Grafting Multiwalled Carbon Nanotubes with Polystyrene to Enable Self-Assembly and Anisotropic Patchiness

Summary

A procedure for the synthesis of polystyrene-grafted multiwalled carbon nanotubes using successive chemical modification steps to selectively introduce the polymer chains to the sidewalls and their self-assembly via anisotropic patchiness is presented.

Abstract

We demonstrate a straightforward protocol to graft pristine multiwalled carbon nanotubes (MWCNTs) with polystyrene (PS) chains at the sidewalls through a free-radical polymerization strategy to enable the modulation of the nanotube surface properties and produce supramolecular self-assembly of the nanostructures. First, a selective hydroxylation of the pristine nanotubes through a biphasic catalytically mediated oxidation reaction creates superficially distributed reactive sites at the sidewalls. The latter reactive sites are subsequently modified with methacrylic moieties using a silylated methacrylic precursor to create polymerizable sites. Those polymerizable groups can address further polymerization of styrene to produce a hybrid nanomaterial containing PS chains grafted to the nanotube sidewalls. The polymer-graft content, amount of silylated methacrylic moieties introduced and hydroxylation modification of the nanotubes are identified and quantified by Thermogravimetric Analysis (TGA). The presence of reactive functional groups hydroxyl and silylated methacrylate are confirmed by Fourier Transform Infrared Spectroscopy (FT-IR). Polystyrene-grafted carbon nanotube solutions in tetrahydrofuran (THF) provide wall-to-wall collinearly self-assembled nanotubes when cast samples are analyzed by transmission electron microscopy (TEM). Those self-assemblies are not obtained when suitable blanks are similarly cast from analogous solutions containing non-grafted counterparts. Therefore, this method enables the modification of the nanotube anisotropic patchiness at the sidewalls which results into spontaneous auto-organization at the nanoscale.

Introduction

Since the discovery of single-walled carbon nanotubes (SWCNTs),^1,2 the scientific communities have applied their outstanding electrical, mechanical and thermal properties³ in a wide range of cutting-edge applications by modulating their surface properties via covalent⁴ and non-covalent⁵ strategies. Examples of those applications include their use as transducers in sensors,^6,7 electrodes in solar cells,⁸ heterogeneous supports in catalysis,⁹ nanoreactors in synthesis,¹⁰ anti-fouling agents in protective films,¹¹ fillers in composite materials,¹²etc. However, the possibility to modulate the surface properties of their more robust, yet industrially available multiwalled counterparts namely, MWCNTs, to control the directionality in their non-covalent interactions at the nanoscale, has remained a difficult task so far.¹³

Supramolecular self-assembly of molecular building blocks is one of the most versatile strategies to control the organization of matter at the nanoscale.^14,15 In this sense, supramolecular interactions involve directional, short-range and mid-range non-covalent interactions such as H-bond, Van der Waals, dipole-dipole, ion-dipole, dipole-induced dipole, π- π stacking, cation-π, anion-π, coulombic, among others.¹⁶ Unfortunately, directionality in self-assembly for larger structures such as MWCNTs is not spontaneous and usually requires external motive forces (e.g. templates or energy dissipation systems).¹⁷ A recent report used non-covalent wrapping of nanotubes with tailored co-polymers to pursue the latter goal,¹⁸ but the use of covalent strategies to offer new alternatives to solve that problem have remained scarcely explored.

Chemical modification of carbon nanotubes can be selectively carried out to introduce different functional groups either to the termini or to the sidewalls of the same.^19,20 One of the most useful approaches to tailor the surface properties in carbon nanostructures is polymer-grafting through standard polymerization routes. Typically, those approaches involve the preliminary introduction of polymerizable or initiator groups (acrylic, vinyl, etc.) on the nanostructure surface and their successive polymerization with a suitable monomer.²¹ In the case of MWCNTs, the covalent introduction of polymer chains on the sidewalls to control their patchiness in an anisotropic fashion has remained a challenge.

