Chem. Mater. 1995, 7, 707—715
`
`707
`
`Kinetic Control of Pore Formation in Macroporous
`Polymers. Formation of “Molded” Porous Materials with
`High Flow Characteristics for Separations or Catalysis
`Frantisek Svec and Jean M. J. Fréchet*
`Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301
`Received October 27, 1994. Revised Manuscript Received February 2, 1995®
`
`The preparation of large macroporous polymer objects with controlled macroporous
`in an unstirred mold through careful control of the
`be carried out
`structures can
`polymerization kinetics. The polymerization is carried out in a mold using a mixture of
`initiator under conditions that afford
`monomers, porogenic solvent and free-radical
`macroporous objects with extremely large channels that provide for the high flow charac-
`In contrast, bead polymers
`teristics required for applications in separation or catalysis.
`prepared from identical polymerization mixtures but in a suspension polymerization process
`type of macroporous structure with large flow-through channels.
`do not exhibit the same
`The main differences between the two processes are the lack of interfacial tension between
`aqueous and organic phases and the absence of dynamic forces resulting from stirring in
`the case of the polymerization in an unstirred mold. Control of the kinetics of the overall
`process through 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 within the unstirred mold. For example, a decrease in the reaction
`temperature that slows down the rate of polymerization and the use of shorter reaction
`times than required for complete monomer
`conversion lead to porous objects with larger
`flow through channels.
`
`Introduction
`
`Macroporous polymers are characterized by their rigid
`porous matrix that persists even in the dry state. These
`polymers are typically produced as spherical beads by
`a suspension polymerization process using a polymer-
`ization mixture that contains both a cross-linking
`and an inert diluent, the porogen. Porogens
`monomer
`can be solvating or nonsolvating solvents for the poly-
`that is formed, or soluble non-cross-linked polymers
`mer
`or mixtures of polymers and solvents. The mechanism
`of pore formation as well as the properties of macroporous
`polymers and their applications have been reviewed
`several times.1-3
`The size distribution of pores within a porous polymer
`a broad range from a few nanometers to
`may cover
`several hundred nanometers. Pores with a diameter of
`less than 2 nm are classified as micropores, pores
`ranging from 2 to 50 nm are mesopores, while pores over
`the
`50 nm are macropores. The larger the pores,
`smaller the surface area.
`Therefore, porous polymers
`with very large pores have relatively low specific surface
`areas, typically much less than 10 m* 12
`The morphology of macroporous polymers is rather
`complex.1,2’4-6 They consist of interconnected micro-
`
`3456/g.
`
`® Abstract published in Advance ACS Abstracts, March 1, 1995.
`(1) Seidl, J.; Malinsky, J.; Dusek, K.; Heitz, W. Adv. Polym. Sci.
`1967, 5, 113.
`(2) Guyot, A.; Bartholin, M. Prog. Polym. Sci. 1982, 8, 277.
`(3) Hodge, P.; Sherrington, D. C., Eds. Syntheses and Separations
`Using Functional Polymers; Wiley: New York, 1989.
`(4) Kun, K. A.; Kunin, R. J, Polym. Sci., A1 1968, 6, 2689.
`(5) Pelzbauer, Z.; Lukas, J.; Svec, F.; Kalal, J. J. Chromatogr. 1979,
`171, 101.
`(6) Revillon, A.; Guyot, A.; Yuan, Q.; daPrato, P. React. Polym. 1989,
`10, 11.
`
`0897-4756/95/2807-0707$09.00/0
`
`spheres (globules) that are partly aggregated in larger
`clusters that form the body of the beads. The size of
`form the bulk of
`the spherical globules that
`the
`macroporous polymer ranges from 10 to 50 nm.
`The
`pores in the macroporous polymer actually consist of the
`irregular voids located between clusters of the globules
`(macropores), or between the globules of a given cluster
`even within the globules themselves
`(mesopores), or
`(micropores). The pore size distribution reflects the
`internal organization of both the globules and their
`clusters within the macroporous polymer and largely
`the composition of the polymerization
`depends on
`mixture and the reaction conditions. The most effective
`variables that control pore size distribution are
`the
`percentage of cross-linking monomer,
`the type and
`amount of porogen, the concentration of the free-radical
`initiator in the polymerization mixture, and the reaction
`temperature.2
`In analogy to conventional sieving processes, the use
`of polymers with large pores is advantageous in promot-
`transfer through a porous polymer.
