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Covalent Grafting of Polydimethylsiloxane over Surface-Modified Alumina Nanoparticles Satyajit Gupta,†,‡ Praveen C. Ramamurthy,*,†,‡ and Giridhar Madras‡ †
Department of Materials Engineering and ‡Institute Nanoscience Initiative, Indian Institute of Science, Bangalore 560012, India
bS Supporting Information ABSTRACT: The synthesis of “smart structured” conducting polymers and the fabrication of devices using them are important areas of research. However, conducting polymeric materials that are used in devices are susceptible to degradation due to oxygen and moisture. Thus, protection of such devices to ensure long-term stability is always desirable. Polymer nanocomposites are promising materials for the encapsulation of such devices. Therefore, it is important to develop suitable polymer nanocomposites as encapsulation materials to protect such devices. This work presents a technique based on grafting between surface-decorated γ-alumina nanoparticles and polymer to make nanocomposites that can be used for the encapsulation of devices. Alumina was functionalized with allyltrimethoxysilane and used to conjugate polymer molecules (hydride-terminated polydimethylsiloxane) through a platinum-catalyzed hydrosilylation reaction. Fourier transform infrared spectroscopy, X-ray-photoelectron spectroscopy, and Raman spectroscopy were used to characterize the surface chemistry of the nanoparticles after surface modification. The grafting density of alkene groups on the surface of the modified nanoparticles was calculated using CHN and thermogravimetric analyses. The thermal stability of the composites was also evaluated using thermogravimetric analysis. The nanoindentation technique was used to analyze the mechanical characteristics of the composites. The densities of the composites were evaluated using a density gradient column, and the morphology of the composites was evaluated by scanning electron microscopy. All of our studies reveal that the composites have good thermal stability and mechanical flexibility and, thus, can potentially be used for the encapsulation of organic photovoltaic devices.
’ INTRODUCTION The development of organic photovoltaic devices (OPVDs) has been an active research area in the past decade.15 OPVDs are flexible and lightweight compared to traditional inorganicmaterial-based devices and thus have excellent prospects in the future. A major hurdle for the introduction of OPVDs in the commercial market is the limited lifetimes of such devices because of the degradation of the active device component due to moisture68 and oxygen.9,10 These materials are also susceptible to environmental aging,11,12 and the underlying mechanisms for the degradation and deterioration of their properties have been well studied. Oxygen can induce photodegradation of conjugated polymers such as poly(p-phenylene vinylene) (PPV) and its derivatives,9,13 when singlet oxygen atoms bind to the vinyl bonds and lead to the formation of two aldehyde molecules.14 In the case of electrochemically prepared poly(3methylthiophene), exposure to oxygen increases the conductance.15 The electrodes used in the devices such as aluminum and calcium undergo oxidation in air16 and water.17 To increase the lifetime of these devices, protection of the devices from deterrents such as moisture and oxygen is critical. To date, most organic devices have been encapsulated by sealing the device in an inert atmosphere such as nitrogen or argon using a metal that can be secured with a bead of UV-cured epoxy resin and a scavenger such as calcium oxide or barium oxide.18 However, this method of encapsulation is difficult to apply to organic devices because it results in rigid devices that cannot be used for applications demanding flexible devices. Neat plastic membranes offer little protection against atmospheric oxygen r 2011 American Chemical Society
and water. Thus, to decrease the permeability of the polymerbased encapsulant, there has been intense interest in developing barrier materials such as polymer nanocomposites for encapsulation. Silicone polymers have been used in various commercial applications including drug-delivery systems, biomedical implants, lubricants, cosmetics, construction sealants, and flexible pressure sensors.19 Recently, we developed a method using aminefunctionalized alumina nanoparticles as a reinforcing agent for an epoxy [diglycidyl ether-terminated poly(dimethylsiloxane)] polymer matrix for the encapsulation of devices.20 In the present study, polymer nanocomposites were prepared by surface functionalization of nanoparticles with allyltrimethylsiloxane (ATMS), followed by conjugation of the silicone polymer (hydride-terminated polydimethylsiloxane) (Scheme 1) through a hydrosilylation reaction catalyzed by platinum catalyst (i.e., Karstedt catalyst).21,22 This is one of the most efficient methods for forming Si—C bonds.23,24 In addition, the intended nanoparticles in the matrix can reduce the permeability of gases (such as water vapor and oxygen) by providing a tortuous pathway. Various groups have reported hydrosilylation reactions over H-terminated silicon surfaces.2528 In this work, surface allyl-decorated alumina nanoparticles were attached to the Si—H group of the polymer through the hydrosilylation reaction. The effect of various loadings of nanoparticles inside the matrix was Received: December 1, 2010 Accepted: April 25, 2011 Revised: April 25, 2011 Published: April 25, 2011 6585
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Scheme 1. Preparation of Polymer Nanocomposites: (Step 1) Surface Functionalization of Alumina and (Step 2) Hydrosilylation of the Hydride Polymer with the Dangling Allyl Groups at the Surface
studied, and it was found that these nanocomposites have properties suitable for use in encapsulation.
