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Molecular dynamics simulation of a novel kind of polymer composite incorporated with polyhedral oligomeric silsesquioxane (POSS)
Computational Materials Science 50 (2011) 3282–3289
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Molecular dynamics simulation of a novel kind of polymer composite incorporated with polyhedral oligomeric silsesquioxane (POSS) Xueyu Song, Yi Sun ⇑, Xiaorong Wu, Fanlin Zeng Department of Astronautic Science and Mechanics, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 15 April 2011 Received in revised form 30 May 2011 Accepted 3 June 2011 Available online 13 July 2011 Keywords: Molecular dynamics Polyhedral oligomeric silsesquioxane Network polymer Wide X-ray scattering
a b s t r a c t Molecular dynamics (MD) simulation is adopted to investigate the physical properties of a novel dental nano-composite resin with poly-functional polyhedral oligomeric silsesquioxane (POSS). An improved method is proposed to model the cross-link polymer. The quality of the proposed technique is verified by wide X-ray scattering (WXRS) and volume–temperature behavior. The excellent agreement proves the accuracy of the models and the force field. Mechanical properties and volume shrinkage of various POSS weight-percentage resins are predicted, and the results are also compared with available experimental data. Further analysis provides some molecular insight about the effect of POSS on the dental composite resin. The improved method could be employed in further research of new cross-link dental materials with complex structures. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction For the last decades, polymer nano-composite materials have achieved amount of attention due to the possibility of tailoring their structure and excellent performance on the nano-scale and with much greater chemical fidelity for standard composites [1]. Especially, hybrid organic–inorganic nano-composites based on the incorporation of POSS into polymeric matrices have received a considerable amount of attention. POSS has a compact hybrid structure (Fig. 1) with an inorganic core consisting of silicon and oxygen atoms (SiO1.5)n, with n = 8, 10, 12, externally surrounded by non-reactive or reactive organic ligands [2–4]. Obviously, such chemical nature of the reactive organic ligand R plays a major role in the control of the morphologies and the functional properties generated in the hybrid materials. Recently, it has been widely used to incorporate with dental composite resins to improve the wear resistance, mechanical properties, biocompatibility, and processing properties of the dental resin materials [5–16]. However, most of the studies about the dental composites resins improved by POSS or other materials are limited on the experimental technology and few research works are to interpret the enhancement mechanism via MD simulation because of their too complex structures [17–19]. Most previous MD simulations are focused on the linear homo-polymers or copolymers, while seldom studies are on network polymers. Some network polymers are
studied based on the MD simulations, where the network structures are directly replaced by linear chains or by simple network segments [20–22]. Such simplification inevitably reduces the computational accuracy because of the lack on the details of the network. Recently, several methods have been proposed to chemically construct the network polymer models along with the chemical reaction path by molecular simulations, and the results are more reasonable [23–30]. The goal of the paper is to investigate the physical properties of a novel dental nano-composite resin incorporated with polyfunctional polyhedral oligomeric silsesquioxane (POSS) by MD simulations. An improved method was developed to construct cross-linking polymer model of the dental nano-composite resin based on the previous experimental works. Subsequently, the quality of the models was verified by MD simulations via the wide X-ray scattering (WXRS) and glass transition temperature Tg, the simulated results were consistent with corresponding experimental data. Finally, mechanical properties and volumetric shrinkage of three different resin models were calculated and analyzed with molecular insight. Their results were also compared with available experimental data.
2. Experimental 2.1. Material synthesis
⇑ Corresponding author. Tel.: +86 0451 86418124. E-mail address: [email protected] (Y. Sun). 0927-0256/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.commatsci.2011.06.009
Bisphenol A glycerolate dimethacrylate (Bis-GMA) and Tri(ethylenglycol) dimethacrylate (TEGDMA) were purchased from Aldrich
X. Song et al. / Computational Materials Science 50 (2011) 3282–3289
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analysis (DSC) was performed on the DSC 2920 (TA-Instruments, USA) at a heating rate of 10 K/min, under nitrogen atmosphere. 3. Molecular simulation 3.1. Molecular models of the novel resins
Fig. 1. Structure of typical POSS with functional ligand R.
Chemical Co., Methacryl POSS was purchased from Hybrid Plastics (Fountain Valley, CA). All monomers were used as received without any further purification. The commonly used visible light photo-initiator camphorquinone (CQ, 97%) and co-initiator 2(Dimethylamino) ethyl methacrylate (DMAEMA, 98%) were selected for this research. Both CQ and DMAEMA were also purchased from Aldrich Chemical Co. A solution containing 49.5 weight percent (wt.%) of Bis-GMA, 49.5 wt.% of TEGDMA, 0.5 wt.% of CQ and 0.5 wt.% of DMAEMA were prepared by mixing sufficiently in a container which was kept away from light. Then multifunctional Methacryl-POSS (POSS-MA) was proportionately added into the solution of neat resins and magnetically blended uniformly. In this study the curing time of each sample was 40 s at room temperature. All specimens were prepared for each type material and immersed in distilled water at 37 °C for 24 h after taking off stainless steel molds, and a careful polish under water with 2400 grit silicon carbide paper longitudinal direction was followed. The final dimensions of the specimens were measured accurately and recorded just before the testing.
2.2. Material characterization The novel dental resins improved with three different contents of Methacryl POSS were synthesized. Fourier-transform infra-red spectroscopy (FTIR) was utilized to evaluate the degree of conversion. Wide-angle X-ray scattering (WAXS) was performed on a D/max-B rotational anode X-ray diffractometer (Japan), with Cu Ka irradiation (wavelength of 1.54 Å), using a nickel filter. Thermal
In this paper, the novel dental nano-composite resins with different loadings of POSS (0, 2 and 5 wt.%) were studied using all atom MD simulation. The matrices of the resins are Bis-GMA and TEGDMA, and the additive is POSS-MA (Fig. 2.). Each matrix molecular has two carbon–carbon double bonds at the terminals, and there are eight carbon–carbon double bonds at the terminals of POSS molecular especially. The dental composite resins are supposed to have three-dimensional network structures due to the polymerization between the carbon–carbon double bonds of the monomers. The essential reaction occurring in the polymerization is explained through the schematic diagram Fig. 3. During the curing, the original carbon–carbon double bond is activated into active functional group shown in Fig. 3 by the visible light. As a result, the double bond converts to a single band and two active sections A and B are introduced. Then, the chemical linking just takes place between them. Thus, three-dimension polymer network system emerges chemically. In this section, three network molecular system models shown in Table 1 were constructed dynamically using Material Studio software. In order to capture the details of the chemical reaction process and to form the network models dynamically, the following procedures were carried out (the procedures were illuminated by Model P02): (i) Three types of molecular models for original Bis-GMA, TEGDMA and POSS-MA were constructed, then energy minimization was performed respectively to obtain their reasonable structures. Needing explanation is that energy minimization was carried out using two different methods; first the steepest descends (convergence value of 1000 kcal/mol) and then the conjugate gradient (convergence value of 10 kcal/mol), relaxing the system to a local state of minimal potential energy. Subsequent energy minimizations were all based on this scheme. Afterward, the original carbon–carbon double bond was changed into activated functional group (shown in Fig. 3) by converting the double bond into the single one. As a result, two active sections A and B were introduced. Thus, the reactive molecular models for Bis-GMA, TEGDMA and POSS-MA were constructed. Before the three-dimension network models were created, all reactive molecular models were optimized into reasonable structures just using energy minimization.
