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Cooperativity and Structural Relaxations in PVDF/PMMA Blends in the Presence of MWNTs: An Assessment through SAXS and Dielectric Spectroscopy Maya Sharma,† Giridhar Madras,† and Suryasarathi Bose‡,* †

Center for Nano Science and Engineering, Indian Institute of Science, Bangalore 560012, India Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India

‡

S Supporting Information *

ABSTRACT: Intermolecular cooperativity and structural relaxations in PVDF/ PMMA blends were studied in this work with respect to different surface modified (amine, ∼NH2; carboxyl acid, ∼COOH and pristine) multiwalled nanotubes (MWNTs) at 1 wt % near blend’s Tg and in the vicinity of demixing using dielectric spectroscopy, SAXS, DSC, and WAXD. Intermolecular cooperativity at Tg and configurational entropy was addressed in the framework of cooperative rearranging region (CRR) at Tg. Because of specific interactions between PVDF and NH2-MWNTs, the local composition fluctuates at its average value resulting in a broad Tg. The scale of cooperativity (ξCRR) and the number of segments in the cooperative volume (NCRR) is comparatively smaller in the blends with NH2MWNTs. This clearly suggests that the number of segments cooperatively relaxing is reduced in the blends due to specific interactions leading to more heterogeneity. The configurational entropy at Tg, as derived from Vogel-Fulcher and Adam− Gibbs analysis, was reduced in the blends in presence of MWNTs manifesting in entropic penalty of the chains. The crystallite size and the amorphous miscibility was evaluated using SAXS and was observed to be strongly contingent on the surface functional groups on MWNTs. Three distinct relaxationsαc due to relaxations in the crystalline phase of PVDF, αm indicating the amorphous miscibility in PVDF/PMMA blends, and αβ concerning the segmental dynamics of PMMAwere observed in the blends in the temperature range Tg < T < Tc. The dynamics as well as the nature of relaxations were observed to be dependent the surface functionality on the MWNTs. The dielectric permittivity was also enhanced in presence of MWNTs, especially with NH2-MWNTs, with minimal losses. The influence of the MWNTs on the spherulite size and crystalline morphology of the blends was also confirmed by POM and SEM.

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INTRODUCTION Blends of poly(vinylidene fluoride)/poly(methyl methacrylate) PVDF/PMMA have been widely studied due to their synergistic properties.1−3 It is well understood that PVDF/ PMMA blends are melt miscible. However, as PVDF concentration increases beyond 60%, it crystallizes and PMMA segregates in the interspherulitic regions in the blends. The exclusion of PMMA from the interlamellar regions has been shown by different studies using SAXS. Upon cooling from the melt, PVDF crystals attain a thermodynamically stable form, i.e., the α-phase (TGTG′). The β-phase of PVDF is industrially important due to its strong peizo and pyroelectric properties, which finds application in sensors and transducers.4,5 Various routes like poling, stretching, annealing, or, more recently, incorporation of nanoparticles (i.e., nanoclay, carbon nanotubes, graphite nanoplatelets) have been adopted to promote the β-phase in PVDF.6−10 These particles influence the crystallization kinetics and the crystalline morphology of PVDF. It has been reported that incorporation of nanoclay significantly promotes nucleation in PVDF and assist in the βphase crystal formation. Similar effects were also reported with © 2014 American Chemical Society

multiwall carbon nanotubes (MWNTs), which enhances the rate of crystallization as well as β-phase crystallization in PVDF.11,12 Besides β-phase crystals, piezoelectricity as well as dielectric permittivity is also enhanced dramatically in the presence of carbon nanotubes due to nomadic charges trapped at PVDF/CNT interfaces.13,14 Intermolecular coupling and segmental dynamics influence the structure and properties of polymeric systems. Often, a single glass transition temperature (Tg) is obtained for miscible blends although, each entity retains its identity down to the molecular length scales. Because of intramolecular interactions, the local composition fluctuates at its average value that causes chains to show a range of relaxations resulting in a broad Tg.15−17 The intermolecular coupling and cooperativity between the chains is the basis of these relaxations and their dependence on temperature.16 Different models have been developed to study the intermolecular coupling near Tg such as Received: November 16, 2013 Revised: January 30, 2014 Published: February 7, 2014 1392