’ EXPERIMENTAL SECTION Materials. Allyltrimethoxysilane (ATMS), platinum dimethyldivinylsilane [Karstedt catalyst: platinum(0)-1,3-divinyl-1,1,3,3tetramethyldisiloxane complex, solution in xylene (∼2% Pt)], silicone polymer (hydride-terminated polydimethylsiloxane) with a number-average molecular weight of 17500, and alumina matrix were obtained from Sigma-Aldrich Company Ltd., Dorset, U.K. The alumina nanoparticles were of gamma phase and had a particle size of 40 nm, a melting point of 2040 °C, and a density of 3.97 g cm3. Analytical-reagent-grade toluene was distilled over pressed sodium and was preserved under inert atmosphere before use.
Surface Functionalization of Alumina Nanoparticles with Allyltrimethoxysilane (ATMS). The surface functionalization of
γ-alumina nanoparticles with ATMS was performed according to the work of Tsubokawa et al.29 (without using any catalyst). Before functionalization, pristine γ-alumina nanoparticles were dried at 110 °C in a vacuum oven for 15 h to remove adsorbed moisture from the surface. Then, nanoparticles (200 mg) were dispersed by sonication in dry toluene, 1 mL of ATMS was added, and the mixture was refluxed for 20 h under argon atmosphere. Dry toluene was used, to prevent homopolycondensation of ATMS and promote local hydrolysis of silane methoxy (Si—O—Me) groups to Si—OH. This Si—OH then undergoes condensation with Al—OH groups at the alumina nanoparticle surface. After completion of the reaction, the powder was then separated by centrifugation, washed with excess toluene to remove the unreacted siloxane (ATMS) moieties, 6586
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Industrial & Engineering Chemistry Research and dried under a vacuum at 110 °C for 3 h to remove residual toluene. Hydrosilylation Reaction for Attachment of the Functionalized Alumina to the Silicone Polymer. Various weight percentages (i.e., 0.21, 0.36, 0.56, 0.96, and 1.11 wt %) of dried allyl-functionalized nanoparticles were mixed with the polymer matrix, dispersed using a magnetic stirrer, and further sonicated for 1 h at 40 °C. The nomenclature H1, H2, H3, H4, and H5 denotes 0.21, 0.36, 0.56, 0.96, and 1.11 wt % allyl-functionalized nanoparticles, respectively. One drop (∼13.8 mg) of Karstedt catalyst was added to the mixture and stirred well. This reaction is instantaneous, and the catalyst undergoes an induction period to generate the active catalyst species (Pt-colloid). This species reacts with allyl and Si—H groups to give the final product. To ascertain the temperature of the reaction, a differential scanning calorimetry (DSC) study (discussed later) was carried out and indicated curing at 130 °C. Therefore, we carried out the reaction at 130 °C for 1 h in finely polished stainless steel molds. The reaction was carried out under high vacuum (30 mmHg) to remove the hydrogen gas that is produced during the hydrosilylation reaction. Characterization. A Perkin-Elmer (Spectrum GX) spectrometer was used to obtain the Fourier transform infrared (FTIR) spectra of both pristine (unfunctionalized) and functionalized alumina nanoparticles, to evaluate the surface chemistry. The FTIR spectra were recorded using potassium bromide (KBr) pellets of both pristine and functionalized alumina powder and analyzed in the range from 400 to 4000 cm1 with a resolution of 4 cm1. The conjugation of ATMS was further investigated by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, elemental (CHN) analysis, and thermogravimetric analysis (TGA). XPS spectra of the samples (dried) were recorded on a Thermo Scientific Multilab 2000 instrument. Deconvolution was carried out by the software provided with the instrument (Thermo Scientific Multilab 2000). Raman spectra were obtained using an Alpha 300S scanning near-field optical microscope from WiTec Focus Innovation. The surface area of the pristine alumina was measured by the BrunauerEmmettTeller (BET) nitrogen sorption method at 77 K with a Belsorp instrument. For the