Dispersion of carbon nanotubes triggered by the helical self-assembly of poly(methyl methacrylate)

Abstract

Single-walled carbon nanotubes (SWCNTs) are promising nanofillers for various advanced materials, but their uniform dispersion in commodity plastics remains elusive due to solubility problems and poor miscibility. Here, we demonstrate that poly(methyl methacrylate) (PMMA) acts as an effective surfactant for the selective dispersion of small-diameter SWCNTs under θ-solvent conditions. The solvent quality critically governs the formation of PMMA hierarchical helical structures, which enables efficient SWCNT encapsulation. Furthermore, we find that the stereoregularity of PMMA, in particular the syndiotacticity, controls the dispersion selectivity based on the nanotube diameter. The combination of experimental studies and DFT calculations reveals that the dynamic helical conformations of PMMA create nanoscale cavities that are conducive to the entrapment of SWCNTs. This work provides important insights into the design of polymer–nanotube hybrids and opens new avenues for the use of commodity plastics in advanced nanocomposites.

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Introduction

Single-walled carbon nanotubes (SWCNTs) have served as unique cylindrical substrates for molecular interactions, reactions, and physical functionalization, and the dispersion and complexation of SWCNTs in an arbitrary matrix is practically necessary for their application.^1–8 Owing to their poor solubility and limited dispersibility in aqueous and organic liquids, SWCNTs are challenging to manipulate and incorporate into diverse matrices, posing a major hurdle for their use in nanocomposites. To overcome this challenge, limited “good solvents” were found, such as neat aprotic or halogenated solvents,^9,10 and aqueous micellar solution,^11–14 avoiding chemical functionalization.¹⁵ Recently, a few additives that solubilize SWCNTs in organic dyes and cellulose derivatives have been found, including organic solvents.^16–19 Despite their high demand, however, fewer reports describe molecular strategies for inks and composites designed with arbitrary host materials.²⁰ In particular, the absence of cosolvents with commonplace polymers makes it difficult to fabricate composites by impregnating well-exfoliated SWCNT networks. A deeper understanding of SWCNTs’ stabilization using commodity plastics in organic solvents is therefore highly desirable. Once stable polymer–SWCNT dispersions are obtained in suitable solvent systems, they can serve as processable inks for fabricating solid-state composites through wet spinning or solution mixing techniques. This approach offers a practical pathway to produce homogeneous and functional bulk materials from otherwise poorly dispersible nanocarbon fillers.²¹

Hierarchical-polymer architectures are based on short- and long-range interactions. A representative example is the double helix in DNA. Polymers can form ideal folding structures in theta (θ) solvents, where the solvation and excluded volume effects are well balanced.²² A θ-solvent is defined as a solvent condition under which polymer–polymer and polymer–solvent interactions are balanced, resulting in ideal chain conformations without swelling or collapse. More practically, polymers close to the θ conditions are recognized as intermediates between “swollen” and “aggregated”. This ideal structure has been used to dissipate polymers with controlled film quality, reflecting their processing history.²³ For example, the crystallization of isotactic poly(methyl methacrylate) (it-PMMA) and its stereocomplex assembly with syndiotactic poly(methacrylic acid) (st-PMAA) into helical forms were achieved in acetonitrile exhibiting the θ conditions close to room temperature (Scheme 1).²⁴ Such control of the hierarchy of intrinsic polymers is essential for designing molecular assemblies with the desired functions.^25–29

Scheme 1 Chemical substructures of isotactic and syndiotactic PMMA.

Herein we demonstrate that PMMA, a well-known plastic, is a surfactant for SWCNT dispersion in organic solvents close to the θ conditions. The effect of the θ solvents on the SWCNT–PMMA dispersion was systematically elucidated by tuning the PMMA solubility. Furthermore, the effect of structural tacticity on the SWCNT dispersibility was examined using tacticity-controlled PMMA. Furthermore, the selectivity of the dispersed SWCNT diameter distribution observed provides a deeper understanding of nanotube recognition with controlled, hierarchical PMMA structures, such as helical forms, along with the design of practical CNT-based polymer composites.