dx.doi.org/10.1021/ma4023718 | Macromolecules 2014, 47, 1392−1402

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Figure 1. Step change in specific heat at Tg for the neat 80/20 PVDF/PMMA blends. A cartoon illustrating the effect of NH2-MWNTs on the CRR is shown as inset. samples into discs and thin films. Hot pressed samples were air cooled to induce crystallization in PVDF. Characterizations. Small angle X-ray scattering (SAXS) studies were done on an Empyrean diffractometer using X-ray wavelength of 1.54 Å. Compressed molded films were investigated at 25 °C over 2θ range of 0−8°. Thermal transitions like crystallization temperature (Tc), melting temperature (Tm), and glass transition (Tg) were studied using a calibrated Mettler Toledo differential scanning calorimetry. Samples were heated from room temperature to 220 °C and subsequently quenched to −50 °C to prevent crystallization followed by a second heating run. After holding for 5 min at 220 °C, the samples were cooled to 25 °C at 2 K/min (in order to stimulate crystallization in PVDF). Fourier transform infrared (FTIR) spectroscopy was carried out using Perkin- Elmer GX in the range of 4000−400 cm−1 using a resolution of 4 cm−1. Wide angle X-ray diffraction (WXRD) studies were analyzed using PANalytical Xpert pro to analyze the structure of PVDF/PMMA blends using a Cu Kα radiation (1.54 Å, 40 k eV) and in the 2θ range of 5−50° with a scan rate of 0.04° s−1. The crystalline morphology was studied using polarizing optical microscope (Olympus BX51, Japan) fitted an automated hot stage (Linkam THMS600) and a CCD camera (ProgRes C3, Germany). Scanning electron microscopy was carried out to evaluate the phase morphology and the preferential localization of MWNTs in annealed and quenched samples. Dielectric measurements in the frequency range of 0.01 ≤ ω ≤ 107 Hz was studied using an Alpha-N Analyzer, Novocontrol (Germany). Dielectric loss spectra were fitted using Havriliak−Nigami empirical equations.

Vogel−Fulcher (VF) and Kohlrausch−William−Watts (KWW).18 It has also been reported that incorporation of nanoparticles in the polymer cause chains to confine in narrow spaces and this influences the relaxations and segmental dynamics at the cost of both configurational and conformational entropy.19 In an earlier study, we have related the molecular relaxations associated with different regions in PVDF using broadband dielectric spectroscopy in PVDF/PMMA blends.20 The segmental relaxations are governed by cooperativity and dynamic heterogeneity in the blends. Different surfacefunctionalized MWNTs resulted in different crystallization kinetics. This significantly influenced the molecular relaxations associated with crystalline regions and the miscible amorphous regions in the blends. These effects were discussed with respect to enhanced local environment and dynamic heterogeneity in the system. In this work, we have studied the segmental dynamics, using dielectric spectroscopy, broadly in two very different regimes; near the Tg of the blend where chain connectivity and intermolecular coupling dominates the structural relaxations and in the vicinity of the demixing temperature where thermal concentration fluctuation and crystallization driven demixing governs the overall structural relaxations in the blends. The molecular relaxations in the glassy region suggests enhanced intermolecular coupling in the presence of MWNTs, which is strongly contingent upon the surface functional groups on the nanotubes. The structural relaxations associated with the crystalline morphology in PVDF/PMMA blends advocate that the surface functional groups on MWNTs significantly influence the molecular level miscibility and the crystalline morphology (α and β phases of PVDF). The latter was systematically studied using SAXS, polarized optical microscopy (POM), and SEM. The structures were evaluated using WXRD and FTIR.

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RESULTS AND DISCUSSION Chain Connectivity, Intermolecular Coupling, and Scale of Cooperativity in the Glassy Region: Effect of MWNTs. The thermal transitions were recorded using DSC for 80/20 PVDF/PMMA blends with different MWNTs and are shown in Figure 1. A single broad Tg confirms the miscibility in the blends. It is generally stated that broadening of Tg is a result of concentration fluctuation and local heterogeneity in the system.22,23 However, concentration fluctuation is not significant in case of strongly interacting blends like PVDF/PMMA.24 In order to take into account the dynamic heterogeneities in the blends, Lodge and McLeish25 introduced the concept of self-concentration. According to this theory,

EXPERIMENTAL SECTION

Materials and Methods. PMMA (Atuglass V825T) and PVDF (Kynar-761) were obtained from Arkema Inc. Nanocyl (Belgium) supplied the different functionalized multiwall carbon nanotubes (MWNTs) used in this work. The physical characteristics of NH2MWNTs, COOH-MWNTs and pristine (p-) MWNTs are listed elsewhere.21 80/20 (w/w) blends of PVDF/PMMA with MWNTs (1 wt %) were prepared by melt mixing, as explained in our previous work.20 A lab scale hydraulic press was used to press the extruded

φeff , i =φs , i + (1 − φs , i)φ 1393

(1)


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Table 1. Thermal Properties of the Blends and with 1 wt % MWNTs composition

Tg (°C)

PVDF PMMA PVDF/PMMA 8020 blend blends with 1 wt % NH2-MWNTs blend with 1 wt % COOH-MWNTs blend with 1 wt % p-MWNTs

