Contrasting Effects of Graphene Oxide and Poly(ethylenimine) on the

Article pubs.acs.org/crystal

Contrasting Effects of Graphene Oxide and Poly(ethylenimine) on the Polymorphism in Poly(vinylidene fluoride) Maya Sharma,† Giridhar Madras,‡ and Suryasarathi Bose*,§ †

Centre for Nano Science and Engineering, ‡Department of Chemical Engineering, and §Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: The nature of interaction between a heteronucleating agent (graphene oxide, GO) and a strongly polar macromolecule (poly(ethylenimine), PEI) with poly(vinylidene fluoride) (PVDF) influencing the crystalline structure and morphology has been systematically investigated in this work. PEI interacts with PVDF via ion-dipole interaction, which helps in lowering the free energy barrier for nucleation thereby promoting faster crystallization. In contrast, besides interacting with PVDF, GO also promotes heteronucleation in PVDF. We observed that both GO and PEI have very different effects on the overall crystalline morphology of PVDF. For instance, the neat PVDF showed a mixture of both α and β phases when cooled from the melt. However, incorporation of 0.1 wt % GO resulted in phase transformation from the stable α-phase to polar β-polymorph in PVDF. In contrast, PEI, which also resulted in faster crystallization in PVDF predominantly, resulted in the stable α- phase. Various techniques like Fourier transform infrared spectroscopy, X-ray diffraction, and differential scanning calorimetry were employed to confirm the phase transformations in PVDF. PEI was further grafted onto GO nanosheets to understand the combined effects of both GO and PEI on the polymorphism in PVDF. The PVDF/PEI−GO composite showed a mixture of phases, predominantly rich in α. These phenomenal effects were further analyzed and corroborated with the specific interaction between GO and PEI with PVDF using X-ray photon scattering (XPS) and NMR. In addition, the dielectric permittivity increased significantly in the presence of GO and PEI in the composites. For instance, PVDF/PEI−GO showed the highest permittivity of 39 at 100 Hz.

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nanotubes (CNTs),24−26 carbon black,27 etc., which transforms or induces the polar phase in PVDF. In a recent review, Bohlén et al.28 reported the various processes that were employed to induce the β phase in PVDF. They also reported the nucleation effect of CNTs resulting in the β-phase in PVDF when cooled from melt. In our earlier studies,19,29,30 we have clearly demonstrated the existence of both phases in PVDF/PMMA blends in the presence of functionalized CNTs. Apart from CNTs, graphene oxide sheets (GO) also induce similar effects in the PVDF matrix,31 where the −CO functional moieties present at the edge of GO interacts with the -CF2 group in PVDF resulting in the formation of the β phase even at very low concentrations (0.1 wt %). The incorporation of GO sheets leads to piezoelectric behavior in PVDF which is of industrial importance.32 PEI is a biocompatible cationic polymer that has been widely used in drug delivery applications.33,34 There are studies that report that conjugation of PEI with GO improves DNA binding and condensation and transfection efficiency.35,36 There are several studies that reflect the effect of a heteronucleating agent on the polymorphism in PVDF; however, the effect of a polar macromolecule on the

INTRODUCTION Poly vinylidene fluoride (PVDF) is the most studied polymeric material due to its significant scientific and technological relevance. PVDF is characterized by its high dielectric constant, piezo- and pyroelectric effects,1,2 membrane characteristics,3,4 excellent thermal stability,5 unusual high mechanical properties6,7 and processability. PVDF exhibits polymorphism and when cooled from either melt or solution different phases such as α, β, γ, and δ are evolved.8−10 The predominant α phase has TGTG′ semihelical conformation, and it is the most stable polymorph that develops upon cooling from melt. The electroactive β-polymorph is polar and is commercially relevant in context to its piezoelectric properties. The β phase exists in trans-conformation, i.e., H and F are on the opposite side of the main backbone chain, resulting in nonzero dipole moment in PVDF. Various strategies have been employed in the past for obtaining the β phase (such as drawing or uniaxial stretching,11−13 thermal annealing,14 high electric field,15 mechanical rolling,16 solvent induced crystallization in certain conditions such as DMF, DMSO,17 by incorporating a polar polymer like PA6,18 PMMA,19 etc., quenching or crystallization under controlled conditions20). Apart from processing, incorporation of nanoparticles also induces the β phase in PVDF.21,22 Several studies are available that clearly show the effect of different nanoparticles like nanoclay,23 carbon © XXXX American Chemical Society

