`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`1
`
`MTX1014
`
`

`

`American Chemical Society
`1155 16th St, N.W.
`Washington, DC 20036
`(202) 872-4600
`TDD (202) 872-8733
`FAX (202) 872-4615
`
`Journal Publishing Operations
`American Chemical Society
`2540 Olentangy River Road
`P.O. Box 3330
`Columbus, OH 43210
`(614) 447-3600, Ext 3171
`TELEX 6842086; FAX (614) 447-3745
`
`Member & Subscriber Services
`American Chemical Society
`P.O. Box 3337
`Columbus, OH 43210
`(614) 447-3776; FAX (614) 447-3671
`(800) 333-9511
`
`PUBLICATIONS DIVISION
`
`Robert H. Marks, Director
`
`Journal Publishing Operations: Charles R. Bertsch,
`Director
`
`Editorial Office: Mary E. Scanlan, Manager; Joseph
`E. Yurvati, Journals Editing Manager; James W.
`Cooper, Assistant Editor
`
`Advertising Office: Centcom, Ltd., 1599 Post Road
`East, P 0. Box 231, Westport, CT 06881
`
`©Copyright 1996 American Chemical Society.
`Copyright permission: Reprographic copying beyond
`that permitted by Section 1 07 or 1 08 of the US
`Copyright Act is allowed for internal use only, pro(cid:173)
`vided that $12.00 per article is paid to the Copyright
`Clearance Center (CCC), 222 Rosewood Dr., Dan(cid:173)
`vers, MA 01923. Republication or reproduction for
`sale of articles or abstracts in this journal is permit(cid:173)
`ted only under license from ACS. Direct these other
`permission requests to the ACS Copyright Office at
`the ACS Washington address. For quotes and infor(cid:173)
`mation on ordering bulk reprints, call (202) 872-4539
`or write to the ACS Customer Service and Sales
`Office at the ACS Washington address.
`The paper used in this publication meets the
`minimum requirements of American National Stan(cid:173)
`dard for Information Sciences-Permanence of
`Paper for Printed Library Materials, ANSI Z39.48-
`1984.
`
`Editorial Information
`
`Instructions for authors and copyright status
`form are printed in the first issue of each volume.
`These instructions are available via the World Wide
`Web by selecting Instructions for Authors and Edi(cid:173)
`tors at URL http://pubs.acs.org. Please conform to
`these instructions when submitting manuscripts.
`Manuscripts for publication should be submit(cid:173)
`ted to the Editor, Leonard V. Interrante.
`Correspondence regarding accepted papers and
`proofs should be directed to the Journal Publishing
`Operations at the address above.
`
`The American Chemical Society and its Editors
`assume no responsibility for the statements and
`opinions advanced by contributors.
`Registered names and trademarks, etc., used in
`this publication, even without specific indication
`thereof, are not to be considered unprotected by law.
`At the end of each document is a unique nine(cid:173)
`character document number that serves as a link
`between the printed and electronic (CJACS Plus
`Images) products and facilitates the retrieval of the
`document in electronic form.
`
`1996 Subscription and Ordering Information
`
`Members may share/donate their personal sub(cid:173)
`scriptions with/to libraries and the like but only after
`5 years from publication.
`
`All
`Canada
`Other
`and
`U.S. Mexico Europe' Countries'
`
`Printed
`Members-1 yr $ 54 $ 76
`Nonmembers $522 $544
`Microfiche
`Members-1 yr $ 54 $ 54
`Nonmembers $522 $522
`Supporting
`$ 28 $ 35
`information on microfiche
`
`' Air service included.
`
`$106
`$574
`
`$ 66
`$534
`$ 35
`
`$122
`$590
`
`$ 66
`$534
`$ 35
`
`New and renewal subscriptions should be sent
`with payment to American Chemical Society, Depart(cid:173)
`ment L-0011, Columbus, OH 43268-0011.
`Nonmember subscribers in Japan must enter
`subscription orders with Maruzen Company Ltd.,
`3-10 Nihonbashi 2-chome, Chuo-ku, Tokyo 103,
`Japan. Tel: (03) 272-7211.
`Microforms and back issue orders should be sent
`to: Microforms & Back Issues Office at the ACS
`Washington office.
`Claims for issues not received will be honored
`only if submitted within 90 days of the issue date
`(subscribers in North America) or within 180 days of
`the issue date (all other subscribers). Claims are
`handled by Member & Subscriber Services.
`Supporting Information is noted on the table of
`contents with a 1111.
