Mechanical properties of dental nanocomposites reinforced with

polyhedral oligomeric silsesquioxane(POSS)

Xiaorong Wu1, a, Weili Xie2, b, Yi Sun1, c, Shouhua Sun1, d 1 Department of Astronautic Science and Mechanics, Harbin Institute of Technology, China

2 Department of Stomatology, Harbin Medical University, China awxr@hit.edu.cn, csunyi@hit.edu.cn,

Abstract: The objective of this study was to incorporate POSS-MA into the polymeric matrix to develop a new kind of nanocomposite. The effect of different weight percentage of POSS incorporated into the matrix on the mechanical properties was also evaluated. Infrared spectroscopic technique and X-ray diffraction were used to characterize their microstructures and double bond conversion. With only 2wt% POSS added, the nanocomposite’s flexural strength increased 15%, compressive strength increased 12%, hardness increased 15% and uncommonly, even the toughness of resins obviously increased. With 5wt% POSS polymerized, compressive strength increased from 192MPa to 251MPa and compressive modulus increased from 3.93GPa to 6.62GPa, but flexure strength began to decline from 87MPa to 75MPa. The nanocomposites incorporated with POSS showed greatly improved mechanical properties, and increased wear resistance and service life. The mechanism of reinforcement was also discussed.

Key words: Mechanical properties. Dental nanocomposites. Polyhedral oligomeric silsesquioxane (POSS). Characterization.

Introduction In recent years, many efforts have been undertaken to develop restorative composites to be showing physico-mechanical properties similar to those of the natural tooth structures. Improvements of currently used commercial dental restorative composite resins are focused on the reduction of the polymerization shrinkage [1], as well as the improvement of mechanical properties, wear resistance, biocompatibility, and processing properties.

POSS (polyhedral oligomeric silsesquioxane) belongs to a very important type of hybrid nanocomposites. A typical POSS monomer is a well-defined molecule represented by the formula (RSiO1.5)8 with an inorganic silica-like core (Si8O12) surrounded by eight organic corner groups. A. Sellinger et al. [2] firstly mentioned the possibility of POSS used in dental restorative materials. Later, some other research works [3-8] had been done.

The main goal of this study was to develop a novel dental nanocomposite, investigate their double bond conversion, mechanical properties, and improve their wear resistance and service life.

Formulation of �anocomposites Materials. POSS-MA0735 was from Hybrid Plastics (Fountain Valley, CA). Bis-GMA and TEGDMA were from Aldrich Chemical Co. The commonly used visible light photo-initiator CQ and co-initiator DMAEMA were selected for this research (Aldrich). The filler was finely milled silanated barium oxide glass powder (BG), provided by Peking University, whose average particle size was 0.8µm.

Synthesis of Materials. A solution containing 49.5wt% Bis-GMA, 49.5wt% TEGDMA, 0.5wt%

Advanced Materials Research Vols. 79-82 (2009) pp 345-348Online available since 2009/Aug/31 at www.scientific.net© (2009) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.79-82.345

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CQ and 0.5wt% DMAEMA was prepared by Table 1 Weight percentage of components(wt%) mixing sufficiently in a container which kept away from light. Then POSS-MA was proportionately added into the solution of neat resins as Table 1 and magnetically blended uniformly. At last, BG used in the dental nanocomposites was slowly put into the mixture and stirred in the vacuum mixer escaping from air bubbles. Methods Characterization. Fourier-transform infrared (FTIR) spectra were collected with an AVATAR360

(Nicolet, USA). Wide-angle X-ray diffraction (WAXD) was performed using a D/max-γ B

rotational anode X-ray diffractometer (Japan). Mechanical Properties. The specimens were prepared in stainless steel molds with a dimension

of 2mm in width by 2mm in depth by 25mm in length. Flexural strength (FS) and flexural elastic modulus (Ef) were obtained by three-point flexure test at a crosshead speed 0.5 mm/ min. Cylindrical samples for compressive strength (CS) and compressive elastic modulus (EC) were measured by compressive tests at a crosshead speed 10mm/ min with the dimension of 3mm in diameter by 7mm in length. Testing was performed on a Universal Testing Machine (ZWICK, Germany).Calculations were made using formulas as follows:

22/3 WTPLFS = . (1)

3 3( / )( / 4 )fE P d L WT= . (2)

2/4 DPCS π= . (3)

20[4 ( )] /[ ( )]C H L H LE L F F D L Lπ= − − . (4)

where P is the load at fracture, L is the distance between two supports, W is the width of the specimen, T is the thickness of the specimen, d is the deflection, at load P, D is the diameter of the specimen, L0 is the length of the specimen, FH (FL) and LH (LL) is the end (start) of corresponding load and distortion of the specimen in the elastic phase of load-distortion curve.

