Enhancement of the Dielectric Constant and Thermal Properties of r-Poly(vinylidenefluoride)/Zeolite Nanocomposites

Ana Catarina Lopes,†,‡ Marco P. Silva,‡ Renato Goncalves,† Manuel F. R. Pereira,§

Gabriela Botelho,† Antonio M. Fonseca,† Senenxtu Lanceros-Mendez,*,‡ and Isabel C. Neves*,†

Departamento de Quımica, Centro de Quımica and Departamento de Fısica, Centro de Fısica, UniVersidadedo Minho, Campus de Gualtar, 4170-057 Braga, Portugal, and Laboratorio de Catalise e Materiais (LCM),Laboratorio Associado LSRE/LCM, Departamento de Engenharia Quımica, 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 toprepare R-poly(vinylidene fluoride) (R-PVDF)/zeolite nanocomposites. Structural and textural characterizationof 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 dielectricresponse and thermal stability. The best dielectric properties were obtained with NaY because of its smallerframework Si/Al ratio, which means a larger number of sodium in the zeolite structure. For this zeolite, thedielectric constant was increased by a factor higher than two, maintaining unaltered the dielectric loss forfrequencies higher than 0.5 MHz. The zeolite HUSY, which presents an extended mesoporosity, shows thelowest onset temperature for thermal degradation. To clarify the origin of the dielectric response of thenancomposites, 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 thezeolite framework.

Introduction

The use of piezoelectric materials has been increasing forsensors and actuators applications because of their ability tocouple electrical and mechanical signals.1 Composite materialsare the preferred solution in many of those applications becausethe electromechanical properties can be tuned to the desiredvalues for specific applications.2 Among the electroactivepolymers, poly(vinylidene fluoride) (PVDF) is still the one withthe most interesting piezoelectric response.1,3 Strong researchefforts are being performed in the preparation of new PVDF-based composites and nanocomposites aiming to improve theelectromechanical coupling, piezoelectric coefficient, and di-electric constant of the polymer. Within these efforts, theincrease in the dielectric constant is one of the necessary stepsto improve performance of the material for the above-mentionedapplications.

The outstanding piezoelectric and dielectric properties ofPVDF already generated various applications in the field ofsensors, actuators, and energy generation/storage.4,5 It is asemicrystalline polymer exhibiting different crystalline phases.4

The electroactive properties and the dielectric response stronglydepend on the crystalline phase or phases present in thepolymer.4,5

Depending on the phase present on the material, the dielectricconstant 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

values are quite high for polymers7 but are still low whencompared with the values obtained for electroactive crystals andceramics.8 Polymers show some advantages with respect to theabove-mentioned materials: they are flexible, easy to tailor, andcan be produced in large sizes and a large variety of formatsand shapes.9 In this way, the increase in the dielectric andelectroactive characteristics of polymers is an important tech-nological and scientific issue.10,11

The methods based on the doping of polymers with preformednanoparticles have become intensively investigated in recentyears because they allow the production of engineering nano-composites with well-defined properties.11-16 The dielectricproperties have been enhanced with different nanofillers suchas metallic nanoparticles,17 carbon nanotubes,18 or nanofibers19

and clays.20,21

In this respect, zeolites as a guest have a great potential forincreasing the dielectric properties because of their inherentstructural characteristics.22 Zeolites are crystalline hydratedaluminosilicates materials whose crystalline structure is formedby channels and cavities of strictly regular dimensions calledmicropores.23 The pore size is comparable to that of smallmolecules, allowing them to reach the acid sites located insidethe zeolite structure while hindering the access of bulkymolecules.24 A net negative charge, which arises on the zeoliteframework, has to be neutralized by the presence of cationswithin the pores. These cations may be any of the metals ormetal’s complexes or alkylammonium cations.23

Zeolites find broad application in heterogeneous catalysis andpolymer catalytic degradation and also attract interest inmaterials science for the development of functional materialsand in nanotechnology.25-28 The most important examples ofthese tridimensional zeolite structures are the faujasite structures,

* Corresponding authors. E-mail: lanceros@fisica.uminho.pt (S.L.-M.);ineves@quimica.uminho.pt (I.C.N.).

† Departamento de Quımica, Centro de Quımica, Universidade do Minho.‡ Departamento de Fısica, Centro de Fısica, Universidade do Minho.§ Universidade do Porto.