nitrogen adsorption/desorption experiments, the pristine alumina sample was degassed at 120 °C for 5 h. TGA was carried out using a high-resolution TGA 2950 apparatus (NETZSCH, STA 409PC) with a heating rate of 10° min1 in an alumina crucible to evaluate the surface functionalization as well as the thermal stability of the composites. A Thermo Finnigan FLASH EA 1112 analyzer was used for elemental analysis. To monitor the curing reaction, differential scanning calorimetry (DSC) was carried out in a Mettler Toledo (DSC 822e) instrument at a heating rate of 10 °C min1 in argon atmosphere at a flow rate of 80 mL min1 in a hermitic aluminum pan. The morphologies of the cross-sectional surface of the frozen (liquid nitrogen) fractured surfaces of the composites were observed using scanning electron microscopy (SEM), which was carried out on an ESEM Quanta instrument. Prior to the SEM analysis, samples were sputter-coated with a thin film of gold using a JEOL (JFC-1100E) ion sputtering device. The pristine alumina powder sample was coated directly over carbon tape and was sputter-coated with gold. The samples were cut into rectangular blocks (4.5 mm 3 mm 2 mm), and silver paste was applied on the opposite sides of the samples to form the contacts. Then, the samples were analyzed in a probe station (SUSS MicroTec) connected to a parametric analyzer (4155C Agilent semiconductor parametric analyzer) setup to measure
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the resistance of the composites. A ramp voltage was applied to the samples, and the resulting current was measured, from which the average resistance was calculated. Experiments were carried out multiple times to ensure reproducibility of the resistance value. Nanoindentation experiments were carried out using a Hysitron triboindenter with a Berkovich tip (a three-sided pyramidal diamond tip) to determine the elastic modulus. As the mechanical properties determined from nanoindentation are sensitive to the tip geometry, the tip area function was calibrated using a standard quartz sample. Because the loads used in indenting polymer films are very small, the tip area function was calibrated in the low depth ranges for determination of the modulus. Using this area function, nanoindentation experiments were carried out on single-crystal aluminum to cross check the standard elastic modulus, E, and hardness, H, values recommended by the manufacturer (75.1 ( 5% GPa and 360 ( 10% MPa, respectively). The standard deviation was within 5% for both values, validating the tip calibration process. Twenty indentations were made in each sample, and the average value was taken as the property of the composite. All experiments were performed in displacementcontrolled mode (at room temperature) with a maximum penetration depth of 2000 nm and a loading and unloading rate of 200 nm/s, with the peak load maintained for 10 s. The elastic modulus was determined by the OliverPharr method.30 The specific gravity of the composites was measured using a density gradient column. The test was carried out as described in standard ASTM D1505. The density gradient column was prepared using ethanol (d = 0.816) and carbon tetrachloride (d = 1.59). The standard apparatus consists of a glass column tube of 1000-mm length containing a solution of ethanol (EtOH) and carbon tetrachloride (CCl4), which has a linear density gradient from the top (lower density) to the bottom (higher density) of the column. The mixtures with EtOH/CCl4 ratios (v/v) of 100/ 0, 90/10, 80/20, 70/30, 60/40, 50/50, 40/60, 30/70, 20/80, 10/ 90, and 0/100 were prepared and carefully poured down the sides of the gradient tube to avoid mixing of the layers. Standard density markers in the form of glass beads (H & D Fitzgerald Ltd., St. Asaph, U.K.) along with the composites were dropped into the column slowly. The specific gravity of the composites was determined by comparing the positions of the composites with those of the standards.
