Piezoelectric β Polymorph in Poly(vinylidene ... - ACS Publications

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J. Phys. Chem. C 2007, 111, 14670-14680

Piezoelectric β Polymorph in Poly(vinylidene fluoride)-Functionalized Multiwalled Carbon Nanotube Nanocomposite Films Swarup Manna and Arun K. Nandi* Polymer Science Unit, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700 032, India ReceiVed: April 23, 2007; In Final Form: August 1, 2007

Poly(vinylidene fluoride) (PVF2)-multiwalled carbon nanotube (MWNT) nanocomposites (PCNCs) are prepared using ester (-COOC2H5)-functionalized MWNT (F-MWNT). Due to the specific interaction of the >CdO group in F-MWNT and the >CF2 group of PVF2, the dispersion of F-MWNT in PVF2 matrix is uniform. Transmission electron microscopy study reveals that the F-MWNTs in the composite are fatter than the pure F-MWNTs indicating that the PVF2 chains are wrapped on the surface of MWNTs. In the nanocomposites, the spherulitic morphology of PVF2 is lost and the fibrils are curled and smaller in length than those of pure PVF2 sample. Field emission scanning electron microscopy study indicates good dispersion of carbon nanotubes in the F-MWNT samples. The solvent-cast films have the β-polymorphic (piezoelectric) structure for F-MWNT concentration g1% (w/w) and have a mixture of R and β polymorphs below that concentration of F-MWNT. In the melt-cooled specimen, there occurs a mixture of R and β polymorphs and the latter is totally absent in the corresponding unfunctionalized MWNT-PVF2 composites (UCP). Possible explanation of β polymorph formation in the PCNCs has been offered. The Tg of F-MWNT-PVF2 nanocomposite was higher than that of pure PVF2, and it is also greater than those of unfunctionalized MWNTPVF2 nanocomposites. In the melt-cooled samples, the β phase increases, with a concomitant decrease of the R phase, with increasing F-MWNT content and reaches a saturation value of ∼50% at 5% F-MWNT content in the PCNC. The storage modulus and loss modulus values increase with increase in F-MWNT concentration, and the percent increase in storage modulus is much greater than that of unfunctionalized MWNT-PVF2 nanocomposites, particularly for higher MWNT content (>0.1% (w/w)). The current (I)-voltage (V) characteristic curves are very interesting, and their nature depends on the amount of F-MWNT in the composite. The 1% and 2% F-MWNT content samples have negative hysteresis in the I-V curves for the complete cycle of forward and reverse bias while the corresponding CP5 samples exhibit a memory effect in both the negative and the positive bias, rendering the material to be useful in fabricating memory devices. The aged CP5 sample (∼3 months) requires a higher inflection voltage to show memory effect, while the UCP5 sample does not exhibit any memory effect because of longer conducting path and higher conductivity.

Introduction In recent years polymer nanocomposites have drawn considerable research interest because of their dramatic improvement in physical, thermal, mechanical, and electroactive properties compared to those in the pristine polymer.1-12 Both inorganic and organic nanofillers are used for this purpose on a variety of polymers by different group of workers. Among the former category, mainly the clay polymer nanocomposites have drawn major attention because of their lower cost and very high reinforcing property.1-7 The dispersion of metal or semiconductor nanoparticles within the polymer matrix also exhibits a large enhancement of the physical and mechanical properties.8-12 Among the organic nanofillers, the carbon nanotube is the most important13-17 because it possesses low mass density and large aspect ratio that yields a unique combination of mechanical, thermal, and electroactive properties.18-20 There are mainly two types of carbon nanotubes, namely, the single walled carbon nanotube (SWNT) and the multiwalled carbon nanotube (MWNT). In both samples, there is this problem of acheiving homogeneous dispersion in low molecular weight liquids * To whom correspondence should be addressed. E-mail: psuakn@ mahendra.iacs.res.in

(solvent) and also in polymers.19,21,22 Functionalization of nanotubes with relatively large functional groups is an easy way to disperse the carbon nanotubes homogeneously.23-25 In the present study, we have functionalized the MWNT with -COOC2H5 groups which render the functionalized multiwalled nanotube (F-MWNT) soluble in N,N-dimethyl formamide (DMF) and also in poly(vinylidene fluoride) (PVF2). We have chosen MWNT because it has a better reinforcement property than that of SWNT.15 PVF2 is a technologically important polymer, and in its crystalline state it exhibits five different polymorphic structure.26 The variety of crystalline polymorphs in PVF2 is probably related to the similar atomic radii of hydrogen and fluorine atoms in the polymer.27 The most common polymorph of PVF2 is R having a monoclinic unit cell with TGTG h chain conformation and is generally produced during crystallization from the melt.28-30 The piezoelectrically active β polymorph has an orthorhombic unit cell with an all-trans conformation.26,31 The γ phase also has an orthorhombic unit cell with a T3GT3G h chain conformation32 and is produced during crystallization from the melt at higher isothermal crystallization temperatures.33 The δ and  polymorphs are the polar and antipolar analogues of R and γ forms, respectively.26,34,35

10.1021/jp073102l CCC: $37.00 © 2007 American Chemical Society Published on Web 09/20/2007

