Inorganic Films as a Moisture

Article pubs.acs.org/IECR

Polyvinylbutyral Based Hybrid Organic/Inorganic Films as a Moisture Barrier Material Satyajit Gupta,†,‡ Sindhu Seethamraju,‡ Praveen Chandrashekarapura Ramamurthy,*,†,‡ and Giridhar Madras‡ †

Department of Materials Engineering, Indian Institute of Science, Bangalore, India Center for Nanoscience and Engineering, Indian Institute of Science, Bangalore, India

‡

S Supporting Information *

ABSTRACT: Flexible and thermally stable, freestanding hybrid organic/inorganic based polymer-composite films have been fabricated using a simple solution casting method. Polyvinylbutyral and amine functionalized mesoporous silica were used to synthesize the composite. An additional polyol“tripentaerythritol”component was also used to increase the −OH group content in the composite matrix. The moisture permeability of the composites was investigated by following a calcium degradation test protocol. This showed a reduction in the moisture permeability with the increase in functionalized silica loadings in the matrix. A reduction in permeability was observed for the composites as compared to the neat polymer film. The thermal and mechanical properties of these composites were also investigated by various techniques like thermogravimetric analysis, differential scanning calorimetry, tensile experiments, and dynamic mechanical analysis. It was observed that these properties detoriate with the increase in the functionalized silica content and hence an optimized loading is required in order to retain critical properties. This deterioration is due to the aggregation of the fillers in the matrix. Furthermore, the films were used to encapsulate P3HT (poly 3 hexyl thiophene) based organic Schottky structured diodes, and the diode characteristics under accelerated aging conditions were studied. The weathered diodes, encapsulated with composite film showed an improvement in the lifetime as compared to neat polymer film. The initial investigation of these films suggests that they can be used as a moisture barrier layer for organic electronics encapsulation application.

■

functionalized mesoporous silica particles were used as the filler material for various silicone resins. Epoxy and hydride terminated silicone resins were used as a polymer matrix, and the composites were fabricated by in situ thermal curing technique. Two diverse routes, amine-epoxy curing11 and hydrosilylation reaction12,13 strategies were followed to obtain a composite matrix for the potential application in organic devices. Inorganic oxide nanoparticles (like Al2O3) can be used as filler in the polymer matrix to reduce the gas permeability.15 The present work is focused on the fabrication and characterization of a polymer composite based matrix, as a moisture barrier material. Polyvinylbutyral was chosen as the polymer matrix, which is a good ceramic binder and widely used in the automotive industries.16 It is commonly used as a component in laminated safety glass and in photovoltaic solar panels. This polymer matrix, having excellent film forming ability, provides good adhesion to various surfaces.17 Other critical properties of the PVB polymer include optical clarity, toughness, flexibility, resilience, and high impact strength at low temperatures. Amine functionalized mesoporous silica (prepared through silane coupling methodology) was used as a reinforcing filler matrix for the hybrid composite. Mesoporous silica serves as a good filler,18 because of the confinement of the polymer chains

INTRODUCTION Moisture and gas barrier materials have versatile applications in a broad range of areas. Barrier materials are critical in food/ beverage and electronics packaging industries. Organic conducting polymer based devices like organic photovoltaics and organic field effect transistors are extensively studied1−3 because of their flexibility, light weight, and cost effectiveness. However, commercialization of these devices is hampered by the instability of the devices toward moisture and oxygen.4−8 The active device components such as the electrode material and the organic conducting polymer are highly susceptible toward degradation due to oxygen and moisture. Therefore, a suitable encapsulant material is required to protect these devices and provide a lifetime of ≥10 years. This requires that the encapsulant has a water vapor transmission rate (WVTR) that should be no more than 10−6 g m−2 day−1 and an oxygen transmission rate (OTR) that should be no more than 10−3 cm3 m−2 day−1.9,10 Hence, development of a barrier material is highly important for both organic and inorganic electronic devices. Polymer based composite materials find a variety of uses ranging from transportation, marine industries, packaging, and space vehicles. The polymer based composite materials which possess mechanical flexibility, good barrier properties, and thermal stability can be used for the encapsulation of electronic devices. Polymer nanocomposite materials based on silicone polymers have been developed which can be used for encapsulation of organic devices.11−14 In the above studies, surface amine/allyl modified alumina nanoparticles and allyl © 2013 American Chemical Society

Received: Revised: Accepted: Published: 4383

August 21, 2012 February 17, 2013 March 1, 2013 March 1, 2013 dx.doi.org/10.1021/ie3022412 | Ind. Eng. Chem. Res. 2013, 52, 4383−4394

Industrial & Engineering Chemistry Research

Article

in the mesopores of silica particles. It also provides a torturous pathway for the incoming molecules and thus reduces the permeability.19 In addition, surface amine groups can also form hydrogen bonds with −OH groups in the polymer matrix (PVB) and enhance the interfacial interaction that would provide good thermal and mechanical stability. Along with mesoporous silica, a polyol“tripentaerythritol” was also used for the composite matrix, which provides multiple polar −OH sites in the composite matrix. This can further reduce the permeability of moisture through hydrogen bonding (polar− polar interaction)20 and hence acts as a trap for moisture. In comparison to the existent organic/inorganic multilayered encapsulation (VITEX)9/ALD (atomic layer deposition)/MLD (molecular layer deposition) based methods of encapsulation, this method of encapsulation is simple, faster, and less energy intensive but needs further optimization to achieve lower WVTR values. The objective of this study was to synthesize functionalized silica based PVB composite to be used as a moisture barrier material for the encapsulation of organic Schottky structured devices. The general studies on the composites were carried out to evaluate the thermo-mechanical characteristics of the composites. The optimum loading of the silica particles that can be used for the application as an encapsulant for organic devices was discussed. The effect of loading of functionalized silica in the hybrid composite materials was evaluated by thermo-mechanical and light absorbance techniques. In addition, the moisture permeability of the composites was determined by the calcium degradation test.21 The composites were used to encapsulate diode devices, and the device properties under accelerated aging conditions were studied.