Fig. 2. Snapshots of molecular models from our simulation used in the cross-linking polymer models: (a) Bis-GMA, (b) TEGDMA and (c) POSS-MA.
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Fig. 3. The essential reaction occurring in the polymerization (The red lines present new covalent links.). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Characteristics of three different cross-linking models in the simulation and experiment. Resin models
Number of molecular
P00 P02 P05
Density (g/cm3)
POSS loadings
POSS
BisGMA
TEGDMA
Simulation (wt.%)
Experiment (wt.%)
Simulation
Experiment
0 1 2
60 64 50
120 128 100
0.00 2.02 5.02
0 2 5
1.15 1.15 1.16
1.21 1.19 1.14
(ii) Secondly, 64 active molecular models of Bis-GMA, 100 active molecular models of TEGDMA and one active POSS-MA molecular model were packed into one cubic lattice with a density of 1.2121 g/cm3 (obtained from the experimental profiles) using amorphous cell module in Material Studio software. To eliminate the surface effect, three-dimension periodic boundary condition was imposed. Then, the initial physical mixture of the novel dental nano-composite resin model was constructed. (iii) Thirdly, 50,000 steps of energy minimization were carried out to relax the unit cell system. Then, the relaxed physical mixture system was evaluated to identify the activated sections in close distance. One activated section’s carbon of A or B was chosen first, afterward, another search of nearby activated section’s carbon was performed. Only the proximal two activated section’s carbons in the reaction cut-off distance range could react, this means that the covalent bond between them will construct. Especially, the cutoff distance from 3 to 10 Å was chosen in this paper because this selection could avoid the reaction of two nearby carbons just in the same activated functional groups. In addition, the larger one of cut-off distance was optional according to the situation. During the reaction, the formation of four-member and fivemember ring should be avoided. Although the real cross-link systems were simulated as far as possible, the physical defects such as chain entanglements were allowable. (iv) Finally, the modified molecular model system was checked again to evaluate the double bonds conversion during the polymerization. If the double bonds conversion was smaller than the experimental result from FTIR, the larger one of the
cut-off distance was enlarged and step iii was performed again. On the other hand, if the conversion was larger than the experimental result from FTIR, the larger one of the cutoff distance was reduced and step iii was performed again. Until the double bonds conversion dropped in the permissible range, the procedure was over and all non-hydrogen atoms in the system were saturated with hydrogen atoms. The final physical and chemical mixture model systems were chosen for further study. All steps were coded as a Perl script and run in the Material Studio software from Accelrys. Eventually, three network molecular system models with different loadings of POSS (0, 2.02 and 5.02 wt.%) were constructed shown in Tables 1 and 2. It is obvious that the loadings of POSS of the network polymers in the simulations are approximate to the experimental ones and the densities of various network polymers also coincide with the experimental results in a certain error range. 3.2. Molecular simulation protocol details All the simulations were performed based on the COMPASS (condensed phase optimized molecular potentials of atomic simulation studies) force field. The COMPASS force field is the first ab initio force field which has been parameterized and validated using condensed-phase properties, it has been used effectively on lots of nano-composites with POSS [18,19,31,32]. In the COMPASS force field, the total energy expression has two main terms of bond interactions and non-bonded interactions. The latter interactions (Van der Waals and electrostatic energy) are very important terms
Table 2 Characteristics about the polymerization of three different cross-linking models. Resin models
P00 P02 P05
Double bond numbers Original total number
Final cured number
360 392 316
210 227 185
Cut off distance (Å)
Simulation conversion (%)
3–5 3–5.27 3–5.2
58.33 57.91 58.545
Note: The polymerization conversion was evaluated by the conversion of the double bonds, i.e., conversion = (cured double bond number)/ (original total double bond number).
X. Song et al. / Computational Materials Science 50 (2011) 3282–3289
in the total energy expression. Since the energy terms depend on the number of atoms in the system, the interaction between too far separated atoms should be neglected. The cut-off in the whole study was selected as 9.5 Å, and atom-based method was selected for Van der Waals energy. However, this method may lead the energy discontinuity for the electrostatic interactions, especially for the system with large electric charge. Thus, Ewald method recognized as the best one with quite high accuracy was chosen for the electrostatic energy in the paper [33]. Periodic boundary conditions were used to simulate the materials. Nosé algorithm is more stable and could keep the regularity of the system. Thus, it was chosen to control the temperature [19,34,35]. Parrinello algorithm enables the change of both symmetry and size of the simulation cell [36,37], hence it was chosen to control the pressure. The velocity form of the Verlet integrator was used to solve the equations of motion with a time step of 0.5 fs. Before MD simulation was carried out, 50,000 steps of energy minimization were carried out to relax the unit cell system. Subsequently, for the selected network system models, MD simulation was performed in constant particle numbers, pressure and temperature (NPT) conditions. The snapshots of the network system models for P00 and P05 were shown in Fig. 4. 4. Results and discussion 4.1. Wide X-ray scattering profiles The wide X-ray scattering (WXRS) profiles of the selected configurations fully balanced were calculated. Fig. 5 shows the comparison between the simulation results and the experimental ones. The simulation results are in good agreement with the experimental results, in terms of peak position and intensity. It could arrive at the conclusion that the network system polymer models created in the study, the force field, and the molecular dynamics protocol are valid. In addition, the network polymers with different loadings of POSS appear the similar trend on the WXRS for the simulation and experiment and just the intensity of WXRS differs slightly. These phenomena indicate that the structures of the novel network polymers with different loading of POSS are similar with each other. For all polymer resins, WXRS patterns show a broad amorphous halo at 2h = 18.50° corresponding to the effective Bragg distance of 4.43 Å. This phenomenon verifies that the network polymer resins are amorphous. There is also a smaller amorphous halo at 2h = 41.18° in the experimental results, while the simulated profiles are not apparent at the same position.