Functionalized Carbon Nanotube Composites Generally, in the preparation of PVF2-clay6,7,36 and PVF2Ag nanocomposites,12 the piezoelectric β-polymorph PVF2 is produced when made from the solution casting method, but in the melt quenching method this β polymorph is not always retained. The electronic properties of carbon nanotubes (CNTs) are interesting, and the conductivity of the CNTs in the composite depends on the percolation threshold.37 So, PVF2CNT nanocomposite would be an interesting material for the combination of piezoelectric and conducting properties, and there is a report where PVF2-carbon nanotube composites yield β polymorph partly together with R polymorph in the solventcast samples.38a Yet, there is no report of pure β polymorph formation in the solvent-cast method and also the formation of any β polymorph in the melt-cooled condition for the PVF2CNT nanocomposite. Here, we report a method where PVF2MWNT nanocomposites prepared under solvent-cast method produce purely the β polymorph and in the melt-cooled condition the β polymorph is produced to a major extent (∼ 50%). The PVF2-MWNT nanocomposites with β polymorphic PVF2 would be very much useful because of significant enhancement in their piezoelectric and pyroelectric responses producing better transducers.38 The PVF2-MWNT nanocomposites also show a significant increase in the dielectric constant at low loadings (CF2 group of PVF2 has a strong, specific interaction with the >CdO group of the ester.40,41 Such a specific interaction between the components may yield an extended all-trans conformation of β PVF2, and the above conformation may be retained because the functionalized nanotubes are then molecularly dispersed in PVF2. In our previous work, we have shown that aliphatic diesters with longer intermittent carbon atoms are able to produce a fibrillar network structure of R PVF2, preventing the spheroid formation.42,43 Here, MWNTs have a more rigid backbone than that in aliphatic diesters, and therefore the pendent -COOR groups of the MWNTs may enforce the PVF2 chain to adopt a trans zigzag conformation of the β polymorph rather than allowing it to transform into lower energy TGTG h conformation of the R polymorph.44 In this paper, we shall discuss the morphological, structural, thermal, and mechanical properties of ester-functionalized PVF2-MWNT nanocomposites (PCNCs) produced under both solvent-cast and melt-cooled conditions. Also, the composite exhibits interesting current (I)-voltage (V) behavior because of additional dipolar charges of the β polymorph, and these I-V characteristic curves are included here. Experimental Section Samples. PVF2 (Sol-1010, Solvay Corporation, USA) was recrystallized from its dilute solution in acetophenone, washed with methanol, and finally dried in vacuum. The molecular weight (M h w) of the sample measured from GPC in N,N-dimethyl formamide (DMF) at 90 °C was found to be 4.48 × 105 having a polydispersity index of 2.09 using polystyrene as the standard. The H-H defect was measured from 19F NMR spectra in DMFd7 and was found to be 4.19 mol %. DMF used in this work was dried over anhydrous MgSO4 and distilled under vacuum. The middle fraction was used in all the work. MWNTs, produced by chemical vapor deposition (CVD), were purchased from Aldrich USA (product no. 636495, lot no. 04619DC). The MWNT was characterized by Raman

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Figure 1. FT-IR spectra of pristine MWNT, acid modified MWNT, and ester-modified MWNT (F-MWNT).

spectroscopy and had a low degree of graphatization.20 The functionalization of the MWNT was carried out in two steps: oxidation45 and modification.46 Oxidation. MWNT (300 mg) was dispersed in 40 mL of a mixture (3:1) of concentrated H2SO4 and concentrated HNO3 and was sonicated for about 8 h in an ultrasonic bath at ∼50 °C. Then the mixture was centrifuged and washed repeatedly with water until pH of the solution reached ∼6.5. The black solution of the MWNTs in water was then filtered through a 0.47 µm pore size PVF2 membrane (hydrophilic), and the filtered material was dried in vacuum at 50 °C for 1 day. Modification. The FT-IR spectra (Figure 1) of the oxidized samples have a new peak at 1718 cm-1 which corresponds to the vibration of >CdO group indicating the formation of the -COOH group. The -COOH groups were then converted to Na+ salt by sonicating 60 mg of the oxidized MWNT in 5 mM NaOH solution for 10 min. To the Na+ salt solution of the MWNT were added a phase transfer catalyst (tetra-n-octylammonium bromide) and 1-bromoethyl acetate, and the mixture was refluxed at ∼90 °C with vigorous stirring. After 8 h of reaction, a black precipitate was observed, and it was extracted in chloroform and was filtered through a PVF2 membrane. The precipitate was washed thoroughly with CHCl3 and dried in vacuum for 2 days. The functional groups introduced into the MWNT were characterized by FT-IR spectroscopy giving characteristic peaks of the >CdO group at 1732 cm-1 and C-H stretching of the ethyl group at 2850, 2920, and 2953 cm-1 (Figure 1) confirming the formation of the ester group. Preparation of Nanocomposites. The functionalized MWNT (F-MWNT) was dispersed in DMF (0.1% w/v) by sonication for 30 min in an ultrasonic bath (60 W, model AV10C, Eyela), which produced a dispersion that was stable for months. PVF2 was dissolved in DMF to make a 2% (w/v) solution by heating the mixture at 70 °C for 2 h under sealed condition. The PVF2 solution was mixed with the F-MWNT solution in the required proportion to produce different compositions of F-MWNT in PVF2 (0.1, 0.5, 1, 2, and 5% (w/w)) of the nanocomposite (PCNC). They were homogenized by sonication for 30 min, and the films were cast by evaporation of the solvent on flat dishes at 70 °C. They were finally dried in vacuum at 70 °C for 3 days. The melt-cooled PCNCs were prepared by melting a portion of the above films in a Mettler FP82HT hot stage for 15 min at 220 °C under nitrogen atmosphere and then cooling

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TABLE 1: Characteristics and Designations of Samples F-MWNT

unfunctionalized MWNT

composition of MWNT % (w/w) solvent-cast melt-cooled solvent-cast melt-cooled 0.1 0.5 1.0 2.0 5.0