’ RESULTS AND DISCUSSION Synthesis. In the present work, an organosilane coupling methodology was used for developing allyl functionalized γ-alumina nanoparticles for the silicone polymer matrix to fabricate composites (Scheme 1). The surface functionalization of γ-alumina was carried out with allyl trimethoxy silane. Silanes are bifunctional molecules having the general chemical formula Y—(CH2)n—Si—R3, where Y represents the headgroup functionality (here, it is —HC=CH2) and (CH2)n represents an alkane chain. The tail group (i.e., Si—R3) is the anchor group by which the silane is grafted to the metal oxide surface, forming a strong M—O—Si bond (M is the metal from the inorganic substrate; here, M = Al),31 whereas the allyl fragment remains at the surface of the metal oxide. As the surface is functionalized with surface pendant —HC=CH2 groups, the surface becomes nonpolar, making the nanoparticles compatible in the polymer matrix. The surface-terminating allyl group reacts with Si—H groups of the silicone polymer in the presence of platinum dimethyldivinylsilane (Karstedt catalyst) and thus forms the polymer nanocomposite. 6587
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Figure 1. FTIR spectra of (A) pristine and (B) functionalized γ-alumina.
Characterization of ATMS-Functionalized Alumina. FTIR Analysis. The FTIR spectra of both modified and unmodified
alumina are shown in Figure 1. Because of the grafting of siloxane molecules onto the surface of the nanoparticles, the respective peaks were observed in the spectrum of functionalized γ-alumina. In pristine alumina, the broad peak at 3465 cm1 is due to surface OH stretching (Al—OH), and the peak at 1640 cm1 is due to the —OH bending vibration. After siloxane modification, the siloxane modification peaks at 2922 and 2849 cm1 are characteristic of asymmetric and symmetric stretching, respectively, of —CH2. The peaks at 3081 and 2979 cm1 are due to CdC—H asymmetric and symmetric stretching, respectively. The peak observed at 1633 cm1 is due to CdC bending. The appearance of a peak in the range of 10001100 cm1 is due to Si—O stretching. The Raman spectrum (Figure S1, Supporting Information) also supports the FTIR results, as the peak observed at 3026 cm1 is characteristic of the ν(C—H) bands of —CdC— H (allylic) bonds and the peak around 28003000 cm1 is characteristic of aliphatic C—H bonds. XPS. Although the FTIR and Raman spectra unambiguously indicate the successful functionalization of ATMS to the alumina surface, the findings were further confirmed by XPS. The highresolution O 1s spectrum (Figure 2) of pristine alumina displays two peaks at 530.8 and 532.3 eV, which can be attributed to oxygen in the O—H component of aluminum oxohydrides and to oxygen in the Al—O component of alumina, respectively. The appearance of the shoulder at 531.7 eV (corresponding to oxygen in Al—O—Si) after surface modification indicates chemical bonding between ATMS and aluminum oxide. BET Analysis. The surface functionalization was also evaluated by N2 adsorption/desorption isotherm. The BET surface area for pristine γ-alumina was found to be 130 m2 g1. Figure 3 shows that, for the pristine alumina sample, the isotherm rises slowly to P/P0 = 0.8. For P/P0 > 0.8, the adsorption branch ascends sharply, but does not reach a stable state even for P/P0 ≈ 1.0. Therefore, this isotherm is a type II isotherm in the IUPAC classification.32 Type II isotherms are sigmoidal and usually occur for physical adsorption in powdered samples and correspond to multilayer formation. The typical hysteresis loop (H3 type) between the desorption and adsorption branches indicates the macropore structure of the sample. The macroporous nature might arise from particle agglomeration, as shown in the SEM (Figure 4) image of pristine alumina.
