Structure of Organic− Inorganic Nanohybrids Incorporating Titanium

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J. Phys. Chem. B 2008, 112, 16478–16484

Structure of Organic-Inorganic Nanohybrids Incorporating Titanium(IV) Oxoalkoxyacylate Nanoclusters: A SANS Study Inna Karatchevtseva,*,† Andre Heinemann,† Veronica Hartley,‡ and Robert Knott† ANSTO, PriVate Mail Bag, Menai NSW 2234, Australia, and School of Chemical, Physical and Earth Sciences, Flinders UniVersity, Bedford Park SA 5042, Australia ReceiVed: August 11, 2008; ReVised Manuscript ReceiVed: October 22, 2008

The small angle neutron scattering (SANS) technique was used to investigate the structure of nanohybrids consisting of a poly(methylmethacrylate) (PMMA) and one of two types of titanium(IV) nanoclusters. Cluster 1, [Ti6O4](OC2H5)8(CH2dCCH3COO)8, with polymerizable MMA ligands, formed covalent bonds with the polymer chains during the copolymerization, whereas cluster 2, [Ti6O4](OC2H5)8(CH3COO)8, had no polymerizable linkers and was blended into the polymeric matrix purely as a filler. In this study, SANS with contrast variation was used to investigate the size, shape and aggregation of the clusters in the hybrid materials, and their effect on the structure of the matrix. A polydispersed core-diffusion zone model was employed to explain the scattering contribution from the titanium clusters in both nanohybrid materials. No significant differences between the structures of the two nanohybrids were found. The fitted models suggest that the interface region between the cluster and matrix (the diffusion zone) is heavily occupied by the PMMA chains; however, they do not penetrate into the core region (titanium cluster). 1. Introduction The modification of polymer properties by the incorporation of inorganic particles or clusters is an important research area and has provided enormous inspiration in the field of organicinorganic hybrids. The availability of an extensive range of novel molecular precursors and associated synthetic methods for tailoring the properties of such materials has created considerable technological interest, with potential applications in many areas, including integrated and nonlinear optics, sensors, and surface engineering (e.g., control of hydrophobicity/hydrophilicity, passivation, functionalization, etc.). Organic-inorganic nanohybrids can now be engineered with a high degree of complexity by combining knowledge of sol-gel and organic chemistry. Several excellent reviews have been published describing the concepts for incorporation of inorganic blocks into organic polymers on a nanoscale.1-3 In the past decade, nanohybrid composites where organic and inorganic components are linked through strong chemical bonds (covalent or iono-covalent), have received major attention. It is commonly found that organic-inorganic hybrid materials prepared via strong covalent bonding typically exhibit properties far superior to those of the noncovalently bonded hybrid materials. Strong bond interaction between the organic and inorganic phases is also believed to play a primary role in the formation of more homogeneous hybrid materials where inorganic domains are more likely to be on a nanoscale size. At the same time, it is usually anticipated that physical mixtures of organic polymers and inorganic components can lead to phase separation and/or particle aggregation resulting in poor mechanical, optical properties, etc. Furthermore, unmodified inorganic particles tend to aggregate in the polymer matrix independent of the type of material and the size.3 * Corresponding author. Tel: + 61 2 97179099. Fax: +61 2 95437179. E-mail: [email protected]. † ANSTO. ‡ Flinders University.

