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J. Phys. Chem. C 2010, 114, 14446–14452
Enhancement of the Dielectric Constant and Thermal Properties of r-Poly(vinylidene fluoride)/Zeolite Nanocomposites Ana Catarina Lopes,†,‡ Marco P. Silva,‡ Renato Gonc¸alves,† Manuel F. R. Pereira,§ Gabriela Botelho,† Anto´nio M. Fonseca,† Senenxtu Lanceros-Mendez,*,‡ and Isabel C. Neves*,† Departamento de Quı´mica, Centro de Quı´mica and Departamento de Fı´sica, Centro de Fı´sica, UniVersidade do Minho, Campus de Gualtar, 4170-057 Braga, Portugal, and Laborato´rio de Cata´lise e Materiais (LCM), Laborato´rio Associado LSRE/LCM, Departamento de Engenharia Quı´mica, Faculdade de Engenharia, UniVersidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal ReceiVed: June 9, 2010; ReVised Manuscript ReceiVed: July 21, 2010
Different commercial Y zeolites derivate from faujasite structure with similar total Si/Al ratio, NaY (Si/Al ) 2.83), HY (Si/Al ) 2.80), and HUSY (Si/Al ) 3.00), but different framework Si/Al ratios were used to prepare R-poly(vinylidene fluoride) (R-PVDF)/zeolite nanocomposites. Structural and textural characterization of different Y zeolites was obtained by XRD, FTIR, elemental analysis and nitrogen adsorption isotherms. Nanocomposite films were prepared containing 14% (w/w) of zeolite and characterized in terms of the dielectric response and thermal stability. The best dielectric properties were obtained with NaY because of its smaller framework Si/Al ratio, which means a larger number of sodium in the zeolite structure. For this zeolite, the dielectric constant was increased by a factor higher than two, maintaining unaltered the dielectric loss for frequencies higher than 0.5 MHz. The zeolite HUSY, which presents an extended mesoporosity, shows the lowest onset temperature for thermal degradation. To clarify the origin of the dielectric response of the nancomposites, zeolites with different sodium amounts were obtained by ion exchange treatment using NaNO3 for HY and NH4NO3 for NaY. The dielectric constant was enhanced with increasing sodium content in the zeolite framework. Introduction The use of piezoelectric materials has been increasing for sensors and actuators applications because of their ability to couple electrical and mechanical signals.1 Composite materials are the preferred solution in many of those applications because the electromechanical properties can be tuned to the desired values for specific applications.2 Among the electroactive polymers, poly(vinylidene fluoride) (PVDF) is still the one with the most interesting piezoelectric response.1,3 Strong research efforts are being performed in the preparation of new PVDFbased composites and nanocomposites aiming to improve the electromechanical coupling, piezoelectric coefficient, and dielectric constant of the polymer. Within these efforts, the increase in the dielectric constant is one of the necessary steps to improve performance of the material for the above-mentioned applications. The outstanding piezoelectric and dielectric properties of PVDF already generated various applications in the field of sensors, actuators, and energy generation/storage.4,5 It is a semicrystalline polymer exhibiting different crystalline phases.4 The electroactive properties and the dielectric response strongly depend on the crystalline phase or phases present in the polymer.4,5 Depending on the phase present on the material, the dielectric constant of PVDF can be up to 12, and the piezoelectric d33 coefficient can be in the range from -20 to -36 pC/N.6 These * Corresponding authors. E-mail:
[email protected] (S.L.-M.);
[email protected] (I.C.N.). † Departamento de Quı´mica, Centro de Quı´mica, Universidade do Minho. ‡ Departamento de Fı´sica, Centro de Fı´sica, Universidade do Minho. § Universidade do Porto.
