Hydrogen-Bond Acidic Hyperbranched Polymers for Surface Acoustic

Nov 13, 2004 - Douglas L. Keeley, and Petar R. Dvornic. Michigan Molecular Institute, 1910 West St. Andrews Road, Midland, Michigan 48640-2696...
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Chem. Mater. 2004, 16, 5357-5364

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Hydrogen-Bond Acidic Hyperbranched Polymers for Surface Acoustic Wave (SAW) Sensors Claire Hartmann-Thompson,* Jin Hu, Steven N. Kaganove, Steven E. Keinath, Douglas L. Keeley, and Petar R. Dvornic Michigan Molecular Institute, 1910 West St. Andrews Road, Midland, Michigan 48640-2696 Received May 26, 2004. Revised Manuscript Received September 14, 2004

A series of novel hyperbranched hydrogen-bond acidic polymers for surface acoustic wave (SAW) sensor applications were prepared by functionalizing hyperbranched polycarbosiloxanes or polycarbosilanes with phenol or hexafluoro-2-propanol groups. Starting polymer, sensor polymer, and reagent structures were confirmed by IR, 1H, 13C, and 29Si NMR, SEC, or GCMS as appropriate. The hyperbranched sensor polymers were coated onto 500 MHz SAW platforms and their responses to the nerve agent simulant dimethyl methylphosphonate (DMMP) were studied. The hyperbranched sensor polymers with phenol groups gave very high initial responses to DMMP which dropped to 30% of the initial levels over a period of 6 months, and the hyperbranched sensor polymers with hexafluoro-2-propanol groups gave lower initial responses that did not change with time. Hence, the long-term performances of hyperbranched phenolic sensor polymers and hyperbranched hexafluoro-2-propanol sensor polymers were found to be comparable.

Introduction Array-based chemical sensor devices that measure gravimetric, optical, chemiresistive, or electrochemical properties are the most portable and cost-effective technology for the detection of potentially toxic volatile organic compounds.1 In such arrays, individual sensing elements are coated with materials that respond to broad classes of chemical vapors. Each sensor material should be sufficiently different from the others to enable the collective array to span a broad range of chemical possibilities.1,2 Polymers are preferred materials for coating the sensing elements because they are readily processable into thin adherent films on a variety of substrates, they are excellent sorbents for organic vapors, vapor diffusion and sensor response are rapid above the polymer glass-transition temperature (Tg), their absorption isotherms are linear over a large range of vapor concentration resulting in linear calibration curves, and they can provide a wide range of selectivities by convenient synthetic variation of structure.3 It has been established through the use of linear solvation energy relationship (LSER) theory4-8 that a polymer* To whom correspondence should be addressed. E-mail: [email protected]. (1) Albert, K. J.; Lewis, N. S.; Schauer, C. L.; Sotzing, G. A.; Sitzel, S. E.; Vaid, T. P.; Walt, D. R. Chem. Rev. 2000, 100, 2595-2626. (2) Grate, J. W. Chem. Rev. 2000, 100, 2627-2648. (3) Crooks, R. M.; Ricco, A. J. Acc. Chem. Res. 1998, 31, 219-227. (4) Grate, J. W.; Abraham, M. H. Sens. Actuators, B 1991, B3, 85111. (5) Abraham, M. H.; Andonian-Haftvan, J.; Du, C. M.; Diart, V.; Whiting, G.; Grate, J. W.; McGill, R. A. J. Chem. Soc., Perkin Trans. 2 1995, 369-378. (6) Grate, J. W.; Snow, A.; Ballantine, D. S.; Wohltjen, H.; Abraham, M. H.; McGill, R. A.; Sasson, P. Anal. Chem. 1988, 60, 869-875. (7) Grate, J. W.; Pratash, S. J.; Abraham, M. H. Anal. Chem. 1995, 67, 2162-2169. (8) McGill, R. A.; Abraham, M. H.; Grate, J. W. CHEMTECH 1994, 24, 27-37.

based sensor array will collect the most chemical information if its polymers cover the full range of solubility interactions including dispersion, dipolediplole, and hydrogen bonding.4,8 Hence, a variety of different polymers is generally required including nonpolar, polarizable, dipolar, hydrogen-bond basic, and hydrogen-bond acidic ones. Conveniently, many polymers exhibiting these types of solubility interactions are commercially available with the notable exception of hydrogen-bond acidic polymers, which are of critical importance for the detection of hydrogen-bond basic entities such as nerve agents (Figure 1)9-13,16a,16b and explosives.14-15,16c,17-19 They also provide greater chemical diversity and result in better performance of the (9) Rose-Pehrsson, S. L.; Grate, J. W.; Ballantine, D. S.; Jurs, P. C. Anal. Chem. 1988, 60, 2801-2811. (10) Grate, J. W.; Rose-Pehrsson, S. L.; Venezky, D. L.; Klusty, M.; Wohltjen, H. Anal. Chem. 1993, 65, 1868-1881. (11) Rebiere, D.; Dejous, C.; Pistre, J.; Lipskier, J. F.; Planade, R. Sens. Actuators, B 1998, 49, 139-145. (12) McGill, R. A.; Nguyen, V. K.; Chung, R.; Shaffer, R. E.; DiLella, D.; Stepnowski, J. L.; Mlsna, T. E.; Venezky, D. L.; Dominguez, D. Sens. Actuators, B 2000, 65, 10-13. (13) Hopkins, A. R.; Lewis, N. S. Anal. Chem. 2001, 73, 884-892. (14) Mlsna, T. E.; Mowery, R.; McGill, R. A. The Design of Aromatic Acid Silicone Polymers and their Evaluation as Sorbent Coatings for Chemical Sensors, Silicones in Coatings II; Paint Research Association: London, 1998; pp 1-19. (15) Linnen, C.; Kobrin, P. H.; Seabury, C.; Harker, A. B.; McGill, R. A.; Houser, E. J.; Chung, R.; Weber, R.; Swager, T. Proc. SPIE 1999, 3710, 328-334. (16) (a) Chang, Y.; Noriyan, J.; Lloyd, D. R.; Barlow, J. W. Polym. Eng. Sci. 1987, 27, 693-702. (b) Barlow, J. W.; Cassidy, P. E.; Lloyd, D. R.; You, C. J.; Chang, Y.; Wong, P. C.; Noriyan, J. Polym. Eng. Sci. 1987, 27, 703-715. (c) Houser, E. J.; McGill, R. A.; Mlsna, T. E.; Nguyen, V. K.; Chung, R.; Mowery, R. L. Proc. SPIE 1999, 3710, 394401. (17) McGill, R. A.; Mlsna, T. E.; Chung, R.; Nguyen, V. K.; Stepnowski, J. P. Sens. Actuators, B 2000, 65, 5-9. (18) Houser, E. J.; McGill, R. A.; Nguyen, V. K.; Chung, R.; Weir, D. W. Proc. SPIE 2000, 4038, 504-510. (19) Briglin, S. M.; Burl, M. C.; Freund, M. S.; Lewis, N. S.; Matzger, A.; Oritz, D. N.; Tokumaru, P. Proc. SPIE 2000, 4038, 530-538.