Therefore, the modification of inorganic particles by the attachment of polymerizable (organic) functionalities is often considered to avoid these problems. Organic fragments are often used to control the structural evolution of nanohybrids, leading to control of the size and spatial orientation of organic and inorganic domains within the nanohybrids. Recently, organic-inorganic nanohybrid materials have been synthesized where well-defined nanosized oxo-metalate clusters effectively cross-link the matrix, resulting in interesting optical, magnetic and catalytic properties.4-10 These nanohybrids include silicon-based cubic polyhedral oligomeric silsesquioxane (POSS) clusters,10,11 tin-based stannate clusters,12 and heteropolyanion clusters of tungsten and silicon.13,14 Stannate clusters have been copolymerized with methacrylate at very low cross-linking density,12 and in the past decade, Schubert and co-workers 15-20 prepared a series of acrylate- and methacrylate-substituted oxotitanate, oxozirconate and mixed oxo(titanate-zirconate) clusters. It has been reported that the cluster dopants acted as reinforcing agents and nanosized cross-linkers could create nanoconfined hybrid materials with the reinforced organic polymer matrix. When the transition metal cluster was covalently bonded to the matrix polymer, the cross-link density of the matrix polymer could be varied to optimize the properties of the nanohybrids. A range of complementary surface-analytical and thermomechanical techniques, including photoacoustic Fourier-transform infrared spectroscopy (PA-FTIR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), modulated thermogravimetric analysis (MTGA), differential scanning calorimetry (DSC), micro thermal analysis (µ-TA), dynamic mechanical analysis (DMA) and transmission electron microscopy (TEM) have been was used by numerous research groups to investigate cluster-polymer nanohybrids properties.1,3,5,8,10,21 Thus, PMMA-titanium cluster hybrid materials were synthesized by polymerizing methyl methacrylate (MMA) with methacrylate-substituted oxoethoxytitanate cluster [Ti6O4(OEt)8 (OMc)8].7,8,21 A two-stage procedure was employed in which

10.1021/jp807174d CCC: $40.75  2008 American Chemical Society Published on Web 12/03/2008

Organic-Inorganic Nanohybrids with Ti(IV) Nanoclusters (a) the cluster was first prepared via the sol-gel reaction of titanium(IV) ethoxide with methacrylic acid (1:2 mol ratio), and (b) the isolated surface-modified clusters were copolymerized with MMA. This research emphasized the importance of covalent linkage between the cluster and polymer matrix in modulating the properties of these materials for technological applications. For example, it has been reported that even a small amount of covalently linked [(Ti6O4)(OC2H5)8(CH2d CCH3COO)8] cluster (0.3 mol %) could increase the refractive index of PMMA from 1.49 to over 1.70 for the same film thickness. This makes it possible to reduce the thickness of optical materials while still retaining comparable optical properties. The tensile strength of this material was also increased by 8%, demonstrating that the hard brittle PMMA matrix can be modified with the cluster to improve its mechanical properties.21 To complement previous research, in this study, the small angle neutron scattering (SANS) technique was used to investigate the structure of nanohybrid materials consisting of a PMMA matrix incorporating one of two types of titanium nanoclusters. SANS with contrast variation, was expected to provide molecular structure and, particularly, explore the important interface region between the organic and inorganic components. Contrast variation involves changing the scattering length density (SLD) of one of the nanohybrid components and observing the change in the SANS profile. For the PMMA-titanium hybrids this is achieved by exchanging the hydrogen for deuterium on the PMMA chain. Cluster 1, [Ti6O4](OC2H5)8(CH2dCCH3COO)8, prepared in this work, contains polymerizable MMA ligands covalently linked to the titanium periphery. During the polymerization of MMA, cluster 1 was covalently bonded to the polymer chains generating a cross-linked network. This approach also enables the cross-link density of the matrix to be modulated, thus assisting with the tailoring of the mechanical properties. As a control, clusters 2, [Ti6O4](OC2H5)8(CH3COO)8, with no polymerizable linkers was also blended into the polymeric matrix acting purely as a filler. An important question here could be how the polymer matrix accommodates the covalently bonded titanium cluster. Will the interface region between the cluster and matrix be occupied only by the polymerizable groups, or will the matrix penetrate the interface region and contact the titanium cluster surface? The synthesis and crystal structures of the titanium clusters of interest with different ligands has been previously studied in detail.16,22,23 The structure of both cluster 1 and cluster 2 can be described by two Ti2O10 (two edge-sharing octahedra) units linked by two corner-sharing octahedra. There are four oxygen atoms in bridging positions between Ti2O10 and the edge-sharing octahedra: two are triply bridging oxygen atoms and two are doubly bridging oxygen atoms. In the structure, all the methacrylate (or acetate) groups bridge two titanium atoms. Additionally, two types of ethoxy groups could be identified: six terminal groups and two bridging groups. In all cases, the 6-fold coordination of all Ti(IV) sites in the clusters provides enhanced hydrolytic stability when compared to the parent alkoxide (Ti(OEt)4). This stability is important when attempts are made to incorporate the inorganic clusters into the organic polymer matrix. In cluster 1, all of eight polymerizable methacrylate groups are available for copolymerization with the MMA monomer and are expected to cross-link the polymer chains upon radical polymerization. 2. Experimental Section 2.1. Materials and Methods. All reagents were analytical grade, and obtained from Sigma-Aldrich, unless otherwise