Received: April 1, 2015 Revised: May 12, 2015

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DOI: 10.1021/acs.cgd.5b00445 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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carried out on a Bruker Discover D8 using Cu Kα radiation (40 kV) in the 2θ range of 10−50°. Thermal properties of composites were measured using DSC measurements using Q2000 from TA Instruments. XPS of composites were obtained using a Kratos Analytical instrument. 13C and 19F NMR of composites were done using Delta 2 (Jeol) single pulse NMR. The dielectric properties of composites were determined using an Alpha-N Analyzer, Novocontrol (Germany) in a broad frequency range of 0.01 ≤ ω ≤ 107 Hz. The onset of crystallization and the evolution of crystalline morphology were studied using a polarizing optical microscope (POM) (Olympus BX51, Japan) fitted with an automated hot stage (Linkam THMS600). A CCD camera (ProgRes C3, Germany) mounted on the microscope allowed the recording of the evolution of morphology as a function of temperature. Surface morphologies of the samples were evaluated using scanning electron microscopy (SEM, Zeiss) at room temperature. The dynamic mechanical analysis studied on various composites is discussed in the Supporting Information (see Figure S1).

polymorphism in PVDF is less understood. It is envisaged that functional moieties on the surface of nanoparticles or in the polar macromolecules have a strong effect on the polymorphism of PVDF. However, the nature of interaction between the functional groups and the PVDF on the overall crystalline morphology has not received much attention. Hence, in this study, we have systematically investigated the effect of GO sheets, PEI, and PEI grafted GO on the crystalline morphology and polymorphism in PVDF by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The possible interactions between GO, PEI, and PVDF were assessed using X-ray photon scattering (XPS) and NMR. As mentioned earlier PEI is a hyper-branched macromolecule with abundant NH2 terminal groups. In order to study the effect of concentration of NH2 terminal groups, a few compositions were prepared using methylene dianiline comprising two terminal NH2 groups. This allowed us to systematically assess the effects on the polymorphism in PVDF.

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RESULTS AND DISCUSSION Figure 1 illustrates schematically the conjugation between PEI and GO and MDA and GO. The amine groups of PEI

EXPERIMENTAL SECTION

Materials. PVDF (Kynar 761) with an average Mw of 44 0000 g/ mol was procured from Arkema, USA. Hyperbranched PEI with an average Mw of 25 000 g/mol, N-hydroxysuccinimide (NHS), N-(3(dimethylamino)propyl)-N′-ethylcarbodiimide hydrochloride (EDC), MES (2-(N-morpholino)ethanesulfonic acid) buffer, and 4,4-methylene dianiline (MDA) were obtained from Sigma-Aldrich. Tetrahydrofuran, N,N-dimethylformamide, and all other solvents were procured from commercial sources and used without any further purification. Synthesis of PEI−GO Conjugate. Graphene oxide (GO) was synthesized using modified Hummer’s method as reported in our previous studies.37,38 The following protocol was adopted to conjugate the hyperbranched PEI onto GO sheets.35 Typically, GO flakes were first dispersed in MES buffer (0.1 M). PEI was separately dissolved in MES buffer. EDC and NHS stock solutions were also prepared in MES buffer. The EDC/NHS solution was added to the GO solution and stirred at 37 °C for 1 h. EDC reacts with COOH groups of GO and forms an intermediate complex. PEI solution was then added to this mixture and stirred for 24 h. The prepared PEI-GO solution was filtered using centrifugation at 10 000 rpm for 10 min. The composite was washed several times with DI water to remove EDC/NHS residues and unreacted PEI chains. The sample was dried in an oven for 12 h at 60 °C. The synthesis of MDA-GO has been reported in our previous study.38 Preparation of PVDF Composites. Solution casting was used for preparing PVDF based composites. For PVDF/GO composites, GO (1 and 0.1 wt %) was dispersed in DMF using bath sonication. PVDF was dissolved in DMF separately, and the dispersed GO solution was then added to the PVDF solution and mixed using a shear mixer at 8000 rpm for 45 min. The PVDF/GO mixture was then poured into a glass petridish and left to dry. Similarly, PVDF/PEI and PVDF/PEIGO composite samples were also prepared. Neat PVDF was also prepared using same protocol and used as the control sample. In PVDF/PEI-GO samples, the concentration of GO was fixed at 0.1 and 1 wt % (determined a priori from TGA analysis). The obtained samples were further molded into thin films (∼100 μm) using compression molding and used for subsequent measurements. Characterization. Characterization of PEI−GO Conjugates. FTIR was performed using a PerkinElmer frontier by accumulating 32 scans over a range of 600−4000 cm−1, and X-ray photon scattering (XPS, Kratos Analytical instrument with a Al monochromatic source 1.486 keV) was used for confirming the grafting of PEI onto the GO sheets. Characterization of Polymer Composites. FTIR of composite films was carried out using ATR mode. XRD of the composites were

