Kinetically Controlled Localization of Carbon Nanotubes in Polylactide

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C: Physical Processes in Nanomaterials and Nanostructures

Kinetically Controlled Localization of Carbon Nanotubes in Polylactide/Poly(vinylidene fluoride) Blend Nanocomposites and Its Influence on the Electromagnetic Interference Shielding, Electrical Conductivity, and Rheological Properties Reza Salehiyan, Mohammadreza Nofar, Suprakas Sinha Ray, and Vincent Ojijo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04494 • Publication Date (Web): 12 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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The Journal of Physical Chemistry

Kinetically Controlled Localization of Carbon Nanotubes in Polylactide/Poly(vinylidene fluoride) Blend Nanocomposites and Its Influence on the Electromagnetic Interference Shielding, Electrical Conductivity, and Rheological Properties Reza Salehiyan1*, Mohammadreza Nofar2, Suprakas Sinha Ray1,3*, Vincent Ojijo1 1DST-CSIR

National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa 2Polymer Research Laboratory, Metallurgical & Materials Engineering Department, Faculty of Chemical and Metallurgical Engineering, Istanbul Technical University, Maslak, Istanbul, 34469, Turkey 3Department of Chemical Sciences, University of Johannesburg, Doorfontein 2028, Johannesburg, South Africa ABSTRACT: This study illustrates the effects of the kinetic parameters (processing time, polyvinylidene fluoride (PVDF) viscosity, carbon nanotube (CNT) aspect ratio, and processing method) on the CNT migration and consequently, the viscoelastic properties, electromagnetic interference shielding effectiveness (EMI SE), dielectric properties, and electrical conductivities of the corresponding polylactide PLA/PVDF/CNT (70/30/0.25 w/w/w) nanocomposites. In the internal mixer, CNTs are pre-mixed with either PLA or PVDF while in the extruder CNTs are only predispersed in the PVDF since the migration route is from PVDF to PLA. The morphology development and CNT migration exhibit time-dependent mechanisms where the properties of the nanocomposites prepared in the internal mixer are relatively higher than those of nanocomposites prepared via the extruder. The viscosity ratio also plays an important role and more CNTs are found at the interface and PLA when low-viscosity PVDF is employed. The highest SE (7.86 dB), dielectric permittivity (935.23 𝜀′𝑝), and electrical conductivity (1.06 × 10-4 S.cm-1 at 0.1 Hz) values are attained when large aspect ratio (L)-CNTs are pre-dispersed with low-viscosity (L)-PVDF. While the lowest properties belong to the blends prepared in the extruder when short aspect ratio (S)-CNTs are pre-dispersed with high-viscosity (H)-PVDF (4.5 dB, 6.00 𝜀′𝑝 and 2.16 × 10-14 S.cm-1 at 0.1 Hz). 1. INTRODUCTION The rapid growth in the daily use of electronic and electric devices has raised serious environmental and health concerns associated with the emittance of electromagnetic (EM) waves. Therefore, the need to filter these hazardous waves has led to a significant effort toward the development of electromagnetic interference (EMI) shielding materials.1 Conventional shields are 1 ACS Paragon Plus Environment

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made of metals, owing to their excellent electrical conductivity, which is a crucial parameter for effective EMI shielding. However, metals are heavy, expensive, prone to corrosion, not too flexible, and difficult to process. In applications where flexibility and lightweight shields are required conductively filled polymers have been introduced.2 EM waves can be reflected if a conductive pathway is built up or absorbed when the dielectric constant is increased due to the dispersion and concentration of the nanoparticles. In context with development of materials for electronic applications carbon nanotubes (CNTs) have attracted significant attention than the other nanoparticle types such as clays and silica, owing to their outstanding properties such as tensile strength, excellent electrical and thermal conductivities.3−5 Nanoparticle incorporation into immiscible polymer blends has been employed by manufacturers and scientists due to its attractive characteristics and convenient route to fabricate materials with superior properties compared to those of the virgin blends.6−7 Segregation of components (immiscibility) in polymer blends can assist the dispersion and distribution of the fillers within the blend. When CNTs are incorporated into immiscible polymer blends the morphologies and hence, the polymer properties are highly dependent on the state of dispersion and localization of the nanoparticles in the blends. High conductivities at low concentrations can be achieved when a network of CNTs are formed at the interface of an immiscible blend.8−12 The ability of nanoparticles to form a network is the key parameter to nanocomposite performance. Nanoparticles can be found in one component, both, or at the interface between the two polymers. Nanoparticles present at the interface or continuous component of an immiscible blend can form a “double percolation” structure, thus reducing the percolation threshold concentration required for the construction of a nanoparticle network.13−16 Therefore, many attempts have been made to manufacture blend nanocomposites with the required CNT network for the most efficient outcome. Apart from percolation threshold reduction, which saves on material costs, the interfacial localization of nanoparticles stabilizes the phase-separated structures of the immiscible blends by acting as a shield to suppress coalescence.6,11,17,18 Such selective localization of the nanoparticles inside immiscible polymer blends depends on various thermodynamic and kinetic parameters. In this regard, particle migration within the immiscible blends plays an important role in the final localization of the nanoparticles.5 Migration refers to the relocation of particles inside an immiscible blend when particles are pre-dispersed in the less thermodynamically favored component. During blending, these pre-dispersed particles would transfer to the more thermodynamically favored component or at the interface. Kinetic driving forces 2 ACS Paragon Plus Environment

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such as mixing time, intensity, or viscosity ratios could play an important role in migration and determination of the final localization of nanoparticles.

