Host Polymer Blends: Understanding the Role of

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PVP/PVDF as Guest/Host Polymer Blends: Understanding the Role of Compositional Transformation on Nanoscale Dielectric Behaviour Through a Simple Solution#Process Route Prateek Prateek, Ritamay Bhunia, Ashish Garg, and Raju Kumar Gupta ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b01092 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019

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ACS Applied Energy Materials

PVP/PVDF as Guest/Host Polymer Blends: Understanding the Role of Compositional Transformation on Nanoscale Dielectric Behaviour Through a Simple Solution‒Process Route Prateek,† Ritamay Bhunia,‡ Ashish Garg‡ and Raju Kumar Gupta*,†,‼ †

Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur

208016, Uttar Pradesh, India ‡

Department of Materials Science and Engineering, Indian Institute of Technology Kanpur,

Kanpur 208016, Uttar Pradesh, India ‼

Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur,

Kanpur 208016, Uttar Pradesh, India *

Corresponding author. Tel: +91-5122596972; Fax: +91-5122590104.

E-mail address: [email protected]

ABSTRACT: We report on the role of polyvinyl pyrrolidone (PVP) additives in PVP/polyvinylidene fluoride (PVDF) polymer blends on compositional transformation dependent dielectric behavior. Our results show that the dielectric performance is controlled by spherulite growth, dipole-dipole interaction, and porosity in PVP/PVDF blends. The breakdown strength, energy density, and efficiency of polymer blends was improved through reducing the content of α-PVDF crystalline phase. The maximum energy density of 4.6 J cm-3 at 3369 kV cm-1 was obtained which was 77% and 283% higher than pristine PVDF and commercially available biaxially oriented polypropylene (BOPP), respectively and was further investigated by piezoforce microscopy (PFM). Keywords: Energy density, PVDF, Composites, Capacitors, PVP, Piezoforce microscopy (PFM)

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Dielectric capacitors have drawn much attention worldwide due to increasing demand to develop high-performance energy storage devices for advanced electrical and electronic applications. These capacitors should combine advantages of ultrahigh power density as well as ultrafast charging-discharging efficiency as compared to other energy systems such as batteries and supercapacitors. Among different capacitors, polymer film capacitors exhibit inherent benefits of easy processing, excellent flexibility, low cost as well as high operating voltages or breakdown strengths. Few of the examples of polymeric dielectric materials are polypropylene (PP), polycarbonate (PC), polyester (PET), polyvinylidene fluoride (PVDF), etc [1, 2]. The energy density (Ud) of an electrostatic capacitor is dependent on dielectric constant (εr) or dielectric polarization (P) of the material as well as an applied field (E), as shown in eq 1,

U d   EdP .

(1)

In the case of linear dielectrics, since εr is independent of the applied field, the energy density is given by eq 2, Ud 

1 2

PE 

1

o r E 2

2

(2)

where εo is the permittivity of vacuum (8.854 × 10-12 F m-1). The present state-of-the-art film capacitor is biaxially oriented polypropylene (BOPP), a linear dielectric, which showed excellent breakdown strength (> 7000 kV cm-1) but suffers from low dielectric constant (~ 2– 3), hence suppressing the energy density to only 1–3 J cm-3 [3, 4]. Discharge efficiency is another important criteria to determine the dielectric performance of capacitors. Higher efficiency requires that the dielectric system should possess nearly linear hysteresis loops with low remnant polarization and hence low hysteresis loss. Drawbacks of reduced efficiency or high ferroelectric loss are the conversion of electrical energy into waste heat, film aging as well as early breakdown [5].