Figure 2. XPS spectra of oxygen (O 1s) (A) before and (B) after functionalization.
Figure 3. N2 adsorption/desorption isotherm for pristine alumina.
TGA. The grafting of ATMS on the alumina surface was further evaluated by thermal analysis. The comparative weight losses for unmodified alumina and aluminaATMS between room temperature and 900 °C are shown in Figure 5. The pristine alumina nanoparticles show some weight loss in this temperature range. The weight loss between 100 and 400 °C is mainly due to desorption of physically adsorbed water33 and some CO2 (also 6588
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Industrial & Engineering Chemistry Research detected by elemental analysis). The weight loss beyond 400 °C is mainly due to the dehydration reaction occurring at the surface of the alumina nanoparticles. In the case of aluminaATMS, the weight loss between 50 and 150 °C is due to the loss of physically adsorbed moisture from the surface. The ATMS-functionalized nanoparticles show less of this type of loss than pristine alumina, because attachment of ATMS makes the surface hydrophobic. The second regime of 160750 °C corresponds to the decomposition of the organic coupling agent. The evaluations of thermogravimetric (TG) analysis were further used to calculate the surface grafting density of ATMS. In the case of aluminaATMS, the weight loss in the range from 160 to 750 °C is due to decomposition of the organic coating, and this weight loss is about 8.63%. Considering the loss
Figure 4. SEM image of pristine alumina particles.
Figure 5. Thermogravimetric analysis of (A) pristine and (B) functionalized alumina.
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due to organic fragments, the grafting density is approximately 16.2 ( 0.7 μmol m2 (∼9 molecules/nm2) using the expression given in a previous study.34 Further TG analysis indicates a surface coverage by ATMS of approximately 74 ( 5%, considering that the average surface —OH group density of γ-alumina is 12.5/nm2. The results of thermogravimetric analysis were further used to calculate the number of ATMS molecules (functionalizing agent) attached to each Al2O3 particle. As a control, ATMS-functionalized alumina nanoparticles were first analyzed to measure the amount of organic material (allyltrimethoxysilane) present. The result of this experiment showed that ATMS accounts for approximately ∼8.63% of the mass of stock alumina. Coupled with the molar mass of ATMS (162.26 g mol1), each alumina particle (assumed to be spherical and to have a mean diameter of ∼40 nm) was found to contain ∼43251 functional sites.35 CHN Analysis. From elemental analysis, it was found (Table 1) that the carbon and hydrogen contents both increase after functionalization with ATMS. This further indicates successful attachment of the organic fragment (ATMS) to the pristine alumina surface. From the results of the CHN analysis, the grafting density was calculated to be approximately 12.7 μmol m2 (∼8 molecules/nm2), using the standard expression.34 CHN analysis indicated an approximate surface coverage by functionalizing agent of 64% (considering that the average surface —OH group density of γ-alumina is 12.5/nm2). TGA and elemental analysis (CHN) were performed to quantify the grafting density of ATMS at the alumina surface. The quantification of ATMS from TGA was also confirmed by elemental analysis. TGA indicated a grafting density of 16.2 ( 0.7 μmol m2, which is close to the value of 12.7 μmol m2 observed from elemental analysis (Table 1). The wetting experiment in water also showed that the particles become hydrophobic because of the allyl groups at the surface. DSC. Differential scanning calorimetry (DSC) was used to evaluate the curing reaction for H1 composition. The heating
Figure 6. DSC thermogram of the curing of a mixture of functionalized alumina, polymer, and platinum catalyst.