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4.2. Glass transition temperature In this study, the volume versus temperature was determined to predicate Tg with the method of NPT MD simulations. The temperature at which the slope changes on the volume–temperature diagram represents Tg of the polymers [18,19,31,38,39]. To ensure the models fully equilibrated, MD simulations were performed on the network systems, where the temperature was first cooled down from 600 K to 50 K, then heated up from 50 K to 600 K, finally cooled down from 600 K to 50 K with the same intervals of 50 K. Only the last cooling process was chosen to evaluate Tg of the network polymer models. Moreover, MD simulation at each isothermal temperature was performed more than 250 ps, and the average volume of more than 10 different configurations was used to determine the volume–temperature diagrams. The volume– temperature diagrams of models with different loading of POSS are shown in Fig. 6. In each case, the break of slope occurs apparently. This indicates that the size of the network polymer models is large enough to evaluate the glass transition behavior [18]. The simulated glass transition temperatures obtained from the volume–temperature diagrams are compared with the available experimental results from DSC (Fig. 7). The simulated results are in agreement with the experimental results and both results are in 50–60 °C. These results further verify the molecular simulation protocol and the method used to calculate Tg from simulations. In addition, the results also show that the incorporation of POSS has a little effect on the glass transition temperature. It is easy to understand such results for the improved dental resins and the original resin have the similar network structures. The simulated results are a little higher than the experimental results, and the similar phenomena have been reported in many literatures [18,19,40]. In general, the glass transition of polymers occurs in a range of temperatures. The cooling rates have an effect on the measure of glass transition temperatures [40], the number of the repeat units in the simulations could also shift the glass transition temperature [41,42]. In the MD simulations, the cooling rate (1011 K/s) was much higher than the experimental one (10 K/ min). All above factors make the simulated results different from the experimental ones in some degree. Thus, the results in this section are reasonable. 4.3. Mechanical properties In general, mechanical properties are mainly due to the behavior exhibited by polymeric systems under different testing modes of
Fig. 4. Snapshots from our simulation of (a) network polymer model P00 and (b) polymer model P05. Note: in model P05, POSS was colored orange with shape of ellipsoid.
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Fig. 5. Simulated and experimental WAXS results: (a) P00, (b) P02 and (c) P05. Fig. 6. Specific volume and temperature curves for: (a) P00, (b) P02 and (c) P05 at 1 atm as determined from NPT dynamics. The arrows locate the position of Tg as determined from MD simulations.
elastics properties. MD simulations have been successfully used to predict elastic properties of polymers, mainly non-network polymers but also some network polymers systems [43–48]. At this section, the static method was performed to predict the properties of the novel composites resin models. The static method also named constant-strain minimization method was first proposed by Theodorou and Suter at 1986. In this method, after an initial energy minimization, the microtension and compression tests are performed with a very small elastic strain (±0.001) on the system cells. Then, another energy minimization is carried out and the stiffness
matrix is derived from the second derivative of potential energy U with respect to the strain e as follows: 2
C ij ¼
d U dr ei dej ¼ i d dej
ð1Þ
For the amorphous isotropic material, the stiffness matrix is decided by two Lame constants (k, l) shown as follows:
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Table 3 Mechanical properties obtained from the calculation for effective isotropic elastic constants of three models. Mechanical properties
Young’s modulus (GPa)
Shear modulus (GPa)
Bulk modulus (GPa)
Poisson’s ratio
P00 P02 P05
3.269 3.591 4.263
1.215 1.316 1.632
3.529 4.425 3.671
0.3456 0.3647 0.3065
Obviously, the matrix is symmetrical and moreover the nondiagonal components of the matrix are zero. Thus, once the stiffness matrix is obtained from Eq. (1), two Lame constants (k, l) would be derived from Eq. (2). As a result, the elastic properties constants could be obtained from them as follows:
Fig. 7. Simulated and experimental glass transition temperature with the loading of POSS.
2
C 66
0 B B B B B B B B @
6 6 6 6 ¼6 6 6 6 4
k þ 2l
k
k
0
3
0
0
07 7 7 07 7 07 7 7 05
l
k
k þ 2l
k
0
0
k
k
k þ 2l
0
0
0
0
0
0
0
0
l 0 0 l
0
0
0
0
0
8 l E ¼ lð3kþ2 Þ > > kþl > > < G¼l > K ¼ k þ 23 l > > > : m¼ k
ð3Þ
2ðkþlÞ
ð2Þ
Here E, G, K and m represent Young’s modulus, shear modulus, bulk modulus and Poisson’s ratio respectively. In this paper, the elastic entire stiffness matrices of three novel composites resins were obtained using constant-strain minimization method based on COMPASS force field at 300 K. Then, the average mechanical properties were calculated from them respectively. The final results of entire stiffness matrix of Model P05 are shown
1
6:112ð0:056Þ
2:515ð0:124Þ
2:608ð0:010Þ
0:140ð0:032Þ
0:531ð0:329Þ
0:216ð0:117Þ
2:313ð0:230Þ
5:43ð0:009Þ
2:725ð0:060Þ
0:300ð0:249Þ
0:120ð0:200Þ
0:220ð0:223Þ C C C 0:221ð0:159Þ C C 0:1363ð0:034Þ C C C 0:094ð0:018Þ A
2:389ð0:281Þ
2:951ð0:122Þ
5:885ð0:165Þ
0:164ð0:178Þ
0:395ð0:330Þ
0:301ð0:089Þ
0:003ð0:095Þ
0:0718ð0:022Þ
1:686ð0:034Þ
0:243ð0:055Þ
0:136ð0:344Þ
0:432ð0:349Þ
0:614ð0:349Þ
0:088ð0:077Þ
1:430ð0:063Þ
0:090ð0:053Þ
0:083ð0:019Þ
0:193ð0:037Þ 0:048ð0:074Þ 0:252ð0:147Þ
Fig. 8. Simulated and experimental elastic modules results with the loading of POSS.