According to Young’s equations13,19

(Equation S1, Supplementary Information, SI), thermodynamic equilibrium localization depends on the surface energies between the polymer components as well as the nanoparticles and polymers. There are reports on the surface modification of nanoparticles, in favor of the final localization in immiscible blends, via thermodynamic driving forces.20−23 In this approach, surface-modified nanoparticles bond or interact with both components, thereby facilitating the interfacial localization.20−23 One model system worth investigating comprises immiscible polylactide (PLA)/poly(vinylidene fluoride) (PVDF) blends. PLA is a biobased, biodegradable, and biocompatible thermoplastic with high stiffness and strength; however, it has several disadvantages including high brittleness; low toughness, heat deflection, and melt strength; a slow crystallization rate, and consequently, poor foamability7,24-33 Therefore, the use of a second polymer with high ductility and toughness could further improve the mechanical features of PLA as well as its viscoelasticity and thus, its processability.34,

35

On the other hand, PVDF is a semi-crystalline thermoplastic with ferroelectric

characteristics (piezoelectric and pyroelectric) and high dielectric properties as well as excellent mechanical and thermal properties that can accelerate the PLA crystallization rate through blending.36 Series of studies on the compatibilization of PLA/PVDF blends by interfacial localization of different particles through thermodynamic forces have been recently published.37−43 The aim of these works was to initiate reactions at the interface between the surface of the particles and both components. Recently, modified CNTs were incorporated into the same (50/50) PLA/PVDF blend to stabilize its morphology.43 Thus, to position the CNTs at the interface of the blends, the nanotubes were modified via a two-step method whereby long chains of poly (methyl methacrylate) (PMMA) were grafted onto the CNT surfaces to encourage physical interaction with PVDF. This was followed by modifications with reactive epoxy groups to initiate an in situ reaction with the PLA carboxylic groups. The resultant nanocomposites displayed stabilized morphologies when reactive CNTs were located at the interface as well as the enhanced electrical conductivity of the blends. Notably, while appreciating the efforts in CNT modification, the exhausting and costly route of these modifications does not always lead to an improvement in the electrical conductivity. Geng et al.44 revealed that the reacted epoxy groups covalently bonded on CNTs could cover the CNTs and disturb the π-electron system, thereby leading to lower conductivities than those of the non-treated CNT nanocomposites.

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Similarly, wrapped CNTs with a thick layer of rubber were formed above the critical concentration of silane coupling agents, which disturbed the tube–tube interactions required for electron passage.45,46 Because of the kinetically controlled migration of the nanoparticles during melt blending, unmodified nanoparticles can also be localized at the interface while stabilizing the morphologies.10,47−49 Apart from the thermodynamic driving forces, other kinetically driven forces, including the viscosity ratio, mixing time, shear forces, mixing sequence, and aspect ratio, play crucial roles in the migration and final localization of the nanoparticles in immiscible polymer blends.8,42,50 In a previous study, CNTs were initially premixed with (80/20 w/w) polystyrene (PS)/poly (phenylene ether) (PPE) to produce the PS/PPE/CNT masterbatch.51 During further processing with the polyamide 66 (PA66), the CNTs migrated to PA66 due to their better compatibility with PA. Such migration resulted in a better CNT dispersion and thus, higher electrical conductivity and rheological properties in the (60/40 w/w) PA66/ (PS/PPE) blends. In another study, the migration of the CNTs from a polyethylene (PE)/CNT masterbatch to PA in a (75/25 w/w) PA/PE blend led to CNT localization at the interface during migration. This preparation enhanced the electrical conductivity and elastic modulus G’ (ω) of the blend compared to those of the blends prepared via simultaneous mixing or through the use of a PA/CNT masterbatch.52 As previously mentioned, time is another parameter involved in the migration of nanoparticles. Göldel et al.53 reported that within 30 s, the CNTs migrated from the styrene-co-acrylonitrile (SAN) to the polycarbonate (PC). Other studies reported that longer times were required for migration of CNTs when they were initially pre-dispersed in the component with a higher viscosity than the other one.54,55 Xu et al.54 showed that it takes 30 min to have most of the CNTs migrated to the PA6 (20 wt.%) dispersed component from high viscose polyphenylene sulfide (PPS) matrix in a (PPS+CNT)/PA6 blend prepared from a PPS+CNT masterbatch. In another study it was shown that in a (1/1/3.9 vol.%.) (PS/PVDF/CNT) blend, migration of CNTs from PS with lower viscosity to higher viscosity PVDF would take longer than 30 min.55 Furthermore, particles with higher aspect ratios have been reported to migrate and penetrate the other component faster than ones with lower ratios through a mechanism termed the “Slim-Fast Mechanism”.56,57 Other processing conditions, such as the shearing intensities, were also studied. Moreover, processing in a twin screw extruder afforded more convenient material development and particle dispersion and migration than internal mixers, while its usage was more practical.58−60 In this study, we comprehensively investigated the effects of different kinetic parameters, namely the processing time, viscosity ratio, CNT aspect ratio, and processing method, on CNT migration in a 4 ACS Paragon Plus Environment