Results and discussion

Concept and DFT calculations

PMMA has been reported to order and wrap CNTs in dimethylformamide, as suggested by an intense wide-angle X-ray diffraction pattern.^30,31 However, the efficient colloidal dispersion of PMMA–CNT mixtures is rarely achieved, which is implied by the endotherm in aprotic solvents estimated by molecular dynamics simulations.^32,33 In this study, we hypothesize that the helical nanospace of PMMA under the θ conditions can encapsulate SWCNTs (Fig. 1a). Syndiotactic PMMA could form a helix cavity with ca. 1 nm inner space in the ideal state, achieved under the θ conditions, and used it for an assembly with C₆₀ confirmed in the solid state.³⁴ Polymer chains adopt a spread random coil configuration stabilized with suitable solvents. However, in poor solvents, the chains tend to aggregate, limiting the opportunity for PMMA to wrap around SWCNTs. Soluble yet adaptive polymer chains were obtained under these conditions, which enable exothermic CNT adsorption in the energy landscape. Dispersion-corrected density functional theory (DFT) calculations further revealed that helical methyl methacrylate (MMA) oligomers formed more stable complexes with (7,7) SWCNTs than their isolated forms (Fig. 1b; see computational details in the ESI†).^35–37 Additionally, a 22-mer of MMA, which can spontaneously form a single helix, showed a significant driving force of complexation with a (7,7) SWCNT and a stabilization energy of −91.9 kcal mol⁻¹. This strongly suggests the preferential formation of PMMA-wrapped SWCNTs. Furthermore, the molecular interactions between the solvents and the PMMA–SWCNT complexes determine the classification of solvents as good and poor.

Fig. 1 (a) A concept for efficient PMMA wrapping of CNTs in θ solvents. (b) Optimized structures of syndiotactic PMMA–SWCNT complexes with different helical pitches. A (7, 7) SWCNT with 9.62 Å in diameter was used for DFT calculations. For [MMA]_x, x indicates the number of MMA units that can form a single helix. The structures were estimated using the dispersion-corrected DFT effective for dynamically arranged molecular assemblies.

Efficient dispersion of CNTs with PMMA

To verify the above hypothesis, we used a mixture of dry tetrahydrofuran (THF) and water (H₂O) that exhibits good-θ-poor solvent transition for PMMA dependent on THF–H₂O ratios around room temperature (23 °C).³⁸ The SWCNT dispersibility was determined by sonication-assisted dispersion, where probe sonication (ca. 10 W, 10 min) and centrifugation (13 190g, 30 min) were applied, followed by sampling of the upper ca. 70% homogeneous supernatant. We used PMMA with ∼65% syndiotacticity, which was synthesized via radical polymerization. THF, a suitable solvent for PMMA, was found to have no colloidal dispersibility of SWCNTs in the presence of 1% PMMA after sonication. Adding 5–20% H₂O to THF before sonication enhances the solubility of the SWCNTs (Fig. 2a). We observed a significant decrease in CNT dispersibility when the H₂O concentration in the THF–H₂O mixture was above 25%.

Fig. 2 PMMA-assisted SWCNT dispersion dependent on the H₂O/THF ratio. (a) Photographs of CoMocat SWCNT dispersion with solvents of different H₂O/THF ratios including 1 wt% PMMA. Absorption spectra of various SWCNTs with PMMA in (b) pure THF, (c) 15% H₂O, and (d) 30% H₂O in a THF–H₂O mixture. (e) Dispersibility dependence of each CNT on H₂O concentration in the THF–H₂O mixture. S₂₂ absorbance is proportional to the dispersion concentration, plotted as a function of H₂O concentration. SWCNTs (mean diameter, observation wavelength) used here are CoMocat (ca. 0.8 nm, 572 nm), NoPo Hipco (ca. 0.8–1.2 nm, 650 nm), arc-discharge (ca. 1.4 nm, 1000 nm), OCSiAl Tuball (ca. 1.6 nm, 1000 nm), OCSiAl Tuball (ca. 1.8 nm, 1000 nm), and Meijo eDIPS EC2.0 (ca. 2.0 nm, 1000 nm).