J. Phys. Chem. B, Vol. 112, No. 51, 2008 16479 stated. Prior to use, solvents were dried and stored in a glovebox over molecular sieves. Perdeuterated MMA (d-MMA) monomer was purchased from PolymerSource (Quebec, Canada) and used as received. The PMMA-titanium cluster nanohybrids were synthesized by copolymerization of MMA monomer in the presence of 0.3 mol % of appropriate titanium cluster using a two step procedure. In step (1) the clusters were first prepared via the sol-gel reaction of titanium(IV) ethoxide, and then step (2) the isolated surface-modified clusters were copolymerized with MMA monomer. The synthesis of titanium clusters was performed in a nitrogen-purged glovebox with the rigorous exclusion of water (H2O < 1 ppm). Cluster 1, [Ti6O4](OEt)8(OMc)8, where OEt indicates OC2H5 and OMc-OOC-C(CH3)dCH2, and cluster 2, [Ti6O4](OEt)8(OAc)8, where OAc indicates OOCCH3, were prepared as previously described.16,22,24 After thorough mixing, the resulting solutions were left to stand at ambient temperature in the glovebox, and after several weeks lead to the formation of colorless, needle-like crystals. To produce PMMA-titanium cluster nanohybrid materials, the appropriate weights of h-MMA, d-MMA and cluster were added to a 20 mm diameter glass tube and mixed using a magnetic stirrer in a temperature controlled bath. All polymerization reactions were thermally initiated with dibenzoylperoxide at 60, 61.8, 63.4, 65, or 67 °C for the corresponding d-MMA content of 0, 25, 50, 75, or 100 wt %. It has been noted in a number of studies that protonated and deuterated monomers can have a significant effect on the reaction kinetics of the synthesis of deuterated polymers.25 The synthesis conditions were modified accordingly for differing h-MMA/d-MMA ratios. Three series of samples were prepared for the SANS experiments: • Series A, h-MMA and d-MMA were copolymerized with h-MMA content of 0, 25, 50, 75 or 100 wt % (which corresponds to 0, 26.5, 51.9, 76.4, and 100% by volume), and samples noted as S1-S5, respectively. With no cluster present, this enabled the scattering from the linear polymer matrix to be examined. • Series B were prepared by copolymerizing cluster 1 (0.3 mol % or approximately 3.3 vol %) with the mixture of h-MMA and d-MMA monomers with the same h-MMA/d-MMA ratios as in series A. Samples are referred as S6-S10, respectively. • Series C were prepared by polymerizing h-MMA and d-MMA monomers mixture with the same ratios as in series A and B in the presence of cluster 2 (0.3 mol % or approximately 2.8 vol %). Samples are referred as S11-S15, respectively. Data were collected on the V4 30m SANS instrument at BENSC (HMI, Berlin).26 Monochromatic neutrons are selected by a mechanical velocity selector with variable mean wavelength, λ, in the range 0.38-3 nm. For all experiments in this study a λ of 0.6 nm was used. The two-dimensional 3He-detector with 64 × 64 pixels of 10 × 10 mm2 can be positioned at any distance between 1 and 16 m from the sample to cover the total Q-range 0.032 < Q < 3.8 nm-1 (where Q is the scattering vector equal to 4π sin θ/λ with 2θ the scattering angle). For these experiments, three instrument configurations were used which provided adequate overlap in Q for subsequent merging of the data. The sample to detector distance was set at 16, 4, and 1 m with corresponding collimation lengths of 16, 8, and 8 m. For the SANS experiments, samples were cut into circular wafers (∼20 mm diameter) with thickness 1.0 ( 0.1 mm. The wafer was attached to a cadmium aperture (10 mm diameter