values are quite high for polymers7 but are still low when compared with the values obtained for electroactive crystals and ceramics.8 Polymers show some advantages with respect to the above-mentioned materials: they are flexible, easy to tailor, and can be produced in large sizes and a large variety of formats and shapes.9 In this way, the increase in the dielectric and electroactive characteristics of polymers is an important technological and scientific issue.10,11 The methods based on the doping of polymers with preformed nanoparticles have become intensively investigated in recent years because they allow the production of engineering nanocomposites with well-defined properties.11-16 The dielectric properties have been enhanced with different nanofillers such as metallic nanoparticles,17 carbon nanotubes,18 or nanofibers19 and clays.20,21 In this respect, zeolites as a guest have a great potential for increasing the dielectric properties because of their inherent structural characteristics.22 Zeolites are crystalline hydrated aluminosilicates materials whose crystalline structure is formed by channels and cavities of strictly regular dimensions called micropores.23 The pore size is comparable to that of small molecules, allowing them to reach the acid sites located inside the zeolite structure while hindering the access of bulky molecules.24 A net negative charge, which arises on the zeolite framework, has to be neutralized by the presence of cations within the pores. These cations may be any of the metals or metal’s complexes or alkylammonium cations.23 Zeolites find broad application in heterogeneous catalysis and polymer catalytic degradation and also attract interest in materials science for the development of functional materials and in nanotechnology.25-28 The most important examples of these tridimensional zeolite structures are the faujasite structures,
10.1021/jp1052997 2010 American Chemical Society Published on Web 08/10/2010
R-Poly(vinylidene fluoride)/Zeolite Nanocomposites SCHEME 1: Faujasite Structure of Y Zeolite29
X and Y.29 The zeolite Y used in this work is a crystalline microporous aluminosilicate based on sodalite cages joined by O bridges between the hexagonal faces. Eight sodalite cages are linked together, forming a large central cavity or supercage with a diameter of 13 Å. The supercages share a 12-membered ring with an open diameter of 7 Å (Scheme 1).29 The most common way to obtain electroactive β-phase PVDF, used in technological application, is by stretching from the R-phase PVDF;6 also, because of its intrinsic interest, it is important to perform a preliminary study on how the different zeolite properties influence the dielectric properties of R-PVDF. In this work, R-PVDF/zeolite nanocomposite films with Y zeolites (NaY, HY, and HUSY) have been prepared by solvent casting. Two of these Y zeolites were subjected to an ion exchange treatment using NaNO3 for HY and NH4NO3 for NaY and then used for the preparation of PVDF/zeolite nanocomposites.30 Dielectric behavior and thermal stability of the nanocomposite films were evaluated. Previously, the structural and the textural properties of Y zeolites were characterized by X-ray powder diffraction, elemental analysis, infrared spectroscopy and N2 adsorption isotherms. Experimental Section Materials and Reagents. PVDF in powder form (Solef 1010) with a density of 1.78 g/cm3 was supplied by Solvay (Belgium). All Y zeolites were obtained from Zeolyst International in powder form and were calcined at 500 °C during 8 h under a dry air stream prior to nanocomposite preparation. The zeolite USY (ultrastabilized Y, CBV 500) was available in the ammonium form. After heating, the ammonium is transformed in NH3 and H+. The NH3 desorbs, and the presence of the protons increases the number of acid sites. The protonic form of USY (HUSY) was obtained after this calcination. The other zeolites were available in the proton form for HY (CBV 400) and sodium form for NaY (CBV100). Chemicals for ion exchange treatment (NaNO3 and NH4NO3) were purchased from Aldrich. The solvent used in this study, N,N-dimethylformamide (DMF), was purchased from Aldrich (analytical grade) and previously dried using molecular sieves. Ion Exchange Treatment in the Zeolites Y. The ion exchange treatment in the zeolites Y was performed according to a previously published procedure.30 The zeolites Y used have similar Si/Al atomic ratios and were used as the starting materials: NaY (Si/Al ratio ) 2.83) and HY (Si/Al ratio ) 2.80). The samples were prepared with 1.0 M solutions of the appropriate nitrate (NaNO3 for HY and NH4NO3 for NaY). The zeolites obtained by ion exchange were calcined at 500 °C during 8 h under a dry air stream. These zeolites were designated H(Na)Y from HY and Na(H)Y from NaY.