J. Phys. Chem. C 2010, 114, 14446–1445214446

10.1021/jp1052997 2010 American Chemical SocietyPublished on Web 08/10/2010

X and Y.29 The zeolite Y used in this work is a crystallinemicroporous aluminosilicate based on sodalite cages joined byO bridges between the hexagonal faces. Eight sodalite cagesare linked together, forming a large central cavity or supercagewith a diameter of 13 Å. The supercages share a 12-memberedring with an open diameter of 7 Å (Scheme 1).29

The most common way to obtain electroactive �-phase PVDF,used in technological application, is by stretching from theR-phase PVDF;6 also, because of its intrinsic interest, it isimportant to perform a preliminary study on how the differentzeolite properties influence the dielectric properties of R-PVDF.In this work, R-PVDF/zeolite nanocomposite films with Yzeolites (NaY, HY, and HUSY) have been prepared by solventcasting. Two of these Y zeolites were subjected to an ionexchange treatment using NaNO3 for HY and NH4NO3 for NaYand then used for the preparation of PVDF/zeolite nanocom-posites.30 Dielectric behavior and thermal stability of thenanocomposite films were evaluated. Previously, the structuraland the textural properties of Y zeolites were characterized byX-ray powder diffraction, elemental analysis, infrared spectros-copy 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 inpowder form and were calcined at 500 °C during 8 h under adry air stream prior to nanocomposite preparation. The zeoliteUSY (ultrastabilized Y, CBV 500) was available in theammonium form. After heating, the ammonium is transformedin NH3 and H+. The NH3 desorbs, and the presence of theprotons increases the number of acid sites. The protonic formof USY (HUSY) was obtained after this calcination. The otherzeolites were available in the proton form for HY (CBV 400)and sodium form for NaY (CBV100). Chemicals for ionexchange treatment (NaNO3 and NH4NO3) were purchased fromAldrich. The solvent used in this study, N,N-dimethylformamide(DMF), was purchased from Aldrich (analytical grade) andpreviously dried using molecular sieves.

Ion Exchange Treatment in the Zeolites Y. The ionexchange treatment in the zeolites Y was performed accordingto a previously published procedure.30 The zeolites Y used havesimilar Si/Al atomic ratios and were used as the startingmaterials: NaY (Si/Al ratio ) 2.83) and HY (Si/Al ratio ) 2.80).The samples were prepared with 1.0 M solutions of theappropriate nitrate (NaNO3 for HY and NH4NO3 for NaY). Thezeolites obtained by ion exchange were calcined at 500 °Cduring 8 h under a dry air stream. These zeolites were designatedH(Na)Y from HY and Na(H)Y from NaY.

Preparation of the r-PVDF/Zeolite Nanocomposites. Thepreparation of nanocomposite films was performed by solventcasting method.31 Nanocomposite films with thickness around40-50 µm were obtained by spreading a solution of 1.0 g ofPVDF with a suspension of 0.14 g of different Y zeolites in 4mL of DMF into a glass slide. The suspension was previouslyplaced in an ultrasound bath for 4 h for homogenization. Afterthis period of time, the polymer was added to the suspensionand kept during 1 h under 100 rpm with a magnetic stirrer. Theresulting nanocomposites films were kept in an oven at acontrolled temperature of 210 °C for 10 min. Finally, thenanocomposite films were removed from the oven and cooledto room temperature. The polymer obtained by this procedureis 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 byinductively coupled plasma atomic emission spectroscopy (ICP-AES) using Philips ICP Spectrometer (PU 7000). Phase analysisof the zeolite samples was obtained by X-ray diffraction (XRD)using a Philips analytical X-ray model PW1710 BASEDdiffractometer system. Scans were taken at room temperatureusing Cu-KR radiation in a 2θ range between 5 and 60°. Thetextural characterization of the zeolites was based on the N2

adsorption isotherms determined at 77 K using a QuantachromeInstruments Nova 4200e apparatus. The samples were previouslyoutgased at 423 K under vacuum. The micropore volumes(Vmicro) and mesopore surface areas (Smeso) were calculated bythe t-method.33 We calculated surface areas by applying the BETequation.33 Mesopores size distributions were obtained from thedesorption branch of the isotherm using the Barrett, Joyner, andHalenda (BJH) method.33 Fourier transform infrared (FTIR)spectra of the zeolites in KBr pellets were measured using aBomem MB104 spectrometer in the range of 4000-500 cm-1

