A versatile approach for the processing of polymer. nanocomposites with self-assembled nanofiber templates

Transcription

1 A versatile approach for the processing of polymer nanocomposites with self-assembled nanofiber templates Supplementary Information Jeffrey R. Capadona, 1,2,3 Otto van den Berg,,1 Lynn A. Capadona, 4 Michael Schroeter, 1,5 Stuart J. Rowan, 1,2,3,6 Dustin J. Tyler, 2,3 and Christoph Weder *1,2,6 1 Department of Macromolecular Science and Engineering, Case Western Reserve University, 2100 Adelbert Road, Cleveland, OH Rehabilitation Research and Development, Louis Stokes Cleveland DVA Medical Center, East Blvd., Cleveland, OH Department of Biomedical Engineering, Case Western Reserve University, Euclid Avenue, Cleveland, OH Polymeric Materials Branch, NASA Glenn Research Center, Brookpark Road, Cleveland, OH On leave from Institute for Polymer Research, GKSS Research Center Geesthacht GmbH, Kantstrasse 55, Teltow, Germany. 6 Department of Chemistry, Case Western Reserve University, 2100 Adelbert Road, Cleveland, OH USA. * To whom correspondence should be addressed. christoph.weder@case.edu These authors contributed equally to this work.

2 Cellulose Whiskers Derived from Tunicates. Sulfate functionalized cellulose whiskers were prepared as previously described 1 ; the protocol is briefly summarized here. The tunicates were gutted, and the harder sections of their mantels were removed for homogeneity. The incrustations on the surface of the tunicates were removed by repeated treatments of mechanical agitation, scrubbing, and heating in a 5% w/w potassium hydroxide solution (80ºC, 24h; this protocol is a slight modification to a procedure reported by Yuan et. al.) 2. After excessive rinsing with water to reach a neutral ph, the tunicate mantels (500g) were placed in water (3 L) with acetic acid (5 ml) and sodium hypochlorite (10 ml, >4% chlorine), and the mixture was heated to 60ºC. In 1 h intervals additional portions of acetic acid (5 ml) and sodium hypochlorite solution (10 ml) were added until the material s color changed from pinkish to white. Next, the bleached de-proteinized walls were washed with de-ionized water and disintegrated with a Waring blender, yielding a fine cellulose pulp. The pulp was hydrolyzed with sulfuric acid according to the method described by Favier et al. with slight modifications 3-5. Sulfuric acid (98%, 960 ml) was slowly added under vigorous mechanical stirring to a cooled suspension of tunicate cellulose pulp in de-ionized water (600 ml, 0ºC). Subsequently, the dispersion was heated (60ºC, 90 min) under continued stirring. The dispersion was then cooled to 0ºC, filtered over a small-pore fritted glass filter, and washed with de-ionized water until the ph was neutral. The resulting whiskers were then dialyzed in two to three successive 24 h treatments with de-ionized water to remove any remaining salts. Finally, the whiskers were redispersed in de-ionized water (~4g in 500 ml) by overnight sonication, and water was added to adjust the concentration to 8 mg/ml. Whiskers used for fabrication of solution-cast nanocomposites were frozen in an acetone/dry-ice cooled stainless steel container, and subsequently lyophilized. The size of the as-prepared whiskers was established by image analysis of TEMs as follows: diameter = /- 3.0 nm; length = / μm 1, resulting in an aspect ratio f = 84. All references to cellulose refer to cellulose derived from tunicates, unless specified otherwise.

3 Low-Aspect-Ratio Cotton Cellulose Whiskers. Cellulose whiskers from cotton were prepared using the procedure by Dong et al. 6 with minor modifications. Briefly, 5.2 g of Whatman No. 1 filter paper was combined with 250 ml de-ionized water and blended in a Waring blender at high speed until a lumpy pulp was formed. In a beaker, sulfuric acid (98%, 140 ml) was slowly added under vigorous mechanical stirring to the cooled filter paper pulp (250 ml, 20ºC). After this addition, the suspension was heated to 50ºC for 3.5 h while stirred. The mixture was then cooled to room temperature, filtered over a small-pore fritted glass filter, and washed with de-ionized water until the ph was neutral. The resulting whiskers were then dialyzed in four successive 24 h treatments with deionized water to remove any remaining salts. The whiskers were then sonicated for approximately 7 h. after the dispersion was left to settle at ambient for at least 16h and whiskers that began to settle were removed by decanting the supernatant dispersion. The concentration of the whiskers in that dispersion was determined gravimetrically to be ~20. mg/ml. This concentration was used for the fabrication of gels and solution cast materials following the same experimental protocols as outlined for tunicate whiskers. The dimensions of the as-prepared whiskers were established by TEM image analysis: average diameter = 20 nm, length = 210 nm, aspect ratio = Determination of Whisker Content in Organogels. The whisker content of the various organogels was determined gravimetrically. Gels were prepared by solvent exchange between an aqueous dispersion containing 0.8 % w/w of cellulose whiskers (3 ml) with organic solvents as indicated (15 ml) according to the above protocol. The gels were weighted in their swollen and dried state and the whisker to total weight ratio was determined as an average of at least 3 independently prepared samples. The results are compiled in Table 1. The whisker content of acetone gels produced in larger scale (vide supra) for the preparation of nanocomposites was determined in a similar manner; the whisker content of the gel was varied between 6 and 40 mg/ml by variation of the aqueous dispersion:acetone ratio. A general trend of increased gel density is observed when changing the solvent from methanol, to ethanol, to isopropanol, and

