d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 456–462

avai lab le at www.sc iencedi rec t .com

journa l homepage: www. int l .e lsev ierhea l th .com/ journa ls /dema

Development of novel dental nanocomposites reinforcedwith polyhedral oligomeric silsesquioxane (POSS)

Xiaorong Wua,∗, Yi Suna, Weili Xieb, Yanju Liua, Xueyu Songa

a Department of Astronautic Science and Mechanics, Harbin Institute of Technology, Chinab Department of Stomatology, Harbin Medical University, China

a r t i c l e i n f o

Article history:

Received 14 April 2009

Received in revised form

18 July 2009

Accepted 16 November 2009

Keywords:

Dental composite resins

Nanocomposites

Shrinkage

Polyhedral oligomeric

silsesquioxane (POSS)

Mechanical properties

a b s t r a c t

Objectives. It has been the focus to develop low shrinkage dental composite resins in recent

ten years. A major difficulty in developing low shrinkage dental materials is that their

deficiency in mechanical properties cannot satisfy the clinical purpose. The aim of this

study is to develop novel dental nanocomposites incorporated with polyhedral oligomeric

silsesquioxane (POSS). It is especially interesting to evaluate the volumetric shrinkage and

mechanical properties, improve the shrinkage, working performances and service life of

dental composite resins.

Methods. The effect of added POSS on the composites’ mechanical properties has been

evaluated. The weight percentages of added POSS are 0, 2, 5, 10 and 15 wt% respectively.

Fourier-transform infra-red spectroscopy and X-ray diffraction were used to characterize

their microstructures. Physico-mechanical properties that were investigated included vol-

umetric shrinkage, flexural strength, flexural modulus, compressive strength, compressive

modulus, Viker’s hardness and fracture energy. Furthermore, the possible reinforced mech-

anism has been discussed.

Results. The shrinkage of novel nanocomposites decreased from 3.53% to 2.18%. The

nanocomposites incorporated with POSS showed greatly improved mechanical properties,

for example, with only 2 wt% POSS added, the nanocompsite’s flexural strength increased

15%, compressive strength increased 12%, hardness increased 15% and uncommonly, even

the toughness of resins was obviously increased. With 5 wt% POSS polymerized, compres-

sive strength increased from 192 MPa to 251 MPa and compressive modulus increased from

3.93 GPa to 6.62 GPa, but flexure strength began to decline from 87 MPa to 75 MPa. This find-

ing indicated that the reinforcing mechanism of flexure state maybe different from that of

compressive state.

Conclusions. The mechanical properties and volumetric shrinkage of dental composite resins

polymerized with POSS can be improved significantly. In current study, the nanocomposite

with 2 wt% POSS incorporated is observed to achieve the best improved effects.

© 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

∗ Corresponding author. Tel.: +86 0451 86414825.E-mail addresses: wuxr2230@163.com, atanackovic@uns.ns.ac.yu (X. Wu).

0109-5641/$ – see front matter © 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.dental.2009.11.161

2 6 ( 2 0 1 0 ) 456–462 457

1

Tltmdtppstrit[

cdnifs

mtaiAg

Pse(tpf

[rtoiodp

fnIar

scBnn

Valley, CA). Bis-GMA (Bisphenol A glycerolate dimethacrylate)and TEGDMA (Tri(ethylenglycol) dimethacrylate, TEGDMA,98%) were from Aldrich Chemical Co. (shown in Fig. 2). Allmonomers were used as received without further purification.

d e n t a l m a t e r i a l s

. Introduction

he development of dental restorative materials, in particu-ar direct resin-based filling composites, has revolutionizedhe field of dentistry over the past 30 years. This develop-

ent has been achieved mainly through organic monomeriscovery, modifications in formulation, the use of newailor-made fillers (e.g. nanofillers or clusters for the com-osite) or advances in light curing equipment and efficienthoto initiators [1]. Despite these developmental advances,ome problems still limit the use of composites in den-al restoration. Most improvements are focused on theeduction of polymerization shrinkage [2–5], as well as themprovement of wear resistance [6–8], mechanical proper-ies [9–12], biocompatibility [13–15], and processing properties16,17].