10.1021/cm040346z CCC: $27.50 © 2004 American Chemical Society Published on Web 11/13/2004

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Figure 1. Selected nerve agents and nerve agent simulant DMMP.

sensor array as a whole.20 A particularly effective means of enhancing hydrogen-bond acidity of a polymer has been to incorporate phenol or fluorinated alcohol groups.5,8,16,18,20-24 These groups have high polarity, but they generally increase the polymer glass-transition temperature (Tg) and lower the rate of vapor diffusion if the Tg is near or above room temperature.11 A common approach to offset this effect has been to introduce silicon-carbon or silicon-oxygen bonds into the polymer backbone, which effectively lowers the Tg.5,14,16c,17-18,21,22 With the notable exception of two systems where dendritic polymers were used,25,26 all hydrogen-bond acidic polymers developed to date for vapor-sensing applications have had linear architectures. However, the former may be expected to possess certain advantages over the latter, including higher molecular density of exo-presented functional groups for improved sensitivity and lower density of segmental entanglements for faster diffusion of vapor molecules in and out of the polymer matrix. In one example, a hyperbranched polycarbosilane was synthesized from the AB2 monomer bis(allyl)2-naphthylmethylsilane, and hydrogen-bond acidic hexafluoro-2-propanol groups were subsequently introduced by the reaction of hexafluoroacetone with the allyl and naphthyl groups.26 This polymer was characterized by FTIR, and it reportedly gave a strong response to the nerve simulant DMMP (Figure 1) when coated onto a surface acoustic wave (SAW) sensor. In another example, dendrimers with halogenated alcohol or phenol end-groups were prepared and used.25 Hyperbranched polymers are traditionally prepared by monomolecular polymerization reactions of AB2, AB3, or generally ABx monomers.27 A less common, but more advantageous approach, however, is the bimolecular polymerization of A2 + B3, A2 + B4, or generally Ax + By systems where A and B are functional groups that react with each other but neither reacts with itself. To avoid cross-linking,28 the polymerization conditions must be controlled so that rp2 e 1/[(x - 1)(y - 1)], where r is the molar ratio of the reacting groups (i.e., r ) [A]/[B]) and p is the extent of the reaction determined (20) Grate, J. W.; Pratash, S. J.; Kaganove, S. N.; Wise, B. M. Anal. Chem. 1999, 71, 1033-1040. (21) Grate, J. W.; Kaganove, S. N.; Nelson, D. A. Chem. Innovation 2000, 30, 29-37. (22) Grate, J. W.; Kaganove, S. N.; Pratash, S. J.; Craig, R.; Bliss, M. Chem. Mater. 1997, 9(5), 1201-1207. (23) Ballantine, S. D.; Rose, S. L.; Grate, J. W.; Wohltjen, H. Anal. Chem. 1986, 58, 3058-3066. (24) Snow, A. W.; Sprague, L. G.; Soulen, R. L.; Grate, J. W.; Wohltjen, H. J. Appl. Polym. Sci. 1991, 43, 1659-1671. (25) Houser, E.; McGill, R. A. U.S. Patent Appl. 20030135005 A1, 2003. (26) Simonson, D. L.; Houser, E. J.; Stepnowski, J. L.; Pu, L.; McGill, R. A. Polym. Mater. Sci. Eng. 2003, 89, 866. (27) (a) Hult, A. Hyperbranched Polymers. In Encyclopedia Of Polymer Science And Technology; John Wiley and Sons: Hoboken, NJ, 2003; Vol. 2, p 722. (b) Dvornic, P. R.; Tomalia, D. A. Curr. Opin. Colloid Interface Sci. 1996, 1, 221-235. (28) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953; pp 347-398.

