Model Fluorous Polyurethane Surface Modifiers Having Co

Sep 7, 2007 - Kenneth J. Wynne*, Umit Makal, Pinar Kurt, and Lara Gamble. Department of Chemical and Life Science Engineering, Virginia Commonwealth ...
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Langmuir 2007, 23, 10573-10580

10573

Model Fluorous Polyurethane Surface Modifiers Having Co-polyoxetane Soft Blocks with Trifluoroethoxymethyl and Bromomethyl Side Chains Kenneth J. Wynne,*,† Umit Makal,† Pinar Kurt,† and Lara Gamble‡ Department of Chemical and Life Science Engineering, Virginia Commonwealth UniVersity, Richmond, Virginia 23284, and NESAC/BIO, Department of Bioengineering, UniVersity of Washington, Seattle, Washington 98195-1750 ReceiVed June 7, 2007. In Final Form: July 23, 2007 Polyurethanes containing poly(2-trifluoroethoxymethyl-2-methyl)-co-(2-bromomethyl-2-methyl)-1,3-propylene oxide (co-polyoxetane) soft blocks, P[3FOx:BrOx-m:n], were prepared and used (0.5-2 wt %) to modify the surface properties of a conventional polyurethane. The substrate polyurethane was composed of an isophorone diisocyanate/ butanediol hard block and a polytetramethylene oxide soft block [IPDI/BD(50%)-PTMO(2000)]. A combination of tapping mode atomic force microscopy (TM-AFM), X-ray photoelectron spectroscopy (XPS), and dynamic contact angle (DCA) studies showed that the fluorous polyurethane surface modifiers confer surface properties similar to those of the parent at 0.5-1.0 wt %. The retention of initial wetting behavior in water was enhanced with higher ratios of 3FOx:BrOx that corresponds to increasing fluorous character. A semifluorinated chaperone is necessary to surface concentrate -CH2Br groups. Negligible Br was detected by XPS when the P[BrOx]-soft block polyurethane was used as a surface modifier (0.5%) and the wetting behavior was similar to that of the bulk polyurethane. Despite being hydrophobic (θadv ) 102°) the P[BrOx]-soft block polyurethane is not a polymer surface modifier under the conditions described herein. The calculated solubility parameters for PTMO and P[BrOx], which are similar, support the notion of BrOx miscibility with the base polyurethane. The combination of miscibility of BrOx repeat units and lack of an end-group-like architecture minimizes BrOx surface concentration in the chosen bulk polyurethane.

Introduction Surface modification of polyurethanes and related segmented copolymers remains of interest in the exploration of improved hydrophobicity, oleophobicity, biocompatibility, and biostability. Ideally, polymer surface modifiers confer the surface property associated with the modifier, while retaining the bulk properties of the majority polymer. The unique cohesive energy density of semifluorinated groups1 leads to distinctive solubility characteristics.2 This has been extensively exploited with “fluorous” tags or “pony tails”3 that permit recycling valuable catalysts4,5 and extraction of specific intermediates and products6 and that provide candidates for novel ionic liquids.7 Using the same fluorous character that confers distinctive solubility characteristics, we describe surface modification using co-telechelic polyurethanes. The objective is twofold. First, low surface energy semifluorinated groups A are used to surface concentrate a second constituent B of the copolymer soft block (Figure 1). Second, as shown in Figure 1, the P[AB] polyurethane is selectively concentrated at the surface of a conventional polyurethane. The approach parallels the application of fluorous tags and ponytails to separations and catalysis, noted above, and * Author to whom correspondence may be sent. E-mail: [email protected]. † Virginia Commonwealth University. ‡ University of Washington. (1) Lee, J. N.; Park, C.; Whitesides, G. M. Anal. Chem. 2003, 75, 6544-6554. (2) Thomas, R. R.; Anton, D. R.; Graham, W. F.; Darmon, M. J.; Sauer, B. B.; Stika, K. M.; Swartzfager, D. G. Macromolecules 1997, 30, 2883-2890. (3) Deelman, B.-J. Handbook of Fluorous Chemistry; Gladysz, J.A., Curran, D. P., Horvath, I. T., Eds.; Wiley-VCH: UK, Weinheim, 2005; Vol. 347. (4) Chu, Q.; Zhang, W.; Curran, D. P. Tetrahedron Lett. 2006, 47, 92879290. (5) Tesevic, V.; Gladysz, J. A. J. Org. Chem. 2006, 71, 7433-7440. (6) Zhang, W.; Curran, D. P. Tetrahedron 2006, 62, 11837-11865. (7) Emnet, C.; Weber, K. M.; Vidal, J. A.; Consorti, C. S.; Stuart, A. M.; Gladysz, J. A. AdV. Synth. Catal. 2006, 348, 1625-1634.