CP0.1 CP0.5 CP1 CP2 CP5

MCP0.1 MCP0.5 MCP1 MCP2 MCP5

UCP0.1 UCP0.5 UCP1 UCP2 UCP5

MUCP0.1 MUCP0.5 MUCP1 MUCP2 MUCP5

at the rate of 5 °C/min to 50 °C. The sample notations of the PCNCs produced by both the methods are presented in Table 1. Microscopy. The dispersion of MWNT in the polymer matrix was studied using a transmission electron microscope (TEM, JEOL 2010EX) operated at an accelerated voltage of 200 kV. A drop of the dilute solution of the homogeneous mixture on the carbon coated copper grid was dried in air at 30 °C. It was finally dried in vacuum at 30 °C for 2 days and was directly observed under the TEM. The TEM study of the melt-cooled samples was carried out by heating the grid of the solvent-cast sample to 220 °C followed by cooling to 50 °C at the rate of 5 °C/min. The morphology of the melt-cooled and solvent-cast and freeze-fractured samples were observed through a field emission scanning electron microscope (FE-SEM, JEOL GSM-5800) at 5 kV. The samples were platinum coated before observation through FE-SEM. The texture of the sample was observed through an optical microscope with a perfectly crossed polarizer at 200× magnification. A Leitz Biomed microscope fitted with a semiautomatic camera (Wild MPS 12) system was used in this work. Spectroscopy. The UV-vis spectra of the PCNCs were taken from solution cast films on a glass cover slip using a HewlettPackard UV-vis spectrometer (model 8453). The spectra were refined by subtracting the reference spectrum of the glass cover slip in each case. FT-IR spectra of the PCNCs were obtained from thin films casted from their hybrid solution in DMF using a Shimadzu FT-IR instrument (FT-IR 8400S). For FT-IR spectra of the meltcooled samples, the cast films were melted at 220 °C and cooled at 5 °C/min to room temperature. WAXS Study. Wide-angle X-ray scattering of both meltcooled and solvent-cast samples was obtained by fixing them in an aluminum holder, and the scattering experiments were carried out using a Seifert X-ray diffractometer (model C3000) fitted with a parallel beam optics attachment. The instrument was operated at a 35 kV voltage and 30 mA current and was calibrated with a standard silicon sample. Nickel filtered Cu KR radiation (λ ) 0.154 nm) was used in the work. The samples were scanned from 2θ ) 2 to 35° in the step scan mode, and the diffraction pattern was recorded using a scintillation counter detector. Thermal Study. A Perkin-Elmer differential scanning calorimeter (Diamond DSC) working under N2 atmosphere was used to measure the thermal properties of PCNCs. The instrument was calibrated with indium before each set of experiment. The samples were taken in aluminum pans and were crimped using a universal crimper. Heating runs were performed at scan rates of 10 and 40 °C/min from 50 to 220 °C. Cooling runs were performed at a scan rate of 5 °C/min from 220 to 50 °C. The melting temperature and enthalpy of fusion were measured with the help of a personal computer attached to the instrument using Pyris software (version 7.0).

The thermal stability of the PCNCs was measured using a TGA/DTA instrument (model SDT Q600 TA Instruments) under nitrogen atmosphere at a heating rate of 20 °C/min. Mechanical Property. The storage modulus, loss modulus, and tan δ of the PCNC films were measured using a dynamic mechanical analyzer (DMA) (TA Instruments, model Q-800). Films of 25 × 5 × 0.08 mm dimension were made from the composites by solution casting on a die, and they were installed on the film tension clamp of the calibrated instrument. The samples were heated from -110 to 150 °C at a heating rate of 10 °C/min. The storage modulus, loss modulus, and tan δ values were measured at a constant frequency of 1 Hz with a static force of 0.01 N. Conductance Measurement. The I-V characteristic curves of the F-MWNT-PVF2 nanocomposites were studied using the solvent-cast films by applying a voltage from -5 to +5 V. The films were gold coated, the voltage was varied incrementally at a rate of 0.5 V every 10 s (i.e., 3 V/min) from -5 to + 5 V, and the current (I) was measured at each applied voltage by an electrometer (Keithley, model 617). The I-V characteristics at 25 °C were measured in both sweep directions. For aged samples, the I-V characteristic curves were obtained by applying a voltage from -10 to +10 V at the same scan rate as above. Results and Discussion UV-vis Spectroscopy. In Figure 2a the UV-vis spectra of PCNCs prepared with F-MWNT and unfunctionalized MWNTs are shown. The absorption spectra of the PCNCs are dominated by Raleigh scattering from the nanophase dispersed in PVF2 matrix and as the wavelength increases the scattering intensity decreases.47 Again, with an increase in the MWNT concentration, the scattering intensity increases for any particular wavelength (for e.g., see curve 500 nm shown in Figure 2b). One interesting feature in the figure is that at the same concentration the scattering intensity is greater for unfunctionalized MWNT than that for F-MWNT. This indicates that the larger sized aggregates of carbon nanotubes are present in the unfunctionalized MWNTs compared to F-MWNTs.38 In other words, the F-MWNTs are better dispersed than the unfunctionalized MWNTs in the PVF2 matrix. The scattering intensity varies linearly with the concentration of F-MWNT (Figure 2b) (homogeneously dispersed) because the functionalized MWNTs have a carbonyl (>CdO) group which specifically interacts with the >CF2 group of PVF2.40,41 Microscopy. The dispersion of F-MWNTs can be visualized directly from Figure 3a-d for different concentrations of F-MWNT and they are well dispersed in all cases. In the TEM micrographs, overlapping of the nanotubes is observed in the composites and the overlapping increases with increase in MWNT concentration. From the TEM micrographs, the average diameters of MWNTs in different samples were measured and the values (averaged over 25 measurements) are 19.1 ( 2.8, 35.9 ( 3.7, 27.7 ( 6.5, and 27.6 ( 6.7 for F-MWNT, CP0.5, CP2, and CP5, respectively. This indicates that the average diameter of F-MWNT increases in the CP0.5 sample and then decreases in CP2 and CP5 samples. These results may be interpreted due to thicker MWNT because of wrapping of PVF2 on the MWNT surface through specific interaction of the >Cd O and >CF2 groups of F-MWNT and PVF2, respectively. The larger increase of F-MWNT diameter at lower F-MWNT content (CP0.5) may be due to the interaction of PVF2 chains to a specific F-MWNT, and as the F-MWNT concentration is