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aperture) for mounting on the computer-controlled sample changer. The data collection time for each sample was adjusted to give adequate counting statistics in the high Q region for three instrument configurations. “BerSANS” data reduction software26 was used to correct the sample data for background effects and then radially average the isotropic data in Q to provide a SANS profile for each instrument setting (scattered intensity I as a function of Q). The three SANS profiles were then merged to provide a single SANS profile for each sample. 2.2. Data Analysis. Five SANS profiles for samples in series B were simultaneously fitted using a procedure developed to extract physical parameters from multiple SANS data sets.27 In a similar manner, five SANS profiles for samples in series C were simultaneously fitted. A polydispersed core-diffusion zone model28 (Figure 1a) was used as a model for the scattering contribution from the titanium clusters together with the dilute scatterer approach. Because the 0% cluster samples (series A) showed considerable I(Q) at low Q, which was essentially independent of h-MMA/d-MMA, these scattering contributions had to be included in the simulation. As shown in Figures 2, 3 and 5, this contribution generated a much higher total scattering intensity than the titanium-cluster contribution. If not included properly, the nonsystematic and random differences from this contribution could overlap all possible parameters to sample property correlations in the experimental data. A proper model in this sense is the one that is complex enough to cover the measured data properties but does not induce correlations between the different model parameters. To our knowledge, no operational approach is known to ensure this generally. An obvious solution is to use a model where all new parameters are related to real physical properties of the sample. Even if the absolute values of these parameters are not of direct interest, this solution has two major advantages. First, in a nonlinear fit it is much easier to identify unphysical fitting results and, second, a natural way of reducing the number of free parameters is provided by establishing all known connections between these physical properties. The excess I(Q) at low Q is considered to be due to porosity and was modeled with the random polyhedron approach: 29,30

dΣ/dΩ ) 64πca(1 - ca)(η1 - η2)231+2γb3(9 + 4b2Q2)-2-λ (1) where dΣ/dΩ is the differential scattering cross-section; η1 is the SLD of the matrix and η2 is the SLD of the pores; and ca is the pore volume fraction. In eq 1 the mean cord length, b, is connected with the mean pore volume V ) 16b3/9π. Assuming the surfaces of these polyhedra are not smooth on the length scale under investigation using SANS, then a fractal parameter γ can be introduced. The variation of γ from zero characterizes the transition from a smooth polyhedron surface to a rougher one. These pore properties are assumed to vary in a nonsystematic way, which would lead to many new independent fitting parameters for every scattering curve. Therefore, any advantage from a contrast variation sequence and simultaneously fit of these data would possibly be destroyed. Taking the information limit into account,31 a reliable parameter estimation for the titanium-cluster model is impossible. This can be demonstrated by calculating the asymptotic-correlationmatrix, which in this case showed considerable artificial correlations between parameters from the spherical particle with diffusion zone and a random polyhedron model. One way of reducing the total number of free parameters, and therefore

Figure 1. Schematic illustration of the polydispersed core-diffusion zone model used in the simultaneous fit to the experimental SANS data (a); visualization of the fitted cluster volume and atom coordinates24 forming a dense titanium cluster core with a SLD depending on the fitted volume (b). The ligand molecules forming the surrounding diffusion zone in the 2-D density plot are not shown.

reducing the ambiguities, is the use of a priori information for establishing functional and nonfunctional connections between these parameters. Because the fitted results are obtained under these constraints, they heavily reflect their physical implication. This volitional implication could turn into a drawback if there is no physical meaning of the applied constraints. Therefore; the model in Figure 1a was used instead of a dΣ/dΩ ∼ Q-(6-d) type function.27 Using eq 1, a simultaneous fit was carried out implementing a complete set of parameter connections and constraints. As mentioned, all results are critically dependent on these factors; therefore, a detailed list is presented: (a) All matrix dependent SLD’s and incoherent scattering contributions are linear functions of the different h-MMA/dMMA ratios. The SLD’s and incoherent scattering of pure

Organic-Inorganic Nanohybrids with Ti(IV) Nanoclusters

Figure 2. Experimental SANS data for series A (cluster-free PMMA matrix) samples with (a) 0, (b) 25, (c) 50, (d) 75, and (e) 100 wt % h-MMA content. Different theoretical h-MMA/d-MMA ratios were prepared for a contrast variation sequence. The inset shows schematically a random polyhedron model used to simulate the pore scattering.

