Ordered Nanoporous Poly(cyclohexylethylene) - Langmuir (ACS

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Langmuir 2003, 19, 6553-6560

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Ordered Nanoporous Poly(cyclohexylethylene)† Johanna H. Wolf and Marc A. Hillmyer* Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455-0431 Received February 3, 2003. In Final Form: April 9, 2003 Nanoporous poly(cyclohexylethylene) (PCHE) monoliths were prepared from ordered poly(cyclohexylethylene)-polylactide (PCHE-PLA) diblock copolymers. The copolymers were prepared by first the catalytic hydrogenation of hydroxyl end-functionalized polystyrene (PSOH) giving hydroxyl end-functionalized PCHEOH. The hydroxyl functionality was utilized to form a metal alkoxide macroinitiator which catalyzed the polymerization of D,L-lactide. The resultant PCHE-PLA block copolymers were characterized by differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and rheological analysis. The Flory-Huggins interaction parameter, χ (estimated from rheological, SAXS, and solubility parameter data), was approximated to be 3 times that of the corresponding PS-PLA material. PCHE-PLA block copolymers containing PLA cylinders were degraded under basic conditions to remove the PLA. The resulting nanoporous monoliths showed increased thermal stability by SAXS and DSC and improved solvent resistance over nanoporous PS. These robust properties make nanoporous PCHE very desirable as a template for the synthesis of nanomaterials.

Introduction Nanoporous polymeric materials, ranging from tracketched polycarbonate thin films1 to cross-linked polystyrene beads,2 find use in diverse applications such as nanoelectrode array sensors,3 separations,2,4 and sizeselective catalysis.5 One of the most exciting uses of nanoporous polymers is for the template synthesis of nanomaterials, a straightforward technique for nanomaterial preparation1 wherein voids of an appropriate nanoporous framework are filled with the desired material, followed by selective removal of the template. What remains is the patterned material with dimensions matching the original template features. In this way, metal,6 inorganic,7 and polymer8 nanomaterials have been prepared from thin films of nanoporous materials and used in composite materials,9 in biomedical and drug delivery applications,10 as conducting nanowires,11 and for chemical sensing.12 For example, Russell and coworkers have patterned cobalt nanowire arrays in crosslinked nanoporous polystyrene thin films that show interesting magnetoelectronic properties.13 Also, Martin * To whom correspondence should be addressed. E-mail: [email protected]. † Part of the Langmuir special issue dedicated to David O’Brien. (1) Martin, C. R. Science 1994, 266, 1961-1966. (2) Janco, M.; Xie, S.; Peterson, D. S.; Allington, R. W.; Svec, F.; Frechet, J. M. J. J. Sep. Sci. 2002, 25, 909-916. (3) Jeoung, E.; Galow, T. H.; Schotter, J.; Bal, M.; Ursache, A.; Tuominen, M. T.; Stafford, C. M.; Russell, T. P.; Rotello, V. M. Langmuir 2001, 17, 6396-6398. (4) Lee, S. B.; Mitchell, D. T.; Trofin, L.; Nevanen, T. K.; Soderlund, H.; Martin, C. R. Science 2002, 296, 2198-2200. (5) Gin, D. L.; Gu, W. Adv. Mater. 2001, 13, 1407-1410. (6) Piraux, L.; Dubois, S.; Duvail, J. L.; Radulescu, A.; DemoustierChampagne, S.; Ferain, E.; Legras, R. J. Mater. Res. 1999, 14, 30423050. (7) Lakshmi, B. B.; Dorhout, P. K.; Martin, C. R. Chem. Mater. 1997, 9, 857-862. (8) Martin, C. R. Acc. Chem. Res. 1995, 28, 61-68. (9) Ash, B. J.; Schadler, L. S.; Siegel, R. W. Mater. Lett. 2002, 55, 83-87. (10) Gref, R.; Minamitake, Y.; Peracchia, M. T.; Trubetskoy, V.; Torchilin, V.; Langer, R. Science 1994, 263, 1600-1603. (11) Menon, V. P.; Lei, J. L.; Martin, C. R. Chem. Mater. 1996, 8, 2382-2390. (12) Huang, J.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc. 2003, 125, 314-315.

and co-workers have successfully templated functionalized silica nanotubes, useful in bioseparations and biocatalysis, from nanoporous alumina membranes.14 Critically important to the success of nanotemplating methodologies is the nature of the nanoporous template. Key features include controlled pore size, pore aspect ratio, thermal stability, and solvent/reagent resistance of the template. The preparation of ordered nanoporous polymer thin films from self-assembled diblock copolymers has been demonstrated.15-18 We developed a route to monolithic nanoporous polystyrene (PS) using PS-polylactide (PLA) diblock copolymer precursors.19,20 Cylinders of PLA in a matrix of PS were obtained upon microphase separation of PS-PLA materials at the appropriate composition. The cylinders of PLA were aligned and chemically etched leaving behind nanoporous PS with several potential advantages: the material is monolithic, the morphology and size of the pores can readily be controlled by varying the degree of polymerization and volume fraction of PLA, and the pore walls are lined with chemically accessible hydroxyl-functionality. These attributes are particularly attractive when using the nanoporous monoliths for template synthesis. In fact, using these porous polymer monoliths as templates, the synthesis of nanomaterials with extremely large aspect ratios (ca. g104) is possible. Further, the hydroxyl functionality on the pore walls can be readily modified to suit a particular template synthesis.21 (13) Bal, M.; Ursache, A.; Russell, T. P. Appl. Phys. Lett. 2002, 81, 3479-3481. (14) Mitchell, D. T.; Lee, S. B.; Trofin, L.; Li, N.; Nevanen, T. K.; Soderlund, H.; Martin, C. R. J. Am. Chem. Soc. 2002, 11864-11865. (15) Thurn-Albrecht, T.; Schotter, J.; Ka¨stle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126-2129. (16) Hashimoto, T.; Tsutsumi, K.; Funaki, Y. Langmuir 1997, 13, 6869-6872. (17) Liu, G.; Ding, J.; Hashimoto, T.; Kimishima, K.; Winnik, F.; Nigam, S. Chem. Mater. 1999, 11, 2233-2240. (18) Lee, J.; Hirao, A.; Nakahama, S. Macromolecules 1988, 21, 274276. (19) Zalusky, A. S.; Olayo-Valles, R.; Wolf, J. H.; Hillmyer, M. A. J. Am. Chem. Soc. 2002, 124, 12761-12773. (20) Zalusky, A. S.; Olayo-Valles, R.; Taylor, C. J.; Hillmyer, M. A. J. Am. Chem. Soc. 2001, 123, 1519-1520.