Functionalized Carbon Nanotube Composites

Figure 2. (a) UV-vis spectra of solvent-cast films of PVF2 and PCNCs prepared with F-MWNT (CP0.1, CP0.5, CP1, CP2, and CP5). Inset: UV-vis spectra of solvent-cast films of PVF2 and PCNCs prepared with unfunctionalized MWNT (UCP0.1, UCP0.5, UCP1, UCP2, and UCP5). (b) Plot of scattering intensity at 500 nm vs MWNT concentration (w/w %).

increased, the above interactive force becomes operative at all directions. This might be a reason for the decrease of the number of PVF2 chains adhering to a particular F-MWNT, hence, lowering the hybrid diameter than that of the CP0.5 sample. In the micrograph shown in Figure 3e, the dispersion of F-MWNT in the sample MCP2 produced under melt-cooled condition is presented and it is very much apparent that the dispersion is as good as in the micrograph of sample CP2 (Figure 3c). This indicates that the dispersion of F-MWNTs remains unaffected if the samples are prepared from the bulk under melt-cooled conditions. In the micrograph (Figure 3f) the dispersion of unfunctionalized MWNT in the sample UCP2 is shown and the MWNTs are not well dispersed as observed in micrograph 3c. The overlapping of MWNT to form agglomerated structure is more prominent here supporting the higher Raleigh scattering than that for F-MWNT composites (Figure 2b). It is interesting to note from the optical micrographs (Figure 4) that PVF2 has spherulitic morphology, but the spherulitic morphology is gradually lost on addition of F-MWNT both for

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14673 solvent-cast and for melt-cooled samples (Figure 1 in Supporting Information). The 0.1% F-MWNT containing composite has very small size spherulites, but from 0.5% F-MWNT concentration onward the spherulites are not at all observed. The loss of birefringes in the PCNCs might be due to the random orientation of PVF2 lamella/ fibrils producing the spherulitic structure of PVF2.6 The difference in morphology can be further visualized from Figure 5a,b, where the FE-SEM pictures of solvent-cast CP1 and pure PVF2 are compared. The R PVF2 spherulites of pure PVF2 have fibrils splaying in radial directions (Figure 5a) but in the F-MWNT nanocomposite the growth of PVF2 fibrils is not radial; they are curled and also smaller in size (Figure 5b). Similar observations were made in the the melt-cooled samples (Figure 2 in Supporting Information). However, there are mixtures of bright and dark fibrils in the MCP1 sample, the former due to R polymorph and the latter due to β-polymorph PVF2, which will be further discussed in the following section. In Figure 5c,d, the morphology of the freeze fractured surface of CP1 and CP5 samples is presented. In contrast to the micrographs of Figure 5a,b, here the carbon nanotubes are clearly observed and they exhibit network morphology. The network density is greater in CP5 sample than that in CP1 sample, but the diameters of the nanotubes of CP1 sample are larger than those of CP5 sample, probably for the same reason as discussed in the case of TEM micrographs. Structure. In Figure 6 the WAXS patterns of PVF2-FMWNT nanocomposites are presented. Figure 6a indicates that the solvent-cast PVF2 has peaks at 17.8°, 18.4°, and 20.0° which are characteristics of the R polymorph of PVF2.48-50 In the nanocomposites, the CP0.1 has a mixture of R- and β-polymorphic structure but for the compositions CP0.5-CP5 the X-ray pattern corresponds to that for pure β polymorph, as only the diffraction peak at 20° is present.48-50 For the meltcooled samples (Figure 6b), the situation is different than that of the solvent-cast samples. Here, the MCP0.1 has purely R-polymorphic structure but MCP0.5-MCP5 have a mixture of both R and β polymorphs. The percentage of β polymorph increases with the increase in F-MWNT content as evident from the decrease of the relative size of the peaks at 18.4° and 26.7° 2θ48-50 compared to those of pure PVF2 (cf. FT-IR study). In Figure 3 of Supporting Information, the WAXS patterns of the nanocomposites with unmodified MWNT are compared for solvent-cast and melt-cooled samples. The solvent-cast sample UCP0.1 has R-polymorphic PVF2 compared to the other compositions that have a mixture of R- and β-polymorphic structure, and the β polymorph increases with increase in F-MWNT concentration. On the other hand, the melt-cooled nanocomposites do not exhibit any of the characteristic peaks of β-polymorphic structure for any of the compositions (Figure 3b in Supporting Information). These results indicate that unfunctionalized MWNT cannot induce the formation of β polymorph in the melt-cooled samples, though the solvent-cast samples produce some β-polymorphic structure at higher MWNT concentration. Similar conclusions are also supported by FT-IR study of the nanocomposite samples (Figure 4a-d in Supporting Information). Also, it is interesting to note that the amorphous peak of CF-CH-CF bending vibration (875 cm-1)51-53 shifts to somewhat higher energy region (879 cm-1) indicating these >CF2 groups in the PVF2 chains are interacting with the >CdO group of the ester. The reason for β polymorph formation in the solvent-cast samples is not definitely known, and it might be possible that

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Figure 3. TEM pictures of F-MWNT and PVF2-F-MWNT nanocomposites: (a) F-MWNT, (b) CP0.5, (c) CP2, (d) CP5, (e) MCP2, and (f) UCP2.