Figure 1. Schematic of a conventional polyurethane (soft block, solid line) and a PSM PU having a copolymer soft block (dashed line) with A and B side chains (one set shown).

incorporates additional features that enhance thermodynamic driving forces for surface concentration. The use of semifluorinated groups is well-known to confer oleophobicity to surfaces in addition to hydrophobicity. This has been demonstrated for acrylates,2 silicones,8 and oxetanes.2,9-12 Perfluorinated polyethers have been used to make silicone surfaces oleophobic as well as hydrophobic.13 The work of Vogl is related to the functional surface modifier concept described herein. Vogl placed fluorinated groups on benzotriazole monomers and generated copolyacrylates wherein the UV absorbing groups were surface concentrated.14 He described this concept as “morphology engineering or surface stratification” and noted that it was “...desirable to synthesize UV stabilizers that migrate to the polymer surface, protecting the polymer where it is most needed.”14 (8) Owen, M. J. J. Appl. Polym. Sci. 1988, 35, 895-901. (9) Malik, A. A.; Carlson, R. P. U.S. Patent 5,637,772, 1997. (10) Thomas, R. R.; Ji, Q.; Kim, Y. S.; Lee, J. S.; McGrath, J. E. Polyurethane 2000 Polymer DiVision Abstracts; 2000. (11) Thomas, R. R.; Anton, D. R.; Graham, W. F.; Darmon, M. J.; Stika, K. M. Macromolecules 1998, 31, 4595-4604. (12) Kim, Y. S.; Lee, J. S.; Ji, Q.; McGrath, J. E. Polymer 2002, 43, 71617170. (13) Thanawala, S. K.; Chaudhury, M. K. Langmuir 2000, 16, 1256-1260.

10.1021/la701684a CCC: $37.00 © 2007 American Chemical Society Published on Web 09/07/2007

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Polyurethane surface modifiers have been generated with semifluorinated end groups or soft blocks. Polyoxetane soft blocks with semifluorinated side chains make conventional polyurethanes water and oil resistant.2,9-12 A base polyurethane modified (2.5-5 wt %) with polyurethanes end-capped with fluorous tails conferred hydrophobic behavior, improved biostability, and reduced platelet adhesion.15-17 Santerre described an important extension of this work to compositions where one chain end comprised a fluorous tail and the other a peptide. When a modified base polyurethane was exposed to aqueous media, surface concentration of the peptide and enhanced cell compatibility were observed.18 Herein, polyurethane surface modification is explored using polymer surface modifiers (0.5-2 wt %) having co-polyoxetane soft blocks. The P[AB] soft block, where “P” indicates the polymerized form of the respective monomers, has the same 1,3-propylene oxide main chain, but different side chains (Figure 1). A focus of this research is studying the effectiveness and/or need for a fluorous group or “chaperone” A to surface concentrate a functional B group. A combination of features distinguish the P[AB] soft block approach for concentrating a desired functional group B at a polymer surface. This combination leverages (a) the tendency of soft blocks to surface concentrate,19-22 (b) the presence of multiple side chains that act as pseudochain ends,23 (c) differing solubility parameters for the P[AB] soft block and other coating constituents that enhance phase separation, and (d) a low P[AB] Tg that may enhance the effectiveness of the B surface function. The fluorous P[AB] soft blocks described herein employ a trifluoroethoxymethyl A group. Surface modification is thus driven by a multiplicity of Rf1 groups. This approach may be compared to the use of polyurethane end groups where the fluorinated “tails” are a mixture of longer Rf end groups including Rf8.15-17 Another comparison may be made to reagents with Rf8, Rf10, and higher semifluorinated pony tails that are designated “green” intermediates or catalysts by virtue of recyclability coupled with enhanced reactivity.4,24 Analytical studies that show the worldwide presence of perfluorooctoate or Rf8 in mammals raises some concern about technology employing longer fluorocarbon tails.25,26 We have previously used the fluorous P[AB] approach to surface concentrate biocidal hydantoin and quaternary alkylammonium moieties.27,28 The present study employs a P[AB] soft block derived from hydroxy-terminated poly(2-trifluoroethoxymethyl-2-methyl)-co-(2-bromomethyl-2-methyl)-1,3-propylene oxide telechelics 1 designated P[3FOx/BrOx-m:n].