`In
`ing rapid mass
`chromatography this may be beneficial7-11 for a variety
`In
`of preparative as well as analytical applications.
`catalysis, convection through a catalyst that has very
`large pores increases the catalyst effectiveness factor,12
`and large pore supports are therefore used in numerous
`
`(7) Afeyan, N. B.; Gordon, N. F.; Maszaroff, I.; Varady, L.; Fulton,
`S. P.; Yang, Y. B.; Regnier, F. E. J. Chromatogr. 1990, 519, 1.
`(8) Tennikova, T. B.; Bleha, M., Svec, F.; Almazova, T. V.; Belenkii,
`B. G. J. Chromatogr. 1991, 555, 97.
`(9) Svec, F.; Fréchet, J. M. J. Anal. Chem. 1992, 64, 820.
`(10) Wang, Q. C.; Svec, F.; Fréchet, J. M. J. Anal. Chem. 1993, 65,
`2243.
`(11) Fréchet, J. M. J. Makromol. Chem., Makromol. Symp. 70/71,
`289, 1993.
`(Í2) Nir, A; Pismen, L. Chem. Eng. Sci. 1977, 32, 35.
`© 1995 American Chemical Society
`
`See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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`

`

`708 Chem. Mater., Vol. 7, No. 4, 1995
`catalytic processes.13 Other areas of application of very
`large pore materials include supports for the growth of
`mammalian cell cultures14 and the production of bio-
`mass.15
`frequently used for the
`Two approaches are most
`preparation of porous polymers with very large pores:
`(i) Polymerization of a mixture containing a large
`volume fraction of a non-solvating diluent.16 (ii) Po-
`lymerization in the presence of a linear polymer poro-
`gen.4·17’18
`Most of the macroporous polymers prepared to date
`have been almost exclusively produced in a shape of
`spherical beads that are used as ion-exchange resins,
`chromatographic separation media, adsorbents, etc.
`Therefore, studies of the mechanism of formation of
`macroporous structures have focused exclusively on
`materials prepared by suspension polymerization.1-3
`For example, an extensive study19 of the effects of
`different variables on the properties of macroporous
`poly(glycidyl methacrylate-co-ethylene dimethacrylate)
`beads prepared by suspension polymerization has ap-
`peared. The average pore size of the copolymers pre-
`pared in this study that involved the use of cyclohexa-
`nol and dodecanol as porogens ranged from 20 to 150
`nm.19
`In our search for enhanced and simpler chromato-
`graphic separation media, we polymerized mixtures
`and porogenic solvents directly
`containing monomers
`within a chromatographic column used as a mold.9-11
`The macroporous material that is obtained contains two
`very different families of pores:10 large channels and
`more conventional diffusive pores. Examination of the
`unusual pore size distribution curve
`of a typical poly-
`(styrene-co-divinylbenzene) rod shows the existence of
`a sharp peak at about 1 000 nm and another small peak
`in a size range corresponding to small mesopores.10 Rods
`prepared from poly(glycidyl methacrylate-co-ethylene
`dimethacrylate) also contains similar bimodal pore size
`distribution including very large pores.9
`Because the rod columns are essentially a single
`“molded” polymer monolith traversed by large channels
`their hydrodynamic
`and permeated by small pores,
`properties are excellent and even high flow rates can
`be used. They are unlike any of the existing porous
`materials that are typically used in packed beds because
`flow through the rod column does not
`involve any
`interstitial space but results entirely from the existence
`of the large flow-through channels that are built into
`the porous polymer monolith.
`The continuous polymer rod media afford excellent
`resolution in the chromatographic separation of pro-
`teins, peptides, and small molecules.7·8·20 Recently, our
`approach has been used for the preparation of continu-
`ous rods of molecularly imprinted polymers capable of
`
`(13) Rodrigues, A. E.; Lopez, J. C.; Lu, Z. P.; Loureiro, J. M.; Dias,
` . M. J. Chromatogr. 1992, 590, 93.
`(14) Vournakis, J.; Ronstadler, P. Bio/Technology 1989, 7, 143.
`(15) Brettenbucher, K.; Siegel, K.; Knupper, A.; Radke, M. Water
`Sci. Technol. 1987, 5, 835.
`(16) Chung, D. Y.; Bartholin, M.; Guyot, A. Angew. Makromol.
`Chem. 1982, 103, 109.
