Porous polymeric particle formation

Fig. 6 Mechanism of large porous polymeric particle formation by pressure quench method using SC CO2. (From RefP. )...

At concentrations above ( ), a phase separation would lead to the formation of particles dispersed in a liquid matrix. The composition of such particles should be given by the binodal line. Thus such particles will still contain enough solvent to undergo a phase separation. Indeed such an internal phase separation can be used to prepare porous polymeric particles with potential for application as chromatography beads [48]. 

Fig. 45A-C. Schematic illustration of the process for generating porous nutshell particles A cell containing a nucleus B polymerization of styrene/DVB in the non-solvent layer C formation of the porous nutshell particle...

Disadvantages of the known porous polymeric membrane preparation processes are that they involve additional process steps after the formation of the fiber to come to a final product. It is therefore desirable to have a more efficient preparation process. A new method to prepare structures of any geometry (Figure 6.13c through f) and large variety of functionality was recently proposed [61]. The authors proposed to incorporate the functionality by dispersion of particles in a polymeric porous structure formed by phase inversion. A slurry of dissolved polymer and particulate material can be cast as a flat film or spun into a fiber and then solidified by a phase inversion process. This concept is nowadays commercialized by Mosaic Systems. The adsorber membranes prepared via this route contain particles tightly held together within a polymeric matrix of different shapes, which can be operated either in stack of microporous flat membranes or as a bundle of solid or hollow-fiber membranes. 

Figure 12 Imprinting of a theophylline derivative immobilized on silica. The template (a carboxypropyl-derivative of theophylline) was immobilized on porous and spherical amino-functionalized silica gel via formation of an amide bond. This construct is then imprinted with traditional imprinting monomers (TFMAA and DVB). Following polymerization, the composite material is treated with HF to dissolve and remove the silica gel, leaving spherical porous MIP particles which mirror the original silica in size, shape, and porosity.

To conclude, a few residual model parameters usually remain undefined, and these have to be evaluated by direct fitting to the available experimental data. The specific parameters are system dependent, and detailed examples are presented in the following sections. Note that, since the model is not accounting for particle formation, the total interphase surface area is often an adjustable quantity. In fact, this quantity could be estimated from experimental data of particle size and number assuming spherical, non-porous particles. However, this is not the case in precipitation polymerization or unstable dispersions where the polymer-rich phase is recovered in the form of irregular fragments, porous structures, or even a bulky phase, thus preventing a reliable estimation of the actual surface area at reaction conditions. 

A common method to slip-cast ceramic membranes is to start with a colloidal suspension or polymeric solution as described in the previous section. This is called a slip . The porous support system is dipped in the slip and the dispersion medium (in most cases water or alcohol-water mixtures) is forced into the pores of the support by a pressure drop (APJ created by capillary action of the microporous support. At the interface the solid particles are retained and concentrated at the entrance of pores to form a gel layer as in the case of sol-gel processes. It is important that formation of the gel layer starts... 

Organic or inorganic entities as well as polymer particles can also be used as template agents in the preparation of porous ceramic membranes following either the polymeric or the colloidal sol-gel route. The strategy to control microstructure in porous material is illustrated in Fig. 7.13. The template agents are trapped during matrix formation and eliminated in a second step with the aim to define the pore size in the final material. 

One can view th e monoliths as a single big porous particle. Thus, some of the preparation procedures use similar ingredients as the procedures used to make tnacroporous particles by suspension polymerization. Consequently, the structures of the monoliths are similar to the pore structure of macroporous particles, as can easily be seen by electron microscopy. Also similar chemistries are available, including styrene- ivinylbenzene and methacrylates, which have been proven to form sufBciently rigid structures to be useful in HPLC. But the tedmology of the formation of the monoliths is less constrained than the suspension polymerization used to form particles, and thus a broader range of chemistries is available. The classic monoliths were based on polyurethanes (20). Recently, silica-based monoliths were formed in a capillary (24). 

To increase the surface-to-volume ratio in the preconcentration channel without the need for particles, the channel can be filled with a polymeric rod. These are formed by in situ polymerization, during which the polymer material also reacts with the wall of the channel. As a result, no frits are needed to hold the material in place. Columns filled with a polymeric rod, so-called monolithic columns, were originally developed for conventional liquid chromatography. They are made by sol-gel technology, " which enables the formation of a highly porous material containing macropores and mesopores in its structure. The use of a monolithic phase circumvents the problems encountered when packing a column with particles. 

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