by averaging 20 scans at a maximum resolution of 4 cm-1. Thethermal stability of the nanocomposite films as well as the purepolymer was studied in a Perkin-Elmer instrument, Pyris1TGA,controlled by a PC under the Windows operating system. Theatmosphere used was high purity nitrogen (99.99% minimumpurity) with a flow rate of 20 mL/min according to theequipment specifications. The sample holders used were cru-cibles of alumina oxide, supplied by Perkin-Elmer, in which∼3 mg sample was degraded. The sample temperature wasmeasured with a thermocouple located at the crucible. To obtainthe actual temperature in the crucible, we performed a calibrationprocedure using the internal standards (i.e., alumel, nickel, andperkalloy) in all experiments. The samples were heated between25 and 800 at 10 °C/min to evaluate the thermal stability. Thecapacity and tan δ, dielectric loss were measured with anautomatic Quadtech 1929 Precision LCR meter at room tem-perature in a home-built sample holder. From the capacity valuesand the sample geometrical characteristics, the real part of thedielectric constant, ε′, was obtained. The applied signal forseveral frequencies in the range of 100 Hz to 1 MHz was 0.5V. The samples were coated by sputtering with circular Auelectrodes of 5 mm diameter onto both sides of the sample.

Results and Discussion

Physicochemical Properties of the Zeolites. The faujasitezeolite structure was evaluated by XRD. The powder XRDdiffraction patterns (not shown) of NaY, HY, and HUSY wererecorded at 2θ values between 5 and 60°. All zeolites exhibitedthe typical and similar pattern of highly crystalline faujasitezeolite structure. We estimated the relative crystallinity of HYand HUSY zeolites by comparing the peak intensities of the

SCHEME 1: Faujasite Structure of Y Zeolite29

R-Poly(vinylidene fluoride)/Zeolite Nanocomposites J. Phys. Chem. C, Vol. 114, No. 34, 2010 14447

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 wereused for the comparison according to ASTM D 3906-80 method.The HY and HUSY XRD patterns present over 70% ofcrystallinity.

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 themost sensitive band in the zeolite Y structure and can be usedto calculate framework Si/Al ratios34 using eq 1

where x ) [1 + (Si/Al)]-1, with 0.1 < x < 0.3, and wDR is thezeolite-specific double-ring vibration mode between 570 and600 cm-1.

We obtained the framework Si/Al ratio by XRD from thecalculated unit cell parameters by using the Breck and Flanigenequation (eq 2)35

where NAl is the framework aluminum number and a0 is thecell parameter. The unit cell parameters were calculated fromthe values of the [5 3 3], [6 4 2], and [5 5 5] reflection peaksaccording to the ASTM D 3942-80 method.

XRD data can be used to estimate the particles size usingthe Debye-Scherrer equation.36

where D is the crystal diameter, K is a constant (0.9), λ is theX-ray wavelength, B is the full width at half-maximum of thepeak in radians, and θ is the Bragg angle. The average par-ticle size of the zeolite Y was estimated from the most intensereflection peak [5 3 3] position of the [h k l]. Table 1 shows thetotal and framework Si/Al ratios calculated by the methodsmentioned above.

All Y zeolites used in this work present similar total Si/Alratio. However, the difference between the Si/Al ratios deter-mined by FTIR and XRD (framework) and those determinedby chemical analysis (total) indicates an irregular distributionof silicon and aluminum throughout the zeolite structure. In thecase of NaY, the total and framework Si/Al ratios are similar,which means that both species are tetrahedrally coordinated inthe framework. For HY and HUSY, the framework Si/Al ratiois higher than the total Si/Al ratio, indicating the presence ofextra-framework alumina species (EFAL).37

Therefore, in HY and HUSY, the number of negative chargesin the zeolite framework is lower than in NaY, which means alower number of ions (Na+ or H+) that is necessary to maintainthe electroneutrality of the solid.

The sodium amount and the acidity of the zeolites are alsovery different. These properties of the zeolite structure can playan important role in the dielectric response and in the polymercatalytic degradation.26,37 NaY presents the lower theoreticalnumber of acid sites due to the higher amount of sodium intheir structure, as indicated in Table 1. However, both H formsof Y zeolites (HY and HUSY) show lower amount of sodiumand exhibit a higher theoretical number of acid sites due to thepresence of a higher number of Brønsted acid sites in theirstructures. Finally, the particle size of all Y zeolites determinedby XRD is of the same order of magnitude.

The nitrogen adsorption-desorption equilibrium isothermsat 77 K for the NaY, HY, and HUSY zeolites are illustrated inFigure 1.

The N2 adsorption isotherms for all zeolites are of Type-Iisotherm, according the IUPAC classification, which is typicalof solids with microporous structure.26 The shapes of bothadsorption and desorption isotherms of the zeolites Y are verysimilar to each other. However, the existence of an hysteresisloop observed in the isotherm of the zeolite HUSY suggeststhat this zeolite has an extended degree of mesoporosity.