−22 11020 46 50 44 47

ΔTg

Δμ (cal mol

−1

)

B (from eq 4)

Va (Å)

ΔCp

Tc (°C)

%X

0.17 0.15 0.15 0.14

150 − 141 145 144 145

40 − 38 33 35 39

20

15 18 15 15

169 157 152 138

23.38 23.66 23.8 23.7

17.3 16.4 18.3 19.1

Figure 2. Dielectric loss as a function of frequency in the vicinity of Tg for (a) PVDF/PMMA blend, (b) with COOH-MWNTs, (c) with NH2MWNTs, and (d) with p-MWNTs.

where φef f,i is the effective concentration of monomers of type i, φ is the average bulk composition and φs,i is the selfconcentration of monomers. Using the effective concentration of the monomers, effective glass transition (Tg,eff,i) temperature can also be calculated from the modified Fox equation, which accounts for the local dynamics, and is given by 1 Tg , eff , i

=

φeff , i Tg , i

+

The overall crystallinity is relatively high for this blend and can be realized as constrained and unconstrained amorphous domains. It is also believed that constrained PVDF domain does not interact with PMMA, which further increases the Tg of the blend. The calorimetric transitions are listed in Table 1. The Tg for the blends with NH2-MWNTs are higher as compared to neat and blends with p-MWNTs and COOHMWNTs. The plausible reason for a higher Tg in the presence of NH2-MWNTs is due to the specific interactions with PVDF. Interestingly, the blends with COOH-MWNTs exhibit a lower Tg in contrast to neat blends. This was also observed in our previous study20 and is possibly due to enhanced miscibility in presence of COOH-MWNTs wherein COOH-MWNTs act as a diluent. Using the Donth fluctuation formula,27 the scale of cooperativity or cooperative volume Va at Tg can be estimated by

1 − φeff , i Tg , j

(2)

φs,i of PMMA and PVDF are taken to be 0.31 and 0.32, respectively.26 Surprisingly, there is a large discrepancy between the calorimetric Tg (see Figure 1) and that obtained from the modified Fox equation for the control 80/20 PVDF/PMMA blends. From DSC, Tg is 45.9 °C whereas it is 21.5 °C based on eq 2. The plausible reason for this variation may be due to restriction of chain mobility in the vicinity of the crystallites.16 1394


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Figure 3. Temperature dependence of segmental relaxation for (a) 80/20 blends and with (b) NH2-MWNTs, (c) COOH-MWNTs, (d) p-MWNTs, and (e) configurational entropy in control and blends with different MWNTs; Solid line in the parts a−d denotes the VFT fit.

Va =

kTg 2ΔCp−1 2

ρ(δT )

reduced in the blends due to specific interaction of PVDF with NH2-MWNTs. It is envisaged that this interaction might lead to dynamic heterogeneities in the system, which further can result in smaller domains (ξCRR)3 leading to high concentration fluctuation inside the domains. A cartoon illustrating the effects of NH2-MWNTs on ξCRR is shown in the inset of Figure 1. The structural relaxations in the vicinity of Tg are shown in Figure 2. With an increase in temperature, the segmental relaxation shifts to higher frequency due to increase in free volume.17 Interestingly, the dielectric relaxations shifts to higher frequency in the presence of NH2-MWNTs and p-MWNTs in striking contrast to neat blends. However, the relaxations are almost unaltered in case of COOH-MWNTs. Broader relaxations in case of blends with different MWNTs suggest increased concentration fluctuations and dynamic heterogeneity in the blends. Although, all blends that are investigated here are

(3)

In eq 3, δT is the half width of Tg, ΔCp is the difference in specific heat at Tg, ρ is the bulk polymer density. Cooperative volume for the blends investigated here is listed in Table 1. Interestingly, blends with NH2-MWNTs exhibited a relatively smaller cooperative volume (∼16.4 Å) as compared to the other blends. According to Adam and Gibbs (AG), a sub system which can rearrange to another configuration, for a given thermal fluctuation is termed as cooperative rearranging region, CRR. As the number of segments inside this cooperative rearranging region (NCRR) is proportional to the cooperative volume (ξCRR),3 it is evident that NCRR is comparatively lesser in blends with NH2-MWNTs. This clearly suggests that the number of segments cooperatively relaxing is 1395


