Design of a pH-Responsive Conductive Nanocomposite Based on MWCNTs Stabilized in Water by Amphiphilic Block Copolymers

Homogeneous water dispersions of multi-walled carbon nanotubes (MWCNTs) were prepared by ultrasonication in the presence of an amphiphilic polystyrene-block-poly(acrylic acid) (PS-b-PAA) copolymer. The ability of PS-b-PAA to disperse and stabilize MWCTNs was investigated by UV-vis, SEM and zeta potential. The results show that the addition of a styrene block to PAA enhances the dispersion efficiency of the graphitic filler compared to pure PAA, possibly due to the nanotube affinity with the polystyrene moiety. Notably, the dispersions show an evident pH-responsive behavior, being MWCNTs reaggregation promoted in basic environment. It is worth noting that the responsive character is maintained in solid composites obtained by drop casting, thus indicating potential applications in sensing.

The bromine terminated polystyrene was used as a macroinitiator in the second step of the reaction. The reaction mixture was diluted with 20 mL anisole and performed at 90 °C using Cu(I)Cl as catalyst. The results can be found in Table S2. The obtained polymers were characterized by 1 H-NMR and GPC. For 1 H-NMR chloroform-d was used as a solvent. THF was used as eluent for GPC and toluene as a reference. To determine the molecular weight of the polystyrene by 1 H-NMR the area of styrene protons was compared to the initiator proton area ( Figure S1). From this, the molecular weight could be calculated. The result only slightly deviated from the GPC results. The PtBA chain length was determined solely by GPC. Figure S1. 1 H-NMR of PS-Br in CDCl3 The copolymer lengths were determined solely by 1 H-NMR. Here, the area of the styrene protons was compared to the tert-butyl proton area. The backbone protons had to be deducted to calculate the chain extension length. One of the 1 H-NMR results is given in Figure S2, red graph.
The tert-butyl group was removed by hydrolysis yielding polyacrylic acid chains. The reaction was performed in 1,4-dioxane and an excess of HCl. The mixture was refluxed at 90 °C for 3 h. The resulting polymer was characterized by 1 H-NMR with d6-DMSO as a solvent ( Figure S2, green graph). The disappearance of the tBA peak (δ = 1.3 ppm) and the new peak at δ = 12.2 ppm show the success of the hydrolysis.

Polymer/MWCNT dispersion characterization via UV-vis spectroscopy (Section 3.2)
The content of MWCNT dispersed in solution was determined by UV-vis spectroscopy. Figure S3. Example of UV-vis spectrum recorded from 300 to 600 nm of MWCNTs (feed of 0.03 mg/mL) dispersed in PS26PAA226 water solution (0.46 mg/mL, pH 5).
where: α is distance between charges NA is the Avogadro number (6.02 × 10 23 mol −1 ) I is ionic strength e is the elementary charge ε0 is the permittivity of free space εr is the dielectric constant of the solvent Kb is the Boltzmann constant T is the temperature κ −1 is the Debye length

Where:
The polymer feed is in g*m -3 During the dispersion process the concentration was 1.0 g × mL −1 which is 1000 g × m −3 Mw is the molecular weight of the polymer in g × mol −1 Np is the number of acrylic acid monomers The distance between the charges is. = Where: lm is the distance between two monomers, which is 2.57 Å. [3] f is the fraction of charged AA units For the polymer PS26PAA580, which has a molecular weight of 44,675 g × mol −1 the relation between Rg and f is shown in Figure S5. Figure S5. Graph of the relation between radius of gyration and fraction of charged AA units based on the system with PS26PAA580.
The fraction of charged acrylic acid units depends on the pH. First of all, it can be assumed that the acrylic acids in the PAA chain behave similarly as acrylic acid monomers in water. With this assumption f can be calculated based on the pKA of acrylic acid (which is 4.25) and the following formula: Filling in the f found for every pH, the relation between Rg and pH with this calculation is shown in Figure  S6. pH However, acrylic acid behaves differently when it is in a polymer chain, because of the proximity of charges, which influences one another. Therefore, the exact relation between f and pH for a PAA chain is more complicated that equation (7) and it is a function of polymer structure and length. However, it can be approximated by the following formula: [4] (8) This gives a different relation between Rg and pH. It is shown in Figure S7. Figure S7. The relation between radius of gyration and pH for PS26PAA580, with the assumption that the proposed approximation of the relation between f and pH is correct.
The radius of gyration was calculated according to the method described above and the surface is calculated as projection of the corresponding sphere. This is multiplied by the molar concentration resulting in the total coverage.