10.1021/la0341862 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/29/2003

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While the matrix PS in the aforementioned nanoporous system is not cross-linked and therefore easily removed, the synthesis of nanomaterials using this matrix is limited by the solubility properties of PS. We have found that relatively few liquids will percolate through the material without dissolving the PS matrix, thus compromising the porosity.19 Similarly, any reaction carried out within a nanoporous polymer template must be performed at a temperature below the softening point of the template material (the PS glass transition temperature (Tg) is ca. 100 °C22) to avoid “melting” the nanoporous structure.19 In principle, the strategy for nanoporous polymer synthesis from block copolymer precursors is general and can be adapted to produce a variety of nanoporous polymeric monoliths. This flexibility would allow for the synthesis of specific nanoporous polymers with the properties necessary for the desired application. As a potential nanoporous matrix with improved properties, we explored the preparation of poly(cyclohexylethylene) (PCHE) monoliths. The saturation of the aromatic ring in PS gives the new polymer increased resistance to oxidative degradation and improved thermal stability (PCHE’s Tg is ca. 147 °C).23 Furthermore, because of its hydrocarbon nature, PCHE is in principle “resistant” to a wider range of liquids than PS. We also expect PCHE-PLA diblock copolymers to have a large Flory-Huggins interaction parameter, χ. This is attractive because the bigger the χ parameter for a block copolymer, the lower the molecular weight necessary for microphase separation, allowing for the synthesis of nanoporous materials with smaller pore sizes.24 In this paper, we describe the preparation of ordered nanoporous PCHE monoliths and demonstrate the significantly improved solvent resistance and higher thermal stability than the corresponding PS materials. We began with the preparation of hydroxyl-terminated PCHE (PCHEOH) by the hydrogenation of well-defined hydroxylterminated PS (PSOH) samples. PCHEOH was used as a macroinitiator precursor for the formation of PCHEPLA diblock copolymers. The majority of the copolymers formed cylinders of PLA in a matrix of PCHE. The PLA in these materials was etched, resulting in ordered nanoporous PCHE monoliths. Experimental Section General Methods. All 1H NMR spectra were obtained using a Varian 300 VI spectrometer. Polymer samples were dissolved in deuterated chloroform (Cambridge) at approximate concentrations of 1.0 wt %. The size exclusion chromatography (SEC) experiments were performed on a Hewlett-Packard 1100 series liquid chromatograph equipped with Jordi poly(divinylbenzene) columns with pore sizes of 10000, 1000, and 500 Å and a HewlettPackard 1047A differential refractometer. The experiments were performed at 40 °C with tetrahydrofuran (THF) as the mobile phase at a flow rate of 1 mL/min. The differential scanning calorimetry (DSC) experiments were carried out using a PerkinElmer DSC-7, calibrated with an indium standard. Nitrogen was used as the purge gas. The samples were scanned at 10 °C/min. Materials. Styrene (Aldrich) was stirred over calcium hydride for at least 12 h followed by distillation to a flask containing dibutylmagnesium. The styrene was stirred for 2 h in that flask and then transferred to a flame-dried buret. Ethylene oxide (Aldrich) was purified in the same manner as styrene. The concentration of sec-butyllithium (Aldrich) was verified by the (21) Tsutsumi, K.; Funaki, Y.; Hirokawa, Y.; Hashimoto, T. Langmuir 1999, 15, 5200-5203. (22) Hahnfeld, J. L.; Dalke, B. D. Encyclopedia of Polymer Science and Engineering, 2nd ed.; Wiley: New York, 1989. (23) Hucul, D. A.; Hahn, S. F. Adv. Mater. 2000, 12, 1855-1858. (24) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998.

Wolf and Hillmyer Table 1. Characterization of PSOH Samples samplea

Mn (kg/mol)b

Xnc

Mn (kg/mol)d

Mw/Mnd

[OH]/[s-Bu]e

PSOH(4.9) PSOH(24) PSOH(40) PSOH(56)

4.9 24 40 56

47 230 390 540

4.1 22 38 48

1.04 1.03 1.03 1.05

1.01 0.95 1.08 1.07

a PSOH(X) ) hydroxyl-terminated polystyrene with a numberaverage molecular weight X kg/mol by 1H NMR analysis. b PSOH number-average molecular weights were determined from 1H NMR end-group analysis and agree with the expected molecular weight from the molar ratio of initiating species to monomer present in the initial reaction solution. c Degree of polymerization calculated by dividing the molecular weight (1H NMR spectroscopy) by 104.15 g/mol. d SEC versus polystyrene standards. e The molar ratio of the methylene (adjacent to the hydroxyl group) protons of the ethoxy end-group to the methyl protons of the s-butyl initiator, a measure of the level of functionalization.

