Ind. Eng. Chem. Res. 2009, 48, 4365–4374
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Nanofibrous Structure of Nonwoven Mats of Electrospun Biodegradable Polymer NanocompositessA Design of Experiments (DoE) Study A. Tsimpliaraki,†,‡ S. Svinterikos,† I. Zuburtikudis,*,‡ S. I. Marras,‡ and C. Panayiotou† Department of Chemical Engineering, Aristotle UniVersity of Thessaloniki, 54124 Thessaloniki, Greece, and Department of Industrial Design Engineering, TEI of Western Macedonia, 50100 Kozani, Greece
The morphology of nanofibrous nonwoven mats of an electrospun biodegradable polymer nanocomposite was studied in order to define the material and process parameter settings capable of giving the targeted nanofibrous structure of the mats. The polymer solution concentration, the flow rate of the injected solution, and the organically modified clay content of the polymer matrix were the investigated factors according to a design of experiments (DoE) within the context of response-surface methodology (RSM). Three responses were defined and were estimated by image processing of the scanning electron microscopy (SEM) micrographs. The first two were the ratio of the average bead-to-fiber diameter Dbead/Dfiber and the number surface density of the beads Nbead and were introduced to indicate the fibrous quality of the mats, while the third, indicative of the fiber thickness, was Dfiber. The developed quadratic models and the individual and coupling effect of the three factors examined are given. The results suggest that the dominant parameter affecting mats’ morphology is polymer solution concentration and that a broader range in the factor settings, especially for concentration, should be used in a subsequent optimization. 1. Introduction In the past few years, the field of polymeric composite materials has received significant attention because of the development of polymer/layered silicate nanohybrids.1-5 These materials are particularly intriguing for both industry and academia, because of their potential applications and the interest they have from a theoretical point of view. Incorporation of a small quantity (typically 0.9 as excellent. For a model to pass this diagnostic test, both R2 and Q2 should be high and preferably not separated by >0.2-0.3.26 The Q2 values for the three transformed responses Y1, Y2, and Y3 were equal to 0.7449, 0.8091, and -0.7037, respectively. The Q2 values for Y1 and Y2 are >0.5 and 10% w/v and even higher. At these values of SC, a very small number of beads will be formed in accordance with the experimental findings. For FR at the L-level ()1.0 mL/h), the same picture evolves. In this case, however, the window of very low bead number density is narrower than that for FR at the K-level. This only suggests that an even higher than 12% solution concentration
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Figure 11. Coefficient plots with their confidence intervals at 95% level for responses Y1 and Y2.
Figure 12. Contour plots for the two nontransformed responses Dfiber and Nbead as a function of SC and CL at the 4 levels of FR.
is necessary, if we are to produce fibers with just a few beads and diameters in the 500 nm scale.
tions higher than 10% are necessary for the production of fibers in the 500 nm scale with the least number of beads.
The response-surface results for flow rates at the M ()2.75 mL/h) and N ()5.4 mL/h) levels are the same as above. The clay content plays only a minor role for both fiber diameter and bead number density. Again, polymer solution concentra-
The similarity of results for the four values of flow rate, which correspond to a 10-fold increase in the flow rate, suggest that this factor (FR) does not play any role on the fiber diameters and on the number surface density of beads. All of the above
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response-surface results are in good agreement with the experimental findings and show the way for further experimentation. For this further experimentation, they indicate the domain of experimental settings that should be used. The polymer solution concentration ought to be set in a more extended region; higher concentrations than the ones used here should be employed. In general, depending on the targeted morphology, the above results can be used in defining in a better way the experimental settings space, so that an optimization within the context of response-surface methodology can be accomplished. 4. Conclusions A DoE study in the framework of response-surface methodology (RSM) was undertaken here in order to identify and quantify the significance and the interaction of parameters involved in the production of biodegradable-polymer nanocomposite mats via electrospinning and to recognize the experimental settings region for a subsequent optimization. Fibrous structure, beading, and spindling of the electrospun morphologies were measured by introducing the proper responses, namely, the fiber diameter and the bead number surface density. The results from the quadratic models developed point to the fact that the polymer solution concentration is the dominant parameter. The dominance of solution concentration is such that it overlaps the effect of any other statistically significant factor. Further experiments with a broader range in the experimental settings (particularly in solution concentration) should be designed to incorporate even more factors such as the applied voltage and/or the room relative humidity in order to improve the process. Studies regarding further optimization analysis in electrospinning and characterization of these fibrous nanocomposite materials, which are also referential to other systems, are clearly a goal for future work. Literature Cited (1) Ray, S. S.; Okamoto, M. Polymer/layered silicate nanocomposites: A review from preparation to processing. Prog. Polym. Sci. 2003, 28, 1539. (2) Kickelbick, G. Concepts for the incorporation of inorganic building blocks into organic polymers on a nanoscale. Prog. Polym. Sci. 2003, 28, 83. (3) Zeng, Q. H.; Yu, A. B.; Lu, G. Q.; Paul, D. R. Clay-based polymer nanocomposites: Research and commercial development. J. Nanosci. Nanotech. 2005, 5, 1574. (4) Okada, A.; Usuki, A. Twenty years of polymer-clay nanocomposites. Macromol. Mater. Eng. 2006, 291, 1449. (5) Okamoto, M. Recent advances in polymer/layered silicate nanocomposites: An overview from science to technology. Mater. Sci. Technol. 2006, 22, 756. (6) Huang, Z. M.; Zhang, Y. Z.; Kotaki, M.; Ramakrishna, S. A review on polymer nanofibers by electrospinning and their applications in nanocomposites. Compos. Sci. Technol. 2003, 63, 2223. (7) Sawicka, K. M.; Gouma, P. Electrospun composite nanofibers for functional applications. J. Nanopart. Res. 2006, 8, 769. (8) Reneker, D. H.; Yarin, A. L.; Fong, H.; Koombhongse, S. Bending instability of electrically charged liquid jets of polymer solutions in electrospinning. J. Appl. Phys. 2000, 87, 4531. (9) Berkland, C.; Pack, D. W.; Kim, K. Controlling surface nanostructure using flow-limited field-injection electrostatic spraying (FFESS) Of poly(D, L-lactide-co-glycolide). Biomaterials 2004, 25, 5649.
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ReceiVed for reView September 2, 2008 ReVised manuscript receiVed February 17, 2009 Accepted March 16, 2009 IE801327Z