J. Phys. Chem. C, Vol. 114, No. 34, 2010 14447 Preparation of the r-PVDF/Zeolite Nanocomposites. The preparation of nanocomposite films was performed by solvent casting method.31 Nanocomposite films with thickness around 40-50 µm were obtained by spreading a solution of 1.0 g of PVDF with a suspension of 0.14 g of different Y zeolites in 4 mL of DMF into a glass slide. The suspension was previously placed in an ultrasound bath for 4 h for homogenization. After this period of time, the polymer was added to the suspension and kept during 1 h under 100 rpm with a magnetic stirrer. The resulting nanocomposites films were kept in an oven at a controlled temperature of 210 °C for 10 min. Finally, the nanocomposite films were removed from the oven and cooled to room temperature. The polymer obtained by this procedure is R-PVDF with 14% (w/w) of the zeolite Y.32 Characterization Procedures. Elemental chemical analysis (Si, Al and Na) of the zeolite samples was performed by inductively coupled plasma atomic emission spectroscopy (ICPAES) using Philips ICP Spectrometer (PU 7000). Phase analysis of the zeolite samples was obtained by X-ray diffraction (XRD) using a Philips analytical X-ray model PW1710 BASED diffractometer system. Scans were taken at room temperature using Cu-KR radiation in a 2θ range between 5 and 60°. The textural characterization of the zeolites was based on the N2 adsorption isotherms determined at 77 K using a Quantachrome Instruments Nova 4200e apparatus. The samples were previously outgased at 423 K under vacuum. The micropore volumes (Vmicro) and mesopore surface areas (Smeso) were calculated by the t-method.33 We calculated surface areas by applying the BET equation.33 Mesopores size distributions were obtained from the desorption branch of the isotherm using the Barrett, Joyner, and Halenda (BJH) method.33 Fourier transform infrared (FTIR) spectra of the zeolites in KBr pellets were measured using a Bomem MB104 spectrometer in the range of 4000-500 cm-1 by averaging 20 scans at a maximum resolution of 4 cm-1. The thermal stability of the nanocomposite films as well as the pure polymer was studied in a Perkin-Elmer instrument, Pyris1TGA, controlled by a PC under the Windows operating system. The atmosphere used was high purity nitrogen (99.99% minimum purity) with a flow rate of 20 mL/min according to the equipment specifications. The sample holders used were crucibles of alumina oxide, supplied by Perkin-Elmer, in which ∼3 mg sample was degraded. The sample temperature was measured with a thermocouple located at the crucible. To obtain the actual temperature in the crucible, we performed a calibration procedure using the internal standards (i.e., alumel, nickel, and perkalloy) in all experiments. The samples were heated between 25 and 800 at 10 °C/min to evaluate the thermal stability. The capacity and tan δ, dielectric loss were measured with an automatic Quadtech 1929 Precision LCR meter at room temperature in a home-built sample holder. From the capacity values and the sample geometrical characteristics, the real part of the dielectric constant, ε′, was obtained. The applied signal for several frequencies in the range of 100 Hz to 1 MHz was 0.5 V. The samples were coated by sputtering with circular Au electrodes of 5 mm diameter onto both sides of the sample. Results and Discussion Physicochemical Properties of the Zeolites. The faujasite zeolite structure was evaluated by XRD. The powder XRD diffraction patterns (not shown) of NaY, HY, and HUSY were recorded at 2θ values between 5 and 60°. All zeolites exhibited the typical and similar pattern of highly crystalline faujasite zeolite structure. We estimated the relative crystallinity of HY and HUSY zeolites by comparing the peak intensities of the
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TABLE 1: Chemical and Structural Characterization of Y Zeolites a
Si/Al Si/Alb Si/Alc EFALd Na (%)a Na (UC)e nA1 × 1020 (sites · g-1)f particle size (nm)g crystallinity (%)g
NaY
HY
HUSY
2.83 2.80 2.82 0 7.76 47.2 2.8 10 100
2.80 4.05 4.10 17 1.95 10.7 14.0 9 75
3.00 3.90 3.96 11 0.12 0.7 20.5 9 80
a Total Si/Al ratio and sodium amount determined from ICP-AES. b Framework Si/Al ratio determined from XRD. c Framework Si/Al ratio determined from FTIR. d EFAL is the number of extra framework aluminum species drawn from the framework Si/Al ratio of the zeolites.35 e Number of sodium ions drawn from the unit cell formula of the zeolites. f nA1 is the theoretical number of acid sites drawn from the unit cell formula of the zeolites obtained by XRD and chemical analysis. g Determined from XRD analysis.
