Surface Modification of Halogenated Polymers. 8. Local Reduction of Poly(tetrafluoroethylene) by the Scanning Electrochemical Microscope − Transient Investigation

Abstract

Local reduction of poly(tetrafluoroethylene) (PTFE) was achieved by scanning electrochemical microscopy (SECM). The PTFE reduction process was analyzed by the current transients, which helped propose general trends for PTFE microfabrication. The SECM was used to investigate quantitatively the kinetics of PTFE phase transformation. In a short time, a nucleation process accounts for the PTFE reduction evolution. The nucleation rate follows a potential dependency similar to that observed for conducting polymer growth or metal deposition. At long time, the expansion of the PTFE carbonization proceeds in a hemi-ellipsoidal fashion, the radial expansion being much easier than the in-depth one. Those expansions can be correlated to the tip current when changing reduction time, reducing species concentration, nature of the electrolytic solution, SECM tip radius, and tip-substrate separation. The stoichiometry of the reduction was estimated, and the existence of two kinetic regimes owing to the redox mediator reduction potential was evidenced, which confirmed previous results by feedback experiments. A rough model was proposed in which the reduction along the in-depth direction is mainly controlled by a diffusive process while the material resistivity controls the radial expansion. The observed variations may be explained by penetration of the reducing species into the rough reduced PTFE material.

Two-step local functionalization of fluoropolymer Dyneon THV microfluidic materials by scanning electrochemical microscopy combined to click reaction

“…1C-2) shows (i) the decrease of the peak corresponding to -CF 2 groups (1150-1200 cm −1 ) indicating the local defluorination of the THV surface and (ii) the apparition of a new peak at 2130 cm −1 which is characteristic of alkyne groups [18]. These results confirm that THV reduction leads to the formation of a conducting polyyne structure according to the general mechanism proposed for the reduction of PTFE [12]:…”

“…As for other fluoropolymers [12][13][14], the local reduction performed at the SECM tip yields the material activation and such a process is socalled carbonization due to the darkening of the material surface. The optical microscopy image (Fig.…”

“…In a recent paper, we have shown that THV surface can be electrochemically functionalized through a three-step approach [11]: (i) SECM-assisted carbonization of the THV, (ii) introduction of azide groups by autografting of 4-azidobenzenediazonium on the carbonized area, and (iii) covalent linkage of alkyne-bearing molecules on the particular spot through click reaction. We propose here a simplified process by developing an original two-step strategy where the first step consists in a local formation of surface alkyne functions through the reduction of THV using the SECM tip [12,13]. The chemical availability of the created alkyne functions was previously demonstrated from their engagement in graft copolymerization processes in the case of polytetrafluoroethylene [13,14].…”

Surface modification of halogenated polymers. 9. Etching of polytetrafluoroethylene with the scanning electrochemical microscope

“…Assuming as in our previous work [12] that the reduction of the PTFE, for scan rates lower than 30 lm/s, is controlled by an apparent heterogeneous charge transfer rate, k het , one may convert the change in current (see Fig. 1, curves d-g) into a change in k het d. This gives 7 · 10 À6 < k het d < 1.4 · 10 À5 cm 2 s À1 , which clearly does not reflect changes in d but rather changes in the surface reactivity.…”

“…Reduction of polytetrafluoroethylene (PTFE) by an electrogenerated radical anion had already been reported using a band microelectrode separated from the polymer by an insulating gap [6]. Reduction of fluoropolymers results in their carbonization [7][8][9][10] and we have used SECM to investigate the reduction mechanism [11,12] or to localize chemical functionalization of PTFE [13][14][15]. With a steady electrode, the shape of the modification is the image of the electrode surface: disk shaped modifications or lines are obtained with, respectively, disk or band microelectrodes [16].…”