Scheme 1. (A) Pristine Silica Functionalization. (B) Composite Hybrid Matrix, Showing the Mechanism with Which It Can Reduce the Permeability. (C) Schematic of the Polymer (PVB) Backbone Structure

■

EXPERIMENTAL SECTION Materials. Aminopropyltrimethoxysilane (APTMS), tripentaerythritol, Butvar B-98 polymer matrix (having hydroxyl content about 18−20%, which is expressed as percent polyvinyl alcohol, acetate content of 0−2.5%, expressed as percent polyvinyl acetate, and butyral content of ∼80%, expressed as percent polyvinyl butyral) of molecular weight about 40 000− 70 000 g mol−1 and mesoporous silica [MSU-H, large pore 2D (pore volume 0.91 cm3 g−1 and pore size ∼7.1 nm) hexagonal, d = 2.6 g cm3 as specified by the manufacturer] were obtained from Sigma-Aldrich Company Ltd. (St. Louis, MO) USA. Reagent grade toluene was obtained from local suppliers (SD Fine Chem), which was distilled over pressed sodium and preserved under an inert atmosphere before use. Absolute ethanol (purity ≥99.9%) was obtained from local suppliers (SD. Fine Chem) and was used as a solvent for the composite preparation without further purification.

residual toluene. These dried functionalized silica powders were used for further characterization and composite fabrication. Step II: Composite Film Fabrication: Solution Casting. The composite fabrication was carried out by the solution casting method. A 5 g portion of PVB polymer (powder formstructure given in Scheme 1C) was added in 70 mL of ethanol (14 mL g−1), dissolved by stirring at 50 °C for 45 min. To the mixture, 0.01 g of the polyol (i.e., 5.37 μmol g−1 with respect to the PVB content and this amount was held constant for all the composites) was added and stirred at 60 °C for 2 h. Thereafter, various weight percentages (0.25, 0.5, and 1.5) of functionalized silica particles (with respect to the polymer matrix and were dried prior to the use) were added, stirred for 15 min, and sonicated for 30 min to assist in dispersion at room temperature (25 °C). The homogeneous solution was transferred into a Teflon coated mold, dried at 40 °C in ambient atmosphere to obtain transparent films and further dried under vacuum. As a control, a neat polymer film was prepared by following the same procedure described above, by dissolving PVB in ethanol at 50 °C and then casted. The films were then pulled out from the cast (average thickness ∼250 μm) and used for further studies. The pristine polymer, 0.25, 0.5, and 1.5 wt % of functionalized silica loading in the polymer are designated as PV0, PV1, PV2, and PV3.

■

COMPOSITE FABRICATION Step I: Synthesis of Amine Functionalized Silica. In order to modify the mesoporous silica surface, the particles were dried at 120 °C for 10 h. Then, the surface functionalization was carried out under dry toluene reflux (Scheme 1A).11,12 The mesoporous silica particles were dispersed in dry toluene for 30 min by sonication, ATPMS was added to the mixture and refluxed under inert atmosphere for 20 h. These treated particles were separated using centrifugation; thoroughly washed multiple times using toluene to remove the unreacted siloxanes; and then dried under vacuum (25 mm Hg) for 8 h at ∼100 °C to remove any

■

CHARACTERIZATION Perkin-Elmer (Spectrum GX) spectrometer was used to record the FTIR (Fourier transform infrared spectroscopy) spectra of both pristine and functionalized silica particles. The spectra 4384

dx.doi.org/10.1021/ie3022412 | Ind. Eng. Chem. Res. 2013, 52, 4383−4394


Article

Calcium Degradation Test. The moisture permeability was measured by the calcium degradation test. A calcium square (1 cm × 1 cm) of 150 nm thickness (which acts as a moisture sensor) was deposited over a cleaned glass slide using a shadow mask, inside a glovebox (MBRAUN, MB 200B, ultra high pure argon atmosphere, H2O < 10 ppm, O2 < 10 ppm), connected with a thermal evaporator (Sigma Instruments), under high vacuum of ∼10−6 mbar. Then aluminum, which acts as an electrode, was deposited (200 nm) on the top of the as deposited calcium layer through a shadow mask. Then the calcium/aluminum deposited glass substrate was covered with the composite films (of same thickness and prepared under similar conditions as discussed previously) and the sides were sealed with a glue (room temperature curing epoxy glue-Lapox L12) to stop the leakage through sides (an image of the setup is given in Figure S1see the Supporting Information). To verify the permeability through the glue, a control experiment was carried out by sealing a glass substrate, with the same glue under similar conditions as maintained for the sealing of the composites. The encapsulated glass slides were inserted in a programmable humidity chamber (RH = 95% and T = 35 °C, Kaleidoscope, India), and the resistance values were measured over time. The change in resistance (R), due to the degradation of the calcium is inversely proportional to the height of the remaining calcium layer (H).21