samples with NaY used as a standard sample (100% crystalline). The total intensities of the six peaks assigned to [3 3 1], [5 1 1], [4 4 0], [5 3 3], [6 4 2], and [5 5 5] reflections were used for the comparison according to ASTM D 3906-80 method. The HY and HUSY XRD patterns present over 70% of crystallinity. The zeolites differ in terms of Si/Al ratio, sodium amount, and acidity. The total Si/Al ratio was determined by ICP-AES, and framework Si/Al ratio was calculated by FTIR and XRD. The infrared region between 570 and 600 cm-1 shows the most sensitive band in the zeolite Y structure and can be used to calculate framework Si/Al ratios34 using eq 1
x ) 3.857 - 0.00621wDR (cm-1)
(1)
where x ) [1 + (Si/Al)]-1, with 0.1 < x < 0.3, and wDR is the zeolite-specific double-ring vibration mode between 570 and 600 cm-1. We obtained the framework Si/Al ratio by XRD from the calculated unit cell parameters by using the Breck and Flanigen equation (eq 2)35
NAl ) 115.2(a0 - 24.191)
(2)
where NAl is the framework aluminum number and a0 is the cell parameter. The unit cell parameters were calculated from the values of the [5 3 3], [6 4 2], and [5 5 5] reflection peaks according to the ASTM D 3942-80 method. XRD data can be used to estimate the particles size using the Debye-Scherrer equation.36
D)
Kλ B cos θ
(3)
where D is the crystal diameter, K is a constant (0.9), λ is the X-ray wavelength, B is the full width at half-maximum of the peak in radians, and θ is the Bragg angle. The average particle size of the zeolite Y was estimated from the most intense reflection peak [5 3 3] position of the [h k l]. Table 1 shows the total and framework Si/Al ratios calculated by the methods mentioned above.
Figure 1. Nitrogen adsorption-desorption equilibrium isotherms of the different zeolites at 77 K: ([) NaY, (2) HY, and (*) HUSY.
All Y zeolites used in this work present similar total Si/Al ratio. However, the difference between the Si/Al ratios determined by FTIR and XRD (framework) and those determined by chemical analysis (total) indicates an irregular distribution of silicon and aluminum throughout the zeolite structure. In the case of NaY, the total and framework Si/Al ratios are similar, which means that both species are tetrahedrally coordinated in the framework. For HY and HUSY, the framework Si/Al ratio is higher than the total Si/Al ratio, indicating the presence of extra-framework alumina species (EFAL).37 Therefore, in HY and HUSY, the number of negative charges in the zeolite framework is lower than in NaY, which means a lower number of ions (Na+ or H+) that is necessary to maintain the electroneutrality of the solid. The sodium amount and the acidity of the zeolites are also very different. These properties of the zeolite structure can play an important role in the dielectric response and in the polymer catalytic degradation.26,37 NaY presents the lower theoretical number of acid sites due to the higher amount of sodium in their structure, as indicated in Table 1. However, both H forms of Y zeolites (HY and HUSY) show lower amount of sodium and exhibit a higher theoretical number of acid sites due to the presence of a higher number of Brønsted acid sites in their structures. Finally, the particle size of all Y zeolites determined by XRD is of the same order of magnitude. The nitrogen adsorption-desorption equilibrium isotherms at 77 K for the NaY, HY, and HUSY zeolites are illustrated in Figure 1. The N2 adsorption isotherms for all zeolites are of Type-I isotherm, according the IUPAC classification, which is typical of solids with microporous structure.26 The shapes of both adsorption and desorption isotherms of the zeolites Y are very similar to each other. However, the existence of an hysteresis loop observed in the isotherm of the zeolite HUSY suggests that this zeolite has an extended degree of mesoporosity. We calculated the micropore volumes (Vmicro) and mesopore surface areas (Smeso) by the t method, and we calculated the total surface areas by applying the BET equation (SBET). The mesopore volume (Vmeso) was calculated as the difference between the total pore volume for P/P0 ) 0.986 (VP/P0)0.986) and the micropore volume. These values are summarized in Table 2. In fact, the mesopores size distribution (Figure 2) confirms that the HUSY sample presents larger pores than the other zeolites. Contrary to the other samples, the HUSY zeolite presents a significant amount of mesopores with a radius larger than 50 Å.