Here we will show how a series of straightforward chemical modification steps^22,23 can be applied to insert PS chains on the sidewalls of MWCNTs in order to modify their surface patchiness and to promote their anisotropic self-assembly²³ at the nanoscale. During the modification route, a first step allows for the selective hydroxylation of pristine MWCNTs at the sidewalls by following a biphasic catalytically mediated oxidation reaction to yield the hydroxylated counterparts namely, MWCNT-OH. A second step uses 3-(trimethoxysilyl)propyl methacrylate (TMSPMA) to introduce silylated methacrylic moieties to the previously created hydroxyl groups (MWCNT-O-TMSPMA). These inserts will provide surface reactive sites during a third step, when styrene monomer is polymerized from the methacrylic moieties thus yielding polymer chains grafted to the sidewalls of the nanotubes at the end (i.e. MWCNT-O-PS).

Protocol

Caution: Please consult all relevant material safety data sheets (MSDS) before use. Several of the chemicals used in this protocol are acutely toxic and carcinogenic. Carbon nanotube derivatives may have additional respiratory hazards compared to other traditional bulk carbon allotropes. It is suspected that carbon nanotubes in aerosol may affect lungs in a similar way than asbestos, though their carcinogenic properties have not been completely elucidated so far. Please use all appropriate safety practices when performing the chemical reactions and product processing steps including the use of fume hood and personal protective equipment (lab coat, gloves, safety glasses, closed shoes, full length pants) while suitable filtering facepiece respirators should be particularly used when potential nanotube dust may be generated (NIOSH approved N95 model, or European EN 149 certified FFP3 versions). Portions of the following procedure involve standard inert atmosphere handling techniques.²⁴

