Chapter 25
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Synthesis of Structured Polymeric Materials Using Compressed Fluid Solvents A. I. Cooper, R. Butler, C. M. Davies, A. K. Hebb, K. Senoo, and C. D. Wood Department of Chemistry, Donnan and Robert Robinson Laboratories, C r o w n Street, Liverpool L 6 9 3 B X , United Kingdom
This paper describes the use of supercritical CO and liquid R134a (1,1,1,2-tetrafluoroethane) as alternative solvents for the synthesis of crosslinked polymer materials with fine control over structural features on micro-, meso-, and macroscopic length scales. 2
Supercritical carbon dioxide (scC0 ) has attracted much interest recently as an alternative solvent for materials synthesis and processing (/-5). We discuss here four areas where die use of compressed fluid solvents may offer unique advantages; [1] The synthesis of macroporous polymer beads by suspension polymerization using s c C 0 as a 'pressure-adjustable' porogen. [2] The synthesis of macroporous polymer monoliths using scCC>2 as the porogenic solvent. [3] The synthesis of emulsion-templated polymers using high internal phase C0 -in-water emulsions (C/W HIPEs) [4] The synthesis of cross-linked polymer microspheres by dispersion polymerization using liquid R134a as the continuous phase. 2
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2
© 2003 American Chemical Society
Gopalan et al.; Supercritical Carbon Dioxide ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
387
388 Synthesis of Macroporous Polymer Beads by Suspension Polymerization using scC0 as a 'Pressure-Adjustable' Porogen 2
Macroporous polymers are important in a wide range of applications such as ion-exchange resins, chromatographic separation media, solid-supported reagents, and supports for combinatorial synthesis (6). Unlike gel-type polymers which swell in die presence o f an appropriate solvent, die cross-link density in macroporous polymers is sufficient to form a permanent porous structure which persists in the dry state (7). Macroporous polymers are usually synthesized as beads (typical diameter = 10-1000 pm) by O/W suspension polymerization in the presence o f a suitable porogen (i.e., an additive, usually an organic solvent, which induces pore formation in die polymer matrix) (8,9). In general, porogens that are 'good solvents for the growing polymer network tend to give rise to smaller pores and higher surface areas than porogens that are 'bad solvents. This is because the degree of solvation imparted by the porogen affects the phase separation process which occurs during polymerization, thus determining the physical structure of the porous channels. To achieve fine control over porosity is not always straightforward. In addition, die synthesis of macroporous polymer beads by suspension polymerization is solvent intensive because a large volume of organic solvent (typically 40-60% v/v) is required as die porogen and an even larger volume of a lower boling point solvent is then used to wash out the porogen phase. 9
9
O/W Suspension Polymerization using scC0 as the Porogen 2
We have developed a method for the synthesis of macroporous polymer beads using no organic solvents whatsoever - just water and C 0 (Figure 1) (10). We have exploited the fact that the solvent strength of s c C 0 can be tuned continuously over a significant range by varying the density. As such, SCCO2 can be thought of as a 'pressure-adjustable' porogen. 2
2
cross-linked polymer beads
TRIM
Figure 1
Gopalan et al.; Supercritical Carbon Dioxide ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
389 In a typical reaction, a mixture o f monomers [trimethylolpropane trimethacrylate (TRIM)], initiator [2,2'-azobisisobutyromtrile (AIBN)], and s c C 0 was suspended in water with stirring in the presence of a stabilizer [0.5% w/v poly(vinyl alcohol)] to inhibit droplet coalescence. Table 1 summarizes the results of a series of polymerizations carried out under various conditions. In the absence of C 0 , the O/W suspension polymerization of T R I M led to non-porous polymer beads with an average diameter o f 180 pm (entry 1). This reaction was repeated in the presence o f s c C 0 over a range of pressures while keeping all other variables constant (entries 2-5). Uniform spherical macroporous polymer beads were formed when C 0 was added to the reaction mixture (Figure 2a). 2
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Table 1. Suspension Polymerization of TRIM using scC0 as the Porogen 2
pressure (bar) 1 2 3 4 5
mean bead diameter
intrusion volume (cm /g) 0.00 0.28 1.05 1.23 0.69 3
(mf 180 80 110 128 114
1 100 200 300 400
b
median pore diameter (nmf —
(19) 2206 110 40
surface area (m /g)