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(70/30 w/w) PLA/PVDF blend and their influence on the electrical conductivity, dielectric permittivity and loss, EMI shielding, and rheological behavior of the nanocomposites. The correlation between the effect of the different processing parameters on the migration, state of dispersion, and rheological properties are presented systematically. The major objectives of this study can be summarized as: (i) monitoring the CNT migration as a function of mixing time in an internal mixer, (ii) the effect of the CNT aspect ratio in the migration mechanism and distribution in an internal mixer, (iii) the effect of the viscosity ratio on CNT migration in an internal mixer, (iv) the effect of the viscosity ratio on CNT migration studied via a two-step mixing process a twin-screw extruder, and (v) comparison between the effects of the processing types (internal mixer and twin-screw extruder) on migration and CNT dispersion. It is expected to observe CNTs localized at the interface; thus, CNT migration is possibly from PVDF toward PLA component. 2. EXPERIMENTAL 2.1. Materials. Two different PVDFs of different viscosities were used in this study. The higherviscosity (H)-PVDF, (MFI = 2.2 g/10 min at 300 °C and 3.8 kg load) was purchased from SynQuest Labs, Inc. (Alachua, FL), while the lower-viscosity (L)-PVDF (MFI = 20-35 g/10 min at 230° C and 3.8 kg load, 𝑀𝑛=71,000, 𝑀𝑤=180,000) was purchased from Sigma-Aldrich (South Africa). Bottle grade semi-crystalline 7032D PLA (MFI = 6 g/10 min at 210 °C and 2.16 Kg load; D-lactide content = 1.4 mol%) was purchased from NatureWorks LLC, USA. The viscosities of the two types of PVDFs are displayed in Figure S1, SI. Two unmodified types of carbon nanotubes (CNTs) having different aspect ratios, short (S)-CNT (9.5 nm × 1µm) and long (L)-CNT (9.5 nm × 1.5µm), with 105 Hz) than that of (H-PVDF+S-CNT)/PLA (≤102 Hz; Figure 4(c)).



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Figure 4. Dielectric (a) permittivity (𝜀′), (b) loss (tan 𝛿), and (c) electrical conductivity(𝜎; S. cm-1) of the (PLA+S-CNT)/H-PVDF and (H-PVDF+S-CNT)/PLA blend nanocomposites mixed for 10 min. H-PVDF, high-density PVDF; S-CNT, short CNTs (low aspect ratio). 3.1.2. Effect of PVDF’s Viscosity. 3.1.2.1. Rheology vs. CNT Localization. As previously mentioned, when the S-CNTs are initially pre-mixed with PLA as the lower-viscosity component, they can build up a conductive network. On the other hand, the assistance of migration from the opposite component (PVDF) toward the interface favors the formation of nanocomposites with better dispersions and higher conductivities.8,51,53,74,75 S-CNT migration is a time-dependent process, whereby after 10 min of mixing the amount of S-CNTs found in PLA was relatively less than that observed when the S-CNTs were pre-mixed with PLA. This explains why the high H-PVDF viscosity retarded the migration of the S-CNTs, whereas after 10 min of mixing the majority of the S-CNTs still remained inside the H-PVDF droplets. Therefore, to further investigate this, L-PVDF was next 14 ACS Paragon Plus Environment

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selected to facilitate migration. The elastic moduli, G’ (ω), of PLA/PVDF blends of different PVDF viscosities and their corresponding nanocomposites are displayed in Figure 5(a). The elastic modulus of the (H-PVDF+S-CNT)/PLA composite is still larger than that of (L-PVDF+S-CNT)/PLA, owing to the inherently higher H-PVDF viscosity. However, interestingly, the magnitude of enhancement of the modulus was larger in the nanocomposites comprising L-PVDF, owing to the higher rate of S-CNT migration toward the interface and PLA (Figure 1(b) and 6(b)). The magnitude of enhancement can be clearly observed when the moduli of the nanocomposites are divided by the moduli of the neat blends as illustrated in Figure 5(b).