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A higher value of βKWW signifies a narrow distribution of relaxation time while a smaller value implies broad distribution. n is a coupling parameter, which is referred to as intermolecular coupling between neighboring non bonded chain units.29 The values of βKWW decreases with the addition of MWNTs (not shown here) manifesting in a broad distribution of relaxation time. βKWW decrease as MWNTs restrict the motion of polymer chains leading to dynamic nanoheterogeneity in the blend. In the recent past, many studies were conducted to study the effect of nanoconfinement on the intermolecular coupling and segmental dynamics. Chehrazi and Qazvini30 studied the intermolecular cooperativity and segmental dynamics of PMMA-SAN−nanoclay and PMMA-SAN−silica composites at the glass transition regime and reported that intermolecular coupling and segmental cooperativity decreases upon incorporation of nanosilica particles. Vyazovkinet et al.31 accounted for the increase in segmental cooperativity with addition of nanoclay in polystyrene nanoclay nanocomposites. The coupling parameter n, as estimated from eq 5e, is shown in Figure 4. Intermolecular coupling is observed to increase in the

miscible as observed from DSC, broadness in the relaxation spectrum is more prominent in blends with NH2-MWNTs due to enhanced dynamic heterogeneity in the blends. Several theoretical models have been proposed to shed more insight into these interesting phenomenons at Tg. Characteristic relaxation at Tg is fitted using the Vogel− Fulcher (VF) equation:28

⎛ BT∞ ⎞ τ = τo exp⎜ ⎟ ⎝ T − T∞ ⎠

(4)

where T∞ is Vogel temperature (the temperature at which relaxation time diverges), το represents relaxation time at infinite temperature; B is material constant which corresponds to apparent activation energy. Figure 3a−d illustrates that all the blends follow the VF equation and the solid line represents the VF fit and the fitting parameters are reported in Table 1. Under the outline of Adams-Gibbs (AG) theory, the configurational entropy has been identified as the controlling factor of the dynamics and is controlled by CRR. It is believed that the size of the CRR (ξCRR) is greater near Tg and diverges when the configurational entropy tends to zero. The addition of MWNTs alters the rate of relaxation as observed from the relaxation spectra. According to AG theory, NCRR is envisaged to decrease with increasing temperature. Thus, the relaxation time with x molecules can be written as,

τ = τo exp(xΔμ/kBT )

(5a)

where Δμ is the elementary activation energy, independent of NCRR. The temperature dependence of x can be expressed in terms of the configurational entropy and is given by, τ = τo exp(NAscΔμ/kBTSc(T ))

(5b)

where, NA is Avogadro’s number, sc (=kB ln 3! for polymers) is the entropy of the minimum number of particles able to rearrange, and Sc(T) is the macroscopic configurational entropy per mol of particles. Following Angell’s approach, ΔCp(T) is proportional to 1/T and as Sc(T) = ∫ TToΔCp/T dT, and taking ΔCp(T) = ΔCp(Tg)T dT, eq 5b reduces to the following expression τ = τ exp[NAscΔμTo/Tg ΔCp(Tg)kB(T − To)]

Figure 4. Intermolecular coupling (n) for various investigated PVDF/ PMMA blends.

presence of both COOH-MWNTs and p-MWNTs with respect to neat blends. Interestingly, the intermolecular coupling decreased in presence of NH2-MWNTs due to specific interactions with PVDF. This results in reduced number of segments cooperatively relaxing due to enhanced heterogeneity. Structure, Crystalline Morphology, and Preferential Localization of MWNTs: POM and SEM. Cooling thermograms of the blends investigated here are shown in Figure 5. The onset of Tc increases with addition of MWNTs manifesting the key role of heteronucleating agent32 (see Table 1). It is evident that the overall crystallinity of the blend decreases with addition of MWNTs as compared to neat blend. The size distribution of the spherulites, as also supported from POM (see next section), also increased significantly with addition of MWNTs, which further manifested in a broad crystallization curve. Interestingly, this effect is more prominent in NH2MWNTs in sharp contrast to both neat and blends with COOH-MWNTs. This might be due to specific interaction between PVDF and NH2-MWNTs. With addition of NH2MWNTs, the Tc increases by ∼5 °C with respect to neat blends. Interestingly, blends with p-MWNTs also showed a similar rise in Tc.

(5c)

which has the same form as VF (eq 4) if B = NAsc Δμ/Tg ΔCp(Tg)kB. Therefore, Sc(T ) = ΔμNA ln 3! /kBT ln τ(T )/τo

(5d)

Figure 3e gives the relation of the configurational entropy as a function of temperature for all the blends investigated here. It is obvious that Sc(T), which is the macroscopic configurational entropy per mole, decreases with temperature and slowly diminishes to zero at T ≪ Tg. The configurational entropy in case of blends with MWNTs are lower. A similar trend is also observed in Δμ; elementary activation energy per mol (see Table 1). Because of specific interaction between PVDF and MWNTs, it is believed that the chains will not be able to take their ideal conformation and lose part of their configurational (and conformational) entropy. The non exponential βKWW parameter describes the distribution of relaxation time and can be obtained from HN fit using the equation below. βKWW = (αγ )1/1.23 ; n = 1 − βKWW