Figure 2. Examples of hydrogen-bond acidic linear polymers used in SAW sensors.

with respect to the lesser functionality.29 A great advantage of this approach is that a very wide range of Ax and By monomers are commercially available, whereas specific ABx monomers such as bis(allyl)-2-naphthylmethylsilane26 generally require laboratory synthesis. The molecular mass of a hyperbranched polymer resulting from an Ax + By synthesis can be controlled by varying the r ratio, and both A and B functional groups can become terminal end units depending on which monomer is used in excess. Here, we describe the preparation of several hydrogenbond acidic hyperbranched polymers for application in SAW arrays. Figure 2 shows five examples of hydrogenbond acidic linear polymers that have been described in the literature: BSP3 (containing phenol groups),22 SXFA (containing fluorinated aliphatic alcohol groups),5 PSpFA16a,16b (poly([4-(1,1,1,3,3,3-hexafluoro-2-hydroxyprop-2-yl)phenyl]ethylene)) and NRL516c (both containing fluorinated aromatic alcohol groups),16b and a fluoropolyol (FPOL).11,30 In this study, these same hydrogenbond acidic groups (Figure 2) have been introduced as end groups into hyperbranched polycarbosilanes and polycarbosiloxanes synthesized using the novel and costeffective Ax + By polymerization approach shown in Reaction Scheme 1.29 The initial vapor responses of the obtained hydrogenbond acidic hyperbranched polymers to the nerve simulant DMMP were studied using a 500 MHz SAW platform. The SAW responses of the coatings over 190 days (approximately 6 months) were also studied. To date, most of the SAW literature has focused only on initial response data, while long-term SAW response data has been essentially ignored.31,32 This study seeks to address this deficiency in the literature data and to describe the syntheses of useful new hyperbranched SAW sensor polymers. (29) (a) Dvornic, P. R.; Hu, J.; Meier, D.; Nowak, R. M. Polym. Prepr. 2004, 45(1), 585-586. (b) Dvornic, P. R.; Hu, J.; Meier, D. J.; Nowak, R. M. U.S. Patent 634172, 2002. (c) Dvornic, P. R.; Hu, J.; Meier, D. J.; Nowak, R. M.; Parham, P. L. U.S. Patent 6534600, 2003. (30) Field, D. E. J. Coatings Technol. 1976, 48(615), 43-47. (31) Osburn, G. C.; Bartholomew, J. W.; Ricco, A. J.; Frye, G. C. Acc. Chem. Res. 1998, 31, 297-305.

Surface Acoustic Wave Sensors

Chem. Mater., Vol. 16, No. 25, 2004 5359 Scheme 1

Experimental Section Materials and Apparatus. Reagents and solvents were purchased from Sigma-Aldrich, Inc. (Milwaukee, WI) or Gelest, Inc. (Tullytown, PA) and used without further purification. High-pressure experiments were carried out in a 300-mL stainless steel glass-lined Parr bomb reactor fitted with a 600 psi Fike Metal Products relief valve, Ashcroft Duralife pressure gauge, and Parr 4835 temperature controller. The reactor was connected to a 100-g hexafluoroacetone lecture bottle (Aldrich) using stainless steel Swagelok fittings and valves. Surface acoustic wave (SAW) sensor units (500 MHz) were obtained from Sawtech (TriQuint Semiconductor, Inc., Hillsboro, OR) and epoxy-mounted onto 15 mm × 10 mm six-pin headers attached to gold leads. Characterization Procedures. 1H, 13C, and 29Si NMR spectra were recorded on a Varian Unity 300 MHz NMR spectrometer equipped with a 5-mm four nuclei probe. Solvent signals were used as internal standards, and chemical shifts are reported relative to tetramethylsilane (TMS). IR spectra were recorded on a Nicolet 20DXB FTIR spectrometer and samples were prepared for analysis by solution casting onto potassium bromide disks. Differential scanning calorimetry (DSC) measurements were performed on a DuPont Instruments Model 912 unit. Mass spectral data were obtained by analysis of sample solutions using an Agilent 6890 Gas Chromatograph coupled to a 5973 Mass Selective Detector. Size exclusion chromatography (SEC) was carried out using a Waters 510 pump, a Waters 717 auto-injector, a Waters CHM column heater, two Polymer Laboratory PLgel columns, and a Polymer Laboratory PL-ELS 1000 evaporative light scattering detector (ELSD). Detector conditions appropriate to the eluting solvent were used, and the column was calibrated with Dow 1683 polystyrene standards. Purification Procedures. Flash column chromatography was carried out using a column packed with silica gel (Davisil, grade 633, 200-425 mesh, 60 Å, 99+%), and various fractions were monitored by thin-layer chromatography using ME Science aluminum-backed silica gel 60 F-254 TLC plates. Preparative HPLC was carried out by dissolving approximately 2 g of a sample in 4 mL acetonitrile. Sixty-microliter injections of the resulting solutions were sequentially separated using a Zorbax SB C18 column (150 × 4.6 mm). A mobile phase gradient (1.0 mL/min nominal flow rate, Waters 616 gradient pump, 717 plus autosampler) proceeding from 60/40 MeCN/ H2O (0.1% H3PO4) to 90/10 over 15 min, and then to 100/0 over 5 min, resulted in adequate separation of the solution