Figure 5. (a) Elastic moduli [G’ (ω)] of the (PVDF+S-CNT)/PLA blend nanocomposites for two different polyvinylidene fluorides (PVDFs) and (b) the relative elastic moduli (𝐺′𝑛𝑎𝑛𝑜𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒𝑠/ 𝐺′𝑛𝑒𝑎𝑡 𝑏𝑙𝑒𝑛𝑑𝑠) of the blend nanocomposites. S-CNT, short CNTs (low aspect ratio). 3.1.3. Effect of CNT Aspect Ratio. 3.1.3.1. Rheology vs. CNT Localization. At this point, it is clear that the PVDF viscosity plays an important role in the rate of CNT migration in a PLA/PVDF blend. Thus, the effect of the CNT aspect ratio on the dispersion and migration of the CNTs in the (LPVDF+CNT)/PLA blend nanocomposites was next investigated. CNTs exist in bundles and their penetration through the interface is highly dependent on the CNT contact angle at the interface. Therefore, higher aspect ratios increase the probability of the formation of coiled structures. Figure 6(a), (b) reveals that when L-PVDF was employed, the CNTs migrated more easily to the interface and PLA matrix and comprised unbundled structures. Conversely, when L-CNT was employed the CNTs became more interconnected. Moreover, CNTs are prone to breakage during processing.76 15 ACS Paragon Plus Environment


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Thus, an increase in the aspect ratio increases the possibility of breakage, which in turn affects migration and dispersion. Therefore, because both CNT types migrate, to some extent, to the interface and PLA, it follows that the elastic moduli of both blend nanocomposites must exhibit non-terminal behavior (Figure 7(a)); this is also an indication of morphological stabilization. The weighted relaxation spectra (Figure 7(b)) of the blend nanocomposites also exhibited slightly different shapes at their longest (form) relaxation times. When L-CNT was employed, more CNTs were found in the PLA and were more interconnected. Thus, a distinct peak associated with the form relaxation time was observed at τ ≈ 15 s. On the other hand, the peak of the nanocomposites made from S-CNTs was not completely formed, indicating that the minor L-PVDF droplets were not completely relaxed. Previous studies have reported that such a feature in the weighted relaxation spectra could be due to the characteristic behavior of either the co-continuous blends or the highly populated droplet morphologies, where the interface is confined and cannot relax completely.77−79 The morphologies of the corresponding blend nanocomposites are illustrated in Figure S2, SI. The morphology of the nanocomposites derived from S-CNTs is rather concentrated and resembles those of the highly populated samples. On the other hand, more L-CNTs could be found inside the PLA with more interconnected structures, which can immobilize the PLA matrix and facilitate droplet breakup due to the reduction in the viscosity ratio of the blend.

(a)

(b)

1µm Figure 6. Transmission electron micrographs of the (a) (L-PVDF+L-CNT)/PLA and (b) (L-PVDF+SCNT)/PLA blends mixed for 10 min. L-PVDF, low-density PVDF; long CNTs (high aspect ratio).


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Figure 7. (a) Elastic moduli [G’(ω)] and (b) weighted relaxation spectra as a function of the frequency of the (L-PVDF+L-CNT)/PLA and (L-PVDF+S-CNT)/PLA blend nanocomposites. L-PVDF, lowdensity PVDF; L-CNT, long CNTs (high aspect ratio). 3.1.3.2. Electromagnetic Interference Shielding Effectiveness. Further, for the closer inspection of the interconnected structures of the L-CNTs inside the PLA, EMI tests, as well as dielectric and electrical conductivity tests, were carried out to measure the efficiency of the materials in forming the conductive network structures. As discussed earlier, the formation of a conductive network is crucial toward attaining a higher EMI-SE. Figure 8 reveals that nanocomposites incorporated with L-CNTs display relatively higher EMI SE values than the nanocomposites with S-CNTs. Interestingly, the values are significantly enhanced in the range 3.5–5.5 GHz, with two maximum peaks at 4 and 5 GHz. Thus, the blend can be more efficient in this range at higher CNT concentrations.