Gilman double-titration method.25 Purified and degassed cyclohexane and toluene were obtained by passage through an activated alumina column to remove protic impurities followed by passage through a supported copper catalyst to remove oxygen using a home-built purification system described previously.26 The solvents were collected into air-free flame-dried flasks using a Schlenk manifold. D,L-Lactide was recrystallized from ethyl acetate and was dried at room temperature under a vacuum, after which its purity was verified by 1H NMR spectroscopy. Ultrahigh-purity-grade hydrogen gas (99.995%) was used without further purification. All other solvents and reagents were used as received without further purification. Synthesis of PSOH. The synthesis of model PSOH by living anionic polymerization has been described in a previous publication.19 A general procedure for the synthesis of 104 g of PSOH(40) from Table 1 is detailed below. The polymerization was performed in a 2-L flask equipped with five internal threaded glass connectors and a Teflon-coated stir bar. Using Ace Glass bushings and FETFE O-rings, the reactor was fitted with three glass plugs, a thermowell, and a three-port Y-connector. One port interfaced the argon/vacuum manifold, a second interfaced a manometer through Teflon valves, and the third port was plugged with a septum. The reactor was evacuated to nearly 10-3 Torr and heated to 270 °C overnight. After the reactor had cooled, the three glass plugs were replaced with an air-free flask containing purified cyclohexane (0.9 L), a buret of purified styrene (103.9 g, 0.999 mol), and a buret of purified ethylene oxide (15.0 g, 0.341 mol) under positive argon pressure. The buret of ethylene oxide was connected to the reaction flask via a flexible Cajon Ultratorr fitting so that the buret could be immersed in an ice bath to keep the ethylene oxide pressure low. The reactor was evacuated and backfilled with argon five times during which the pressure was monitored to check for leaks in the system. The cyclohexane was added to the reactor followed by the addition of sec-butyllithium (2.2 mL of 1.20 M sec-butyllithium in cyclohexane) with an airtight syringe through the septum. The reaction solution was heated to 45 °C in a water bath. The styrene was added, and the solution turned an orange-red color indicating the presence of poly(styryllithium) chain ends. After 4 h, the reaction solution was cooled to room temperature and the ethylene oxide was added, turning the reaction solution colorless. The solution was stirred for 12 h, after which the reactor was opened under positive argon pressure and the reaction was terminated with acidic methanol. The polymer was precipitated in a 50:50 (by volume) mixture of 2-propanol and methanol, isolated by vacuum filtration, and dried under vacuum at 110 °C for 12 h. The isolation yield was 99.6 g (95.9%). SEC analysis gave Mn ) 38.0 kg/mol and Mw/Mn ) 1.04. 1H NMR end-group analysis gave 40.0 kg/mol and hydroxyl end-group functionality of 1.08. Hydrogenation of PSOH. The procedure used for the hydrogenation of PSOH is similar to a previously reported method for the hydrogenation of PS.27 The hydrogenation of PSOH(24) (25) Gilman, H.; Cartledge, F. K. J. Organomet. Chem. 1964, 2, 447454. (26) Schmidt, S. C.; Hillmyer, M. A. Macromolecules 1999, 32, 47944801.

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Table 2. Characterization of PCHEOH Samples samplea

Mn (kg/mol)b

Xnc

Mn (kg/mol)d

Mw/Mnd

H2 conversion (%)e

functionalityf

PCHEOH(5.2)g

4.5 25 28 45 42

41 227 254 409 373

4 22 19 33 40

1.05 1.03 1.03 1.03 1.04

99.4 98.1 99.8 98.0 96.9

1.15 1.02 0.91 0.94 1.44

PCHEOH(25)ag PCHEOH(25)bh PCHEOH(42)h PCHEOH(59)h

a PCHEOH(X) ) hydroxyl-terminated poly(cyclohexylethylene) with number-average molecular weight X kg/mol calculated using the molecular weight of the PSOH starting materials (see Table 1). b PCHEOH molecular weights determined from 1H NMR spectroscopy end-group analysis. This value is lower than the expected molecular weight for the samples with over 100% functionality and is higher than expected for other samples because some of the hydroxyl end-groups may have been lost during hydrogenation. c Degree of polymerization calculated from molecular weight determined by 1H NMR spectroscopy. d SEC versus polystyrene standards. e Calculated by 1H NMR spectroscopy from the molar ratio of the unsaturated aromatic protons of polystyrene to the aliphatic protons of the hydrogenated species. f Functionality calculated by X (PSOH)/X (PCHEOH); see Table 1. g Hydrogenated with Pd on CaCO catalyst. h Hydrogenated with DHC n n 3 catalyst.

to form PCHEOH(25)b in Table 2 is described below. The PSOH(24) (6.0 g, 4.2 × 10-5 mol) was dissolved in 200 mL of purified cyclohexane. This solution was placed in a 300-mL high-pressure solution delivery vessel (SDV). The solution was degassed by bubbling argon through the dip tube outlet valve for 30 min. The catalyst, Pd on CaCO3 (Aldrich) (15.4 g, 2.5:1 catalyst to PSOH(24) by weight), was sealed in a Pressure Products Industries 300-mL baffled, high-pressure reactor fitted with a Dyna/Mag magnetic drive mixer and a gas dispersion impeller. The catalyst was baked under a vacuum at 100 °C overnight, after which it was activated with 100 psig H2 for 1 h at 100 °C. The reactor was vented and flushed three times with argon and left under 20 psig argon at 100 °C. The SDV was attached to the reactor through the dip tube outlet valve via 1/4 in. stainless steel tubing. The solution was transferred to the sealed reactor through the dip tube outlet valve under 140 psig argon. The argon was vented from the reactor, the temperature was increased to 120 °C, and stirring was begun at a rate of 1500 rpm. The reactor was charged with 500 psig H2. The drop in H2 pressure with time indicated the progress of the reaction. The reactor was recharged to 500 psig H2 several times, and the reaction was allowed to run overnight to ensure complete saturation. The remaining H2 was vented from the reactor. The catalyst was removed by a highpressure filter can fitted with 0.22-micron filter paper (Millipore) under 40 psig argon. The polymer product was precipitated into 1 L of methanol, collected by vacuum filtration, and dried at 150 °C under a vacuum for 12 h. The mass of polymer recovered was 4.8 g (80%). The yield was low due to difficulty in removing the polymer from the catalyst slurry. SEC showed that the low molecular weight distribution was preserved, Mw/Mn ) 1.03, in the PCHE product. The molecular weight shifted to higher elution volume due to the smaller hydrodynamic volume of the hydrogenated polymer chains in the SEC mobile phase, THF.28 The 1H NMR spectrum showed a conversion of 98.1% and 1.02 hydroxyl end-group functionalization. The same procedure was used for hydrogenations performed with Dow Hydrogenation Catalyst (DHC) except that the reaction temperature was 160 °C. DHC is 5 wt % Pt supported on wide-pore silica.28 Synthesis of PCHE-PLA. All block copolymers were synthesized by the general reaction scheme outlined previously.19 Polymerizations were performed in high-pressure vessels equipped with internal threads and Teflon bushings fitted with a Viton O-ring (Chemglass). The vessels were treated with a 10/90 (by volume) solution of dichlorodimethylsilane/methylene chloride and dried at 120 °C for 1 h. Lactide polymerizations were assembled and sealed in a drybox and performed in dry toluene with [lactide]o e 1.0 M. A molar ratio of PCHEOH to triethylaluminum (Et3Al) of at least 1.5 was used to form an aluminum alkoxide macroinitiator. The preparation of CL(44, 0.37) from Table 3 is described here. A Teflon-coated stir bar was placed in a silanized 48-mL high-pressure reaction vessel along with 1.0 g (4.4 × 10-5 mol) of PCHEOH(25)b from Table 2 and 6.0 mL of dry toluene. Using a syringe, 22 µL of a 1.0 M Et3Al solution in hexanes (2.2 × 10-5 mol) was added. The solution was stirred (27) Ness, J. S.; Brodil, J. C.; Bates, F. S.; Hahn, S. F.; Hucul, D. A.; Hillmyer, M. A. Macromolecules 2002, 35, 602-609. (28) Zhao, J.; Hahn, S. F.; Hucul, D. A.; Meunier, D. M. Macromolecules 2001, 34, 1737-1741.