`It is available as photocopy
`($12.00 for up to 3 pages and $1.50 per page for
`each additional page, plus $2.00 for foreign postage)
`or as 24x microfiche ($12.00, plus $1.00 for foreign
`postage). Canadian residents should add 7% GST.
`See supporting information notice at the end of jour(cid:173)
`nal article for number of pages. Orders must give
`complete title of article, name of authors, journal,
`issue date, and page numbers. Prepayment is
`required and prices are subject to change. For infor(cid:173)
`mation on microforms, contact Microforms & Back
`Issues Office at the ACS Washington address or
`phone (202) 872-4554. Supporting information
`except structure factors also appears in the microfilm
`edition.
`Supporting information is also available via the
`Internet using either World Wide Web or gopher pro(cid:173)
`tocols. WWW users should select Electronic Sup(cid:173)
`porting Information at URL http://pubs.acs.org. A
`README document is available for gopher users in
`the directory Supporting Information under ACS Pub(cid:173)
`lications at pubs.acs.org. Detailed instructions for
`using this service, along with a description of the file
`formats, are available at these sites. To download
`the Supporting Information, enter the journal sub(cid:173)
`scription number from your mailing label. For addi(cid:173)
`tional information on electronic access, send elec(cid:173)
`tronic mail to gopher@ acsinfo.acs.org or phone
`(202) 872-4434.
`
`EDITOR
`
`LEONARD V. INTERRANTE
`Department of Chemistry
`Rensselaer Polytechnic Institute
`Troy, New York 12180
`
`Associate Editors
`Dennis W. Hess
`Department of Chemical Engineering
`Lehigh University
`Bethlehem, Pennsylvania 18015
`
`Gary E. Wnek
`Department of Chemistry
`Rensselaer Polytechnic Institute
`Troy, New York 12180
`
`ADVISORY BOARD
`
`Harry R. Allcock
`Neal Armstrong
`David Avnir
`Thomas Bein
`John S. Bradley
`Jeffrey Brinker
`Abraham Clearfield
`John Corbett
`R. Corriu
`James Crivello
`Larry R. Dalton
`Mark E. Davis
`Bruce Dunn
`John G. Ekerdt
`Arthur B. Ellis
`J. M. J. Frechet
`Francis Garnier
`E. P. Giannelis
`Michael Gratzel
`John Greedan
`Martha Greenblatt
`Mark J. Hampden-Smith
`Robert Haushalter
`Norman Herron
`Allan Jacobson
`Klavs F. Jensen
`Jack W. Johnson
`Mercouri G. Kanatzidis
`Richard B. Kaner
`Howard E. Katz
`
`Kenneth Klabunde
`Jacques Livage
`T. E. Mallouk
`Stephen Mann
`Seth Marder
`James E. Mark
`Tobin J. Marks
`Egon Matijevic
`James Moore
`Jeffrey Moore
`Donald Murphy
`Geoffrey A. Ozin
`David A. Payne
`Bernard Raveau
`Elsa Reichmanis
`Jean Rouxel
`Lynn Schneemeyer
`R. Schellhorn
`U. Schubert
`Takeo Shimidzu
`D. F. Shriver
`Arthur Sleight
`Steven Suib
`John M. Thomas
`Stanley Whittingham
`Michael D. Ward
`G. M. Whitesides
`C. Grant Willson
`Aaron Wold
`Fred Wudl
`
`Chemistry of Materials (ISSN 0897-4756) is
`published monthly by the American Chemical
`Society at 1155 16th St., N.W., Washington,
`DC 20036. Second-class postage paid at
`Washington, DC, and additional mailing
`offices. POSTMASTER: Send address
`changes to Chemistry of Materials, Member
`& Subscriber Services, P.O. Box 3337,
`Columbus, OH 43210.
`Canadian GST Reg. No. 127571347
`Printed in the USA.