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Figure 8. Electromagnetic interference shielding effectiveness (EMI SE) of the (L-PVDF+LCNT)/PLA and (L-PVDF+S-CNT)/PLA blend nanocomposites in the (a) broadband and (b) X-band ranges. L-PVDF, low-density PVDF; L-CNT, long CNTs (high aspect ratio). 3.1.3.2. Dielectric Properties and Electrical Conductivity. The data in Figure 9 reveals that the dielectric (Figure 9(a), (b)) and electrical (Figure 9(c)) properties are in line with the SE results. This again highlights the role of the conductive path in the facilitation of charge carrier migration and wave scattering. Clearly, the nanocomposites prepared from L-CNTs are more conductive (more than one order of magnitude) with longer plateau regions (>105 Hz). A comparison of the results in Figure 9(c) and Figure 4(c) reveals that the conductivities of the blend nanocomposites increased by about four orders of magnitude with an extended frequency-independent region when L-PVDF was replaced with H-PVDF in the (PVDF+S-CNT)/PLA blend nanocomposites. This manifests the prominent effect of the viscosity ratio on the extent of CNT migration and formation of the conductive path. Interestingly, the conductivity of the (L-PVDF+L-CNT)/PLA is slightly higher than that of the (50/50) PVDF/acrylonitrile butadiene styrene (ABS) blend incorporated with 3 wt.% identical CNTs.77 In this study, however, only 0.25 wt.% identical L-CNTs were used in a (70/30) PLA/PVDF blend.


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Figure 9. Dielectric (a) permittivity (𝜀′), (b) loss (tan 𝛿), and (c) electrical conductivity (𝜎; S. cm-1) of the (L-PVDF+L-CNT)/PLA and (L-PVDF+S-CNT)/PLA blend nanocomposites mixed for 10 min. LPVDF, low-density PVDF; S-CNT, short CNTs (low aspect ratio); L-CNT, long CNTs (high aspect ratio). 3.2. Nanocomposites Processed Using a Two-Step Method in a Twin-screw Extruder. 3.2.1. Effect of PVDF’s Viscosity. 3.2.1.1 CNTs Localization. The migration and corresponding properties of the blends produced by the two-step method via twin screw extrusion were next investigated to establish their potential for industrial application. Moreover, a comparison of these data with the results observed for the composites prepared in an internal mixer (section 3.1) revealed the effect of the shearing intensity on the migration and morphology development of the (PVDF+SCNT)/PLA blend nanocomposites. Figure 10 illustrates that the CNTs are mostly present at the interface inside the PVDF minor droplets. Compared to those in the blend nanocomposites prepared in an internal mixer, less CNTs migrated to the PLA. However, the nanocomposites prepared from LPVDF presented more uniform morphologies, whereby the CNTs at the interface acted like a shield 19 ACS Paragon Plus Environment


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and prevented the coalescence of the droplets. The effect of the viscosity ratio on the CNT migration inside the PVDF+S-CNT/PLA nanocomposite is noteworthy. To prove the localization of the CNTs in the blend nanocomposites, the DSC results were then employed to observe the effect of CNT localization on the crystallization temperature of the blends on cooling (Figure S3, SI). The data revealed that the CNTs at the L-PVDF interface acted as nucleating agents and crystallized the LPVDF. On the other hand, the CNTs inside the H-PVDF droplets accelerated the crystallization of HPVDF. Thus, the H-PVDF crystallization temperature was four degrees (143.23 °C) higher than that of the L-PVDF (139.39 °C) in the nanocomposites.

(a’)

(a)

1 µm

(b’)

(b)

Figure 10. Transmission electron micrographs of the (a, a’) (H-PVDF+S-CNT)/PLA and (b, b’) (LPVDF+S-CNT)/PLA blends prepared via the two-step method in the twin screw extruder: (a, b) lowmagnification images with 1 µm scale bars and (a’, b’) high-magnification images with 500 nm scale bars. H-PVDF, high-density PVDF; L-PVDF, low-density PVDF; S-CNT, short CNTs (low aspect ratio). 20 ACS Paragon Plus Environment

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3.2.1.2. Rheology. The rheological properties also indicated that the enhancement of the nanocomposite elastic moduli was more evident when L-PVDF was employed. This was attributed to the greater interfacial localization of the S-CNTs during migration (Figure 11). Although the elastic modulus of the PLA/L-PVDF blend was lower than that of the PLA/H-PVDF composite at lowfrequency regions (Figure (a), (b)), the corresponding rheological behavior of both nanocomposites displayed similar responses. Thus, the nanocomposite form relaxation times displayed an increasing trend, as opposed to their respective neat blends which presented distinct peaks τ ≈10 s (Figure 11(c)). This was again attributed to the restricted motion of the PVDF droplets, as a result of the CNTs present at the interface inside the droplets, which prolonged the relaxation process of the droplets.

