Polysulfone-Cellulose Nanocomposites - ACS Symposium Series

Chapter 14

Polysulfone-Cellulose Nanocomposites 1

2

Sweda Noorani , John Simonsen , and Sundar Atre

3

1

Departments of Chemical Engineering and Wood Science and Engineering, Wood Science and Engineering, and Industrial and Manufacturing Engineering, Oregon State University, Corvallis, OR 97331

Downloaded by COLUMBIA UNIV on August 9, 2012 | http://pubs.acs.org Publication Date: July 13, 2006 | doi: 10.1021/bk-2006-0938.ch014

2

3

Microchannel devices are a rapidly evolving process technology with a wide variety of applications. One of these is separation processes such as kidney dialysis, which promises to significantly improve treatment and patient lifestyle. Current membranes for dialysis are hollow fiber membranes typically using polysulfone (PSf). However, this geometry and membrane composition is not optimal for microchannel devices. The incorporation of cellulose nanocrystals (CNXLs) into PSf holds the potential to improve PSf membrane performance in separation devices by improving mechanical properties without loss of separation specificity or flux. However, the incorporation of CNXLs into water insoluble films without aggregation has been problematic. A solvent exchange process was developed that successfully transferred an aqueous CNXL dispersion into the organic solvent N methylpyrrolidone (NMP), which is a solvent for polysulfone (PSf). Films were prepared from the solution of PSf in NMP with dispersed CNXLs by a phase inversion process into hot water. Films were then examined by scanning electron microscopy (SEM), optical microscopy and atomic force microscopy (AFM). The interaction between the polymer matrix and the CNXL filler was studied by means of thermogravimetric analysis (TGA), which suggested a close interaction between the polymer and filler at the 2% filler loading.

© 2006 American Chemical Society

In Cellulose Nanocomposites; Oksman, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

209

210


Introduction Polymeric composite materials filled with nanosized rigid particles have attracted great interest in recent years (7, 2). In addition, the use of nanoparticles of natural origin has also been of interest (3). The advantages of natural fillers are their low density, renewable character, high specific strength and biodegradability associated with the highly specific properties of nanoparticles (4). Cellulose, a semi-crystalline polymer, is one of the most abundantly available polymers on earth. Cellulose is present in virtually all plants and is also produced by certain varieties of bacteria and some sea animals (5). Depending on its origin, the molecular weight and the degree of polymerization vary. The crystalline portions of cellulose can be obtained by acid hydrolysis (6, 7) and are called cellulose whiskers or cellulose nanocrystals (CNXLs). CNXLs prepared in our lab were obtained from cotton and had a length of 200-350 nm, cross section of 5 - 20 nm and aspect ratio between 10-70 nm. The density of CNXL calculated from X-ray diffraction data is 1.566 g/cc (6). The stiffness of CNXLs has been measured at 145 GPa (8). The strength has not been measured directly, but theoretical estimates are about 7500 MPa (9). Polymer nanocomposite films incorporating CNXLs are being developed in our research group specifically to be utilized in microchannel-based devices for separation technologies. Such devices require thin walled membranes with high stiffness and flux. Rapid separation times and scalable throughputs are made possible by several microchannel features such as high mass transfer rates, short residence time distribution, control of the diffusion length, and mixing (10, 11). These benefits translate into a small device size which is ideal for integration into portable and distributed systems (72). Figure 1 shows some prototype polysulfone (PSf) microchannel devices designed for bioseparations (75). One potential application for such microchannel devices is kidney dialysis. By reducing the size and cost of the device, this technology offers the potential to allow dialysis patients to perform the operation at home, resulting in significantly improved patient treatment and lifestyle (14). Incorporation of CNXLs as a reinforcing material in a PSf matrix has the potential to provide the necessary mechanical and separation properties for an optimal microchannel separation device. As a preliminary study, CNXLs were incorporated into a PSf matrix and the morphology and thermal properties of these films were investigated.

Experimental Polymer Matrix The PSf resin (UDEL P-1700) was donated by Solvay Advanced Polymers (Alpharetta, G A). The melt flow index is listed as 6.5 g/10 min, tensile strength



Figure L Prototype microchannel device for bio-separations


212 as 70.3 MPa, tensile modulus 2.8 GPa, elongation at break, 50-100%, specific gravity 1.24. The polymer was received in the form of pellets and was used without further purification. C N X L Production CNXLs were prepared by acid hydrolysis of pure cotton cellulose (Whatman #1 filter paper), ground to pass 20 mesh, at 45 °C for 45 minutes in 65% sulfuric acid. The solution obtained was centrifiiged and neutralized with 5% NaHC0 until the solution pH was 6 - 9 . This solution was then sonicated using a Branson Sonifier 250 (Branson Ultrasonics Corp., Danbury, CT) for about 10 minutes. After sonicating, the solution was filtered in an Ultrasette™ Tangential flow ultrafilter, 300 kD MWCO, from Pall Corp. (Ann Arbor, MI). Filtration was continued until the final conductivity of the ultrafilter permeate was < 5 //S/cm.