(detection at 280 nm, Waters 486 absorbance detector). After 5 min at 100/0, the system was re-equilibrated at 60/40 MeCN/ H2O (0.1% H3PO4) for a further 5 min before the next injection. The various collected fractions were examined using GC-FID analysis. Cleaning and Coating Procedures. SAWs and their covers were washed for 30 s with HPLC-grade acetone, dried in a stream of nitrogen, washed for 30 s in HPLC-grade chloroform, and dried again with nitrogen. They were then air plasma-cleaned for 30 min in a Model PDC-3XG Harrick Plasma Cleaner-Sterilizer attached to a vacuum pump and Huntington Laboratories vacuum gauge. The SAW coating process was performed inside a custom-built benchtop clean box. The SAWs were mounted onto a Femtometrics frequency readout circuit board containing a sealed reference SAW. The mounted circuit board was placed 6 in. beneath a Badger 150 airbrush connected to a cylinder of ultrahigh purity nitrogen with the outlet pressure set to 20 psi. The airbrush contained a 0.05% w/v chloroform solution of the polymer to be coated. Polymer solution was sprayed onto the SAW from the airbrush in a 1 s on/1 s off sequence, and airbrush spraying was continued until a frequency increase of 500 kHz was observed. The SAW cover was then attached and sealed with epoxy. SAW Test Procedure. Coated SAW units were tested in a Femtometrics Individual Vapor Detector (IVD) containing a beat frequency reference SAW used during the coating procedure and an uncoated thermal reference SAW. One IVD unit could test up to six coated SAWs simultaneously. The system was tested for leaks by running in air with an Agilent ADM2000 flowmeter. After equilibriation to 28 ( 0.3 °C, the system was subjected to a test cycle of 5 min of purified air, 5 min of DMMP vapor, and 5 min purified air. DMMP vapor was introduced using a VICI Dynacalibrator Model 340 vapor generator via a six-port automated valve attached to an HP 6024A DC power supply. The DMMP vapor tube within the generator was held at 50 °C, and the flow was controlled such that a concentration of 0.5 mg/m3 of DMMP was generated within the unit. The IVD SAW response data sets were transferred to a laptop computer for analysis via an RS232 port connection. Responses are quoted as positive difference frequencies relative to the reference SAW. Synthesis of Hyperbranched Polymers. R-Silanetetraylω-[(dimethylsilyl)oxy]poly[oxy(dimethylsilanediyl)ethylene( 1,1,3, 3-tetramethyldisiloxane-1,3-diyl)ethylene(1,1-dimethyldisiloxane-1,3,3,3-tetrayl)] (1), designated HB-PCSOX. A 100-mL one-necked round-bottomed flask was equipped with a Tefloncoated magnetic stirrer bar and a condenser with a nitrogen

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inlet at its top. The flask was charged with 10.58 g tetrakis(dimethylsiloxy)silane (32.19 mmol), 4.00 g 1,3-divinyltetramethyldisiloxane (21.46 mmol), 20 mL anhydrous tetrahydrofuran (THF), and 1 drop of platinum divinyltetramethyldisiloxane complex in xylene (Karstedt’s catalyst, 2.1-2.4% Pt). The mixture was refluxed at 65 °C for 24 h and then stirred with activated charcoal at room temperature for 24 h. The charcoal residue was removed by gravity filtration and the anhydrous THF and excess monomer were removed in vacuo to give a quantitative amount of silane (SiH) terminated hyperbranched polycarbosiloxane 1. IR (thin film): ν (cm-1) 2133 (SiH).1H NMR (CDCl3): δ (ppm) 0.04-0.21 (m; SiCH3), 0.46 (s; (CH2)2), 0.51 (s; (CH2)2), 4.73 (m; SiH). 13C NMR (CDCl3): δ (ppm) -1.2 (SiCH3), 9.4-9.7 ((CH2)2). 29Si NMR (CDCl3): δ (ppm) -108.7 to -107.1 (SiO4), -24.5 (OSi(CH3)2O), -10.5 to -8.2 (SiH), 3.9-7.0 (CH2CH2Si(CH3)2O). SEC (toluene elutant): Mw ) 2913, Mn ) 1350, polydispersity ) 2.16. R-{Ethylenebis[(dimethylsilanediyl)propane-1,3-diylsilanetetrayl]}-ω-(2-propenyl)poly[propane-1,3-diyl(dimethylsilanediyl)ethylene(dimethylsilanediyl)propane-1,3-diylsilanetetrayl (2), designated HB-PCS. A 10-mL one-necked round-bottomed flask was equipped with a Teflon-coated magnetic stirrer bar and a condenser with a nitrogen inlet at its top. The flask was charged with 1.00 g 1,1,4,4-tetramethyldisilylethylene (6.85 mmol), 2.19 g tetraallylsilane (11.42 mmol), 5 mL anhydrous THF, and 1 drop of platinum divinyltetramethyldisiloxane complex in xylene (Karstedt’s catalyst, 2.1-2.4% Pt). The mixture was refluxed at 65 °C for 24 h, and then stirred with activated charcoal at room temperature for 24 h. The charcoal residue was removed by gravity filtration and the anhydrous THF and excess monomer were removed in vacuo to give a quantitative amount of allyl-terminated hyperbranched polycarbosilane 2. IR (thin film): ν (cm-1) 3067 (CdCH), 2948, 2904, 2867 (CH2), 1242, 1213, 1154, 1036 (SiCH2). 1H NMR (CDCl3): δ (ppm) 0.36 (s; SiCH2CH2Si), 0.57-0.60 (m; SiCH2CH2CH2Si), 1.34-1.37 (m; SiCH2CH2CH2Si), 1.55-1.63 (d; SiCH2CHdCH2), 4.81-4.92, 5.72-5.81 (2m; CH2CHdCH2). 29Si NMR (CDCl ): δ (ppm) 2.2, 2.9 (SiCH CHdCH ), 6.4 3 2 2 (SiCH2). SEC (THF elutant): Mw ) 6322, Mn ) 3214, polydispersity ) 1.97. DSC (10 °C/min, nitrogen): -87 °C (Tg). R-Silanetetrayl-ω-({[3-(2-hydroxyphenyl)propyl]dimethylsilyl}oxy)poly[oxy(dimethylsilanediyl)ethylene(1,1,3,3-tetramethyldisiloxane-1,3-diyl)ethylene(1,1-dimethyldisiloxane-1,3,3,3-tetrayl)] (3). A 50-mL one-necked round-bottomed flask was equipped with a Teflon-coated magnetic stirrer bar and a condenser. The flask was charged with 1.38 g 2-allylphenol (10.3 mmol), 2.00 g of silane-terminated hyperbranched polycarbosiloxane 1 (8.42 mmol SiH), 15 mL THF, and 1 drop of platinum divinyltetramethyldisiloxane complex in xylene (Karstedt’s catalyst, 2.1-2.4% Pt). The mixture was heated under nitrogen at 50 °C for 24 h. The solvent was evaporated to give the product as a yellow oil in quantitative yield. IR (thin film): ν (cm-1) 3448 (OH) 2956, 2911, 2875 (CH2), 1591, 1503, 1490, 1456, 1407, 1328 (Ar), 1253, 1172, 1132, 1070 (SiCH2, SiOSi). 1H NMR (CDCl3): δ (ppm) 0.06 (s; SiCH3), 0.43 (m; SiCH2CH2Si), 0.58-0.63 (m; SiCH2CH2CH2Ar), 1.58-1.70 (m; SiCH2CH2CH2Ar), 2.57-2.62 (m; SiCH2CH2CH2Ar), 6.726.75 (d; ArH), 6.83-6.87 (t; ArH), 7.03-7.10 (m; ArH). SEC (THF elutant): Mw ) 17170, Mn ) 6514, polydispersity ) 2.63. R-Silanetetrayl-ω-{[(3-{2-hydroxy-5-[2,2,2-trifluoro-1-(4-hydroxyphenyl)-1-(trifluoromethyl)ethyl]phenyl}propyl)dimethylsilyl]oxy}poly[oxy(dimethylsilanediyl)ethylene(1,1,3,3-tetramethyldisiloxane-1,3-diyl)ethylene(1,1-dimethyldisiloxane-1,3,3,3tetrayl)] (4). A 100-mL one-necked round-bottomed flask was equipped with a Teflon-coated magnetic stirrer bar and a condenser. The flask was charged with 0.619 g 1,1,1,3,3,3hexafluoro-2-(3-propenyl-4-hydroxyphenyl)-2-(4-hydroxyphenyl)propane 7 (1.65 mmol), 0.321 g of a silane-terminated hyperbranched polycarbosiloxane 1 (1.58 mmol SiH), 25 mL THF, and 1 drop of platinum divinyltetramethyldisiloxane complex in xylene (Karstedt’s catalyst, 2.1-2.4% Pt). The mixture was refluxed for 6 days and monitored by IR for the disappearance of the SiH band at 2133 cm-1. The solvent was evaporated to give the product as a yellow oil in quantitative yield. IR (thin