Table 3. Characterization of PCHE-PLA Samples

samplea CL(10, 0.43) CL(24, 0.74) CL(30, 0.14) CL(38, 0.28) CL(39, 0.30) CL(42, 0.34) CL(44, 0.37) CL(68, 0.32) CL(86, 0.26) CL(90, 0.29) CL(111, 0.40)

Mn Mn (kg/mol) (kg/mol) PCHEb PLAc 5.2 5.2 25 25 25 25 25 42 59 59 59

5.1 19 5.2 13 14 17 19 26 27 32 52

Nd

fPLAe Mw/Mnf D (nm)g

83 180 260 320 320 350 360 560 720 760 900

0.43 0.74 0.14 0.28 0.30 0.34 0.37 0.32 0.26 0.29 0.40

1.17 1.29 1.06 1.11 1.14 1.17 1.14 1.11 1.14 1.16 1.37

12.6 29.9 10.2 19.6 25.8 30.2 30.7 48.0 31.0 45.2 59.3

a CL(X, Y) ) PCHE-PLA with total molecular weight (X kg/ mol) and fPLA (Y). b See Table 2. c The number-average molecular weight of PLA was determined from 1H NMR spectra. d The degree of polymerization was calculated from the number-average molecular weights determined from 1H NMR analysis, the polymer densities at 187 °C (FPCHE ) 0.849 g/cm3 and FPLA ) 1.114 g/cm3) (refs 27 and 36), and a reference volume of 215 Å3 where Ni ) Mni/(FiVrefNA) and N ) NPLA + NPCHE. e fPLA ) NPLA/(NPLA + NPCHE). f SEC versus polystyrene standards. This value includes the PCHE homopolymer contamination. g Cylinder diameter calculated from the SAXS principal peak position at RT, q*, where the intercylinder spacing a ) 4π/(x3 q*) and D ) 2a(x3/2π)1/2fPLA1/2 except for sample CL(24, 0.74) which is lamellar and has the characteristic domain spacing listed.

at room temperature for about 30 min to allow the formation of the macroinitiator. D,L-Lactide (0.9 g, 6.25 × 10-3 mol) was added, and the reactor was sealed and placed in a 90 °C oil bath for 4 h. The reaction was quenched with acidic methanol, and the polymer was precipitated in 400 mL of methanol and isolated by vacuum filtration. The product was dried at 115 °C under a vacuum for 10 h. The conversion of lactide was 66% based on isolation yield assuming complete recovery of the PCHE. This agreed with the 1H NMR result giving a total molecular weight of 44 kg/mol. The SEC trace showed that the molecular weight distribution remained low (Mw/Mn ) 1.14), but there was a small shoulder attributed to unreacted PCHE homopolymer. Integration of the SEC peaks showed that in CL(44, 0.37) the homopolymer contamination was at most 1.4 wt %. The validity of this comparison was shown through the method of standard additions. For a set of PCHE-PLA samples deliberately contaminated with a known amount of PCHE homopolymer, the experimentally determined weight percent of the impurity was higher than the true value. 1H NMR Characterization of PSOH, PCHEOH, and PCHE-PLA. The following 1H NMR resonances are representative of the polymers described in this paper. All resonances are reported in parts per million (δ) downfield from tetramethylsilane (0.0 ppm). All of the resonances observed were either broad (b) or contained multiple overlapping peaks (m). PSOH: 6.3-7.2 (m, aromatic protons), 3.3 (b, PS-CH2-CH2-OH), 1.2-1.7 (m, -CH(C6H5)-CH2-), 0.9 (b, -CH2- of initiator), 0.7 (m, -CH3 of initiator). PCHEOH: 3.6 (b, PCHE-CH2-CH2-OH), 1.0-2.0 (b, -CH(C6H11)-CH2-), 0.7-1.0 (b, cyclic -CH2-). PCHE-