4 may be related to the solvation energy of the non-solvent; however, this relationship was not further explored. Fabrication of Cellulose Whisker Film by Solution Casting. An aqueous dispersion containing 0.8 % w/w of cellulose whiskers (3 ml) whiskers was cast into a Teflon petri dish, which was placed into a vacuum oven (60 C, 15 mbar, 48 h) to evaporate the water and dry the resulting film, which had a thickness of ca. 70 μm. Fabrication of PBD/Whisker Nanocomposites by thetemplate Approach. Cellulose whisker acetone gels, prepared as described in METHODS (comprising 15 mg/ml whiskers) were placed at RT into solutions containing various concentrations of PBD in toluene (polymer concentration = 1 30 % w/v) for 16 h. The gels were subsequently removed from the polymer solution, sliced into pieces to facilitate rapid solvent evaporation, and dried at ambient for up to 2 h to remove (most of) the solvent, before the material was compression-molded as described above (80 C at 6000 psi for 2 min) to yield 460 μm thin nanocomposite films. While this aspect was not further investigated, it appears that it is advantageous to avoid complete drying. The whisker content of the nanocomposite was determined gravimetrically from the weights of the wet acetone gels and the final nanocomposites. Due to the high molecular weight of the PBD, solutions above 15 % w/v polymer in toluene proved to be too viscous for appropriate polymer diffusion, and low-whisker density films were difficult to obtain. Fabrication of PBD/Whisker Nanocomposites by Solution Casting. Lyophilized whiskers were dispersed in DMF at a concentration 3.1 mg/ml as previously described 1. The PBD was dissolved in toluene (~5 % w/v, 45.7 mg/ml) by stirring for 16 h. Nanocomposites were prepared by combining appropriate amounts of the whisker suspension and PBD solution under vigorous stirring (the PBD rapidly precipitated upon mixing), to produce nanocomposites containing 2.3 % or 13.8 % v/v whiskers. The mixtures were cast into Teflon petri dishes and placed into a vacuum oven (80 C, 15 mbar, 24 h) to evaporate the solvent and dry the resulting films, before the materials were compression-molded as described above (80 C at 6000 psi for

5 2 min) to yield 460 μm thin nanocomposite films. The 13.8 % v/v whisker nanocomposite appeared heterogeneous to the unassisted eye (Fig. 3b). Fabrication of EO-EPI/Whisker Nanocomposites by the Template Approach. Cellulose whisker acetone gels, prepared as described in METHODS (comprising 6 or 14 mg/ml whiskers) were placed at RT into solutions containing various concentrations of EO-EPI copolymer in THF (polymer concentration = 1 25 % w/w) for 16 h. An extensive immersion time (16 h) was chosen to ensure complete equilibration; this parameter was not varied here, but it should depend on the shape and size of the gel. The gels were subsequently removed from the polymer solution, dried at ambient for up to 2 h to remove (most of) the solvent, before the material was compression-molded between spacers (80 C at 6000 psi for 2 min) to yield μm thin nanocomposite films. The whisker content within the nanocomposite was determined gravimetrically from the weights of the wet acetone gel and the final material. Fabrication of EO-EPI/Whisker Nanocomposites by Solution Casting. Lyophilized whiskers were dispersed in dimethyl formamide (DMF) at a concentration 5 mg/ml as previously described 1. The EO-EPI copolymer was dissolved in DMF (5% w/w) by stirring for two days. Nanocomposites were prepared by combining the desired amounts (to yield materials containing 0.9% % v/v whiskers) of the colloidal whisker dispersion and polymer solution, and solution-casting the resulting homogeneous mixture into Teflon petri dishes. The dishes were placed into a vacuum oven (60 C, 15 mbar, 48 h) to evaporate the solvent and dry the resulting films, before the material was compression-molded between spacers in a Carver laboratory press (80 C at 6000 psi for 2 min) to yield μm thin nanocomposite films. Fabrication of PS/Whisker Nanocomposites by the Template Approach. Cellulose whisker acetone gels, prepared as described in METHODS (comprising 15 mg/ml whiskers) were placed at RT into solutions containing various concentrations of PS in DCM (polymer concentration = 5 40 % w/v) for 16 h. The gels were subsequently removed from the polymer solution, dried at ambient for up to 2 h to remove (most of) the solvent, before the material was