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Thus, an ideal polymer as a dielectric for energy storage applications should have high dielectric constant as well as breakdown strength and should exhibit linear hysteresis loops which are necessary for high energy density and hence discharge efficiency. Although linear hysteresis loops with low remnant polarization and coercive field are necessary for high efficiency, enhanced breakdown strength and maximum polarization are crucial parameters for high energy density. A right combination between maximum polarization, breakdown strength, and linearity helps in achieving both high energy density and efficiency. In search of a highly efficient dielectric polymer, ferroelectric polymers such as PVDF as well as PVDF based copolymers/terpolymers have been extensively utilized for energy storage applications because of high dielectric constant (usually > 10 at 1 kHz) than that of BOPP [6, 7]. PVDF is a semicrystalline polymer (crystallinity: 50–70%) and is known to exhibit five different phases including α, β, γ, δ, and ε depending on the conditions, with α, β, and γ being most frequently discussed. Some of the previous works include the use of PVDF and PVDF based copolymers/terpolymers in polymer nanocomposites, polymer blends, and multi-layered polymer composites [3]. A careful literature review over a past recent years suggests two important considerations. First, the use of nanofillers in polymer matrix deteriorates the dielectric performance due to agglomeration and further restrict enhancement in the dielectric properties. Second, PVDF based copolymers/terpolymers have been used by several researchers as polymer matrix due to their high dielectric constant and breakdown strength. However, the use of PVDF for high energy density applications has dwindled due to comparatively low breakdown strength and high remnant polarization, which results in comparatively lower energy density. Also, the cost of PVDF based copolymers/terpolymers is comparatively very high as compared to PVDF, which is a detriment to the possibility of their large scale commercialization [8]. Thus, there is a need to fabricate cost-effective polymer



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dielectrics with a high dielectric constant (>5–10), energy density and efficiency while minimizing the loss tangent and hysteresis loss. In the present work, we have used PVDF as a polymer matrix and polyvinyl pyrrolidone (PVP) as an additive in PVP/PVDF polymer blend to tailor the film quality and dielectric properties of the blend. In the literature, PVP has been utilized as a functionalizing agent for nanofillers to enhance their compatibility with the polymer matrix [9]. Use of PVP also plays a crucial role in electrospinning by altering the solution properties and making the blend more stretchable [10]. Our results show that with PVP addition, ferroelectric behavior of PVDF is surprisingly altered to that like a linear dielectric as the hysteresis loss dramatically reduced and discharged energy density as well as efficiency significantly enhanced with the addition of PVP. The blends with different PVP contents in PVDF are summarized in Table 1, and detailed experimental procedures and device fabrication steps are mentioned in the Supporting Information. In brief, the PVP and PVDF of known amounts were stirred in N, N– Dimethylformamide (DMF) at 60 oC for 24 h and spin-coated on indium tin oxide coated glass (ITO) substrates at 1000 rpm for 1 min followed by drying at 60 oC on hot-plate for 4 h, vacuum drying for 24 h and then, annealing at 180 oC for 10 min. Table 1. Different amounts of PVP in PVPx/PVDF* films. S. No.

device notation

PVP (mg)

PVDF (mg)

DMF (mL)

1 2 3 4 5

PVDF PVP1/PVDF PVP5/PVDF PVP10/PVDF PVP15/PVDF

0 1 5 10 15

100 100 100 100 100

1 1 1 1 1

*PVPx/PVDF, x represents the amount of PVP (mg) per mL of DMF

Surface morphology of the films was investigated by atomic force microscopy (AFM) and field emission scanning electron microscope (FESEM). Figure 1 shows the AFM micrographs of pure PVDF and PVP/PVDF blends, which indicate that morphology of the 4 ACS Paragon Plus Environment

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films is affected by the mixing of PVP in the PVDF matrix. While pristine PVDF film exhibit relatively non-uniform morphology (Figure 1 a), the film uniformity subsequently increases on blending with PVP1 and PVP5 (Figures 1 b and c). A similar observation is also made in FESEM images (Figure S1) which show that the spherulites are usually composed of radiating fibrils along with daughter fibrils as space fillers with same crystallographic orientation and absence of pin-holes in all the films [11]. The crosslinking between PVP and PVDF changes the chain orientation of PVDF that helps in tuning the spherulite sizes, which in turn is related to the dielectric performance. The PVP5/PVDF blend film contained the largest spherulite or bound spherulites touching each other (Figure 1 c) and start to distort upon further addition of PVP in PVDF matrix, as seen in the morphological images of PVP10/PVDF and PVP15/PVDF (Figures 1 d and e). In the case of PVP15/PVDF blended films, the spherulite structure was completely destroyed. In all samples, we did not observe any significant change in roughness (28–33 nm).