Figure 11. (a) Elastic moduli [G’ (ω)] of the (PVDF+S-CNT)/PLA blend nanocomposites for two different PVDFs prepared by a two-step method in the extruder; (b) relative elastic moduli 21 ACS Paragon Plus Environment


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[𝐺′𝑛𝑎𝑛𝑜𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒𝑠/𝐺′𝑛𝑒𝑎𝑡 𝑏𝑙𝑒𝑛𝑑𝑠] of the blend nanocomposites; and (c) weighted relaxation spectra of the neat PLA/H-PVDF and PLA/L-PVDF blends and their corresponding low aspect ratio CNT (S-CNT)incorporated nanocomposites. H-PVDF, high-density PVDF; L-PVDF, low-density PVDF; S-CNT, short CNTs (low aspect ratio); L-CNT, long CNTs (high aspect ratio). 3.2.1.3. Electromagnetic Interference Shielding Effectiveness. On the other hand, the EMI SEs of the blend nanocomposites were significantly different (Figure 12), especially in the broadband ranges where a maximum peak emerged in the range 4–5 GHz for the nanocomposites prepared via the LPVDF+S-CNT pre-mixture. The average EMI SE of the (L-PVDF+S-CNT)/PLA nanocomposite was higher than that of (H-PVDF+S-CNT)/PLA, indicating the ability of the former blend to form a more conductive network. Moreover, the interfacial localization of the S-CNTs also enhanced the internal reflections of the EM waves.

Figure 12. Electromagnetic interference shielding effectiveness (EMI-SE) of the (H-PVDF+SCNT)/PLA and (L-PVDF+S-CNT)/PLA blend nanocomposites in the (a) broadband and (b) X-band range prepared from a two-step process. H-PVDF, high-density PVDF; L-PVDF, low-density PVDF; S-CNT, short CNTs (low aspect ratio). 3.2.1.4. Dielectric Properties and Electrical Conductivity. To evaluate the conductive network of the blend nanocomposites with respect to the viscosity of the PVDF used in the preparation of the premixtures, the dielectric and electrical properties of the blends were next measured (Figure 13). Interestingly, both the dielectric permittivity and tan 𝛿 of the nanocomposites prepared from the H22 ACS Paragon Plus Environment

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PVDF+S-CNT pre-mixture were quite low and approached the values of the neat blends. This is indicative of an insulating (non-conductive) material (Figure 13(c)). The conductivity of the (HPVDF+S-CNT)/PLA nanocomposite gradually increased with increasing frequency, a trend similar to that observed for the neat blends; this is typical insulating material behavior.80 In contrast, nanocomposites prepared from the L-PVDF+S-CNT pre-mixture exhibited conductive behavior with a plateau region at frequencies ≤104 Hz. These findings indicate that the S-CNTs were unable to construct a conductive path in the (H-PVDF+S-CNT)/PLA nanocomposite. This was attributed to the discontinuous arrangement of the S-CNTs throughout the blend when H-PVDF was used, owing to the hindrance effect of its higher viscosity on the migration of the S-CNTs toward the PLA and interface. This phenomenon does not occur in the (L-PVDF+S-CNT)/PLA nanocomposite.

Figure 13. Dielectric (a) permittivity (𝜀′), (b) loss (tan 𝛿), and (c) electrical conductivity (𝜎; S. cm-1) of the (H-PVDF+S-CNT)/PLA and (L-PVDF+S-CNT)/PLA blend nanocomposites prepared from a two-step process. H-PVDF, high-density PVDF; L-PVDF, low-density PVDF; S-CNT, short CNTs (low aspect ratio). 23 ACS Paragon Plus Environment


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3.3. Summary 3.3.1. Migration Mechanism. The nanocomposite migration mechanisms of the nanotubes can be discussed using the kinetic factors in a dynamic process where mixing sequence is considered. As previously discussed, in a sequential mixing procedure particles can transfer (migrate) from a less thermodynamically preferred polymer to the other one. Further, particles can be trapped at the interface if such a blending process (two-step method) is stopped at a specific time. It is found that migration of CNTs from PVDF into the PLA and interface depends on both the PVDF viscosity and mixing time (Sections 3.1 and 3.2.1). The general concept for particle migration from one component to another is based on the translational motion of the particles during the coalescence and collision of the droplets in a mixer. Moreover, stretching-folding is the main mechanism of the morphology development in immiscible blends inside the mixers.81 When a molten blend passes through the narrow gap between the screws, the droplets stretch and become thinner. Such a process occurs multiple times throughout the entire course of mixing, implying the importance of the time on the morphology evolutions. At some point, the thickness of the stretched threads reaches a critical condition, whereby contact between the CNTs inside the threads (PVDF herein) and the interface becomes possible.51 The CNTs can either migrate into the other polymer when the stretched droplets break as a result of Rayleigh instability or they can become trapped at the interface if the mixing time is not adequate. Moreover, the possibility of the breaking the trapped CNTs due to collision and dispersing into the continuous polymer (PLA) could also be taken into account. The proposed mechanism of CNT migration from the minor PVDF droplets to the interface and PLA is illustrated in Scheme 1.