J. Phys. Chem. B, Vol. 112, No. 51, 2008 16481 (d) The pores are expected to have different mean volumes and surface properties, within a physical expedient range (nm3 < V < µm3) and (-2 < γ < 2). (e) The mean radius, R, and standard derivation, σR, of the titanium cluster core are constant within one sample series (series B or series C), however, could vary for samples from different series. (f) The characteristic length of the diffusion zone, L, defined as the distance where the diffusion zone SLD at the corediffusion zone border changes its value to 1/e of |(ηm - η2)|, is constant for all samples within series B or series C but could differ between series B and C samples. (g) The total volume of the titanium cluster is assumed to be sufficiently small that its SLD contribution does not change the matrix SLD. This approximation was found to be more than properly fulfilled by the 100% h-MMA sample from series C where no evidence of titanium clusters was found. For all other samples the volume fractions obtained from the fits were indeed very low. (h) The SLD of the titanium cluster core is theoretically determined by the mean scattering length of all atoms bi within the cluster averaged over the mean cluster volume V ) 4π(R4 + 3R2σR2 + 2σR4)/(3R) by calculating Σbi/V in every fitting step. Figure 1b is a visualization of the fitted cluster volume together with the cluster coordinates.24 The SLD-density plot shows the 2-D representation of the diffusion-zone surrounding the titanium cluster core. (i) The SLD profile of the cluster neighborhood was modeled with a diffusion zone profile (Figure 1a). This profile depends only on three parameters ηm, η2, and L (see dot-point f). The core-diffusion zone border SLD η2 dependency from different d-MMA/h-MMA ratios was obtained during the fits. A constant η2 would for example mean no penetration of MMA into the diffusion-zone. All these constraints, applied as functional and nonfunctional dependencies, were used during the simultaneous fits of the data for series B and series C samples yielding parameter values presented in Table 1. Additionally, for parameters with functional constraints, confidence intervals with a p-value of 0.95 were calculated. Finally, the asymptotic-correlation-matrix was computed. It showed no artificial correlations between these parameters thus the values do not depend on each other in a trivial way. All these calculations are necessary to establish the reliability of the results. Figures 3 and 5 show good agreement of the fitted curves with the experimental data, with a maximum sum of the squared residuals of χ2 ∼ 0.03 per curve indicating that excellent data-fit agreement was achieved. 3. Results and Discussion

Figure 3. Experimental SANS data (∆) and fit results (- - -) for series B samples prepared with different theoretical h-MMA/d-MMA ratios: (a) 0, (b) 25, (c) 50, and (d) 75 wt % of h-MMA content. The inset shows the SLD profiles obtained from the fits and the scattering from the 100 wt % (e) h-MMA sample.

d-MMA and h-MMA were used to fix the slope of these functions. (b) The mean pore SLD is assumed to be the same for all samples. (c) The total volume of the pore phase, with SLD ) η1 in eq 1, determined by the mean volume V ) 16b3/9π and the volume fraction ca, changes the average matrix SLD for the titanium clusters and for the pores itself. This implies the necessity of a self-consistent fitting procedure.

Samples in series A (S1-S5) are pure PMMA matrix with a h-MMA content of 0, 25, 50, 75, or 100 wt %, respectively. In the absence of a titanium cluster these samples were used to examine the scattering from the linear polymer matrix. Figure 2 represents the reduced and radially integrated data for this series of samples. It can be seen that the PMMA matrix shows an excess of scattering intensity I(Q), at low Q, which is essentially independent of h-MMA/d-MMA ratio. This was attributed to sample porosity rather than phase separation effects. It is understood that the porosity in samples could have resulted from inadequate mixing technique during the bulk polymerization of MMA when the viscosity of the reaction medium increased dramatically. Consistency in samples preparation is, therefore, important for the SANS experiments. However, the challenge here was to synthesize

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Figure 4. Fitted values and estimated errors for SLD of matrix and diffusion zone for sample series B (a) and (b) and sample series C (c) and (d). The nearly identical slopes in (a) and (b), and (c) and (d), respectively, confirm the penetration of PMMA into the diffusion zone.