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TABLE 2: Textural Properties of the Zeolites Y SBET (m2/g) Vmicro (cm3/g) Smeso (m2/g)a VP/P0)0.986 (cm3/g) Vmeso (cm3/g) a
NaY
HY
HUSY
787 0.347 18.9 0.382 0.035
665 0.302 25.0 0.349 0.047
750 0.269 23.6 0.355 0.086
From t plot.
Figure 2. Mesopore size distribution profiles of the zeolites: NaY ([), HY (2), and HUSY (*).
TABLE 3: Chemical and Structural Analysis of the Modified Zeolites samples
Si/Ala
Si/Alb
Na (%)a
Na (UC)c
particle size (nm)b
H(Na)Y Na(H)Y
2.94 2.89
4.19 3.09
2.24 2.52
12.3 15.3
9 9
a Total Si/Al ratio and sodium amount determined from ICP-AES.30 b Framework Si/Al ratio determined from XRD.30 c Number of sodium ions drawn from the unit cell formula of the zeolites.
NaY and HY zeolites were modified by ion exchange treatment to understand the effect of sodium amount on the dielectric response of the nanocomposites. The chemical and structural properties of the modified zeolites were characterized by XRD and chemical analyses recorded under the same conditions as those used for NaY and HY (Table 3). All samples exhibited the typical and similar XRD pattern of highly crystalline Y zeolite. The XRD pattern of modified samples showed no reduction in the intensity of the peaks and no variation in the zeolite lattice parameters after the ion exchange process, suggesting full crystallinity retention of the starting zeolite Y.30 However, the ion exchange treatment slightly affects the Si/Al ratio of the zeolites. The modified zeolites present different Si/Al ratios compared with the ratios determined by XRD and those determined by chemical analysis, indicating an irregular distribution of silicon and aluminum throughout the zeolite structure.30 The amount of sodium depends on the ion exchange treatment used in the starting zeolites. In the case of HY, when a sodium salt solution is used to transform to proton-type zeolite (HY) in the sodium-type Y zeolite H(Na)Y, the amount of sodium increases after the treatment, as expected. The amount of sodium decreases when the sodium-type Y zeolite (NaY) is transformed into proton-type zeolite Na(H)Y. After the ion exchange treatment in NaY, the loss of sodium is very pronounced, reaching 67%. This result can be explained by the selectivity of the zeolite Y for proton exchange because each cation compensates the negative charge arising from an Al atom in the framework.37
The particle size of NaY and HY (Table 1) and those of modified zeolites are of the same order of magnitude, which gives evidence of the preservation of the zeolite structure. Therefore, the ion exchange treatment described in our previous work30 did not modify the zeolite structure. Morphological and Dielectric Characterization of the PVDF/Zeolite Nanocomposite Films. All nanocomposite films were prepared with a zeolite concentration of 14% (w/w). The zeolite amount was chosen in a way to allow the study of the zeolite effect in PVDF electrical response and thermal stability but not enough to affect the original spherulitic microstructure of PVDF. Figure 3 shows the SEM micrographs for (a) R-PVDF, (b) NaY, and (c) R-PVDF/NaY. Figure 3a shows the typical spherulitic microstructure of the polymer19 in its R-phase, and Figure 3b presents a typical morphology of the faujasite structure of NaY. The SEM micrograph of the nanocomposite (Figure 3c) shows that a good dispersion of the zeolites was achieved. The addition of zeolite does not change the spherulitic microstructure of the polymer, and the resulting nanocomposites show the well-defined particles of the NaY. The R-phase of the polymer was confirmed by FTIR.32 Effect of the Different Zeolites. Figure 4 shows the dielectric measurements performed on the pure polymer and the nanocomposite films. The variations of the ε′, real part of the dielectric function, with frequency for the R-PVDF and the nanocomposite films are presented in Figure 4a. In all cases, there is no change in the frequency dependence of the dielectric response, but a general increase in the value of ε′ can be observed for the nanocomposites with respect to the pure R-PVDF polymer sample (6.86). NaY zeolite shows the highest dielectric constant (doubling the value of the pure polymer), followed by HY and HUSY zeolites. The effect on the dielectric response of the different zeolites in the PVDF matrix is illustrated in Table 4 for a frequency of 10 kHz. With respect to the tg δ, dielectric