Transparent Macroporous Polymer Monoliths

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  • Transparent Macroporous Polymer Monoliths

    J. H. G. Steinke, I. R. Dunkin, and D. C. Sherrington*

    Department of Pure and Applied Chemistry, University of Strathclyde,Thomas Graham Building, 295 Cathedral Street, Glasgow G1 1XL, Scotland, U.K.

    Received December 19, 1995; Revised Manuscript Received June 12, 1996X

    ABSTRACT: Free-radical polymerization of trimethylolpropane trimethacrylate (TRIM) was carried outin the presence of a variety of porogens at room temperature, yielding transparent (macro)porous polymermonoliths. N2 BET and surface area and Hg intrusion porosimetry data from the resulting polymersshowed a variety of different pore sizes and pore size distributions. In the latter, three distinct maximaat 13, 50-60, and 500 nm were found for small pores (500 nm) did not exhibit any regularity. Variation of initiator concentration, ratio ofmonomer to porogen, polymerization temperature, and cross-linker had no influence in obtainingtransparent monoliths. Correlation of the transparency of the monoliths with the refractive index andsolubility parameter of the corresponding porogens led to threshold values for the refractive index (1.42)above which, and for the total solubility parameter (20 (MPa)1/2) below which, a transparent polymermonolith is generally obtained, if one allows for exceptions. A correlation between pore size distributionand transparency, however, could not be found.

    Introduction

    Thin films,1 liquid crystals,2 multiphase polymercomposites,3 xerogels,4 glass fibers,5 and organic-inorganic glasses6 are prominent examples of polymerswhose potential areas of application (window insula-tion,4 film coatings,1 photo7- and electrochromic8 devices,photo9- and electroluminescent10 displays, LC displays,11and optical sensors12) require materials with highoptical transparency and thus the optical properties ofthese polymers have been studied extensively.In contrast to the wealth of information available on

    optical properties of polymers prepared by the sol-gelprocess,13-15 only little is known about their organiccounterparts, i.e., highly cross-linked polymer networksbased on multifunctional cross-linkers such as divinyl-benzene (DVB) or ethylene dimethacrylate (EDMA).This is somewhat surprising, in particular when

    considering the structural similarites betweenmacroporous polymer networks, e.g., poly(HIPE) on theone hand16 and porous silica on the other.4 In both casesa high degree of control over the pore structure (poresize, pore size distribution, shape of pores, and porevolume) can be achieved. Both types of polymericnetworks suffer from becoming brittle upon drying. Inboth cases this problem is overcome by the right choiceof co-network (IPN).6,17

    Obviously, porous silica networks exhibit higherthermal stability compared to carbon-based polymers.An advantage of porous organic networks on the otherhand is the facile introduction of a wide variety offunctional groups either via suitably chosen co-monomers or via polymer-analogue functionalizationscarried out as a postmodification step. Control of both,pore structure and functionalities within the pores, incombination with optical clarity, could be of interest fora number of reasons. Photochemical reactions takingplace within the pores would present a novel approachof carrying out synthetic transformations in a defined(restricted) reaction space. So far, zeolites, container

    compounds, micelles, and dendrimers have been studiedtoward this goal, none of which offers much flexibilityand control over pore structure and the presence offunctional groups.18 Furthermore, a transparent mono-lith could be used to study the chromatographic proc-ess19 in real time by UV-vis spectroscopy or any otheranalytical technique requiring optically transparentconditions. Up to now, this has only been done eitherindirectly, by using fluorescent dyes as indicators,20 ordirectly by NMR spectroscopy,21 which lacks sensitivityand its time scale is slow compared to optical tech-niques. It is even conceivable to use chemically selectivetransparent monoliths, obtained through molecularimprinting for example, as optical sensors offering allthe advantages associated with optical spectroscopy.22The lack of studies concerned with the optical proper-

    ties of (macro)porous polymer networks and our interestin designing novel optical sensors prepared using theconcept of molecular imprinting led us to investigate thesynthesis of transparent (macro)porous polymer mono-liths in more detail. The results of this study havealready enabled us to synthesize selective, transparentpolymer monoliths. These monoliths have been turnedinto anisotropic networks by performing a photoselectivereaction on the polymer itself. The underlying conceptand the synthesis have been reported elsewhere.22Here we should like to present our efforts in the

    syntheses of transparent polymer monoliths with con-trolled pore structures based on trimethylolpropanetrimethacrylate (TRIM).