TABLE 2: Fitted Parameter Values and Estimated Errors Individual for All Samples in Series B (Sample S6-S10) and Series C (Samples S11-S15) sample h-MMA h-MMA fit h-MMA ID (vol %) (vol %) error (%)

volume fraction error of vol of titanium fraction oxo-clusters

Series B S6 S7 S8 S9 S10

0

S11 S12 S13 S14 S15

0

26.5 51.9 76.4 100

0 (fixed) 41.7 64.8 90.5 100 (fixed)

0.3 0.4 0.4

0.011 0.033 0.021 0.033 0.033

0.001 0.004 0.005 0.008 0.060

0.010 0.007 0.019 0.033 0.0003

0.036 0.006 0.005 0.008 0.014

Series C

Figure 5. Experimental SANS data (0) and fit results (- - -) for series C samples prepared with different theoretical h-MMA/d-MMA ratios: (a) 0, (b) 25, (c) 50, and (d) 75 wt % of h-MMA content. The inset shows the SLD profiles obtained from the fits and the scattering from the 100 wt % (e) h-MMA sample.

TABLE 1: Fitted Parameter Values and Estimated Errors for Series B and C Samples series B

series C

parameter

fitted value

error

R (nm) σR (nm) L (nm) η cluster (1010 cm-2) R (nm) σR (nm) L (nm) η cluster (1010 cm-2)

1.03 0.19 1.66 1.11 0.93 0.38 1.90 1.11

0.07 0.03 0.04 0.06 0.09 0.08 0.05 0.06

a series of samples that were essentially identical except for the h-MMA/d-MMA ratio. Even though this was achieved to a reasonable extent, it is clear from the S5 sample (100%

26.5 51.9 76.4 100

0 (fixed) 27.4 57.1 63.2 100 (fixed)

0.2 0.2 0.3

h-PMMA, Figure 2e) that there were some variability in the chemistry and/or sample preparation and, hence, the final nanostructure as observed by SANS. Samples in series B (S6-S10) were prepared by incorporation of cluster 1 ([Ti6O4](OEt)8(OMc)8, OMc-OOC-C(CH3)dCH2) into the mixture of MMA monomers with various h-MMA/dMMA ratios. Figure 3 shows the reduced and radially integrated SANS profiles for the series B samples. The full set of five curves was used for the simultaneous fit; however, only four curves are presented for clarity. The fifth curve is included as an insert (Figure 3e). Curves from a simultaneous fit are plotted and the results are presented in Tables 1 and 2. The polydispersed core-diffusion zone model (Figure 1a) was applied in the simultaneous fit of SANS data for this series of samples. According to this model, the core, represented by titanium cluster, is tightly packed with SLD independent of the h-MMA/d-MMA ratio, suggesting that this region (core) is inaccessible to the PMMA matrix. The fitted value for the cluster radius, R, is 1.03 ( 0.07 nm (Table 1). The diffusion zone is formed outside the dense core where upon copolymerization the eight methacrylate groups from the titanium cluster undergo covalent linking with PMMA matrix molecules. Fitted values and estimated errors for the SLD of the matrix and diffusion zones for samples in series B are summarized in Figure 4a,b. The fitted value for the characteristic