Absorption spectroscopy was used to quantify the dispersion yield, where the absorbance was proportional to the solute concentration, according to Beer's law. The absorbance derived from the optical transitions between the first or second van Hove singularity points in the SWCNTs was used for comparison. We observed H₂O-dependent dispersibility for various SWCNTs with different diameter distributions synthesized using five different SWCNTs: the Hipco method (Nopo Nanotechnologies, batch #: Aug19), the arc-discharge (Sigma-Aldrich), and chemical vapor deposition (OCSiAl Tuball with a mean diameter of 1.6 nm, batch #: 01RW03. N1.616 and 1.8 nm, lot # 53-15122014; 2.0 nm, Meijo eDIPS EC2.0) (Fig. 2b–d). These results confirm that PMMA exhibits selective dispersion of SWCNTs with diameters smaller than 1.6 nm (Fig. 2e). The amount of H₂O in the THF–H₂O mixture likely controlled the diameter selectivity in the dispersion, where CoMocat SWCNTs (ca. 0.8 nm) specifically dissolved in around 10% H₂O. In contrast, larger-diameter SWCNTs such as Hipco and arc-discharge dispersed even in THF with a higher H₂O content. It should be noted that the peak absorption coefficients have been previously examined for narrow diameter-SWCNTs (0.24–0.64 μg⁻¹ mL cm^–1),³⁹ which could help determine the concentration of SWCNTs in the dispersion on the basis of the Lambert–Beer law. For example, the absorbance of 0.4 for ca. 1 nm SWCNTs and a 10 mm optical length yield the estimated concentration of approximately 1.6 μg mL⁻¹.

Selective CNT dispersion depending on CNT diameters

To further clarify the role of polymer morphology, we examined the dependence of the SWCNT dispersibility on the PMMA solubility in various solvents. The absorbance dependence of the solvents used shows that suitable solvents, such as dry THF and toluene, cannot disperse SWCNTs in the presence of PMMA. In contrast, the transition to the θ solvents improves SWCNT dispersibility. We used several solvents suggested as the θ-close solvents around room temperature: 2-heptanone (T_θ = 12 °C),⁴⁰ acetonitrile (T_θ = 31 °C),⁴⁰ dimethyl sulfoxide (DMSO) (T_θ = 35 °C),⁴¹ and ethyl acetate (T_θ = 45 °C).⁴² All the θ solvents used here showed SWCNT dispersibility (Fig. 3a–f) while they exhibited a unique dispersion dependence on the SWCNT diameter distribution. 2-Heptanone and ethyl acetate enabled the preferential dissolution of small-diameter SWCNTs such as Hipco (ca. 0.8–1.2 nm). SWCNTs with a mid-diameter range, such as arc-discharge (ca. 1.4 nm) and Tuball SWCNTs (i.e., 1.6 and 1.8 nm), were dispersed in acetonitrile with PMMA. Finally, all SWCNTs showed better dispersibility in DMSO, where each fine peak in the absorption spectra was observed, suggesting better exfoliation of the SWCNTs. Additionally, the suppression of the S₁₁ absorption of large-diameter SWCNTs, such as Tuball and eDIPS, above 1600 nm indicated moderate charge carrier doping of the SWCNTs.⁴³ Because the solvents used here are unlikely to be oxidative, the intermolecular interactions of PMMA with the SWCNTs are expected to contribute to doping. While PMMA conformation plays a primary role, indeed, our previous work suggests that solvent–nanotube interactions (e.g., CH/π interaction and weak charge transfer) may enhance dispersion in polar solvents such as DMSO.¹⁸ While it is quite difficult to identify specific interactions behind the efficient SWCNT dispersion, the solvent–SWCNT interactions should be considered in future mechanistic studies. We further confirmed that the transition from the θ to poor solvent dramatically decreased the CNT dispersibility for the DMSO–H₂O mixture.

Fig. 3 Absorption spectra of PMMA-assisted SWCNT dispersion in 2-heptanone (gray), acetonitrile (blue), DMSO (red–brown), and ethyl acetate (green). SWCNTs used here are (a) CoMocat (ca. 0.8 nm mean diameter), (b) NoPo Hipco (ca. 0.8–1.2 nm), (c) arc-discharge (ca. 1.4 nm), (d) OCSiAl Tuball (ca. 1.6 nm), (e) OCSiAl Tuball (ca. 1.8 nm), and (f) Meijo eDIPS EC2.0 (ca. 2.0 nm). The noise peak around 1400 nm in the panel (e) was derived from the water impurity.