    Experimental SectionChemicals. All solvents used in polymerizations were

    purified and dried according to standard procedures. Theywere stored under argon and were kept over dried molecularsieves (4 ). Trimethylolpropane trimethacrylate (TRIM) oftechnical quality (90%) and ethylene dimethacrylate (EDMA,98%), were obtained from Aldrich Chemical Co. Ltd., and eachcompound was fractionally distilled prior to use. Azobis-(isobutyronitrile) (AIBN) and azobis(2,4-dimethylvaleronitrile)(ADVN) were of analytical grade. 2-Acrylamido-2-methylpro-panesulfonic acid (AMPSA, 99%) was obtained from Aldrich,recrystallized twice from a mixture of ethanol and water, anddried over P2O5 under vacuum. Michlers ketone (MK, 98%),supplied by Aldrich Chemical Co. Ltd., was recrystallized twicefrom ethanol/toluene and subsequently at least twice from pureethanol. It was dried over P2O5 under vacuum.

    * To whom correspondence should be addressed. Current address: Melville Laboratory for Polymer Synthesis,

    Department of Chemistry, University of Cambridge, PembrokeStreet, Cambridge CB2 3RA, UK.

    X Abstract published in Advance ACS Abstracts, July 15, 1996.

    5826 Macromolecules 1996, 29, 5826-5834

    S0024-9297(95)01875-4 CCC: $12.00 1996 American Chemical Society

  • Polymerization. The compositions of the monomer mix-tures are summarized in Tables 1, 3, 5, and 7. Standardcompositions consisted of a monomer to porogen ratio of 30:70 (v/v) and 1 mol % of free-radical initiator (to monomer).The preparation of the polymer monoliths was carried out asfollows. The initiator was weighed into a polymerizationampule. The ampule was evacuated, filled with nitrogen, andsealed before placing it just above the surface of a dry ice/ethanol slush. Cross-linker and monomers were added next,and then the porogen. A stream of dry nitrogen was bubbledthrough the polymerization mixture for 10 min. The ampulewas immersed into the slush, and nitrogen bubbling wasdiscontinued after another 5 min. The polymerization tubewas thereafter immersed in liquid nitrogen. Three freeze-pump-thaw cycles were carried out under vacuum. After thelast cycle, the ampule, which was still under vacuum, wassealed and left to warm to room temperature. The polymer-ization mixture was homogenized and finally polymerized andcured for a specified time and temperature (see Tables 1, 3, 5,and 7).Post-treatment. The polymers were dried inside the

    ampule under reduced pressure at a specified temperature andfor a specified period of time (see Tables 1, 3, 5, and 7). Theywere then ground, and the bed volume of the polymers wasmeasured using calibrated tubes, first in the dry and then inthe swollen state, allowing at lesat 72 h to reach equilibriumconditions with occasional agitation of the gels during thisperiod of time.Texture Determination. The pore structure for a pore

    radius larger than 3 nm was determined by mercury porosim-etry23 using pressure of up to 450 MPa (Micromeritics 9220).A typical pore size distribution profile (for polymer T1) isshown in Figure 1. The surface area was obtained fromadsorption measurements with nitrogen according to the BETmethod (Micromeritics Accusorb 2100E).24 Polymer sampleswere degassed in their sample holders overnight at 60 C.Porosimetry and BET data are summarized in Tables 2, 4, 6,and 8.Texture Calculations. Percentage values of pore size

    contributions for pores smaller than 10 000 nmwere calculatedfrom their corresponding pore volume (excluding any porevolume found for pore sizes greater than 10 000 nm).Transparency Assessment. The optical quality (turbid-

    ity) of polymer monoliths was assessed after polymerizationand curing of the polymer network had taken place. For thispurpose, written text was viewed through the polymer sample(about 1.5 cm diameter). A polymer monolith, through whichprinted letters appeared essentially undistorted and clear wasclassified as transparent. If images became distorted andweaker but were still recognizable when viewed through apolymer, it was then classified as translucent. Polymersamples that did not allow any pattern at all to be discernedwere categorized as opaque. In order to be able to plot physicaldata versus the optical appearance (turbidity) of each polymermonolith, transparent polymers were set arbitrarily to 1,translucent polymers to 0.5, and opaque monoliths were giventhe value 0.Determination of Unreacted Carbon-Carbon Double

    Bonds. The amount of unreacted methacrylate groups was

    determined by solid-state 13C-CP-MAS NMR spectroscopy ona 25 MHz Bruker MSL-100 solid-state spectrometer.25 Thespectra were obtained by using approximately 200 mg ofsample packed in a zirconia rotor, sealed with a KEL-F endcapand spun at ca. 5000 Hz. The recycle delay between each pulsewas 1.5 s and the CP contact time was 1 ms. For all spectrathe chemical shifts were externally referenced to TMS (0.0ppm).