Figure 1. Schematic illustration showing the synthesis of (top) PEIGO and (bottom) MDA-GO.

covalently reacts with the carboxylic groups of graphene oxide resulting in the formation of amide bonds. PEI−GO and MDA−GO are further characterized by FTIR, as shown in Figure 2. The FTIR spectrum of GO showed the presence of −OH (3233 cm−1), −COOH (1722 cm−1), CC (1624 cm−1), and C−O−C (1043 cm−1). After conjugation with PEI, the peak corresponding to COOH group diminishes.35 Additionally, the characteristic peak of CC becomes broad, and a new peak appears at 1540 cm−1 corresponding to amide

Figure 2. FTIR spectrum of GO, PEI−GO, and MDA−GO. B



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II (C−N stretch + C(O)−N−H bend), which further confirms the covalent grafting of PEI chains onto GO sheets. The synthesis and characterization of MDA−GO have already been reported in our earlier study.38 Upon grafting of MDA onto GO sheets, the band around 1000−1150 cm−1 becomes broader indicating the nucleophilic substitution reaction at the epoxy groups. Furthermore, the presence of C−OH and C−N bonds is confirmed by the bands present in the range of 1000−1300 cm−1, as shown in Figure 2. The covalent grafting of PEI onto GO sheets was further confirmed using XPS. As observed from Figure 3a, the XPS

Figure 3. C 1s core level XPS spectrum of (a) GO, (b) PEI−GO.

Figure 4. (a) FTIR spectra of PVDF and PVDF based composites. (b) Enlarged FTIR spectra in the range of 800−1400 cm−1.

spectra of graphene oxide mainly show the C1s and O1s peaks. The PEI−GO composite powder shows an additional N1s peak in the XPS spectrum, which suggests the presence of nitrogen in the composite powder. In the XPS spectra of GO, distinct peaks that correspond to C−C at 285.58 eV, CO at 287.69 eV, and OC−OH at 289 eV are well evident. After conjugation with PEI, the intensity of oxygen containing groups decreases significantly (Figure 3b), and an additional peak appearing at 286.17 eV corresponds to the C−N bond. This further confirms the successful grafting of PEI chains onto GO sheets.39 Polymorphism in PVDF: FTIR, XRD, and DSC. Polymorphism in neat PVDF and in the presence of different agents is assessed here by FTIR (Figure 4a−b). Interestingly, PVDF obtained from solution casting and subsequently compression molded showed a mixture of both α and β phases. From the FTIR spectra of neat PVDF (Figure 4a), the characteristic peak at 763 (CF2 bending), 612 (wagging mode of CF2), and 976 cm−1 (twisting mode of CF2) indicates the presence of the α phase. In addition, the presence of a broad peak at 838 cm−1 (CH2 rocking) indicates the presence of the β phase in PVDF. As we mentioned in our earlier studies,16,40 the characteristic peaks of γ and β phases lie in the range of 833−840 cm−1 and are often very difficult to distinguish. As both of these phases correspond to the polar phase of PVDF, for simplicity, it is considered as a single polar phase. Thus, the neat PVDF consists of a mixture of phases, predominantly the β phase. Interestingly, the PVDF/GO composite exhibited predom-