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PLA: The PCHE resonances are the same as above except that the diblocks have a (b, PCHE-CH2-CH2-O-PLA) resonance at 4.2 ppm. PLA resonances are 5.2 (m, -C(O)-CH(CH3)-(O)-) and 1.6 (m, -C(O)-CH(CH3)-(O)-). Alignment of Microstructure. The microstructure of the PCHE-PLA diblock copolymers was aligned through channel die processing as described previously.19 The powder samples were pressed into a 0.5-mm-thick stainless steel mold between two Teflon sheets at 200 °C. The microstructure was aligned using a home-built channel die lined with Teflon (2 mm wide and 55 mm long).29 A pressed polymer piece was placed into the center of the channel and the plunger, also lined with Teflon, was placed on top of the sample. The system was heated to 200 °C in a laboratory press for several minutes to soften the polymer. The plunger was pressed onto the sample causing the polymer to flow toward the ends of the channel. A typical compression ratio was 8. This procedure caused the PLA cylinders to align parallel to the channel. The samples were allowed to cool slowly (over several hours) before removal from the channel die. The degree of alignment was determined by SAXS. Degradation of PLA in PCHE-PLA Samples. PLA was hydrolytically degraded from samples processed in the channel die by placing them in a 0.5 M solution of sodium hydroxide in a 40:60 (by volume) methanol/water mixture. The samples were heated to 65 °C in an oil bath (well below the glass transition temperature of PCHE and above that of PLA) for 5-10 days. The samples were then removed from the degradation solution, washed with methanol and water, and dried overnight at room temperature under a vacuum. The mass loss of these pieces was consistent with removal of all of the PLA, and the 1H NMR analysis showed the absence of the PLA methine resonance at 5.2 ppm. SAXS. All samples were aligned in a channel die as described above before SAXS analysis. SAXS measurements were taken at the University of Minnesota on a home-built line. Copper KR X-rays with a wavelength of 1.542 Å were generated through a Rigaku RU-200BVH rotating anode X-ray machine fitted with a 0.2 × 2 mm2 microfocus cathode and Franks mirror optics. The sample was mounted in a brass block (temperature controlled by electrical heating and water cooling) in a vacuum-sealed chamber. Two-dimensional (2-D) diffraction patterns were recorded using a Siemens multiwire detector and were corrected for detector response before analysis. The 2-D scattering patterns were azimuthally integrated to a one-dimensional (1-D) plot of intensity versus q ) 4πλ-1 sin(θ/2), where λ and θ are the radiation wavelength and scattering angle, respectively. The detector distance was 230 cm for most samples, and data were typically collected for a 10-15 min period for diblock copolymer samples and a 0.5-1 min period for degraded samples. The samples were examined with the X-ray beam probing the samples perpendicular to the preferred direction of the PLA microstructure or cylindrical pores. To quantify the alignment of the microstructure, the 2-D scattering patterns were reduced to a 1-D plot of intensity versus azimuthal angle, β, by summing the intensity of a region, ∆q, equal to the full width at half-maximum of the principal peak (q*). Scanning Electron Microscopy (SEM). SEM samples were prepared by cutting the nanoporous monoliths with a razor blade and mounting them onto brass shims with colloidal graphite (Ted Pella). The samples were sputter-coated with 2-3 nm of platinum (estimated from a calculated deposition rate and experimental deposition time). SEM analysis was performed on a Hitachi S-900 FE-SEM using accelerating voltages of 3-5 keV. Dynamic Mechanical Analysis. All dynamic mechanical data were obtained from a Rheometrics Solid Analyzer RSA II in a shear sandwich geometry (0.5 mm thickness). A strain sweep was performed and showed that strain amplitudes of 0.75% placed the samples in the linear viscoelastic regime. Temperature ramps were conducted at a rate of 1 °C/min at a frequency of 0.5 rad/s. The dynamic storage (G′) and loss (G′′) moduli were recorded as a function of temperature. The samples were thermostated in a controlled nitrogen atmosphere and were not heated above 230 °C to avoid thermal degradation of PLA. (29) Drzal, P. L.; Barnes, J. D.; Kofinas, P. Polymer 2001, 42, 56335642.

Wolf and Hillmyer

Figure 1. (a) Anionic polymerization of styrene to form endfunctionalized PSOH. (b) Hydrogenation of PSOH to PCHEOH. (c) Formation of macroinitiator and controlled polymerization of D,L-lactide to produce PCHE-PLA.

Results and Discussion Synthesis of Hydroxyl-Functionalized Poly(cyclohexylethylene). Model PCHEOH samples were prepared by the catalytic hydrogenation of well-defined PSOH. The PSOH starting materials were synthesized by the anionic polymerization of styrene followed by the addition of ethylene oxide to cap the living chain ends with hydroxyl end-groups (Figure 1a).20,30,31 The polymers exhibited narrow molecular weight distributions by SEC (Mw/Mn e 1.05), and analysis of 1H NMR spectra showed near quantitative hydroxyl end-group incorporation (Table 1). Further evidence for the end-capping efficiency was the successful synthesis of PS-PLA diblock copolymers devoid of PS homopolymer (SEC) using each of the PSOH starting materials.19 The number-average molecular weights of the PSOH samples determined using 1H NMR spectroscopy by end-group analysis were used in all subsequent calculations. Preservation of the hydroxyl end-groups upon hydrogenation23,32 of the PSOH samples is imperative, as this functionality acts as a reactive handle in the subsequent formation of the PLA diblock copolymers. The PSOH samples were hydrogenated using two catalysts, Pd on CaCO3 and DHC (see Experimental Section), to give the corresponding saturated materials (Figure 1b). SEC showed that the hydrogenated polymers retained the narrow molecular weight distributions of their PSOH precursors (Figure 2 and Table 2). The reactions resulted in essentially complete hydrogenation, verified by analysis of the 1H NMR spectra (Figure 3). The preservation of hydroxyl end-groups upon hydrogenation was verified by 1H NMR spectroscopy. The methylene protons alpha to the hydroxyl of the ethylene oxide end-group give a 1H NMR signal at 3.6 ppm, while the saturated PCHE repeat units are observed at 0.7-2.0 ppm. The relative intensities of these peaks were used to calculate the number-average molecular weight by comparing the molar ratio of the end-groups to the PCHE repeat units. If each of the hydroxyl end-groups was preserved during the hydrogenation reaction, the PCHE number-average degree of polymerization calculated from 1H NMR spectroscopy will equal that of the PSOH starting material. Therefore, the functionality of the PCHEOH (30) Quirk, R. P.; Mathers, R. T.; Wesdemiotis, C.; Arnould, M. A. Macromolecules 2002, 35, 2912-2918. (31) Quirk, R. P.; Ma, J.-J. J. Polym. Sci., Part A: Polym. Chem. 1988, 26, 2031-2037. (32) Gehlsen, M. D.; Bates, F. S. Macromolecules 1994, 27, 36113618.

