AmericanChemicalSociety.com

Cylinder-forming block copolymers have been used to prepare membranes containing cylindrical nanoscale pores with uniform diameters.  Such nanoporous membranes will make it possible to design novel membranes for efficient chemical catalysis and separations.  For these future applications, in-depth understanding of the chemical properties of the nanopore surface is essential.

In the last 12 months (September 2009 ~ August 2010; no-cost extension), we have addressed the following issues related to the surface chemistry of nanoporous membranes derived from cylinder-forming polystyrene-poly(methylmethacrylate) diblock copolymers (PS-b-PMMA; the PMMA volume fraction is 0.3) based on our previous achievements (Langmuir 2007, 23, 12771.; Langmuir 2008, 24, 8959.; Anal. Chem. 2009, 81, 851.; Polymer 2009, 50, 2273.; Langmuir 2010, 26, 2119.):

(1) Our CV studies (Langmuir 2007, 23, 12771.; Langmuir 2008, 24, 8959.) showed changes in surface chemical properties due to the aqueous-phase amidation based on EDC and N-HSS of the surface -COOH groups.  In these studies, the progress of the surface reaction could be qualitatively discussed from a change in CV before and after the reaction, but the surface reaction yield could not be quantitatively determined from the CV data due to the influence of ionic screening.  

Here, we have quantified the surface -COOH density based on the cation-exchange of the surface -COO- groups (manuscript in preparation).  The protons of surface -COOH groups on vertically-oriented nanopores were replaced with positively-charged fluorescent molecules (Rhodamine 6G) at neutral pH, where the -COOH groups should be deprotonated.  Subsequently, the loaded fluorescent molecules were released by immersing the nanopores in HCl solution (pH 2), and were measured from the fluorescence intensity of the solution.  Under the assumption of 1:1 ion pair formation, we can determine the density of the surface -COOH groups on the nanopores.  The surface reaction yield can be discussed as decreases in free -COOH groups upon amidation or esterification.  Since the UV irradiation induces the crosslinking of the PS matrix, reactions in organic solutions can be used for the surface functionalization of UV/AcOH-treated PS-b-PMMA films.  

Prior to the surface functionalization, the surface -COOH density was (0.84 ± 0.19) nm-2 regardless of the nanopore diameter examined (15 and 20 nm obtained from PS-b-PMMA having molecular weights of 43000 and 57000, respectively).  Upon amidation with ammonia (in ethanol) and upon esterification with CH3CH2OH via -COOH activation with oxalyl chloride, free -COOH density decreased to (0.14 ± 0.03) and (0.20 ± 0.05) nm-2, corresponding to 83 % and 76 % surface reaction yield, respectively.  In contrast, the surface -COOH density after EDC-mediated, aqueous-phase amidation was (0.75 ± 0.22) nm-2, corresponding to the surface reaction yield of 11 %, which was consistent with the low reaction yield estimated from our previous CV data (Langmuir 2008, 24, 8959.).  

Based on these results, we obtained the following two conclusions.  First, the cation-exchange-based method can be used to quantitatively determine the density of the redox moieties linked onto the etched PMMA nanodomains.  Second, the surface density of the redox moieties can be tuned by controlling the surface reaction conditions.

(2) In addition, we compared the pH-dependence in CVs of charged redox species on nanopore-array gold electrodes based on track-etched polycarbonate membrane with 10-nm diameter pores (Analyst 2010, 135, 172.) and microelectrodes modified with COOH-terminated SAMs (Supramol. Chem. 2010, 22, 450).   The former, as with gold electrodes coated with PS-b-PMMA-derived nanopores (Langmuir 2007, 23, 12771.; Langmuir 2008, 24, 8959), exhibited pH-dependent CV changes with similar reversibility, which could be explained by changes in effective pore size due to the electrical double layer extending from the charged pore surface.  In contrast, the latter showed CVs reflecting changes in electrode reaction kinetics affected by electrode surface charge, as supported by the consistency between the experimental data and theoretical quasi-reversible steady-state voltammograms reflecting the Frumkin effect.