ð4Þ
1:441ð0:077Þ
in Eq. (4) and the average mechanical properties are shown in Table 3. It is obvious that the entire stiffness matrix of Model P05 based on COMPASS force field satisfies the main features of isotropic amorphous material essentially. Although the non-diagonal components of stiffness matrix are not zero strictly, the diagonal components are much higher than the non-diagonal ones and the entire matrixes are almost symmetric along the diagonal components. This phenomenon indicates that there is residual stress in the molecular systems. This is not surprising for the temperature tested here is below the glass transition temperature of the systems. Thus, the results are reasonable. From Table 3, we could see that with the incorporation of POSS, Young’s modulus, shear modulus and Bulk modulus of composites resins are enhanced dramatically. Moreover, the higher of POSS contents, the more improvement of Young’s modulus and shear modulus. By incorporating with 0.2 wt.% of POSS, Young’s modulus and shear modulus are improved 9.85% and 8.31% respectively. By incorporating with 05 wt.% POSS, Young’s modulus and shear modulus are improved 30.4% and 34.3% respectively. Especially, the novel dental composite with 02 wt.% of POSS behaves the maximum Bulk modulus. However, Poisson’s ratio of composites resins is influenced slightly by the incorporation of POSS. Actually,
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Fig. 9. Simulated and experimental volume shrinkage with the loading of POSS.
Fig. 10. The schematic of the influence of POSS on the volume of novel composites resins.
the novel dental composite with 0.5 wt.% of POSS behaves smaller Poisson’s ratio than the initial dental composite without POSS. Besides, previous experimental research were done on the composites resins with additional 60 wt.% finely milled silanized barium oxide glass powder (BG) [49]. The paper compared the Young’s modulus obtained from the simulation with the compression elastic modules obtained from experimental compression tests (Fig. 8). It is obvious that the trend of elastic modules results with the loading of POSS is consistent while the experimental results are systematically higher than the simulation results. Since BG could improve the elastic modules of composites resins, above results are reasonable and also validate the methodology used to calculate the mechanical properties of network models. 4.4. Volumetric shrinkage During the photo-polymerization of dental composite materials, Van der Waals interactions acting on molecular monomers turn into covalent interactions. The distances among monomers are decreased distinctly. Thus, the dental composite materials exhibit volume shrinkage during the photo-polymerization. It is a very important parameter, which may cause micro-leakage and other negative effects on teeth. Usually, volume shrinkage is calculated using the following formula [50]:
DV ¼ ð1
quncured Þ qcured
ð5Þ
In this paper, we calculated the densities of uncured and cured network polymer models fully equilibrated. Then, the volume shrinkage was evaluated and the results shown in Fig. 9 indicate
that the incorporation of POSS could reduce the volume shrinkage distinctly. Moreover, with the increase of POSS loading, the composites exhibit lower volume shrinkage. The volume shrinkage of the composite incorporated with 0.2 wt% POSS is reduced 14.3% than initial resin composites. The volume shrinkage of the composite incorporated with 0.5 wt.% POSS is reduced 15.9% than initial one. Besides, previous experimental research investigated the volume shrinkage of the composite resins with additional 60 wt.% of BG powder [49]. The paper compared the simulation results with the experimental ones (Fig. 9). Although, their numerical values are different, both results behave the similar trend. Besides, some literatures have reported that typical resins of dental methacrylate based monomers undergo volumetric shrinkage between 6% and 10% and the highly filled composites resins still exhibited shrinkage of 2.6–7.1% [17,51]. Thus, above results are reasonable and further validate the methodology to predict the volume shrinkage. In addition, from Fig. 2 we find that POSS molecular with a three-dimension compact hybrid structure is more rigid and has much larger volume than BisGMA and TEGDMA. Moreover, in two-dimension, BisGMA and TEGDMA could be simplified to linear bars and POSS could be simplified to a diamond with four short bars. Fig. 10 shows the influence of POSS on the packing of novel composites resins. It is easy to understand that the original composite resin could be packed more close-grained because BisGMA and TEGDMA have similar linear bar structures. However, the packing of improved composite resin is influenced by the incorporation of POSS distinctly. As a result, the volume shrinkage of composite resin is reduced by POSS. 5. Conclusions In this study, an improved method is developed to construct cross-linking polymer models of a novel dental nano-composite resin. Atomistic MD simulation is performed to investigate the effects of introducing poly-functional POSS as embedded parts on this dental resin material. The features of densities and double bond conversion indicate the rationality of the network polymer models. The excellent agreement both of WXRS profiles and volume–temperature between the simulation and the experiment verifies the accuracy of the models and the choice of the force field. With the incorporation of POSS, Young’s modulus, shear modulus and Bulk modulus of composites resins are enhanced dramatically. The incorporation of POSS could reduce volume shrinkage
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distinctly. The results are also compared with available experimental data. Further analysis is used to interpret the working mechanisms of POSS on the composite resins. A simple model clarifies the results in molecular insight about the effect of POSS on the packing of dental composite resin. The developed method could be employed in further research of new dental materials with network structures. Acknowledgements This work was supported by the National Natural Science Foundation of China (10472028), the Doctoral Program Foundation of Ministry of Education of China (20070213054) and the Excellent Youth Foundation of Heilongjiang Province. References [1] Naida Lacevic, Richard H. Gee, Andrew Saab, Robert Maxwell, J. Chem. Phys. 129 (2008) 124903. [2] A. Provatas, J.G. Matisons, Trends Polym. Sci. 5 (1997) 327. [3] G. Li, L. Wang, H. Ni, C.U. Pittman, J. Inorg. Organomet. Polym. 11 (2001) 123. [4] Tudor C. Ionescu, Feng Qi, Clare McCabe, Alberto Striolo, John Kieffer, Peter T. Cummings, J. Phys. Chem. B 110 (2006) 2502. [5] Eduardo C. Bianchi, Paulo R. de Aguiar, Manoel C.S. Alves, César A. de Freitas, Ana R. Rodrigues, Oscar B. de Carvalho, Polímeros – Ciência e Tecnologia. 17 (2007) 130. [6] Yong-Keun Lee, Huan Lu, Makoto Oguri, John M. Powers, J. Biomed. Mater. Res. Part B: Appl. Biomat. 82B (2007) 313. [7] Y. Tanimoto, K. Nemoto, Compos. Interface. 11 (2004) 15. [8] Yujie Zhang, Daohong Zhang, Chuangye Qin, Jingwei Xu, Polym. Compos. 30 (2009) 176. [9] S.G. Pereira, R. Osorio, M. Toledano, et al., Dent. Mater. 23 (2007) 1030. [10] H.K. Xu Hockin, J.L. Moreau, Sun Limin, L.C. Chow, Biomaterials 29 (2008) 4261. [11] A. Shahdad Shakeel, F. McCabe John, Bull Steven, Rusby Sandra, W. Wassell Robert, Dent. Mater. 23 (2007) 1079. [12] I. Yudovin-Farber, N. Beyth, A. Nyska, E.I. Weiss, J. Golenser, A.J. Domb, Biomaromolecules 9 (2008) 3044. [13] K. Moharamzadeh, R. Van Noort, I.M. Brook, et al., J. Mater. Sci.: Mater. Med. 18 (2007) 133. [14] J.D. Eick, R.E. Smith, C.S. Pinzino, et al., J. Dent. 34 (2006) 405.