inantly the β phase which is manifested by a peak centered at ∼840 cm−1; however, a small fraction of α is also evident (see Scheme 1). Achaby et al.31 reported the formation of pure β phase with 0.1 wt % GO in PVDF matrix. The transformation of α to β phase is due to the specific interaction between the fluorine group (>CF2) of PVDF and carbonyl groups (>CO) present on the GO surface. In a recent article, Huang et al.41 reported all β phase with 0.1 wt % reduced graphene oxide (rGO) in PVDF. The formation of the β phase can be explained by the adsorption of PVDF chains onto GO sheets. This adsorption is highly dependent on the interaction between PVDF chains and GO sheets. The adsorption facilitates the conversion of TGTG′ chains to TT conformation. Similar results were found in PVDF/CNT composites in which, using density functional theory, the authors confirmed the adsorption of PVDF chains on the surfaces of CNTs and formation of the β phase during crystallization.24 We observed similar results at 0.1 and 1 wt % GO. The characteristic peak of C−C asymmetric stretching modes of PVDF/GO shifted to 871− 874 cm−1 as against 875 cm−1 in neat PVDF. This can be attributed to specific interaction between GO and PVDF chains. The formation of 100% β phase can be ascribed to the heteronucleating nature of GO. The stiff GO sheets provide nucleation sites for PVDF (explained in subsequent sections) leading to faster nucleation and resulting in the formation of the β phase. In contrast, the PVDF/PEI composites show characteristic peaks of mainly α, and a small fraction of β phase C



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Scheme 1. Cartoon Illustrating the Contrasting Effects of Graphene Oxide and Poly (ethyleneimine) on the Polymorphism in PVDF

phase (TGTG) in PVDF. By adsorbing onto PEI surfaces, PVDF lowers its surface energy leading to faster crystallization. PVDF/PEI−GO composites show similar peaks in the FTIR spectra as that of PVDF/GO composites (the GO concentration is same in both the composites). The C−C asymmetric stretching modes of PVDF shifted from 875 to 872 cm−1 in PVDF/PEI−GO composites suggesting possible interaction between PEI−GO and PVDF chains (Figure 4b). However, the PVDF/PEI−GO composites show predominantly α phase as compared to PVDF and PVDF/GO composites, which are rich in the β phase. In order to study the effect of concentration of NH2 terminal groups interacting with PVDF, we prepared a composite with MDA as described earlier. Interestingly, the PVDF/MDA−GO exhibits similar spectra as in PVDF/PEI (Figure 4b), which further confirms that the concentration of NH2 groups significantly influence the ion-dipole interaction in PVDF composites. Although the molecular architecture of both PEI and MDA are very different, and moreover the concentration of NH2 groups are also different, we could not appreciate any change in the overall crystalline morphology in PVDF. This study clearly demonstrates that both PEI and MDA act as nucleating agents through ion-dipole interactions, thereby hastening the crystallization process. This finally yields a more stable α phase. It is important to note that both PEI and MDA interacts with PVDF via ion-dipole interaction; however, this could not influence the crystal structure. In contrast, a stiffer nucleating agent like GO can result in only the β phase in PVDF possibly due to the combined effect of heteronucleation activities and specific interaction with PVDF. The PVDF chains