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Scheme 1. Schematic illustration of the proposed migration mechanism of the carbon nanotubes (CNTs) inside the immiscible polymer blends, from the minor dispersed component toward the interface and matrix component. In this scheme: (i) represents an ideal static PVDF droplet with CNTs embedded within. Once the droplet is between the gap of the screws it becomes stretched along the direction of the applied force (ii). As the stretching-folding process proceeds, the CNT bundles begin to recoil and align themselves along the stretched droplet (iii). The stretched droplet cracks and tears beyond the critical thickness as a result of the local Laplace pressure at the curvature. This may also occur in conjunction with CNT breakage, whereby the broken CNTs are left at the interface (iv). Further instabilities at the interface curvature, owing to gradients in the normal stresses and Laplace pressure, could break the portions of the CNTs wetted by the matrix at the interface, not to mention that portions of CNTs indicated in (iv) can also break during the mixing as a result of collision with other droplets (v). Notably, the high viscosity ratio of the blends further lengthens the stretching-breakage and finally, the migration cycle processes. On the other hand, at high shear rates, e.g. in the extruder, a short residence time limits this stretching and breakage process,82 thereby reducing the extent of complete migration.



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3.3.2. Comparison between Internal Mixing and Extrusion A summary of the results is presented in Table 2 with an emphasis on the processing methods. Table 2. Summary of the rheological, shielding effectiveness (SE), and electrical properties of the blend nanocomposites prepared in an internal mixer and twin screw extruder.

Internal mixer (10 min) Extrusion method

Material

𝐺′0.01(Pa)

τ (s)

(PLA+S-CNT)/H-PVDF (H-PVDF+S-CNT)/PLA (L-PVDF+S-CNT)/PLA (L-PVDF+L-CNT)/PLA (L-PVDF+S-CNT)/PLA (H-PVDF+S-CNT)/PLA

236 98.3 49.6 127 24 18.5

38.85 22.36 14.78 -

SEmax (dB) 7.86 7.86 6.36 -

𝜀′𝑝 812.43 27.35 432 935.23 170.57 6.00

𝜎1 × 10 ―1 Hz (S.cm-1) 6.9 × 10-5 6.84 × 10-10 7.85 × 10-6 1.06 × 10-4 4.45 × 10-7 2.16 × 10-14

𝑓𝑐 (Hz) 176 × 103 76.8 234 × 102 126 × 103 61 × 102 -

The 10 min mixed nanocomposites from the internal mixer were selected; τ represents the longest relaxation peak, SEmax represents the SE of the maximum peak, p denotes the values of the plateau region, and fc indicates the frequency in which the conductivities deviate from the plateau region.

The data reveal that the nanocomposites prepared in an internal mixer displayed higher values than their counterparts prepared via the extrusion method. This confirms the aforementioned suppositions on the different mixing profiles and droplet deformations in the different mixers. The data also reveal that CNT migration is more time- than shear-dependent. However, using an extrusion method has more economic justification where high-throughput polymer nanocomposite production of polymer nanocomposites is required. Interestingly, the highest conductivity and dielectric permittivity values were achieved when the L-CNTs were pre-mixed with L-PVDF and subsequently mixed with PLA for a total time of 10 min. This occurred despite the relatively lower elastic modulus of the afforded nanocomposite compared to those of the (PLA+S-CNT)/H-PVDF nanocomposites and was attributed to the inherent higher H-PVDF viscosity. These results confirm the synergistic effect of the viscosity ratio, mixing time, and aspect ratio on the formation of a conductive network in immiscible PLA/PVDF blends. 4. CONCLUSIONS This study was done to investigate the kinetics of CNT migration from the PVDF dispersed component to the PLA matrix. The CNTs were better dispersed inside the PLA, owing to the much lower viscosity of the latter over that of PVDF. Therefore, a series of tests was carried out to evaluate the extent of migration with respect to the mixing time, viscosity ratio, aspect ratio, and mixer type. Subsequently, shielding effectiveness, dielectric permittivity, and electrical conductivity tests were carried out to assess the effect of migration and the final CNT distribution. Each property was closely 26 ACS Paragon Plus Environment

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related to the ability of the material to form a conductive path. The results revealed that CNT migration is initially a time-dependent process, where the viscosity ratio plays an important role in the time-consuming process. Moreover, the use of L-PVDF facilitated the migration of the CNTs toward the interface and PLA. Therefore, the electrical properties, as well as EMI shielding effectiveness, were higher when L-PVDF was employed, owing to the presence of more CNTs at the interface. The highest conductivity was obtained when CNTs with a large aspect ratio were pre-mixed with L-PVDF. This indicated the role of both the aspect and viscosity ratios on the construction of the conductive network. Overall, the nanocomposites prepared in an internal mixer presented relatively higher properties compared to those of the other samples, due to the longer residence time as opposed to the very short residence time inside the extruder. This again confirmed that migration in high-viscosity ratio blends is more proportional to the residence time than the shear rates. Interestingly, the (HPVDF+S-CNT)/PLA nanocomposites exhibited a more typical insulating polymer behavior than that of its counterpart prepared inside the internal mixer in 10 min. This again confirmed the more important role of the residence time over that of the shear rates. All the EMI SEs and dielectric and electrical conductivities were in line with the state of the CNT dispersion/distribution. Finally, these findings provide insight into the process mechanism for further optimization of the parameters to produce nanocomposites with low percolation thresholds. Based on these findings, further studies are underway to develop an EMI shielding material by optimizing the CNT distribution and content in the (L-PVDF+L-CNT)/PLA nanocomposite prepared from a masterbatch method. AUTHOR INFORMATION Corresponding Authors *E-mails: [email protected] (RS); [email protected], [email protected] (SSR) Phone: +27 12 841 2388. Website: www.csir.co.za/nano ORCID Reza Salehiyan: 0000-0001-5345-5162 Mohammadreza Nofar: 0000-0002-4364-2930 Suprakas Sinha Ray: 0000-0002-0007-2595 Vincent Ojijo: 0000-0002-3473-6580 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT. The authors would like to thank the Department of Science and Technology (CGER8x) and the Council for Scientific and Industrial Research (CGER74s) for their 27 ACS Paragon Plus Environment