`(17) Hilgen, J.; deJong, G. J.; Sederel, W. L. J. Appl. Polym. Sci.
`1975, 19, 2647.
`(18) Guyot, A.; Revillon, A.; Yuan, Q. Polym. Bull. 1989, 21, 577.
`(19) Horak, D.; Svec, F.; Ilavsky, M.; Bleha, M.; Baldrian, J.; Kalal,
`J. Angew. Makromol. Chem. 1981, 95, 117.
`(20) Wang, Q. C.; Svec, F.; Fréchet, J. M. J. J. Chromatogr. 1994,
`669, 230.
`
`Svec and Fréchet
`
`isomers and en-
`
`molecular recognition of positional
`antiomers.21
`All of these rods were prepared from polymerization
`mixtures essentially identical to those that are used for
`the preparation of macroporous beads by suspension
`polymerization, yet beads prepared in parallel experi-
`ments by suspension polymerization do not contain any
`of the very large micrometer-size pores found in the
`molded continuous media.19 This indicates that some-
`what different mechanisms of pore formation must
`operate during the preparation of macroporous rods by
`our approach and of beads by the standard suspension
`polymerization technique.
`This report explores the effects of polymerization
`conditions on the porous properties of rods prepared by
`polymerization of a mixture containing glycidyl meth-
`acrylate and ethylene dimethacrylate in a steel tube and
`provides an explanation for the formation of much larger
`pores during the polymerization in a tube as compared
`to the porous beads resulting from a suspension polym-
`erization.
`
`Experimental Section
`Preparation of Polymers. Polymerization Mixture.
`Azobisisobutyronitrile (1% of the weight of monomers, Kodak)
`was dissolved in 4 vol parts of a mixture consisting of 60%
`glycidyl methacrylate (2-methyl-2-propenoic acid oxiranyl-
`methyl ester, CAS reg. no. 106-91-2, Aldrich) and 40% ethylene
`dimethacrylate (2-methyl-2-propenoic acid 1,2-ethanediyl es-
`ter, CAS reg. no. 97-90-5, Sartomer). Cyclohexanol (Aldrich)
`followed by the addition
`was admixed slowly to the monomers
`of dodecanol (Aldrich); the total volume of the alcohols was 6
`parts. The mixture was purged with nitrogen for 15 min. The
`stock polymerization mixture was stored in a closed flask in a
`refrigerator at a temperature of 5 °C and consumed within 7
`repeated with two
`days. Typically, polymerizations were
`different fresh mixtures and with duplicate experiments done
`for each polymerization mixture.
`Suspension Polymerization. The polymerization mix-
`ture (4 parts) was added to a 1% aqueous solution of poly-
`vinylpyrrolidone) (Aldrich) MW 360 000 (6 parts) and deaer-
`ated. The polymerization was carried out in a 250 mL glass
`reactor (Büchi BEP 280) equipped with an anchor stirrer and
`a heating jacket. The beads were washed with water, ex-
`tracted in a Soxhlet apparatus with methanol for 24 h and
`dried at 60 °C.
`Polymerization in Bulk Solution. A stainless steel tube
`(50-mm x 8-mm i.d., Labio) was charged with 2.5 mL of the
`polymerization mixture then sealed with rubber nut plugs. The
`polymerization was allowed to proceed in a water thermostat.
`The tubes either stood vertically in the bath or
`the contents
`were subjected to an end-over-end rotation while immersed.
`After the chosen polymerization time elapsed, the rubber plugs
`replaced at one end by the column end fitting and the
`were
`forced out of the steel tube by applying a pressure of
`rod was
`THF using a chromatographic pump. The length of the rod
`was measured using a ruler. The soluble compounds were
`removed from the rod by extraction in a Soxhlet apparatus
`with methanol for 24 h and the rod was dried at 60 °C. The
`conversion was calculated from the weight of the extracted dry
`rod.
`In a modified procedure, the polymerization mixture was
`placed in a 5 mL polypropylene syringe barrel, the piston was
`left in the upper position, and the syringe was submerged in
`a water bath. Once the polymerization was completed, the
`end of the barrel was cut off and the rod was pushed out of
`the plastic tube using the syringe piston.
`
`(21) Matsui, J.; Kato, T.; Takeuchi, T.; Suzuki, M.; Yokoyama, K.;
`Tamiya, E.; Karube, I. Anal. Chem. 1993, 65, 2223.