Ordered Nanoporous Poly(cyclohexylethylene)

Figure 2. SEC data for (a) PSOH(24) from Table 1, (b) PCHEOH(25)b from Table 2, (c) CL(44, 0.37) from Table 3 shifted to a higher molecular weight on formation of the PLA diblock and showing signal from a small amount of unreacted poly(cyclohexylethylene) starting material, and (d) degraded CL(44, 0.37) which matches the PCHEOH starting material upon removal of the PLA.

Figure 3. 1H NMR spectra of (a) PCHEOH(25)b from Table 2, (b) CL(44, 0.37) from Table 3 with methine resonance at 5.2 ppm indicating the presence of PLA, and (c) degraded CL(44, 0.37) with the absence of the PLA peak at 5.2 ppm. The insets show (a) the ethoxy end-group signal at 3.6 ppm for PCHEOH(25)b which (b) shifts to 4.2 ppm in the diblock copolymer and (c) shifts back to 3.6 ppm on degradation of the PLA.

can be determined by comparison of Xn(PSOH) to Xn(PCHEOH). In all cases, the saturated polymers retained a high degree of functionality (Table 2). Unfortunately, this calculation is complicated because accurate integration of the end-group resonances relative to the PCHEOH aliphatic peaks in the 1H NMR spectra is difficult (especially at high molecular weight), resulting in calculated functionalities greater than 1 (as in the data given in Table 1). While both hydrogenation catalysts gave products with virtually complete conversion, assuming accurate integration values, the DHC catalyst resulted in a loss of up to 9% of the end-functionality by 1H NMR analysis (Table 2).

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Synthesis of PCHE-PLA. PCHE-containing diblock copolymers, typically prepared by the hydrogenation of unsaturated diblock precursors,32 can be prepared using PCHEOH materials as macromolecular initiators. The first step in the formation of a PCHE-PLA diblock copolymer is to react the hydroxyl-terminated PCHEOH with triethylaluminum to form the corresponding macroinitiator.26,33-35 This PCHE macroinitiator was exposed to D,L-lactide, resulting in the growth of PLA from the end of the PCHE (Figure 1c). We have used this type of change of mechanism polymerization to synthesize several PLA-containing block copolymers.26,33,36 1H NMR spectroscopy was used to verify the presence of PLA and to determine the volume fraction of PLA (fPLA) in the copolymers (Table 3).28,37 Using this methodology, we synthesized PCHE-PLA diblock copolymers with fPLA values ranging from 0.14 to 0.74 (Table 3). SEC showed an increase in molecular weight and broadening of the molecular weight distribution upon formation of the diblocks. When the diblock copolymer SEC trace is compared to that of the PCHEOH starting material, unreacted poly(cyclohexylethylene) homopolymer in the system is evident (Figure 2). Typically, homopolymer contamination comprised a small (ca. 5%) weight percent of the total sample by integration of the SEC peak areas (see Experimental Section). At this time, we are unsure if the homopolymer contamination is a result of unreacted hydroxyl groups during the formation of the macroinitiator (perhaps due to the steric effects of the bulky PCHE) or if the hydroxyl functionality was lost during the hydrogenation reaction (beyond detection of the aforementioned 1H NMR analysis). Despite the homopolymer contamination, pressed samples are optically clear, suggesting a lack of macrophase separation.38 DSC analysis of a representative PCHE-PLA diblock (CL(44, 0.37) in Table 3) shows a Tg for PCHE at 140 °C and a Tg for PLA at 50 °C, comparable to the bulk Tg values of 147 and 57 °C, respectively.23,39 With the goal of nanoporous material synthesis in mind, we targeted the cylindrical microstructure. The ordered state symmetry of the PCHE-PLA materials was determined by SAXS analysis. SAXS samples were prepared by alignment of the microstructure in a channel die at 200 °C followed by slow cooling (see Experimental Section). We assume SAXS data at 20 °C are indicative of the equilibrium microstructure at the Tg of PCHE (the majority component in most samples). All scattering patterns were consistent with a cylindrical microstructure except for that of CL(24, 0.74), which displayed a lamellar microstructure. A representative 1-D SAXS pattern for CL(44, 0.37) is shown in Figure 4. By synthesizing a range of materials with various fPLA values, we were able to create cylinders of PLA in a PCHE matrix ranging from 13 to 60 nm in diameter with center to center distances ranging from 19 to 87 nm (Table 3). PCHE-PLA χ Parameter. The χ parameter for a diblock copolymer of known degree of polymerization, N, (33) Wang, Y.; Hillmyer, M. A. Macromolecules 2000, 33, 7395-7403. (34) Kricheldorf, H.; Berl, M.; Scharnagle, N. Macromolecules 1988, 21, 286-293. (35) Dubois, P.; Jerome, R.; Teyssie, P. Makromol. Chem., Macromol. Symp 1991, 42/43, 103-116. (36) Schmidt, S. C.; Hillmyer, M. A. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 2364-2376. (37) Witzke, D. R.; Narayan, R.; Kolstad, J. J. Macromolecules 1997, 30, 7075-7085. (38) The indices of refraction, nD, for PCHE and PLA are 1.51 and 1.47, respectively. See ref 23 and: Bartus, J.; Weng, D.; Vogl, O. Polym. Int. 1994, 34, 4333-4342. (39) Jamshidi, K.; Hyon, S.-H.; Ikada, Y. Polymer 1988, 29, 22292234.