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financial support. The authors also appreciate the support shown by Prof. Selcuk Paker and Prof. Ferid Salehi from the Istanbul Technical University who assisted the performance of the EMI shielding and conductivity measurements in their laboratories. SUPPLEMENTARY INFORMATION. This document reports the calculation of the interfacial tensions, scanning electron micrographs of the (L-PVDF+L-CNT)/PLA and (L-PVDF+S-CNT)/PLA nanocomposites, and differential scanning calorimetry results of the nanocomposites from the masterbatch method. REFERENCES (1) Kato, Y.; Horibe, M.; Ata, S.; Yamada, T.; Hata, K. Stretchable Electromagnetic-Interference Shielding Materials Made of a Long Single-Walled Carbon-Nanotube–Elastomer Composite. RSC Adv. 2017, 7, 10841-10847. (2) Thomassin, J.-M.; Jerome, C.; Pardoen, T.; Bailly, C.; Huynen, I.; Detrembleur, C. Polymer/Carbon Based Composites as Electromagnetic Interference (EMI) Shielding Materials. Mater. Sci. Eng. R 2013, 74, 211-232. (3) Krause, B.; Barbier, C.; Kunz, K.; Pötschke, P. Comparative Study of Singlewalled, Multiwalled, and Branched Carbon Nanotubes Melt Mixed in Different Thermoplastic Matrices. Polymer 2018, 159, 75-85. (4) Marcourt, M.; Cassagnau, P.; Fulchiron, R.; Rousseaux, D.; Lhost, O.; Karam, S. A Model for the Electrical Conductivity Variation of Molten Polymer Filled with Carbon Nanotubes Under Extensional Deformation. Compos. Sci. Technol. 2018, 168, 111-117. (5) Pawar, S. P.; Rzeczkowski, P.; Pötschke, P.; Krause, B.; Bose, S. Does the Processing Method Resulting in Different States of an Interconnected Network of Multiwalled Carbon Nanotubes in Polymeric Blend Nanocomposites Affect EMI Shielding Properties? ACS Omega 2018, 3, 5771-5782. (6) Salehiyan, R.; Ray, S. S. In Processing of Polymer-based Nanocomposites; Ray, S. S., Eds.; Springer, Cham, Switzerland, 2018; Vol. 278, pp 167-197. (7) Nofar, M.; Sacligil, D.; Carreau, P. J.; Kamal, M. R.; Heuzey, M.-C. Poly (lactic acid) Blends: Processing, Properties and Applications. Int. J. Biol. Macromol. 2018, 125, 307-360. (8) Salehiyan, R.; Ray, S. S. Tuning the Conductivity of Nanocomposites Through Nanoparticle Migration and Interface Crossing in Immiscible Polymer Blends: A Review on Fundamental Understanding. Macromol. Mater. Eng. 2019, 304, No. 1800431. (9) Liebscher, M.; Domurath, J.; Krause, B.; Saphiannikova, M.; Heinrich, G.; Pötschke, P. Electrical and Melt Rheological Characterization of PC and Co‐continuous PC/SAN Blends Filled with CNTs: Relationship Between Melt‐Mixing Parameters, Filler Dispersion, and Filler Aspect Ratio. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 79-88. (10) Roman, C.; García-Morales, M.; Gupta, J.; McNally, T. On the Phase Affinity of MultiWalled Carbon Nanotubes in PMMA:LDPE Immiscible Polymer Blends. Polymer 2017, 118, 1-11. (11) Wu, D.; Lin, D.; Zhang, J.; Zhou, W.; Zhang, M.; Zhang, Y.; Wang, D.; Lin, B. Selective Localization of Nanofillers: Effect on Morphology and Crystallization of PLA/PCL Blends. Macromol. Chem. Phys. 2011, 212, 613-626. 28 ACS Paragon Plus Environment

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Kinetically Controlled Localization of Carbon Nanotubes in Polylactide

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