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Langmuir 1999, 15, 1418-1422
Design and Growth of Robust Layered Polymer Assemblies with Molecular Thickness Control P. Kohli and G. J. Blanchard* Michigan State University, Department of Chemistry, East Lansing, Michigan 48824-1322 Received June 30, 1998. In Final Form: December 4, 1998
We report on two alternating copolymers synthesized from maleimide and vinyl ether monomers, where the vinyl ether possesses a pendant phosphonate functionality. Partial hydrolysis of the phosphonate groups in conjunction with Zr-phosphonate layer growth chemistry produces robust, stable polymer multilayer structures where the layer thickness is 16 Å for polymers synthesized with N-phenylmaleimide (NPM) monomer and 31 Å for polymers synthesized with N-biphenylmaleimide (NBM) monomer. We present the syntheses and characterization of the polymers and layered assemblies containing up to 10 layers. X-ray photoelectron spectroscopy data on layer-dependent Zr loading suggest interlayer bonding that is similar to that seen for Zr-alkanebisphosphonate multilayers.
* Author to whom correspondence should be addressed. Telephone: (517) 355-9715 x224; Fax: (517) 353-1793; e-mail: blanchard @photon.cem.msu.edu.
dination,19 charge-transfer,20,21 hydrogen bonding,22 and alternate adsorption of oppositely charged polyelectrolytes.23-25 Many of these methods are well suited to the deposition of multilayers of small molecules and can be used for the deposition of polymers under certain circumstances. For several of the linking methods, however, the stability of the resulting structures is limited under conditions of high temperature or solvent exposure. Only the approaches that use covalent or strongly ionic interlayer linking chemistry can withstand thermal and solvent attack. We combined the advantages of metal phosphonate interlayer linking chemistry with the physical robustness of vinyl ether-maleimide alternating copolymers26 to produce well-controlled polymer multilayer structures. We report here on the synthesis and layer-by-layer deposition of poly(N-phenylmaleimide-(2-vinyloxy)ethylphosphonate) (NPM-VEP) and poly(N-biphenylmaleimide-(2vinyloxy)ethylphosphonate) (NBM-VEP) on oxidized silicon and silica substrates using Zr-phosphonate interlayer linking chemistry. We pursued a strategy of layer-by-layer growth because the resulting materials can, in principle, possess significant advantages over polymer materials deposited by conventional spin coating methods. For example, layer-by-layer growth offers better control over
(1) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D.; Science 1991, 254, 1485. (b) Li, D.; Ratner, M. A.; Marks, T. J.; Zhang, C. H.; Yang, J.; Wong, G. K. J. Am. Chem. Soc. 1990, 112, 7389. (2) Kepley, L. J.; Crooks, R. M.; Ricco, A. Anal. Chem. 1992, 64, 3191. (3) Swalen, J. P.; Allara, D. L.; Andrade, J. P.; Chandross, E. A.; Garoff, S.; Israelachvilli, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Tu, H. Langmuir 1987, 3, 932. (4) Metzger, R. M.; Wiser, D. C.; Laidlaw, R. K.; Takassi, M. A.; Mattern, D. L.; Panetta, C. A. Langmuir 1990, 6, 350. (5) Dulcey, C. S.; Geoger, J. H., Jr.; Krauthamer, V.; Stenger, D. A.; Fare, T. L.; Calvert, J. M. Science 1991, 252, 551. (6) Calvert, J. M.; George, J. H., Jr.; Peckerar, M. C.; Perhsson, P. E.; Schnur, J. M.; Scheon, P. E. Thin Solid Films 1992, 210/211, 359. (7) Kumar, A.; Biebuyck, H. A.; Abbott, N. L.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9188. (8) Netzer, L. Sagiv, J. J. Am. Chem. Soc. 1983, 105, 674. (9) Tillman, A.; Ulman, A.; Penner, T. L. Langmuir 1989, 5, 101. (10) Liu, Y.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 2114. (11) Kohli, P.; Taylor, K. K.; Harris, J. J.; Blanchard, G. J. J. Am. Chem. Soc. 1998, 120, 11962. (12) Lee, H.; Kepley, L. J.; Hong, H.-G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618. (13) Lee, H.; Kepley, L. J.; Hong, H.-G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1998, 92, 2597.