We calculated the micropore volumes (Vmicro) and mesoporesurface areas (Smeso) by the t method, and we calculated thetotal surface areas by applying the BET equation (SBET). Themesopore volume (Vmeso) was calculated as the differencebetween the total pore volume for P/P0 ) 0.986 (VP/P0)0.986)and the micropore volume. These values are summarized inTable 2.

In fact, the mesopores size distribution (Figure 2) confirmsthat the HUSY sample presents larger pores than the otherzeolites. Contrary to the other samples, the HUSY zeolitepresents a significant amount of mesopores with a radius largerthan 50 Å.

TABLE 1: Chemical and Structural Characterization of YZeolites

NaY HY HUSY

Si/Ala 2.83 2.80 3.00Si/Alb 2.80 4.05 3.90Si/Alc 2.82 4.10 3.96EFALd 0 17 11Na (%)a 7.76 1.95 0.12Na (UC)e 47.2 10.7 0.7nA1 × 1020 (sites ·g-1)f 2.8 14.0 20.5particle size (nm)g 10 9 9crystallinity (%)g 100 75 80

a Total Si/Al ratio and sodium amount determined fromICP-AES. b Framework Si/Al ratio determined from XRD.c Framework Si/Al ratio determined from FTIR. d EFAL is thenumber of extra framework aluminum species drawn from theframework Si/Al ratio of the zeolites.35 e Number of sodium ionsdrawn from the unit cell formula of the zeolites. f nA1 is thetheoretical number of acid sites drawn from the unit cell formula ofthe zeolites obtained by XRD and chemical analysis. g Determinedfrom XRD analysis.

x ) 3.857 - 0.00621wDR (cm-1) (1)

NAl ) 115.2(a0 - 24.191) (2)

D ) KλB cos θ

(3)

Figure 1. Nitrogen adsorption-desorption equilibrium isotherms ofthe different zeolites at 77 K: ([) NaY, (2) HY, and (*) HUSY.

14448 J. Phys. Chem. C, Vol. 114, No. 34, 2010 Lopes et al.

NaY and HY zeolites were modified by ion exchangetreatment to understand the effect of sodium amount on thedielectric response of the nanocomposites. The chemical andstructural properties of the modified zeolites were characterizedby XRD and chemical analyses recorded under the sameconditions as those used for NaY and HY (Table 3).

All samples exhibited the typical and similar XRD patternof highly crystalline Y zeolite. The XRD pattern of modifiedsamples showed no reduction in the intensity of the peaks andno variation in the zeolite lattice parameters after the ionexchange process, suggesting full crystallinity retention of thestarting zeolite Y.30 However, the ion exchange treatmentslightly affects the Si/Al ratio of the zeolites. The modifiedzeolites present different Si/Al ratios compared with the ratiosdetermined by XRD and those determined by chemical analysis,indicating an irregular distribution of silicon and aluminumthroughout the zeolite structure.30

The amount of sodium depends on the ion exchange treatmentused in the starting zeolites. In the case of HY, when a sodiumsalt solution is used to transform to proton-type zeolite (HY)in the sodium-type Y zeolite H(Na)Y, the amount of sodiumincreases after the treatment, as expected. The amount of sodiumdecreases when the sodium-type Y zeolite (NaY) is transformedinto proton-type zeolite Na(H)Y. After the ion exchangetreatment in NaY, the loss of sodium is very pronounced,reaching 67%. This result can be explained by the selectivityof the zeolite Y for proton exchange because each cationcompensates the negative charge arising from an Al atom inthe framework.37

The particle size of NaY and HY (Table 1) and those ofmodified zeolites are of the same order of magnitude, whichgives evidence of the preservation of the zeolite structure.Therefore, the ion exchange treatment described in our previouswork30 did not modify the zeolite structure.

Morphological and Dielectric Characterization of thePVDF/Zeolite Nanocomposite Films. All nanocomposite filmswere prepared with a zeolite concentration of 14% (w/w). Thezeolite amount was chosen in a way to allow the study of thezeolite effect in PVDF electrical response and thermal stabilitybut not enough to affect the original spherulitic microstructureof 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 thepolymer19 in its R-phase, and Figure 3b presents a typicalmorphology of the faujasite structure of NaY. The SEMmicrograph of the nanocomposite (Figure 3c) shows that a gooddispersion of the zeolites was achieved. The addition of zeolitedoes not change the spherulitic microstructure of the polymer,and the resulting nanocomposites show the well-defined particlesof the NaY. The R-phase of the polymer was confirmed byFTIR.32

Effect of the Different Zeolites. Figure 4 shows the dielectricmeasurements performed on the pure polymer and the nano-composite films.