`
`2
`
`

`

`744
`
`Chem. Mater. 1996, 8, 744-750
`
`Monolithic, "Molded", Porous Materials with High Flow
`Characteristics for Separations, Catalysis, or Solid-Phase
`Chemistry: Control of Porous Properties during
`Polymerization
`
`Camilla Viklund, Frantisek Svec, and Jean M. J. Frechet*
`Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301
`
`Knut Irgum
`Department of Analytical Chemistry, University of Umea, S-901 87 Sweden
`
`Received September 19, 1995. Revised Manuscript Received December 18, 1995°
`
`The porosity and flow characteristics of macroporous polymer monoliths that may be used
`to prepare separation media, flow-through reactors, catalysts, or supports for solid-phase
`chemistry can be controlled easily during their preparation. Key variables such as
`temperature, composition of the pore-forming solvent mixture, and content of cross-linking
`divinyl monomer allow the tuning of average pore size within a broad range spanning 2
`orders of magnitude. The polymerization temperature, through its effects on the kinetics
`of polymerization, is a particularly effective means of control, allowing the preparation of
`macroporous polymers with different pore size distributions from a single composition of
`the polymerization mixture. The choice of pore-forming solvent is also important, larger
`pores being obtained in a poor solvent due to an earlier onset of phase separation. Increasing
`the proportion of the cross-linking agent present in the monomer mixture not only affects
`the composition of the final monoliths but also decreases their average pore size as a result
`of early formation of highly cross-linked globules with a reduced tendency to coalesce. The
`synergy of different effects has also been observed under specific polymerization conditions
`using two monomer pairs, styrene-divinylbenzene and glycidyl methacrylate-ethylene
`dimethacrylate polymerized in close molds. Mercury intrusion porosimetry measurements,
`inverse size exclusion chromatography, and back pressure measured at different flow rates
`with the macroporous monoliths were used for the characterization of the porous properties.
`A good correlation between pore size and flow resistance that follows the Hagen-Poiseuille
`equation used previously to describe flow through a straight tube has been found.
`
`Introduction
`The preparation of macroporous polymer beads is
`generally achieved as a result of the phase separation
`which occurs during the polymerization of a monomer
`mixture containing appropriate amounts of both a cross(cid:173)
`linking monomer and a porogenic solvent. This process,
`invented in the late 1950s, has been commercialized,
`and macroporous bead materials are widely used today
`for the preparation of ion-exchange resins, catalysts,
`adsorbents, chromatographic media, etc. 1 In contrast
`to common cross-linked polymers that must be swollen
`in a good solvent in order to acquire porous properties,
`these resin beads remain porous even in the dry state.
`The porous properties of such particulate resins can be
`controlled by many variables. The most important ones
`are generally thought to be the concentration of cross(cid:173)
`linking monomer in the monomer mixture and the type
`and percentage of porogenic solvent present in the
`polymerizing system. Other variables such as content
`of initiator, reaction time, and polymerization temper-
`
`ature are considered to be of minor importance for the
`control of the porous properties of these materials. 1•2
`In 1992 we described a totally new type of macroporous
`material obtained by solution polymerization in an
`unstirred mold. 3 This macroporous material is obtained
`as a monolith with a shape that conforms to that of the
`mold and with a highly unusual pore structure allowing
`direct flow of a liquid through its large pores. Because
`direct flow through the monolithic molded medium, as
`opposed to flow around a classical bead, may offer new
`opportunities for applications as varied as chromatog(cid:173)
`raphy or chemistry on solid support, it is important to
`control accurately the porous structure ofthe monoliths.
`We have recently shown that polymer beads prepared
`by classical suspension polymerization and polymer
`monoliths prepared by our process using the same
`monomer mixture, pore-forming solvent, initiator, and
`temperature
`have
`vastly
`different macro(cid:173)
`porous structures. 4•5 Only the molded monoliths contain
`the large channels that allow a liquid to flow through
`
`0 Abstract published in Advance ACS Abstracts, February 15, 1996.
`(1) Seidl, J.; Malinsky, J.; Dusek, K.; Heitz, W. Adv. Polym. Sci.
`1967, 5, 11. Guyot, A.; Bartholin, M. Prog. Polym. Sci. 1982, 8, 277.
`Hodge, P., Sherrington, D. C., Eds. Syntheses and Separations Using
`Functional Polymers; Wiley: New York, 1989. Kun, K. A.; Kunin, R.
`J. Polym. Sci., Al1968, 6, 2689. Sederel, W. L.; DeJong, G. J. J. Appl.
`Polym. Sci. 1973, 17, 2835.
`
`(2) Horak, D; Svec, F.; Bleha, M.; Kalal, J. Angew. Makromol. Chem.
`1981, 95, 109. Horak, D; Svec, F.; Ilavsky, M., Bleha, M.; Baldrian, J.;
`Kalal, J. Angew. Makromol. Chem. 1981,95, 117.
`(3) Svec, F.; Frechet, J. M. J.Anal. Chem., 1992,64,820. Svec, F.;
`Frechet, J. M. J. US Patent No. 5,334,310, 1994.