(14) Akhter, S.; Lee, H.; Hong, H.-G.; Mallouk, T. E.; White, J. M. Acc. Chem. Res. 1992, 25, 420. (15) Putvinski, T. M.; Schilling, M. L.; Katz, H. E.; Chidsey, C. E. D.; Mujsce, A. M.; Emerson, A. B. Langmuir 1990, 6, 1567. (16) Katz, H. E.; Schilling, M. L.; Chidsey, C. E. D.; Putvinski, T. M.; Hutton, R. S. Chem. Mater. 1991, 3, 699. (17) Yang, H. C.; Aoki, K.; Hong, H.-G.; Sackett, D. D.; Arendt, M. F.; Yau, S.-L.; Bell, C. M.; Mallouk, T. E. J. Am. Chem. Soc. 1993, 115, 11855. (18) Thompson, M. E. Chem. Mater. 1994, 6, 1168. (19) Bell, C. M.; Keller, S. W.; Lynch, V. M.; Mallouk, T. E. Mater. Chem. Phys. 1993, 35, 225. (20) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1997, 13, 1385. (21) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1998, 14, 2768. (22) Sun, L.; Kepley, L. J.; Crooks, R. M. Langmuir 1992, 8, 2101. (23) Decher, G.; Hong, J.-D. Makromol. Chem., Macromol. Symp. 1991, 46, 321. (24) Decher, G.; Hong, J.-D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (25) Decher, G.; Hong, J.-D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 504. (26) Kohli, P.; Scranton, A. B.; Blanchard, G. J.; Macromolecules 1998, 31, 5681.
Introduction The design and synthesis of thin films and chemically modified surfaces has been an area of intense research activity because of the potential utility of these structures. Various applications, including optical second harmonic generation,1 chemical sensing,2 electrical or environmental isolation,3 electronic rectification,4 and photoreactivity5-7 have been either proposed or demonstrated, underscoring the importance of materials advances in this area. Of particular significance to interfacial materials and thin films is the ability to grow layered materials where there is good control over the layer thickness and uniformity. Layer-by-layer deposition of films can provide spatial resolution and directionality, and both of these structural properties can be critical to the macroscopic properties of the system. To achieve controlled, layer-by-layer growth requires the development of efficient and robust means of connecting individual layers. Several different techniques have been devised for linking individual molecular layers, including covalent,8-11 ionic-covalent,12-18 coor-
10.1021/la9807917 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/21/1999
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Scheme 1. Synthetic Route for Poly(NPM-VEP). See Text for Details
6
the thickness of ultrathin polymer layers (on the order of tens of angstroms), the formation of macroscopic defects such as bubbles or pinholes is minimized, and the layers formed can be both chemically and thermally robust. Experimental Section The alternating copolymers of N-phenylmaleimide (NPM) and (2-vinyloxy)-ethylphosphonate (VEP) and of N-biphenylmaleimide (NBM) and VEP are prepared by radical copolymerization using azobisisobutyronitrile (AIBN) as the initiator.26,27 The reaction between NPM and VEP is shown in Scheme 1 and is the same save for the maleimide monomer for poly(NBM-VEP) synthesis. No monomer homopolymerization was seen for our reaction conditions. VEP is prepared by the reaction of excess tri(isopropyl)phosphite with 2-chloroethylvinyl ether at 170 °C for 5 days in an argon atmosphere.28 Distillation of the product yields ∼72% VEP. 1H NMR (300 MHz, d-CHCl3): δ 1.3 (d, 12 H of isopropyl), 2.1 (m, 2 H of CH2 adjacent to phosphonate group), 3.9 (m, 2 H adjacent to oxygen of vinyl ether), 4.0 and 4.2 (dd, 1 H each of vinyl group), 4.6 (m, 2 H of isopropyl group), 6.3 (dd, 1 H from vinyl group adjacent to oxygen). The monomer NEM was purchased from Aldrich Chemical Co. and used after recrystallization from hexanes. NBM was synthesized according to the following procedure. 