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Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci
Molecular dynamics simulation of a novel kind of polymer composite incorporated with polyhedral oligomeric silsesquioxane (POSS) Xueyu Song, Yi Sun ⇑, Xiaorong Wu, Fanlin Zeng Department of Astronautic Science and Mechanics, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 15 April 2011 Received in revised form 30 May 2011 Accepted 3 June 2011 Available online 13 July 2011 Keywords: Molecular dynamics Polyhedral oligomeric silsesquioxane Network polymer Wide X-ray scattering
a b s t r a c t Molecular dynamics (MD) simulation is adopted to investigate the physical properties of a novel dental nano-composite resin with poly-functional polyhedral oligomeric silsesquioxane (POSS). An improved method is proposed to model the cross-link polymer. The quality of the proposed technique is verified by wide X-ray scattering (WXRS) and volume–temperature behavior. The excellent agreement proves the accuracy of the models and the force field. Mechanical properties and volume shrinkage of various POSS weight-percentage resins are predicted, and the results are also compared with available experimental data. Further analysis provides some molecular insight about the effect of POSS on the dental composite resin. The improved method could be employed in further research of new cross-link dental materials with complex structures. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction For the last decades, polymer nano-composite materials have achieved amount of attention due to the possibility of tailoring their structure and excellent performance on the nano-scale and with much greater chemical fidelity for standard composites [1]. Especially, hybrid organic–inorganic nano-composites based on the incorporation of POSS into polymeric matrices have received a considerable amount of attention. POSS has a compact hybrid structure (Fig. 1) with an inorganic core consisting of silicon and oxygen atoms (SiO1.5)n, with n = 8, 10, 12, externally surrounded by non-reactive or reactive organic ligands [2–4]. Obviously, such chemical nature of the reactive organic ligand R plays a major role in the control of the morphologies and the functional properties generated in the hybrid materials. Recently, it has been widely used to incorporate with dental composite resins to improve the wear resistance, mechanical properties, biocompatibility, and processing properties of the dental resin materials [5–16]. However, most of the studies about the dental composites resins improved by POSS or other materials are limited on the experimental technology and few research works are to interpret the enhancement mechanism via MD simulation because of their too complex structures [17–19]. Most previous MD simulations are focused on the linear homo-polymers or copolymers, while seldom studies are on network polymers. Some network polymers are
studied based on the MD simulations, where the network structures are directly replaced by linear chains or by simple network segments [20–22]. Such simplification inevitably reduces the computational accuracy because of the lack on the details of the network. Recently, several methods have been proposed to chemically construct the network polymer models along with the chemical reaction path by molecular simulations, and the results are more reasonable [23–30]. The goal of the paper is to investigate the physical properties of a novel dental nano-composite resin incorporated with polyfunctional polyhedral oligomeric silsesquioxane (POSS) by MD simulations. An improved method was developed to construct cross-linking polymer model of the dental nano-composite resin based on the previous experimental works. Subsequently, the quality of the models was verified by MD simulations via the wide X-ray scattering (WXRS) and glass transition temperature Tg, the simulated results were consistent with corresponding experimental data. Finally, mechanical properties and volumetric shrinkage of three different resin models were calculated and analyzed with molecular insight. Their results were also compared with available experimental data.
2. Experimental 2.1. Material synthesis
⇑ Corresponding author. Tel.: +86 0451 86418124. E-mail address: [email protected] (Y. Sun). 0927-0256/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.commatsci.2011.06.009
Bisphenol A glycerolate dimethacrylate (Bis-GMA) and Tri(ethylenglycol) dimethacrylate (TEGDMA) were purchased from Aldrich
X. Song et al. / Computational Materials Science 50 (2011) 3282–3289
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analysis (DSC) was performed on the DSC 2920 (TA-Instruments, USA) at a heating rate of 10 K/min, under nitrogen atmosphere. 3. Molecular simulation 3.1. Molecular models of the novel resins
Fig. 1. Structure of typical POSS with functional ligand R.
Chemical Co., Methacryl POSS was purchased from Hybrid Plastics (Fountain Valley, CA). All monomers were used as received without any further purification. The commonly used visible light photo-initiator camphorquinone (CQ, 97%) and co-initiator 2(Dimethylamino) ethyl methacrylate (DMAEMA, 98%) were selected for this research. Both CQ and DMAEMA were also purchased from Aldrich Chemical Co. A solution containing 49.5 weight percent (wt.%) of Bis-GMA, 49.5 wt.% of TEGDMA, 0.5 wt.% of CQ and 0.5 wt.% of DMAEMA were prepared by mixing sufficiently in a container which was kept away from light. Then multifunctional Methacryl-POSS (POSS-MA) was proportionately added into the solution of neat resins and magnetically blended uniformly. In this study the curing time of each sample was 40 s at room temperature. All specimens were prepared for each type material and immersed in distilled water at 37 °C for 24 h after taking off stainless steel molds, and a careful polish under water with 2400 grit silicon carbide paper longitudinal direction was followed. The final dimensions of the specimens were measured accurately and recorded just before the testing.
2.2. Material characterization The novel dental resins improved with three different contents of Methacryl POSS were synthesized. Fourier-transform infra-red spectroscopy (FTIR) was utilized to evaluate the degree of conversion. Wide-angle X-ray scattering (WAXS) was performed on a D/max-B rotational anode X-ray diffractometer (Japan), with Cu Ka irradiation (wavelength of 1.54 Å), using a nickel filter. Thermal
In this paper, the novel dental nano-composite resins with different loadings of POSS (0, 2 and 5 wt.%) were studied using all atom MD simulation. The matrices of the resins are Bis-GMA and TEGDMA, and the additive is POSS-MA (Fig. 2.). Each matrix molecular has two carbon–carbon double bonds at the terminals, and there are eight carbon–carbon double bonds at the terminals of POSS molecular especially. The dental composite resins are supposed to have three-dimensional network structures due to the polymerization between the carbon–carbon double bonds of the monomers. The essential reaction occurring in the polymerization is explained through the schematic diagram Fig. 3. During the curing, the original carbon–carbon double bond is activated into active functional group shown in Fig. 3 by the visible light. As a result, the double bond converts to a single band and two active sections A and B are introduced. Then, the chemical linking just takes place between them. Thus, three-dimension polymer network system emerges chemically. In this section, three network molecular system models shown in Table 1 were constructed dynamically using Material Studio software. In order to capture the details of the chemical reaction process and to form the network models dynamically, the following procedures were carried out (the procedures were illuminated by Model P02): (i) Three types of molecular models for original Bis-GMA, TEGDMA and POSS-MA were constructed, then energy minimization was performed respectively to obtain their reasonable structures. Needing explanation is that energy minimization was carried out using two different methods; first the steepest descends (convergence value of 1000 kcal/mol) and then the conjugate gradient (convergence value of 10 kcal/mol), relaxing the system to a local state of minimal potential energy. Subsequent energy minimizations were all based on this scheme. Afterward, the original carbon–carbon double bond was changed into activated functional group (shown in Fig. 3) by converting the double bond into the single one. As a result, two active sections A and B were introduced. Thus, the reactive molecular models for Bis-GMA, TEGDMA and POSS-MA were constructed. Before the three-dimension network models were created, all reactive molecular models were optimized into reasonable structures just using energy minimization.