Organic-Inorganic Nanohybrids with Ti(IV) Nanoclusters length of the diffusion zone, L, is 1.66 ( 0.04 nm (Table 1), which is well beyond the length (0.25 nm) of the polymeric repeat unit (MMA) in its fully extended conformation.32 This suggests a strong penetration of PMMA into the diffusion zone. The nearly identical slope in both graphs (Figure 4a,b) also confirms the penetration of PMMA into the diffusion zone. Additionally, the fitted h-MMA content for S7, S8, and S9 samples are found to be respectively 42, 65 and 91 vol % (Table 2). The derivations from the as-prepared values (26.5, 51.9, and 76.4 vol %) is reasonable for this type of sample preparation. Finally, samples in series C were prepared by introduction of cluster 2 ([Ti6O4](OEt)8(OAc)8) into the mixture of MMA monomers with various h-MMA/d-MMA ratios. Cluster 2 was not expected to cross-link the polymer chains as it does not have the functional groups to participate in copolymerization. For that reason, it should be included into the developing polymer matrix as filler only. Figure 5 shows reduced and radially integrated SANS profiles for the series C samples. As previously, the full set of five curves was used for the simultaneous fit. However, it was noted that sample S15 (100% h-PMMA) showed no evidence of titanium clusters (calculated volume fraction of titanium cluster was 0.0003 as provided in Table 2). This again could have resulted from the inadequate mixing during the bulk copolymerization of MMA and cluster 2, providing unsatisfactory dispersion of titanium cluster within the PMMA matrix in the sample volume observed by the 10 mm diameter neutron beam. The results of simultaneous fitting are provided in Tables 1 and 2. Again, for the samples in series C, the simultaneous fit to the contrast variation sequence could not be obtained without the inclusion of a diffusion zone between the core (titanium cluster) and the PMMA matrix, and thus, the polydispersed corediffusion zone model (Figure 1a) was used to investigate the nanostructure of the hybrid materials. The fitted value for the radius of the titanium cluster, R, is 0.93 ( 0.09 nm, which is surrounded by a diffusion zone with characteristic length, L, of 1.9 ( 0.05 nm (Table 1). Fitted values and estimated errors for the SLD of the matrix and diffusion zones for series C samples are shown in Figure 4c,d. The nearly identical slopes confirm the penetration of PMMA into the diffusion zone. Furthermore, fitted values for the h-MMA content in samples S12, S13, and S14 are calculated at 27, 57 and 63 vol % (Table 2), respectively. The derivations from the as-prepared values (26.5, 51.9, and 76.4 vol %) are not significant as for series B samples. SANS results for samples in series B and C indicate an insignificant difference between the nanostructures of two titanium clusters-PMMA hybrid materials prepared by the in situ polymerization method. This finding is somewhat surprising as in the case of cluster 2 the aggregation of clusters was anticipated because they were expected to act as filler only. Previous research indicates that unmodified inorganic particles tend to aggregate in the polymer matrix independent of the typekind of material and the size.3 Even for the copolymerized PMMA-titanium clusters nanohybrids, earlier study conducted using the SANS technique also suggested the clusters aggregate to a certain extent, in nanohybrids prepared with 0.5-2 mol % of clusters.7 One possible explanation why a larger difference between the two structures was not observed is that the concentration of titanium clusters in these hybrid materials is possibly too small, within 0.007-0.03 volume fraction for most samples (Table 2), to cause their aggregation and /or play a major role in the hybrid structure formation.