Selective CNT dispersion depending on PMMA's tacticity

Our assumption for the dispersion of SWCNTs is based on the formation of PMMA nanostructures with nanoscale cavities associated with the flexibility of the PMMA main chains. Based on this assumption, we determined the structural selectivity of CNT dispersions. Considering the THF–H₂O dispersion (Fig. 1b), the CNTs’ concentration is proportional to the absorbance and depends on their diameter and H₂O ratio. Peak absorbance plots show a clear dispersibility dependence on the diameter, where better PMMA solubility leads to the efficient dispersion of CNTs with smaller diameters. This suggests that the extended PMMA design altered the dispersibility of the CNTs according to the diameter distribution. We sought to verify this hypothesis using tacticity-enriched PMMA. The tacticity of commercial and synthesized PMMA was estimated by calculating meso–racemo populations derived from α-methyl proton peaks (1.0–1.5 ppm) in the ¹H-NMR spectra (Fig. S1–S6†). Here, we confirmed that an increase in the diad-based syndiotacticity from 72 to 92% leads to narrowing of the diameter distribution in the dispersed CNTs. This systematic improvement in diameter selectivity is associated with the rigidity of the polymer nanospace, which is controlled by an increase in syndiotacticity (Fig. 4a–c). In particular, small-diameter SWCNTs were enriched when the syndiotacticity increased. Around 80–83% syndiotacticity, almost no arc-discharge (1.4 nm) SWCNTs were dispersed, while the efficient dispersibility of smaller-diameter SWCNTs (CoMocat and Hipco) with PMMA was preserved (Fig. 4d). This result strongly suggests that the improved rigidity of the PMMA architectures with ca. 1 nm nanochannels controls the dispersibility of the SWCNTs. It should be noted that efficient dispersions were achieved by dynamic helical forms such as PMMA in DMSO, poly(butyl methacrylate) in 2-propanol, and poly(ethyl methacrylate) in 2-propanol (Fig. S7–S9†).

Fig. 4 Absorption spectra of CNT dispersion with PMMA having the syndiotacticity of (a) r = 72, (b) r = 80, and (c) r = 92. (d) The peak absorbance dependence on PMMA's syndiotacticity for the dispersion of different SWCNTs.

We further assessed the dispersibility dependent on the tacticity-enriched PMMA concentration (Fig. 5a and b), revealing that a low PMMA concentration of around 0.01–0.1 wt% leads to maximum yields (Fig. 5c and d). This does not contradict the conformational effects of PMMA, which forms a helix in a limited manner in low-concentration theta solvents, such as dry DMSO. Additionally, we found that isotactic PMMA can assist in the dispersion of SWCNTs with a morphology different from that of syndiotactic PMMA. The absorption peaks for the isotactic (m = 97) PMMA-assisted dispersions were broadened compared to those of the syndiotactic (r = 92) PMMA dispersion, suggesting that the colloidal materials were composed of bundled SWCNTs. Sharp excitonic peaks indicated the presence of well-exfoliated SWCNTs in the dispersion, whereas peak broadening was accompanied by nanotube bundling.⁴⁴ To complement our absorption-based assessment, we performed photoluminescence excitation (PLE) mapping. This confirmed the presence of bright emission bands for small-diameter single-walled carbon nanotubes (SWCNTs) dispersed in syndiotactic-rich polystyrene (PS) (Fig. 5e). This indicates effective exfoliation and individualization. Conversely, quenched photoluminescence and signs of intertube energy transfer were observed in isotactic PMMA dispersions (Fig. 5f), suggesting the presence of bundled tubes. These results support our hypothesis that polymer tacticity governs the dispersion state of SWCNTs. These macroscopic expectations were further confirmed by transmission electron microscopy (TEM) observation (Fig. 5g and h). TEM indicated that the fibres of the SWCNTs were fully wrapped by polymer shells that were a few nanometers thick. Additionally, isolated and bundled SWCNTs were preferentially observed in syndiotactic and isotactic PMMA dispersions, respectively. Consequently, we elucidated that the stereoregularity of PMMA controls the morphology of SWCNTs in the dispersion, ensuring high dispersion yields. For more quantitative evaluation of bundling, techniques such as small-angle X-ray scattering or statistical atomic force microscopy analysis are considered powerful approaches, as previously reported by Fagan.⁴⁵ Incorporating such analyses will be an important direction for future work to further validate and deepen the mechanistic understanding of polymer–nanotube interactions.

Fig. 5 NoPo Hipco SWCNT dispersion with syndiotactic and isotactic PMMA dispersants. Absorption spectra depend on the concentration of (a) syndiotactic (r = 92) and (b) isotactic (m = 97) PMMA. Po Hipco SWCNT dispersion with syndiotactic and isotactic PMMA dispersants. PLE mapping for the SWCNT dispersion with (e) syndiotactic (r = 92) and (f) isotactic (m = 97) PMMA. Right scale bars indicate photoluminescence intensities. Concentration dependence on the dispersibility of SWCNTs for (c) syndiotactic and (d) isotactic PMMA. The dispersibility was determined as the absorbance representatively at 990 nm. TEM micrographs of dispersed SWCNTs with (g) syndiotactic PMMA and (h) isotactic PMMA. Scale bars in TEM micrographs indicate 10 nm.