4-Phenylaniline (1 g) was dissolved in CHCl3 (10 mL) and added dropwise to a solution of maleic anhydride (0.71 g) in CHCl3 (5 mL) over a period of 1 h. The reaction was allowed to stir for an additional 2 h. The resulting amic acid appeared as a bright yellow precipitate and was separated from the supernatant by filtration. Biphenylamic acid (0.4 g) was cyclized by heating to ∼60 °C for 2 h in a solution of 6.4 mL of acetic anhydride and 0.072 g of anhydrous sodium acetate. The resulting solution was cooled to room temperature and added to 50 mL of ice water. The NBM product was filtered, dried, and recrystallized from hexanes (80% yield). 1H NMR (300 MHz, d6-dimethylsulfoxide (DMSO)): δ 7.20 (2 H, s), δ 7.46 (5 H, m), δ 7.70 (2 H, d), δ 7.77 (2 H, d). The copolymer is synthesized by reacting equimolar amounts of the maleimide and VEP in CHCl3 at 60 °C under a N2 atmosphere for ∼18 h using AIBN as the initiator. 1H NMR (300 MHz, d6-DMSO): δ 1.0-1.4 (6 H, VEP isopropyl groups), δ 1.82.2 (2 H, VEP CH2 adjacent to phosphonate group), δ 3.0-4.0 (2 H, succinimide ring + 2 H adjacent to vinyl ether oxygen + 2 H from ethyl group), δ 4.3-4.7 (1 tertiary isopropyl H + 1 tertiary H adjacent to oxygen), δ 7.0-7.4 (5 H, phenyl ring). 13C NMR (d-CHCl3, 75.46 MHz): δ 24, 29.5, 38-42, 46-54, 71, 77, 125135, 174-180. For our experimental conditions, statistically one of the isopropyl groups terminating each phosphonate oxygen is hydrolyzed to yield a hydroxyl group during the course of the polymerization. This displacement is likely due to the presence of HCl formed in CHCl3 solution by AIBN. Gel permeation chromatography (GPC) characterization of the resulting NPMVEP polymer shows Mn ) 7200, Mw ) 10 800, yielding a polydispersity of 1.5. The reaction of either poly(NPM-VEP) or poly(NBM-VEP) with bromotrimethylsilane in anhydrous CH2Cl2 at room temperature for ∼2 h yields a polymer that is partially (27) The copolymerization of maleimide and vinyl ether usually results in formation of alternating copolymers; for example, see: (a) Olson, K. G.; Bulter, G. B. Macromolecules 1984, 17, 2480. (b) Olson, K. G.; Bulter, G. B. Macromolecules 1984, 17, 2486. (28) Rabinowitz, R. J. Org. Chem. 1961, 26, 5152.
Figure 1. Schematic representation of layered growth of poly(NPM-VEP) indicating direct priming chemistry of silanol groups with POCl3 and growth of layered assemblies using partial hydrolysis of the -PO3HR functionalities to control the available concentration of active PO32- sites layer by layer. hydrolyzed, making it useful for the formation of multilayer assemblies.29 The characterization of these films was accomplished using optical null ellipsometry (Rudolph Auto-EL II), 13C (100.6 MHz) and 31P (161.9 MHz) solid-state cross-polarization/magic angle spinning (CPMAS) NMR (Varian VXR 400 MHz), FTIR (Nicolet Magna 550), and UV-visible (Unicam model UV-2) spectroscopies. We discuss the results of these measurements in the following section.