J. Phys. Chem. B, Vol. 112, No. 51, 2008 16483 4. Conclusions A SANS study was undertaken to investigate the structures of organic-inorganic nanohybrid materials incorporating one of two types of titanium(IV) oxoalkoxyacylate nanoclusters. In the SANS experiments, very little difference was found between the structures of two titanium cluster-PMMA nanohybrids prepared by the in situ polymerization method. A polydispersed core-diffusion zone model together with the diluted scatterer approach was used to analyze the scattering contribution from the titanium clusters. There was a small but significant difference in the thickness of the diffusion zone between the titanium clusters covalently linked to the surrounding matrix, and the titanium clusters simply “dissolved” in the surrounding matrix. The fitted results indicate that the PMMA matrix polymer chains occupy a significant volume fraction of the diffusion zone, however, do not penetrate up to the core (titanium cluster). However, if there is any steric hindrance caused by the covalently bonded polymer molecules to the surface of the titanium cluster then it is below the resolution limit of these experiments. For SANS experiments, sample porosity is of particular concern and special care should be taken to minimize the size and volume fraction of the pores. An estimate of the pores contribution to the sample scattered intensity was made during data analysis as it was not possible to completely eliminate the porosity in samples. Acknowledgment. We are grateful to John Healy for his valuable assistance in data collection. Travel funds for V.H. and R.K. were provided by the Australian Access to Major Research Facilities Program. V.H. acknowledges the Australian Institute for Nuclear Science and Engineering for financial assistance. References and Notes (1) Kickelbick, G.; Schubert, U. Monatsh. Chem. (Chemical Monthly) 2001, 132, 13. (2) Schubert, U. Chem. Mater. 2001, 13, 3487. (3) Kickelbick, G. Prog. Polym. Sci. 2003, 28, 83. (4) Sanchez, C.; Ribot, F.; Doeuff, S. Transition metal oxo polymers synthesized Via sol-gel chemistry; in Inorganic and Organometallic Polymers with Special Properties; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992. (5) Kickelbick, G.; Schubert, U. Mater. Res. Soc. Symp. Proc. 1998, 519, 401. (6) Schubert, U.; Kicklebick, G.; Husing, N. Mol. Cryst. Liq. Cryst. 2000, 354, 107. (7) Moraru, B.; Husing, N.; Kickelbick, G.; Schubert, U. Chem. Mater. 2002, 14, 2732. (8) Gao, Y.; Roy Choudhury, N.; Matisons, J.; Schubert, U.; Moraru, B. Chem. Mater. 2002, 14, 4522. (9) Sanchez, C.; De A A Soler-Illia, G. J.; Rozes, L.; Caminade, A. M.; Turrin, C. O.; Majoral, J. P. Mater. Res. Soc. Symp. Proc. 2000, 628, CC6.2.1-6. (10) Pynn, J.; Matyjaszewski, K. Macromolecules 2000, 33, 217. (11) Sellinger, A.; Laine, R. M. Macromolecules 1996, 29, 2327. (12) Ribot, F.; Banse, F.; Sanchez, C.; Lahcini, M.; Jousseaumme, B. J. Sol-Gel Sci. Technol. 1997, 8, 529. (13) Mayer, C. R.; Thouvenot, R.; Lalot, T. Chem. Mater. 2000, 12, 257. (14) Hubert-Pfalzgraf, L. G.; Pajot, N.; Papiernik, R.; Parraud, S. Mater. Res. Soc. Symp. Proc. 1996, 435, 137. (15) Schubert, U.; Huesing, N.; Lorenz, A. Chem. Mater. 1995, 7, 2010. (16) Schubert, U.; Arpac, E.; Glaubitt, W.; Helmerich, A.; Chau, C. Chem. Mater. 1992, 4, 291. (17) Trimmel, G.; Fratzl, P.; Schubert, U. Chem. Mater. 2000, 12, 602. (18) Schubert, U.; Trimmel, G.; Moraru, B.; Tesch, W.; Fratzl, P.; Gross, S.; Kickelbick, G.; Huesing, N. Mater. Res. Soc. Symp. Proc. 2000, 628, CC 2.3.1-11. (19) Moraru, B.; Kickelbick, G.; Huesing, N.; Schubert, U.; Fratzl, P.; Peterlik, H. Chem. Mater. 2002, 14, 2732. (20) Schubert, U. J. Chem. Soc., Dalton Trans. 1996, 16, 3343.

16484 J. Phys. Chem. B, Vol. 112, No. 51, 2008 (21) Hartley, V. Novel nanostructured materials based on inorganicorganic hybrid systems. Honours Thesis. Flinders University Australia, 2004. (22) Gautier-Luneau, I.; Mosset, A.; Galy, J. Z. Kristallogr. 1987, 180, 83. (23) Laaziz, P. I.; Larbot, A.; Guizard, J.; Durand, J.; Cot, L.; Joffre, J. Acta Crystallogr. 1990, C46, 2332. (24) Doeuff, S.; Dromzee, Y.; Taulelle, F.; Sanchez, C. Inorg. Chem. 1989, 28, 4439. (25) Davis, T. P.; O’Driscoll, K. F. Makromol. Chem. Rapid Commun. 1989, 10, 509. (26) Keiderling, U.; Wiedenmann, A. Physica B 1995, 213-214, 895.

Karatchevtseva et al. (27) Heinemann, A. To be published. (28) Heinemann, A.; Hermann, H.; Wiedenmann, H.; Mattern, N.; Wetzig, K. J. Appl. Crystallogr. 2000, 33, 1386. (29) Hermann, H. J. Phys. A Math. Gen. 1994, 27, L935. (30) Heinemann, A.; Hermann, H.; Wetzig, K.; Haessler, H.; Baumbach, H.; Kroening, M. J. Mater. Sci. Lett. 1999, 18, 1413. (31) Vestergaard, B.; Hansen, S. J. Appl. Crystallogr. 2006, 39, 797. (32) Bicerano, J. Prediction of Polymer Properties, 3rd ed.; Marcel Dekker: New York, 2002.

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