The variations of the ε′, real part of the dielectric function,with frequency for the R-PVDF and the nanocomposite filmsare presented in Figure 4a. In all cases, there is no change inthe frequency dependence of the dielectric response, but ageneral increase in the value of ε′ can be observed for thenanocomposites with respect to the pure R-PVDF polymersample (6.86). NaY zeolite shows the highest dielectric constant(doubling the value of the pure polymer), followed by HY andHUSY zeolites. The effect on the dielectric response of thedifferent zeolites in the PVDF matrix is illustrated in Table 4for 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 dielec-tric losses at lower frequencies but remains at the same levelas PVDF for frequencies higher than 5 × 105 Hz. It is essentialto notice the importance of this result because a relevant increasein the dielectric constant is achieved without any increase inthe dielectric loss for frequencies higher that 5 × 105 Hz. Finally,the R-PVDF/HY nanocomposite shows higher losses in theentire frequency range. This behavior is related to the differentmicrostructural properties of the zeolites.

In this context, it is important to notice that the dielectricconstant for the zeolites shows a similar frequency dependenceas 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, thecommon models used for the description of the dielectricbehavior of dielectric-dielectric composites including highdielectric constant filler38 are not applied. In this study, the effectseems to be related to confined mobility of charged ions inconfined space of the micropores or to interfacial polarizationeffects. The electrical force originated by the application of theelectric field is reflected in a stretching of ionic bond andconsequently an induced dipole moment. Some cations are ableto overcome the negative electrostatic attraction and move withinthe zeolite structure. This fact probably leads to the buildup ofinterfacial polarization because of the charges that accumulatewithin the zeolite structure.39

TABLE 2: Textural Properties of the Zeolites Y

NaY HY HUSY

SBET (m2/g) 787 665 750Vmicro (cm3/g) 0.347 0.302 0.269Smeso (m2/g)a 18.9 25.0 23.6VP/P0)0.986 (cm3/g) 0.382 0.349 0.355Vmeso (cm3/g) 0.035 0.047 0.086

a From t plot.

Figure 2. Mesopore size distribution profiles of the zeolites: NaY ([),HY (2), and HUSY (*).

TABLE 3: Chemical and Structural Analysis of theModified Zeolites

samples Si/Ala Si/Alb Na (%)a Na (UC)c particle size (nm)b

H(Na)Y 2.94 4.19 2.24 12.3 9Na(H)Y 2.89 3.09 2.52 15.3 9

a Total Si/Al ratio and sodium amount determined fromICP-AES.30 b Framework Si/Al ratio determined from XRD.30

c Number of sodium ions drawn from the unit cell formula of thezeolites.

R-Poly(vinylidene fluoride)/Zeolite Nanocomposites J. Phys. Chem. C, Vol. 114, No. 34, 2010 14449

In this view, the lower dielectric constant of HY and HUSYnanocomposites with respect to NaY nanocomposite is relatedto the lower number of counterions that is necessary to maintainthe electroneutrality.

Furthermore, the difference in values of Si/Al between thesezeolites does not seem to be the only factor involved inexplaining the different dielectric performance of these materials,which leads us to conclude that the higher number of protonsin the HY and HUSY zeolites has a lower effect on the dielectricresponse than the sodium ions in NaY. The actual effect on thedielectric response of the sodium amount is presented in thenext section.

A similar behavior can be observed of R-PVDF and R-PVDF/HUSY nanocomposites. This can be explained by the biggestinteraction of HUSY with the polymer matrix due the significantamount of mesopores. R-PVDF chains can enter in HUSYzeolite cavities, hindering the cations mobility, which explainsthe lower values of the dielectric constant and dielectric losses.The R-PVDF/HY nanocomposite is the one with the largestlosses due to strong electrostatic force between proton andzeolite 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 largermass of the Na ions. With increasing frequency of electric field,the sodium ions are not able to follow the field, so its behaviorbecomes 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 thedielectric response can also be attributed to this factor. Figure5 shows the dielectric response of nanocomposite films with14 wt % of zeolites subjected to ion exchange.

It can be observed in Figure 5 that NaY zeolite, the one withthe largest amount of sodium ions per unit cell (47.2), showsthe highest dielectric constant. The remaining nanocompositesshow similar dielectric response, which also reflects similarnumbers of sodium ions per unit cell: 10.7 for HY, 12.3 forH(Na)Y, and 15.3 for Na(H)Y (Table 1).