Hartmann-Thompson et al. film): ν (cm-1) 3409 (OH), 2959, 2911, 2877 (CH2), 1614, 1517, 1442 (Ar), 1384 (CF3), 1254, 1205, 1174, 1134, 1073, 1049 (SiCH2, SiOSi). 1H NMR (CDCl3): δ (ppm) 0.05 (s; SiCH3), 0.40 (m; SiCH2CH2Si), 0.47 (m; SiCH2CH2CH2Ar), 1.64 (m; SiCH2CH2CH2Ar), 2.53 (m; SiCH2CH2CH2Ar), 6.70-6.82 (m; ArH), 7.03-7.12 (m; ArH), 7.17-7.29 (m; ArH). SEC (THF elutant): Mw ) 11241, Mn ) 1400, polydispersity ) 8.03. R-Silanetetrayl-ω-{[(3-{2-hydroxy-3-[2,2,2-trifluoro-(1-{3[2,2,2-trifluoro-1-hydroxy-1-(trifluoromethyl)ethyl]phenyl})-1(trifluoromethyl)ethoxy]propoxy}propyl)dimethylsilyl]oxy}poly[oxy(dimethylsilanediyl)ethylene(1,1,3,3-tetramethyldisiloxane1,3-diyl)ethylene(1,1-dimethyldisiloxane-1,3,3,3-tetrayl)] (5). A-100 mL one-necked round-bottomed flask was equipped with a Teflon-coated magnetic stirrer bar and a condenser. The flask was charged with 0.20 g 1-[1-oxypropenyl-(2-hydroxyl)propyloxy-1,1-bis(trifluoromethyl)methyl]-3-[1,1,1, 3,3,3-hexafluoro2-hydroxyprop-2-yl]benzene 8 (0.38 mmol), 0.088 g of a silaneterminated hyperbranched polycarbosiloxane 1 (0.37 mmol SiH), 20 mL THF, and 1 drop of platinum divinyltetramethyldisiloxane complex in xylene (Karstedt’s catalyst, 2.1-2.4% Pt). The mixture was refluxed for 5 days and monitored by IR for the disappearance of the SiH band at 2130 cm-1. The solvent was evaporated to give the product as a yellow oil in quantitative yield. IR (thin film): ν (cm-1) 3314 (OH), 2959, 2911, 2875 (CH2), 1671, 1406 (Ar), 1262, 1221, 1154, 1135, 1075, 1049 (SiCH2, SiOSi).1H NMR (CDCl3): δ (ppm) 0.07 (s; SiCH3), 0.39 (m; SiCH2CH2Si), 0.47 (m; SiCH2(CH2)2OCH2CH(OH)CH2OC(CF3)2ArC(CF3)2OH), 1.54-1.70 (m; SiCH2CH2CH2OCH2CH(OH)CH2OC(CF3)2ArC(CF3)2OH), 3.40-3.46 (t; Si(CH2)2CH2OCH2CH(OH)CH2OC(CF3)2ArC(CF3)2OH), 3.553.64 (m; Si(CH2)3OCH2CH(OH)CH2OC(CF3)2ArC(CF3)2OH), 4.02-4.12 (m; Si(CH2)3OCH2CH(OH)CH2OC(CF3)2ArC(CF3)2OH), 7.52-7.58 (m; ArH), 7.69-7.83 (m; ArH), 8.01-8.12 (m; ArH). SEC (THF elutant): Mw ) 4879, Mn ) 2439, polydispersity ) 2.00. R-{Ethylenebis[(dimethylsilanediyl)propane-1,3-diylsilanetetrayl]}-ω-[5,5,5-trifluoro-4-hydroxy-4-(trifluoromethyl)pent-1en-1-yl]poly[propane-1,3-diyl(dimethylsilanediyl)ethylene(dimethylsilanediyl)propane-1,3-diylsilanetetrayl (6). Allylterminated hyperbranched polycarbosilane 2 (18 g) was added to a 300 mL high-pressure steel Parr bomb reactor with a glass liner. The reactor was purged and then charged with 10 g hexafluoroacetone. After 24 h at 90 °C, excess hexafluoroacetone was pumped out of the reactor and destroyed by bubbling through an appropriate quantity of sodium borohydride solution in triglyme. The product was isolated as a yellow oil in quantitative yield. IR (thin film): ν (cm-1) 3600 (free OH), 3489 (hydrogen-bonded OH), 2956, 2911, 2867 (CH2), 1379 (CF3), 1279, 1209, 1146, 1054 (SiCH2). 1H NMR (CDCl3): δ (ppm) 0.33 (s; SiCH2CH2Si), 0.55-0.57 (m; SiCH2CH2CH2Si), 1.331.36 (m; SiCH2CH2CH2Si), 2.72-2.79 (d; SiCHdCHCH2C(CF3)2OH), 5.79-6.16 (m; SiCH)CHCH2C(CF3)2OH), 6.39-6.58 (m; SiCH)CHCH2C(CF3)2OH). 29Si NMR (CDCl3): δ (ppm) -6.8 (SiCHdCHCH2C(CF3)2OH), 4.9 (SiCH2). SEC (THF as elutant): Mw ) 9867, Mn ) 2494, polydispersity ) 3.96. DSC: (10 °C/min, nitrogen): -37 °C (Tg). Preparation of End Group Precursor Reagents. 1,1,1, 3,3,3-Hexafluoro-2-(3-propenyl-4-hydroxyphenyl)-2-(4-hydroxyphenyl)propane (7). This compound was prepared by a modification of the procedure reported for di-allyl compound 2,2′bis(3-allyl-4-hydroxyphenyl)hexafluoropropane.33 One equivalent of 4,4′-hexafluoroisopropylidenediphenol was reacted with one equivalent of allyl bromide in the presence of toluene, water, sodium hydroxide, and tetramethylammonium bromide at 60 °C, and the product of this reaction was heated to 200 °C, undergoing a Claisen rearrangement. Compound 7 was obtained as a yellow oil in 46% yield after purification by flash column chromatography (1:1 v/v CH2Cl2-hexane, gradient to 100% CH2Cl2). IR (thin film): ν (cm-1) 3585 (OH), 3393 (OH), 3081(CdCH), 3015, 2970 (CH2). 1H NMR (CDCl3): δ (ppm) (32) Barie, N.; Rapp, M.; Ache, H. J. Sens. Actuators, B 1998, 46, 97-103. (33) Abraham, M. H.; Hamerton, I.; Rose, J. B.; Grate, J. W. J. Chem. Soc., Perkin Trans. 2 1991, 1417-1423.

