Nanostructured bacterial cellulose poly(4-styrene sulfonic acid) composite membranes

Transcription

1 Nanostructured bacterial cellulose poly(4-styrene sulfonic acid) composite membranes with high storage modulus and protonic conductivity Tiago D. O. Gadim, a Andrea G. P. R. Figueiredo, b Nataly C. Rosero-Navarro, a Carla Vilela, b José A. F. Gamelas, c Ana Barros-Timmons, b Carlos Pascoal Neto, b Armando J. D. Silvestre, b Carmen S. R. Freire,* b Filipe M. L. Figueiredo* a a Department of Materials & Ceramic Engineering, CICECO, University of Aveiro, Aveiro, Portugal. lebre@ua.pt b Department of Chemistry, CICECO, University of Aveiro, Aveiro, Portugal. E- mail: cfreire@ua.pt c Chemical Process Engineering and Forest Products Research Center, Chemical Engineering Department, University of Coimbra, Pólo II, R. Sílvio Lima, Coimbra, Portugal Supporting Information This section provides additional information on the structure, the morphology/microstructure and the protonic conductivity of pure bacterial cellulose membrane. The obtained impedance spectra are presented and interpreted. Finally, the preparation of Nafion membranes is described. 1

2 Figure S1. Snapshot of the as-prepared BC/PSSA/PEGDA (1:5:0.4) nanocomposite membrane with the highest content of cross-linker. The Universidade de Aveiro Logo is a registered trademark of Universidade de Aveiro. Used with permission. 2

3 A) B) C) (001) (011) (010) Figure S2. Schematic representation of the cellulose structure type I : A) the polymeric chain; B) 2 chains linked by hydrogen bounds; C) the sheets formed by the polymeric chains and staked by weak electrostatic forces. The hydrogen atoms are omitted for clarity. (constructed on Mercury 3.0 (Build RC5) based on data by Nishiyama et. al [Nishiyama, Y.; Sugiyama, J.; Chanzy, H.; Langan, P.; Crystal Structure and Hydrogen Bonding System in Cellulose Iα from Synchrotron X-ray and Neutron Fiber Diffraction, J. Am. Chem. Soc., (2003), 125, ]. 3

4 Figure S3. SEM image showing the nanofibrilar microstructure of pure BC. Cross sections are shown in A), B) and C), whereas D) depicts the surface. Figure S4. Protonic conductivity of the pure BC membrane treated with HCl. 4

5 Impedance spectroscopy Figure S5 shows typical impedance spectra collected under high and low RH for membranes with low and high polymer contents. The spectra of the BC/PSSA/PEGDA (1:5:0.4) at 30% RH are dominated by one single semi-circle with amplitude corresponding to the membrane ohmic resistance and decreasing with increasing temperature. A second contribution is apparent at low frequency for the higher temperatures/lower impedance, which varies with the test signal amplitude (Figure S6, Supporting Information) and may thus be ascribed to the electrode processes. The increase of the RH leads to a drastic decrease of the impedance, confirming that this is essentially due to protonic conduction. The high frequency semi-circle tends to disappear with decreasing membrane resistance (either with increasing temperature or with increasing RH). This can be understood assuming a simple parallel RC circuit to describe the proton relaxation in the membrane with a frequency 0 =(R b C b ) -1 (where R b is the resistance and C b is the capacitance of the bulk membrane) that increases (beyond the limit of the LCR meter) with decreasing R b. Most of the impedance spectra of the membrane with the highest PSSA/PEGDA do not display the full semicircle due to the much lower resistance, which is at least 3 orders of magnitude lower than for the BC/PPSA/PEGDA (1:5:0.4) composite. In fact, for the high temperature and RH, when the resistance is the lowest, an inductance (of about 1 H) due to the wires of the experimental set-up becomes increasingly apparent at high frequency. In these cases, the membrane resistance is given by the value of Z at the high frequency intercept with the real axis, obtained by fitting the data taking into account the contribution of the inductance. In the intermediate cases without the high frequency intercept, the value of Z at the maximum frequency is assumed as the membrane resistance. 5

6 Figure S5. Typical impedance spectra collected with test signal amplitude of 100 mv through the plane of two composite membranes (without and with 40% cross-linker) under variable temperature and relative humidity conditions. The relevant frequencies are indicated. Figure S6. Impedance spectra for the BC/PSSA/PEGDA (1:5:0) composite obtained at 94 C / 98% RH with two different test signal amplitudes (0.1 and 0.5 V). The inset shows a detail of the high frequency part of the spectra. 6

7 Preparation of bacterial cellulose membranes Bacterial cellulose membranes were prepared by growing the microorganism Gluconacetobacter sacchari at 30 ºC during 48 h, in static conditions, in HS (Hestrin Schramm) liquid medium (20 g/l glucose, 5 g/l peptone, 5 g/l yeast extract, 2.7 g/l Na 2 HPO 4 and 1.15 g/l citric acid, ph 5),before inoculation of 5 ml into liquid production medium in 250 ml Erlenmeyer flasks, as detailed in Trovatti, E. Serafim, L., Freire, C. S. R., Silvestre, A. J. D., Neto, C. P., Carbohydrate Polymers 86 (2011) The process is aerobic and thus the interpenetrating nanofibrils are formed at the surface of the media in contact with air in the form of a mat. After producing the first mat, the bacteria continue to produce an additional mat layer on top of the first one, and then on until the production is stopped. The result is a multilayer structure (Fig. S3). After the production an intensive washing procedure is required, firstly at 80 ºC for 1 h using a NaOH 0.5 M solution (repeated three times), followed by ph stabilization with water. With the ph stable, the mats are washed with a 1% leaches solution during 2 h, finally, washed again with distilled water. The membranes are stored in water at 4 ºC until use. Preparation of Nafion membranes A pure Nafion membrane was prepared by casting on a petri dish (5 cm in diameter) a commercial Nafion suspension (20% w/w M n 1100, Aldrich), previously dried at 45 ºC to evaporate the solvent, and re-dissolved in N,N-dimethylacetamide. The casted membrane was washed with de-ionized water, then in a 3 vol.% aqueous H 2 O 2 and again with de-ionized water, during one hour under boiling conditions for all steps. The membrane could then be protonated in boiling aqueous sulfuric acid (0.5 M) for 1 h, and finally washed with de-ionized water, being kept in the same media until use. The typical thickness of these membranes in the dry state is slightly in excess of 50 m. 7