`(4) Svec, F.; Frechet, J. M. J. Chem. Mater. 1995, 7, 707.
`(5) Svec, F.; Frechet, J. M. J. Macromolecules 1995, 28, 7580.
`
`0897-4 7 56/96/2808-07 44$12.00/0
`
`© 1996 American Chemical Society
`
`3
`
`

`

`Control of Porous Properties during Polymerization
`
`Chem. Mater., Vol. 8, No. 3, 1996 745
`
`the medium at a low applied pressure. Typical macro(cid:173)
`porous beads obtained by suspension polymerization
`contain a more extensive network of smaller pores than
`the monolith but they allow mass transfer only through
`flow around the beads using the large interstitial spaces
`that unavoidably result from their spherical shape.
`Mass transfer is increased considerably in the perfusive
`beads that have been introduced recently for use in
`HPLC, diagnostics, and enzyme immobilization.6 These
`beads have some pores that are large enough to allow
`up to about 5% of the mobile phase to flow through, and
`even this small convection has a positive effect on the
`chromatographic separations. 7
`The process used to prepare the monoliths and the
`beads differ only in the lack of interfacial tension
`between an aqueous and an organic phase, and the
`absence of dynamic forces resulting from stirring in the
`case of the polymerization in an unstirred mold. The
`kinetics of the overall process within the unstirred mold
`is also one of the most important variables that con(cid:173)
`tributes to the formation of large pores and allows the
`control of the macroporous structure involving these
`flow-through channels.5
`Once polymerized in a tubular or a flat mold, the
`porous material is an excellent support for the im(cid:173)
`mobilization of biological catalysts or for use as a
`separation medium for high-performance liquid chro(cid:173)
`matography (HPLC) of a broad spectrum of molecules,
`large and small. 8 Our approach has also been used for
`the preparation of rods with molecularly imprinted
`templates that have been used in molecular recognition
`by HPLC and in separations by capillary electrophore(cid:173)
`sis.9
`This study examines in detail the effects of the most
`important variables such as temperature, concentration
`of cross-linking monomer in the polymerization mixture,
`and composition of the porogenic solvent, on the porous
`structure obtained with the two chemically different
`systems of styrenic and methacrylate monomers.
`
`Experimental Section
`
`Preparation of Polymers. Polymerization Mixtures.
`Azobisisobutyronitrile (1 wt % with respect to monomers,
`Kodak) was dissolved in 4 vol parts of a mixture consisting of
`glycidyl methacrylate and ethylene dimethacrylate (both from
`Sartomer). In an alternative procedure, azobisisobutyronitrile
`(1 wt% with respect to monomers) was dissolved in 4 vol parts
`of a mixture consisting of styrene (Aldrich) and divinylbenzene
`(80% of divinyl monomer, Dow Chemicals). The porogenic
`solvents, mixtures of cyclohexanol with dodecanol or of dade(cid:173)
`canol with toluene, respectively (all Aldrich), were admixed
`slowly to the monomers. The total volume of the porogenic
`
`(6) Afeyan, N. B.; Gordon, N. F.; Mazsaroff, I.; Varady, L.; Fulton,
`S. P.; Yang, Y. B.; Regnier, F. E. J. Chromatogr. 1990,519, 1. Afeyan,
`N. B.; Fulton, S. P.; Regnier, F. E. J. Chromatogr. 1991, 544, 267.
`Fulton, S. P.; Afeyan, N. B.; Regnier, F. E. J. Chromatogr. 1991,547,
`452.
`(7) Liapis, A. I. Math. Modeling Sci. Comput. 1993, 1, 397. Rod(cid:173)
`rigues, A. E.; Lu, Z. P.; Loureiro, J. M.; Carta, G. J. Chromatogr. 1993,
`653, 93. Liapis, A. I.; McCoy, M.A. J. Chromatogr. 1994, 660, 85.
`(8) Frechet, J. M. J. Mahromol. Chem., Mahromol. Symp. 1993, 70 I
`71, 289. Wang, Q. C.; Svec, F.; Frechet, J. M. J. J. Chromatogr. 1994,
`669, 230. Wang, Q. C.; Svec, F.; Frechet, J. M. J. Anal. Chem. 1993,
`65, 2243. Svec, F.; Frechet, J. M. J. J. Chromatogr. A 1995, 702, 89.
`Tennikova, T. B.; Svec, F. J. Chromatogr. 1993, 646, 279.