`
`2
`
`

`

`Pore Formation in Macroporous Polymers
`
`Pore diameter, nm
`Figure 1. Differential pore size distribution curves
`of the
`poly(glycidyl methacrylate-co-ethylene dimethacrylate) beads
`( ) and rod ( ) prepared at a temperature of 70 °C. For
`conditions see Table 1.
`
`Porous Properties. Following washing or solvent extrac-
`tion, the porous properties of the beads or
`rods were deter-
`mined by mercury intrusion porosimetry and the specific
`surface areas calculated from nitrogen adsorption/desorption
`isotherms using a custom made combined BET-sorptometer
`and mercury porosimeter (Porous Materials, Inc., Ithaca, NY).
`Prior to the measurements, the rods were cut to small pieces
`with a razor
`blade.
`Gas Chromatography. Gas chromatographic determina-
`tions were carried out in a HP capillary column (crosslinked
`i.d. 0.17 mm,
`methylsilane gum, o.d. 0.32 mm,
`length 25 m,
`temperature gradient from 100 to 280 °C in 15 min) using a
`Hewlett-Packard 5890 chromatograph equipped with a HP-
`76739 automatic autosampler and TCD detector and helium
`as a carrier gas. The data were
`collected by a HP-3393 inte-
`grator.
`
`Results and Discussion
`Suspension polymerization is generally treated in the
`literature as a variant of bulk polymerization in which
`each droplet of the dispersed phase containing monomer
`is an individual bulk reactor.22 Therefore, one might
`have anticipated that the properties of the products of
`both suspension and bulk polymerizations would be
`nearly identical. As indicated above, this is not the case,
`and properties such as pore size distribution are actually
`entirely different. Figure 1 shows the considerable
`discrepancy that exists between the pore size distribu-
`tions of macroporous glycidyl methacrylate—ethylene
`dimethacrylate copolymers prepared by both suspension
`and the bulklike rod polymerization at 70 °C from an
`identical polymerization mixture containing 12% dodec-
`anol, 48% cyclohexanol, 24% glycidyl methacrylate, and
`16% ethylene dimethacrylate. The median pore size
`diameter for the beads is 85 nm while for the rod it is
`to the median pore diameter, the
`In contrast
`315 nm.
`and the pore volumes do not
`specific surface areas
`exhibit such marked differences (Table 1). Since the
`reaction conditions in both polymerizations were
`com-
`parable, this unprecedented difference in median pore
`size diameter has to result
`from the polymerization
`technique itself.
`While suspension polymerization has already been
`times,1’2 little is
`analyzed in the literature several
`
`e
`
`709
`
`Chem. Mater., Vol. 7, No. 4, 1995
`Table 1. Polymerization Conditions and Properties of
`the Macroporous Poly(glycidyl methacrylate-co-ethylene
`dimethacrylate)0
`C d
`VV
`n
`temp,
`dodecanol,6
`*g>
` L/p,max>
`°C
`expt polymerization
`mL/g
`%
`m2/g
`nm
`96.0
`1.23
`53
`70
`0
`suspension
`1
`1.29 173.6
`63
`70
`6
`2
`suspension
`PP tube
`1.40 128.4
`91
`70
`6
`3
`steel tube
`137.2
`1.33
`93
`70
`4
`6
`PP tube
`809
`62.8
`1.33
`55
`6
`5
`steel tube
`65.6
`935
`1.10
`55
`6
`6
`steel tube
`214
`103.1
`50—7<y 1.33
`6
`7
`102.8
`85
`1.39
`70
`12
`8
`suspension
`PP tube
`94.2
`283
`1.58
`70
`12
`9
`steel tube
`315
`1.46
`102.7
`70
`12
`10
`PP tube
`1.24
`1530
`38.7
`55
`12
`11
`steel tube
`81.2
`1527
`1.18
`55
`12
`12
`50-7(/
`steel tube
`1690
`1.50
`172.3
`12
`13
`0 Reaction conditions: polymerization mixture: glycidyl meth-
`acrylate 24%, ethylene dimethacrylate 16%, porogenic solvent
`(cyclohexanol + dodecanol) 60% b Percentage of dodecanol in the
`polymerization mixture. c Pore volume. d Specific surface area.
`e Median pore diameter, f Temperature was
`raised from 50 to 70
`°C in steps by 5 °C lasting 1 h each and kept at 70 °C for another
`4 h.
