Many applications of porous materials in areas such as catalysis, adsorption, ion exchange, chromatography, and solid phase synthesis rely on the intimate contact with a surface that supports the active sites.

In order to obtain large surface area, a significant number of smaller pores should be incorporated into the polymer.

Types of pores

The most substantial contribution to the overall surface area comes from micropores, with sizes smaller than 2 nm, followed by the mesopores ranging from 2 to 50 nm [1,2]. Larger pores (macropores) contribute very little to the surface area. However, these pores are essential to allow liquid to flow through the material at a reasonably low pressure. This pressure, in turn, depends on the overall porous properties of the material [3].

From those kinds of pores, there are two main types in porous structure of monolith:

• the through pores enabling an easy flow of the mobile phase and
• the mesopores filled with the “stagnant” mobile phase in which the solute molecules should migrate to access the active adsorption sites.

The volume of the mesopores and the morphology of the mesopore space significantly affect the mass transfer resistance and hence the chromatographic band broadening. It also controls the phase ratio in the column and may influence the hydrophobicity of the monolithic material and the retention properties [4].

Therefore, the pore size distribution of the monolith must be adjusted properly to fit each type application. Important parameters such as temperature, composition of the pore-forming solvent mixture, and content of crosslinking monomer allow the tuning of the average pore size within a broad range, from tens to thousands of nanometres [3,5,6].

Temperature

The effect of temperature on the kinetics of polymerization allows the preparation of macroporous polymers with different pore size distributions from a single composition of the polymerization mixture. The effect of temperature can be readily explained in terms of the nucleation rates, and the shift in pore size distribution induced by changes in the polymerization temperature can be explained by the difference in the number of nuclei that result from these changes [3,6]. With higher temperature, monoliths with smaller pores are prepared.

Pore-forming solvents

Influence of the 60% (top) and 64% (bottom) of 1-propanol in the porogenic solvents on the porous properties of monolith

The choice of pore-forming solvent (porogen) is the mostly used tool for the control of porous properties without changing the chemical composition of the final monolith. In general, larger pores are obtained in a poorer solvent due to an earlier onset of phase separation. The porogenic solvent controls the porous properties of the monolith through the solvation of the polymer chains in the reaction medium during the early stages of the polymerization [3,5].

Properties of pore-forming solvents

Specific attention was paid to the design of the porogen mixtures for preparation of methacrylate monolithic columns. Ideally, this system should provide:

• preparation of a homogenous, single phase polymerization mixture from a charged, water soluble monomer and hydrophobic monomers;
• direct uniform incorporation of these monomers with widely differing polarities into a macroporous polymer monolith;
• exact control of the porosities of the monolithic material over a broad range; and finally
• facile initial washing and equilibration of the capillary column.

Extensive studies led to the development of a ternary porogen solvent system consisting of water, 1-propanol, and 1,4-butanediol in various proportions [7]. Besides the traditional ternary mixture binary porogenic solvents with only alcohols have also been adopted. Compared with ternary porogenic solvents, the design with binary ones allows for fine control of the pore size and tailoring of the specific surface area of the monolithic polymers. Monoliths prepared with binary porogenic solvents have a different pore distribution from those prepared with ternary porogenic solvents – with larger surface area and containing more of the small pores [8].

Cross-linker

In contrast, increasing the proportion of the cross-linking agent present in the monomer mixture affects the chemical composition of the final monoliths. At the same time, it also decreases their average pore size as results of early formation of highly cross-linked globules with a reduced tendency of coalesce. The experimental results imply that, in this case, the pore size distribution is controlled by limitations in swelling of cross-linked nuclei [3].

The control of porous properties of the organic polymer monolithic materials is a complex process influenced by various parameters.

References

1. K.K. Unger, Porous silica, J. Chromatogr. Library, 16, 1979, Elsevier, p. 15.
2. IUPAC, Manual of Symbols and Terminology, Apendix 2, Part I, Colloid and Surface Chemistry, Pure Appl. Chem., 31 (1972) 578.
3. C. Viklund, F. Svec, J.M.J. Frechet, U. Irgum, Chem. Mater, 8 (1996) 744.
4. D. Moravcová, P. Jandera, J. Urban, J. Planeta, J. Sep. Sci., 23 (2003) 1005.
5. B.P. Santora, M.R. Gagne, K.G. Moloy, N.S. Radu, Macromolecules, 34 (2001) 658.
6. F. Svec, J.M.J. Frechet, Macromolecules, 28 (1995) 7580.
7. E.C. Peters, M. Petro, F. Svec, J.M.J. Frechet, Anal. Chem., 69 (1997) 3646.
8. L. Zhang, G. Ping, L. Zhang, W. Zhang, Y. Zhang, J. Sep. Sci., 26 (2003) 331.

Typical structure of (polymethacrylate) organic polymer monolith

The generally accepted mechanism of pore formation in organic polymer monolihts during a typical polymerization in the presence of a precipitant is following [1,2]:

The organic phase contains both monovinyl and divinyl monomers, initiator and porogenic solvent. The free-radical initiator decomposes at a particular temperature and the initiating radicals start the polymerization process in solution.

