Testing ‘Double Percolation’ of Carbon Nanotubes in Copolymer Latexes by means of Reverse Addition Fragmentation Transfer Polymerisation.

Introduction Polymer composites containing carbon nanotube (CNT) networks have undergone extensive study over the last few years1,2. Electronic conductance in these films relies on a connected network of nanotubes or by saturation of CNT's into the polymer matrix. Recent studies on latex systems and polymer blends have revealed that conductivity at lower NT loadings can be achieved by a concept known as 'Double Percolation'(figure 1) where the threshold is controlled by the composition and morphology of the polymers. In this way the percolation threshold may be reduced by preferentially dispersing the NT's into the more compatible minor phase.

Figure 1 illustrating a percolated network in a copolymer.
This concept of double percolation is being applied to copolymers where the major and minor blocks can be produced in a latex (controlled morphology) using controlled radical polymerisation methods such as RAFT (Reverse Addition Fragmentation Transfer) followed by dispersion of the CNT's therein. Owing to studies by Van den Brande and Van Mele3, Koning et. al.4,5 and Hermant6; several candidate co-polymers have been identified, notably copolymers containing blocks of polystyrene (PS), poly-t-butylmethacrylate (PtBMA), polymethylmethacrylate (PMMA), polydimethylsiloxane (PDMS) and poly(2-dimethylamino)ethyl methacrylate (PDMAEMA).

Experimental Block Copolymers are prepared using emulsion RAFT polymerization and bulk RAFT polymerization. ATRP was carried out for some PDMAEMA-co-PS or PMMA copolymers.
Block copolymers and composites are analysed using Differential Scanning Calorimetry, Thermal Gravimetric Analysis and Gel Permeation Chromatography utilizing Universal Calibration and Triple detection. Latexes and solutions are analysed by Dynamic Light Scattering and UV-vis spectroscopy. Percolation thresholds are acquired by 2 and 4 point conductivity measurements on Nanocomposite thin films.

Discussion RAFT polymerization offers a flexible route to well defined block copolymers latexes. A significant challenge was using PDMS in emulsion systems, which was unrealistic for R-ATRP due to emulsion stability and catalyst/ligand problems. In the case of blocks where block A and block B are both miscible phases for CNT's 'Double Percolation' does not hold and a higher threshold is obtained (Figure 2).

Figure 2 Conductivity measurements of films of PS, PMMA and PMMA-b-PS.
Co-polymer blocks consisting of PDMS-b-hydrophilic polymer can be used as a CNT dispersing surfactant. MWCNT's can be dispersed by these surfactants at lower concentrations than sodium dodecyl sulfate, SDS can have undesirable effects on compression molded films. Aqueous dispersion directly into a block-copolymer can offer an improved compatibility route to CNT dispersion into latexes or a direct route to nancocomposite polymer films.

References
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3 N. Van den Brande, B. van Mele, P Geerlings and G van Lier, (Thesis) Experimental and Theoretical Investigation of novel (semi-)conducting nanocomposites based on carbon nanotubes, 2009, Vrije Universiteit Eindhoven.
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