is also evident in the composites. Park et al. reported the existence of both α and β phases of PVDF in PEI/PVDF elctrospun fibers.42 However, the presence of the α phase is dominant in PVDF/PEI composites as compared to neat PVDF (Figure 4a). Additionally, there is a new peak that appears in PVDF/PEI composites at 1382 cm−1 corresponds to the -CH2 bending of alkanes, which comes from PEI. C−C asymmetric stretching mode (1071 cm−1) and C−C asymmetric vibrations (875 cm−1) of PVDF are shifted to 1069 and 871 cm−1, respectively, in PVDF/PEI composites. The CF2 stretching mode shifts from 1167 to 1179 cm−1 (Figure 4b) in the composite samples, which suggests strong specific interaction between -NH2 groups of PEI and CF2 of PVDF. This presumably resulted in the formation of the α phase in PVDF. In the PVDF/PEI composite, CF2 asymmetric vibrations at 1276 cm−1 become broad and split into two peaks, and a new peak appears at 1294 cm−1 (Figure 4b). A plausible reason for these shifts in the wavenumbers in the presence of PEI could be attributed to ion dipole interaction between PVDF and PEI chains that leads to better dispersion of PEI in the PVDF matrix. Liang et al.43 reported that ion-dipole interaction is the most widely accepted mechanism for the direct formation of polar phases of PVDF from its composites containing charged substances. They also concluded that the ion-dipole is derived from the interaction between the positively charged surfaces and the CF2 dipoles in PVDF. As a result, the ion-dipole interaction resulted in a mixture of both phases but dominantly TGTG′ conformation (discussed in subsequent section). Hasegawa et al.8 reported that the energy of the β phase (TT conformation) is higher than that of the α D



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adsorb onto the basal plane of GO and finally take TT conformation. To get more insight into the crystal structure of PVDF and PVDF based composites, XRD scans (shown in Figure 5) were

specific interactions exist, PEI acts as a diluent resulting in a more stable α phase. The thermal properties of the composite are studied using DSC as illustrated in Figure 6. Interestingly, from DSC heating cycles (Figure 6), we can conclude that neat PVDF itself shows polymorphism manifested by the presence of double melting endotherm but a single peak in the cooling cycle. This indicates that compression molded samples resulted in a mixture of both phases in PVDF. However, after the first heating cycle in DSC, where the processing history is erased, a single exothermic peak is obtained. The crystallization temperature (Tc) of the PVDF/ GO composites is observed to be higher than of neat PVDF.44,45 This observation was expected as GO sheets can act as heteronucleating agent, thereby influencing the crystallization temperature. It is worth mentioning that most of the literature where it has been reported that GO acts as a heteronucleating agent has also reported complete phase transformation from α to electroactive β polymorph in PVDF. In our case, the PVDF was rich in β and the GO sheets further promoted the β phase resulting in complete phase transformation. It is envisaged that the specific interaction between GO and PVDF results in adsorption of PVDF chains further promoting this phenomenal phase transformation. While this effect is reflected from the FTIR and calorimetric measurements, the heteronucleating effect is also realized from DSC. Interestingly, GO enhanced the calorimetric crystallization temperature, although the presence of oxygen moieties in general acts as a diluent in PVDF. The latter phenomenon can be appreciated from our previous studies with COOH functionalized CNTs in PVDF/PMMA blends.29 On the contrary, increase in Tc was observed in blends incorporated with NH2 functionalized CNTs. The specific interaction between carboxylic groups of GO and CF2 group of PVDF might influence the crystal structure differently. Using density function theory, Yu et al.24 explained that the TT molecular chain prefers to be absorbed on the CNT surface compared with the TGTG′ molecular chain, and the configuration in which H atoms and the CNT surface are face-to-face is more stable than that where F atoms and CNT surface are face-to-face. Further, in the PVDF/PEI composite, Tc is observed to increase with 0.1 wt % PEI (Figure 6a). The increase in Tc can be due to the presence of a specific interaction between -NH2 groups of PEI and -CF2 of PVDF

Figure 5. XRD of PVDF based composites.

recorded. The neat PVDF showed peaks corresponding to the α phase at 18.4° (020), 20.0° (110), and at 20.6° (200) corresponding to the β-phase. The PVDF/GO composite shows a peak at 20.6° (200) indicating the presence of only the β-phase. This is also supported by spectroscopic techniques. PVDF/PEI composites show peaks corresponding to the α phase (Figure 5). XRD scans of PVDF/PEI−GO show a broad peak exhibiting both phases, α and a hump at 20.6° (200) corresponding to the electroactive β-phase of PVDF. PVDF/ MDA−GO shows similar XRD scans as that of PVDF/PEI− GO composites. Hence, from both FTIR and XRD it is clear that PEI facilitates the α phase and GO facilitates only the β polymorph in PVDF. However, the combination (PEI−GO) results in a mixture of crystals but rich in the α phase. As explained earlier, besides specific interaction, the adsorption of PVDF chains on the basal plane of GO promotes the electroactive β polymorph in PVDF. In contrast, although

