Solvent porogen effects

Solvent porogen effects for macroporous resins are often explained in terms of the degree of solvation imparted to the incipient polymer netwoik, the point at which phase separation takes place, and the resultant degree of in filling between primary particles [26]. This may play a role in some amorphous MOPs (for example, micro/ mesoporous PPV [13]) however other systems such as HCPs (Sect. 2.1) do not undergo phase separation in this way [21, 22]. This basic mechanistic difference also accounts for the apparent independence of surface area on monomer concentration for conjugated microporous PAE networks [ 19], for example, in comparison with macro-porous polymer resins where surface area may be strongly concentration dependent. 

Monodispersed poly (methyl methacrylate-ethyleneglycol dimethacrylate) is prepared by a multistep swelling and polymerization method. When a good solvent such as toluene is applied as a porogen, the seed polymer severely affects the pore structure, whereas no effects are observed with poor solvents, such as cyclohexanol, as a porogen, in comparison with the conventional suspension polymerization (68,69). 

Fig. 3. Effect of dodecanol in the porogenic solvent on the differential pore size distribution of molded poly(glycidyl methacrylate-co-ethylene dimethacrylate) monoliths (Reprinted with permission from [62]. Copyright 1996 American Chemical Society). Conditions polymerization time 24 h, temperature 70 °C, polymerization mixture glycidyl methacrylate 24%, ethylene dimethacrylate 16%, cyclohexanol and dodecanol contents in mixtures 60/0 (curve 1), 57/3 (curve 2), 54/6 (curve 3), and 45/15 vol.% (4)...

The morphology of the monoliths is closely related to their porous properties, and is also a direct consequence of the quality of the porogenic solvent as well as the percentage of crosslinking monomer and the ratio between the monomer and porogen phases. The presence of synergistic effects of these reaction conditions was verified using multivariate analysis [65]. 

The results of the MIP analyses of the bulk polymers are illustrated in Figure 1.7. It could be demonstrated that the polymerization time is capable of influencing the shape of the pore distribution itself, rather than shifting a narrow macropore distribution (and thus the pore-size maximum) along the scale of pore diameter (see effect of the porogenic solvent in Section 1.3.2.2 and Figure 1.5). On... 

There are two processes by which the bulk imprinted polymers are formed covalent imprinting and noncovalent imprinting. In the former, the template molecule is first covalently functionalized with the monomer, and then copolymerized with the pure monomer. After that the covalent bond is broken and the template molecule is removed by extraction. In order to facilitate the extraction step, a so-called porogenic solvent is used. It effectively swells the polymer matrix. 

Fig. 6.23. Effect of thermal (a) and UV initiation (b), type of comonomer, and percentage of 1-dodecanol in the polymerization mixture on the mode pore diameter of quinidine-functionalized chiral monoliths. (Reprinted with permission from [56]. Copyright 2000 American Chemical Society). Reaction conditions polymerization mixture, chiral monomer 25 8 wt%, glycidyl methacrylate ( ) or 2-hydroxyethyl methacrylate ( ) 16 wt%, ethylene dimethacrylate 16 wt%, porogenic solvent 60 wt% (consisting of 1-dodecanol and cyclohexanol), polymerization time 20 h at 60°C (a) and 16 h at room temperature (b).

Fig. 6.25. Effect of the percentage of 1-propanol in the porogenic mixture on the porous properties of monolithic polymers (Reprinted with permission from [64], Copyright 1998 American Chemical Society). Reaction conditions polymerization mixture ethylene dimethacrylate 16.00 wt.%, butyl methacrylate 23.88 wt.%, 2-acrylamido-2-methyl-l-propanesulfonic acid 0.12 wt.%, ternary porogen solvent 60.00 wt.% (consisting of 10 wt.% water and 90 wt.% of mixtures of 1-propanol and 1,4-butanediol), azobisisobutyronitrile 1 wt.% (with respect to monomers), polymerization time 20 h at 60°C.

Another in situ preparation of molecularly imprinted columns employs dispersion polymerisation, whereby agglomerated polymer particles are obtained [16]. The procedure is similar to the rod preparation a mixture of the chemicals for the polymer preparation, such as a template, a functional monomer, a cross-linker, a porogen and an initiator is put in a column and heated to effect polymerisation. This method also requires polar solvents, such as cyclohexanol-dodecanol and isopropanol-water, to obtain aggregated polymer particles of well-defined micro-sises. A crucial difference with the rod preparation lies in the volume of the porogen used larger volumes of porogens are used in dispersion polymerisation. 

Not only the rigidity is crucial to the efficiency of MIPs, but also the accessibility as many recognition sites as possible should be accessible for rebinding. The material should therefore be porous. This is realised by dissolving monomers, cross-linkers and print molecules in a porogenic solvent prior to polymerisation. The effect of the solvent on the polymer morphology can be monitored by measuring physical parameters such as surface area, pore diameter and pore volume. 

Low-weight organic molecules, such as volatile organic compounds (VOCs) [25], e.g. hydrocarbons without functionalities or anaesthetics, can be used as print molecules for non-covalent MIPs. If the print molecule is a suitable organic solvent, the print molecule itself is the porogen during the polymerisation process. Enhanced imprinting effects are promoted by n-n interactions between aromatic moieties in monomers and analytes, such as polycyclic aromatic hydrocarbons (PAHs) or aromatic VOCs (xylene or toluene, for example). 

Fig. 21.5. Tetrahydrofuran imprinted polyurethane layer of 300 nm with embedded phthalide indicator shows a reasonable sensor effect with tetrahydrofuran - no effect occurs with ammonia vapour. Other solvents as template/porogen shift the sensitivity of the MIPs towards the analyte (former template) to be.

The role of solvent and other additives in pore formation in MOPs is likely to be very important but is poorly understood. Templating effects are commonly invoked for microporons MOFs and similar considerations would apply to crystalline microporons COFs [14, 20], for example. Likewise, the influence of solvents and other species as porogens in porous polymer synthesis is well-known [26] but mostly understood at a qualitative level. Indeed, there is relatively little detailed information to hand for MOPs and this issue requires further attention. 

Achieving a suitable particle size with better yield is important in precipitation polymerization as many parameters affect its mechanism. We have prepared GA based MIPs by the precipitation polymerization and observed the effect of porogen on particle size and specific molecular recognition properties (Pardeshi et al, 2014], MIP, M-lOO prepared in the porogen acetonitrile and MIP, M-75 prepared in a mixture of acetonitrile-toluene (75 25 v/v), resulted in the formation of microspheres with approximately 4 pm particle size and surface area of 96.73 m g and nanoparticles (0.8-1000 nm] and a surface area of 345.9 m g" respectively. The results have shown that effect of toluene on the particle size of MIPs depends on the type of cross-linker used and its solubility parameter. Matching the solubility parameter of solvent mixture and cross-linkers is important to obtain the desired particle size in MIPs. The MIPs selectively recognized GA in presence of its structural analogues. Pure GA with percent recovery of 75 ( 1.6) and 83.4 ( 2.2) was obtained from the aqueous extract of herb Emblica officinalis by M-lOO and M-75, respectively. 

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