`
`known on how to control the properties of macroporous
`polymers obtained by a bulk polymerization within a
`mold. Therefore, we have studied this type of polym-
`erization more
`thoroughly and investigated the effect
`of reaction variables such as composition of the poro-
`genic solvent, reaction time, and reaction temperature
`on the porous properties of the molded rods. We did
`take into consideration two other variables,
`the
`not
`concentration of cross-linking agent and the monomers
`to porogenic solvent ratio.
`On the basis of our experience, we chose a standard
`composition of monomer mixture including 40% ethyl-
`ene dimethacrylate and 60% glycidyl methacrylate for
`all experiments. This composition is deemed ideal
`because any lower concentration of the cross-linking
`agent could impair the mechanical properties of the final
`polymer rods, while a higher one would decrease the
`content of reactive epoxide groups that are needed for
`the subsequent functionalization of the rods. The 4:6
`monomers/porogenic solvent ratio has already proven
`to be the most advantageous for the preparation of
`macroporous poly(glycidyl methacrylate-co-ethylene
`dimethacrylate) materials.23
`Effect of Polymerization Time. The influence of
`reaction time on conversion is well demonstrated for all
`reactions. Since the polymerization at 70 °C proceeds
`too fast to be monitored readily, we chose a temperature
`of 55 °C at which the rate of polymerization is low
`enough to be readily monitored (Figure 2). Although
`the conversion of monomers
`to polymer is close to
`quantitative after about 10 h, some additional structural
`changes still occur within the rod if the system is kept
`longer at the polymerization temperature. However, no
`reaction times
`observed at
`significant changes are
`exceeding 22 h.
`The length of the completely polymerized rod pre-
`pared under the conditions specified in Table 2 in a
`tubular mold charged with 2 mL of the polymerization
`It would be expected that the length
`mixture is 35 mm.
`of the rod itself would not depend on the polymerization
`
`(22) Yuan, H. G.; Kalfas, G.; Ray, W. H., J. Macromol. Sci., Rev.
`Macromol. Chem. Phys. 1991, C31, 215.
`
`(23) Horak, D.; Pelzbauer, Z.; Svec, F.; Kalal, J. J. Appl. Polym.
`Sci. 1981, 26, 3205.
`
`3
`
`

`

`710 Chem. Mater., Vol. 7, No. 4, 1995
`Table 2. Porous Properties of Poly(glycidyl methacrylate-co-ethylene dimethacrylate) Rods Prepared Using Different
`Polymerization Times0
`
`Svec and Fréchet
`
`BET
`mercury porosimetry
`ipol,6 min
`YV mL/g
`Vp,c mL/g
`Sg,d m2/g
`Dpitnax/ nm
`Dptnied;e 1
`Sg,d m2/g
`Dptsurf)^ nm
`Dp,vol,g nm
`217.5
`3.759
`618
`523.9
`0.688
`702
`60
`3.44
`6.33
`3.453
`870
`149.2
`0.360
`317.3
`811
`3.28
`6.53
`75
`2.926
`996
`0.335
`136.0
`966
`283.5
`100
`3.40
`6.37
`2.532
`127.8
`1124
`255.7
`0.296
`3.29
`6.71
`1201
`130
`2.347
`0.287
`6.82
`249.1
`123.8
`1090
`150
`1150
`3.22
`974
`125.0
`200
`239.8
`1.673
`0.239
`1099
`3.28
`6.32
`73.0
`1.312
`300
`0.165
`6.22
`149.7
`966
`1128
`3.28
`934
`600
`0.153
`1.257
`1125
`3.33
`6.18
`79.2
`138.0
`65.6
`1.093
`0.139
`1320
`6.78
`1154
`935
`3.33
`120.1
`0.140
`1.108
`1800
`66.0
`119.8
`940
`6.49
`1148
`3.29
`° Reaction conditions: polymerization mixture: glycidyl methacrylate 24%, ethylene dimethacrylate 16%, cyclohexanol 54%, dodecanol
`6%; polymerization temperature 55 °C. 6 Polymerization time.c Pore volume. d Specific surface area.6 Median pore diameter. / Pore diameter
`at the maximum of the distribution curve.
`* Median pore diameter based on pore volume. h Median pore diameter based on surface area.