Figure 6. (a) DSC cooling curves and (b) heating. E



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that leads to better dispersion of PEI in the PVDF matrix. Well dispersed flexible PEI polymer chains can help in nucleation and crystallization of PVDF and result in an increase of Tc. Moreover, positively charged nucleating agents led to a remarkable increase in the crystallization temperature of PVDF (lower supercooling) as compared to that of neat PVDF.46 It can be seen from Table 1 that the maximum shift in Tc is ca. 6 Table 1. Thermal Properties of PVDF and PVDF Based Composites sample PVDF PVDF PVDF PVDF PVDF PVDF PVDF PVDF PVDF

+ + + + + + + +

0.1 wt % GO 1.0 wt % GO 0.1 wt % PEI 1 wt % PEI 0.1 wt % PEI−GO 1 wt % PEI−GO 0.1 wt % MDA−GO 1 wt % MDA−GO

Tc1 (°C) 138 143 142 142 144 142 137 143 139

Tc2 (°C) 140

138

140

Tm1 (°C)

Tm2 (°C)

Xc (%)

166 168 168 167 167 167 167 167 168

172 173 174 174 173 173 172 172 173

39 41 45 44 41 35 44 43 47

Figure 7. SAXS Lorentz corrected Kratky plots for PVDF composites.

Table 2. SAXS Results for PVDF Composites

°C as compared to PVDF. This indicates that the addition of the positive nucleating agent lowers the free energy barrier of nucleation, thus accelerating the crystallization of PVDF significantly, manifested in a higher Tc. PVDF/PEI−GO composites also show an increase in Tc (Figure 6a). Similar effects were observed for PVDF/MDA−GO composites as shown in Figure 6a. The formation of the α phase in PVDF/ PEI composites can also be explained using specific interactions between PVDF and PEI chains. Crystallization and melting temperatures, enthalpy of crystallization (ΔHc), and the degree of crystallinity (Xc) for all composites are reported in Table 1. Xc can be estimated using the following relation,

sample

Lw (A)

La

Lc

PVDF PVDF + 0.1 wt % GO PVDF + 0.1 wt % PEI PVDF + 0.1 wt % PEI−GO

80 80 80 82

46 44 48 49

34 36 32 33

while PVDF/PEI-GO has a higher Lw. This increase in the amorphous region with the addition of PEI is due to coarser spherulites of PVDF possibly due to inclusion of PEI in the interlamellar spaces of PVDF, which in turn increases the periodic spacing of the bundles. Similar effects were obtained for PVDF/PMMA blends in our previous studies19 where Lw is observed to increase with increasing PMMA concentration. A decrease in Lc with addition of GO shows the compactness of spherulites. To get better insight into the morphology, polarizing optical microscopy of the composites was done. From Figure 8, we can clearly see that the crystal size of PVDF/ GO composites is comparatively less as compared to PVDF, which probably is due to small crystals facilitated by faster kinetics in the presence of GO. Neat PVDF shows larger

Xc = [(ΔHc)/(ΔHo)]× 100%

where ΔHc corresponds to the heat of crystallization and ΔH0 is the heat of fusion for 100% crystalline PVDF. The % crystallinity in the composites are reported in Table 1. The crystallinity in the composites is increased with addition of PEI, which supports the hypothesis that PEI chains improve the crystallization in the PVDF matrix. Interestingly, PVDF showed double melting endotherms, as seen from Figure 6b. This phenomenon further supports the existence of both α and β phases. Moreover, the existence of two crystalline phases in PVDF and PVDF composites are already established from FTIR and XRD studies. The melting temperature of the composites is reported in Table 1. Small angle X-ray scattering (SAXS) studies were performed on PVDF and the composites to get a clear insight into the crystalline morphology. As an example, the SAXS profile for the PVDF and its composites with 0.1 wt % GO or PEI is depicted in Figure 7, and the SAXS parameters are listed in Table 2. The peak maxima of the Lorentz corrected Kratky plots is used to calculate the long period (Lw) of the composites (Lw = Lc + La), where La is the amorphous region thickness and Lc is the crystalline lamella thickness. From Table 2, it is observed that La increases with the addition of PEI in the composites with respect to neat PVDF, whereas with addition of 0.1 wt % GO, La decreases. In presence of PEI-GO, the La further increases manifesting in amorphous miscibility. The long period Lw of PVDF, PVDF/GO, and PVDF/PEI is approximately identical,