MWNT has a nucleating effect (see thermal section). The MWNT surface has zigzag carbon atoms, which match with the all-trans conformation of β PVF2, and as a result may induce crystallization of PVF2 in the β-polymorphic structure. The crystallization rate is slow during solvent casting, so the β polymorph is produced to a large extent by the slow attachment of PVF2 segments on to the β nuclei adhered on the MWNT surface. In the F-MWNT composites, the formation of β phase is complete at higher F-MWNT concentration due to specific interaction, and in the unfunctionalized MWNT, the partial β phase formation is due to the lack of any specific interaction. But in the melt-cooled state, rapid crystallization occurs due to higher nucleation density of lower energy R nuclei44 in the bulk

phase of PVF2 and consequently R polymorph is produced for the unfunctionalized MWNT sample. In the melt-cooled FMWNT composites, due to the specific interaction between the >CdO group of ester and the >CF2 group of PVF2, the nucleation in β lattice is easily achieved on the F-MWNT surface and the interaction of PVF2 segments to the F-MWNT surface promotes the growth of the β crystal. But in the bulk (i.e., at a far distance from the F-MWNT surface where the interactive force is not operative) the R crystallites are produced to some extent due to the rapid nucleation of R PVF2. Thermal Properties. Thermogravimetric Analysis. The degradation temperatures (onset of inflection) for pure PVF2, CP0.1, CP0.5, CP1, CP2,

Functionalized Carbon Nanotube Composites

Figure 4. Optical micrographs of the solvent-cast (a) PVF2 and (b) 1% F-MWNT-PVF2 composite (CP1).

and CP5 are 455.3, 466.2, 470.7, 470.8, 467.1, and 468.2 °C, respectively. The same trend of the degradation temperature is evident from the differential thermogravimetric (DTG) curves (Figure 5 in Supporting Information). Hence, the thermal

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14675 stability of the PCNCs has increased to some extent and is almost independent of the F-MWNT concentration studied here. This increase in thermal stability is probably due to the better packing of the all trans conformation than that of the TGTG h conformation of PVF2. Differential Scanning Calorimetry. In Figure 7 the DSC thermograms of PVF2-F-MWNT nanocomposites are shown for heating of the solvent-cast and melt-cooled samples, respectively. It is apparent from Figure 7a that the pure PVF2 has two R polymorph melting peaks and the two peaks arise due to melt recrystallization.27,54 Under the melt-cooled condition (cooling rate 5 °C/ min) the presence of a single melting peak (Figure 7b) of pure PVF2 indicates the absence of melt recrystallization, though in both cases R polymorph is produced as evidenced from FT-IR and X-ray data. On the other hand, CP0.1 shows two peaks; the lower melting peak can be attributed to R polymorph and the higher one to the β polymorph as assigned from WAXS and FT-IR results. All the other composite samples exhibit two peaks for the melting of two polymorphs. This is confirmed from the similar texture of the thermograms on heating at a higher heating rate (40 °C/min, Figure 6 in Supporting Information) indicating that the two peaks are not arising from melt recrystallization. After the melt is kept at 220 °C for 10 min, they are cooled at a cooling rate of 5 °C/min. The pure PVF2, CP0.1, and CP5 all exhibit one exotherm whereas CP0.5, CP1, and CP2 all exhibit two exotherms, probably corresponding to the two different polymorph formation (Figure 7a in Supporting Information). The reason for formation of a single exotherm in CP5 sample is not clear, and probably both the R and the β polymorphs are crystallizing simultaneously, as evident from the skew nature of the exotherm. The faster crystallization of β polymorph than that of the R polymorph might be due to the nucleating effect of F-MWNT, which helps to produce zigzag chain conformation in β PVF2.

Figure 5. FE-SEM pictures of solvent-cast (a) pure PVF2, (b) CP1, and freeze fractured surfaces of (c) CP1 and (d) CP5 samples.

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Figure 6. (a) WAXS patterns of solvent-cast PVF2-F-MWNT nanocomposites at indicated compositions. (b) WAXS patterns of meltcooled PVF2-F-MWNT nanocomposites at indicated compositions.

In the case of unfunctionalized MWNT, the nanocomposite crystallizes only into a single peak as shown in Figure 7b of Supporting Information. Here, R PVF2 crystallizes at progressively higher temperatures with increasing MWNT concentration indicating that the MWNT acts only as a nucleating agent for the R polymorph and not induce the PVF2 chain to crystallize into β polymorph as evident from WAXS and FT-IR results. The probable cause is the interacting force between PVF2 and MWNT, which is much lower in the unfunctionalized samples than that in the functionalized samples yielding R polymorph in the former and mixed R and β phases in the latter. The attached PVF2 chains on F-MWNT surface produce the β-polymorphic structure whereas the PVF2 chains in the bulk produce the R-polymorphic crystals, as they do not experience any orienting influence of the F-MWNT as discussed earlier. The enthalpy of fusion values of the melt-cooled nanocomposites measured at the higher heating rate (40 °C/min) are found to be 58, 57, 57, 56, 53, and 46 J/gm for PVF2, MCP0.1, MCP0.5, MCP1, MCP2, and MCP5, respectively. A small decrease in the enthalpy values with increase in the F-MWNT concentration may be attributed to some disorder in the composite crystal, and the F-MWNT may be responsible for the formation of such a disordered structure. The ratio of the R and β polymorphs produced under meltcooled conditions for the F-MWNT nanocomposites has been calculated from the deconvolution of the two peaks in the DSC thermograms (Figure 7b), and the enthalpies of the R and β phases are plotted as a function of F-MWNT content in Figure 8. It is apparent that the β phase increases, with a concomitant decrease of R phase, with increasing F-MWNT content in the