`
`100
`
`50
`
`0
`
`£ coH
`
`>coÜ
`
`0
`
`1000
`Polymerization time, min
`Figure 2. Kinetics of the polymerization of glycidyl meth-
`acrylate and ethylene dimethacrylate at a temperature of 55
`°C. For conditions see Table 2.
`
`2000
`
`time as the polymerization ought to take place through-
`the entire volume of the mixture in the tube.
`out
`However, this is not the case, and if tubular molds were
`held vertically during the polymerization reaction, the
`rods obtained after 60 and 75 min of polymerization
`were significantly shorter (21 and 25 mm, respectively)
`and occupied only the bottom part of the mold. The
`liquid remaining on the top of the rods under these
`removed with a syringe and analyzed
`conditions was
`by gas chromatography. Even after 150 min of polym-
`erization a small amount of the liquid was still found,
`but this is most likely due to the oxygen inhibition of
`the polymerization at the surface of the rod because the
`tube was not completely filled with the polymerization
`mixture and the residual space can contain some air.
`The composition of all the liquids collected was generally
`close to that of the original polymerization mixture. The
`liquid did not contain any polymeric components as
`confirmed by the lack of precipitation during dilution
`of the samples with methanol for GC analysis.
`Table 2 summarizes the porous properties of the rods.
`During the early stage of the polymerization, the pore
`volume is very high reaching almost 4 mL/g. This
`represents a porosity of about 82% at the low conversion
`of 15.7%. The pore volume decreases as the polymeri-
`zation progresses, eventually reaching a value slightly
`above 1 mL/g that represents a porosity of about 60%.
`This porosity is directly related to the volume of the
`porogenic solvents used for the polymerization and
`confirms that all of the monomers
`have been consumed.
`
`Conversion, %
`Figure 3. Effect of conversion on the specific surface area
`determined by BET method ( ) and mercury intrusion poro-
`simetry (a) during the polymerization at a temperature of 55
`°C. For conditions see Table 2.
`
`1
`
`1000
`100
`10
`Pore diameter, nm
`Figure 4. Differential pore size distribution curves
`of the
`polyCglycidyl methacrylate-co-ethylene dimethacrylate) rods
`after 1 h ( ) and 14 h ( ) of polymerization at a temperature
`of 55 °C. For conditions see Table 2.
`
`10000
`
`The specific surface area, calculated from both mercury
`porosimetry and BET measurements, also decreases
`with the polymerization time. Figure 3 documents that
`the specific surface area decreases linearly within the
`range of conversions from 20 to 100%.
`Figure 4 shows the pore size distribution curves
`for
`rods formed after 60 and 1320 min, respectively. Al-
`1 (corresponding to the
`though the maximum of curve
`most porous rod formed within 1 h) is located at 618
`
`4
`
`

`

`Pore Formation in Macroporous Polymers
`I
`
`10000
`
`Chem. Mater., Vol. 7, No. 4, 1995
`
`711
`
`5000
`
`0
`
`m®oa <
`
`o
`
`¡5
`
`o o eeE
`
`5
`
`10
`
`40
`70
`Conversion, %
`Figure 5. Diameter of the largest detectable pores in the poly-
`(glycidyl methacrylate-co-ethylene dimethacrylate) rods pre-
`pared at a temperature of 55 °C as a function of conversion.
`For conditions see Table 2.
`
`100
`
`Figure 7. Differential pore size distribution curves
`of the
`poly(glycidyl methacrylate-co-ethylene dimethacrylate) rods
`prepared from mixtures containing 6% (a, ) and 12% do-
`decanol ( , ) by a polymerization at a temperature of 55 °C
`(closed points) and 70 °C (open points). For general conditions
`see Experimental Section.
`
`document that the very large pores that are character-
`istic of rods in the early stages of the polymerization
`and which contribute considerably to the median pore
`size, disappear as the polymerization progresses while
`the influence of the small pores on the average diameter
`becomes increasingly important. Table 2 shows that the
`size of the pore diameter Dp,max reaches a plateau after
`In contrast, the calculated
`about 2 h of polymerization.
`median pore diameter £)p,med initially increases, then
`reaches a maximum also after about 2 h, and then
`decreases again continuously.