This result demonstrates that effectively the number of sodiumions is the main contribution to the increase in the dielectricconstant with respect to the pure polymer because of thedifferent mobility of charges in the zeolite structure when theelectric field is applied. There is a larger affinity to protons thanto sodium within the zeolite structure, as demonstrated by theresults of Table 3, resulting in a successful exchange of sodiumions by protons in NaY to Na(H)Y and a not so successfulexchange of protons by sodium ions in HY to H(Na)Y. This isexplained by the stronger interaction between the zeoliteframework and the protons than between the zeolite and thesodium ions. In this way, a larger field is required for the releaseof proton (∆δ ) 0.73 V) than for sodium ion release (∆δ )0.60 V).40 Hence, field-induced mobility for protons is lowerand therefore does not contribute to ionic or interfacial polariza-tion as significantly as sodium ions.

An important issue is the behavior of the dielectric loss. Thedielectric loss is proportional to the imaginary part of thedielectric constant and therefore to the real part of the conduc-tivity.41 It is observed in Figure 5 that the dielectric loss forPVDF/NaY is the lowest among all nanocomposites, close tothe one of the polymer. This fact supports the idea of field-induced confined movements of the sodium ions within thezeolite structure, contributing to the real part of the dielectricfunction but not so much to the dielectric loss.

The effect of different mobility of cations also explains theresults previously obtained for the different zeolites: NaY zeolite,

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) dielectricloss obtained for the pure polymer and the different nanocompositefilms: (b) PVDF, ([) PVDF/NaY, (2) PVDF/HY, and (*) PVDF/HUSY.

TABLE 4: Effect of the Different Types of Zeolites at aConcentration of 14 wt % on the Dielectric Response of theNanocomposite Films at 10 kHz

samples R-PVDF R-PVDF/NaY R-PVDF/HY R-PVDF/HUSY

ε′ 6.86 17.92 13.79 9.61tg δ (× 10-2) 2.43 8.97 9.59 4.33

14450 J. Phys. Chem. C, Vol. 114, No. 34, 2010 Lopes et al.

which has the greater number of sodium ions per unit cell (47.2),shows the higher dielectric constant. HY zeolite has anintermediate value for the dielectric response due to the presenceof 10.7 sodium ions per unit cell, and the HUSY zeolite, whichhas the lower value of sodium, has the lowest dielectric constant.

Thermal Stability of the Nanocomposites. Dynamic ther-mogravimetric analyses were performed to evaluate the thermalstability of the nanocomposites when compared with the purepolymer. Figure 6 shows the weight loss of the R-PVDF andthe nanocomposite films as a function of the temperature,measured at 10 °C/min.

Table 5 presents the values obtained for the onset temperatureof dynamic thermograms obtained at 10 °C/min.

The onset temperature of R-PVDF is the highest in compari-son with all nanocomposites. It is known that during the thermaldegradation of PVDF, carbon-hydrogen bond scission occursbecause of the lower bond strength of C-H compared with C-F(410 and 460 kJ mol-1, respectively).42 Because of the highenergy linkage of C-F, this polymer has a very high thermalstability.

The addiction of the zeolites enhances the catalytic degrada-tion process of R-PVDF, as obtained for other polymers.26,30,43-45

Zeolites have a very large internal surface area with a largenumber of catalytic sites that are initially unavailable forthe degradation of large molecules, such as polymers.46 Fromthe thermogravimetric results, it can be observed that for thecatalyzed process the degradation takes place at quite lowertemperatures when compared with pure PVDF because of thepolymer 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 onsettemperature. The differences obtained for the onset temperaturecan be explained by the difference in the number of acid sitesand the porosity of the zeolites.

HUSY zeolite, which has the highest theoretical number ofacid sites and a significant number of mesopores with a radiuslarger than 50 Å (Tables 1 and 2), gives the lowest value ofonset temperature (330 °C). The mesoporosity of HUSY zeolite,determined by N2 adsorption, leads to more accessibility of thecatalytic sites by the large molecules of the polymer than themicroporosity of HY and NaY zeolites.37,44

The variations of the thermal stability of the nanocompositesinduced by the ion exchange within the zeolites were alsoinvestigated. The protons are commonly associated with the acidsites present in the zeolite structure. It is known that theimportant sites in zeolites are the Brønsted acid sites, whichconsist of hydrogen bonds to an oxygen atom that connects tothe tetrahedral-coordinated Si4+ and Al3+ cations [O-Al-OH-Si-O] (Scheme 2). These sites are responsible for the strongacidic catalytic behavior observed for these zeolites.26,44,47

The catalytic degradation reaction with R-PVDF/NaY nano-composite was slower because of the lower acidity of the zeolite.NaY zeolite has a lower number of protons reflecting in theacidity behavior.30 The ion exchange treatment in the modifiedY zeolites leads to an intermediate behavior in the onsettemperature on the catalytic degradation of the nanocomposites,as expected.30

Figure 5. (a) Real part of the dielectric constant and (b) dielectricloss 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.