`(9) Matsui, J.; Kato, T.; Takeuchi; T., Suzuki, M.; Yokoyama, K;
`Tamyia, E.; Karube, I. Anal. Chem. 1993., 65, 2223. Sellergren, B. Anal.
`Chem. 1994, 66, 1578. Sellergren, B. J. Chromatogr. A 1994, 673, 133.
`Nilsson, K; Linde!, J.; Norrlow, 0.; Sellergt·en, B. J. Chromatogr. A
`1994, 680, 57.
`
`mixture was 6 parts. The stock polymerization mixtures were
`stored in closed flasks in a freezer at a temperature of -18 oc
`and consumed within 2 days.
`Polymerization in Solution. The polymerization mix(cid:173)
`tures were purged with nitrogen for 15 min in order to remove
`oxygen. The stainless steel tubular molds (100 mm x 8 mm
`i.d.) were sealed at one end with rubber septa fitted over a
`piece of polyethylene film, filled with the mixture, then sealed
`on the other end, and placed in a vertical position into a water
`bath. The polymerization was allowed to proceed for 24 h at
`55-80 °C. 10 The seals were removed, the tube was provided
`with fittings, attached to the high-pressure pump, and 25 mL
`tetrahydrofuran was pumped through the column at a flow
`rate of 0.1-1 mL/min depending on the flow resistance of the
`rod to remove the porogenic solvents and any other soluble
`compounds that remained in the polymer rod after the
`polymerization was completed. After all ofthe in situ hydro(cid:173)
`dynamic and chromatographic measurements were completed,
`the polymer was forced out of the steel tube by applying a
`pressure of THF using the pump and dried prior to porosity
`studies.
`Porous Properties in Dry State. The porous properties
`of the monoliths were determined by mercury intrusion
`porosimetry and the specific surface areas calculated from
`nitrogen adsorption/desorption isotherms using a custom-made
`cmnbined BET sorptometer and mercury porosimeter (Porous
`Materials, Inc., Ithaca, NY). Prior to the measurements, the
`materials were cut into small pieces using a razor blade.
`Chromatographic and Hydrodynamic Measurements.
`Inverse size-exclusion chromatography was carried out using
`a Nicolet IBM LC 9560 ternary gradient liquid chromatograph
`equipped with a Hewlett-Packard 1050 UV detector. The
`peaks of toluene, o-terphenyl, and polystyrene standards with
`molecular weights ranging from 1 250 to 2 950 000 were
`monitored in tetrahydrofuran as the mobile phase at 254 nm.
`Before and after these measurements, the outlet capillary was
`removed from the bottom fitting of the tube and the back
`pressure produced by a flow of tetrahydrofuran through the
`material was recorded.
`
`Results and Discussion
`
`The use of porous monolithic materials for applica(cid:173)
`tions such as dead-end filtration, membrane chroma(cid:173)
`tography, and HPLC in which a liquid or gas have to
`flow through the medium requires that the flow be
`achieved at a reasonably low pressure. Because the
`pressure depends on the porous properties of the mate(cid:173)
`rial, the pore size distribution of the monolith should
`be adjusted to match each of the applications. Typically,
`the material must contain a sufficient volume of large
`channels with a diameter of about 1 ,urn, and, for some
`of the applications, additional diffusive pores smaller
`than 100 nm. The key variables that allow the control
`of the pore size5 are the polymerization temperature,
`the composition ofporogenic solvent, and the percentage
`of cross-linking monomer.
`Effect of Polymerization Temperature. Recently,
`we have formulated general guidelines for use of tem(cid:173)
`perature in the control of pore sizes within macroporous
`polymers. 5 As a rule, the higher the polymerization
`temperature, the smaller the pores. We have now found
`this to be true for both the poly(styrene-co-divinylben(cid:173)
`zene) and the poly(glycidyl methacrylate-co-ethylene
`dimethacrylate) monoliths prepared in tubular molds.
`Indeed our findings summarized in Figure 1 confirm
`that adjustments in the polymerization temperature
`
`(10) The time required for heating the polymerization mixture in
`the 8 mm tubular molds to the reaction temperature is assumed to be
`much shorter than the polymerization time5 and is not considered to
`affect the properties of the final monolith.
`
`4
`
`

`

`746 Chem. Mater., Vol. 8, No. 3, 1996
`
`10
`
`3
`
`2
`
`0
`
`10
`
`10
`
`1000
`100
`Pore diameter, nm
`
`10000
`
`2
`
`10
`
`10000
`
`1000
`100
`Pore diameter, nm
`Figure 1. Differential pore size distribution curves of molded
`poly(glycidyl methacrylate-co-ethylene dimethacrylate) and
`poly(styrene-co-divinylbenzene) monoliths polymerized at dif(cid:173)
`ferent temperatures. Conditions: Polymerization time 24 h.