loss (Figure 3b, Table 4), the R-PVDF/HUSY is the nanocomposite with the lowest loss, equivalent to R-PVDF. The R-PVDF/NaY shows higher dielectric losses at lower frequencies but remains at the same level as PVDF for frequencies higher than 5 × 105 Hz. It is essential to notice the importance of this result because a relevant increase in the dielectric constant is achieved without any increase in the dielectric loss for frequencies higher that 5 × 105 Hz. Finally, the R-PVDF/HY nanocomposite shows higher losses in the entire frequency range. This behavior is related to the different microstructural properties of the zeolites. In this context, it is important to notice that the dielectric constant for the zeolites shows a similar frequency dependence as the one presented in Figure 4 for the composites, with ε′ ranging from ∼4 to ∼2 (e.g., 4.19 at 10 kHz). In this case, the common models used for the description of the dielectric behavior of dielectric-dielectric composites including high dielectric constant filler38 are not applied. In this study, the effect seems to be related to confined mobility of charged ions in confined space of the micropores or to interfacial polarization effects. The electrical force originated by the application of the electric field is reflected in a stretching of ionic bond and consequently an induced dipole moment. Some cations are able to overcome the negative electrostatic attraction and move within the zeolite structure. This fact probably leads to the buildup of interfacial polarization because of the charges that accumulate within the zeolite structure.39
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Figure 3. Scanning electron micrographs (SEM) of (a) R-PVDF, 1000×; (b) NaY, 5000×; and (c) R-PVDF/NaY nanocomposite, 1000×.
Figure 4. (a) Real part of the dielectric constant and (b) dielectric loss obtained for the pure polymer and the different nanocomposite films: (b) PVDF, ([) PVDF/NaY, (2) PVDF/HY, and (*) PVDF/ HUSY.
TABLE 4: Effect of the Different Types of Zeolites at a Concentration of 14 wt % on the Dielectric Response of the Nanocomposite Films at 10 kHz samples
R-PVDF
R-PVDF/NaY
R-PVDF/HY
R-PVDF/HUSY
ε′ tg δ (× 10-2)
6.86 2.43
17.92 8.97
13.79 9.59
9.61 4.33
In this view, the lower dielectric constant of HY and HUSY nanocomposites with respect to NaY nanocomposite is related to the lower number of counterions that is necessary to maintain the electroneutrality. Furthermore, the difference in values of Si/Al between these zeolites does not seem to be the only factor involved in explaining the different dielectric performance of these materials, which leads us to conclude that the higher number of protons in the HY and HUSY zeolites has a lower effect on the dielectric response than the sodium ions in NaY. The actual effect on the dielectric response of the sodium amount is presented in the next section.
A similar behavior can be observed of R-PVDF and R-PVDF/ HUSY nanocomposites. This can be explained by the biggest interaction of HUSY with the polymer matrix due the significant amount of mesopores. R-PVDF chains can enter in HUSY zeolite cavities, hindering the cations mobility, which explains the lower values of the dielectric constant and dielectric losses. The R-PVDF/HY nanocomposite is the one with the largest losses due to strong electrostatic force between proton and zeolite structure, which leads to an energetic loss. The R-PVDF/ NaY nanocomposite shows significant losses to low frequencies; however, these are lower than R-PVDF/HY because of the larger mass of the Na ions. With increasing frequency of electric field, the sodium ions are not able to follow the field, so its behavior becomes similar to the pure polymer. Effect of Sodium. Modified zeolites, Na(H)Y and H(Na)Y, differ from the starting zeolites only in the number of cations (Na+ or H+) that compensate the negative charges of the zeolite. This means that the difference in the results obtained for the dielectric response can also be attributed to this factor. Figure 5 shows the dielectric response of nanocomposite films with 14 wt % of zeolites subjected to ion exchange. It can be observed in Figure 5 that NaY zeolite, the one with the largest amount of sodium ions per unit cell (47.2), shows the highest dielectric constant. The remaining nanocomposites show similar dielectric response, which also reflects similar numbers of sodium ions per unit cell: 10.7 