Table 1. Various Parameters Obtained from CHN and TG Analyses of Pristine and Functionalized Alumina analysis technique CHN analysis
TGA
parameter
pristine alumina
carbon (%)
0.4069
6.395
hydrogen (%) surface grafting density (μmol m2)
1.4197
1.7896 12.7
surface grafting density (μmol m2)
16.2 ( 0.7
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functionalized alumina
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Industrial & Engineering Chemistry Research program was from 0 to 200 °C at a rate of 10 °C min1. The sample was cooled back to 0 °C and again heated to 250 °C to verify possible reversible reactions. From the DSC thermogram, it was observed (Figure 6) that the curing reaction, indicated by an exothermic peak, occurred at about 130 °C in the first cycle and did not reappear during the second heating. From the thermogram, the curing peak onset, Tonset, was observed at about 104 °C, and Tmidpoint, was observed at 130 °C. A small peak
Figure 7. FTIR spectra of various composites.
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around 92 °C could be due to possible side reactions such as selfcoupling.3639 Characterization of Composites. FTIR Spectroscopy, Fractured Surface SEM, and Resistance Measurements. The hydrosilylation reaction was also monitored by FTIR spectroscopy (Figure 7). After the hydrosilylation reaction, the disappearance of the peak at 2126 cm1 confirmed that all Si—H groups reacted.40 SEM (Figure 8) micrographs of cryogenically fractured surfaces of the composites (H1, H3, and H5) exhibited deformation and cleavages, due to interfacial binding within the polymer matrix. The resistances of all of the composites were found to be on the order of (69) 1010 Ω. This electrical insulation behavior of nanocomposites is very important, as it avoids electrical interference with devices when they are encapsulated. Thermal Stability of the Composites. The thermal stability of the encapsulant is critical in maintaining the effectiveness of the encapsulation in protecting a device over its lifetime. Samples were heated to 900 °C at a heating rate of 10 °C min1. From the thermograms (Figure 9), it was observed that 5% degradation (TD5%) of samples H1H5 occurred at a temperature of ∼370 °C, whereas 10% degradation (TD10%) occurred at ∼400 °C. This stability is higher than that reported in our previous study of epoxy nanocomposites.20 TD10% decreases with increasing loading, as explained in the next section. Such thermal stability of the composites is due to covalent bond formation between the allyl-decorated nanoparticles and the polymer matrix.
Figure 8. Fracture surface SEM images of composites, showing interfacial bindings. 6590
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Figure 9. TGA of various composites.
Figure 11. Indentation modulus values of various composites.
Figure 10. Loadpenetration depth curves of various composites.
Figure 12. Density measurements of various composites.
Nanoindentation: Mechanical Characteristics. Nanoindentation is a technique that has been used widely for the primary characterization of hard materials (elastoplastic properties of hard materials), but it can also be used for soft, elastomeric materials.41 In nanoindentation, controlled indentation to a maximum penetration depth of 2000 nm was employed. From the unloading curves, it is evident that a large elastic recovery takes place after the removal of the load (Figure 10). Atomic force microscopy of the indented regions showed no impression, indicating that the material completely recovers upon unloading. This shows the ability of the material to recover completely after it is unloaded, which is an important characteristic of an encapsulant. The modulus values were calculated using the OliverPharr method (Figure 11), which gives a relative modulus value (normally higher than the actual value, but capable of differentiating between elastic moduli of soft materials41). However, the modulus values were used to verify how the modulus values of various composites vary with increasing loading. It was observed (Figure 9) that, with an increase in the loading, the modulus value decreases. This is because the functionalized nanoparticles are far apart at low loadings, and the Si—H groups can reach perfect orientation with the surface-bound allyl groups. Thus, Si—H groups can easily react with the surface-decorated allyl (CdC) groups through the hydrosilylation reaction by