6 compression-molded as described above (120 C at 6000 psi for 2 min) to yield μm thin nanocomposite films. The whisker content within the nanocomposite was determined gravimetrically as described above. Fabrication of PS/Whisker Nanocomposites by Solution Casting. Lyophilized whiskers were dispersed in dimethyl formamide (DMF) at a concentration 3.1 mg/ml as previously described 1. The PS was dissolved in dry DMF (5% w/w) by stirring for 16 h. Nanocomposites were prepared by combining appropriate amounts (to yield a nanocomposite containing 10.3% v/v whiskers) of the colloidal whisker dispersion and the polymer solution (both in DMF), and solution-casting the resulting homogeneous mixture into a Teflon petri dish. The dish was placed into a vacuum oven (60 C, 15 mbar, 48 h) to evaporate the solvent and dry the resulting film, before the material was compression-molded as described above (120 C at 6000 psi for 2 min) to yield a 400 μm thin nanocomposite film. Formation of Singlee Walled Carbon Nanotube (SWCNT) Organogels. SWCNT organogels were prepared from aqueous dispersions using a solvent-exchange sol-gel process in which gelation was induced through addition of a water miscible non-solvent (acetone) to the SWCNT dispersion, analogously to the whisker system. First, SWCNTs (single-walled, polyaminobenzene sulfonic acid functionalized) were dispersed in water following the manufacturer s protocol. Briefly, SWCNTs (26.2 mg) were added to a glass vial containing nanopure water (1.05 ml), and sonicated (30 min). An additional volume of water (4.19 ml) was subsequently added and the mixture was sonicated for 48 h until no more sedimentation was observed. A typical example of the fabrication of a SWCNT acetone gel is as follows: the aqueous SWCNT dispersion (~2.5 ml) was placed into a 20 ml vial, and acetone (15 ml) was gently added on top of the aqueous whisker dispersion. Unlike the whisker systems described above, some mixing proved difficult to avoid, perhaps due to the lower viscosity of the SWCNT dispersion. However, a SWCNT-free organic layer was formed on top of the aqueous dispersion, clearly indicated by the absence of coloration. This layered system was left to stand

7 until the lower portion had assembled into a mechanically coherent SWCNT acetone gel (typically 5-7 days). The acetone gels thus produced were released form the glass vials and the SWCNT content of the gel was determined gravimetrically. Fabrication of PS/SWCNT Nanocomposites by the Template Approach. SWCNT acetone gels, prepared as described above, were placed at RT into solutions containing various concentrations of PS in DCM (polymer concentration = 5, or 10 % w/v) for 16 h. The gels were subsequently removed from the polymer solutions, dried at ambient for up to 2 h to remove (most of) the solvent, before the materials were compression-molded as described above (120 C at 6000 psi for 2 min) to yield μm thin nanocomposite films. The SWCNT content within the nanocomposite was determined gravimetrically as described above. Electrical Conductivity of PS/SWCNT Nanocomposites. For electrical conductivity measurements a Keithley 2400 series source measure unit was used in combination with a prefabricated two-point gold electrode with spacings of 0.7 cm. A strip of nanocomposite of precisely known dimensions (~2.5 mm wide, 0.14 mm thick, and ~6.2 mm long) was held in place between a glass microscope slide and the prefabricated electrode, using a metal clamp. The two-point electrode consisted of two self-adhesive gold-coated copper strips fixed onto a microscope slide.

8 References 1. van den Berg, O., Capadona, J. R., & Weder, C. Preparation of homogeneous dispersions of tunicate cellulose in organic solvents. Biomacromolecules 8, (2007). 2. Yuan, H., Nishiyama, Y., Wada, M., & Kuga, S. Surface acylation of cellulose whiskers by drying aqueous emulsions. Biomacromolecules 7, (2006). 3. Favier, V., Canova, G. R., Shirvastava, S. C., & Cavaille, J. Y. Mechanical percolation in cellulose whisker nanocomposites. Polym. Eng. Sci. 37, (1997). 4. Favier, V., et al. Nanocomposite materials from latex and cellulose whiskers. Poly. Adv. Technol. 6, (1995). 5. Favier, V., Chanzy, H., and Cavaille, J. Y. Polymer nanocomposites reinforced by cellulose whiskers. Macromolecules 28, (1995). 6. Dong, X. M., Kimura, T., Revol, J.-F., & Gray, D.G. Effects of microcrystallite preparation conditions on the formation of colloid crystals of cellulose. Cellulose 5, (1998).

