Porogen mechanism

The effects of the concentration of divinylbenzene on pore-size distribution and surface areas of micropores, mesopores, and macropores in monosized PS-DVB beads prepared in the presence of linear polymeric porogens have been studied (65). While the total surface area is clearly determined by the content of divinylbenzene, the sum of pore volumes for mesoforms and macropores, as well as their pore-size distribution, do not change within a broad range of DVB concentrations. However, the more cross-linked the beads, the better the mechanical and hydrodynamic properties. 

The various morphological variants available in bead form can be repHcated in thin films ( 2 cmx8cmx50-100 pm) produced simply by photo-initiated free radical polymerization of comonomer mixtures introduced by capillary action into an appropriate mold formed with microscope sHdes [48]. With appropriate choice of comonomers, and porogen in the case of macroporous films, reasonably mechanically robust self-supporting films can be removed from the mold for further exploitation (Fig. 1.9). 

The polymerization mixture for the preparation of rigid, macroporous monolithic materials in an unstirred mold generally contains a monovinyl compound (monomer), a divinyl compound (crosslinker), an inert diluent (porogen), as well as an initiator. The mechanism of pore formation of such a mixture has been postulated by Seidl et al. [101], Guyot and Bartholin [102], and Kun and Kunin [103] and can be summarized as in the following text. 

The fact that adding a better solvent to the mixture results in a shift of the distribution to smaller pore sizes has been explained by the mechanism of pore formation, postulated for macroporous resins in the late 1960s [101-103]. The addition of a poor solvent causes the phase separation to occur early, whereas the precipitated polymer nuclei are swollen with monomers, which present a better solvating agent than the porogen. Due to the high monomer concentration within the globuli. 

The mechanical properties of these membranes were improved by including a crosslinker, methylene bisacrylamide, in the aqueous phase, and by using a styrene/butyl acrylate (BA) mixture as the continuous phase [185]. The styrene/BA mixture had to be prepolymerised to low conversion to allow HIPE formation. The permeation rate of the membrane was improved by including a porogen (hexane) in the organic phase, generating a permanent porous structure [186]. The pervaporation rate was indeed increased, however a drop in selectivity for water from water/ethanol mixtures was also observed. 

By far the most used systems are matrices based on methacrylate, methacrylamide and styrene and the most common crosslinkers are ethyleneglycol dimethacrylate and divinylbenzene. Methacrylamide based species are the most hydrophilic and styrene ones the most hydrophobic, with the methacrylate systems falling in between. This therefore provides quite a wide choice and imprinted polymers with high mechanical stability and chemical inertness, suitable for example in HPLC applications, are readily achievable. To some extent the chemical nature of the matrix is of less importance than its morphology, and in this respect the type and level of crosslinker used, together with the nature and proportion of the porogen are more crucial (see later). All of these experimental parameters are of course inter-dependent. 

The influence of temperature on the porosity of a molded monolith prepared with a precipitating porogen is described by a simple rule the higher the temperature of polymerization, the smaller are the pores (Fig. 4.6) [383]. This tendency may be understood in terms of the classical mechanism of porosity formation via nucleation and subsequent aggregation of the primary nuclei. Phase separation occurs when the growing polymer becomes insoluble in the monomer—diluent mixture. 

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