were recorded using potassium bromide (KBr) pellet method for both samples in the range 400 to 4000 cm−1 with a spectral resolution of 4 cm−1. Raman spectra were collected using NXR FT-Raman Module (ThermoScientific). Solid-state 29Si (one pulse experiment) and 13C CPMAS (cross-polarization magic angle spinning) for both pristine silica and functionalized silica were recorded on a 300 MHz Bruker DSX. In the case of 29Si, the relaxation delay was 12 s. In 13C, the spinning rate was 7 kHz, the contact time was 1200 μs, relaxation delay was 5 s, and decoupling sequence spinal 64 was used. The surface area of pristine and functionalized silica was measured by the Brunauer−Emmett−Teller (BET) nitrogen sorption method at 77 K with a Belsorp instrument from Japan. For the nitrogen adsorption/desorption experiments, samples were degassed at 130 °C for 6 h. The quantification of APTMS was carried out by elemental (CHN) analysis and thermogravimetric analysis (TGA). The TGA (Perkin-Elmer) analysis was carried out in order to evaluate the surface functionalization as well as the thermal stability of the composites, at a heating rate of 10 °C min−1. A Perkin-Elmer 2400 Series II CHNS/O system was used for elemental (CHN) analysis. In order to monitor the thermal history of the composites as well as the pristine polymer, a differential scanning calorimetry (DSC) technique was carried out on a Mettler Toledo (DSC 822e) instrument. Analysis was carried out in argon atmosphere at a flow rate of 80 mL min−1 in a hermitic aluminum pan at heating rates of 10 °C min−1 (the DSC instrument was calibrated using standard indium and zinc specimens). The tensile studies for PVB/functionalized silica composite were performed using Zwick/Roell instrument by 0.15 mm s−1 as test speed and average value is reported. ASTM D638 was used as a standard for the tensile studies. The morphologies of the cross-sectional surface of the fractured surfaces (after tensile experiment) of the pure polymer and the composites were observed in field emission scanning electron microscope (FESEM), in Sirion. In case of pristine silica, powder sample was directly coated over carbon tape and morphology was observed using scanning electron microscopy (SEM), which was carried out using ESEM Quanta. Prior to the SEM analysis, samples were sputter-coated with thin film of gold using JEOL (JFC-1100E) ion sputtering device. JEOL 2000 FX-II TEM (transmission electron microscope) was used to observe the size and morphology of the silica at an accelerating voltage of 200 kV. The specimen was dispersed in ethanol and ultrasonicated and drop casted on a copper grid with a carbon-reinforced plastic film and then dried. UV−visible spectra of the pristine polymer and composite films were obtained in Hitachi U 3000 in a wavelength, ranging from 200 to 1050 nm. Optical microscopy of the composites was taken in transmittance mode in Axiovert 200 MAT inverted microscope. Dynamic mechanical analysis (DMA) of the composites were carried out in MetraVib DMA+100, in tensile mode at a constant frequency of 1 Hz, in a temperature range from 50 to 100 °C. A static force of 0.5 Newton and dynamic force of 2 Newton was applied on the samples. The static contact angle measurement of the PVB composite films were carried out at 25 °C, in an OCA 30 goniometer (Dataphysics, Germany) containing stepper motor for controlling the volume (3 μL) of the liquid (Millipore water) supplied from a microsyringe. Five readings were taken for each sample in different positions. A programmable humidity chamber (Kaleidoscope, India, Model Number-KEW/PHC-80) was used for weathering study.

R ∝ (1/H )

(1)

The permeation rate of moisture (P) through the composite film is proportional to the slope of 1/R (conductance) over the measured period (T). This can be expressed as21 P = −n

M(H2O) M(Ca)

δρ

L d(1/R ) B dT

(2)

where n is molar equivalence for the reaction between water and calcium, M(H2O) is the molecular weight of water, and M(Ca) is the molar mass of calcium, δ is the density, and ρ is the resistivity of calcium. L and B are the length and width of the as deposited calcium. In this study, we have used the same value for both L and B. The swelling studies of the composites were carried out in deionized (DI) water (∼5 mL) at 25 °C. The composite films were cut into 8 mm × 8 mm (l × w) and were dipped in DI water for 36 h. The solvent (water) attached to the surface was removed using tissue paper, and the weight change was measured using an analytical microbalance (Essae Teraoka Limited with a resolution of 0.01 mg). Organic Device Encapsulation and Weathering Study. An organic device fabrication was carried out inside a glovebox. One weight percent of poly(3-hexylthiophene) (P3HT) (P3HT was synthesized in a previously reported method)22,23 was dissolved in chlorobenzene by stirring for 12 h, under inert atmosphere. Then, the solution was filtered using a 0.45 μm Nylon filter to remove agglomerates. The filtrate was used for device fabrication. At first 25.4 mm × 25.4 mm indium tin oxide (ITO) coated glass slides, obtained from Delta Technologies, Limited, USA, were pretreated with concentrated HCl to remove the ITO from the sides using a mask at the center. The ITO layer area in the middle of the slides becomes 15 mm × 25.4 mm. Then these ITO coated glass slides were cleaned using isopropanol and acetone, and then, they were 4385



Article

subjected to RCA1 solution (5:1:1 of H2O:H2O2:NH4OH). Finally they were rinsed with water and dried with dry argon. A solution (150 μL) of P3HT in chlorobenzene was spin coated (1000 rpm speed for 10 s) on a cleaned ITO coated glass slide. Then it was annealed at 120 °C for 10 min. Then aluminum (contact area of 4 mm2) electrodes were deposited over P3HT layer using a shadow mask (Figure S2, see the Supporting Information) by thermal evaporation inside a glovebox. Devices were encapsulated using the composite film as well as neat polymer film. The glue used for encapsulation was Lapox L12, which is the same glue as used for calcium test. The current−voltage (I−V) characteristics of the fabricated devices were measured using a source meter (Keithley, Model 2420). The accelerated aging of the devices were carried out inside a humidity chamber (Figure 2).

■

RESULTS AND DISCUSSION Fabrication of the Composite Films. In the present work, solution casted PVB film as well as PVB/amine functionalized silica based hybrid composite films were fabricated using ethanol as solvent. The amine functionalized silica particles were used for the reinforcement of the polymer matrix. The functionalization of mesoporous silica was carried out using aminopropyltrimethoxysilane as a coupling agent. In addition to functionalized silica particles, a polyoltripentaerythritolwas used for the composite matrix fabrication. Hence, it is envisioned that the synergistic effect (Scheme 1B) of the inorganic phase−silica particles and polyol, employed to fabricate the composite matrix, that can reduce the permeability of the water vapor.

■

Figure 1. FTIR spectra of (A) pristine and (B) functionalized silica.