Figure 1. AFM images of (a) PVDF, (b) PVP1/PVDF, (c) PVP5/PVDF, (d) PVP10/PVDF, and (e) PVP15/PVDF films. 5 ACS Paragon Plus Environment


Figure 2 a shows the X-ray diffraction (XRD) spectra of pristine PVDF and PVP/PVDF composite films. It is observed that the XRD peaks mainly consist of α-phase with small amount of γ-phase. Also, there are no peaks of ferroelectric β-phase. The characteristics peaks of α-PVDF are at 17.6, 18.3, and 26.6o assigned to (100), (020), (021) planes, respectively [12, 13]. Although a small superposed peak at 2θ = 20.02o can be ascribed to γ-phase which may be present with α-PVDF, identification of γ-phase from XRD characteristic peaks is contentious as no samples with pure γ-phase have been synthesized till date [13].

Figure 2. (a) XRD spectra and (b, c) FTIR spectra of PVDF and PVP/PVDF blend films.

The XRD spectra show that the mixing of PVP suppresses the α-peak intensities at 17.6 and 18.2o, the peak at 17.6o decreases at a much faster rate from PVDF to PVP15/PVDF. This qualitatively indicates that dipole-dipole interaction between PVP and PVDF reduces the crystalline α-phase content and increases the amorphous content, as also evident from AFM images. The Fourier transform infrared (FTIR) spectra of PVDF and the interaction between PVP and PVDF in PVP/PVDF blended films are shown in Figures 2 b and 2 c. The spectra show absorption bands at 410, 486, 532, 615, 763, 796, 976 cm-1 which are characteristic peaks of α-phase (TG+TG-) while those at 429 and 1234 cm-1 are representative of γ-phase of PVDF. Also, the peak at 510 cm-1 is usually contributed for a mixture of β and γ-phases [14]. The 6 ACS Paragon Plus Environment

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peaks at 2890 and 2924 cm-1 represent aliphatic symmetric vibration (νsym) (CH) and asymmetric vibration (νasym) (CH) and adjacent shoulder peaks at 2982 and 3024 cm-1 correspond to νsym (CH2) and νasym (CH2) vibrations, respectively which constitute half of the PVDF molecular structure [15, 16]. Different peaks of PVP are mentioned in Table S1, while their interaction with PVDF is explained as follows. The absorption peak at 1647 cm-1 is attributed to the stretching vibration (νstr) of carbonyl (–C=O) group of pure PVP which shifts to 1675 cm-1 in PVP/PVDF because of the PVP-PVDF dipole-dipole interactions [17-19]. Furthermore, a broad spectrum of free hydroxyl (–OH) groups at 3450 cm-1 at higher PVP loading in PVP10/PVDF and PVP15/PVDF shows that intermolecular hydrogen bonding is significant with the addition of PVP.[20] The influence of PVP in blended films is also confirmed from the appearance of a sharp peak at 1675 cm-1, which is absent in pure PVDF film [12, 14, 17, 21]. The relative content of α-phase, F(α) is calculated by considering that PVDF crystalline content consists of only α and -phases with minimal or trace amount of β-phase as shown in eq 3 [14], F ( ) 

A 100% (K / K ) A  A

(3)

where, Aα and Aγ are the absorbances and Kα (0.365 µm-1), and Kγ (0.395 µm-1) are the absorption coefficients of α and γ-phases of PVDF, respectively at 763 and 1234 cm-1 [14, 22, 23]. The quantitative analysis suggests that the α-phase decreases as follows: 67.5 (PVDF) > 64.4 (PVP1/PVDF) > 63.7 (PVP5/PVDF) > 62.1 (PVP10/PVDF) > 53.1% (PVP15/PVDF). However, there are no remarkable changes in the peaks attributed to -phases. From these observations, we can comment that the content of -phase decreases with the increment of PVP concentration. The crystalline phase of PVDF and blends are also studied by deconvoluting the peaks