(5e) 1396


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only the α crystal form whereas the blends with NH2- and pMWNTs revealed both the crystal forms (α and β). It is now understood that both NH2- and p-MWNTs promote β crystal forms in the blends. These crystal forms are defective resulting in a wider spherulites distribution. Further information can be determined from SAXS and is discussed in the next section. Figure 6 illustrates the polarizing optical microscope (POM) images of control and 80/20 PVDF/PMMA blends with different MWNTs. The samples were allowed to crystallize at Tc for 1 h to study the evolution of crystalline morphology. In neat blends, a typical Maltese cross pattern can be observed attaining a crystal−amorphous morphology with narrow size distribution. With addition of different MWNTs, smaller spherulites evolve with a wider distribution in size. The less developed birefringent spherulites also develop along with bright and fully developed spherulites. The latter is a characteristic of α-form crystal structure with respect to PVDF. From Figure 6, it is evident that the size of the spherulite significantly decreases with MWNTs in comparison to neat blends. Moreover, as MWNTs act as heteronucleating agent, the crystallization temperature is elevated in the blends. For instance, neat blends crystallize at 150 °C whereas blends

Figure 5. Crystallization thermograms of 80/20 PVDF/PMMA blends with different MWNTs.

We systematically investigated the crystal structure of the blends in presence of different MWNTs by FTIR and WAXD (see Supporting Information for more details). Interestingly, the control blends and blends with COOH-MWNTs exhibited

Figure 6. POM images 80/20 PVDF/PMMA blends: (a1−a3) neat blends at 150 °C, (b1−b3) blend with a-MWNTs at 155 °C, (c1−c3) blend with cMWNTs at 152 °C, (d1−d3) blend with p-MWNTs at 155 °C, White scale bar represents 100 μm images taken at 20×. Parts a3−d3 indicate 1 h annealed samples. 1397


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Figure 7. Lorentz-corrected Kratky plots for 80/20 PVDF/PMMA blends.

blends. In our earlier studies, we observed that MWNTs preferentially localize in the PVDF phase of the blends due to specific interactions between MWNTs and PVDF.20 Blend samples were annealed for 3 h at Tc and subsequently quenched to arrest the phase morphology. PMMA is selectively etched out by dissolving in glacial acetic acid and were analyzed under SEM. Figure 8 shows the PMMA etched out SEM micrographs of 80/20 PVDF/PMMA blend with and without MWNTs. Because of selectively etching of PMMA from 80/20 PVDF/ PMMA, porous spherulitic structures are well evident. Figure 8a shows the neat 80/20 blends with large spherulites with clear interspherulitic amorphous regions. Spherulite size decreases with incorporation of MWNTs in the blend. As discussed earlier, due to heteronucleating nature of MWNTS, the crystal nuclei increases as well as defective crystals develops resulting in a decrease in spherulitic size. Spherulitic morphology of the sample is clearly observed in high magnification images. Structural Relaxations in Demixed PVDF/PMMA Blends: Effect of MWNTs. Dielectric spectroscopy measurements were done to examine the different relaxations in the blends in the presence of different MWNTs. In our earlier study,20 all possible segmental relaxation in neat homopolymers (PVDF and PMMA) were discussed. Furthermore, the segmental relaxations in PVDF/PMMA blends were contingent upon the content of PMMA and also on the surface functional groups. The blends investigated here is highly crystalline (80 wt % PVDF; Xc = 40% from DSC) and the surface functional groups on the MWNTs has led to various crystal structures as seen from DSC, WAXD and SAXS analysis. Hence, probing the structural relaxations in 80/20 PVDF/PMMA blends in the presence of different MWNTs will allow us to systematically evaluate the role of surface functionalities on the segmental relaxations. The dielectric loss spectra (ε″) as a function of frequency are illustrated in Figure 9. Interestingly, 80/20 blend shows three distinct relaxations in the measured frequency window which was interestingly, absent in 70/3020 and 60/40 blends. This observation is a clear mandate to the fact that structural

with COOH-MWNTs, NH2-MWNTs, and p-MWNTs crystallize at 152, 155, and 155 °C, respectively. Small angle X-ray scattering (SAXS) studies were done to assess the crystal lamella region of PVDF with its blends. SAXS profiles of neat PVDF and PVDF/PMMA blends with different surface-functionalized MWNTs are shown in Figure 7 and the parameters are listed in Table 2. In general, PMMA segregates Table 2. SAXS Parameter for PVDF, Neat Blend, and Blends with MWNTs composition

Lw = 2π/qmax (Å)

Lc (Å)

La (Å)

PVDF PVDF/PMMA 8020 blend blend with NH2-MWNTs blend with COOH-MWNTs blend with p-MWNTs