Polyhedral oligomeric silsesquixanes (POSS) is one typi-al organic–inorganic hybrid nanocomposite, which has beeneveloped since the end of last century. POSS is really aanostructural chemical whose molecule is 1.5 nm isotropic

n structure. POSS monomer is represented by the empiricalormula (RSiO1.5)n with an inorganic silica-like core (SiO1.5)urrounded by organic corner groups R.

It has been revealed that the performances of POSS-odified polymers are usually attractive. They not only have

he organic characters such as good processability, toughnessnd low cost, but also hold excellent inorganic performancesn mechanics, thermodynamics, anti-oxidation, etc. [18–20].t present, it is a very important way to get functional andood performance materials modified by POSS.

Sellinger and Laine [21] first mentioned the possibility ofOSS being used in dental restorative materials. In a lattertudy, the research group of Culbertson and co-workers [22]valuated the systems of POSS incorporated with neat resinswithout filler). The results showed that miscibility betweenhe POSS component and the matrix, especially the diluents,layed a very important role in improving the properties of theormulated thermosets.

Based on the research of Culbertson and co-workers’ group22], Fong et al.’s team [23] explored novel polymeric dentalestorative composites, in which POSS-MA was used to par-ially (or completely) replace Bis-GMA. The results showed thatnly a small percentage of POSS-MA substitution of Bis-GMA

n the resin systems could improve the mechanical propertiesf the composites. However, it was a pity that there was noistinct improvement in such important properties as com-ressive strength, hardness and toughness.

Dodiuk-Kenig et al. [24] believed that the type of the graftedunctional group of the caged silica was the dominant factor inanotailoring of improved dental composites and adhesives.

t showed that the mechanical properties of dental compositesnd adhesives were improved by acrylated POSS but deterio-ated by octaphenyl grafted POSS.

Many other research works [25–27] have been done on thetructural design of the POSS molecule, synthesizing the pro-

ess of modified resins, as well as properties characterization.ut we have to admit that considerably complicated tech-iques of synthesis and rigorous conditions of reaction areeeded for most new POSS monomers. Therefore, this seri-

Fig. 1 – Molecular anatomy of multifunctional methacrylPOSS.

ously limits the development and application of POSS in dentalrestorations.

In this paper, multifunctional methacryl POSS cage mix-ture (n = 8, 10, 12) (from Hybrid Plastics) was explored to beadded into dental composite resins to develop novel dentalnanocomposites with improved properties. Multifunctionalmethacryl POSS molecule’s anatomy (showed by Fig. 1) con-sists of an inorganic “cage” built with silicon and oxygen, andeach “Si” is attached by a methacrylate functional group. Addi-tionally, the methacryl POSS is soluble in the composite resins.

The aim of the study was to develop novel dental nanocom-posites with POSS incorporated, evaluate their polymerizationshrinkage, mechanical properties, and improve working per-formances and service life.

2. Materials and methods

2.1. Formulation of nanocomposites

Methacryl POSS was obtained from Hybrid Plastics (Fountain

Fig. 2 – Molecular structures of the monomers andphoto-initiation system.

458 d e n t a l m a t e r i a l s 2

Table 1 – Weight percentage of the components used indental nanocomposites (wt%).

Composites code Resin matrix wt% POSS wt% BG wt%

P00 40 0 60

P02 38 2 60P05 35 5 60P10 30 10 60P15 25 15 60

The commonly used visible light photo-initiator CQ (cam-phorquinone, 97%) and co-initiator (2-(dimethylamino) ethylmethacrylate, DMAEMA, 98%) were selected for this research.Both CQ and DMAEMA were also from Aldrich Chemical Co.The filler used in this study was finely milled silanized bar-ium oxide glass powder (BG) with an average particle size of0.8 �m, provided by the materials laboratory of the School ofStomatology at Peking University.

2.2. Synthesis of materials

A solution containing 49.5 wt% Bis-GMA, 49.5 wt% TEGDMA,0.5 wt% CQ and 0.5 wt% DMAEMA was prepared by mixingsufficiently in a container kept away from light. Then multi-functional methacryl POSS was proportionately added to thesolution of neat resins as Table 1 and magnetically blendeduniformly. Finally, BG was slowly put into the mixture andstirred in the vacuum mixer allowing for the escape of air bub-bles. In this study the curing time of each sample was 40 sat room temperature, except special explanation. Five speci-mens were prepared for each type of material and immersedin distilled water at 37 ◦C for 24 h after taking out of the stain-less steel molds, followed by a careful polish in a longitudinaldirection under water with 2400 grit silicon carbide paper. Thefinal dimensions of the specimens were then measured accu-rately and recorded immediately before testing.