platinum catalyst (Figure S2, Supporting Information). However, with an increase in loading, as the functionalized particles come closer, steric factors and particle agglomeration hinder the coupling of Si—H with the surface-bound allyl group (Figure S3, Supporting Information), and other competitive reactions such as self-coupling3638,42 become predominant. Thus, not all of the Si—H groups attach to the surface, resulting in the deterioration43,44 of mechanical properties (modulus value) and thermal stability. The TGA of composites (Figure 7) shows a decrease in 10% degradation temperature of the composites with increased loading. Density Gradient Column. The specific gravities of the various composites were measured45 using standard glass beads of known density as a reference. The specific gravity values indicate how densely the material is packed within a given volume. This indicates that the nanoparticles are well integrated into the polymer matrix without the formation of any entrapped voids (i.e., free space). Figure S4 (Supporting Information) shows the calibration curve drawn from various standard density beads with a high correlation (R2 > 0.99) for the best-fit line. The specific gravity of the composites was calculated using this equation. In Figure 12, the relative changes in specific gravity with nanoparticle loading are shown. The specific gravities of samples H1H5 were measured to be 1.3988, 1.2379, 1.2950, 1.4000, and 1.3936 g cm1, respectively. With an increase in the loading, a steady 6591
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Industrial & Engineering Chemistry Research increase in specific gravity was expected, but it can be seen that the specific gravity of composite H1 is lower than that of composites H2 and H3 because of the formation of voids and free space. This might be because not all of the polymer molecules bonded to the nanoparticles, as discussed in a previous study.43 With a further increase in loading, an increase in specific gravity (with specific gravity values closer to that of H1) is observed because of a relative increase in the number of nanoparticles inside the matrix. Stability of Composites at Various pH Values. The stability of alumina nanoparticles in extreme acidic and basic solutions was determined by placing the nanocomposites in 1 N hydrochloric acid solution and 1 N sodium hydroxide solution, respectively.43 The samples were immersed in deionized water for 24 h for saturation and then put into excess acidic or basic solution for >120 h. The samples were then dipped into deionized water again to remove the dissolved alumina. However, no weight loss was observed under either extreme condition, indicating that the polymer matrix is effectively protecting the alumina nanoparticles from dissolution in excess acidic or basic solutions. To verify the stability of the nanocomposites, FTIR spectroscopy was carried out on the composites before and after the experiment. A small change in the overlapped double peak at 10001100 cm1 was visible for both HCl- and NaOH-exposed nanocomposites. This might indicate the formation of silicone oligomers. However, this change is very small and is found only at extremely harsh conditions of treatment. Therefore, the nanocomposite might be very stable under milder conditions that are normally used. The observation of no weight loss also indirectly indicates that these composites can reduce the diffusion of water molecules through the matrix while protecting the organic device.
’ CONCLUSIONS In this work, γ-alumina nanoparticles with surface-pendant allyl groups were synthesized using 3-allyltrimethoxy silane and were used to fabricate the nanocomposites. The surface functionalization was confirmed by FTIR, Raman, and XP spectroscopies. Quantification of the ligands at the surface of the nanoparticles was successfully carried out by TGA and CHN analysis. Thermal and mechanical characterizations of the composites were thoroughly performed using TGA and nanoindentation. From TGA, it can be concluded that the composites are stable (10% degradation at ∼400 °C). Nanoindentation reflects the flexibility of the composites, and modulus values at various loadings were also extracted from indentation measurements. SEM study showed that interfacial binding is present within the matrix. Resistance measurements showed that the composites are good insulators. All of these studies imply that the optimum loading of allyl-functionalized nanoparticles in the composite matrix is very critical for the purpose of encapsulation. The stability of the polymer matrix at two extreme pH values shows that the polymer matrix is effectively protecting the alumina nanoparticles. These results indicate that the as-synthesized composite is a promising material for encapsulating organic devices such as photovoltaic devices and field-effect transistors. ’ ASSOCIATED CONTENT
bS
Supporting Information. Raman spectra of ATMSfunctionalized alumina (Figure S1), schematics of the reaction mechanism (Figures S2 and S3), and a calibration plot with standard density beads for measuring the density of composites
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(Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
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[email protected]. Fax:þ91-80-2360-0472. Tel.: þ91-80-2293-2627.