from differential scanning calorimetry (DSC) curves, as shown in Figure S2. The lowering of melting peaks suggests that the amorphous content increases with the further addition of PVP due to lack of ordered crystalline content and hence intermolecular forces as compared to the amorphous phase [24]. This might be due to the hindrance created by the large aromatic groups of PVP in crystallizing PVDF [25]. Also, at smaller loading of PVP1, the PVP1/PVDF consists of a heterogeneous system with small imperfect crystals and pure crystallized domains of PVDF along with the presence of small content of amorphous phase (also see discussion in Supporting Information) [26, 27]. The percent crystallinity first increases from PVDF to PVP1/PVDF and decreases after that as calculated using eq S1. Moreover, the amorphous to crystalline content follows a reverse trend, i.e., decreases from PVDF to PVP1/PVDF and increases with the addition of PVP to PVP15/PVDF (Figure S2 g). Thus, PVP helps in tailoring the crystallinity of the blends, which significantly influences the dielectric behaviour. We investigated the effect of PVP addition on the dielectric performance of PVP/PVDF polymer blends, and the results are studied, as shown in Figure 3. Figure 3 a shows the frequency variation of dielectric constant and loss tangent for various PVP contents. Dielectric constant at 1 kHz increased from 9.7 for pure PVDF to 10.2 for PVP1. This is associated with more interfaces and hence accumulation of interfacial charges in the blend. However, a further increase in PVP concentration results in a decrease in dielectric constant. The dielectric constant at 1 kHz decreased from 9.5 for PVP5/PVDF to 9.2 and 9.1 for PVP10/PVDF and PVP15/PVDF, respectively. This trend in dielectric constant can be explained as follows. When PVP is added in the PVDF matrix, the spherulites formation or number of interfaces, dipoledipole interaction between C‒F (PVDF)‧‧‧‧‧‧C=O (PVP) are the controlling parameters for tuning the dielectric performance. At lower PVP content in PVP1/PVDF, the spherulites of smaller sizes forms and hence the number of interfaces per unit volume of the film increases. The increment in the number of interfaces per unit volume leads to an increase in interfacial


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polarization as well as dielectric constant, but here, the interaction between PVP and PVDF does not play a dominant role. In contrast, PVP5/PVDF samples have completely grown spherulites suggesting that while the number of interfaces is reduced, PVP-PVDF interaction also becomes significant enough to restrict the dipole rotation with the frequency which leads to a decrease in the dielectric constant. Further addition of PVP in PVP10/PVDF and PVP15/PVDF lowers down the dielectric constant further, and that might be associated with the distorted spherulites as well as enhanced interaction between polymers which restrict the dipolar rotation. Also, XRD spectra show that the intensity of shoulder peaks at 17.6 and 18.3o reduced with PVP addition and become insignificant in PVP10/PVDF and PVP15/PVDF. This indicates that amorphous content of -phase of PVDF increases with PVP addition, as also observed in FTIR spectra, which contributes to a reduction in dielectric constant. Furthermore, dielectric constant decreases with the frequency, which is due to limited dipolar mobility at higher frequencies. The loss tangent of all the devices was less than ~0.03 at lower frequencies (102‒104 Hz). The increase at higher frequencies is primarily due to high-frequency relaxation due to weaker dipoles active in this frequency range. Apart from dielectric constant and loss tangent, operative electric field capacity, as well as the dielectric performance of the capacitors, is determined by an important parameter termed as breakdown strength which is calculated using two-parameter Weibull analysis as given in eq 4 [7],   E   P  1  exp   E     b 