3

Nanocomposite Processing A solvent exchange process was used to transport the CNXLs from water to the organic solvent N-methylpyrrolidone (NMP). CNXL aqueous dispersion and NMP were mixed together. The amount of NMP added depended on the percentage CNXL required in the final composite. The water was removed via a Buchi Rotovapor R l 10 (Flawil, Switzerland) rotary evaporator. The temperature of water bath was adjusted to 50-80 °C, a vacuum was applied, and the process continued until all the water was removed from the mixture as determined by the weight of the remaining dispersion. The dispersion was then sonicated for about 10 minutes. PSf was added to this solution mixture and stirred until the polymer was completely dissolved. The dispersion was sonicated again. This dispersion was used to make films by the phase inversion process: Solutions were cast on a glass plate and immersed into hot water (70 °C). The films were left in the hot water bath for at least 24 h to remove the remaining solvent from the films. The percentage of CNXLs in the films was varied between 0 and 16 wt %. The thickness of the final films was ~ 20 μιη and was measured by calibrated optical microscopy. Thermogravimetric Analysis (TGA) Thermogravimetric measurements were carried out with a TA Instruments Q500 thermogravimetric analyzer (New Castle, DE). The temperature range was room temperature to 600 C at a 10 ° C/min ramp under nitrogen flow. 0


213 Atomic Force Microscopy (AFM) A Digital Images Dimension 3100 atomic force microscope (Veeco Instruments Corp., Santa Barbara, CA) was used to obtain height and tapping mode images of the surfaces of the films.


Scanning Electron Microscopy An AMRay 1000 scanning electron microscope (SEM) using lOkV secondary electrons was used to image the morphology of the nanocomposite films. The specimens were frozen in liquid nitrogen,fractured,mounted, coated with gold/palladium and thefracturesurface was observed at a 300 angle.

Optical Microscopy Images were obtained on an Eclipse E400 (Nikon USA, Melville, NY).

Results and Discussion Morphology of Films The fracture surface of the unfilled PSf appeared to be a smooth leaf-like structure (Figure 2A). Thefracturesurface of films containing CNXLs showed a radically different topology. At the 2% loading, the fracture surface of the film was rough and non-uniform with ~ 1 μ fracture zones (Figure 2B). Clearly the presence of the nanoparticles was altering the fracture behavior of the material. Increasing the CNXL loading to 11% gave a fracture surface similar to the 2% sample, except the fracture zones were somewhat smaller and more numerous (Figure 2C). This would be expected if the nanoparticles were altering the crack propagation process since more nanoparticles should give greater crack trajectory modifications. At the 16% CNXL loading level, the fracture surface was extremely rough, uneven, and showed apparent domains of agglomerated CNXLs (Figure 2D). This loading of CNXLs evidently exceeds the ability of the PSf to wet the filler surface, giving rise to the generally "cheesy" behavior. Figures 3 (A - C) show the polarized optical micrographs of PSf films with 0%, 2% and 11% filler loadings respectively. At lower filler loadings (2%) the



Figure 2. Fracture surfaces of PSffilmfilledwith A) 0%; B) 2%) 11%; D) 16% CNXLs.



Figure 3. Polarized optical micrograph of PSf film containing A) 0%; B) 2%; andC) 11% CNXLs.



216 CNXL is less agglomerated while at higher filler loadings (11%) the CNXL is highly agglomerated. Note that the individual CNXLs are not visible in an optical microscope, being smaller than the wavelengths of visible light. However, the small particles still diffract and scatter light and produce out-offocus "spots" in optical images. Agglomerates larger than - 500 nm are easily visible and are apparent in Figures 5 Β and C, especially C, where large agglomerates are apparent. A F M images also show agglomeration at the higher CNXL loadings (Figure 4). These images were acquired using tapping mode AFM. The intent was to asses the feasibility of "seeing" or "sensing" the CNXLs imbedded in the film using tapping mode A F M and phase imaging. The phase image responds to a number of variables and currently there is no theory that allows the analysis or prediction of phase images from first principles (15). Nevertheless, phase imaging has become a useful tool for the atomic force microscopist due to its versatility and image quality. One of the variables which effects the phase image is the hardness of the material being imaged. Typically the harder a substance is the brighter it displays in a phase image. Thus regions of "hard" and "soft" materials can be discerned in phase images. Since the CNXLs are much harder than the PSf matrix, they should be visible in the phase image, even though they are not on the surface. It was expected that as the force, or "hardness" of the tapping increased, more deeply imbedded CNXLs should appear in the phase image. There is some evidence to support this hypothesis. Comparing Figure 5 with Figure 4C, one can find additional bright areas which may be more deeply imbedded CNXLs. The only difference between the images is the level of the setpoint, which determines the degree of "hardness" of the tapping mode. Thus we conclude that this technique, while certainly not quantitative, does hold some merit for qualitatively analyzing polymer nanocomposite films.

