Fabrication of 3D Gradient of Polymer Brush Via Bipolar Electrochemical Method

Conducting substrates placed between driving electrodes are polarized by external electric field. When the external electric field is large enough, the substrate can behave as a bipolar electrode (BPE) possessing both anodic and chathodic surfaces with potential gradient. Bipolar electrochemical method is an ideal candidate for creating gradient materials by using its potential gradient. For example, we have reported the use of BPE for the gradient modification of conducting polymer surfaces [1]. In this study, we demonstrate the precise control of electrochemical introduction of small molecules onto BPE using bipolar electrochemistry, and following surface-initiated atom transfer radical polymerization (SI-ATRP) to give polymer-grafted surfaces with density gradient.

Bipolar electrolysis and surface analysis; An aqueous solution of 1 mM of 4-(2-hydroxyethyl)phenyldiazonium tetrafluoroborate (for the attachment of phenethylalcohol moiety through cathodic reduction) and 1 mM of hydroquinone (as a sacrificial reagent at anodic oxdation) was used as electrolyte in a setup equipped with a pair of platinum electrodes (20 x 20 mm) as driving electrodes, and ITO electrode (20 x 5 mm) as BPE. After the bipolar electrolysis was conducted, the obtained phenethylalcohol-modified ITO plate were washed with solvents, and subjected into a solution of dichloromethane containing 0.5 M 2-bromoisobutyryl bromide and 0.05 M triethylamine for 3 h to achieve the surface with ATRP initiator. To estimate the contents of the ATRP-initiator on the substrate, the elemental ratio of C, O, N, and Br was calculated by X-ray photoelectron spectroscopy (XPS). The XPS analysis for each position of the initiator-modified substrate showed the gradual increase of bromine contents (derived from initiator moiety) with approaching to the cathodic surface. This result indicates that the electrochemical reactions successfully took place on the BPE, accompanying with the formation of density gradient of ATRP initiator. Moreover, by tuning the parameters such as voltage and reaction time, the steepness of the profile of bromine contents was successfully controlled.

SI-ATRP; Surface-initiated polymerization of methyl methacrylate (MMA) was conducted under general conditions of ARGET-ATRP with the ITO substrate prepared above ([MMA]₀/[free Initiator]₀/[CuBr₂]₀/[bipyridine]₀/[sodium ascorbate]₀=1000/1/0.035/3.5/3.5), in the presence of the free initiator to estimate the degree of polymerization of PMMA. The density gradient of the grafted polymer was analyzed by infrared reflection absorption spectroscopy (IR-RAS). IR-RAS results measured at each position showed the gradual increase of absorption derived from PMMA from anodic surface to cathodic surface in a similar manner to the XPS results of the initiator-modified substrate. Thus the fabrication of polymer-grafted surface with density gradient was successfully demonstrated. Finally, the measurement of thickness of the polymer-grafted surface by stylus type surface roughness tester revealed that the thickness was dependent on the density of the initiator modified, showing 3D gradient of the polymer brush.

Reference

[1] N. Shida, Y. Ishiguro, M. Atobe, T. Fuchigami, S. Inagi, ACS Macro Lett., 2012, 1, 656-659.