Fig. 2. Snapshots of molecular models from our simulation used in the cross-linking polymer models: (a) Bis-GMA, (b) TEGDMA and (c) POSS-MA.
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Fig. 3. The essential reaction occurring in the polymerization (The red lines present new covalent links.). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Characteristics of three different cross-linking models in the simulation and experiment. Resin models
Number of molecular
P00 P02 P05
Density (g/cm3)
POSS loadings
POSS
BisGMA
TEGDMA
Simulation (wt.%)
Experiment (wt.%)
Simulation
Experiment
0 1 2
60 64 50
120 128 100
0.00 2.02 5.02
0 2 5
1.15 1.15 1.16
1.21 1.19 1.14
(ii) Secondly, 64 active molecular models of Bis-GMA, 100 active molecular models of TEGDMA and one active POSS-MA molecular model were packed into one cubic lattice with a density of 1.2121 g/cm3 (obtained from the experimental profiles) using amorphous cell module in Material Studio software. To eliminate the surface effect, three-dimension periodic boundary condition was imposed. Then, the initial physical mixture of the novel dental nano-composite resin model was constructed. (iii) Thirdly, 50,000 steps of energy minimization were carried out to relax the unit cell system. Then, the relaxed physical mixture system was evaluated to identify the activated sections in close distance. One activated section’s carbon of A or B was chosen first, afterward, another search of nearby activated section’s carbon was performed. Only the proximal two activated section’s carbons in the reaction cut-off distance range could react, this means that the covalent bond between them will construct. Especially, the cutoff distance from 3 to 10 Å was chosen in this paper because this selection could avoid the reaction of two nearby carbons just in the same activated functional groups. In addition, the larger one of cut-off distance was optional according to the situation. During the reaction, the formation of four-member and fivemember ring should be avoided. Although the real cross-link systems were simulated as far as possible, the physical defects such as chain entanglements were allowable. (iv) Finally, the modified molecular model system was checked again to evaluate the double bonds conversion during the polymerization. If the double bonds conversion was smaller than the experimental result from FTIR, the larger one of the
cut-off distance was enlarged and step iii was performed again. On the other hand, if the conversion was larger than the experimental result from FTIR, the larger one of the cutoff distance was reduced and step iii was performed again. Until the double bonds conversion dropped in the permissible range, the procedure was over and all non-hydrogen atoms in the system were saturated with hydrogen atoms. The final physical and chemical mixture model systems were chosen for further study. All steps were coded as a Perl script and run in the Material Studio software from Accelrys. Eventually, three network molecular system models with different loadings of POSS (0, 2.02 and 5.02 wt.%) were constructed shown in Tables 1 and 2. It is obvious that the loadings of POSS of the network polymers in the simulations are approximate to the experimental ones and the densities of various network polymers also coincide with the experimental results in a certain error range. 3.2. Molecular simulation protocol details All the simulations were performed based on the COMPASS (condensed phase optimized molecular potentials of atomic simulation studies) force field. The COMPASS force field is the first ab initio force field which has been parameterized and validated using condensed-phase properties, it has been used effectively on lots of nano-composites with POSS [18,19,31,32]. In the COMPASS force field, the total energy expression has two main terms of bond interactions and non-bonded interactions. The latter interactions (Van der Waals and electrostatic energy) are very important terms
Table 2 Characteristics about the polymerization of three different cross-linking models. Resin models
P00 P02 P05
Double bond numbers Original total number
Final cured number
360 392 316
210 227 185
Cut off distance (Å)
Simulation conversion (%)
3–5 3–5.27 3–5.2
58.33 57.91 58.545
Note: The polymerization conversion was evaluated by the conversion of the double bonds, i.e., conversion = (cured double bond number)/ (original total double bond number).
X. Song et al. / Computational Materials Science 50 (2011) 3282–3289
in the total energy expression. Since the energy terms depend on the number of atoms in the system, the interaction between too far separated atoms should be neglected. The cut-off in the whole study was selected as 9.5 Å, and atom-based method was selected for Van der Waals energy. However, this method may lead the energy discontinuity for the electrostatic interactions, especially for the system with large electric charge. Thus, Ewald method recognized as the best one with quite high accuracy was chosen for the electrostatic energy in the paper [33]. Periodic boundary conditions were used to simulate the materials. Nosé algorithm is more stable and could keep the regularity of the system. Thus, it was chosen to control the temperature [19,34,35]. Parrinello algorithm enables the change of both symmetry and size of the simulation cell [36,37], hence it was chosen to control the pressure. The velocity form of the Verlet integrator was used to solve the equations of motion with a time step of 0.5 fs. Before MD simulation was carried out, 50,000 steps of energy minimization were carried out to relax the unit cell system. Subsequently, for the selected network system models, MD simulation was performed in constant particle numbers, pressure and temperature (NPT) conditions. The snapshots of the network system models for P00 and P05 were shown in Fig. 4. 4. Results and discussion 4.1. Wide X-ray scattering profiles The wide X-ray scattering (WXRS) profiles of the selected configurations fully balanced were calculated. Fig. 5 shows the comparison between the simulation results and the experimental ones. The simulation results are in good agreement with the experimental results, in terms of peak position and intensity. It could arrive at the conclusion that the network system polymer models created in the study, the force field, and the molecular dynamics protocol are valid. In addition, the network polymers with different loadings of POSS appear the similar trend on the WXRS for the simulation and experiment and just the intensity of WXRS differs slightly. These phenomena indicate that the structures of the novel network polymers with different loading of POSS are similar with each other. For all polymer resins, WXRS patterns show a broad amorphous halo at 2h = 18.50° corresponding to the effective Bragg distance of 4.43 Å. This phenomenon verifies that the network polymer resins are amorphous. There is also a smaller amorphous halo at 2h = 41.18° in the experimental results, while the simulated profiles are not apparent at the same position.