Viker’s hardness (HV) was determined using a depth-sensing microindentation technique on Indentation Testing Machine (ZWICK, Germany).

(a) P00 (Wposs=0wt %) (b) P02 (Wposs=2wt %) Fig.1 FTIR spectra of the resins irradiated for 40s.

Composites Resin Matrix POSS BG 40:00:60 40 0 60 38:02:60 38 2 60 35:05:60 35 5 60 30:10:60 30 10 60 25:15:60 25 15 60

346 Multi-Functional Materials and Structures II

“Fracture energy” (Nmm) can express the fracture toughness of the materials, and it can be calculated by the enclosed area of stress-strain fracture curve obtained from three-point flexure test.

Results and Discussion Degree of conversion. Compared with Fig.1 (a) P00 and (b) P02, we could see that the weaken degree of C=C absorption of P02 at 1635 cm-1was bigger than that of P00. It accounted for the degree of conversion of P02 was higher than that of P00. Fig.2 WAXD profiles of the nanocomposites

WAXD. Wide angle X-ray diffraction (WAXD) data (Fig. 2) indicated that all nanocomposites were amorphous. No POSS crystal peaks could be identified, which confirmed that the POSS-MA had polymerized with resins matrix.

Mechanical properties. Flexural strength, a very important property for dental restoration, can reflect the ability that the materials withstand the complex stress. The results showed in Fig.3. With only 2wt% POSS added, the nanocompsite’s flexural strength increased 15%, compressive strength increased 12%, compressive modulus increased 4% and flexural modulus showed no change. With 5wt% POSS polymerized, compressive strength increased from 192MPa to 251MPa and compressive modulus increased from 3.93GPa to 6.62 GPa, but flexure strength began to decline from 87MPa to 75MPa. This finding indicated that the reinforcement mechanism of flexure state maybe different from that of compressive state.

Wear resistance. Hardness is another important mech- anical property for dental materials. It can reflect the ability of resisting plastic distortion and wear resistance of the dental restoration. In this study, the hardness of novel nanocomposite was increased from 349MPa to 400MPa, and consequently closer to the hardness of dentins (570~ 600MPa)[9]. It is probably because the special rigid Fig.3 Mechanical properties: (a) flexural cubic structure of POSS improved the hardness of the strength, (b)compressive strength and matrix. (c)compressive modulus.

Toughness. Traditionally, with increasing strength and stiffness of reinforced polymers, the toughness of the materials usually declined. Uncommonly, in this study, POSS monomer was also found to increase the toughness of resins obviously. It was observed that with only 2 wt% POSS, fracture energy increased 56%, which can express the fracture toughness of the materials. This is an important finding for the clinical perspective because that, in some structures of dentures, toughness of material is a desired property. The improvement may be owing to the methacrylate functional groups attached in POSS-MA0735 molecules. Their flexible long chain structures may provide

Advanced Materials Research Vols. 79-82 347

Table 2 Properties of the dental nanocomposites better toughness for the resin matrix. In addition, interface action between nano POSS particles and matrix would be stronger for their formed cross-linked net works. It needs absorb more energy during fracture. This is another possible reason for POSS to increase the toughness of the matrix. Summary In our current study, it indicated that the dental

Materials (40-x): x: 60 =resin matrix: POSS: nanocomposite with 2wt% POSS could fillers (wt %) improve their mechanical properties, increase their wear resistance and service life, which was observed to achieve the best improved effects.

References ［1］ Xiaorong Wu, in: Development of novel low shrinkage dental nanocomposite, 2nd

International Conference on Smart Materials and Nanotechnology in Engineering (SMN2009), in press.