TABLE 5: Onset Temperatures of the Pure Polymer andthe Nanocomposites Obtained by DynamicThermogravimetric Curves

samples onset temperature (°C)

R-PVDF 457PVDF/HY 369PVDF/NaY 449PVDF/HUSY 330PVDF/H(Na)Y 380PVDF/Na(H)Y 382

SCHEME 2: Schematic Representation of ZeoliticBrønsted Acidic Hydroxyl Group47

R-Poly(vinylidene fluoride)/Zeolite Nanocomposites J. Phys. Chem. C, Vol. 114, No. 34, 2010 14451

Conclusions

Highly flexible films of R-PVDF/zeolite nanocomposites weresuccessfully prepared by solvent casting with DMF solution. Yzeolites used (NaY, HY, and HUSY) showed a differentframework Si/Al ratio and a different number of sodium ions.All nanocomposites studied exhibited a dielectric constantincrease compared with pure R-PVDF (6.86); however, thisoccurs in different proportions depending on the type of Yzeolite (23.26 to NaY, 13.79 to HY, and 9.49 to HUSY for 14wt % zeolite at 10 kHz) because of the different type of ionsthat compensates the negative charge of zeolite structure.

Nanocomposites of R-PVDF with zeolites subjected to ionexchange were successfully prepared as well. Comparisonbetween nanocomposites prepared with NaY and Na(H)Y andbetween HY and H(Na)Y leads us to conclude that the sodiumamount for the enhancement of the dielectric constant of thenanocomposites studied in this work is a key property. Thelarger the amount of sodium, the larger the dielectric constant.This happens because of the fact that interaction forces betweenthe zeolite and the protons are stronger than those between itand sodium ions, which leads to a better field-induced mobilityof sodium ions within zeolites structure and therefore anincreased dielectric response.

A reduction in the onset degradation temperature wasobserved for the nanocomposites when compared with purePVDF. This reduction is partially related to the number of acidsites 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 wassupported by the Centro de Quımica and Centro de Fısica(University of Minho, Portugal) and FCT (Portugal) throughPOCTI and FEDER projects (ref: POCTI-SFA-3-686, PTDC/CTM/69316/2006, PTDC/CTM/69362/2006 and NANO/NMed-SD/0156/2007).

References and Notes

(1) Bar-Cohen, Y. ElectroactiVe Polymer (EAP) Actuators as ArtificialMuscles: Reality, Potential and Challenges, 2nd ed.; SPIE Press: Belling-ham, 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. Forum2006, 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.; Gregorio, 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, Productand Process Engineering; CRC Press: Boca Raton, FL, 2002.

(12) Schwartzberg, A. M.; Zhang, J. Z. J. Phys. Chem. C 2008, 112,10323–10337.

(13) Mendes, S. F.; Costa, C. M.; Sencadas, V.; Serrano Nunes, J.; Costa,P.; Gregorio, R., Jr.; Lanceros-Mendez, S. Appl. Phys. A: Mater. Sci.Process. 2009, 4, 899–908.

(14) Odegarm, G. M. Acta Mater. 2004, 52, 5315–5330.(15) Pastoriza-Santos, I.; Perez-Juste, J.; Kickelbick, G.; Liz-Marzan,

L. M. J. Nanosci. Nanotechnol. 2006, 6, 453–458.(16) Perez-Juste, J.; Correa-Duarte, M. A.; Liz-Marzon, L. M. Appl. Surf.

Sci. 2004, 226, 137–143.(17) Miranda, D.; Sencadas, V.; Sanchez-Iglesias, A.; Pastoriza-Santos,

I.; Marzan, L. M.; Ribelles, G.; Lanceros-Mendez, S. J. Nanosci. Nano-technol. 2009, 9, 2910–2916.

(18) Simoes, R.; Silva, J.; Vaia, R.; Sencadas, V.; Costa, P.; Gomes, J.;Lanceros-Mendez, S. Nanotechnology 2009, 20, 035703.

(19) Costa, P.; Silva, J.; Sencadas, V.; Costa, C. M.; Van Hattum,F. W. J.; Rocha, J. G.; Lanceros-Mendez, S. Carbon 2009, 47, 2590–2599.