`It should be emphasized that any direct comparison
`of the BET and mercury porosimetry data would not be
`a different range of
`appropriate as each method covers
`pores. This can be confirmed by the comparison of the
`pore diameter data summarized in Table 2. While the
`mercury porosimetry monitors efficiently the significant
`changes affecting the pore diameters, the BET data do
`not show any change in the median pore diameters
`calculated from both pore volumes and surface areas
`during the polymerization. On the other hand,
`the
`surface areas measured by BET involve also the pores
`smaller than those detected by the mercury intrusion
`method. Therefore, the BET specific surface areas
`are
`those calculated from the
`twice as
`about
`large as
`mercury intrusion porosimetry. However, Figure 3
`shows that the trends in changes of specific surface
`are similar for both the BET and the mercury
`areas
`porosimetry measurements.
`Effect of the Porogenic Solvent.
`It was observed
`earlier that the addition of dodecanol to cyclohexanol
`used as the porogenic solvent results in the formation
`of larger pores in poly(glycidyl methacrylate-co-ethylene
`dimethacrylate) beads.19 This is also confirmed in this
`study. The median pore diameter for beads prepared
`at 70 °C in the presence of 0, 6, and 12% of dodecanol is
`53, 63, and 85 nm, respectively (Table 1). Figure 7
`shows the shift induced by dodecanol in the maxima of
`the pore size distribution curves
`for molded rods pre-
`pared at two different temperatures. For example, the
`median pore size of rods prepared with 6 and 12%
`dodecanol at 70 °C increases from 91 to 283 nm,
`respectively.
`
`100
`
`0
`
`-100
`
`-200
`
`-300
`
`S
`E
`
`1E
`
`o ,
`
`a
`
`20
`
`0
`
`10
`time, h
`Figure 6. Difference between the calculated median pore
`diameter Dp,med and the pore diameter corresponding to the
`maximum of the distribution curve
`.Dp,max for the poly(glycidyl
`methacrylate-co-ethylene dimethacrylate) rods prepared at a
`temperature of 55 °C as a function of polymerization time. For
`conditions see Table 2.
`
`nm, the rod also contains a substantial amount of very
`large pores with diameters up to 10 mm.
`In contrast,
`2 is located at 1154 nm, but it
`the peak for curve
`is
`and without pores over 2 mm in diameter. The
`narrower
`almost 4-fold difference in the pore volumes of the two
`molded rods obtained after 1 and 14 h, respectively, is
`also reflected in the much larger area beneath curve
`1,
`particularly in the range of large pores. The pore size
`distribution narrows
`as the polymerization progresses
`because the largest pores disappear. Figure 5 shows
`the size of the largest pores detected by mercury
`porosimetry at different stages of the polymerization of
`the rod and documents that their size decrease is a
`function of the conversion.
`Mercury porosimetry measurements provide two kinds
`of pore diameters:
`the calculated median pore diameter
`•Dp.med and the pore diameter that corresponds to the
`maximum read from the distribution curve
`£)p,max.
`Figure 6 shows that
`the difference Z)p,med —
`Z)p,max
`decreases smoothly within the whole range of conver-
`sions. At low conversion, the median size exceeds the
`peak value. However, this already changes after about
`90 min of polymerization as the median size decreases
`and the difference becomes negative. The data also
`
`5
`
`

`

`712 Chem. Mater., Vol. 7, No. 4, 1995
`
`Svec and Fréchet
`
`Figure 8. Differential pore size distribution curves
`of the
`poly(glycidyl methacrylate-co-ethylene dimethacrylate) rods
`prepared by a 22 h polymerization at a temperature of 55 °C
`( ), 12 h at 70 °C ( ) and at a temperature increased during
`the polymerization from 50 to 70 °C in steps by 5 °C lasting 1
`h each and kept at 70 °C for another 4 h ( ). Conditions:
`polymerization mixture: glycidyl methacrylate 24%, ethylene
`dimethacrylate 16%, cyclohexanol 54%, dodecanol 6%.
`Effect of the Polymerization Temperature. An
`effect similar to that of dodecanol was also observed
`when the temperature was changed in otherwise identi-
`cal preparations. The lower the polymerization tem-
`perature, the larger the pores. Figure 7 shows the effect
`of temperature on the pore size distributions of continu-
`ous rods prepared at 55 and 70 °C. This temperature
`effect may be very useful in practice because it allows
`the control of the pore size distribution of the molded
`rods without requiring any change in the composition
`of the polymerization mixture. However, the lower limit
`of the polymerization temperature depends on the
`initiator used.