    Results and Discussion

    Few publications are available which mention trans-parent and macroporous polymers. Transparency isachieved usually by refractive index matching. Damenand Neckers26 added a molecularly imprinted porousstyrene-divinylbenzene copolymer to a benzene solutionand carried out a photochemically allowed dimerizationreaction. Although they did not explicitly mention thetransparent nature of the suspension, it can be assumedthat benzene matched the refractive index of themacroporous polymer. Similarly, Wulff and Kirstein27measured the optical rotation of chiral cavities in ahighly cross-linked macroporous polymer. A blend of1,2-dichloroethane, tetrachloroethylene, and o-dichloro-benzene ensured transparency although the polymerwas prepared initially by using tetrahydrofuran (THF)as pore-forming agent. Mosbach et al.28 initiated thepolymerization of methacrylate monomers at 4 Cphotochemically at 366 nm. One has to assume eitherthat the swollen polymer stayed transparent throughoutthe polymerization or that until gelation occurred asufficient number of radicals are formed to guaranteethe further formation of the polymer network.Detailed investigations into the pore structure and

    accessibility of highly cross-linked macroporous poly-(meth)acrylamides by Shea et al.29 showed thatmacroporosity depended strongly on porogen and cross-linker. Fluorescence labeling experiments, suggestingthe presence of transparent polymer networks, revealedthe macroporous polymers to be more permeable topolar solvents than to nonpolar ones. However, differ-ences in the transparency due to choice of cross-linker,cross-linker concentration, or porogen have not beenstated explicitly.A paper by Rosenberg and Flodin in 1987 is probably

    the first, among only a few, in which the authors in factmention a transparent macroporous polymer.30 Theeffect of the porogen on the microstructure of a highlycross-linked TRIM polymer was studied. It was foundthat transparent polymers were encountered when thesolubility parameter of the porogen exceeded a thresholdvalue of 19.4 (MPa)1/2. Furthermore, they establishedthat no macroporosity was introduced when the solubil-ity parameters of polymer and porogen differed toomuch from each other.This was the background on which we based our

    study. Our initial goal was to explore compositions andreaction conditions which would allow us to synthesizetransparent polymers with defined morphology as partof a concept leading to novel optical sensors.22 However,the subject became interesting in its own right, and sowe set out to elucidate relationships between transpar-ency and morphology of these polymer monoliths.Polymer monoliths were prepared by precipitation

    polymerization at room temperature. For comparison,a small number of polymers were also prepared at 60C. Classification of the polymer monoliths into threecategories (transparent, translucent, and opaque) wasregarded as being sufficient, because for the finalapplication in mind, only a truly transparent polymer

    Figure 1. Plot of incremental intrusion versus pore diameterfor T1 from Hg porosimetry data.

    Macromolecules, Vol. 29, No. 18, 1996 Transparent Macroporous Polymer Monoliths 5827

  • could be considered. Depending on the porogen used,the mechanical stability of the polymers varied frombrittle in shear (usually high surface area) to hard andglassy (usually low surface area), although all showedgood rigidity in compression.Influence of Porogen (Polymers T1, T4-T6, and

    T10-T19). The choice of porogen was intended to covera wide range of solvents from polar species such asmethanol to relatively nonpolar ones such as toluene.A change of porogen had the most pronounced effect ontransparency and morphology of the TRIM- and EDMA-based polymers. Data on the optical appearance ofthese polymers are summarized in Tables 1 and 2.Figure 2 shows the plot of transparency of the polymermonolith versus refractive index of the correspondingporogen. Transparent polymers are observed for poro-gens with a refractive index higher than 1.42 andopaque polymers below this threshold. However, di-

    methyl sulfoxide (DMSO) with a refractive index of1.475 also gives an opaque polymer, and two otherporogens, cyclohexanone and dimethylacetamide (DMAc),which also have refractive indices higher than 1.42,yielded translucent polymers. A very similar observa-tion concerning a threshold value for the refractive indexwas made by Sellergren and Shea.31 While their mainobjective was to study the influence of polymer mor-phology on the ability to resolve enantiomers usingmolecularly imprinted highly cross-linked networks (80mol % EDMA), they found that depending on whichporogen was used either transparent, translucent, oropaque polymers were obtained... and concluded thatthe transparent polymers were prepared using porogenswith a refractive index higher than 1.40. Unfortu-nately, their study did not include the porogens that wefound not to follow this general trend. Nevertheless,both observations clearly indicate that a strong mis-

    Table 1. Compositions and Reaction Conditions for TRIM and EDMA Polymers (T1-T19) Obtained in the Presence ofDifferent...

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