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3.37 (d; CH2), 5.08-5.16 (m; CHdCH2), 5.90-6.03 (m; CHd CH2), 6.77-7.18 (4m; ArH), 7.15 (s; ArH). 13C NMR (CDCl3): δ (ppm) 35.2 (CH2), 114.1 (CHdCH2), 135.9 (CHdCH2), 115.0, 116.9, 122.5, 125.0, 125.9, 126.2, 130.0, 131.8, 132.4, 154.6, 155.9 (ArC, C(CF3)2 and CF3). GC-MS: 376 (M+). 1-[1-Oxypropenyl-(2-hydroxyl)propyloxy-1,1-bis(trifluoromethyl)methyl]-3-[1,1,1,3,3,3-hexafluoro-2-hydroxyprop-2-yl]benzene (8). A 100-mL one-necked round-bottomed flask was equipped with a Teflon-coated magnetic stirrer bar and a condenser with a nitrogen inlet at its top. The flask was charged with 3.13 g allylglycidyl ether (26.8 mmol), 34.14 g 1,3-bis(2-hydroxyhexafluoroisopropyl)benzene (78.9 mmol), 0.96 g triethylamine (9.49 mmol), and 25 mL THF. The mixture was stirred at room temperature for 5 days and then refluxed for an additional day. THF was evaporated to give a crude product. After an unsuccessful attempt at purification by flash column chromatography, the product was purified by preparative HPLC (0.30 g, 1%). 1H NMR (CDCl3): δ (ppm) 3.55-3.57 (d; CH2CHdCH2), 3.58-3.76 (m; CH2OCH2CHdCH2), 4.004.02 (d; CH2OC(CF3)2Ar), 4.05-4.12 (m; CH2CH(OH)CH2), 5.09-5.29 (4m; CH)CH2), 5.82-5.95 (m; CH)CH2), 7.74-7.79 (t; ArH), 7.97-8.00 (d; ArH), 8.15 (s; ArH). 13C NMR (CDCl3): δ (ppm) 69.3 (OCH2CHOH), 69.8 (OCH2CHdCH2), 71.9 (CHOH), 72.8 (C(CF3)2OCH2), 116.7 (CH)CH2), 121.5, 122.1, 125.3, 125.9, 129.3, 132.7 (ArC), 127.9, 130.1, 131.5, 136.1 (CF3 and C(CF3)2), 130.4 (CHdCH2). GC-MS: 524 (M+), 393 ((CF3)2COHArC(CF3)2).