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4.2. Glass transition temperature In this study, the volume versus temperature was determined to predicate Tg with the method of NPT MD simulations. The temperature at which the slope changes on the volume–temperature diagram represents Tg of the polymers [18,19,31,38,39]. To ensure the models fully equilibrated, MD simulations were performed on the network systems, where the temperature was first cooled down from 600 K to 50 K, then heated up from 50 K to 600 K, finally cooled down from 600 K to 50 K with the same intervals of 50 K. Only the last cooling process was chosen to evaluate Tg of the network polymer models. Moreover, MD simulation at each isothermal temperature was performed more than 250 ps, and the average volume of more than 10 different configurations was used to determine the volume–temperature diagrams. The volume– temperature diagrams of models with different loading of POSS are shown in Fig. 6. In each case, the break of slope occurs apparently. This indicates that the size of the network polymer models is large enough to evaluate the glass transition behavior [18]. The simulated glass transition temperatures obtained from the volume–temperature diagrams are compared with the available experimental results from DSC (Fig. 7). The simulated results are in agreement with the experimental results and both results are in 50–60 °C. These results further verify the molecular simulation protocol and the method used to calculate Tg from simulations. In addition, the results also show that the incorporation of POSS has a little effect on the glass transition temperature. It is easy to understand such results for the improved dental resins and the original resin have the similar network structures. The simulated results are a little higher than the experimental results, and the similar phenomena have been reported in many literatures [18,19,40]. In general, the glass transition of polymers occurs in a range of temperatures. The cooling rates have an effect on the measure of glass transition temperatures [40], the number of the repeat units in the simulations could also shift the glass transition temperature [41,42]. In the MD simulations, the cooling rate (1011 K/s) was much higher than the experimental one (10 K/ min). All above factors make the simulated results different from the experimental ones in some degree. Thus, the results in this section are reasonable. 4.3. Mechanical properties In general, mechanical properties are mainly due to the behavior exhibited by polymeric systems under different testing modes of
Fig. 4. Snapshots from our simulation of (a) network polymer model P00 and (b) polymer model P05. Note: in model P05, POSS was colored orange with shape of ellipsoid.
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Fig. 5. Simulated and experimental WAXS results: (a) P00, (b) P02 and (c) P05. Fig. 6. Specific volume and temperature curves for: (a) P00, (b) P02 and (c) P05 at 1 atm as determined from NPT dynamics. The arrows locate the position of Tg as determined from MD simulations.
elastics properties. MD simulations have been successfully used to predict elastic properties of polymers, mainly non-network polymers but also some network polymers systems [43–48]. At this section, the static method was performed to predict the properties of the novel composites resin models. The static method also named constant-strain minimization method was first proposed by Theodorou and Suter at 1986. In this method, after an initial energy minimization, the microtension and compression tests are performed with a very small elastic strain (±0.001) on the system cells. Then, another energy minimization is carried out and the stiffness
matrix is derived from the second derivative of potential energy U with respect to the strain e as follows: 2
C ij ¼
d U dr ei dej ¼ i d dej
ð1Þ
For the amorphous isotropic material, the stiffness matrix is decided by two Lame constants (k, l) shown as follows:
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Table 3 Mechanical properties obtained from the calculation for effective isotropic elastic constants of three models. Mechanical properties
Young’s modulus (GPa)
Shear modulus (GPa)
Bulk modulus (GPa)
Poisson’s ratio
P00 P02 P05
3.269 3.591 4.263
1.215 1.316 1.632
3.529 4.425 3.671
0.3456 0.3647 0.3065
Obviously, the matrix is symmetrical and moreover the nondiagonal components of the matrix are zero. Thus, once the stiffness matrix is obtained from Eq. (1), two Lame constants (k, l) would be derived from Eq. (2). As a result, the elastic properties constants could be obtained from them as follows:
Fig. 7. Simulated and experimental glass transition temperature with the loading of POSS.
2
C 66
0 B B B B B B B B @
6 6 6 6 ¼6 6 6 6 4
k þ 2l
k
k
0
3
0
0
07 7 7 07 7 07 7 7 05
l
k
k þ 2l
k
0
0
k
k
k þ 2l
0
0
0
0
0
0
0
0
l 0 0 l
0
0
0
0
0
8 l E ¼ lð3kþ2 Þ > > kþl > > < G¼l > K ¼ k þ 23 l > > > : m¼ k
ð3Þ
2ðkþlÞ
ð2Þ
Here E, G, K and m represent Young’s modulus, shear modulus, bulk modulus and Poisson’s ratio respectively. In this paper, the elastic entire stiffness matrices of three novel composites resins were obtained using constant-strain minimization method based on COMPASS force field at 300 K. Then, the average mechanical properties were calculated from them respectively. The final results of entire stiffness matrix of Model P05 are shown
1
6:112ð0:056Þ
2:515ð0:124Þ
2:608ð0:010Þ
0:140ð0:032Þ
0:531ð0:329Þ
0:216ð0:117Þ
2:313ð0:230Þ
5:43ð0:009Þ
2:725ð0:060Þ
0:300ð0:249Þ
0:120ð0:200Þ
0:220ð0:223Þ C C C 0:221ð0:159Þ C C 0:1363ð0:034Þ C C C 0:094ð0:018Þ A
2:389ð0:281Þ
2:951ð0:122Þ
5:885ð0:165Þ
0:164ð0:178Þ
0:395ð0:330Þ
0:301ð0:089Þ
0:003ð0:095Þ
0:0718ð0:022Þ
1:686ð0:034Þ
0:243ð0:055Þ
0:136ð0:344Þ
0:432ð0:349Þ
0:614ð0:349Þ
0:088ð0:077Þ
1:430ð0:063Þ
0:090ð0:053Þ
0:083ð0:019Þ
0:193ð0:037Þ 0:048ð0:074Þ 0:252ð0:147Þ
Fig. 8. Simulated and experimental elastic modules results with the loading of POSS.
ð4Þ
1:441ð0:077Þ
in Eq. (4) and the average mechanical properties are shown in Table 3. It is obvious that the entire stiffness matrix of Model P05 based on COMPASS force field satisfies the main features of isotropic amorphous material essentially. Although the non-diagonal components of stiffness matrix are not zero strictly, the diagonal components are much higher than the non-diagonal ones and the entire matrixes are almost symmetric along the diagonal components. This phenomenon indicates that there is residual stress in the molecular systems. This is not surprising for the temperature tested here is below the glass transition temperature of the systems. Thus, the results are reasonable. From Table 3, we could see that with the incorporation of POSS, Young’s modulus, shear modulus and Bulk modulus of composites resins are enhanced dramatically. Moreover, the higher of POSS contents, the more improvement of Young’s modulus and shear modulus. By incorporating with 0.2 wt.% of POSS, Young’s modulus and shear modulus are improved 9.85% and 8.31% respectively. By incorporating with 05 wt.% POSS, Young’s modulus and shear modulus are improved 30.4% and 34.3% respectively. Especially, the novel dental composite with 02 wt.% of POSS behaves the maximum Bulk modulus. However, Poisson’s ratio of composites resins is influenced slightly by the incorporation of POSS. Actually,
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Fig. 9. Simulated and experimental volume shrinkage with the loading of POSS.