`(a, top) Polymerization mixture glycidyl methacrylate 24%,
`ethylene dimethacrylate 16%, cyclohexanol 54%, dodecanol 6%,
`temperature 80 (1), 70 (2), and 55 oc (3). (b, bottom) Polym(cid:173)
`erization mixture styrene 20%, divinylbenzene 20%, dodecanol
`40%, toluene 20%, temperature 80 (1), 70 (2), and 60 oc (3).
`
`alone may be used to shift the maximum of the pore
`size distribution profile (modal pore diameter) within a
`range of 2 orders of magnitude. Tables 1 and 2 show
`that this shift is accompanied by changes in the volume
`fraction of the smaller pores, and, consequently, changes
`in specific surface areas. Obviously, a larger volume
`fraction of smaller pores translates into a higher overall
`surface area for the porous material.
`The effect of temperature can be readily explained in
`terms of the nucleation rate. The free-radical initiator
`decomposes at a particular temperature and the result(cid:173)
`ing radicals initiate a polymerization in solution. How(cid:173)
`ever, the polymers that are formed soon become in(cid:173)
`soluble and precipitate in the reaction medium as a
`result of both their cross-linking and the choice of
`porogen, which is typically a poor solvent for the
`polymer. Precipitation leads to the formation of nuclei
`which grow to the size of globules as the polymerization
`proceeds further. The globules and their clusters
`constitute the elemental morphological units of the
`macroporous polymer. Because higher reaction tem(cid:173)
`peratures lead to the formation of a larger number of
`free radicals by decomposition of the initiator, a larger
`number of growing nuclei and globules are formed.
`Since the volume of monomer used is the same for each
`
`Viklund et al.
`
`polymerization, the formation of a larger number of
`globules is compensated by their smaller size. Because
`macro porous materials are composed of arrays of inter(cid:173)
`connected globules, smaller voids, or pores, are obtained
`if the globules are smaller and more numerous. There(cid:173)
`fore, the shift in pore size distribution induced by
`changes in the polymerization temperature can be
`accounted for by the difference in the number of nuclei
`that result from such changes.
`Temperature also affects the solvent quality, or
`solvency, that controls the phase separation of polymers
`from solution. With the exception of polymers with a
`lower critical solution temperature, the mixing of a
`polymer with a solvent is an endothermic process, and
`therefore an increase in temperature promotes dissolu(cid:173)
`tion of the polymer. Therefore, the phase separation
`required for the formation of a macroporous structure
`is likely to occur when the nuclei reach a higher
`molecular weight ifthe polymerization is run at a higher
`temperature. As a result, both the nuclei and the voids
`between them would be larger. Since this is actually
`not the case and indeed the opposite effect of temper(cid:173)
`ature on pore size is observed, changes in the thermo(cid:173)
`dynamic quality of the solvent resulting from temper(cid:173)
`ature increases seem to be of no consequence to the pore
`formation process.
`If a very poor solvent such as dodecanol is used for
`the polymerization of styrene and divinylbenzene, the
`temperature effect is suppressed by the even larger
`effect of porogen on the phase separation that occurs
`during polymerization (vide infra). As a result, for
`polymerizations in which dodecanol is the sole porogen,
`the size of large through pores increases as the tem(cid:173)
`perature is raised in the range 60-80 oc. This finding
`does not apply to polymerizations done in mixtures of
`dodecanol and toluene (a very good solvent) for which
`the pore size is again controlled by the nucleation rate
`and decreases as the temperature increases (Table 2).
`Effect of Composition of Porogenic Solvent.
`Phase separation of cross-linked nuclei is a prerequisite
`for the formation of the macro porous morphology. The
`polymer phase separates from the solution during
`polymerization because of its limited solubility in the
`polymerization mixture that results either, or both, from
`a molecular weight that exceeds the solubility limit of
`the polymer in the given solvent system or from
`insolubility derived from cross-linking.
`The effect of the thermodynamic quality of the para(cid:173)
`genic solvent system on the properties of the porous
`polymers can be documented for both monomer pairs
`studied.