1. Selective Hydroxylation of Multiwalled Carbon Nanotubes ²²

1. Biphasic Catalytically Mediated Oxidation of MWCNTs
   1. Preparation of the Organic Phase
      1. Add 2.5 g of pristine MWCNTs to a watch glass, Petri dish or beaker.
      2. Move the container into an 80 °C vacuum oven and cover with a suitable watch glass. Turn on the vacuum pump of the oven and set the system to ca. 200 mmHg.
      3. Dry the nanotubes in the vacuum oven for 12 h. After this step, the water content in the MWCNTs will be eliminated.
      4. Pour the dried MWCNTs into a 100 mL round bottom flask with a glass funnel and a spatula and remove the funnel once finished.
      5. Introduce a magnetic stirring bar to the round bottom flask and add ~ 50 mL of 99% dichloromethane into the flask using a beaker. Stir at 60 rpm using a stirring plate until agglomerates disappear.
      6. If agglomerates remain in solution after stirring for 10 min, move the flask into an ultrasonic bath and turn on the system for 5 min. Repeat sonication if necessary. This solution will be the organic phase in the oxidation reaction stage.
   2. Preparation of the Aqueous Phase
      1. In a 25 mL beaker, dissolve 0.6 g of tetrapropylammonium bromide (TPABr) in 5 mL of distilled water using a magnetic stirring bar and a stirring plate.
      2. In a 10 mL volumetric flask, dissolve 0.253 g KMnO₄ in distilled water (0.16 M solution) and keep the solution in a beaker at hand. This solution will be used in Step 1.1.2.4.
      3. Take 5 mL of glacial acetic acid (HOAc) and transfer the volume drop-wise to the TPABr solution. Using a graduated pipette and constant stirring in an ice bath may be helpful.
         NOTE: The mixture process is exothermic, therefore, the addition should be carried out slowly to avoid possible injuries by projected droplets or an excessive warming.
      4. Transfer a 5 mL aliquot of the 0.16 M KMnO₄ aqueous solution to the acidified TPABr solution. Using a clean graduated pipette and stirring may be helpful.
      5. Keep the previous solution covered with a watch glass until needed. This solution will be the aqueous phase in the oxidation reaction stage.
   3. Oxidation Reaction
      1. Pour the aqueous phase into the round bottom flask that contains the organic phase. This mixture will be the biphasic reaction mixture.
      2. Using the stirring plate, agitate the biphasic system at 80 rpm or more and leave the mixture under stirring at room temperature for 24 h. After this time, the violet aqueous phase turns pale pink and the nanotubes become hydroxylated (MWCNT-OH).
2. Purification of MWCNT-OH
   1. Liquid-Liquid Extraction
      1. Transfer the biphasic reaction mixture to a 150 mL or 200 mL separatory funnel with the stopcock at the closed position. Use a glass funnel and a spatula if needed.
      2. Using a Beral pipette, a Pasteur pipette with a rubber bulb attached or a beaker, add ~ 0.25 mL of concentrated hydrochloric acid into the separatory funnel.
      3. Use the separatory funnel.
         1. Close the separatory funnel with a Teflon stopper (do not use glass stoppers for carbon nanotubes) and agitate vigorously for 5 s in a fume hood with the sash window down. During agitation, always hold the stopper strongly attached to the funnel with one hand and grab the stopcock with the other hand.
         2. Immediately after the agitation step, turn the funnel upside down and release the pressure build-up by opening the stopcock with the respective hand.
            NOTE: Make sure to address the pressure direction to the rear of the fume food.
         3. Repeat the last two agitation/pressure-release steps in cycles of 3 - 5.
         4. Once the last cycle is finished, immediately remove the stopper and leave the funnel clamped and undisturbed until phase separation is evident. After liquid-liquid separation, the aqueous phase remains above the organic phase. The MWCNT-OH will remain at the organic phase.
      4. If no phase separation occurs, add 1 - 5 mL of brine (10 - 20% NaCl in water, m/v) to the separatory funnel using a Beral pipette or a Pasteur pipette, mix gently and leave the mixture undisturbed until phase separation. Repeat until needed.
      5. Recover the organic phase on a 150 mL beaker keeping the stopcock open until the phase boundary reaches the stopcock position. A 150 mL Erlenmeyer flask can be alternatively used to recover the organic phase; however, retrieval of nanotubes from this container type is usually more difficult.
      6. Keep the recovered solution protected with a watch glass until filtration.
      7. Discard the aqueous phase appropriately.
   2. Setting of the Vacuum Filtration System
      1. Set the vacuum filtration system seating the stopper and base firmly into the neck of a clamped Kitasato flask.
      2. Keep a 47 mm diameter membrane filter at hand and, depending on the membrane material and indications from the manufacturer, optionally follow this step and the next two ones (Steps 1.2.2.2 - 1.2.2.4). Prepare a 100 mm diameter Petri dish with ethanol (EtOH) filling 50% of its capacity.
      3. With smooth-tip forceps, place the 47 mm diameter membrane filter on the Petri dish with EtOH. Leave the membrane in EtOH for 2 - 5 min. Make sure to use a membrane with a pore size 0.45 - 20 µm. Smaller pore sizes will clog the filtration, while larger ones will reduce filtering efficiency.
      4. Remove the membrane from the EtOH using the smooth-tip forceps.
      5. Center the membrane on the sintered glass area of the filter support surface starting by the edge of the membrane. The optional wetting with EtOH may facilitate this step.
      6. Center the flange of the funnel on top of the assembly. Do not disturb the membrane. Lock the funnel and base together with the spring clamp.
      7. Connect the system to the vacuum source. Do not turn the vacuum on until needed.
   3. Filtration of the MWCNT-OH
      1. Pour the retrieved organic phase into the vacuum filtration funnel aided by a glass rod. Apply vacuum to filter the sample until dryness.
      2. Wash the solid with 40 - 50 mL of 95% methanol (MeOH) using a wash bottle. Using a glass rod may be helpful in this task. Repeat this process at least 5 times. Keep the vacuum on during this step.
      3. After washing, leave the system under vacuum for 20 min covered with a glass watch to dry the solid.
      4. In order to recover the solid from the membrane, follow the next Steps 1.2.3.5 - 1.2.3.8.
      5. Carefully, remove the spring clamp, then vertically remove the funnel without disturbing the membrane and the solid.
      6. Very carefully, remove the membrane together with the solid from the filter support surface aided by a spatula and the smooth-tip forceps.
      7. Transfer the membrane with the solid to a 100 mm diameter Petri dish or watch glass.
      8. Aided by two smooth-tip forceps and a spatula, recover the solid from the membrane surface until all the product is deposited on the Petri dish/watch glass.
   4. Drying and Storage of MWCNT-OH
      1. Cover the Petri dish/watch glass with a suitable watch glass (larger diameter is required).
         NOTE: Do not use sealing film to avoid electrostatic charging.
      2. Introduce the Petri dish/watch glass into a vacuum oven heated at 80 °C. Turn on the vacuum pump of oven and set the system to ca. 200 mm Hg.
      3. Dry the solid for 24 h. After this step, the solvent content in the product will be eliminated. If the product will not be used immediately, store the solid in a desiccator until needed.