Manna and Nandi

Figure 7. DSC melting endotherms (scan rate 10 °C/min) of PVF2F-MWNT nanocomposites at indicated compositions for (a) solventcast samples and (b) melt-cooled samples.

Figure 8. Plot of enthalpy of R and β phases with F-MWNT content (w/w %) (obtained from thermograms of Figure 7b).

composite and reaches a saturation (∼50% β and 50% R) for 5% F-MWNT content in the PCNC. One scenario that may be consistent with the data is that the F-MWNT is nucleating the β polymorph on its surface and R polymorph is nucleating in the bulk. Dynamic Mechanical Analysis. Figure 9a-c shows the temperature dependency of the storage modulus, loss modulus, and tan δ plots of F-MWNT-PVF2 composites. In Figure 8 of Supporting Information, the above properties are compared for unfunctionalized MWNT samples. It is apparent from Figure 9a that the storage modulus (G′) decreases with increase in temperature for all the samples, though the decrease is not linear. With increasing F-MWNT concentration the storage modulus increases gradually, and CP5 exhibits the highest storage

Functionalized Carbon Nanotube Composites

Figure 9. Mechanical property-temperature plots of PVF2-F-MWNT nanocomposites at indicated compositions: (a) storage modulus, (b) loss modulus, and (c) tan δ.

modulus. In Figure 9b, the loss modulus temperature plots exhibit two peaks with increase in temperature. With increase in the F-MWNT content in the PCNCs, the loss modulus also increases as in the case of storage modulus indicating that the cause for the modulus increase is almost same in both the cases. In the loss modulus temperature plots, the lower temperature peak arises at ∼ -40 °C and the higher temperature peak arises at ∼10 °C. The lower temperature peak may be attributed to the glass transition temperature (Tg), and in the composites the Tg values (Table 2a) are higher than that of pure PVF2, though the increase of Tg decreases with F-MWNT content >0.1%. The increase of Tg with incorporation of F-MWNT might be attributed to the specific interaction of F-MWNT with PVF2 segments. However, with increase in F-MWNT concentration, the interactive force between F-MWNT and PVF2 becomes progressively isotropic making the segmental motion relatively free thereby decreasing the Tg. This observation is comparable

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14677 to the F-MWNT diameter values in the solvent-cast composites obtained from TEM investigation. The reason for the lower value of Tg in the CP5 sample than that of pure PVF2 is not known and it might be due to complete conversion into β phase, which has a resultant dipole acting along the C-F direction in the chain. The intrachain repulsion between the dipoles may be responsible for easier segmental Brownian motion causing a lowering of the Tg. This explanation appears to be true because the Tg progressively decreases with increase in the MWNT content yielding higher amount of β phase. In Figure 9c, the tan δ versus temperature plots also exhibit two peaks; the lower peak temperature is due to the glass transition of the samples. The peak temperatures from the loss modulus and tan δ plots are not exactly equal and (a difference ∼5 °C) might be due to two different modes of measurement. The former is related to the dissipation of energy as heat and the latter is related to the reduction of vibration of the material (i.e., damping). Now, we would like to discuss about the higher peak temperature (Tg) in the loss modulus plot (Figure 9b). The origin of this peak might be due to the relaxation of the PVF2 segments in the crystallineamorphous interphase.12, 55 This transition temperature initially increases with increase in F-MWNT concentration and then decreases at higher F-MWNT content, as in the case of Tg data. The reason would be the same as discussed in the Tg of the samples. It is now pertinent to compare the Tg values of unfunctionalized and F-MWNT nanocomposites from Table 2. Comparison of the Tg values indicates that the unfunctionalized MWNT composites have lower Tg values than those of F-MWNT composites, and they do not exhibit any definite trend in variation with composition. This may be due to the lower interaction than with F-MWNT composites causing inhomogeneous dispersion of MWNT (cf. Figure 3f). In Table 2, the storage modulus (G′) values of the composites are compared for different composite compositions both for F-MWNT and for unfunctionalized MWNT nanocomposites. In composites, there is an increase in the storage modulus compared to that of pure PVF2 with increasing F-MWNT concentration at each temperature. Again, with increase in temperature, the percentage increase of G′ is higher for both type of nanocomposites. The highest increase in G′ is found to be 120% than that of pure PVF2 at 50 °C in the F-MWNT-PVF2 nanocomposite (CP5) and may be attributed to the large surface area, aspect ratio, and homogeneous dispersion of F-MWNT. If one compares the percentage increase of G′ of both the composites, the functionalized samples have greater enhancement of the mechanical property than that of unfunctionalized samples except for CP0.1 sample. This might be due to the interaction of >CdO group of F-MWNT with >CF2 group of PVF2 making the dispersion better and intimate, causing the energy transfer more efficient than that of the unfunctionalized MWNT composites. It may be noted that during the functionalization of MWNT shortening of MWNT length occurs (cf. Figure 3c and e) causing a decrease in the aspect ratio.23 As a result, the mechanical property enhancement in CP0.1 is poorer than that of unfunctionalized MWNT. Another cause of higher G′ increase of UCP0.1 than that of CP0.1 may be due to the better dispersion of unfunctionalized MWNT at this lower MWNT concentration. With increase in unfunctionalized MWNT content, the percent increase in G′ decreases at each temperature due to the poorer dispersion compared to that in F-MWNT sample, and for UCP2 and UCP5 the dispersion is so poor (Figure 3f) that there is practically no enhancement in mechanical property. Current (I)-Voltage (V) Behavior. In Figure 10 the I-V characteristic curves of solvent-cast F-MWNT-PVF2 nano-