Conclusions

We demonstrated an efficient colloidal dispersion of SWCNTs with a narrow diameter distribution utilizing the PMMA's θ conditions. Systematic studies on the dependence of dispersion on solvents and PMMA's tacticity highlighted the role of PMMA's hierarchical helical structures. At the present stage, surfactants used for efficient colloidal SWCNT dispersions have been limited to PMMA, limiting their broad applicability. However, we are currently exploring the dispersion behaviour of various helical polymers in their θ conditions. This study paves the way for using commodity plastics in composites for incorporating morphology-controlled SWCNTs.

The ability to uniformly disperse SWCNTs in PMMA under θ-solvent conditions not only offers mechanistic insights into polymer–nanotube interactions but also opens promising pathways for advanced material applications. Potentially, such composites can exhibit enhanced mechanical strength, thermal conductivity, and electrical properties. Such properties are particularly attractive for applications in high-performance composites and flexible electronics. Future efforts toward solid-state processing and composite fabrication will help realize these application potentials.

Experimental

Materials

We used single-walled carbon nanotubes as received, purchased from Merck (Sigma-Aldrich) (CoMocat and arc-discharge), NoPo (Hipco), OCSiAl (Tuball of two different batches, #53-15122014 and #01RW02.N1.208), and kindly supplied by Osaka Soda (eDIPS EC2.0). Dry tetrahydrofuran and dry acetonitrile were purchased from Kanto Chemical. Toluene, 1-butanol, and 2-propanol were purchased from Wako Pure Chemical Industries. 2-Heptanone was purchased from Nacalai Tesque. Radically polymerized PMMA (r ∼ 65) and isotactic PMMA (m ∼ 97, M_w: 1.42 × 10⁵, M_w/M_n: 1.71) were purchased from Merck (Sigma-Aldrich). Syndiotactic PMMA (syndiotactic >85%, practically measured r ∼ 92, M_w: 1.07 × 10⁵, M_w/M_n: 2.05) was obtained from Polymer Source Inc. (Canada). PMMA with controlled tacticity was synthesized by radical polymerization in designed solvents (see the ESI†).

Methods

Dispersion of SWCNTs (5 mg) in solvents (20 mL) was prepared using tip-sonication (QSonica Q125), followed by centrifugation (13 190g, 10 minutes), followed by the collection of supernatant for further evaluation. Absorption spectra were recorded using a UV-Vis-NIR absorption spectrophotometer with PMT, InGaAs, and PbS detectors (Shimadzu UV3600plus) with 10 mm quartz cuvettes (GL Science S15-IR-10). PLE spectroscopy was conducted using a JASCO FP-8750. NMR spectra were obtained using a JEOL JNM-ECX400P. The molecular weight distribution was estimated using gel permeation chromatography (Shimadzu Prominence/LC-20AD system). Transmission electron microscopy was conducted using a JEOL JEM-3100FEF with a high resolution-type carbon-coated Cu microgrid (Ouken Shoji). We performed DFT calculations under periodic boundary conditions (PBC), as implemented in the Vienna Ab initio Simulation Package (VASP v.6.3.0) code, to investigate whether helical syndiotactic poly(methyl methacrylate) (st-PMMA) can wrap a carbon nanotube (see the ESI† for details).

Author contributions

Y. N. and T. K. supervised the project; A. D. I. and K. Y. designed and performed the experiments and characterized the compounds; A. D. I., T. A. and H. A. synthesized polymers with designed tacticity; S. F. and T. Y. performed theoretical calculations; all the authors contributed to data analysis and approved the manuscript; Y. N. and T. K. secured financial support.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The data supporting the findings of this study are available within the article and its ESI.†

Acknowledgements

We thank Sakiko Fujita for conducting the TEM observation and JASCO Corporation for performing the photoluminescence excitation spectroscopy. This study was financially supported by the MEXT Leading Initiative for Excellent Young Researchers (LEADER) and JSPS KAKENHI grant numbers JP22H05134 and JP23H04876 for Transformative Research Area (A). This paper is also based on results obtained from a project, JPNP20004, subsidized by the New Energy and Industrial Technology Development Organization (NEDO).

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Footnote

† Electronic supplementary information (ESI) available: Supplementary results (NMR and absorption spectra) and experimental and computational methods. See DOI: https://doi.org/10.1039/d5nr01706h

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