PART I: CHARACTERIZATION OF AMINE FUNCTIONALIZED SILICA FTIR and Solid State NMR: 13C MAS NMR and 29Si NMR. The FTIR spectra of pristine and amine functionalized silica particles (Figure 1A and B) were carried out in order to validate the functionalization at the silica surface. In the spectra, a few new peaks were observed after functionalization which were absent in the pristine silica, which is due to the grafting of siloxane molecules onto the surface of the nanoparticles. In the FTIR (Figure 1A) spectra of pristine silica, the broad peak at 3436 cm−1 is due to the presence of −OH groups at the silica particle surface and peak at 1637 cm−1 is due to bending of −OH groups. After functionalization (Figure 1B), the peak observed at 2937 cm−1 is characteristic of asymmetric −CH2 stretching vibrations and at 1494 cm−1 is due to −CH2 bending (scissoring). The peak appearing at 1562 cm−1 is due to N−H bending (scissoring) vibrations. The Raman study (Figure S3 see the Supporting Information) also shows a peak around 2900 cm−1, characteristic of the aliphatic domain (C−H). The siloxane grafting was further analyzed by solid state NMR. Figure 2A shows the solid-state 29Si spectra of pristine mesoporous silica, which shows three signals at −100 (Q2geminal silanols), −109.7 (Q3-free silanols), and −114.9 (Q4siloxane groups) ppm.13 After functionalization, new peaks were observed (Figure 2B) in the range from −50 to −80 ppm indicating that the silica surface is chemically modified.24 For the functionalized silica, the peaks appearing at −58.5 (T2bidentrate) and −67.3 (T3-tridentrate) ppm can be assigned to the chemical grafting of the organic ligand in two different modes24 of the siloxane−APTMS.

In addition to solid state 29 Si NMR, 13C NMR was also carried out. Figure 2C shows a peak around 10.8 ppm, which is ascribed to the carbon atom directly bonded to silicon (−Si− CH2−). The peak at around 24.5 ppm corresponds to methylene carbon where as the peak observed around 44.8 ppm is due to the methylene group carbon atoms (−CH2− NH2) attached to the amine group. These results of solid state 29 Si NMR and 13C NMR indicate the grafting of APTMS at the surface25 and support the results obtained from FTIR. BET Isotherm Analysis. BET analysis shows a decrease in the Vm [the monolayer adsorption amountcm3 (STP) g−1], pore volume, C constant26 (related to the enthalpy of adsorption and a dimensionless quantity; the decrease in the C constant indicates a decrease in the interaction between adsorbate nitrogen molecules and the particle surface), and the surface area reduced by ∼84% after functionalization, which is due to the attachment of the APTMS at the surface. All the parameters obtained from BET analysis are as shown in Table 1. The adsorption/desorption isotherm (Figure 3A) resembles type IV and has an H2 type of loop due to the mesoporous nature of the particles as per IUPAC classification.27 After functionalization, the mesoporosity is retained, as observed in the BET adsorption/desorption curve for the functionalized silica. It can be also observed that the shape of hysteresis loops before and after functionalization are almost similar,13 which shows that the pore shape may not be significantly changed after siloxane functionalization. The morphology of the mesoporous silica (SEM image) and the TEM images of used silica are as shown in Figure 3B and C. 4386



Article

Figure 2. (A) 29Si NMR of pristine silica, (B) 29Si NMR of functionalized silica, and (C) 13C NMR spectra of functionalized silica.

Quantification of Surface Grafting: TGA and Elemental (C, H, and N) Analysis. Thermogravimetric analysis (Supporting Information Figure S4) of pristine and functionalized silica was used further to calculate the extent of grafting at the surface. Pristine silica shows some weight loss in the region of 40−125 °C, due to loss of adsorbed moisture. The huge weight loss of functionalized silica as compared to pristine silica of ∼17.2% in the region 125−730 °C is due to the degradation of the organic fragment. Surface APTMS groups were calculated to be 4.39 ± 0.3 μmol m−2, using the following equation28

Table 1. Various Parameters Obtained from the BET Surface Area Analyzer and CHN Analysis Experiments parameters surface area (aBET) [m2 g−1] total pore volume (P/Po = 0.99) [cm3 g−1] Vm [cm3 (STP) g−1] C mean pore diameter [nm] carbon (%) from CHN analysis hydrogen (%) from CHN analysis nitrogen (%) from CHN analysis

pristine silica

functionalized silica

679.6 0.9982 156.2 124.8 5.87 0.38 1.42

111.4 0.2894 25.6 33.4 10.39 12.20 3.09 4.43

Figure 3. (A) Adsorption/desorption isotherm of the pristine and functionalized silica, (B) SEM image of the neat silica, and (C) TEM image of neat silica before functionalization. 4387



Article

Figure 4. Stress/strain curves of the polymer and the composites.

Table 2. Mechanical, Thermal Properties and Contact Angle Values for PVB, Amine Functionalized SilicaPVB Composites sample

Young’s modulus (MPa)

PV0 PV1 PV2 PV3

6 11 9 10

± ± ± ±

yield stress (MPa) (avg)

1 2 1 1

Q TGA = WTGA /(MOF × Ssilica)

22.5 27.0 32.5 44.0

± ± ± ±

elongation at break (%) (avg)

degradation temperature (°C) −10%

contact angle (deg)

± ± ± ±

362 360 362 356

87 ± 1 85 ± 0.5 83.6 ± 0.3 82 ± 0.3

1 1 3 2

11 61 94 30

(3)

■

PART II: COMPOSITE CHARACTERIZATION

Tensile Studies of PVB−Amine Functionalized Silica/ PVB Composites and Fractured Surface SEM. The stress/ strain curves for neat polymer and the composites are as shown in the Figure 4, and the obtained Young’s modulus, yield stress, and the percent elongation at break values are tabulated in Table 2 (the tensile experiments were carried out at 25 °C). It is observed, that the yield stress increased from 22.5 to 44 MPa, for PV0 to PV3, which shows an increase of 2 times the yield stress value with respect to PV0. For PV1 and PV2, the increase in yield stress is in the range of 1.2 and 1.4 times, respectively, as compared to PV0. There exists an effective interaction and integrity between amine functionalized silica and the PVB matrix, and it transmits the tensile load effectively. It can be also observed that Young’s modulus increased from neat PVB (PV0) to PV3. The modulus values for PV1, PV2, and PV3 are almost similar showing that the materials are stiffened with the silica particle addition within the proportional limits.16 The percent elongation at break was 0.11 for PV0 and 0.94 for PV2. In comparison, PV1 and PV2, the decreasing strain percent value may be due to the brittleness of the PVB matrix upon the addition of excess silica particles due to agglomeration. This is also evident from the existence of plateaus, observed in PV3 (Figure 4) which is due to the possibility of slippage by functionalized silica fillers upon higher loadings in the composite matrix. The “agglomeration/cluster” effect in PV3

