Thermoresponsive Polymer Grafting from the Electrochemically Surface-Modified Graphite

    It has been already achieved in our previous studies that the phenolic hydroxyl groups are introduced on carbon fiber by the electrooxidation of the LiNO₃ containing electrolyte using the carbon fiber as an anode.  In this study, we have first applied our procedure to the oxidation of graphite rod surface.  The graphite rod was found to be readily oxidized under anodic conditions.  In the XPS spectrum of the untreated graphite was observed a sharp peak at 285 eV.  On the other hand, this peak broadened and shifted to high energy range after 0.5 F/mol of electricity based on LiNO₃ was passed.  The peak separation analysis of the C 1s spectrum of the oxidized graphite indicates that the spectrum can be charactrized as a combination of 4 peaks attributed to C-C (C=C) at 284.8±0.1 eV, C-O at 286.9±0.3 eV, C=O at 288.5±0.3 eV, and C(=O)O at 289.8±0.2 eV,  and the intensity of the peak at 286.9 eV is highest.  These results indicate that phenolic hydroxyl groups are mainly introduced on the graphite surface under the anodic oxidations.

    The polymerization with reversible addition-fragmentation chain transfer (RAFT) is an extremely versatile process for the synthesis of end-functional polymers and block copolymers.  Trithiocarbonate derivatives have been reported to be effective as a chain transfer reagent (RAFT regent) for polymerization of styrene, acrylates, and acrylamides.  On the anodically oxidized graphite were fixed 4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoyloxy groups by esterification with the corresponding acid chlorides.  From the resulting RAFT reagent-modified graphite, several acrylamide monomers such as N-isopropyl acrylamide (NIPAAm) were polymerized under RAFT polymerization conditions to afford the corrsoponding thermoresponsive polymer-grafted graphites.  In the mechanism of RAFT polymerization the propagating macroradical species migrate between the RAFT reagents fixed on the graphite and those in the solutions, and it is known that the molecular weights and repeat units sequence structures of the grafted polymers are almost same as those of the polymers recovered from the polymerization solution.  Therefore the polymers were recovered from the polymerization solution and characterized.  For instance, the molecular weight and dispersity index of the grafted PNIPAAm were estimated by GPC to be 6750 and 1.24 respectively.

    The RAFT reagent-modifified graphite and the polymer-grafted graphites were also analyzed by XPS.  In S 2p spectrum of the RAFT reagent-modified graphite the peak attributed to thiocarbonylsulfanyl group was clearly observed at 163.6 eV.  This peak was weakened and slightly shifted to higher energy range in the spectra of PNIPAAm-grafted graphite.  Moreover the peaks attributed to the amide groups of PNIPAAm sequence were observed at 400.3 eV in N 1s spectra of the copolymer-grafted graphites.  These results clearly indicate that the PNIPAAm is successfully grafted from the graphite by RAFT polymerization.

    The electron transfer to the substrates on the thermoresponsive polymer grafted graphite in aqueous media was then observed by cyclic voltammetry using 1,4-benzoquinone as a model substrate.  Increases in peak current were observed from 15°C to 25°C in both of reduction and oxidation waves of 1,4-benzoquinone by using the PNIPAAm-grafted graphite as a working electrode.  When the substrate was contacted with the PNIPAAm-grafted graphite below 5°C and then the temperature was raised to 45°C, the reduction peak potential was observed at -0.40 V.  On the other hand, the peak potential was -0.32 V when the substrate was contacted with the graphite at 45°C.  Trapping of the substrates into the polymer field on the graphite or blocking of the substrates from the graphite surface were confirmed by the change of the peak current and redox potential during the phase separation of the thermoresponsive polymers on the graphites.