(20) Dillon, D. R.; Tenneti, K. K.; Li, C. Y.; Ka, F. K.; Sics, I.; Hsiao,B. S. Polymer 2006, 47, 1678–1688.

(21) Peng, Q.-Y.; Cong, P.-H.; Liu, X.-J.; Huang, S.; Li, T.-S. Wear2009, 266, 713–720.

(22) Corma, A.; Garcia, H. Eur. J. Inorg. Chem. 2004, 6, 1143–1164.(23) Corma, A. Chem. ReV. 1995, 95, 559.(24) Guisnet, M.; Magnoux, P. Appl. Catal. 1989, 54, 1–27.(25) Parpot, P.; Teixeira, C.; Almeida, A. M.; Ribeiro, C.; Neves, I. C.;

Fonseca, A. M. Microporous Mesoporous Mater. 2009, 117, 297–303.(26) Neves, I. C.; Botelho, G.; Machado, A. V.; Rebelo, P.; Ramoa, S.;

Pereira, M. F. R.; Ramanathan, A.; Pescarmona, P. Polym. Degrad. Stab.2007, 92, 1513–1519.

(27) Corma, A. Chem. ReV. 1997, 97, 2373–2420.(28) Van Santen, R. A.; Kramer, G. J. Chem. ReV. 1995, 95, 637–660.(29) Database of Zeolite Structures from the International Zeolite

Association (IZA-SC). www.iza-structure.org/databases/ (accessed 14 May2010).

(30) Neves, I. C.; Botelho, G.; Machado, A. V.; Rebelo, P. Eur. Polym.J. 2006, 42, 1541–1547.

(31) Rahimpour, A. Appl. Surf. Sci. 2009, 255, 7455–7461.(32) Sencadas, V.; Gregorio, R., Jr.; Lanceros-Mendez, S. J. Macromol.

Sci., Part B: Phys. 2009, 48, 514–525.(33) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity;

Academic Press: San Diego, 1982.(34) (a) Ghesti, G. C.; Macedo, J. L.; Parente, V. C. I.; Dias, J. A.;

Dias, S. C. L. Microporous Mesoporous Mater. 2007, 100, 27–34. (b) Lutz,W.; Ruscher, C. H.; Heidemann, D. Microporous Mesoporous Mater. 2002,55, 193.

(35) Breck, D. W.; Flanigen, E. M. Molecular SieVes; Society ofChemical Industry: London, 1968, p 47.

(36) Silva, B.; Figueiredo, H.; Quintelas, C.; Neves, I. C.; Tavares, T.Microporous Mesoporous Mater. 2008, 116, 555–560.

(37) Neves, I. C.; Botelho, G.; Machado, A. V.; Rebelo, P. Mater. Chem.Phys. 2007, 104, 5–9.

(38) Firmino Mendes, S.; Costa, C. M.; Sencadas, V.; Serrado Nunes,J.; Costa, P.; Gregorio Jr, R.; Gomes Ribelles, J. L.; Lanceros-Mendez, S.Appl. Phys. A: Mater. Sci. Process. 2009, 96, 899–908.

(39) Zhou, W.; Zhao, K. S. Colloids Surf., A 2008, 317, 10–16.(40) Stephenson, M. J.; Holmes, S. M.; Dryfe, R. A. W. Angew. Chem.,

Int. Ed. 2005, 44, 3075–3078.(41) Kasap, S. O. Principles of Electrical Engineering Materials and

DeVices; McGraw-Hill International: Boston, 2000.(42) Botelho, G.; Lanceros-Mendez, S.; Goncalves, A. M.; Sencadas,

V.; Rocha, J. G. J. Non-Cryst. Solids 2008, 354, 72–78.(43) Madroskaya, L. Y.; Longinova, N. N.; Panshyn, A.; Lobanov, A. M.

Polym. Sci. U.S.S.R. 1983, 25, 2490–2496.(44) Marcilla, A.; Siurana, A. G.; Berenguer, D. Appl. Catal., A 2006,

301, 222–231.(45) Marcilla, A.; Beltran, M. I.; Navarro, R. J. Anal. Appl. Pyrolysis

2005, 74, 361–369.(46) Garforth, A. A.; Fiddy, S.; Lin, Y. H.; Ghanbari-Siakhali, A.;

Sharratt, P. N.; Dwyer, J. Thermochim. Acta 1997, 294, 65–69.(47) Van Santen, R. A.; Kramer, G. J. Chem. ReV. 1995, 95, 637.

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