’ ACKNOWLEDGMENT The authors acknowledge the Institute Nanoscience Initiative (INI) and Indian Institute of Science, Bangalore, India. S.G. acknowledges the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for financial support. The authors gratefully acknowledge Prof. Upadrasta Ramamurty and K. Eswar Prasad of the Department of Materials Engineering, Indian Institute of Science, Bangalore, India, for nanoindentation studies. ’ REFERENCES (1) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Plastic Solar Cells. Adv. Funct. Mater. 2001, 11, 15–26. (2) Spanggaard, H.; Krebs, F. C. A Brief History of the Development of Organic and Polymeric Photovoltaics. Sol. Energy Mater. Sol. Cells 2004, 83, 125–146. (3) Ramamurthy, P. C.; Malshe, A. M.; Harrell, W. R.; Gregory, R. V.; McGuire, K.; Rao, A. M. Polyaniline/Single-Walled Carbon Nanotube Composite Electronic Device. Solid State Electronics 2004, 48, 2019–2024. (4) Ramamurthy, P. C.; Harrell, W. R.; Gregory, R. V.; Sadanadan, B.; Rao, A. M. Mechanical Properties of Polyaniline/Multi-Walled Carbon Nanotube Composite Films. Mater. Res. Soc. Symp. Proc. 2004, 791, 1–6. (5) Ramamurthy, P. C.; Harrell, W. R.; Gregory, R. V. The Influence of Solvent Content in Polyaniline on Conductivity and Electronic Device Performance. Electrochem. Solid State Lett. 2003, 6, G113–G116. (6) Ye, R. B.; BaBa, M.; Suzuki, K.; Ohishi, Y.; Mori, K. Effects of O2 and H2O on Electrical Characteristics of Pentacene Thin Film Transistors. Thin Solid Films 2004, 464465, 437–440. (7) Qiu, Y.; Hu, Y. C.; Dong, G. F.; Wang, L. D.; Xie, J. F.; Ma, Y. L. H2O Effect on the Stability of Organic Thin-Film Field-Effect Transistors. Appl. Phys. Lett. 2003, 83, 1644–1646. (8) Zhu, Z. T.; Mason, J. T.; Dieckmann, R.; Malliaras, G. G. Humidity Sensors Based on Pentacene Thin-Film Transistors. Appl. Phys. Lett. 2002, 81, 4643–4645. (9) Cumpston, B. H.; Parker, I. D.; Jensen, K. F. In Situ Characterization of the Oxidative Degradation of a Polymeric Light Emitting Device. J. Appl. Phys. 1997, 81, 3716–3720. (10) Bliznyuk, V. N.; Carter, S. A.; Scott, J. C.; Klarner, G.; Miler, R. D.; Miller, D. C. Electrical and Photoinduced Degradation of Polyfluorene Based Films and Light-Emitting Devices. Macromolecules 1999, 32, 361–369. (11) Morgado, J.; Friend, R. H.; Cacialli, F. Environmental Aging of Poly(p-phenylenevinylene) Based Light-Emitting Diodes. Synth. Met. 2000, 114, 189–196. (12) Xing, K.; Fahlman, M.; L€ogdlund, M.; dos Santos, D. A.; Parente, V.; Lazzaroni, R.; Bredas, J.-L.; Gymer, R. W.; Salaneck, W. R. The Interaction of Poly(p-phenylenevinylene) with Air. Adv. Mater. 1996, 8, 971–974. (13) Scurlock, R. D.; Wang, B.; Ogilby, P. R.; Sheats, J. R.; Clough, R. L. Singlet Oxygen as a Reactive Intermediate in the Photodegradation of an Electroluminescent Polymer. J. Am. Chem. Soc. 1995, 117, 10194–10202. (14) Sutherland, D. G. J.; Carlisle, J. A.; Elliker, P.; Fox, G.; Hagler, T. W.; Jimenez, I.; Lee, H. W.; Pakbaz, K.; Terminello, L. J.; Williams, S. C.; Himpsel, F. J.; Shuh, D. K.; Tong, W. M.; Lia, J. J.; Callcott, T. A.; 6592
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