Results and Discussion The primary focus of this paper is on the synthesis and properties of layered poly(NPM-VEP) and poly(NBM-VEP) where the layer-by-layer growth of the material is controlled with molecular thickness resolution. We consider first the formation of the layers and the chemical basis for layer formation using this polymer. We then consider the nature of the polymer itself, specifically whether it is an alternating copolymer or a random copolymer, because this issue bears on the extent to which these polymer layers can be considered homogeneous. We discuss the potentially complex structural issues associated with the formation of these polymer layers, including the Zr/P ratio in the films as a function of number of polymer layers. These data in conjunction with FTIR measurements point to the incomplete reaction of hydrolyzed phosphonate groups and thus the possibility of interlayer ZP bonding between nonadjacent layers. The deposition of polymer layers is shown schematically in Figure 1. For ellipsometry measurements we have used Si substrates. To grow these layered materials, the silicon substrate is treated to produce a ∼15 Å thick native oxide layer. The resulting silanol groups are phosphorylated using POCl3 in dry acetonitrile, followed by hydrolysis and zirconation in ZrOCl2 solution. This procedure is a departure from other reports on the preparation of Si and (29) McKenna, C. E.; Schmidhauser, J. J. Chem. Soc., Chem. Commun. 1979, 739.
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Figure 3. Absorbance of poly(NBM-VEP) as a function of number of layers. The bands at 200 and 260 nm both exhibit a linear dependence of absorbance on number of layers. For the 260 nm band, the absorbance is ∼ 0.01 au/layer (inset).
Figure 2. (a) Ellipsometric data for 10 layers of poly(NPMVEP). (b) Ellipsometric data for eight layers of poly(NBM-VEP). The layer thicknesses are 16 Å/layer for poly(NPM-VEP) and 31 Å/layer for poly(NBM-VEP).
SiOx substrates for ZP layer growth. Typically the bare substrates are oxidized (Si) and hydrolyzed (Si and SiOx) in the same manner as we have done here. After the preparation of a hydrolyzed SiOx layer, treatment with either triethoxyaminopropylsilane or methoxydimethylaminopropylsilane to yield an aminated surface is the typical procedure. Reaction of the amino-terminated surface with POCl3 and H2O produces an aminophosphonic acid surface. In this work, we treat the surface silanol groups directly with POCl3 then H2O to produce a surface with the same properties as those achieved with the more widely used silane-based chemistry. Polymer layers are formed on the primed and zirconated substrate by adsorption of either poly(NPM-VEP) or poly(NBM-VEP) from acetonitrile at 50 °C over ∼12 h. Our X-ray photoelectron spectroscopy (XPS) data (vide infra) suggest that Zr4+ forms a strong complex with two phosphonic acid groups of the copolymers, whereas the remaining, partially hydrolyzed phosphonates are used in the formation of the next layer after the remaining isopropyl groups are displaced with bromotrimethylsilane (Figure 1). We demonstrated this procedure to form 10 layers of poly(NPM-VEP) and eight layers of poly(NBMVEP) on oxidized silicon. We do not intend to imply that these are limiting values for the number of layers that can be formed with this chemistry. Indeed, we see no evidence that would suggest any decrease in reactivity with the addition of polymer layers. We present measurements of the ellipsometric thickness as a function of the number of layers in Figure 2. Each data point represents the average of 27 measurements. Because the true refractive indices of the system are unknown, we used n ) 1.54 + 0i as the refractive index for the calculation of
thicknesses. We have used this value of n for other ZP systems. For poly(NPM-VEP), the slope of the line is 16.0 ( 0.9 Å with an intercept of 9.8 ( 5.5 Å. The intercept corresponds to the thickness of the oxidized layer on the Si substrate. For poly(NBM-VEP), we recover a thickness of 31.7 ( 1.3 Å per layer with an intercept of 12.2 ( 6.7 Å. The ellipsometric thickness data for these two polymer films can be used in concert with absorption and FTIR data to provide some insight into the organization of the layers. The most obvious conclusion that can be drawn from these data is that simple steric factors play an important role in determining the morphology of the individual layers. The observed linear increase in thickness with number of layers for poly(NBM-VEP) is consistent with the linear dependence of the film absorbance on number of layers for films grown on a quartz substrate (Figure 3). The absorptive resonance centered at 260 nm is associated with the