for HY, 12.3 for H(Na)Y, and 15.3 for Na(H)Y (Table 1). This result demonstrates that effectively the number of sodium ions is the main contribution to the increase in the dielectric constant with respect to the pure polymer because of the different mobility of charges in the zeolite structure when the electric field is applied. There is a larger affinity to protons than to sodium within the zeolite structure, as demonstrated by the results of Table 3, resulting in a successful exchange of sodium ions by protons in NaY to Na(H)Y and a not so successful exchange of protons by sodium ions in HY to H(Na)Y. This is explained by the stronger interaction between the zeolite framework and the protons than between the zeolite and the sodium ions. In this way, a larger field is required for the release of proton (∆δ ) 0.73 V) than for sodium ion release (∆δ ) 0.60 V).40 Hence, field-induced mobility for protons is lower and therefore does not contribute to ionic or interfacial polarization as significantly as sodium ions. An important issue is the behavior of the dielectric loss. The dielectric loss is proportional to the imaginary part of the dielectric constant and therefore to the real part of the conductivity.41 It is observed in Figure 5 that the dielectric loss for PVDF/NaY is the lowest among all nanocomposites, close to the one of the polymer. This fact supports the idea of fieldinduced confined movements of the sodium ions within the zeolite structure, contributing to the real part of the dielectric function but not so much to the dielectric loss. The effect of different mobility of cations also explains the results previously obtained for the different zeolites: NaY zeolite,
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J. Phys. Chem. C, Vol. 114, No. 34, 2010 14451 TABLE 5: Onset Temperatures of the Pure Polymer and the Nanocomposites Obtained by Dynamic Thermogravimetric Curves samples
onset temperature (°C)
R-PVDF PVDF/HY PVDF/NaY PVDF/HUSY PVDF/H(Na)Y PVDF/Na(H)Y
457 369 449 330 380 382
SCHEME 2: Schematic Representation of Zeolitic Brønsted Acidic Hydroxyl Group47
Figure 5. (a) Real part of the dielectric constant and (b) dielectric loss obtained for the pure polymer and the nanocomposite films: (b) PVDF, (2) PVDF/HY, (9) PVDF/H(Na)Y, ([) PVDF/NaY, and (f) PVDF/Na(H)Y.
Figure 6. Dynamic thermogravimetric curves obtained for the samples: (b) PVDF, (2) PVDF/HY, ([) PVDF/NaY, (9) PVDF/H(Na)Y, and (f) PVDF/Na(H)Y, and (*) PVDF/HUSY.
which has the greater number of sodium ions per unit cell (47.2), shows the higher dielectric constant. HY zeolite has an intermediate value for the dielectric response due to the presence of 10.7 sodium ions per unit cell, and the HUSY zeolite, which has the lower value of sodium, has the lowest dielectric constant. Thermal Stability of the Nanocomposites. Dynamic thermogravimetric analyses were performed to evaluate the thermal stability of the nanocomposites when compared with the pure polymer. Figure 6 shows the weight loss of the R-PVDF and the nanocomposite films as a function of the temperature, measured at 10 °C/min.
Table 5 presents the values obtained for the onset temperature of dynamic thermograms obtained at 10 °C/min. The onset temperature of R-PVDF is the highest in comparison with all nanocomposites. It is known that during the thermal degradation of PVDF, carbon-hydrogen bond scission occurs because of the lower bond strength of C-H compared with C-F (410 and 460 kJ mol-1, respectively).42 Because of the high energy linkage of C-F, this polymer has a very high thermal stability. The addiction of the zeolites enhances the catalytic degradation process of R-PVDF, as obtained for other polymers.26,30,43-45 Zeolites have a very large internal surface area with a large number of catalytic sites that are initially unavailable for the degradation of large molecules, such as polymers.46 From the thermogravimetric results, it can be observed that for the catalyzed process the degradation takes place at quite lower temperatures when compared with pure PVDF because of the polymer catalytic cracking. Therefore, for all nanocomposites, the weight loss starts at temperatures between 330 and 450 °C, showing that a significant reduction occurs in the onset temperature. The differences obtained for the onset temperature can be explained by the difference in the number of acid sites and the porosity of the zeolites. HUSY zeolite, which