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theory estimations of χN at the TODT to calculate a minimum χ value for the PCHE-PLA system at 230 °C. The mean-field theory estimate for (χN)ODT at fPLA ) 0.43 is 11.41 This corresponds to a minimum χ value of 0.13 at 230 °C. For comparison, the χ for PS-PLA at 230 °C is significantly lower (ca. 0.08).19 The phase behavior of CL(10, 0.43) as a function of temperature was also probed by SAXS to 200 °C. The scattering pattern indicative of a cylindrical microstructure was unaffected, verifying the lack of either an orderdisorder or order-order phase transition for CL(10, 0.43) below 200 °C. Using the principal domain spacing, D, of the polymer microstructures obtained from SAXS, we also obtained χ using the degree of polymerization, N, and statistic segment length, a, and eq 1. Figure 4. SAXS pattern for (a) aligned CL(44, 0.37) in Table 3 and (b) degraded CL(44, 0.37) showing that the cylindrical microstructure was preserved on degradation. The principal peaks (d100) are marked by diamonds, and the expected reflections for a cylindrical microstructure (d110, d200, d210, d300, d220, and d310) are marked by triangles.

Dlam ) 1.10aN2/3χ1/6

(1)

This relationship was developed for diblock copolymers with a lamellar morphology in the strong-segregation regime (χN g 100).43 The value of Dlam for CL(24, 0.74) (the only lamellar sample we prepared) was determined at 187 °C by SAXS analysis and the relationship given in eq 2

Dlam )

2π q*

(2)

where q* is the principal peak scattering. The statistical segment length for PCHE-PLA was determined from a volume fraction weighted average19 of statistical segment length of each block (aPCHE ) 7.1 Å and aPLA ) 10.6 Å)44,45 resulting in a ) 9.2 Å. This calculation gives χPCHE-PLA(187 °C) ) 0.33, which is over 3 times that of PS-PLA, χPS-PLA(187 °C) ) 0.10.19 A third estimation of χPCHE-PLA can be made based on solubility parameters using eq 3,

χ) Figure 5. Temperature dependence of the dynamic storage modulus, G′ (circles), and the loss modulus, G′′ (diamonds), for CL(10, 0.43) in Table 3 (1 °C/min, 0.5 rad/s, and 0.75% strain). The decrease in modulus around 140 °C is a result of the polymer softening at its glass transition temperature. Up to 230 °C, there is no evidence of an order-disorder transition.

and volume fraction, f, can be calculated at the experimentally determined order-disorder transition (ODT) temperature (TODT) using mean-field theory estimation of χN at the ODT.40,41 The ODT can be detected by measuring the dynamic elastic modulus, G′, as a function of temperature at low frequency and low strain. The TODT is indicated by a sharp decrease in G′ as the microphaseseparated blocks become disordered.42 Representative rheological data on the lowest molecular weight diblock sample, CL(10, 0.43), are shown in Figure 5. Up to 230 °C, the only drop in modulus is around 140 °C and is attributed to the polymer softening at the Tg of the PCHE component. Assuming that at a high enough temperature the dislike polymer segments will eventually mix (i.e., these materials are characterized by an upper critical solution temperature), we can use mean-field (40) Leibler, L. Macromolecules 1980, 13, 1602-1617. (41) Matsen, M. W.; Bates, F. S. Macromolecules 1996, 29, 10911098. (42) Rosedale, J. H.; Bates, F. S. Macromolecules 1990, 23, 23292338.

Vref (δ - δb)2 RT a

(3)

where Vref is the segment reference volume (see Table 3) and δi is the solubility parameter of polymer i. Since a definitive value of δPCHE is unknown, χPCHE-PLA can be predicted from a relationship based on the difference in solubility parameters of common blocks of related diblock copolymers,46

∆δac(T) ) ∆δab(T) + ∆δbc(T)

(4)

where ∆δij(T) is the difference in solubility parameters between polymer i and j at a given temperature and provided δa > δb > δc. This calculation utilizes data from two studies using diblock copolymers that each contain one segment of the copolymer in question and a second common block: poly(ethylene-alt-propylene) (PEP)-PLA and PCHE-PEP. For both of these polymers, χ(T) has been determined, and at 187 °C χPEP-PLA ) 0.35 and χPCHE-PEP ) 0.031.36,47 The ∆δij(T) values necessary in eq (43) Semenov, A. N. Sov. Phys. JETP 1985, 61, 733-742. (44) Gehlsen, M. D.; Weimann, P. A.; Bates, F. S.; Harville, S.; Mays, J. W.; Wignall, G. D. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 1527-1536. (45) Joziasse, C. A.; Veenstra, H.; Grijpma, D. W.; Pennings, A. J. Macromol. Chem. Phys. 1996, 197, 2219-2229. (46) Almdal, K.; Hillmyer, M. A.; Bates, F. S. Macromolecules 2002, 35, 7685-7691. (47) Cochran, E. W.; Bates, F. S. Macromolecules 2002, 35, 73687374.

Ordered Nanoporous Poly(cyclohexylethylene)

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Figure 6. SEM image of (a) degraded CL(44, 0.37) in Table 3 showing hexagonally packed pores and (b) degraded CL(42, 0.34) in Table 3 fractured parallel to the pores showing the preferred direction of the pores.