N-succinimidobiphenyl chromophore. Finding a linear relationship between film thickness and number of layers is certainly not a surprising result for simple ZP systems where the organobisphosphonate layer constituents are arranged in a sufficiently orderly manner to allow quantitative predictions of layer thickness to be made and verified experimentally. For these polymers, where the layer structure and interlayer bonding arrangements are expected to be highly irregular, it may therefore be somewhat surprising to find that the layer growth appears to be so regular. The simplest interpretation of these data is that the linear alternating copolymers lie approximately flat on the interface surface and the density of the layers is significant. We can estimate the density from the absorbance measurements. For substituted biphenyl chromophores, the extinction coefficient of the 260 nm resonance is typically on the order of ∼18,000 L/mol cm.30 This value for the extinction coefficient corresponds to an absorption cross section of 3 × 10-17 cm2/chromophore. The data shown in Figure 3 yield an absorbance of 0.01 per bilayer (single layer on each side of the substrate), consistent with a surface density of 1.65 × 1014 chromophores/cm2 layer. If half of the polymer phosphonate groups are hydrolyzed at the time of deposition, the interlayer bonding density would be ∼8 × 1013/cm2 layer. Modeling a silica surface as a (30) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules, 2nd ed.; Academic: New York, 1971.
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Figure 4. Schematics of poly(NPM-VEP) and poly(NBM-VEP). The distances indicated for the oligomers were determined using molecular mechanics calculations. The structures presented in the figure are not energetically optimized and are intended for illustrative purposes only.
cubic close-packed array of silanols with a 5 Å separation between silanol groups yields a surface silanol density of 4 × 1014 sites/cm2. This simple calculation certainly overestimates the silanol density for a flat surface and at the same time fails to account for surface roughness, and these two oversimplifications act in opposition to one another. Given that this is intended to be a qualitative estimate, the agreement between measured chromophore density and estimated bonding site density is remarkable, and it suggests that something on the order of a third of the available bonding sites on the substrates are occupied. If the estimate of polymer layer density is close to correct, the measured thickness of 16 Å/layer for poly(NPM-VEP) and 31 Å/layer for poly(NBM-VEP) suggests that on average, each polymer layer is about 1 molecule thick. Molecular mechanics calculations on the basic oligomer unit suggest a 15 Å/layer thickness for the poly(NPMVEP) and 21 Å/layer for poly(NBM-VEP), as indicated in Figure 4. It is likely that the thicknesses we measure that are greater than those predicted by molecular mechanics calculations are reflective of the complex and are likely to be entangled structure of the polymer molecules within each layer. An important consideration for these materials is the nature of the ZP chemistry used to connect polymer layers. One way to determine the nature of interlayer bonding is through XPS measurements of the layer constituents. The XPS data we report here (Figure 5) suggest that the Zr(PO3R)2 layers are very similar for polymer and alkanebisphosphonate multilayers. For ZP multilayers formed using bisphosphonated alkanes, the spacing between the active phosphonate sites is determined either by the substrate or by the organic gallery constituent, but in either case, phosphonate groups are sufficiently close to allow the formation of Zr(PO3R)2 sheets that resemble the structure of solid Zr(PO3R)2. We show in Figure 5a a survey scan of an eight-layer film of poly(NPM-VEP) on an oxidized Si substrate. We assign the peaks as follows; Si (2p resonances at 98.7 eV for elemental Si and 102.5 eV for SiOx), P (2s at 191.5 eV, 2p at 135 eV), Zr (3d5/2 at 183.5 eV, 3d3/2 at 185.8 eV), C (1s at 285 and 289 eV), N (1s at 401.5 eV), and O (1s at 532.5 eV). In addition, there are Auger resonances for O at ∼750 eV and C at ∼1 keV. Indications of changes exist in the interface thickness as a function of layer addition through the elemental ratios. As the number of poly(NPM-VEP) layers increases, we observe a change in the relative amount of Zr4+ from 0.46% for a monolayer to 3.27% for eight layers. We also observe a change in the C1s/Si2p ratio, which varies from 3.3 for
Figure 5. XPS data for layers of poly(NPM-VEP). (a) Survey scan indicating the elements present. (b) Ratio of Zr/P determined from XPS data as a function of number of layers.
