polymer networks can inspire new monomer designs for improving or maintaining desirable properties. New and more efficient copolymer formulations can be developed, accordingly.
2
Materials and methods
2.1
Materials
Urethane-dimethacrylate monomers (UDMA) were synthesized from oligoethylene glycols monomethacrylates (OEGMMA) and diisocyanates (DI) according to the procedure previously reported . OEGMMA: DEGMMA, TEGMMA and TTEGMMA were obtained through a trans-esterification reaction of methyl methacrylate (MMA, Acros, Geel, Belgium) with the corresponding glycols: diethylene (DEG, Acros, Geel, Belgium), triethylene (TEG, Acros, Geel, Belgium) and tetraethylene (TTEG, Acros, Geel, Belgium) ones, according to the procedure described previously . The Bis-GMA monomer was synthesized from 2,2-Bis[4-(2,3-epoxypropoxy)phenyl]propane (BADGE, DER 330, The Dow Chemical Company, Midland, MI, USA, EV = 0.57 mol/100 g epoxy groups), methacrylic acid (MAc, Sigma–Aldrich, St. Louis, MO, USA) and α-picoline (catalyst, Fluka, Taufkirchen, Germany) according to the procedure reported in . 2-Hydroxyethyl methacrylate (HEMA, Sigma–Aldrich, St. Louis, MO, USA), 1,6-hexamethylene diisocyanate (HMDI, Fluka, Taufkirchen, Germany), 2,2,4(2,4,4)-trimethylhexyl-1,6-diisocyanate (TMDI, Sigma–Aldrich, St. Louis, MO, USA), isophorone diisocyanate (IPDI, Sigma–Aldrich, St. Louis, MO, USA), 4,4′-methylenebis(cyclohexyl isocyanate) (CHMDI, Sigma–Aldrich, St. Louis, MO, USA), 2,4-toluene diisocyanate (TDI, Sigma–Aldrich, St. Louis, MO, USA), 4,4′-methylenebis(phenyl isocyanate) (MDI, Sigma–Aldrich, St. Louis, MO, USA) and triethylene glycol dimethacrylate (TEGDMA, Sigma–Aldrich, St. Louis, MO, USA) were used as received.
The structure of all the monomers was confirmed in 1H NMR experiments (300 MHz spectrometer, Varian UNITY/INOVA, Palo Alto, CA, USA), performed in CDCl 3 solution, using tetramethylsilane (TMS) as a reference (Sigma–Aldrich, St. Louis, MO, USA).
2.2
Curing procedure
The monomers were mixed with: 0.4 wt.% of camphorquinone (CQ, Sigma–Aldrich, St. Louis, MO, USA)–the photosensitizer, and 1 wt.% of N,N-dimethylaminoethyl methacrylate (DMAEMA, Sigma–Aldrich, St. Louis, MO, USA) – the reducing agent, and poured into moulds. Petri dishes (120 mm in diameter and 4 mm thick) as well as PTFE O-rings placed on a glass surface (15 mm in diameter and 1 mm thick) were used for this purpose. The samples were covered with PET film in order to reduce the effects of oxygen inhibition and then irradiated for thirty minutes. Photopolymerization was initiated with a high pressure mercury vapor lamp (FAMED-1, model L-6/58, Lodz, Poland, power 375 W ), emitting UV/VIS light, where CQ absorbs in the 420-500 nm range .
Liquid monomers were photopolymerized with the above mentioned lamp at room temperature. Solid monomers (HEMA/HMDI, HEMA/CHMDI, HEMA/TDI and HEMA/MDI) were mixed with the initiation system, introduced into moulds and photopolymerized in the molten state as previously stated. Before irradiation, each monomer was fused at a temperature lower than the temperature of its thermal polymerization ( Table 1 ), as exhibited in earlier DSC experiments .