`decomposition rate of the free-radical
`Figure 8 shows the pore size distribution curves
`for rods
`prepared in steel tubes under different temperature
`In addition to rods prepared by polymeri-
`conditions.
`zation at fixed temperature of 55 and 70 °C, Figure 8
`also includes a curve obtained for a rod prepared using
`a step gradient of polymerization temperature. The
`temperature was increased during polymerization from
`50 to 70 °C in 5 °C steps lasting 1 h each, and then the
`the final
`temperature of 70 °C for
`rod was kept at
`another 4 h. This approach was chosen to reduce the
`possibility of a steep radial temperature gradient that
`could lead to a faster polymerization in the areas
`close
`to the walls of the tube with formation of a “shell”,
`surrounding the liquid in the center of the tube. As
`in this
`expected, the maximum of the distribution curve
`rod prepared in a step gradient of temperature is
`between the maxima for the rods prepared at 55 and
`70 °C because the average polymerization temperature
`was between these two temperatures.
`Since the polymerization temperature affects the pore
`it
`is possible to use
`size distribution,
`temperature
`changes during the polymerization to fine-tune the
`porosity of the rod. For example, a rod prepared by
`polymerization at 55 °C for 1 h then at 70 °C for 14 h
`with the mold standing vertically in the bath throughout
`the process shows a different porosity profile at its two
`extremities (Figure 9).
`While both parts of the rod contain large pores
`1200 nm in diameter, the top of the rod
`centered near
`
`Pore diameter, nm
`Figure 9. Differential pore size distribution curves of the top
`( ) and the bottom part ( ) of the poly(glycidyl methacrylate-
`co-ethylene dimethacrylate) rod polymerized 1 h at a temper-
`ature of 55 °C followed by 14 h at 70 °C. Conditions:
`polymerization mixture: glycidyl methacrylate 24%, ethylene
`dimethacrylate 16%, cyclohexanol 54%, dodecanol 6%.
`
`Pore diameter, nm
`
`Pore diameter, nm
`Figure 10. Differential pore size distribution curves
`of (a)
`top ( ) and bottom part ( ) of the poly(glycidyl methacrylate-
`co-ethylene dimethacrylate) rod polymerized 1 h at a temper-
`ature of 55 °C followed by 14 h at 70 °C; (b) rod polymerized
`1 h at a temperature of 55 °C ( ) and rod polymerized 14 h at
`70 °C ( )
`. Conditions: polymerization mixture: glycidyl
`methacrylate 24%, ethylene dimethacrylate 16%, cyclohexanol
`54%, dodecanol 6%.
`formed in later stages of the polymerization
`which was
`contains a very significant volume of smaller pores.
`Figure 10a shows a magnification of the same porosity
`profiles for the top and bottom portions of the rod.
`While the volume of pores in the range 30—200 nm is
`
`6
`
`

`

`Pore Formation in Macroporous Polymers
`only 6.7% in the bottom part, it accounts for a remark-
`able 25.5% at the top with a distinct maximum centered
`at about 80 nm. Figure 10b shows that the same
`type
`of porosity profiles can be obtained for a rod prepared
`by polymerization at 55 °C for 1 h only and then
`processed and for a rod obtained after the standard
`polymerization time of 14 h at 70 °C.
`This observation is readily explained if one considers
`that much unpolymerized material remains at the top
`of the polymerized rod after a polymerization time of
`If
`the low temperature of 55 °C.
`only 1 h at
`the
`polymerization is then completed at 70 °C, the porous
`solid that forms on top of the previously polymerized
`rod has the pore profile typical of rod prepared at 70
`°C. Although this technique has not yet been explored
`in detail, it is extremely promising for the preparation
`of rods with a gradient of porosity that should prove
`useful in electrophoresis or novel modes of separation.
`The axial heterogeneity observed in the preceding
`experiment resulted from the insufficient polymeriza-
`tion time that left a pool of unpolymerized material on
`In contrast, the polymerized rod
`top of the growing rod.
`occupies the entire volume of the mold if the polymer-
`ization is allowed to proceed for 3 h at 55 °C. Therefore,
`a control experiment was carried out involving polym-
`erization for 3 h at 55 °C followed by 8 h at 70 °C.
`In
`this case, porosimetric measurements reveal no differ-
`ence in porosity profiles for the top and the bottom parts
`of the rod. No evidence of axial heterogeneity is
`uncovered and both parts of the rod have porosity
`profiles that are very similar to that of a rod obtained