83 93 86 83 86

33 35 29 32 30

50 58 57 51 56

in the interlamellar and interspherulitic regions. It is thus envisaged that within the spherulite, the high density crystalline lamellae and the low density amorphous interlamellar region are orderly stacked. The peak maxima are used to calculate Lw (long period), Lc (crystalline lamella thickness) and La (amorphous lamella thickness). It is interesting to note that Lw (Lc + La), increases in PVDF/PMMA blend (92.7 Å) as compared to the neat PVDF (82.6 Å). The increase in Lw with addition of PMMA manifests the presence of PMMA in the interlamellar regions facilitating in amorphous miscibility. Furthermore, the Lw decreases with addition of MWNTs in the blend. Blends with NH2-MWNT and p-MWNT exhibit almost similar Lw whereas it decreases significantly in case of COOH-MWNTs,. Moreover, the crystalline lamellar thickness (Lc) decreases significantly in blends of NH2- and p-MWNTs. It is worth recalling that β crystal forms were obtained in the blends in the presence of NH2- and p-MWNTs and possibly could be one of the reasons behind smaller Lc. Selective dissolution experiments are done for confirming the localization of MWNTs in a given phase in PVDF/PMMA 1398


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Figure 8. SEM micrographs of PMMA-etched samples of 8020 PVDF/PMMA blend (a1, a2) neat, (b1, b2) blend with NH2-MWNTs, (c1, c2) blend with COOH-MWNTs, (d1, d2) blend with p-MWNTs.

relaxations in the blends are strongly dependent on the PMMA content. At the onset of crystallization temperature, for instance, at 145 °C in the case of neat blends, a single relaxation is noted. However, the relaxations become distinct at T < Tc. On the basis of our earlier observations, the relaxation observed in the lower frequency regime is identified as αc

assigned to relaxations concerning the amorphous segments within the crystalline lamellae; relaxation in the frequency range (101−103) is identified as αm, assigned to the molecular motions in the amorphous miscible region of PVDF/PMMA blend;33,34 and the relaxation in the frequency domain (104− 106 Hz) is attributed to PMMA segmental relaxation (αβ). 1399


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Figure 9. Dielectric loss as a function of frequency with temperature as a parameter for (a) PVDF/PMMA blend, (b) blend with COOH-MWNTs, (c) blend with NH2-MWNTs, and (d) blend with p-MWNTs.

One immediate conclusion that can be drawn at this point is NH2-MWNTs promotes β-phase in PVDF (∼70% as estimated from FTIR, see Figures S1, S2 in Supporting Information). Because of imperfect nature of the crystals and a different degree of orientation in an external field, the relaxation concerning αc is often not resolved and merges with other relaxations leading to a broad peak. Furthermore, a broad distribution of crystal size as seen from POM and SEM studies is also evident in the blends with NH2-MWNTs. The segmental relaxations of PMMA also shifted toward higher frequency with addition of NH2-MWNTs suggesting a distribution of cooperative volumes. Interestingly, 80/20 blends with pMWNTs also show three well resolved relaxations in the loss spectra (Figure 9d) although, the relaxations concerning αc is not as prominent as in COOH-MWNTs but, its presence is discerned. One probable reason could be that p-MWNTs also facilitate in β phase (∼55% as estimated from FTIR, see Figures S1, S2 in Supporting Information). Interestingly, relaxations concerning αm are very pronounced in case of p-MWNTs, which corresponds to more miscible amorphous domains in the blend. At low temperatures (110 °C), αc and αm relaxations merge to form a broad relaxation. All the three relaxations shifted toward lower frequency with decreasing temperature. In summary, it can be concluded from DS and SAXS that both p-MWNTs and NH2-MWNTs promote β-phase crystals in PVDF manifesting in smaller crystallite lamellae (Lc). Although the fraction of β phase crystals in PVDF is slightly higher in the case of NH2-MWNTs as discerned from WAXD

Interestingly, all the three relaxations shift toward lower frequency with decreasing temperature. At low enough temperature (i.e., 100 °C) αm and αβ merge to form a single broad relaxation. 80/20 PVDF/PMMA blends with COOHMWNTs also show three distinct relaxations as in the case of neat blends. However, interestingly the relaxations are even more distinct than the neat blends. But unlike neat blends, the relaxations shifted to higher frequency. The plausible reason for this high frequency shift is due to formation of defective crystals in the presence of COOH-MWNTs. These small and defective crystals orient faster than those of neat blends and shift the relaxations to higher frequency. Faster crystallization kinetics in presence of MWNTs, restricts the motion of chains and decreases the global size of the amorphous region.30,34,35 This phenomenon promotes local heterogeneity and is manifested in a much faster relaxation rate. The decrease in the global size of the amorphous regions is also supported by SAXS where in the Lw is significantly less in case of blends with COOH-MWNTs. Another important observation is that the relaxations concerning αc and αm are well resolved in the blends with COOH-MWNTs in contrast to neat blends. The relaxations related to αm stems from amorphous miscibility in PVDF/PMMA blends. The well resolved relaxation also suggests that COOH-MWNTs promote miscibility in PVDF/PMMA blends and is further supported by DSC and melt-rheology. Interestingly, the relaxations in 80/ 20 PVDF/PMMA blends with NH2-MWNT are not well resolved unlike the neat and blends with COOH-MWNTs. 1400


