Photochemisch induzierte Veränderung der Festigkeit von orangischen Nano-Agglomeraten und deren Messung
Final Report Abstract
In the first period of this project, it could be demonstrated that interparticle forces in polystyrene (PS) nanoparticle agglomerates can be influenced by vacuum-ultra violet (VUV) photochemical treatment in the aerosol state. However, the experimental system did not produce large enough quantities of particles for extensive quantitative analytic work and did not allow enough tunability of the oxidative functionalization, and the in-depth investigation of the process had to remain incomplete. Furthermore, the adopted mode of partcle generation was not fit for any kind of up-scaling. In the second stage of the project, the set-up was greatly improved (i) to produce large larger quantities and higher concentrations of particles by adapting the photochemical oxichlorination process to this process, and (II) to achieve a satisfactory tunability of the electronic excitation. The radiant power was quantified for each parameter set using actinometry (Kühn et al. 2004) and could be varied in the range from 0,02 to 0,7 W. By varying the oxygen concentration in the carrier gas, the formation of different functional groups at the surface of the particles was controllable. In the presence of oxygen, the oxichlorination manifold (Braun et al. 1986, 1991) lead to the formation of hydroxyl, carbonyl and carboxyl groups. In the absence of oxygen, however, the C-centered radicals, generated photocnemically, underwent disproportionation (Lopez Gejo et al. 2005, 2006) leading to subsequent cross-linking or reacted with CCI4 (Köhler 2002) used as a solvent in PS particle generation. Relative surface concentrations of hydroxyl, carbonyl, and chlorine groups were measured by FTIR and XPS-spectroscopy. Agglomerates formed by VUV-photochemically oxidized PS nanoparticles were analyzed for stability using the method of impact fragmentation (Seipenbusch et al. 2002). The size of the primary particles was varied to investigate the dependence of the energy necessary for fragmentation on this parameter. The results revealed a linear relationship between the fragmentation energy and the primary particle size pointing to van der Waals forces as the dominant interparticle force. The fragmentation experiments showed that the agglomerate strength could be tuned by varying the radiant power as the primary reaction parameter of the VUVphotochemically induced oxidation process. Hence, agglomerate strength increased with increasing radiant power. Under reductive conditions, Increasing the radiant powers generates larger concentration of C-centered radicals and subsequently higher rates of chlorination correlating with a strong decrease in agglomerate strength. Higher surface concentrations of hydroxyl, carbonyl and carboxyl groups imply stronger hydrophilicity properties that were quantified by contact angle measurments of compressed powder samples collected before and after photochemical treatment. The variation of the contact angle was found to correlate inversely to agglomerate strength and revealed equally good tunability. In addition, hydrophilicity could be correlated with the thermodynamic stability of aqueous suspensions of the oxidized or cross-linked and chlorinated particles, and functionalized particles could be transformed by secondary functionalization both in gas and condensed phase. With the result of this project, we demonstrated a simple and technically feasible method to functionalize the surface of polymer nanoparticles in a continuous process. Quality and extent of surface functionalization control the strength of agglomerates, the hydrophilicity and the reactivity of the particles. Polymer nanoparticles are used in several domains of application, most remarkably in the life sciences, agricultural and nutrional engineering and material sciences. For example, the possibility to alter the hydrophilicity of polymer nanoparticles is an interesting feature in this context, since bioavailability is tied to the hydrophilic properties. Aerosol processes for nanoparticle synthesis and functionalization are very attractive, exhibiting a number of advantages to corresponding liquid phase processes. Functionalized particles of highest purity may be manufactured implying the least number of unit operations and offering a high potential of integration into a vast variety of forms of products (product engineering). Furthermore, fast rates of reactions and absence of any solvent satisfy the aims of "Green Chemical Engineering" and "Process Intensification". Considering these advantages, a continuous method for the functionalization of nanoparticles in gas phase that can be set in line with their generation has a considerable industrial potential.
Publications
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Froeschke S., Weber A.P., Kasper G., Salas-Vicente J., López-Gejo J., Bossmann S.H., Braun A.M. Modification of the bond strengths in organic nanoparticle agglomerates by photochemistry. EAC, Sept. 2004 Budapest; Suppl. J Aerosol Sei., Book of Abstracts Vol.1, p.95/96.
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Froeschke, S., Weber, A.P., Kasper, G., Salas-Vincente, J., López-Gejo, J., Bossmann, S.H., Braun, A.M. Measurement and manipulation of nano-agglomerate strength by photochemistry. Handling of highly dispersed powders. Berichte aus der Verfahrentechnik, Shaker Verlag, (2004), ISBN 3-8322-3244-3.
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Salas Vicente J., López-Gejo J., Bossmann S.H., Braun A.M., Froeschke S., Weber A.P., Kasper G. Surface Modification of Organic Particles by VUV-Photochemically Initiated Oxidation. Minisymposium Photochemie - Optische Spektroskopie, March 2004, Kleinwalsertal (2004).
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López Gejo J., Applications of the VUV photochemically initiated oxidation for waste gas treatment and surface functionalization. Dissertation, Fakultät für Chemieingenieurwesen und Verfahrenstechnik, Universität Karlsruhe (TH), Karlsruhe, (2005)
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López-Gejo J., Glieman H., Schimmel T., Braun A.M., vacuum -ultraviolet photochemically initiated modification of poly-styrene surfaces: chemical changes. Photochem. Photobiol., 81(2005), 777-782
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Braun A.M., vacuum -ultraviolet photochemically initiated modification of poly-styrene surfaces: morphological changes and mechanistic investigations. Photochem. Photobiol. Sei. 5(2006), 948-954
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Froeschke, S., Weber, A.P., Kasper, G., Salas-Vincente, J., Löpez-Gejo, J,, Bossmann, S.H., Braun, A.M. Interparticle bond strength distribution in nanoparticle agglomerates - Measurement and Manipulation. Proc. PARTEC 2004, Nürnberg.
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Kirchhof M. J., Froeschke S., Schmid H.-J., Weber A. P. Characterization of firststage sintering kinetics; Proceedings European Aerosol Conference 2005, 28 August - 2 September, Ghent, Belgium, 691.
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Kirchhof M.J., Froeschke S., Schmid H.-J., Weber A.P. etermination of first-stage nanoparticle sintering contacts by impact fragmentation, EAC, Sept. 2004 Budapest; Suppl. J Aerosol Sei., Book of Abstracts Vol.1, p.193/194.
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Kirchhof M.J., Froeschke S., Schmid H.-J., Weber A.P. Investigation of early stages of airborne nanoparticle sintering by impact fragmentation, AlChE 2004 Annual Meeting, November 7-12; Austin Convention Center; Austin, TX, USA.
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Kockmann N., Dreher S., Woias P., Wengeler R., Nirschl H., Heim M., Kasper G., Braun A.M., Mall-Gleissle S., Schaber K., integrated process development in micro process engineering, mstnews (1) 13-15 (2007).
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Rothenbacher S., Seipenbusch M., Weber A. P., Kasper G., Salas-Vicente J., Braun A. M. Aerosol-Photochemistry: Strength Manipulation of Organic Nano- Agglomerates. Partec 2007.
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Seipenbusch M., Rothenbacher S., Weber A. P., Salas-Vicente J., Braun A. M. Kasper G. Photochemical Alteration of the Stability of Polymer Nano-Agglomerates.2007 NSTI Nanotechnology Conference and Trade Show, May 20-24, 2007, Santa Clara, California, U.S.A.