［2］ A. Sellinger and R. M. Laine. Silsesquioxanes as Synthetic Platforms 3- Photocurable Liquid Epoxides as Inorganic/Organic Hybrids Precursors. Chemistry of Materials(1996), 8, p.1592

［3］ F. Gao, Y. Tong, S. R. Schricker and B. M. Culbertson. Evaluation of neat resins based on methacrylates modified with methacryl-POSS, as potential organic-Inorganic hybrids for formulating dental restoratives. Polym. Adv. Technol(2001), 12, p.355

［4］ H. Fong, S. H. Dickens and G. M. Flaim. Evaluation of dental restorative composites containing polyhedral oligomeric silisesquioxane methacrylate. Dental Materials(2005), 21, p.520

［5］ H. Dodiuk-Kenig, Y. Maoz, K. Lizenboim and I. Eppelbaum. The effect of grafted caged silica (polyhedral oligomeric silesquioxanes) on the properties of dental composites and adhesives. Journal of Adhesion Science and Technology(2006), 20, p.1401

［6］ Qiang Li, Yan Zhou and Xiaodong Hang. Synthesis and characterization of a novel arylacetylene oligomer containing POSS units in main chains. European Polymer Journal(2008), 44, p.2538

［7］ HoSouk Cho, Kaiwen Liang, Sabornie and Chatterjee. Synthesis, morphology, and viscoelastic properties of polyhedral oligomeric silsesquioxane nanocomposites with epoxy and cyanate ester matrices. Journal of Inorganic and Organometallic Polymers and Materials(2005), 15(4), p.541

［8］ I.E. dell’Erba and R.J.J. Williams. Synthesis of oligomeric silsesquioxanes functionalized with (b-carboxyl) ester groups and their use as modifiers of epoxy networks. European Polymer Journal(2007), 43, p.2759

［9］ Zhiqing Chen. Biomaterial of oral cavity. [M]. Peking: Chemical Industry Press, 2004.

Materials Flexural Modulus

(Gpa)

HV (1/10) (MPa)

Fracture Energy (Nmm)

40:00:60 5.31 349 9.07 38:02:60 5.30 400 14.17 35:05:60 5.56 359 4.67 30:10:60 4.24 339 3.64 25:15:60 3.55 258 1.95

348 Multi-Functional Materials and Structures II

Multi-Functional Materials and Structures II 10.4028/www.scientific.net/AMR.79-82 Mechanical Properties of Dental Nanocomposites Reinforced with Polyhedral Oligomeric

Silsesquioxane(POSS) 10.4028/www.scientific.net/AMR.79-82.345

DOI References

[2] A. Sellinger and R. M. Laine. Silsesquioxanes as Synthetic Platforms 3- Photocurable Liquid Epoxides as

Inorganic/Organic Hybrids Precursors. Chemistry of Materials(1996), 8, p.1592

doi:10.1021/cm9601493 [3] F. Gao, Y. Tong, S. R. Schricker and B. M. Culbertson. Evaluation of neat resins based on methacrylates

modified with methacryl-POSS, as potential organic-Inorganic hybrids for formulating dental restoratives.

Polym. Adv. Technol(2001), 12, p.355

doi:10.1002/pat.117 [4] H. Fong, S. H. Dickens and G. M. Flaim. Evaluation of dental restorative composites containing

polyhedral oligomeric silisesquioxane methacrylate. Dental Materials(2005), 21, p.520

doi:10.1016/j.dental.2004.08.003 [5] H. Dodiuk-Kenig, Y. Maoz, K. Lizenboim and I. Eppelbaum. The effect of grafted caged silica

(polyhedral oligomeric silesquioxanes) on the properties of dental composites and adhesives. Journal of

Adhesion Science and Technology(2006), 20, p.1401

doi:10.1163/156856106778456609 [6] Qiang Li, Yan Zhou and Xiaodong Hang. Synthesis and characterization of a novel arylacetylene

oligomer containing POSS units in main chains. European Polymer Journal(2008), 44, p.2538

doi:10.1016/j.eurpolymj.2008.06.018 [7] HoSouk Cho, Kaiwen Liang, Sabornie and Chatterjee. Synthesis, morphology, and viscoelastic properties

of polyhedral oligomeric silsesquioxane nanocomposites with epoxy and cyanate ester matrices. Journal of

Inorganic and Organometallic Polymers and Materials(2005), 15(4), p.541

doi:10.1007/s10904-006-9008-0 [8] I.E. dell’Erba and R.J.J. Williams. Synthesis of oligomeric silsesquioxanes functionalized with (b-

carboxyl) ester groups and their use as modifiers of epoxy networks. European Polymer Journal(2007), 43,

p.2759

doi:10.1016/j.eurpolymj.2007.04.017