2.3. FTIR characterization

Fourier-transform infra-red spectroscopy (FTIR) was utilizedto evaluate the degree of conversion. The mid-IR spectrawere collected with an AVATAR360 (Nicolet, USA) instrumentequipped with 4000–400 cm−1 wave speed. A blank KBr pelletwas used for the collection of the background spectrum. Thesamples involved in this part of the study were neat resinswithout filler.

2.4. X-ray characterization

Wide-angle X-ray diffraction (WAXD) was performed using aD/max-�B rotational anode X-ray diffractometer (Japan) withCu Ka irradiation (wavelength of 1.54 Å) using a nickel filter.The X-rays were collimated with a pinhole collimator. Datapoints were collected over the 2� range of 10–90◦ at 0.02◦ inter-vals. The samples involved in this part of the study were neatresins without filler.

2.5. Shrinkage

According to ISO3521, a pycnometer was used to measure thedensity (�) of uncured and cured resin specimens. The volu-

6 ( 2 0 1 0 ) 456–462

metric shrinkage was calculated using the formula:

Shrinkage �V% =(

1 − �uncured

�cured

)× 100% (1)

The samples involved in this part of the study were pre-pared under a two-step photo-initiated curing process. First,the samples were exposed to visible light and cured for 60 s,then the visible light switched off, 60 min later, the secondphoto-initiated curing began and lasted for another 30 s.

2.6. Flexural strength and flexural modulus

The specimens were prepared in stainless steel molds witha dimension of 2 mm in width by 2 mm in depth by 25 mmin length. Flexural strength (FS) and flexural elastic modulus(Ef) were obtained by three-point flexure testing (according toISO4049: 2000) at a crosshead speed 0.5 mm/min. The test wasperformed on a T1-FR010TH A50 (ZWICK Materials TestingMachine, Germany). Calculations were made using formulasas follows:

FS = 3PL

2WT2(2)

Ef =(

P

d

)(L3

4WT3

)(3)

where P is the load at fracture, L is the distance between twosupports (which was set to be 20 mm), W is the width of thespecimen, T is the thickness of the specimen and d is thedeflection at load P.

2.7. Compressive strength and compressive modulus

Compressive strength (CS) and compressive elastic modulus(EC) were measured by compressive testing at a crossheadspeed of 10 mm/min with cylindrical samples with the dimen-sions 3 mm in diameter by 7 mm in length. The test wasperformed on a T1-FR010TH A50 (ZWICK Materials TestingMachine, Germany). Calculations were made using formulasas follows:

CS = 4P

�D2(4)

EC = 4L0(FH − FL)�D2(LH − LL)

(5)

where D is the diameter of the specimen, L0 is the length ofthe specimen, FH(FL) and LH(LL) is the end(start) of the corre-sponding load and distortion of the specimen in the elasticphase of load–distortion curve.

2.8. Hardness

Vicker’s hardness (HV) was determined using a depth-sensingmicroindentation technique on BZ2.5/TSIS (Universal Hard-

ness Testing Instrument, Germany). The specimens wereindented with a Vickers indenter, whose diamond press headwas in right square pyramid shape with 136◦ relatively angle.Vicker’s hardness (HV) data was obtained by dividing the peak

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 456–462 459

s. (a

la

H

w

2

“istesg

3

Imnibpti

3

Tmrp(1tr

POSS crystal peaks could be identified, which confirmed thatthe multifunctional POSS had polymerized within the resinmatrix.

Fig. 3 – FTIR spectra of the resins irradiated for 40

oad (Pmax) over the maximum projected contact area (Amax)s follows:

V = Pmax

Amax= 1.8544 × Pmax

d2(6)

here d is the average length of two indented diagonals.

.9. Fracture energy

Fracture energy” (Nmm) is the dissipative outside energy dur-ng fracture, which can be calculated by the enclosed area oftress–strain fracture curve obtained from three-point flexureest. In the physics, the square of toughness is namely thenergy release rate. Therefore, fracture energy can be the mea-urement of the toughness of the materials, and usually, thereater the fracture energy, the tougher the material.