Results and Discussion Silicon-Containing Hyperbranched Polymers. Silane-terminated (SiH) hyperbranched polycarbosiloxane, HB-PCSOX, 1 {(CH2)2SiMe2OSiMe2(CH2)2SiMe2OSi(OSiMe2)3}n and allyl-terminated (SiCH2CHdCH2) hyperbranched polycarbosilane, HB-PCS, 2 {SiMe2(CH2)2SiMe2(CH2)3Si[(CH2)3]3}n were synthesized using hydrosilylation chemistry, as shown in Reaction Scheme 1. To avoid gelation and to meet the condition29 1/[(x 1)(y - 1)] g r g (x - 1)(y - 1) for Ax + By nonlinear polymerizations where x ) 2 and y ) 4, an r ) [A]/[B] molar ratio of 0.3 was used. The reactions were catalyzed by platinum divinyltetramethyldisiloxane complex in xylene (Karstedt’s catalyst) and THF was used as solvent. The products were purified by stirring with activated charcoal to remove the catalyst and heating under vacuum to remove the volatile solvent and excess monomers. For polymer 1, an SiH band was observed at 2133 cm-1 in the IR spectrum, a characteristic SiH multiplet was observed at 4.73 ppm in the 1H NMR spectrum, a 29Si NMR signal was observed in the -10 ppm SiH region, and the spectra suggested that all vinylsilyl groups had been consumed. Conversely, for polymer 2, IR, 1H NMR, and 29Si NMR all showed that all SiH groups had been consumed and that only

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Hartmann-Thompson et al. Scheme 3

Scheme 4

Scheme 5

allylsilyl groups remained. The latter were observed at 3067 cm-1 in the IR spectrum, in the 1.6 and 4.8-6.0 regions of the 1H NMR spectrum, and in the 2 ppm region of the 29Si NMR spectrum. Traces of ionic impurities in siloxanes (e.g., Brønsted acids and bases) catalyze the cleavage of SiO bonds and drastically reduce thermal stability and decomposition temperatures.34 Hence, to avoid the possibility of acidic hexafluoro-2-propanol groups (Figure 2) catalyzing the decomposition of SiO bonds within the hyperbranched polymer backbone, a hyperbranched polycarbosilane 2 (rather than a polycarbosiloxane) was chosen as the scaffold for polymer 6. Hyperbranched Polymers with Hydrogen-Bond Acidic End-Groups. Hyperbranched polycarbosiloxanes 3, 4, and 5 were prepared by reacting SiHterminated hyperbranched polycarbosiloxane 1 with allyl-functionalized reagents containing the desired hydrogen-bond acidic functional groups, as shown in Reaction Scheme 2. In contrast to this, 6 was prepared by reacting allyl-terminated hyperbranched polycarbosilane 2 with hexafluoroacetone gas in a high-pressure steel Parr Bomb (see Reaction Scheme 3). Polymers were all yellow oils, and SEC showed that high molecular mass was retained, indicating that neither hydrosilylation nor high-pressure exposure to hexafluoroace(34) Dvornic, P. R.; Lenz, R. W. High-Temperature Siloxane Elastomers; Hu¨thig and Wepf Verlag: Basel, Switzerland, 1990; p 62.

tone damaged the underlying hyperbranched polymeric backbone. DSC results showed that the conversion of the allyl groups in hyperbranched polymer 2 to the hexafluoro-2-propanol groups in polymer 6 raised the Tg from -87 °C to -37 °C. The hydrosilylations shown in Reaction Scheme 2 were carried out in THF with Karstedt’s catalyst (as described in the Experimental Section) and were monitored by IR for the disappearance of the SiH band in the 2100 cm-1 region. Products 3, 4, and 5 were obtained in near quantitative yields, and IR and 1H NMR showed that all allyl and SiH groups were consumed giving the expected SiCH2CH2CH2 linkages. 2-Allylphenol used in the preparation of 3 was commercially available, but compounds 7 and 8 used in the preparation of polymers 4 and 5, respectively, were synthesized as shown in Reaction Schemes 4 and 5. Reagent 7 was synthesized by modifying a two-step procedure reported for the analogous diallyl compound (Scheme 4)33 and purified by flash column chromatography. Reagent 8 was prepared from 1,3-bis(2-hydroxyhexafluoroisopropyl)benzene and allylglycidyl ether in an epoxy ring-opening reaction (Reaction Scheme 5) and purified by preparative HPLC. The purity of products 7 and 8 was evaluated by GC-MS. In both cases, only one component was observed, with a mass corresponding to the target monosubstituted structure (7 or 8). The integrals in the 1H NMR spectra of 7 and 8 confirmed that one out of two hydroxyl groups in each starting material had

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Figure 3. Initial SAW responses for polymers 3 (peak 2740 Hz), 4 (peak 3629 Hz), 5 (peak 530 Hz), and 6 (peak 638 Hz). Units for x axes: seconds; units for y axes: Hz. Table 1. Initial SAW Responses of Hyperbranched Sensor Polymers 3-6 hyperbranched polymer (3) (4) (5)b (6)

percent OH content by mass

initial responsea (Hz)

4% 5% 2% 4%

2740 3629 530 638

a All materials coated to a thickness corresponding to a frequency increase of 500 kHz. b Percent OH content quoted for hydrogen-bond acidic hydroxyls (C(CF3)2OH) rather than all hydroxyl groups (C(CF3)2OH and CHOH).