`Table 1 shows that porous polymers prepared from a
`cyclohexanol-dodecanol mixture with a higher content
`of dodecanol have larger pores. This results from phase
`separation (nucleation) occurring earlier in the system
`that contains more dodecanol because it is a more potent
`precipitant for poly(glycidyl methacrylate-co-ethylene
`dimethacrylate) than cyclohexanol. Table 1 also shows
`that the effect of dodecanol is smaller for a polymeri(cid:173)
`zation done at 55 oc since the polymerization rate at
`this temperature is so slow that the pore size is always
`large. In contrast, the effect of dodecanol is clearly
`dominant at temperatures of 70-80 oc. For example,
`Figure 2a shows that the mode (pore diameter at the
`highest peak) of the pore size distribution curve for
`
`5
`
`

`

`Control of Porous Properties during Polymerization
`
`Chem. Mater., Vol. 8, No. 3, 1996 747
`
`Table 1. Porous Properties of the Molded Poly(glycidyl methacrylate-co-ethylene dimethacrylate)a
`
`pore vol,'%
`-500
`-1000
`
`<50
`
`Vp,dmUg
`
`Sg,g m 2/g
`
`EDMA,b
`dodecanol, b
`temp, oc
`vol%
`vol%
`>1000
`Dp,mean/ nrn
`Dp,mode,e nm
`16
`0
`1.49
`45.9
`19.0
`18.7
`16.4
`55
`17.0
`1010
`1250
`3
`1.41
`9.3
`8.5
`11.3
`55
`16
`1700
`1930
`70.9
`7.9
`1.40
`12.7
`13.3
`16
`6
`12.2
`1900
`65.0
`9.0
`55
`1690
`15
`10.0
`16
`1840
`1.46
`66.0
`14.6
`9.4
`55
`7.9
`1540
`16
`0
`1.46
`0.7
`0
`76.0
`23.3
`70
`43.2
`120
`150
`3
`1.57
`68.9
`30.4
`70
`16
`21.7
`260
`0.7
`0
`150
`16
`6
`25.7
`56.0
`17.4
`19.3
`480
`1.61
`0.9
`70
`470
`16
`15
`4.8
`2570
`1.39
`84.6
`5.5
`5.1
`70
`6.0
`2490
`0
`1.29
`0.6
`3.8
`60.0
`35.6
`80
`16
`68.1
`80
`90
`16
`3
`40.3
`59.1
`60
`1.55
`0.7
`0
`80
`58.1
`70
`27.0
`16
`6
`100
`1.59
`0.9
`0.4
`71.7
`80
`54.2
`100
`12.1
`16
`15
`1330
`80
`1.89
`72.9
`7.3
`7.7
`23.6
`1440
`6
`1900
`1.40
`65.1
`12.7
`13.3
`55
`16
`12.2
`9.0
`1690
`48.6
`23.0
`27.5
`24
`6
`540
`860
`1.85
`0.9
`55
`24.0
`a Reaction conditions: polymerization mixture: monomers (glycidyl methacrylate + ethylene dimethacrylate) 40%, porogenic solvent
`(cyclohexanol + dodecanol) 60%. b Percentage of dodecanol and ethylene dimethacrylate in the polymerization mixture, respectively.
`'Percentage of pore volume in the range less than 50, 50-500, 500-1000, and over 1000 nm. d Total pore volume. e Pore diameter at the
`highest peak in the pore size distribution profile. fMean pore diameter. g Specific surface area.