2. Grafting MWCNT-OH with Polystyrene Chains ^22,23

1. Modification of MWCNT-OH with Silylated Methacrylic Moieties
   1. Pour 2.5 g MWCNT-OH and 5 g of hydroquinone into a clean 100 mL Schlenk flask using a glass funnel and a spatula, remove the funnel and introduce a magnetic stirring bar.
   2. Set the Schlenk flask into reflux mode according to the following indications:
      1. Connect the Schlenk flask to the vacuum/inert gas manifold system using a suitable rubber tubing attached to the flask's glass joint. Keep the flask's manifold closed.
      2. Connect a suitable condenser to the tap water system using suitable rubber tubing, making sure to set the water income through the glass joint closest to the male joint.
      3. Lubricate the male joint of the condenser with an evenly applied fine layer of vacuum grease.
         NOTE: The joint should not be over-greased to avoid product contamination.
      4. Attach the condenser's male joint to the Schlenk flask's neck. Close the condenser's female joint with a rubber stopper.
      5. Carefully, open the flask to the vacuum. Make sure that the nanotubes are not sucked up into the vacuum line during this process.
      6. Fill the Schlenk flask with N₂ gas.
      7. Repeat the last two steps two more times.
      8. Keep the system under positive N₂ gas flow.
   3. Aided by a clean cannula and a suitable syringe, slowly inject 50 mL dry toluene through the rubber stopper.
   4. Using the stirring plate and a thermal bath, agitate the mixture vigorously at 80 rpm or more and leave the mixture under stirring for 5 - 10 min at room temperature.
   5. Using a clean cannula and a syringe, inject 5 mL of TMSPMA through the rubber stopper.
   6. Set the temperature to 100 °C and leave the reaction under reflux and stirring for 12 h. After this time, silylated methacrylic moieties are chemically couple to the hydroxyl groups on the MWCNTs (namely MWCNT-O-TMSPMA).
   7. Once the reaction is over, carefully remove the rubber stopper, close the N₂ flow and wait until the flask is at room temperature.
   8. Set a vacuum filtration system (Step 1.2.2).
   9. Filter the solid (Step 1.2.3). Wash with MeOH in excess.
   10. Dry and store the MWCNT-O-TMSPMA (Step 1.2.4).
2. Polymerization of Styrene on Modified MWCNTs
   1. Pour 2.5 g of the MWCNT-O-TMSPMA and 75 mg of azobisisobutyronitrile (AIBN) into a clean 100 mL Schlenk flask using a glass funnel and a spatula and remove the funnel.
      Caution: AIBN powder should not be exposed to heat or strongly oxidizing agents, as it may spontaneously explode.
   2. Introduce a magnetic stirring bar to the Schlenk flask.
   3. Set the Schlenk flask into reflux mode (Step 2.1.2).
   4. Using a clean cannula and a syringe, inject 50 mL dry toluene through the rubber stopper.
   5. Using the stirring plate and the thermal bath, agitate the mixture at 80 rpm or more and leave the mixture under stirring for 5 - 10 min at room temperature.
   6. Aided by a clean cannula and a syringe, inject 7.5 mL of styrene through the stopper.
   7. Set the temperature to 70 °C and leave the reaction under reflux, stirring and N₂ for 12 h. After this time, PS chains are grafted to the nanotube sidewalls (MWCNT-O-PS).
   8. Open the system to the air, close the N₂ flow and cool the flask to room temperature.
   9. Set a vacuum filtration system (Step 1.2.2).
   10. Filter and wash the solid (Step 1.2.3). Use acetone instead of MeOH during the washing step. Wash the solid five times more with 20 mL THF to remove unbound polystyrene.
   11. Dry and store the MWCNT-O-PS (Step 1.2.4).
       NOTE: The length of the polymer chain grafted to the nanotubes can be tuned by changing the concentration of styrene in toluene during the polymerization step; lower concentrations produce shorter chains. The polymerization reaction time can be used to adjust the polymer chain size; shorter reaction times reduce the length of the chains. See references 22 and 23 for details.