reacted to give the desired mono-allyl products (Reaction Schemes 4 and 5). Initial SAW Responses of Hyperbranched Polymers 3-6. The SAW surfaces were thoroughly cleaned and then airbrush-coated with hyperbranched sensor polymers 3-6 delivered from chloroform solutions to a thickness corresponding to a frequency increase of 500 kHz. The cleaning process was critical for attaining good adhesion of the polymer to the SAW surface and for removing any species on the SAW surface that could interfere with the sensing ability of the coating. The coated SAWs were then tested for their response to DMMP vapor, and the results obtained are shown in Table 1 and Figure 3. DMMP is a commonly used nerve agent simulant8 since it has a structure that closely resembles those of real nerve agents (see Figure 1). The left-hand sides of the “shark-fin” peaks in the SAW response plots correspond to the 5 min (300 s) that the SAWs were exposed to DMMP vapor, the right-hand sides correspond to the recovery after DMMP exposure when

exposure to pure air is restored, and the height of the peak in Hz represents the SAW response. This is the difference frequency between the coated SAW and the reference SAW quoted as a positive number. Ideally, SAWs should respond reversibly. The hydrogen-bond acidic OH groups of the sensing polymers interact with the hydrogen-bond basic groups of the analyte, resulting in a mass increase and an associated detectable change in frequency of the SAW sensor. Upon subsequent purging with air, the analyte is removed from the polymer coating on the SAW surface, and the frequency should return to its original value. Indeed, the plots in Figure 3 show quick return to the baseline and good reversibility. Although the polymers had comparable hydroxyl content by mass (i.e., density of sensing groups, see Table 1), the polymers containing phenol hydroxyl groups (3 and 4) gave excellent responses (2740 and 3629 Hz, respectively) and the polymers containing fluorinated aliphatic hydroxyl groups (5 and 6) gave lower ones (530 and 638 Hz, respectively). Polymer 5 contains two kinds of hydroxyl groups (CH2CH(OH)CH2 and (CF3)2OH, see Reaction Scheme 2), but only the latter has the hydrogen-bond acidity required for a response to DMMP. It appears from the data that the initial response of the phenol-containing polymers was significantly stronger than that of the hexafluoro-2propanol-containing ones. SAW frequency responses depend on several complex and inter-related variables (e.g., partition coefficient and solubility of vapor in polymer, diffusion parameters, hydrogen-bond enthalpies, as well as number of sensing

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Figure 4. Changes in SAW responses with time for four hyperbranched hydrogen-bond acidic sensor polymers 3-6 of Figure 3.

groups per unit mass), and quantitative comparisons should be made with caution. Long-Term SAW Responses of Hyperbranched Polymers 3-6. Changes in SAW responses of the examined hyperbranched polymers with time were studied by testing every few days over a period of 190 days (see Figure 4). Between the tests, the sealed SAW units were removed from the individual vapor detector (IVD) unit and stored on a benchtop at room temperature. The testing of polymer 6 was discontinued after 100 days when its response became indistinguishable from the baseline noise. The two phenolic hyperbranched polymers 3 and 4 lost 70-80% of their SAW response after 190 days, while polymer 5 maintained a steady SAW response during the entire 190 day period. Polymers 3-5 approached a plateau SAW response level of 400-900 Hz. Several possible mechanisms could explain the decrease of SAW response with time. SAW frequency responses depend on the number and availability of sensor groups per unit mass, hydrogen-bond acidity, partition coefficient, and solubility and diffusion of vapor in polymer. The number of sensor groups could be decreased through loss of material from the SAW

Hartmann-Thompson et al.

surface since low Tg polymers are often liquids (as opposed to more physically robust solids) and are vulnerable to repeated vapor challenges. Because a SAW sensor is a gravimetric device, such loss of coating material would result in a detectable decrease in response. To function successfully, the hydroxyl sensor groups must undergo reversible hydrogen-bonding interactions with incoming vapors. The number of sensor groups could be decreased by irreversible chemical reaction with oxygen or carbon dioxide from the air, with other functional groups in the polymer or at the SAW surface, or with the incoming analyte. Many of these reactions would result in a detectable increase in polymer mass. The availability of the hydoxyl groups for hydrogen bonding must also be considered. If they are hydrogenbonded to each other or to silanol (SiOH) groups on the quartz SAW surface, it is thermodynamically favorable for them to break those bonds and form new hydrogen bonds to the incoming DMMP vapor.35 Physical changes in the morphology of the coating could decrease the solubility of vapor in the polymer and decrease the rate of diffusion. SAW frequency responses are a function of modulus as well as mass,36 and SAW sensor technology works on the assumption that modulus remains constant during a sensing event. For all these reasons, before any conclusions can be drawn, it is clear that more rigorous studies of possible aging mechanisms must be carried out. Acknowledgment. The loan of the 500 MHz SAW units, SAW test equipment, and vapor generator used in this study from BAE Systems (Austin, TX) is gratefully acknowledged. BAE Systems also provided partial funding for some of the work reported here. We also thank Dr. Edward S. Wilks for his advice on IUPACstyle nomenclature. IUPAC currently has no official nomenclature for hyperbranched polymers. CM040346Z (35) (a) Pimental, G. C.; McClellan, A. L. The Hydrogen Bond; W. H. Freeman and Company: San Francisco, CA, 1960. (b) Joesten, M. D.; Schaad, L. J. Hydrogen Bonding; Marcel Dekker: New York, 1974. (36) Wohltjen, H. Anal. Chem. 1979, 51, 1458-1475.