14678 J. Phys. Chem. C, Vol. 111, No. 40, 2007

Manna and Nandi

TABLE 2: Comparison of Storage Modulus (G′) (a) Solvent-Cast F-MWNT-PVF2 Nanocomposites Tg (from loss modulus data) sample

Tg(I) (°C)

Tg(II) (°C)

G′ at -100 °C (MPa)

PVF2 solvent cast CP0.1 CP0.5 CP1 CP2 CP5

-46.2 -36.5 -37.9 -39.8 -42.2 -49.0

9.6 23.7 25.0 21.2 15.7 13.2

4825 5205 5962 6270 7595 7671

% increase

G′ at -40 °C (MPa)

7.9 23.6 30 57.4 59.0

2742 3440 4031 4042 4519 4157

% increase

G′ at 50 °C (MPa)

% increase

25.4 47 47 65 52

807 1119 1429 1374 1314 1778

38.6 77 70 63 120

% increase

G′ at 50 °C (MPa)

% increase

(b) Solvent-Cast Unfunctionalized MWMNT-PVF2 Nanocomposites Tg (from loss modulus data) sample

Tg(I) (°C)

Tg(II) (°C)

G′ at -100 °C (MPa)

PVF2 solvent cast UCP0.1 UCP0.5 UCP1.0 UCP2.0 UCP5.0

-46.2 -41.5 -40.7 -45.3 -40.6 -44.7

9.6 13.7 15.4 11.7 16.3 19.9

4825 7566 5431 5497 4390 4230

composites are presented. It is apparent from the figure that pure PVF2 does not show any variation of current with the voltage applied (5 V) indicating the completely insulating nature of the R-polymorph PVF2. On addition of F-MWNT the slopes of I-V curves gradually increase with increasing MWNT concentration. For the CP1 and CP2 samples, the conductance reaches 10-9 A at the applied voltage of 5 V and may be approximately considered to behave like a semiconducting material. The I-V behavior of the CP5 sample is really interesting and at ∼5 V the conductance suddenly rises to 10-8 A. Such an inflection in conductance might be attributed for the increasing conducting paths due to increased concentration of carbon nanotube in the composite. In Figure 11a-c, the I-V characteristic curves for the complete cycle (0 f +5 V f -5 V f 0 V) of applied voltage are shown for CP1, CP2, and CP5 samples. Figure 11a clearly illustrates that CP1 has a negative hysteresis, that is the current at the initial applied voltage is greater than that at the same voltage in the reverse path. This is also true for the CP2 sample. Possibly, the β-polymorph PVF2 in the composite has some charge separation during its preparation. On the application of positive bias the dipole direction adds to yield higher current initially, but in the reverse process the dipole direction opposes the current flow yielding lower

Figure 10. Current (I, nA) vs voltage (V, V) plot of solvent-cast F-MWNT-PVF2 nanocomposite at indicted compositions at 25 °C.

% increase 57 12.5 14

G′ at -40 °C (MPa) 2742 4744 3564 3280 2800 2428

73 30 20 2

807 1784 1343 984 1060 468

121 66 22 31

current. In the case of CP5 sample (Figure 11c), a completely different behavior is observed. Here, we observe a memory effect in both the positive and the negative bias, that is in the reverse process the system appears to retain some charge within the composite yielding higher current. One possible explanation may be derived from the increasing concentration of the conducting F-MWNT in the composite. Because of the higher concentration of MWNT, the conducting charges of the composite may hop into the MWNT surface where the charges become stabilized through the π electrons of graphene rings of F-MWNT. Such resonatingly stabilized charges require lower potential to be annihilated, thereby causing higher current in the reverse bias. It is to be noted that the threshold voltage values for annihilation of stored charges are 1.7 and -1.6 V while the inflection voltage values are 4.7 and -4.9 V in the forward and reverse bias, respectively. On further increase of the F-MWNT concentration to 7%, the system (CP7) behaves like a good conductor (I ) 80 µA) and does not exhibit any memory effect even when the bias is applied to 10 V (Figure 11d). In Figure 12a, the I-V characteristic curves of the aged samples (aged for 3 months) are presented where the inflection voltage values of the CP5 sample have shifted to 6.9 and -6.7 V and the threshold voltage values for annihilation of charges shift to 2.2 and -2.1 V in the two opposite bias. The increase in inflection voltage may be attributed to the loss of alignments of β-PVF2 dipoles due to thermal movement during aging, and at higher voltage they again become aligned showing an inflection of current and memory effect. The threshold voltage of charge annihilation however remain almost the same, and the slightly increased values (2.2 and -2.1 V) compared to the freshly prepared CP5 sample might be attributed to some disorderliness due to aging. In the other samples, aging does not exhibit any significant variation of I-V characteristics (Figure 12b). The UCP5 sample however does not exhibit any memory effect (Figure 12c) though it has β-polymorphic structure. Probably, the long carbon nanotubes in unfunctionalized samples increase the conducting path, hence the very high conductivity (I ) 3 µA), inhibiting any memory effect. Also, the contact between unfuntionalized MWNT and β PVF2 is not as intimate as in the case of functionalized MWNT and β PVF2. Thus, the memory effect of the composite (CP5) is really interesting and may find use in fabricating memory devices.