Figure 8. Polarizing optical microscopy of PVDF based composites (a) PVDF, (b) PVDF/GO, (c) PVDF/PEI−GO. All the images were taken at 50× magnification (white scale is 20 μm). F



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Figure 9. SEM of PVDF based composites (a) PVDF, (b) PVDF/GO, (c) PVDF/PEI-GO.

Figure 10. C 1s core level XPS spectrum of (a) PVDF, (b) PVDF/GO, (c) PVDF/PEI, (d) PVDF/PEI−GO.

G



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spherulites and a compact typical Maltese cross pattern as compared to both PVDF/GO and PVDF/PEI−GO composites, which are smaller, coarse, and have a wide distribution of spherulites. The crystalline temperature obtained from POM is in accordance with DSC results. For instance, PVDF films and PVDF/PEI−GO composite films show almost the same crystallization temperature (i.e., 147 and 148 °C, respectively), whereas PVDF/GO shows a slightly higher crystallization temperature, i.e., 150 °C. Further, the composite films were investigated under SEM to evaluate the surface morphology. SEM micrographs of neat PVDF are shown in Figure 9a. PVDF spherulites (as indicated) are evident from the SEM micrograph with an average spherulite size of ∼12 μm. In Figure 9b, impinging of spherulites is observed which can be facilitated by faster kinetics in PVDF in the presence of GO. These results are in clear agreement with DSC and POM studies, where fast nucleation was observed in the case of PVDF/GO composites. Neat PVDF shows larger spherulites as compared to PVDF/ PEI−GO composites, which are smaller and have a wide distribution of spherulites. In order to understand the nature of interactions between GO, PEI, and PVDF, XPS measurement were done, as shown in Figure 10. The C 1s carbon core structure of PVDF consists of two prominent peaks, at 285.87 eV corresponds to sp3 carbon, C−C, and C−H2 species, and at 289.30 eV corresponds to C−F2 species. One extra peak at 287.6 eV is due to the impurities in the polymer present prior to loading (Figure 10a).47 Figure 10b shows the XPS of PVDF/GO composites. The C1s core spectrum clearly shows the aromatic CC binding energy (284 eV), aliphatic C−C binding energy (285 eV), carbonyl carbon CO at 287.1 eV, and the binding energy of C-F2 species at 289.08 eV.48 This shift in binding energies with addition of GO can be due to interaction between GO and PVDF in PVDF/GO composites and due to the presence of oxygenated carbon.49 Thangavel et al. showed similar results and reported a shift in the -CF2 and -CH2 peaks by 0.2−0.3 eV in PVDF/GO composites. PVDF/PEI composites also show three characteristic peaks in C1s core level spectrum XPS spectrum. The binding energy at 284 eV corresponds to the C−C structure of hyperbranched PEI and the carbon bonded to the amine groups (−C−NR2, R is C or H, at 285.4 eV).50 The peak at 289.08 eV corresponds to CF2 species. These shifts in binding energy can be due to the interaction between NH2- species and CF2 in PVDF. PVDF/ PEI−GO composites show characteristic peaks at 284, 285.37, and at 289.9 eV that correspond to CC of GO, aliphatic C− C of PVDF at C−F2 species in PVDF, respectively.51 The difference in binding energies in CF2 in C 1s core level can be explained by a change in the polarization of C−F in the presence of -COOH and -NH2 species. In PVDF/GO, CF2 is in -COOH and -C−OH environments, which act as electron withdrawing groups and influence the polarization of C−F leading to a decrease in the binding energy. In PVDF/PEI, CF2 is in proximity to -NH2 species, which act as electron donating groups and enhance the polarization of C−F that leads to an increase in binding energy.52 The F1s fluorine core structure yield a single prominent peak at 687.17 eV in XPS spectrum that corresponds to CF2 groups53 as shown in Figure 11a. This peak slightly shifts to a higher side (687.4 eV) in the presence of PEI in PVDF/PEI composite (Figure 11b). This may refer to specific interaction between fluorine moieties with PEI chains.