. Results and discussion

n this paper, 40/60 (wt%) was chosen as the ratio of resinatrix/inorganic filler. The composite resins with such ratio

ot only had proper viscosity to keep the filler from deposit-ng, but also had sufficient fluidity to make resins easy toe processed. In this paper, the effects of dental nanocom-osites with different weight percentage (0–15 wt%) POSS onhe polymerization shrinkage and mechanical properties werenvestigated.

.1. Degree of conversion

he FTIR spectra shown in Fig. 3 were the mid-IR measure-ents of the C C absorption at 1635 cm−1. Uncured and cured

esins were evaluated, which deadline stood for uncured sam-les and dashed stood for cured ones. From FTIR spectra

Fig. 3) it can be seen that the intensity of C C absorption at635 cm−1 was obviously weaker after curing. It proved thathe methacrylate double bonds had partly conversed and theesins had polymerized during curing. Compared with Fig. 3(a)

) P00 (Wposs = 0 wt%) and (b) P02 (Wposs = 2 wt%).

P00 and Fig. 3(b) P02, it could be seen that the weakeneddegree of C C absorption of P02 was greater than that of P00. Itaccounted for why the degree of conversion of P02 was greaterthan that of P00. The methacrylate double bond conversion isone of the important properties in dental composite resins.Generally, the mechanical property of resins can be improvedwith an increase in conversion. It was also proved later in thispaper.

3.2. WAXD

Wide angle X-ray diffraction was employed to investigatewhether POSS crystal peaks appear in the nanocompositesof the WAXD profiles. In this study, wide-angle X-ray diffrac-tion data (Fig. 4) indicated that all POSS nanocomposites wereamorphous, even when the mass fraction of the methacrylPOSS monomers in the resin mixture was as high as 15%, no

Fig. 4 – WAXD profiles of the nanocomposites.

l s 2

460 d e n t a l m a t e r i a

3.3. Shrinkage

Most dental composite resins are free-radical initiation pho-topolymers, and volumetric shrinkage during polymerizationis usually high. Volumetric shrinkage causes microleakage, awell-known effect of contraction gaps at the interface betweenresin and tooth. Saliva, fluid, food residue and microorgan-isms trapped in the gaps lead to decayed teeth and damagedenamel, which is a major problem in current restorative andesthetic dentistry.

The change in the distances among atoms caused by poly-merization is probably the major reason for the shrinkage. Vander Waals forces acting on molecules of the monomers turninto covalent bonds and the distances among atoms becomeshorter from 0.3 to 0.5 nm to 0.154 nm. Therefore, the compactarrangements make the volume shrink during polymeriza-tion. Another possible reason of the shrinkage is the change inthe free volume which exists owing to molecules in the poly-mer not being fully compacted. The free volume of moleculeswith liquid state before curing is greater than their loose state.After curing, monomers polymerize with matrix to form across-linked net framework [28] and the movement of chainsis limited by plenty of cross-linked points. Therefore, the freevolume becomes smaller and causes remarkable shrinkage.

In this paper, the volumetric shrinkage of novel systemswith POSS was evaluated. The results of the shrinkage of com-posite resins synthesized are shown in Fig. 5. Shrinkage wasfrom 3.53% to 3.10% when only 2 wt% POSS was incorporated,and it reduced from 3.53% to 2.18% when 15 wt% POSS wasused. The results proved that multifunctional methacryl POSScan effectively reduce the shrinkage of resins and that shrink-age is reduced with the increasing percentage of POSS. Thepossible reason is that the special nanocubic structure of POSSmay limit the change of the resins’ free volume after a cross-linked net framework is formed. Therefore, the shrinkage ofresins with POSS incorporated was decreased.

3.4. Mechanical properties

The nanocomposites incorporated with POSS showed greatlyimproved mechanical properties. The test results are shown in

Fig. 5 – Shrinkage of the nanocomposites with differentweight percentage of POSS.

6 ( 2 0 1 0 ) 456–462

Fig. 6. With only 2 wt% POSS added, the nanocompsite’s flex-ural strength increased 15%, compressive strength increased12%, compressive modulus increased 4% and flexural modulusshowed no change. With 5 wt% POSS polymerized, com-pressive strength increased from 192 MPa to 251 MPa andcompressive modulus increased from 3.93 GPa to 6.62 GPa,but flexural strength began to decline from 87 MPa to 75 MPa.This finding indicated that the reinforcing mechanism of theflexure state maybe different from that of the compressivestate.