(4)

where, P is the cumulative probability of breakdown, E is the experimental breakdown strength, Eo is the characteristic breakdown strength at 63.2% cumulative failure probability, and β is the shape parameter which measures the dispersion of Eo. A higher value of β indicates a highly concentrated and more reliable data distribution. Figures 3 b and 3 c show the Weibull distribution and Eo of different devices. It can be observed that Eo of all the composites is higher 9 ACS Paragon Plus Environment


than that of pure PVDF having Eo and β of 2607 kV cm-1 and 8.9, respectively. The Eo and β increases from 3254 kV cm-1 and 5.6 for PVP1/PVDF to 3369 kV cm-1 and 10.4, respectively for PVP5/PVDF. However, further addition of PVP in PVP10/PVDF and PVP15/PVDF results in a decrease in Eo to 2896 and 2743 kV cm-1, respectively while corresponding β values are 16.2 and 5.0, respectively. The increase in Eo from PVP1/PVDF to PVP5/PVDF shows that PVP5 helps in the formation of robust interfaces without the presence of voids and pores as compared to PVDF [28]. However, a further increase in PVP content in PVP10/PVDF and PVP15/PVDF add porosity to the blends leading to inhomogeneous local electric fields and hence reduced breakdown strength.

Figure 3. (a) Dielectric constant and loss tangent, (b) Weibull distribution plots, (c) characteristic breakdown strengths, (d) schematics of P–E loops, (e) energy density, and (f) efficiency of different PVDF and PVP/PVDF based polymer composite films.


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For high energy density as well as efficiency (eq 5), the enhanced breakdown strength, reduced remnant polarization as well as loss tangent are highly desired.



Ud Ud  Ul

100%

(5)

To study the effect of PVP on dielectric performance in PVP/PVDF polymer blends, polarization (P) ‒ electric field (E) loops of all the devices were measured (Figure 3 d). The P‒ E loops of different composites at their Eo are shown in Figure 3 e, while those at different fields are shown in Figure S3. The P‒E curves reveal that remnant polarization decreases with PVP content in the samples and the loops depict linear behavior, which is beneficial for high energy density due to low hysteresis losses. Figure 3 f shows energy density and efficiency of polymer blends as a function of the electric field. It can be observed that PVP5/PVDF sample exhibits a maximum energy density of 4.6 J cm-3 due to improved breakdown strength and lower hysteresis loss at higher fields. Moreover, the energy efficiencies of different devices calculated from bipolar loops (Figure S3 and Table S2) are also approximately similar to that obtained from unipolar loops (Figure 3 e). It is also previously observed that PVP5/PVDF has bounded spherulites, which means that complete growth of spherulites has taken place and start overlapping with each other, with a minimal number of pores, as well as defects and PVP content, is tuned in such a way that there is a balance between the competing factors, i.e., interfacial polarization and dipole-dipole interaction between PVP and PVDF. Also, amorphous content increases with PVP content in PVP10/PVDF and PVP15/PVDF, which results in a decrease in the dielectric constant as well as energy density. The lower PVP content helps in the formation of spherulites domains along with the increase in the amorphous content of PVDF, while higher PVP contents of PVP10 and PVP15 deteriorates the spherulites domains which help in easy dipole rotation with the applied electric field. Moreover, Table S3 comparing different parameters of hysteresis loops (Pm, Pr, +Ec and -Ec at Eo) shows that Ec and Pr reduce significantly after PVP5/PVDF, responsible for improved energy density 11 ACS Paragon Plus Environment

and


efficiency. The blends approach a linear dielectric behavior which is responsible for the reduced ferroelectric loss and improved energy efficiency as discussed below. Further, we have studied dual AC resonance tracking piezo response force microscopic (DART-PFM) measurements to co-relate the nano-scale ferroelectric polarization performance with the morphology of PVDF and PVP/PVDF blended films and bulk electrical polarisation studies. Local electroactive responses were scrutinized at different places of the entire surface area (10×10 µm2) of each film. The studies were performed under low humidity (~37–39%) conditions to eliminate the possibilities of electrostatic effect generated during tip-sample surface electrical interaction [29]. It is also confirmed by ~180o phase change between two saturation states [30]. The investigated locations are shown by cross (x) signs, which are at different heights, and the corresponding phase and amplitude signal responses are shown in Figure 4. The results show that ferroelectric loops are observed only in spherulite fiber type region (depicted by red cross signs) while yellow cross signs represent the regions where rather linear or narrow loops were formed. As discussed earlier, there were porous regions among small spherulites in PVDF, and with the inclusion of PVP in PVDF, the nucleation of spherulites started with higher growth of the spherulites which reduced the porosity up-to PVP5/PVDF blended films (Figure 1). These variations in structural morphology are related to the electrical polarization at nano as well macro scale as observed in PFM and P-E hysteresis loops (Figure 3 d-f) studies, respectively. The well-defined broad loops in phase and amplitudes responses formed in yellow cross regions indicate the formation of polar nature in PVDF whereas blended films depict progressively linear loops were, as observed in yellow cross regions, at least until PVP5 sample. The coercive voltage and vibrational amplitude responses vary from point to point for each sample due to spatial differences in the concentration of polar and amorphous phases under the conducting tip. In case of PVP1/PVDF and PVP5/PVDF blended films, most of the points showed the formation of properly shaped hysteresis loops in