Fig. 10. The schematic of the influence of POSS on the volume of novel composites resins.
the novel dental composite with 0.5 wt.% of POSS behaves smaller Poisson’s ratio than the initial dental composite without POSS. Besides, previous experimental research were done on the composites resins with additional 60 wt.% finely milled silanized barium oxide glass powder (BG) [49]. The paper compared the Young’s modulus obtained from the simulation with the compression elastic modules obtained from experimental compression tests (Fig. 8). It is obvious that the trend of elastic modules results with the loading of POSS is consistent while the experimental results are systematically higher than the simulation results. Since BG could improve the elastic modules of composites resins, above results are reasonable and also validate the methodology used to calculate the mechanical properties of network models. 4.4. Volumetric shrinkage During the photo-polymerization of dental composite materials, Van der Waals interactions acting on molecular monomers turn into covalent interactions. The distances among monomers are decreased distinctly. Thus, the dental composite materials exhibit volume shrinkage during the photo-polymerization. It is a very important parameter, which may cause micro-leakage and other negative effects on teeth. Usually, volume shrinkage is calculated using the following formula [50]:
DV ¼ ð1
quncured Þ qcured
ð5Þ
In this paper, we calculated the densities of uncured and cured network polymer models fully equilibrated. Then, the volume shrinkage was evaluated and the results shown in Fig. 9 indicate
that the incorporation of POSS could reduce the volume shrinkage distinctly. Moreover, with the increase of POSS loading, the composites exhibit lower volume shrinkage. The volume shrinkage of the composite incorporated with 0.2 wt% POSS is reduced 14.3% than initial resin composites. The volume shrinkage of the composite incorporated with 0.5 wt.% POSS is reduced 15.9% than initial one. Besides, previous experimental research investigated the volume shrinkage of the composite resins with additional 60 wt.% of BG powder [49]. The paper compared the simulation results with the experimental ones (Fig. 9). Although, their numerical values are different, both results behave the similar trend. Besides, some literatures have reported that typical resins of dental methacrylate based monomers undergo volumetric shrinkage between 6% and 10% and the highly filled composites resins still exhibited shrinkage of 2.6–7.1% [17,51]. Thus, above results are reasonable and further validate the methodology to predict the volume shrinkage. In addition, from Fig. 2 we find that POSS molecular with a three-dimension compact hybrid structure is more rigid and has much larger volume than BisGMA and TEGDMA. Moreover, in two-dimension, BisGMA and TEGDMA could be simplified to linear bars and POSS could be simplified to a diamond with four short bars. Fig. 10 shows the influence of POSS on the packing of novel composites resins. It is easy to understand that the original composite resin could be packed more close-grained because BisGMA and TEGDMA have similar linear bar structures. However, the packing of improved composite resin is influenced by the incorporation of POSS distinctly. As a result, the volume shrinkage of composite resin is reduced by POSS. 5. Conclusions In this study, an improved method is developed to construct cross-linking polymer models of a novel dental nano-composite resin. Atomistic MD simulation is performed to investigate the effects of introducing poly-functional POSS as embedded parts on this dental resin material. The features of densities and double bond conversion indicate the rationality of the network polymer models. The excellent agreement both of WXRS profiles and volume–temperature between the simulation and the experiment verifies the accuracy of the models and the choice of the force field. With the incorporation of POSS, Young’s modulus, shear modulus and Bulk modulus of composites resins are enhanced dramatically. The incorporation of POSS could reduce volume shrinkage
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distinctly. The results are also compared with available experimental data. Further analysis is used to interpret the working mechanisms of POSS on the composite resins. A simple model clarifies the results in molecular insight about the effect of POSS on the packing of dental composite resin. The developed method could be employed in further research of new dental materials with network structures. Acknowledgements This work was supported by the National Natural Science Foundation of China (10472028), the Doctoral Program Foundation of Ministry of Education of China (20070213054) and the Excellent Youth Foundation of Heilongjiang Province. References [1] Naida Lacevic, Richard H. Gee, Andrew Saab, Robert Maxwell, J. Chem. Phys. 129 (2008) 124903. [2] A. Provatas, J.G. Matisons, Trends Polym. Sci. 5 (1997) 327. [3] G. Li, L. Wang, H. Ni, C.U. Pittman, J. Inorg. Organomet. Polym. 11 (2001) 123. [4] Tudor C. Ionescu, Feng Qi, Clare McCabe, Alberto Striolo, John Kieffer, Peter T. Cummings, J. Phys. Chem. B 110 (2006) 2502. [5] Eduardo C. Bianchi, Paulo R. de Aguiar, Manoel C.S. Alves, César A. de Freitas, Ana R. Rodrigues, Oscar B. de Carvalho, Polímeros – Ciência e Tecnologia. 17 (2007) 130. [6] Yong-Keun Lee, Huan Lu, Makoto Oguri, John M. Powers, J. Biomed. Mater. Res. Part B: Appl. Biomat. 82B (2007) 313. [7] Y. Tanimoto, K. Nemoto, Compos. Interface. 11 (2004) 15. [8] Yujie Zhang, Daohong Zhang, Chuangye Qin, Jingwei Xu, Polym. Compos. 30 (2009) 176. [9] S.G. Pereira, R. Osorio, M. Toledano, et al., Dent. Mater. 23 (2007) 1030. [10] H.K. Xu Hockin, J.L. Moreau, Sun Limin, L.C. Chow, Biomaterials 29 (2008) 4261. [11] A. Shahdad Shakeel, F. McCabe John, Bull Steven, Rusby Sandra, W. Wassell Robert, Dent. Mater. 23 (2007) 1079. [12] I. Yudovin-Farber, N. Beyth, A. Nyska, E.I. Weiss, J. Golenser, A.J. Domb, Biomaromolecules 9 (2008) 3044. [13] K. Moharamzadeh, R. Van Noort, I.M. Brook, et al., J. Mater. Sci.: Mater. Med. 18 (2007) 133. [14] J.D. Eick, R.E. Smith, C.S. Pinzino, et al., J. Dent. 34 (2006) 405.
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