has the highest theoretical number of acid sites and a significant number of mesopores with a radius larger than 50 Å (Tables 1 and 2), gives the lowest value of onset temperature (330 °C). The mesoporosity of HUSY zeolite, determined by N2 adsorption, leads to more accessibility of the catalytic sites by the large molecules of the polymer than the microporosity of HY and NaY zeolites.37,44 The variations of the thermal stability of the nanocomposites induced by the ion exchange within the zeolites were also investigated. The protons are commonly associated with the acid sites present in the zeolite structure. It is known that the important sites in zeolites are the Brønsted acid sites, which consist of hydrogen bonds to an oxygen atom that connects to the tetrahedral-coordinated Si4+ and Al3+ cations [O-Al-OHSi-O] (Scheme 2). These sites are responsible for the strong acidic catalytic behavior observed for these zeolites.26,44,47 The catalytic degradation reaction with R-PVDF/NaY nanocomposite was slower because of the lower acidity of the zeolite. NaY zeolite has a lower number of protons reflecting in the acidity behavior.30 The ion exchange treatment in the modified Y zeolites leads to an intermediate behavior in the onset temperature on the catalytic degradation of the nanocomposites, as expected.30
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Conclusions Highly flexible films of R-PVDF/zeolite nanocomposites were successfully prepared by solvent casting with DMF solution. Y zeolites used (NaY, HY, and HUSY) showed a different framework Si/Al ratio and a different number of sodium ions. All nanocomposites studied exhibited a dielectric constant increase compared with pure R-PVDF (6.86); however, this occurs in different proportions depending on the type of Y zeolite (23.26 to NaY, 13.79 to HY, and 9.49 to HUSY for 14 wt % zeolite at 10 kHz) because of the different type of ions that compensates the negative charge of zeolite structure. Nanocomposites of R-PVDF with zeolites subjected to ion exchange were successfully prepared as well. Comparison between nanocomposites prepared with NaY and Na(H)Y and between HY and H(Na)Y leads us to conclude that the sodium amount for the enhancement of the dielectric constant of the nanocomposites studied in this work is a key property. The larger the amount of sodium, the larger the dielectric constant. This happens because of the fact that interaction forces between the zeolite and the protons are stronger than those between it and sodium ions, which leads to a better field-induced mobility of sodium ions within zeolites structure and therefore an increased dielectric response. A reduction in the onset degradation temperature was observed for the nanocomposites when compared with pure PVDF. This reduction is partially related to the number of acid sites and their accessibility in the zeolite structure. Acknowledgment. A.C.L. and R.G. thank FCT (Portugal) for the attribution of their respective grants (ref: SFRH/BD/ 45265/2008 and UMINHO/BII/057/2009). This work was supported by the Centro de Quı´mica and Centro de Fı´sica (University of Minho, Portugal) and FCT (Portugal) through POCTI and FEDER projects (ref: POCTI-SFA-3-686, PTDC/ CTM/69316/2006, PTDC/CTM/69362/2006 and NANO/NMedSD/0156/2007). References and Notes (1) Bar-Cohen, Y. ElectroactiVe Polymer (EAP) Actuators as Artificial Muscles: Reality, Potential and Challenges, 2nd ed.; SPIE Press: Bellingham, WA, 2004. (2) Dias, C. J.; Das-Gupta, D. K. IEEE Trans. Dielectr. Electr. Insul. 1996, 5, 706–734. (3) Lovinger, A. J. DeVelopments in Crystalline Polymers; Basset, D. C., Ed.; Elsevier: London, 1982. (4) Castro, H. F.; Lanceros-Mendez, S.; Rocha, J. G. Mater. Sci. Forum 2006, 202–206. (5) Zhicheng, Z.; Mike-Chung, T. C. Macromolecules 2007, 40, 9391– 9397. (6) Gomes, J.; Serrado-Nunes, J.; Sencadas, V.; Lanceros-Mendez, S. Smart Mater. Struct. 2010, 19, 065010-065017. (7) Sencadas, V.; Moreira, V. M.; Lanceros-Mendez, S.; Pouzada, A. S.; Gregorio, R., Jr. Mater. Sci. Forum 2006, 699, 514–516. (8) Ueno, E. M.; Grego´rio, R., Jr. J. Mater. Sci. 1999, 34, 4489–4500. (9) Das-Gupta, D. K. Ferroelectrics 1981, 33, 95–101. (10) Nalwa, H. S. J. Macromol. Sci., ReV. Macromol. Chem. Phys. 1991, C13, 341. (11) Mazumdar, S. K. Composites Manufacturing: Materials, Product and Process Engineering; CRC Press: Boca Raton, FL, 2002.
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