In the above equation, QTGA is the grafting density in micromoles per squared meter, WTGA is the weight loss (∼17.2%) obtained from TGA in the specified temperature range, MOF is the molecular weight of corresponding organic fragment, and Ssilica is the specific surface area of the used mesoporous silica (679 m2 g−1obtained from BET). Elemental (CHN) analysis indicates an increase in the carbon content (Table 1) as well as appearance of nitrogen content after functionalization. Carbon content as detected in pristine silica could be due to the adsorbed CO2. From CHN analysis, the grafting APTMS density was found to be 4.8 μmol m−2 by using the following equation28 QCHN = δC/(MC × NC × Ssilica)

2 1 3 2

(4)

CHN

Where Q is the grafting density in micromoles per squared meter, δC is the increase in carbon content (%), after functionalization, as observed (Table 1) from elemental (CHN) analysis. MC is the atomic weight of carbon, NC is the number of carbon atoms, and Ssilica is the specific surface area of used mesoporous silica (679 m2 g−1obtained from BET). The estimation from elemental (CHN) analysis (4.8 μmol m−2) shows a similar value obtained from TGA analysis (4.39 ± 0.3 μmol m−2), which are normalized with respect to surface area obtained from BET. 4388



Article

Figure 5. Fractured surface morphology of the pure polymer and the composites obtained from the tensile testing.

ethanol. TEM images showed the presence of aggregated silica in the composite matrix (Supporting Information Figure S7). Thermal and Optical Properties of the Composites: TGA, DSC and UV - Visible spectroscopy. Thermogravimetric analysis (Figure 6) of the composites shows that these

could be possible due to hydrogen bonding between the functionalized silica particles, containing unreacted −OH and surface decorated −NH2 groups as the number density increases inside the matrix (See the Supporting Information, Figure S5). The fractured surface (obtained from the tensile testing) images show (Figure 5) more cracks and cleavages in the composites compared to polymer, which could be due to the increased interfacial interaction between amine functionalized mesoporous silica and the polymer matrix. Thus the mechanical properties of the composites increased from PV0 to PV1 to PV2 but decreases for PV3 due to agglomeration as evidenced from the fractured surface image (Figure 5, PV3 (inset)). In order to verify the homogeneity of the functionalized silica in the composites, EDAX (energy dispersive analysis X-ray spectroscopy) was also carried out in the fractured surfaces of PV2. The Si relative wt % was found to be similar in various places (two EDAX spectra are shown in the Supporting Information, Figure S6). This indicates a relative uniformity in the silica content in the matrix. In addition, to verify the presence of silica in the composite matrix, TEM was carried out for the composite (PV3) after dissolving the composite in

Figure 6. Thermogravimetric analysis of the polymer and the composites. 4389



Article

composite materials are stable up to ∼360 °C (10% degradation). This is an important property for an encapsulation/passivation layer as it has to protect the device or important components over a long period of time. No improvement in the degradation temperature was observed as the loading was increased (see the inset of Figure 6, the high temperature region). Rather a significant decrease in the degradation temperature (356 °C) was observed for PV3 with respect to other compositions. This effect could be due to “agglomeration/cluster” formation of the silica particles inside the hybrid matrix and hence a loss of integrity, which is also reflected in the mechanical properties (Table 2), as discussed earlier. In addition, the basic amine groups at the surface of the silica particles may also catalyze the degradation, by the nucleophilic attack at the butyral fragments of the polymer. Since, in PV3 the relative amine group density is high due to higher functionalized silica loadings, so the reduction in the degradation temperature is more pronounced. DSC was carried out, in order to verify the effect of silica loadings on the thermal history of the composites. In the DSC analysis of the polymer as well as the composites, only glass transition temperature (Tg) at ∼73 °C was observed (Figure 7)

which could be due to the aggregation effect, as observed from the tensile studies. It can be also observed that the relative changes in the modulus values are small, which is due to the small relative increase of silica content in the composites. From DMA experiment, tan δ, which is a ratio of loss modulus to storage modulus [E″/E′], can be obtained. This is related to the damping coefficient of the material. At the glass transition (Tg) of a material, the tan δ (damping coefficient) value shows a maxima and hence a peak is observed in the curve of tan δ as a function of temperature. Hence the glass transition (Tg) temperatures of the composites can be observed from Figure 8 (tan δ vs temperature). It shows that the tan δ

Figure 8. Variation of tan δ with temperature.

value has shifted to a higher temperature with an increase in the silica loading (PV1 71.7 °C, PV2 72.6 °C, and PV3 74.6 °C). This could be due to the better integration between functionalized silica nanoparticle and the polymer matrix at higher temperature under mechanical force. The better integration may be due to the hydrogen bonding interaction between silica surface −NH2 groups and the polar components of the polymer matrix. This improvement in the Tg was not resolved from the DSC analysis of the composites. Contact Angle Measurement. Contact angle measurement values for the polymer and the nanocomposites are shown in the Table 2. On comparing with the neat polymer (87°), composites showed a reduced contact angle value (Figure 9). The reduction in the contact angle is due to the addition of hydrophilic amine decorated silica particles in the matrix, which enhances the wettability of the matrix. This enhanced wettability can be further exploited in development of sandwich/multilayered architecture with other polymer matrix. Surface Morphology of the Composites. Optical microscopy and scanning electron microscopy (SEM) were used to observe the surface topography of the films, which are shown in Figure S11 (see the Supporting Information). Images show the surface morphology and texture of the freestanding films. The images obtained from optical microscopy can be correlated with the images of SEM, which shows the film surface is not uniform. The increase in the surface roughness could be also responsible for the enhancement of the wettability as observed from the contact angle.29 Swelling Study in Water. The composites show no swelling or weight increase, after they were immersed in the