Thermogravimetric analysis Unfilled PSf films and composites filled with various CNXL loadings were tested using TGA. The degradation of pure PSf was observed to begin at -500 °C and that of pure CNXLs at 250 °C (Figure 6). The 11% CNXL loading showed a bimodal distribution with the expected -250 °C CNXL degradation step and the - 500 °C PSf step occurring relatively independently at the degradation temperatures expected for the pure components. However, the 2% CNXL loading showed a broad CNXL degradation step shifted to a higher temperature (Figure 6). This may indicate that the CNXL and PSf components are associated, altering the thermal degradation of the CNXL. Thus at CNXL loadings > 2% there may be agglomeration and the agglomerated CNXL phases may be degrading independently of the PSf matrix. This leads to the supposition that the surface area of the CNXL component is large enough that at some CNXL loading between 2% and 11% the PSf fails to fully "wet" the surface.



Figure 4. AFMtapping mode images with heigth image on the left of each panel and phase image on the right of A) 0% CNXL; B) 2% CNXL; C) 11% CNXL; D) 16% CNXL.


-a


Figure 5. AFM image of 11% CNXL -filled PSƒusing the hard tapping technique. Compare to Figure 6C.



300

Temperature ( °C)

400

500

600

Figure 6. TGA curve under nitrogen forPSƒfilms containing A) 0%; B) 2%; C) 11%; D) 16% CNXLs.

200


220

Conclusions


CNXLs were incorporated into a PSf matrix by a novel solvent exchange process. A solid composite film was obtained via phase immersion casting. Microscopic observations revealed that the CNXLs were dispersed well at lower filler loadings. SEM indicated that the incorporation of CNXLs radically altered the fracture behavior of the composites. In addition, the 16% CNXL loading appeared to be highly agglomerated and incompletely wet by the matrix. A F M images indicated that "hard" tapping with phase imaging was a promising technique for visualizing films such as these. TGA suggested strong PSf/CNXL interactions at 2% filler loading and agglomeration or poor PS17CNXL interactions at 11% and higher filler loadings.

References 1. 2. 3. 4. 5. 6. 7.

8. 9. 10. 11. 12.

13. 14. 15.

Favier, V.; Canova, G.; Shrivastava, S.; Cavaille, J. Polym. Eng. Sci. 1997, 37, (10), 1732-1739. Morin, Α.; Dufresne, A. Macromol. 2002, 35, 2190-2199. Ruiz, M.; Cavaille, J.; Dufresne, Α.; Gerard, J.; Graillat, C. Comp. Interf 2000, 7, (2), 117-131. Samir, Α.; Said, Μ. Α.; Alloin, F.; Sanchez, J.-Y.; Dufresne, A. Polymer 2004, 45, (12), 4149-4157. Ward, K.; Ott., Ε.; H.M. Spurlin; M.W. Grafflin, Occurence of Cellulose. Interscience Publishers: NY, 1954; Vol. Part I, ρ 9-27. Battista, Ο. Α., Microcrystal Polymer Science. 1975. Revol, J.-F.; Giasson, J.; Guo, J.-X.; Hanley, S. J.; Harkness, B.; Marchessault, R. H.; Gray, D. G., Cellul.: Chem., Biochem. Mater. Aspects 1993, 115-22. Eichhorn, S. J.; Young, R. J.; Davies, G. R., Biomacromol. 2005, 6, 507513. Marks, R. E., Cell wall mechanics of tracheids. 1967. Martin, P. M.; Matson, D. W.; Bennett, W. D., Chem. Eng. Comm. 1999, 173, 245-254. Martin, P. M.; Matson, D. W.; Bennett, W. D., J. Vac. Sci. Tech. A. 1999, 17, 2264-2269. Wegeng, R. S.; Drost, M . K.; Brenchley, D. L. In Process Intensification Through Miniaturization or Micro Thermal and Chemical Systems in the 21st Century, 3rd International Conference on Microreaction Technology, Frankfurt, Germany, 1999; Frankfurt, Germany, 1999. Urval, R. Powder Injection Molding of Multi-scale Devices. M.S., Oregon State University, Corvallis, OR, 2004. Curtis, J. Dialys. Transpl. 2004, 33, (2), 64-71. Cohen, S. H.; Lightbody, M . L., Atomic Force Microscopy/Scanning Tunneling Microscopy 2. Plenum Press: New York, 1997.


Polysulfone-Cellulose Nanocomposites - ACS Symposium Series

Recommend Documents