Flexural strength, a very important property for dentalrestorations, can reflect the ability that the materials towithstand complex stress. In current research, it was foundthat with only 2 wt%, POSS copolymerized in resins, flexuralstrength increased 15% and compressive strength increased12% (see Fig. 6). This result accorded with the above part ofdegree of conversion. A lot of researches [30–32] indicated thatthe dispersion of nanoparticles was the key factor for improv-ing the mechanical properties of the matrix. Libor et al. [28]proved that POSS monomer did not simply mix physically withresins but polymerize with matrix to form a cross-linked netframework. That makes POSS disperse uniformly in the matrixand is less aggregated, which was also proved by the above-mentioned wide-angle X-ray diffraction data. It can probablyexplain the results that flexural strength and compressivestrength of nanocomposites were improved by introducingPOSS.

When the weight percentage of POSS was more than10 wt%, it was found that the mechanical properties ofnanocomposites decreased sharply (Fig. 6). The reason maythat be the formation of a cross-linked polymeric network withmore POSS was so rapid, that it limited the speed of the mobil-ity of reactive species. And more POSS monomers could not bepolymerized and then congregated. Therefore, the mechanicalproperties of nanocomposites deteriorated seriously.

3.5. Hardness

While strength itself is of great importance in determining amaterial’s service performance, bulk properties are only partof the story. The surface behavior of a restoration, such ashardness, is another important mechanical property. Hard-ness can reflect the ability of resisting plastic distortion andmay scale with wear for certain materials. The hardness ofmaterials should be as close as possible to that of the naturaltooth. In this study, the hardness of novel nanocomposites wasincreased from 349 MPa to 400 MPa (Fig. 7), and consequentlycloser to the hardness of dentin (570–600 MPa). It is probablybecause the special rigid cubic structure of POSS improved thehardness of the matrix.

3.6. Fracture energy

It was found that with only 2 wt% POSS, fracture energyincreased 56% (Fig. 8), which can be a measurement of thetoughness of the materials. Traditionally, with increasing

strength and stiffness of reinforced polymers, the toughnessof the materials usually declined. Uncommonly, in this study,POSS monomer was also found to be obviously helpful inincreasing the toughness of resins. This is an important find-

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 456–462 461

posites with different weight percentage of POSS.

ioigsImwat

Fig. 6 – Mechanical properties of the nanocom

ng for the clinical perspective because, in some structuresf dentures, toughness of material is a desired property. The

mprovement may be down to the methacrylate functionalroups attached in POSS molecules. Their flexible long chaintructures may provide better toughness for the resin matrix.n addition, interface action between nano-POSS particles and

atrix would be stronger for their formed cross-linked net-

orks. It needs to absorb more energy during fracture. This isnother possible reason why POSS increases the toughness ofhe matrix.

Fig. 7 – Hardness of the dental nanocomposites.

Fig. 8 – Fracture energy of the dental nanocomposites.

4. Conclusion

The authors synthesized POSS-modified dental compositeresins. The mechanical behavior and volumetric shrinkageof the composite resins copolymerized with multifunctionalmethacryl POSS were examined for their dependence on the

weight fraction of POSS monomer (0–15 wt%). In this study, thenanocomposite with 2 wt% POSS incorporated was observed toachieve all-around improved effects, whose flexural strengthincreased 15%, compressive strength increased 12%, compres-

l s 2

r

liquid crystalline polymer enhanced by nano-clay in nylon-6

462 d e n t a l m a t e r i a

sive modulus increased 4%, hardness increased 15%, fractureenergy increased 56% and volumetric shrinkage decreasedfrom 3.53 to 3.10.

The dental composite resins polymerized with POSScan improve their mechanical properties and volumetricshrinkage; increase the dental nanocomposites’ working per-formances and service life.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (10472028), the Doctoral Program Foun-dation of Ministry of Education of China (20070213054), theNatural Science Foundation of Heilongjiang Province (D0329)and the Excellent Youth Foundation of Heilongjiang Province.We thank the help of Dr. Daming Gu, Department of AppliedChemistry, Harbin Institute of Technology, for supplying thelaboratory.

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