Macroporous polymers morphology

Santora BP, Gagne MR, Moloy KG, Radu NS. Porogen and cross-linking effects on the surface area, pore volume distribution, and morphology of macroporous polymers obtained by bulk polymerization. Macromolecules 2001 34 658-661. 

Macroporous styrene-divinyl benzene (S-DVB) copolymers are widely used as supports for chemical reactions (1). The surface area, pore volume, and pore size of these materials can be manipulated by a judicious choice of reaction conditions (2). It is recognized that reaction cosolvent and the ratio of monomer to cosolvent are Important variables and considerable speculation has been offered regarding the relationship between polymerization conditions and polymer morphology (3). On the basis of these studies a model has emerged to account for macroporosity in these materials (4). The continuous or gel phase is found to consist of aggregated microspheres. The macropores are defined by voids created by these aggregated raicrospheres. 

Change in polymer morphology from macroporous to microporous resulted in improved selectivity but a decrease in overall retention, which is desirable in terms of chromatographic performance. For some MIPs uv initiation is not suitable, e.g. when the template is uv sensitive, and thermal initiation is required. In this situation ABDV is favoured since it thermally decomposes at a lower temperature than AIBN (45°C). 

Figure 3 A diagram showing the effect of porogen and cross-linker on the final polymer morphology (I) gel-type resin (II) macroporous resin and (III) microspheres and microgel powder. Adapted from Ref. 3.

Fig. 5-1. Schematic representation of the morphology of (A) microporous and (B) macroporous polymer matrix. In the microporous structure there is a lower number of cross-links with the pores being located between the polymer chains. In the case of the macroporous structure the degree of cross-linking is much higher and hence the pores used for chromatographic separations are external to the polymer chains.

Figure 3. Schematic representation of the micro- and nanoscale morphology of gel-type (a) and macroreticular (b) resins [13], Level 1 is the representation of the dry materials. Level 2 is the representation of the microporous swollen materials at the same linear scale swelling involves the whole polymeric mass in the gel-type resin (2a) and the macropore walls in the macroreticular resin (2b). The morphology of the swollen polymer mass is similar in both gel-type and macroreticular resins (3a,b). Nanopores are actually formed by the void space surrounding the polymeric chains, as shown in level 4, and are a few nanometer wide. (Reprinted from Ref [12], 2003, with permission from Elsevier.)...

I 1 Structure, Morphology, Physical Formats and Characterization of Polymer Supports 1.2.3.2 Collapsible Macroporous Resins... 

To overcome these problems a gradient oven was presented which allows one to find rapidly the real phase separation gap for a given set of polymer and solvent. These results may serve as general guidelines for the preparation of a wide variety of solvent-modified and macroporous thermosets with tailored morphologies via CIPS. 

The morphology of a typical inorganic monolith is fundamentally different from that of organic polymers (Figure 1.4a). The sttucture is rather sponge- than brush-like and is consttucted by interconnected silica rods in the low micrometer size. This composition leads to a discrete distribution of flow channels, which can be deduced by comparison of macropore disttibution of a typical organic... 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

An additional porous polymer is poly(glycidyl methacrylate-ethyleneglycol dimethacrylate) (see Figure 2.46) that is synthesized by suspension polymerization in the presence of an inert porogen in the polymerization reaction, obtaining a material with an internal macroporous morphology characterized by an interconnected pore network, which permeates the extensively cross-linked polymer matrix [209],... 

The solid supports used in this study were macroporous co-polymers of vinylpyridine and styrene crosslinked with divinylbenzene. Polymers of this type in the form of beads are available commercially (e.g. Reillex 425) and were also prepared for this study by Purolite. For spectroscopic studies, a more convenient sample morphology was required and thin-film polymers of similar stoichiometry were synthesised by the group of Sherrington at the University of Strathclyde. Full details of the methods used to prepare thin film polymers are reported elsewhere.11 To generate the ion exchange resin, the pyridyl functionalities of the polymer were quatemised with methyl iodide (Eq 1). 

Fig. 16.3. Scanning electron micrographs of cross-sections of a MIP-filled capillary column. The super-porous morphology of the polymer monolith can be seen. Micrometre-sized globular units of macroporous MIP surrounded by interconnecting super-pores (left). A superpore of about 7 pm in width (above, right). Covalent attachments of the MIP to the capillary wall (below, right). Reprinted from [39] Copyright (1997), with permission from American Chemical Society.

---