Figure 11. F 1s core level XPS spectrum (a) PVDF, (b) PVDF/PEI, (c) PVDF/PEI-GO.

Figure 12a displays the solid state 13C NMR spectra of PVDF and its composites. Two intense resonance peaks at 44.88 and

Figure 12. 13C NMR spectra of (a) PVDF, (b) PVDF/GO, (c) PVDF/PEI, and (d) PVDF/PEI−GO.

121.87 ppm denote the CH2 and CF2 groups, respectively.54 The chemical shifts in the PVDF/GO are 44.79 for the CH2 group resonance and 120.9 ppm for the resonance of CF2 (Figure 12b). This shift corresponds to the interaction between GO and PVDF chains. NMR results are in agreement with XPS where the binding energy of CF2 group decreases in the PVDF/ GO composites. Similarly, PVDF/PEI and PVDF/PEI−GO H



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helps in adsorbing PVDF chains on the surface resulting in faster crystallization. This resulted in the β polymorph in PVDF. In contrast, the PEI chains though facilitating faster crystallization resulted in a more stable α form. The interaction between PVDF and GO or PEI was explained using XPS and NMR. In summary, the interaction between the nucleating agents and PVDF dictates the polymorphism in PVDF.

show CF2 group resonance at 121.4 and at 121.79 ppm, respectively. Hence, it is now understood that the nature of interaction with PVDF dictates the overall crystal structure in PVDF. The physisorption of PVDF onto the basal planes of GO might promote the electroactive β polymorph in PVDF in contrast to PEI which rather segregates in the interlamellar spaces in the crystal. The latter phenomenon results in a more stable α phase when cooled from the melt. Although PEI also enhances the crystallization temperature of PVDF, the nature of interaction governs the polymorphism in PVDF. This study clearly demonstrates this fact and will be useful in evaluating the polymorphism in PVDF in the presence of different agents. The different crystal structure thus generated in the presence of GO and PE was further evaluated with respect to its dielectric behavior. The real part of permittivity of PVDF and PVDF based composites is shown in Figure 13. Permittivity of

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ASSOCIATED CONTENT

S Supporting Information *

Dynamic mechanical analysis (DMA) of the PVDF composites. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00445.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors like to acknowledge DST 1362 for their financial support. Authors would like to acknowledge the MNCF facilities at CeNSE, AFMM, and NMR facilities at IISc. Authors also would like to acknowledge Prof. Raghunathan for extending the SAXS facility.

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REFERENCES

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Figure 13. Dielectric properties of PVDF composites.

the composites is higher than PVDF suggesting enhanced charge storing capacity in the composites. For instance, permittivity of PVDF is 8 at 100 Hz, whereas in PVDF/GO and PVDF/PEI composites, it has increased to 21 and 15, respectively, at 100 Hz as illustrated in Figure 13. Interestingly, with the addition of PEI−GO, the permittivity increased to 37 at 100 Hz. The high permittivity values in composites are due to charge trapping at the interfaces known as the MWS effect. In PVDF/PEI−GO composite, synergistic improvement in dielectric permittivity was observed because of the large difference in the permittivity between three components, i.e., PVDF, PEI, and GO, which helps in trapping more charge at the interfaces. Although the dynamic modulus of the composites slightly decreased in the presence of GO, the flexibility of the films was well retained, and hence these composites can further be explored for flexible electronic devices given that GO promotes the piezoelectric β-phase (see Figure S1 in the Supporting Information).

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CONCLUSIONS In this study, the effect of GO, PEI, MDA−GO, and PEI−GO on the polymorphism of PVDF was studied in detail. PVDF/ GO showed all trans conformation in PVDF. However, in PVDF/PEI, PVDF/PEI−GO, and PVDF/MDA−GO a mixture of α and β phases was observed. The nature of the specific interaction with the PVDF chains resulted in different polymorphs. It is envisaged that specific interaction with GO I



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