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studies, the segmental relaxations are significantly different from those of p-MWNTs. These observations allow us to understand the molecular origin of the segmental relaxations in PVDF/ PMMA blends. For instance, this study clearly demonstrates that the molecular relaxations concerning αc depend largely on the % β fraction in PVDF. This is manifested from the segmental relaxations of the blends with p-MWNTs and NH2MWNTs where the % β fraction of PVDF is lower in the former than in the latter. The variation of electrical conductivity, dielectric permittivity (ε′), and loss tangent (δ) as a function of frequency is shown in Figure 10. Interestingly, the conductivity values decayed as crystallization progressed in the blends. All the blends investigated here showed a similar trend. For instance, for blends with NH2-MWNTs, the variation in conductivity is shown in Figure 10a manifesting that the networks of MWNTs are disrupted during the crystallization process. This is also illustrated as a cartoon in the inset of Figure 10a. ε′ and δ are lower in the neat blends (i.e., ∼6 and 0.12 at 1 kHz respectively) with respect to neat PVDF due to the presence of 20% PMMA in the blend (see Figure 10b). PMMA acts as a dilutant and decreases the polarization as well as losses in the blend. Interestingly, with addition of different functionalized MWNTs, ε′ enhances with nearly same δ. It is interesting to note that blends with NH2-MWNTs have comparatively high permittivity with low loss. The comparative high permittivity value with addition of MWNTs is due to trapping of charge carriers at the crystal−amorphous interface (MWS effect). Further, dielectric permittivity increases with an increase in temperature due to more alignment of dipoles with enhanced MWS effect.

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CONCLUSIONS The intermolecular cooperativity and structural relaxations in PVDF/PMMA blends were studied in the presence of different surface-functionalized MWNTs broadly in two different temperature regime; near the blend’s Tg where chain connectivity effects dominate the structural relaxations and in the vicinity of demixing. 80/20 blend show three distinct dielectric relaxations in the loss spectra; αc of crystalline PVDF, αm due to amorphous miscible phase and αβ of PMMA. However, in case of NH2-MWNTs, relaxations were not fully resolved possibly due to presence of defective β-crystals in the blend. With MWNTs, relaxations were observed to be shifting toward high frequency in the entire temperature range studied in this work. DSC was utilized to confirm the single phase miscibility in the blends. A broad Tg was obtained for all the blends investigated here indicating the presence of dynamic heterogeneities. All the loss relaxations were fitted using H−N empirical equation. The structural relaxations at Tg in case of COOH-MWNTs were almost unaltered. Although, all the blends that were investigated here are miscible (as observed from DSC), the broadness in the relaxation spectrum is more prominent in blends with NH2-MWNTs due enhanced dynamic heterogeneity in the blends. β-phase is obtained for NH2- and p-MWNTs, as revealed from XRD and FTIR studies. The long period was also decreased for blends with surfacefunctionalized MWNTs, as compared to neat blends and PVDF possibly due to decrease in crystallinity. The morphology of the blends was evaluated using POM and SEM, which shows a decrease in spherulite size in the presence of MWNTs.

Figure 10. (a) Electrical conductivity in the blends before and after crystallization of PVDF; (b and c) dielectric permittivity and dielectric loss tangent as a function of frequency for investigated 80−20 blends, respectively.