a monolayer to 22.0 for eight layers. Both pieces of information point to layered growth. For the Zr data, the change in measured composition is not due to changes in the metal ion loading density, but rather is a consequence of the screening of the substrate by the polymer layers. We can extract information on the interlayer chemistry from the XPS Zr/P ratio. The Bein group reported that, for up to three layers of 1,10-decanediylbis(phosphonic acid) (DDBPA) on carbon fibers, the experimental Zr/P ratio is ∼1.1.31 They calculated the Zr/P ratio for an ideal system with stoichiometric amounts of Zr and P and reported that the measured ratio should decrease from ∼1 for a monolayer to about 0.6 for a three-layered structure. The Mallouk group calculated a Zr/P ratio of 0.63 ( 0.08 for multilayers formed from a Zr phosphonate and polycation polymer layered structure, where they used an inelastic mean free path of 15 Å for both Zr and P photoelectrons.32 We recover experimentally for our polymer multilayers a layer-dependent Zr/P ratio that is consonant with these predictions. Our data for poly(NPMVEP) show the same elemental ratio as those for multilayers of 1,12-dodecanediylbis(phosphonic acid). We show these data in Figure 5b. The Zr/P ratio decreases as the number of layers increases for both systems. For a phosphonated and zirconated substrate, we recover a Zr/P ratio of 1.6 for each substrate. This ratio is the same for direct phosphonate-primed substrates and for those primed using aminopropylsilane chemistry. These results suggest that the phosphonate group density is substantially the same for both surface treatments. We note also, as has been considered before, that the elemental ratios recovered depend on stoichiometry, layer thickness, and (31) Hoekstra, K. J.; Bein, T. Chem. Mater. 1996, 8, 1865. (32) Kim, H.-N.; Keller, S. W.; Mallouk, T. E. Chem. Mater. 1997, 9, 1414.
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Figure 6. FTIR spectrum of a bilayer of poly(NPM-VEP) on oxidized Si. Band assignments are as indicated in the text.