`
`Table 2. Porous Properties of the Molded Macroporous Poly(styrene-co-divinylbenzene)a
`
`DVB,b
`toluene,b
`temp, oc
`vol%
`vol%
`Dp,mode/ nm
`Dp,mean/ lllll
`2.34
`4.0
`14.2
`60
`20
`0
`8.6
`4830
`5660
`79.4
`2.4
`20
`10
`6790
`4370
`1.81
`85.7
`3.7
`1.9
`8.7
`60
`8.4
`15
`20
`60
`1750
`1.86
`62.1
`11.1
`12.1
`14.4
`1540
`14.7
`20
`20
`2.23
`27.4
`60
`48.4
`600
`680
`2.7
`60.0
`9.9
`20
`0
`10.1
`70
`12.0
`5660
`7090
`1.79
`84.9
`2.1
`2.9
`20
`10
`70
`7365
`7355
`1.83
`85.1
`1.0
`2.2
`11.7
`8.4
`15
`1250
`2.12
`60.2
`9.7
`20
`1160
`21.7
`8.4
`70
`14.5
`20
`20
`70
`54.7
`270
`290
`2.18
`0.8
`9.9
`76.6
`12.7
`8030
`9590
`2.11
`2.6
`9.4
`80
`20
`0
`88.0
`9.1
`0
`1070
`2.22
`17.2
`20.6
`20
`10
`950
`44.4
`17.8
`80
`19.1
`20
`15
`82.0
`180
`180
`2.21
`1.5
`8.3
`78.2
`12.0
`80
`20
`20
`4.2
`67.1
`28.3
`160.5
`2.01
`80
`70
`60
`0.5
`14.5
`1250
`2.12
`20
`15
`60.2
`21.7
`8.4
`9.7
`1160
`70
`28
`15
`30.4
`550
`540
`2.02
`0.3
`52.7
`42.4
`4.6
`70
`a Reaction conditions: polymerization mixture: monomers (styrene + divinylbenzene) 40%, porogenic solvent (dodecanol + toluene)
`60%. b Percentage of toluene and divinylbenzene in the polymerization mixture, respectively. ' Percentage of pore volume in the range
`less than 50, 50-500, 500-1000, and over 1000 nm. d Total pore volume. e Pore diameter at the highest peak in the pore size distribution
`.profile. f Mean pore diameter. g Specific surface area.
`monoliths polymerized at a temperature of 70 oc
`decreases from 2570 nm to 480, 260, and 150 nm for
`mixtures that contains 15, 10, 5, and 0 vol% dodecanol,
`respectively.
`Dodecanol is also a poor solvent for poly(styrene-co(cid:173)
`divinylbenzene) while toluene is an excellent solvent.
`Table 2 shows that the addition of even a relatively
`small percentage of toluene to the polymerization
`mixture results in a dramatic decrease in pore sizes.
`Figure 2b shows an example of actual pore size distri(cid:173)
`bution profiles.
`The effect of adding a better solvent to shift the
`distribution toward smaller pore sizes can be readily
`explained by considering that phase separation occurs
`in the later stages of polymerization where cross-linking
`dominates the phase-separation process. The addition
`of a poorer solvent to the polymerizing system results
`in an earlier phase separation of the polymer. The new
`phase swells with the monomers because these are
`thermodynamically much better solvents for the poly(cid:173)
`mer than the porogenic solvent. As a result of this
`preferential swelling, the local concentration of mono(cid:173)
`mers in the swollen gel nuclei is higher than that in
`the surrounding solution and the polymerization reac(cid:173)
`tion proceeds mainly in these swollen nuclei rather than
`in the solution. Those newly formed nuclei obtained in
`
`pore vol,'%
`-500
`-1000
`
`<50
`
`>1000
`
`Vp,dmUg
`
`Sg,g m 2/g
`
`the solution are likely to be adsorbed by the large
`preglobules formed earlier by coalescence of many nuclei
`and further increase their size. Overall, the globules
`that are formed in such a system are larger and,
`consequently, the voids between them (pores) are larger
`as well. As the solvent quality improves, the good
`solvent competes with monomers in the solvation of
`nuclei, the local monomer concentration is lower and
`the globules are smaller. As a result, porous polymers
`formed in more solvating solvents have smaller pores. 1•5
`Obviously, 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.
`Effect of Cross-Linking Monomer. In contrast to
`the effects of temperature and porogenic solvent that
`affect the porous properties of the resulting material but
`not its composition, variations in the monovinyl!divinyl
`monomer ratio not only induce the formation of different
`porous structures butalso lead to materials with dif(cid:173)
`ferent compositions. A higher content of divinyl mono(cid:173)
`mer directly translates into the formation of more highly
`cross-linked polymers in the early stages of the polym(cid:173)
`erization process and therefore lead to earlier phase
`separation. Although this is similar to the effect of poor
`solvent, the nuclei are more cross-linked and because
`
`6
`
`

`

`748 Chern. Mater., Vol. 8, No. 3, 1996
`
`Viklund et al.
`
`4
`
`3
`
`2
`
`1000
`100
`Pore diameter, nm
`
`10000
`
`6
`
`3
`
`0
`
`10
`
`15
`
`10
`
`5
`
`0
`
`10
`
`100
`
`1000
`
`10000
`
`100000
`
`Pore diameter, nm
`Figure 2. Effect of dodecanol (a, top) and toluene (b, bottom)
`in the porogenic solvent on differential pore size distribution
`curves of molded poly(glycidyl methacrylate-co-ethylene dimeth(cid:173)
`acrylate) and poly(styrene-co-divinylbenzene) monoliths. Con(cid:173