Nuclei formation

The polymers that are formed by solution polymerization precipitate after they became insoluble in the reaction medium as a result of both their cross-linking and the choice of porogen. In this process, the monomers are thermodynamically better solvating agents for the polymer than the porogen. Therefore, the precipitated insoluble gels like species (nuclei) are swollen with the monomers that are still present in the surrounding liquid. The polymerization then continues both in solution and within the swollen nuclei.

Branched or even cross-linked polymer molecules that can still be formed in the solution, are captured by the growing nuclei and further increase their size. The nuclei enlarged by the continuing polymerization, associate in clusters being held together by polymer chains that cross-link the neighbouring nuclei.

Later stage

In the later stages of the polymerization, the size of the clusters is large enough to allow contact with some of their neighbours thereby forming a scaffolding-like interconnected matrix within the polymerizing system [3].

Control of the kinetics of the overall process through the changes in reaction time, temperature, and overall composition allows the fine tuning of the macroporous structure and provides an understanding of the mechanism of large pore formation [3,4].

References

1. J. Seidl, J. Malinsky, K. Dusek, W. Heitz, Adv. Polym. Sci., 5 (1967) 11.
2. K.A. Kun, R. Kunin, J. Polym. Sci. A1, 6 (1968) 2689.
3. F. Svec, J.M.J. Frechet, Chem. Mater, 7 (1995) 707.
4. C. Viklund, F. Svec, J.M.J. Frechet, U. Irgum, Chem. Mater, 8 (1996) 744.

As a new type of chromatographic stationary phase, monoliths have been subjected to intensive study in the last years. They differ from other supports mainly in their characteristic structure, which results in the improved chromatographic properties.

Packed (a) and monolithic (b) chromatographic column. TrAC 21 (2002) 166.

While most of the chromatographic supports are particle shaped, monoliths consist of a single piece of highly porous material. In contrast to porous particle, the pores inside the monolith are open, forming a highly interconnected network of channels. Monoliths can be prepared in various ways and can have an inorganic or an organic based skeleton [1,2,3,4,5,6].

Silica-based monoliths

The first being silica-based monolithic columns, generally prepared using sol‑gel technology. This technology can be applied to create a continuous sol‑gel network throughout the column former by gelation of a sol solution within the capillary [7,8]. Alternatively, it can be used to glue LC silica-based particles, once the capillary has been packed conventionally, producing a continuously bonded bed [9].

Organic polymer-based monoliths

The second category is rigid organic polymer-based monolithic columns, and these include acrylamide-based [10,11], methacrylate-based [12], and styrene‑based polymers [13]. The polymer network is generally formed inside the capillary by a step-wise chain polymerization reaction.

Polymerization reaction mixtures usually consist of a combination of monomers and cross-linker, initiator and a porogenic mixture of solvents. A variety of monomers can be employed to fabricate the final monolith, being both charged and hydrophilic, to generate electroosmotic flow for capillary electrochromatography, or uncharged and hydrophobic, to allow reversed-phase interactions used in HPLC. The cross-linker concentration can be adjusted to change the degree of cross-linking which influences the overall porosity. An initiator is needed to begin the step-wise chain reaction, and it is often 2,2’‑azo‑bis‑isobutyronitrile (AIBN). The polymerization can be initiated using UV light or thermal treatment.

Precipitation of the polymer occurs after it becomes insoluble in the reaction medium. Solubility is influence both the cross-linking and choice of porogen (a poor solvent for the polymer), which is commonly a mixture of alcohols.

The formation of the monolith can be achieved in-situ within either untreated or pre-treated capillaries. The pre-treatment of the capillary often involves surface preparation for the introduction of double-bond functionality, allowing covalent bonding of the monolith to the capillary wall, which is of particular importance for HPLC application where the monoliths needs to withstand high pressures.

Preparation of organic polymer monoliths

The polymerization mixture is forced into the capillary and generally initiated thermally. The reaction then continues by free radical polymerization to form a macroporous rigid monolithic polymer. The unreacted components, such as porogenic solvents, are then washed away.

References

1. S. Hjerten, J.-L. Liao, R. Zhang, J. Chromatogr., 473 (1989) 273.
2. T.B. Tennikova, B.G. Belenkii, F. Svec, J. Liq. Chromatogr., 13 (1990) 63.
3. M. Merhar, A. Podgornik, M. Barut, M. Zigon, A. Strancar, J. Sep. Sci., 26 (2003) 322.
4. H. Zou, X. Huang, M. Ye, Q. Luo, J. Chromatogr A, 954 (2002) 5.
5. A.-M. Siouffi, J. Chromatogr A, 1000 (2003) 801.
6. E. F. Hilder, F. Svec, J. M. J. Fréchet, J. Chromatogr A, 1044 (2004) 3.
7. K. Nakanishi, N. Soga, J. Am. Ceram. Soc., 74 (1991) 2518.
8. K. Nakanishi, N. Soga, J. Non-Cryst. Solids., 139 (1992) 1.
9. R. Asiae, X. Huang. D. Farman, Cs. Horváth, J. Chromatogr. A., 806 (1998) 251.
10. S . Hjerten, J.-L. Liao, J. Chromatogr. 457 (1988) 333.
11. F. M. Plieva J. Andersson, I. Y. Galaev, B. Mattiasson, J. Sep. Sci., 27 (2004) 828.
12. F. Svec, J. Sep. Sci., 27 (2004) 747.
13. H . Oberacher, C.G. Huber, TrAC 21 (2002) 166.