9 Figure S1. Image of Cellulose Whisker Organogels. Whisker organogels were prepared by solvent exchange of an aqueous whisker suspension (3.0 ml, whisker concentration = 8.0 mg/ml) with 15 ml of acetone (left) or isopropanol (right).

10 E' (MPa) Compressed Aerogel Film cast from Water Aeorogel Foam Temperature ( o C) Figure S2. DMTA Traces of Neat Cellulose Whisker Materials. Materials include: an aerogel prepared by supercritical extraction of acetone gel ( ), a dense cellulose whisker film prepared by compressing the aerogel for 30 sec at 6000 psi and ambient temperature into a thin film ( ), and a whisker film cast from an aqueous dispersion (whisker content 8.0 mg/ml) and dried in vacuum ( ).

11 Figure S3. Photograph of an Aqueous Cellulose Whisker Dispersion. Whisker dispersions were prepared by re-dispersing a supercritically dried whisker-acetone gel in water (3.0 mg/ml, viewed through crossed polarizers).

12 Figure S4. Scanning Electron Microscopy (SEM) Image of a Cellulose Whisker Aerogel. This reference material was prepared by immersion of an acetone gel in neat toluene as a representative non-solvent (but without polymer) for 16 h, re-exchange of toluene against acetone for another 16h, and supercritical extraction with CO 2 (scale bar = 400 nm).

13 Figure S5. Photograph of a Solution-Cast PBD/Whisker Nanocomposite. PBD/whisker nanocomposite containing 13.8 % v/v whiskers prepared by solution casting from a toluene/dmf mixture and subsequent compression molding. Opaque portions are rich in whiskers, while transparent portions consist mainly of PBD.

14 10 10 A E' (Pa) % v/v 14 % v/v 23 % v/v 29 % v/v Temperature ( o C) Figure S6. DMTA Traces of EO-EPI/Whisker Nanocomposites. Tensile modulus of EO-EPI/whisker nanocomposites as a function of temperature and composition. Results indicate an increase in storage modulus across the entire temperature range with increased whisker density. All samples were prepared by the template approach, except 0 % whisker films, which were prepared using traditional solution casting method.

15 Figure S7. Transmission Electron Microscopy (TEM) Image of Cellulose Whiskers Isolated from Cotton. TEM of cotton cellulose whiskers isolated from Whatman filter paper, demonstrating an average length of 210 nm, a width of 20 nm, and an aspect ratio of 10.5 (scale bar = 500 nm).

16 a Volume Fraction of Whiskers in Nanocomposite Polystyrene Solution Concentration (m/v) b 120 º c B 60 º º E' (Pa) % v/v Whiskers 2.5% v/v Whiskers % v/v Whiskers 7.2% v/v Whiskers 10.3% v/v Whiskers 10.3% v/v Whiskers - solution cast 16.7% v/v Whiskers Temperature ( o C) Figure S8. PS/Whisker Nanocomposites. a, Volume fraction of cellulose whiskers in nanocomposites prepared by immersion of whisker acetone gels (15 mg/ml whiskers) into DCM solutions of PS of a range of concentrations and subsequent compaction. b, AFM phase images of ultra-microtomed PS/whisker nanocomposite comprising 7.2 % (v/v) whiskers in PS (horizontal scale bar = 2.5 μm, vertical scale = phase shift). c, DMTA traces of PS/whisker nanocomposites as a function of temperature and composition. Results indicate an increase in storage modulus across the entire temperature range with increased whisker density. All samples were prepared by the template approach, except the one reference marked solution cast.

17 G' (Pa) Volume Fraction of Whiskers Figure S9. Shear Moduli G of PBD/Whisker Nanocomposites. PBD/whisker nanocomposites were fabricated by either solution casting ( ) or the template approach ( ). Solid lines represent predictions by the percolation model. In case of inhomogeneous, solution-cast PBD reference samples, vertically shaded circles represent data from transparent portions of the film (Fig. S5), while horizontally shaded circles refer to opaque samples.

18 Solvent Whisker Content in Organogel (% w/w) Methanol Acetone Tetrahydrofuran Ethanol Acetonitrile Isopropanol Table S1. Densities of Cellulose Whisker Organogels. Whisker content in organogels prepared by solvent exchange between an aqueous dispersion containing 0.8 % w/w of cellulose whiskers (3 ml) and organic solvents as indicated (15 ml).

19 Whisker Content (v/v) G (MPa) 0.0% % % % (solution cast reference) 1.2, 4.4, 685 Table S2. Shear Moduli of Polybutadiene / Whisker Nanocomposites. Shear moduli G of nanocomposites prepared by this template approach were determined by DMTA at 25 C. A significant increase of G is observed upon introduction of the whiskers. Reference data for a solution-cast PBD/whisker nanocomposite reflect severe sample inhomogeneity if conventional processing is attempted (See Supplementary Fig. S5).