| Monomer | MW (g/mol) | T m (°C) | η (Pa s) | d m (g/cm 3 ) |
|---|---|---|---|---|
| HEMA/HMDI | 428.5 | 77 a | – | 1.131 (0.006) |
| DEGMMA/HMDI | 516.6 | – | 14.37 (0.86) | 1.130 (0.007) |
| TEGMMA/HMDI | 604.7 | – | 8.71 (0.45) | 1.129 (0.006) |
| TTEGMMA/HMDI | 692.8 | – | 6.64 (0.26) | 1.126 (0.005) |
| HEMA/TMDI | 470.6 | – | 6.22(0.42) | 1.111 (0.005) |
| DEGMMA/TMDI | 558.7 | – | 2.80 (0.21) | 1.109 (0.005) |
| TEGMMA/TMDI | 646.8 | – | 1.44 (0.09) | 1.104 (0.003) |
| TTEGMMA/TMDI | 735.0 | – | 1.18 (0.07) | 1.104 (0.004) |
| HEMA/IPDI | 482.5 | – | 12.33 (0.83) | 1.139 (0.005) |
| DEGMMA/IPDI | 570.7 | – | 8.80 (0.51) | 1.137 (0.006) |
| TEGMMA/IPDI | 658.8 | – | 6.69 (0.27) | 1.129 (0.006) |
| TTEGMMA/IPDI | 746.9 | – | 4.10 (0.11) | 1.127 (0.004) |
| HEMA/CHMDI | 522.7 | 108 a | – | 1.137 (0.007) |
| DEGMMA/CHMDI | 610.8 | – | 16.66 (1.29) | 1.138 (0.007) |
| TEGMMA/CHMDI | 698.9 | – | 12.85 (0.93) | 1.128 (0.006) |
| TTEGMMA/CHMDI | 787.0 | – | 9.68 (0.58) | 1.126 (0.007) |
| HEMA/TDI | 434.4 | 98 a | – | 1.189 (0.007) |
| DEGMMA/TDI | 522.6 | – | 13.75 (0.84) | 1.181 (0.006) |
| TEGMMA/TDI | 610.7 | – | 10.27 (0.61) | 1.177 (0.006) |
| TTEGMMA/TDI | 698.8 | – | 6.96 (0.39) | 1.177 (0.005) |
| HEMA/MDI | 510.6 | 89 a | – | 1.201 (0.006) |
| DEGMMA/MDI | 598.7 | – | 38.97 (2.43) | 1.203 (0.005) |
| TEGMMA/MDI | 686.8 | – | 23.59 (1.34) | 1.191 (0.007) |
| TTEGMMA/MDI | 774.9 | – | 8.91 (0.37) | 1.189 (0.006) |
| Bis-GMA | 512.6 | – | 1200 b | 1.152 (0.007) |
| TEGDMA | 286.3 | – | 0.011 b | 1.089 (0.001) |
2.3
Viscosity
The monomer viscosity ( η , Pa s) was measured by means of a rotating spindle viscometer (Brookfield Fungilab Viscometer, Visco Star Plus L, Barcelona, Spain) at 25 °C. Viscosity was measured using the appropriate spindle, at various spindle speeds, which allowed for recording viscosity values between 10 and 90% torque.
2.4
Glass transition temperature
The glass transition temperature ( T g ) values were taken from our previous studies . The rectangular samples of polymers (length × width × thickness: 50 mm × 5 mm × 2 mm) were examined by using Dynamic Mechanical Analysis (Polymer Laboratories MK II DMA apparatus, Shropshire, UK). Experiments were performed in bending mode and a frequency of 1 Hz. The T g was taken as the temperature at the tan delta peak maximum.
2.5
Density and polymerization shrinkage
The densities of monomers ( d m ) were measured utilizing a liquid pycnometer at 25 °C according to ISO 1675 . The polymer densities ( d p ) were determined according to the Archimedes’ principle, on the Mettler Toledo XP Balance with 0.01 mg accuracy (Greifensee, Switzerland) with the density determination kit at 25 °C. Water was used as the immersing liquid. The volumetric shrinkage of photopolymerized samples was determined by the following equations:
where S e is experimentally determined polymerization shrinkage, S t – polymerization shrinkage extrapolated to the full conversion, MW – a monomer molecular weight.
The degree of conversion ( DC ) was calculated according to the following formula:
2.6
Water sorption
Water sorption was measured according to ISO 4049 . Disc-like specimens (diameter × thickness: 15 mm × 1 mm) of each UDMA polymer network were dried in a pre-conditioning oven at 37 °C until their weight was constant. This result was recorded as m 0 (Mettler Toledo XP Balance with 0.01 mg accuracy, Greifensee, Switzerland). The specimens were then immersed in distilled water and maintained at 37 °C for a week. After this time, the samples were removed, blotted dry and weighed ( m 1 ). Water sorption ( WS ) was calculated using the following formula:
where V is the initial volume of the sample.
2.7
Mechanical properties
The flexural modulus ( E ) and the flexural strength ( σ ) were determined in accordance with ISO 178 in three-point bending tests, using a universal testing machine (INSTRON, model TT-CM, Norwood, MA, USA) . Rectangular samples of UDMA polymers (length × width × thickness: 80 mm × 10 mm × 4 mm) were cut from moulds, prepared as previously mentioned. The E and the σ were calculated following the relationships, respectively:
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