3348
Ind. Eng. Chem. Res. 2007, 46, 3348-3357
Preparation of Polyetherimide Nanoparticles by an Electrospray Technique Faezeh Bagheri-Tar, Muhammad Sahimi, and Theodore T. Tsotsis* Mork Family Department of Chemical Engineering and Materials Science, UniVersity of Southern California, UniVersity Park, Los Angeles, California 90089-1211
An experimental investigation was conducted using an electrospray of polyetherimide (Ultem-1000 PEI) solutions in dichloroethane to produce fairly monodisperse, fine PEI particles. The effects of three key experimental parameters were investigatedsnamely, the applied voltage, the liquid flow rate, and the polymer concentration. The liquid flow rate has the most important effect in determining the particle size. An optimal range of flow rates often exists. Particles obtained within the optimal range have a narrower size distribution and a better morphology (dense and spherical), compared with those produced with other liquid flow rates. The ultimate goal of this research is to prepare carbon molecular sieve (CMS) particles by in-situ pyrolysis of the electrospray-generated PEI nanoparticles to be utilized in the preparation of membranes. Here, the CMS particles, which are prepared ex-situ via the pyrolysis of the electrospray PEI particles, are compared, in regard to their properties, with the CMS particles that were generated via the conventional grinding of pyrolyzed PEI pellets. They are determined to have generally similar structural properties. I. Introduction Several processes associated with electric power and energy generation involve the separation of mixtures that contain hydrogen (H2), carbon dioxide (CO2), and methane (CH4). Currently, H2 and CO2 separation is achieved using either a adsorption/desorption technique (temperature or pressure swing methods) or absorption. However, both approaches are energyand capital-intensive. Membrane-based separations, as a result, are making significant inroads in this area. Strong interest exists in membranes with better selectivity, and better thermal and chemical stability, than the existing polymeric membranes. Mixed-matrix membranes, which are prepared by incorporating porous molecular-sieve particles into a polymeric matrix, have, as a result, been attracting attention for the separation of gas mixtures. The reason for this interest is that such membranes promise to retain the processability characteristics of polymeric membranes, while exhibiting the desirable separation properties of the molecular sieves. Although great strides have been made,1-3 the efforts, so far, have not been completely satisfactory. One reason for the lack of success has been the apparent incompatibility between the base polymeric matrix (typically, glassy polymers) and the molecular-sieve materials that are used (typically, zeolites). To overcome the incompatibility of the polymeric matrix and the molecular-sieve component, our approach is to utilize carbon molecular sieve (CMS) particles rather than inorganic sieves (e.g., zeolites). In addition, the CMS particles are prepared via the pyrolysis of the same polymeric material that is used as the base matrix in the preparation of the mixed-matrix membranes. Another challenge in the preparation of mixed-matrix membranes is that the sieve particles must have adequately small dimensions and a narrow particle size distribution (PASD). The reason is that the base polymer has generally a very low permeability, and membranes prepared by such a material must be asymmetric with a fairly thin (at most, a few micrometers (µm) thick) top permselective layer. Therefore, the purpose of this research is to prepare CMS fine particles to be utilized in the preparation of mixed-matrix membranes. Nanoparticles of * To whom correspondence should be addressed. Tel.: (213) 7402069. Fax: (213) 740-8053. E-mail:
[email protected].
materials, such as CMS, are also of fundamental interest, relating to whether their chemical and physical properties are remarkably different from those of the same material in the bulk form. They also show potential, in addition to their stated use during membrane preparation, for use as catalyst supports and adsorbents, in the preparation of sensors, and as pigments, and structural, electronic, and magnetic materials. The technique that we utilize in our own work for the preparation of fine CMS particles is the pyrolysis of polymeric precursor particles prepared by electrospray (electrohyrodynamic atomization (EHDA). This process uses electrostatic forces to disperse a liquid stream into fine charged droplets, through the Coulombic interaction of charges in the liquid and the applied electric field. To perform EHDA, one feeds an electroconductive liquid through a metal capillary charged at an appropriately high electric potential (typically tens of kilovolts), relative to a ground electrode positioned a few centimeters away (see Figure 1). The liquid, as it exits the metal capillary, is accelerated by the high electric field, resulting in jets that are dispersed into droplets. The EHDA technique has been shown to be capable of generating fine droplets, as well as nanoparticles. For example, the preparation of ZnS particles