Functionalized Carbon Nanotube Composites

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14679

Figure 12. I-V plot of the solvent-cast F-MWNT-PVF2 nanocomposite (aged for 3 months) for the complete cycle of applied voltage at 25 °C for (a) CP5, (b) CP2, and (c) UCP5 sample.

Conclusion

Figure 11. I-V plot of the solvent-cast F-MWNT-PVF2 nanocomposite for the complete cycle of applied voltage at 25 °C for (a) CP1, (b) CP2, (c) CP5, and (d) CP7 samples.

Compared to the unfunctionalized samples, the ester-functionalized MWNTs are highly dispersed in PVF2 samples produced by solvent-cast method. The diameter of the MWNT increased in the hybrids (highest increase for CP0.5 sample) indicating an effective wrapping of the MWNT by PVF2 chains. The spherulitic morphology of PVF2 is lost in the F-MWNT composites; also, the PVF2 fibrils become curled and shorter in the composites compared to those of pure PVF2 samples under both solvent-cast and melt-cooled conditions. FE-SEM study indicates good dispersion of the MWNTs in the composite for

14680 J. Phys. Chem. C, Vol. 111, No. 40, 2007 functionalized samples. Both X-ray and FT-IR studies indicate β polymorph formation of PVF2 in the solvent-cast samples, but in the melt-cooled samples a mixture of R and β polymorph is produced for the F-MWNT composites. The melt-cooled unfunctionalized MWNT nanocomposite produce only R polymorph. The DSC thermograms of solvent-cast F-MWNT-PVF2 composites exhibit two peaks for PVF2: the lower peak is for R polymorph and the higher peak is for the β polymorph, which increases with increase in F-MWNT concentration. For CP2 sample, the formation of 100% β polymorph is complete. In the melt-cooled samples, however, a maximum of 50% β-polymorph PVF2 was achieved for 5% F-MWNT concentration. With addition of F-MWNT, the storage modulus increases and a maximum 120% increase of storage modulus is observed for CP5 sample. The Tg’s of the nanocomposites are increased, and the increase is greater in the F-MWNT-PVF2 composites than those in the unfunctionalized samples. The I-V characteristic curves are very interesting, and their nature depends on the amount of F-MWNT in the composite. The CP1 and CP2 samples have a negative hysteresis in the I-V curves of forward and reverse bias, while the CP5 sample exhibits a memory effect in both the negative and positive bias. The aged CP5 sample requires higher inflection voltage to show memory effect, while the UCP5 sample does not exhibit any memory effect because of higher conductivity. Acknowledgment. We gratefully acknowledge the Department of Science and Technology, New Delhi for the financial support for this work. S.M. acknowledges CSIR, New Delhi for providing the fellowship. Supporting Information Available: (1) Optical micrographs of melt-cooled F-MWNT nanocomposites. (2) FE-SEM pictures of melt-cooled PVF2 and MCP1. (3) WAXS patterns of solvent-cast and melt-cooled unfunctionalized MWNT nanocomposites. (4) FT-IR spectra of solvent-cast and melt-cooled F-MWNT and unfunctionalized MWNT nanocomposites. (5) TGA and DTG curves for F-MWNT nanocomposites. (6) DSC melting endotherms (scan rate 40 °C/min) of F-MWNT nanocomposites for solvent-cast samples and melt-cooled samples. (7) DSC cooling exotherms from the melt at 220 °C at 5 °C/ min for F-MWNT and unfunctionalized MWNT nanocomposites. (8) Mechanical property-temperature plots unfunctionalized MWNT nanocomposites. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Giannelis, E. P.; Krishnamoorti, R.; Manias, E. AdV. Polym. Sci.1999, 138, 107. (2) Alexandre, M.; Dubois, P. Mater. Sci. Eng. ReV. 2000, R28 (1-2), 1. (3) Vaia, R. A.; Krishnamoorti, R. In Polymer Nanocomposites: Synthesis, Characterization, and Modeling; Krishnamoorti, R., Vaia, R. A., Eds.; American Chemical Society: Washington, DC, 2001; p 1. (4) Maiti, P.; Nam, P. H.; Okamoto, M.; Hasegawa, N.; Usuki, A. Macromolecules 2002, 35, 2042. (5) Park, J. H.; Jana, S. C. Macromolecules 2003, 36, 2758. (6) Shah, D.; Maiti, P.; Gunn, E.; Schmidt, D. F.; Jiang, D. D.; Batt, C. A.; Giannelis, E. P. AdV. Mater. 2004, 16, 1173. (7) (a) Priya, L.; Jog, J. P. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 1682. (b) Priya, L.; Jog, J. P. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 31. (8) Mbhele, Z. H.; Salemane, M. G.; van Sittert, C. G. C. E.; Nedeljkovic, J. M.; Djokovic, V.; Luyt, A. S. Chem. Mater. 2003, 15, 5019. (9) Hussain, I.; Brust, M.; Papworth, A. J.; Cooper, A. I. Langmuir 2003, 19, 4831. (10) Gaddy, G. A.; McLain, J. L.; Korchev, A. S.; Slaten, B. L.; Mills, G. J. Phys. Chem. B 2004, 108, 14858.

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