4 can be calculated from the known χ(T) values and eq 3 using a Vref of 215 Å3.48,49 At 187 °C, ∆δPCHE-PEP ) 0.92 J1/2/cm3/2 and ∆δPEP-PLA ) 3.21 J1/2/cm3/2. From previous work, δPCHE < δPEP < δPLA,36,47 and therefore ∆δPCHE-PLA is the largest solubility parameter difference and is equal to 4.13 J1/2/cm3/2. Using this value of ∆δPCHE-PLA results in χPCHE-PLA(187 °C) ) 0.58. Given the accuracy of this analysis, the χPCHE-PLA(187 °C) values from both SAXS and solubility parameter analysis are in fair agreement. From the relatively large value of χ estimated for PCHE-PLA, we can anticipate phase separation at fairly low degrees of polymerization. For example, using a value of χPCHE-PLA ) 0.33 and the mean-field theory estimation of χN at the ODT at fPLA ) 0.43, we predict phase separation into cylinders at N ) 33. This corresponds to a cylinder diameter of approximately 5 nm. Synthesis and Characterization of Nanoporous PCHE. The PCHE-PLA samples from Table 3 that form cylinders of PLA in a matrix of PCHE were utilized for the synthesis of nanoporous materials. The materials were typically aligned using a channel die (see Experimental Section). As reported previously, the degree of alignment was assessed by calculation of the second-order orientation factor, F2, from the 2-D SAXS patterns.19 Typical F2 values for channel die processed PCHE-PLA copolymers exceed a value of 0.8, comparable to the degree of alignment found in the related PS-PLA. The aligned monolithic materials were immersed in a basic degradation solution at 65 °C. This mild treatment selectively degraded the PLA, leaving the PCHE matrix. Mass loss on degradation was consistent with that expected based on the PLA content of the diblocks. 1H NMR spectroscopy and SEC measurements showed that the degraded samples matched the PCHEOH starting materials (Figure 2 and Figure 3). The etched polymers were analyzed by SAXS, and retention of the SAXS principal peak and the higher order reflections demonstrated that the cylindrical microstructure was preserved through the process of PLA degradation (Figure 4), although there was a slight shift of the principal peak to larger q* postdegradation. The F2 values of the degraded monoliths were comparable with those of the diblock copolymer materials. The degraded materials were then analyzed by SEM to obtain real-space images of the putative nanoporous (48) Fetters, L. J.; Lohse, D. J.; Richter, D.; Witten, T. A.; Zirkel, A. Macromolecules 1994, 27, 4639-4647. (49) Hattam, P.; Gauntlett, S.; Mays, J. W.; Hadjichristidis, N.; Young, R. N.; Fetters, L. J. Macromolecules 1991, 24, 6199-6209.

structures. The degraded samples were first fractured and then coated with Pt to avoid charging of the samples. Observation of the cuts perpendicular to the cylindrical axis revealed the hexagonal close packing of the pores, and when the samples cut parallel to the cylindrical axis were examined, the orientation of the channels was obvious. Figure 6 shows two representative samples with good alignment and a relatively narrow pore size distribution. The diameter of the pores in Figure 6a is about 20 nm, in reasonable agreement with the pore diameter determined by SAXS, 31 nm (see Table 3), after accounting for the Pt coating (2-3 nm). We expected nanoporous PCHE to be more solvent resistant than the nanoporous PS analogues. This was verified by comparison of the solubility of nanoporous PCHE and nanoporous PS materials in several liquids. While methanol is able to percolate through nanoporous PS (24 kg/mol) without affecting the porous structure,19 nanoporous PS completely dissolves in pyridine, ethyl acetate, and acrylic acid. When CL(44, 0.37) was exposed to those liquids, the matrix did not dissolve and the liquids percolated into the pores of the PCHE monoliths. This was verified by the mass gain of the filled monoliths based on estimated pore volume and the densities of the liquids. After filling, the monoliths were dried under reduced pressure to their original weight and analyzed by SAXS. Scattering data from the solvent-treated PCHE monoliths were identical to those of the original nanoporous materials, verifying that the nanoporous structure was retained after being subjected to these liquids. We also proposed that the nanoporous PCHE materials would have high thermal stability due to the high Tg of PCHE. To probe the thermal stability of the nanoporous PCHE material, CL(44, 0.37) was monitored by SAXS while it was slowly heated within the sample chamber. The room-temperature 2-D SAXS pattern of a nanoporous material taken perpendicular to the cylinder axes showed the expected two-spot scattering pattern. As the polymer softened with increasing temperature, the two-spot scattering pattern was lost at ca. 136 °C presumably as a result of pore collapse. The same nanoporous PCHE sample was also analyzed by DSC. Upon steady heating of the sample, an exotherm was observed at ca. 150 °C. We also attribute this to the collapse of the nanoporous structure.19 When the sample was cooled and the experiment run again, a Tg was observed at 140 °C. In related experiments, the nanoporous PS analogue was shown to be stable only to 94 °C.19 As expected, the nanoporous

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PCHE material is stable at temperatures over 40 °C higher than that of nanoporous PS. Conclusions We have successfully synthesized hydroxyl-functionalized PCHEOH by the catalytic hydrogenation of endfunctionalized PSOH. The hydroxyl functionality was effectively utilized as a reactive handle for the formation of PCHE-PLA diblock copolymers. These materials are microphase separated as determined by SAXS, DSC, and rheological analysis. We estimated a value of the FloryHuggins interaction parameter, χ, of approximately 3 times that of PS-PLA. For the PCHE-PLA system, there is stronger segregation and microphase separation can occur at lower molecular weights than for PS-PLA. The PCHE-PLA systems consisting of a hexagonally packed array of PLA cylinders were utilized as templates for the synthesis of nanoporous PCHE monoliths by selective PLA

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etching. This method allows for the synthesis of monolithic nanoporous PCHE with regularly spaced pores, narrow pore size distributions, and tunable pore sizes. The nanoporous PCHE has a high softening temperature and is resistant to solvents that dissolve analogous PS monoliths. Porous PCHE should be useful as a template for the synthesis of nanomaterials. Acknowledgment. This research was supported by the Industrial Partnership for Research in Interfacial and Materials Engineering (IPrime) at the University of Minnesota, the David and Lucile Packard Foundation, and the National Science Foundation (DMR-0094144). We thank Andrew Zalusky for synthesis of several PSOH samples, technical assistance, and helpful discussions. Jason Ness and Roberto Olayo-Valles are also acknowledged for many helpful comments. LA0341862