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both phase and amplitudes responses, which clearly show strong evidence of superior ferroelectric nature along with the high value of saturation polarization, yet with reasonably lower ferroelectric loss. Beyond this optimum concentration of PVP, the blended films (PVP10/PVDF and PVP15/PVDF) show deteriorated performances in PFM studies as depicted by very broad loops from regions marked with red crosses. Thus, PFM studies lend an important conclusion that the progressive shrinking of hysteresis loops towards more linear loops indicate low hysteresis loss during the domain switching. Among all the samples, PVP15/PVDF showed the lowest ferroelectric loss or maximum efficiency in bulk polarization measurement. Table S4 shows a comparison of the dielectric performance of our composites with previous literature on polymer composites, and it is observed that while PVDF based copolymers as well as terpolymers exhibited high dielectric properties, increase in the performance is not as significant as compared to corresponding pristine polymers.



Figure 4. PFM graph, phase and amplitude versus voltage curves of (a-c) PVDF, (d-f) PVP1/PVDF, (g-i) PVP5/PVDF, (j-l) PVP10/PVDF, and (m-o) PVP15/PVDF composite thin film samples. 14 ACS Paragon Plus Environment

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In contrast, our work shows a 77% percent increase in the energy density of PVP5/PVDF as compared to pristine PVDF due to reduced hysteresis losses which are made possible due to control of morphology as well as the content of amorphous phase vis-à-vis -phase. In conclusion, we have studied the role of PVP on morphological changes and dielectric performance of PVP/PVDF polymer blends. The smaller amount of PVP addition helped in the formation and growth of spherulites structure from PVP1 to PVP5, which distorted with the further addition of PVP in PVP10/PVDF and PVP15/PVDF. The formation of a greater number of smaller spherulites increased the number of interfaces in the blends, which improved the interfacial polarization. However, while PVP5/PVDF exhibited completely grown bounded spherulites, the enhanced dipole-dipole interaction between PVP and PVDF led to a decrease in dielectric constant. The improved interaction and film quality helped in increasing the breakdown strength resulting in the maximum energy density of PVP5/PVDF with an energy density of 4.6 J cm-3 at 3369 kV cm-1 field, 77% higher than that of pristine PVDF. The efficiency was found to improve with PVP content. The PFM study revealed that hysteresis loops progressively become linear with reduced hysteresis loss resulting in enhanced energy efficiency. The present work shows that energy density of the PVDF can be significantly improved by reducing the content α-PVDF phase without its conversion to any other crystalline phase conversion and hence increasing the amorphous phase content which assists in achieving more linear type of dielectric behavior with reduced hysteresis losses leading to enhancement in energy density as well as efficiency.

■ ASSOCIATED CONTENT Experimental Procedures, FESEM, DSC, P-E loops, and Tables of PVP/PVDF polymer blends. The Supporting Information is available free of charge on the ACS Publications website.



■ ACKNOWLEDGEMENTS Financial support from Department of Atomic Energy (DAE), BRNS, India for grant 34/14/14/2014−BRNS and Department of Science and Technology (DST) Grant No. DST/TMD/CERI/C140(G) under Clean Energy Research Initiative is acknowledged. RKG acknowledges financial assistance from DST, India, through the INSPIRE Faculty Award (Project No. IFA-13 ENG-57).

■ CONFLICT OF INTEREST The authors declare no conflict of interest.

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Host Polymer Blends: Understanding the Role of

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