Figure 7. DSC thermogram of the polymer and the composites.

in the specified temperature range (25−250 °C). It can be also observed from DSC that Tg of the composites (PV1, PV2, PV3) did not change with respect to the neat polymer (PV0). Therefore, the increase in loading seems to have no effect on the glass transition temperature. The DMA analysis (discussed later) shows that Tg of the composites improves with the increase in loadings of functionalized silica. The DSC trace of the as received powdered PVB is shown in Figure S8 (Supporting Information). In the UV−visible spectra of the composites, a reduction of transmittance was observed with an increase in loading (Supporting Information Figure S9). This could be due to the increased silica particle density inside the matrix. DMA Analysis. The DMA results indicate an improvement in the E′ (storage modulus) which is related to the elastic properties of the material) from PV1 to PV2 (see the Supporting Information Figure S10), i.e. in the increase in the functionalized silica content. Due to the better integrity and homogeneity of the distribution of compatible functionalized silica in the polymer matrix, the storage modulus showed an improvement from PV1 to PV2. PV3 did not show any improvement in storage modulus as compared to PV1 and PV2, 4390



Article

the slope of (Ro/R) vs time varies at each point, the derivative (d(1/R)/dt) at every point was used and the permeability values at each time were calculated (Figure 11) depicting the

Figure 9. Contact angle values of the polymer and the composites films. Figure 11. Permeability value obtained from the slope of each point.

water for a period of ∼36 h. Similar behavior was previously observed for PVB/TiO2 composites.30 The samples were also kept in a humidity chamber (90% RH at 25 °C) for 72 h, and no increase in weight was observed. This swelling behavior is a critical property for the hybrid composites indicating that the hybrid is resistant to moisture and thus can be used for encapsulation. Permeability Measurement of the Composites: Calcium Degradation Test. Permeability studies of the composites were carried out in order to observe the effect of silica loadings in the matrix. The variation of the inverse of normalized resistance (Ro/R) with time for the composites as well as for the glue, at ≥95% RH and T = 35 ± 0.2 °C is shown in Figure 10. This degradation curve shows the moisture

moisture permeability behavior through the polymer. The average values of permeation rates at various time intervals are given in Table 3. The reduction in permeability is due to the Table 3. Average WVTR (g m−2 day−1) for the Composites at Various Time Intervals average WVTR (g m−2 day−1) composites PV PV PV PV

0 1 2 3

0−75 s

75−200 s

200−800 s

800 s−end

0.24 1.58 0.26 0.10

3.24 3.05 0.08 0.10

5.6 0.07 0.11

0.12 0.67

increase in number density of the mesoporous silica particles in the polymer matrix, which provides a resistance for the water vapor. For PV1, the permeability is higher (less extended torturous pathway) since the number density of silica is less. As the number density increases for PV2 and PV3, permeability decreases due to more extended torturous pathway, which retards the movement of moisture molecules. In addition, as the number density of functionalized silica increases, the number of amine groups also increases in the matrix. The amine group density of as prepared PV0, PV1, PV2, and PV3 can be calculated as 0, 37.37, 74.75, and 224.25 μmol, respectively. These values have been calculated from data obtained from TGA, which showed a surface APTMS grafting density as 4.39 ± 0.3 μmol m−2 and BET, which showed a surface area of 679.6 m2 g−1. In Figure 12, the variation of WVTR of the neat polymer matrix and the composites on the amine group density has been shown. This increase in the polar amine groups in the matrix also contributes to the relative reduction in the permeability of the composites. The polar −NH2 groups at the surface can also interact with moisture molecules through hydrogen bonding and can retard the diffusion through the matrix. In the case of comparison between PV0 and the composites, in addition to surface amine groups, the polyol also contributes to the reduction of permeability. The relative variation in

Figure 10. Ro/R plot for the two composites and the glue.

permeation behavior/rate through the composite matrix. With the increase in the functionalized silica content, a reduction in the moisture permeability was observed. The moisture permeability for the neat polymer was found to be higher than the composites. A huge reduction in permeability was observed in the composites from PV1 to PV2 and PV3. Since 4391



Article

Figure 12. WVTR of various composites with amine group density of the respective composites.

Figure 13. Diode characteristic of the devices with and without encapsulation after 0.5 h at 90% RH at 25 °C.

■

permeability is not significant between PV2 and PV3 (Table 3), which could be due to aggregation of functionalized silica, even though the amine group density increased in the matrix. After the permeability test, the sealed film was peeled out from the glass slide and SEM images were obtained for the degraded calcium (calcium oxides/hydroxides species formed after reaction with moisture, see Supporting Information Figure S12). This suggests that the permeation could be through pinhole mediated moisture flow channels. The low permeability at such a high percent RH and temperature (accelerated conditions) suggests that these composites can be used as a barrier material at ambient conditions at which the devices are operated for practical applications. Diode Characteristics of the Devices under Accelerated Weathering. In order to illustrate the composite as a moisture barrier material, organic device encapsulation was carried out. The encapsulated (with PV0 and PV3) and nonencapsulated organic Schottky structured diode devices (see Supporting Information Figure S2) were exposed to a RH of 90% at a temperature of 25 °C and current−voltage (I−V) characteristics were measured using a source-meter. After a time period of 0.5 h, the reduction in current densities with respect to the initial current densities was calculated in a voltage range of 0−1 V. The percentage change [100(Ii −If)/Ii; where Ii is the initial current density and If is the final current density] in the current density for each device was calculated with respect to the initial current density over the 0−1 V range (Figure 13). This gives the reduction in current density with time under aging condition (90% RH and 25 °C) for all the devices. It can be observed from Figure 13 that after 0.5 h, the relative reduction in current density is less (20 ± 5%) for the device encapsulated with PV3 than the diode, encapsulated with PV0 (55% reduction) within the voltage range from 0 to 1 V. While in the nonencapsulated device, current density drops down rapidly to almost 97 ± 1%. This is due to the moisture barrier composites, which protects the device from moisture permeation, leading to device failure. The behavior of PV3 and PV0 is inconsistent with the result observed from calcium degradation test, i.e. the silica loaded composite reduces the permeation of moisture and, therefore, shows lower reduction in current density of the diodes.