Results

TGA data were collected from pristine nanotubes, hydroxylated nanotubes, nanotubes modified with silylated methacrylic moieties and polystyrene-grafted nanotubes (Figure 1). FT-IR results were collected from hydroxylated nanotubes and nanotubes modified with silylated methacrylic moieties (Figure 2). TEM images were collected from pristine nanotubes and polystyrene-grafted nanotubes (Figure 3). TGA d...

Discussion

In this method, there are some steps which result critical to guarantee a successful grafting process. First, the biphasic catalytically mediated oxidation reaction (Step 1.1) should be carried out with recently dispersed carbon nanotubes (Step 1.1.1.5). If dispersion results unviable according to the recommendations in the protocol, the use of an ultrasonic tip sonicator would be helpful if using the same indications (Step 1.1.1.6). Using shorter MWCNTs may also help in solving dispersion issues. Second, setting of the ...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Tetrapropylammonium bromide, 99 % (TPABr)	Sigma-Aldrich	88104	Irritant, toxic
Potassium permanganate, 99 % (KMnO4)	Sigma-Aldrich	223468
Acetic acid, 99.5 %	Sigma-Aldrich	45726
Pristine multiwalled carbon nanotubes, 99 % (MWCNTs)	Bayer Technology Services	Donated sample	Harmful dusts. >1 micrometer in length and 13–16 nm in outer diameter. Alternative supplier: Nanocyl, Catalog N. NC7000, website: http://www.nanocyl.com/
Sodium Chloride, 98 % (NaCl)	Sigma-Aldrich	S3014	Technical grade can also be used
Ethanol, 99.8 % (EtOH)	Sigma-Aldrich	32221	Technical grade can also be used
Methanol, 99.8 % (MeOH)	Sigma-Aldrich	322415	Highly toxic. Technical grade can also be used
Hydroquinone, 99 %	Sigma-Aldrich	H9003
Toluene, 99.8 %	Sigma-Aldrich	244511	Anhydrous
3-(Trimethoxysilyl)propyl methacrylate, 98 % (TMSPMA)	Sigma-Aldrich	440159	Air sensitive, toxic
Azobisisobutyronitrile, 99 % (AIBN)	Sigma-Aldrich	755745	Explosive
Styrene, 99 %	Sigma-Aldrich	S4972	Purified using an alumina gel preparative column and stored at 4 °C
Acetone, 99.5 %	Sigma-Aldrich	179124	Technical grade can also be used
Tetrahydrofuran, 99.9 % (THF)	Sigma-Aldrich	494461
Dichloromethane, 99.5 %	Sigma-Aldrich	443484	Highly toxic
Hydrochloric acid, 37 %	Sigma-Aldrich	435570	Harmful fumes