46192-G5
Surface Chemistry of Diblock-Copolymer-Based Nanoporous Materials
In the last 20 months (January 2007 ~ August 2008), we have established a way to assess the surface functional groups on nanopores derived from a diblock copolymer (DBCP), and also demonstrated the chemical functionalization of the nanopore surface. As DBCPs, we have focused on cylinder-forming polystyrene-poly(methylmethacrylate) diblock copolymers (PS-b-PMMA; the PMMA volume fraction is 0.3), because this type of DBCP has been studied extensively to fabricate nanoporous membranes. Cylindrical nanopores (19-25 nm in diameter) have been formed by selectively removing the cylindrical PMMA domains from annealed PS-b-PMMA monoliths via UV irradiation and subsequent acetic acid (AcOH) treatment. Cyclic voltammetry (CV) have been employed to clarify the surface functional groups and the surface reactivity inside the nanopores, which cannot be measured using conventional spectroscopic and microscopic techniques. These results are published as two research articles in Langmuir (2007, 23, 12771.; 2008, 24, 8959.). In addition, we have systematically studied the effects of substrate surface roughness and film thickness on the orientation of the cylindrical PMMA domains using AFM and CV, which will be reported in the near future.
0. Experimental.
PS-b-PMMA having molecular weights of 57,000 (57K) and 71,000 (71K) were used. Since their PMMA volume fractions were close to 0.3, their minor PMMA blocks formed cylindrical domains having 19 nm (57K PS-b-PMMA) and 24 nm (71K PS-b-PMMA) in diameter. A thin PS-b-PMMA film (20-200 nm thick) was spin-cast on a gold substrate. After annealed at 170 °C for 60 hours, the phase-separated PMMA domains were removed via UV irradiation (254 nm, 40 J/cm2) and subsequent ultrasonication in AcOH. The film thickness was measured using spectroscopic ellipsometry, and the surface morphology was measured using atomic force microscopy (AFM).
1. Verification of the Nanopore Formation in PS-b-PMMA-Derived Membranes.
We have demonstrated that etching conditions of the PMMA domains in PS-b-PMMA films via the UV/AcOH treatments can be easily optimized using CV (Langmuir 2007, 23, 12771.). In addition, we have systematically investigated the effects of the substrate roughness and film thickness on the orientation of cylindrical PMMA domains (i.e., the nanopores) using CV (manuscript in preparation). The redox current was larger on gold surface consisting of nanoscale grains (~30 nm in diameter) rather than one consisting of a large (> 200 nm in diameter) terrace, suggesting that the nanoscale grains induced the vertical orientation of the PMMA domains. This result was consistent with AFM data. On the other hand, with increasing film thickness, CV data changed from peak-shaped to sigmoidal, indicating a decrease in the density of open nanopores due to the interconnection of the nanopores within the film. These results indicated that CV provides a simple means for assessing the PS-b-PMMA-derived nanoporous membranes.
2. Identification of the Surface -COOH Groups on PS-b-PMMA-Derived Nanopores.
We have shown for the first time that the surface of PS-b-PMMA-derived nanopores are COOH-terminated (Langmuir 2007, 23, 12771.). The redox current in a CV of Fe(CN)63-, which is determined by the transport of Fe(CN)63- through the nanopores, decreased with increasing solution pH from 4.6 to 6.3, and a negligible change in CV of uncharged 1, 1’-ferrocenedimethanol. The decrease in redox current of Fe(CN)63- at the higher pH was due to electrostatic repulsion between Fe(CN)63- and the electrical double layer formed in the neighborhood of the negatively-charged nanopore surface. Indeed, the reduction of effective pore radius measured from CVs of Fe(CN)63- was correlated to the change in the thickness of the electrical double layer. The pH range that showed the decrease in redox current of Fe(CN)63- was consistent with the presence of -COOH groups on nanopore surface .
3. Chemical Functionalization of the Surface -COOH Groups on PS-b-PMMA-Derived Nanopores.
Subsequently, we have demonstrated the chemical functionalization of the nanopore surface via amidation of the surface -COOH groups (Langmuir 2008, 24, 8959.). The surface functionalization led to conversion of the surface charge of the nanopores and also to the shrinkage of effective pore radius, as verified using CV for PS-b-PMMA-derived nanoporous films immobilized on gold substrates. For native PS-b-PMMA-derived nanoporous films, the redox current of anionic Fe(CN)63- decreased with increasing solution pH due to the deprotonation of the surface -COOH groups, whereas those of cationic Ru(NH3)63+ and uncharged 1,1’-ferrocenedimethanol were similar regardless of pH. In contrast, upon amidation of the nanopore surface with ethylenediamine, the redox current of Ru(NH3)63+ decreased with decreasing pH and those of Fe(CN)63- and 1,1’-ferrocenedimethanol were independent of pH. The decrease in redox current of Ru(NH3)63+ at acidic pH was consistent with the presence of -NH2 groups on the nanopore surface as a result of the covalent immobilization of ethylenediamine. Furthermore, the redox current of 1,1’-ferrocenedimethanol decreased upon amidation of the nanopores with a long amine-terminated PEG, reflecting the shrinkage of the effective pore radius.