CONCLUSIONS The properties of PVB film and PVB/functionalized mesoporous silica hybrid composite films were investigated for potential use as an encapsulant for Schottky structured organic devices. Thermal analysis of the films shows no significant changes after functionalization but APTMS functionalized silica showed improved mechanical properties. The fracture surface morphology shows more crack and cleavages (observed after tensile testing) with an increase in the functionalized silica loadings as compared to the neat polymer matrix. It indicates that the filler is compatible with the polymer matrix (higher interfacial interaction between amine functionalized silica particles and the polymer matrix). However, higher loading leads to agglomeration and results in the brittleness of the films, as observed from tensile testing. The degradation in the properties was observed due to aggregation behavior; hence, an optimization in functionalized silica loading is important for application purpose. The study reveals that the optimum loading is PV2. After this composition, various properties started deteriorating, due to aggregation of functionalized silica. An increase in silica loadings showed a reduction in water vapor permeability. As compared to neat polymer PV0 (WVTR of 3.24 g m−2 day−1), more than one order reduction in WVTR at 35 °C has been observed in case of PV2 (WVTR of 0.08 g m−2 day−1) and PV3 (WVTR of 0.10 g m−2 day−1), as observed from calcium degradation test. The encapsulated devices were found to be stable than nonencapsulated devices under accelerated weathering condition. The device encapsulated with the neat polymer showed two times decrease in current density, as compared to the composite film encapsulated Schottky structured devices. The composite films did not show any swelling in water. Coupled with this property, an improved moisture barrier property of the composites as compared to the neat polymer (PV0) indicates that the composite material could serve as a good encapsulant. Furthermore, the hybrid layers can be integrated/sandwiched between two other polymer layers (like Surlyn, PET etc.) for further reduction in permeability.

■

ASSOCIATED CONTENT

S Supporting Information *

Figure S1: Calcium degradation test setup (A) and moisture sensing device for calcium degradation test (B). Figure S2: 4392



Article

of an Electroluminescent Polymer. J. Am. Chem. Soc. 1995, 117, 10194−10202. (8) 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.; Ederer, D. L. Photo-Oxidation of Electroluminescent Polymers Studied by Core-Level Photoabsorption Spectroscopy. Appl. Phys. Lett. 1996, 68, 2046−2048. (9) Burrows, P. E.; Graff, G. L.; Gross, M. E.; Martin, P. M.; Hall, M.; Mast, E.; Bonham, C. C.; Bennett, W. D.; Michalski, L. A.; Weaver, M. S.; Brown, J. J.; Fogarty, D.; Sapochak, L. S. Gas Permeation and Lifetime Tests on Polymer-Based Barrier Coatings. Proc. SPIE 2001, 4105, 75−83. (10) Lewis, J. S.; Weaver, M. S. Thin-film permeation-barrier technology for flexible organic light-emitting devices. IEEE J. Sel. Top. Quantum Electron. 2004, 10, 45−57. (11) Gupta, S.; Ramamurthy, P. C.; Madras, G. Synthesis and Characterization of Flexible Epoxy Nanocomposites Reinforced with Amine Functionalized Alumina Nanoparticles: A Potential Encapsulant for Organic Devices. Polym. Chem. 2011, 2, 221−228. (12) Gupta, S.; Ramamurthy, P. C.; Madras, G. Covalent Grafting of Polydimethylsiloxane over Surface-Modified Alumina Nanoparticles. Ind. Eng. Chem. Res. 2011, 50, 6585−6593. (13) Gupta, S.; Ramamurthy, P. C.; Madras, G. Synthesis and Characterization of Silicone Polymer/Functionalized Mesostructured Silica Composites. Polym. Chem. 2011, 2, 2643−2650. (14) Gupta, S.; Ramamurthy, P. C.; Madras, G. Mechanistic Overview of the Curing Behavior of Hydride Terminated Polydimethylsiloxane with Allyl Functionalized Alumina by Calorimetry and Rheometry. Thermochim. Acta 2011, 524, 74−79. (15) Lin, J. S.; Chung, M. H.; Chen, C. M.; Juang, F. S.; Liu, L. C. Microwave-Assisted Synthesis of Organic/Inorganic Hybrid Nanocomposites and their Encapsulating Applications for Photoelectric Devices. J. Phys. Org. Chem. 2011, 24, 193−202. (16) Dhaliwal, A. K.; Hay, J. N. The Characterization of Polyvinyl Butyral by Thermal Analysis. Thermochim. Acta 2002, 391, 245−255. (17) Nakane, K.; Kurita, T.; Ogihara, T.; Ogata, N. Properties of Poly(vinyl butyral)/TiO2 Nanocomposites Formed by Sol−Gel Process. Compos. Part B: Eng. 2004, 35, 219−222. (18) Zhang, F. A.; Lee, D. K.; Pinnavaia, T. J. PMMA/Mesoporous Silica Nanocomposites: Effect of Framework Structure and Pore Size on Thermomechanical Properties. Polym. Chem. 2010, 1, 107−113. (19) Jin, J.; Lee, J. J.; Bae, B. S.; Park, S. J.; Yoo, S.; Jung, K. H. Silica Nanoparticle-Embedded Sol−Gel Organic/Inorganic Hybrid Nanocomposite for Transparent OLED Encapsulation. Org. Electron. 2012, 13, 53−57. (20) Jung, K.; Bae, J. Y.; Park, S. J.; Yoo, S.; Bae, B. S. High Performance Organic Inorganic Hybrid Barrier Coating for Encapsulation of OLEDs. J. Mater. Chem. 2011, 21, 1977−1983. (21) Paetzolda, R.; Winnacker, A.; Henseler, D.; Cesari, V.; Heuser, K. Permeation Rate Measurements by Electrical Analysis of Calcium Corrosion. Rev. Sci. Instrum. 2003, 74, 5147−5150. (22) McCullough, R. D. The Chemistry of Conducting Polythiophenes. Adv. Mater. 1998, 10, 93−116. (23) Kumar, P.; Ranjith, K.; Gupta, S.; Ramamurthy, P. C. Electrochemical Copolymerization of Thiophene Derivatives; A Precursor to Photovoltaic Devices. Electrochim. Acta 2011, 56, 8184−8191. (24) Bauer, F.; Ernst, H.; Decker, U.; Findeisen, M.; Glasel, H. J.; Langguth, H.; Hartmann, E.; Mehnert, R.; Peuker, C. Preparation of Scratch and Abrasion Resistant Polymeric Nanocomposites by Monomer Grafting onto Nanoparticles, 1 FTIR and Multi-Nuclear NMR Spectroscopy to the Characterization of Methacryl Grafting. Macromol. Chem. Phys. 2000, 201, 2654−2659. (25) Rana, S.; Mallick, S.; Parida, K. M. Facile Method for Synthesis of Polyamine-Functionalized Mesoporous Zirconia and Its Catalytic Evaluation toward Henry Reaction. Ind. Eng. Chem. Res. 2011, 50, 2055−2064.