mean free path of the X-ray photoelectrons. For a single polymer layer we recover a Zr/P of 1, which decreases to 0.66 for eight layers. Our data are fully consistent with other reports on the Zr/P ratio in similar systems, and on the basis of these data, it appears that the interlayer bonding for the polymer multilayers is the same as for the presumably more ordered Zr-DDBPA layers. These XPS data are also significant in that they demonstrate our ability to react the protected isopropyl phosphonate functionalities stoichiometrically. This is a concern because of the statistical nature of the hydrolysis/deprotection chemistry we use. To test the stability of the poly(NPM-VEP) and poly(NBM-VEP) multilayers, we immersed films of both polymers in n-hexadecane at ∼100 °C and in boiling ethanol, for 2 h in each case. These solvents were chosen to test the solubility of the polymers in both polar and nonpolar environments. In all cases, we found no change in either ellipsometric thickness or FTIR spectra resulting from exposure to solvent. This result is consistent with ZP multilayers formed from small molecules. We believe that the primary reason for the insolubility of the polymer multilayers is the same as that for the small molecule systems: the Zr(O3PR)2 structure is characterized by its sparing solubility in most solvents. It is important to characterize the polymers within the multilayer structure. We show in Figure 6 the FTIR spectrum of a bilayer of poly(NPM-VEP) adsorbed on a Si substrate. The bands in the 2700 cm-1-3100 cm-1 region are the CH stretching resonances. Although there is a substantial body of literature relating band position to layer structure for alkanethiols on gold, the analogous information is not available from the data for this polymer because of the relatively small amount of aliphatic CH2 functionality in this system. Carbonyl group stretching resonances are seen in the ∼1700 cm-1-1800 cm-1 region and the PdO and P-O bands lie in the 1000 cm-1-1200 cm-1 range. Although these data confirm the presence of the functional groups and are consistent with the formation of layers of poly(NPM-VEP), there is little explicit structural information on the polymer contained within them. In an effort to better understand the nature of the polymer layers, we acquired 13C and 31P CPMAS NMR spectra of copolymers on high surface area silica.33 We show in Figure 7 the 13C CPMAS spectrum of poly(NPMVEP) adsorbed on silica. The resonances in the region of 10-50 ppm belong to the aliphatic carbons in the polymer backbone and side chains. The carbonyl- and phenylcarbon resonances associated with the N-phenylsuccin(33) The procedure and conditions for adsorption of poly(NPM-VEP) layer on silica having a surface area of 450 m2/gm are same as described above for the adsorption of polymer on Si substrate except that the silica mixture was stirred during phosphorylation, zirconation, and deposition of polymeric layers.
Figure 7. 13C NMR spectrum of poly(NPM-VEP) grown on high surface area porous silica. Resonance assignments are as indicated in the inset. The absence of discernible progressions in the spectrum indicates the alternating nature of the polymer. The initial ratio of fully to partly hydrolyzed phosphonate is ∼2 monohydrolyzed to 1 dihydrolyzed group based on NMR integration of iPr methyl group protons relative to phenyl ring protons.
imide moiety are observed in the 150-180 ppm region and 120-140 ppm region, respectively. The form of these data, especially the absence of discernible progressions, suggests that poly(NPM-VEP) is an alternating copolymer. We recognize that the characterization of poly(NPM-VEP) copolymerization by NMR is limited by the spectral line widths, but these findings are consistent with the fact that neither monomer homopolymerizes under our experimental conditions. The characterization of the polymer layers using optical null ellipsometry, FTIR, and 13C NMR demonstrates the presence of multiple polymer layers. It is also important to evaluate the extent to which these layers resist chemical and thermal exposure. We used a 10-layer sample for these experiments. Exposure of this sample to boiling ethanol (78 °C) for 1 h and to boiling hexadecane (120 °C) for 2 h produced no significant changes in ellipsometric thickness or FTIR spectra. These data indicate significant mechanical and chemical stability for these layers. Conclusion We synthesized poly(NPM-VEP), a chemically and thermally stable alternating copolymer. By taking advantage of a strategy of partial, stepwise hydrolysis, we demonstrated the ability to form robust layered polymer structures. Because it is possible to construct these materials with molecular layer resolution, control over the chemical identity of each layer can be established by simple exposure to different polymers for each layer. We expect that the N-substitution of the succinimide moiety will lead to control over polarity of the layers and control over the length of the vinyl ether phosphonate will allow adjustment in layer thickness. We anticipate that these novel materials will find utility in the design of chemically selective surfaces with controlled porosity. Acknowledgment. We are grateful for support of this work through Grant CHE 95-08763 from the National Science Foundation. We thank Dr. P. Askeland of the MSU Composite Materials and Structures Center for his assistance in acquiring the XPS data. We are grateful to Professor G. L. Baker for several stimulating conversations and for the use of his GPC equipment. LA9807917