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Macromolecules

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Article

(23) Wetton, R. E.; MacKnight, W. J.; Fried, J. R.; Karasz, F. E. Macromolecules 1978, 11 (1), 158−165. (24) Shenogin, S.; Kant, R.; Colby, R. H.; Kumar, S. K. Macromolecules 2007, 40 (16), 5767−5775. (25) Lodge, T. P.; McLeish, T. C. B. Macromolecules 2000, 33 (14), 5278−5284. (26) Zhang, H.; Lamnawar, K.; Maazouz, A. Rheol. Acta 2012, 51 (8), 691−711. (27) Donth, E. Acta Polym. 1979, 30 (8), 481−485. (28) Vogel, H. Physikalische Zeitschrift 1921, 22, 645. (29) Ngai, K. L.; Plazek, D. J. J. Polymer Sci., Part B: Polym. Phys. 1986, 24 (3), 619−632. (30) Chehrazi, E.; Taheri Qazvini, N. Iran. Polym. J. 2013, 22 (8), 613−622. (31) Vyazovkin, S.; Dranca, I. J. Phys. Chem. B 2004, 108 (32), 11981−11987. (32) Bhattacharyya, A. R.; Pötschke, P.; Häußler, L.; Fischer, D. Macromol. Chem. Phys. 2005, 206 (20), 2084−2095. (33) Mijovic, J.; Sy, J.-W.; Kwei, T. K. Macromolecules 1997, 30 (10), 3042−3050. (34) Kim, J.; Kwak, S.; Hong, S. M.; Lee, J. R.; Takahara, A.; Seo, Y. Macromolecules 2010, 43 (24), 10545−10553. (35) Vijayan, P. P.; Puglia, D.; Kenny, J. M.; Thomas, S. Soft Matter 2013, 9 (10), 2899−2911.

ASSOCIATED CONTENT

S Supporting Information *

FTIR and X-ray crystal structure analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge DAE-BRNS, India (DAEO 0159), and JATP, India (JATPO 136), for the financial support and the facilities at CeNSE (MNCF), IISc. We would also like to thank Prof. Raghunathan for extending the SAXS facility at RRI, Bangalore.

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REFERENCES

(1) Saito, H.; Stuehn, B. Macromolecules 1994, 27 (1), 216−218. (2) Lin, D.-J.; Lin, C.-L.; Guo, S.-Y. Macromolecules 2012, 45 (21), 8824−8832. (3) Papavoine, C. H. M.; Maas, W. E. J. R.; Veeman, W. S.; Werumeus Buning, G. H.; Vankan, J. M. J. Macromolecules 1993, 26 (24), 6611−6616. (4) Murayama, N.; Nakamura, K.; Obara, H.; Segawa, M. Ultrasonics 1976, 14 (1), 15−24. (5) Yang, C.; Irwin, P. C.; Younsi, K. IEEE Trans. Dielectr. Electr. Insul. 2004, 11 (5), 797−807. (6) He, F.; Lau, S.; Chan, H. L.; Fan, J. Adv. Mater. 2009, 21 (6), 710−715. (7) Patro, T. U.; Mhalgi, M. V.; Khakhar, D. V.; Misra, A. Polymer 2008, 49 (16), 3486−3499. (8) Mago, G.; Fisher, F. T.; Kalyon, D. M. J. Nanosci. Nanotechnol. 2009, 9 (5), 3330−3340. (9) Lund, A.; Gustafsson, C.; Bertilsson, H.; Rychwalski, R. W. Compos. Sci. Technol. 2011, 71 (2), 222−229. (10) Yadav, S.; Mahapatra, S.; Cho, J.; Park, H.; Lee, J. Fibers Polym. 2009, 10 (6), 756−760. (11) Nam, Y. W.; Kim, W. N.; Cho, Y. H.; Chae, D. W.; Kim, G. H.; Hong, S. P.; Hwang, S. S.; Hong, S. M. Macromol. Symp. 2007, 249− 250 (1), 478−484. (12) Manna, S.; Nandi, A. K. J. Phys. Chem. C 2007, 111 (40), 14670−14680. (13) He, L.; Xia, G.; Sun, J.; Zhao, Q.; Song, R.; Ma, Z. J. Colloid Interface Sci. 2013, 393 (0), 97−103. (14) Dang, Z. M.; Wang, L.; Yin, Y.; Zhang, Q.; Lei, Q. Q. Adv. Mater. 2007, 19 (6), 852−857. (15) Ngai, K. L.; Roland, C. M. Macromolecules 1993, 26 (25), 6824− 6830. (16) Ngai, K. L.; Roland, C. M. Macromolecules 1993, 26 (11), 2688− 2690. (17) Campbell, J. A.; Goodwin, A. A.; Simon, G. P. Polymer 2001, 42 (10), 4731−4741. (18) Williams, G.; Watts, D. C. Trans. Faraday Soc. 1970, 66 (0), 80− 85. (19) Pazmino Betancourt, B. A.; Douglas, J. F.; Starr, F. W. Soft Matter 2013, 9 (1), 241−254. (20) Sharma, M.; Sharma, K.; Bose, S. J. Phys. Chem. B 2013, 117 (28), 8589−8602. (21) Roy, S.; Suwas, S. Mater. Sci. Eng.: A 2013, 574 (0), 205−217. (22) Roland, C. M.; Ngai, K. L. Concentration fluctuations and segmental relaxation in miscible polymer blends. In Application of Scattering Methods to the Dynamics of Polymer Systems, Ewen, B., Fischer, E. W., Fytas, G., Eds.; Springer-Verlag: New York, 1993; Vol. 91, pp 75−79. 1402


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