Schematic for weathering study of devices. Figure S3: Raman spectra of neat and functionalized silica. Figure S4: thermogravimetric analysis of neat and functionalized silica. Figure S5: possible inter particle hydrogen bonding interactions. Figure S6: EDAX spectras of PV2 in various regions and compositional analysis. Figure S7: TEM image of PV3 after dissolution in ethanol. Figure S8: DSC thermogram of as received powdered PVB. Figure S9: UV−visible spectra of the polymer and the composites. Figure S10: Variation of storage modulus (E′) with temperature. Figure S11: SEM images (top) of the films and optical microscopy images (bottom) of neat polymer and the composites. Figure S12: SEM image of the degraded calcium layer after the calcium degradation test: a fingerprint for permeation pathway. This material is available free of charge via the Internet at http://pubs.acs.org/.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91-80-23600472. Tel.: +91-80-2293-2627. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS S.G. acknowledges the Council of Scientific and Industrial Research (C.S.I.R), New Delhi, for financial support and fellowship. The authors gratefully acknowledge Dr. Roy Mohapatra and Mr. Harsabardhan of the Aerospace Engineering Department, Indian Institute of Science, Bangalore, for DMA studies, Tribology lab, Mechanical Engineering department, for technical support, and Dr. G. S. Avadhani for TEM studies, Department of Materials Engineering, Indian Institute of Science, Bangalore, and Indian Institute of Science, Bangalore. In addition, the authors gratefully acknowledge the financial support from DST No SR/S3/ME/022/2010-(G) and technical support from IISc advanced facility for microscopy and microanalysis (AFMM), and NMR research centre.

■

REFERENCES

(1) Chen, C. H.; Cheng, W. T.; Tsai, M. L.; Huang, K. T. Fabrication and Performance of Copper Phthalocyanine/Squaraine Dye/Perylene Composite with Bulk Heterojunctions by the Solution Blending Process. Ind. Eng. Chem. Res. 2012, 51, 3630−3638. (2) Lu, X. W.; Wu, W.; Chen, J. F.; Zhang, P. Y.; Zhao, Y. B. Preparation of Polyaniline Nanofibers by High Gravity Chemical Oxidative Polymerization. Ind. Eng. Chem. Res. 2011, 50, 5589−5595. (3) Watanabe, S.; Fukuchi, Y.; Fukasawa, M.; Sassa, T.; Uchiyama, M.; Yamashita, T.; Matsumoto, M.; Aoyama, T. Electron Donor and Acceptor Spatial Distribution in Structured Bulk Heterojunction Photovoltaic Devices Induced by Periodic Photopolymerization. Langmuir 2012, 28, 10305−10309. (4) Smith, P. F.; Gerroir, P.; Xie, S.; Hor, A. M.; Popovic, Z.; Hair, M. L. Degradation of Organic Electroluminescent Devices. Evidence for the Occurrence of Spherulitic Crystallization in the Hole Transport Layer. Langmuir 1998, 14, 5946−5950. (5) 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. (6) Pietro, R. D.; Fazzi, D.; Kehoe, T.; Sirringhaus, H. Spectroscopic Investigation of Oxygen and Water Induced Electron Trapping and Charge Transport Instabilities in n-Type Polymer Semiconductors. J. Am. Chem. Soc. 2012, DOI: 10.1021/ja304198e. (7) Scurlock, R. D.; Wang, B.; Ogilby, P. R; Sheats, J. R.; Clough, R. L. Singlet Oxygen as a Reactive Intermediate in the Photodegradation 4393



Article

(26) Brunaur, S.; Emmett, P. H.; Teller, E. J. Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc. 1938, 60, 309−319. (27) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Reporting Physisorption Data For Gas/Solid Systems - With Special Reference To The Determination of Surface Area and Porosity. Pure Appl. Chem. 1985, 57, 603−619. (28) Bordelanne, O.; Delville, M. Metal Oxide Modification via Transition Metal Complexes: Hybrid Materials Characterizations and Potential Applications in Molecular Recognition. Sol. Stat. Sci. 2002, 4, 851−858. (29) Gaume, J.; Taviot-Gueho, C.; Cros, S.; Rivatona, A.; Thérias, S.; Gardette, J. L. Optimization of PVA Clay Nanocomposite for UltraBarrier Multilayer Encapsulation of Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2012, 99, 240−249. (30) Nakane, K.; Ohashi, J.; Suzuki, F. Preparation and Properties of a Composite of Poly(vinyl butyral) and Titania. J. Appl. Polym. Sci. 1999, 71, 185−188.

4394


Inorganic Films as a Moisture

Recommend Documents