Earlier, we showed that the -CH2Br side chain synergistically enhances the hydrophobicity of the 3FOx group.29 Herein we investigate P[3FOx/BrOx-m:n] polyurethanes as polymer surface modifiers (PSMs). A combination of tapping mode atomic force (14) Stoeber, L.; Sustic, A.; Simonsick, W. J.; Vogl, O. J. Macromol. Sci. Pure Appl. Chem. 2000, 37, 943-970. (15) Tang, Y. W.; Santerre, J. P.; Labow, R. S.; Taylor, D. G. J. Appl. Polym. Sci. 1996, 62, 1133-1145. (16) Tang, Y. W.; Santerre, J. P.; Labow, R. S.; Taylor, D. G. J. Biomed. Mater. Res. 1997, 35, 371-381. (17) Massa, T. M.; McClung, W. G.; Yang, M. L.; Ho, J. Y. C.; Brash, J. L.; Santerre, J. P. J. Biomed. Mater. Res., Part A 2007, 81A, 178-185. (18) Ernsting, M. J.; Bonin, G. C.; Yang, M.; Labow, R. S.; Santerre, J. P. Biomaterials 2005, 26, 6536-6546.

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microscopy (TM-AFM), X-ray photoelectron spectroscopy (XPS), and dynamic contact angle (DCA) analysis is utilized to estimate the surface concentration of the surface modifier and fidelity of surface properties vis-a`-vis the (P[3FOx/BrOx-m:n]) polyurethanes alone. We report that the synergistic copolymer effect, which enhanced hydrophobicity for P[3FOx/BrOx-m:n]-soft block polyurethanes, is not observed when 3FOx/BrOx-m:n polyurethanes are used as 2 wt % surface modifiers. Furthermore, the -CH2Br side chain alone in a P[BrOx] polyurethane is not surface concentrated at all. Experimental Section In designating polyurethanes such as IPDI-BD(40)/P[3FOx/ BrOx-1:1-4700] the hard block composition is followed by hard block wt % in parentheses. The soft block composition is last followed by soft block number average molecular weight. Compositions and corresponding designations for the polyurethane coatings are shown in Table 1. For modified base polyurethanes, designations such as 0.5%PU-5 are used to indicate that the composition is 99.5 wt % PU-1 and 0.5 wt % PU-5. Materials. 3-(2,2,2-Trifluoroethoxymethyl)-3-methyloxetane (3FOx) and 3-bromomethyl-3-methyloxetane (BrOx) were synthesized by published procedures30 or were generously provided by Gencorp Aerojet (Sacramento, CA) or OMNOVA Solutions (Akron, OH). Monomers were distilled under vacuum before use: 3FOx ≈ 100 °C/5 mmHg; BrOx ≈ 85 °C/5 mmHg. Boron trifluoride dietherate (BF3O(C2H5)2), was used as received (Aldrich). Methylene chloride, tetrahydrofuran (THF), dimethylformamide (DMF), and methanol were either used as received or dried and stored over 4 Å molecular sieves (Acros). Isophorone diisocyanate (IPDI, 98%), butane diol, poly(tetramethylene oxide) (PTMO-2000) and dibutyltin dilaurate catalyst (T-12) were obtained from Aldrich and used as received. Synthesis. The synthesis and characterization of isophorone diisocyanate/butane diol polytetramethylene oxide polyurethane [IPDI BD(50)/PTMO(2000)] followed a procedure described previously.31 The synthesis of P[3FOx] and P[BrOx] telechelics has also been reported;31 the method follows closely that originally described by Malik.32 The P[3FOx/BrOx-m:n] telechelics were prepared in a manner identical to that for the homo-telechelics.31 The experimentally determined ratio of 3FOx/BrOx was close to the feed ratio.31 The preparation and characterization of IPDI-BD(40)/P[3FOx/ BrOx-m:n] polyurethanes have been reported.31 IPDI-BD(40)/ P[3FOx/BrOx-m:n] polyurethanes were typically purified by reprecipitation of THF solutions into deionized water/methanol. Without adequate purification, coatings can contaminate the water surface, thereby changing surface tension and contact angles. Coating Process. Coatings for DCA analysis were prepared by dip coating from THF solutions onto glass cover slips (Corning, 24 mm × 40 mm × 1.2 mm). Care was taken to distribute the (19) Garrett, J. T.; Runt, J.; Lin, J. S. Macromolecules 2000, 33, 6353-6359. (20) Garrett, J. T.; Siedlecki, C. A.; Runt, J. Macromolecules 2001, 34, 70667070. (21) Ratner, B. D.; Cooper, S. L.; Castner, D. G.; Grasel, T. G. J. Biomed. Mater. Res. 1990, 24, 605-620. (22) Tingey, K. G.; Andrade, J. D. Langmuir 1991, 7, 2471-2478. (23) Jalbert, C.; Koberstein, J. T.; Hariharan, A.; Kumar, S. K. Macromolecules 1997, 30, 4481-4490. (24) Tesevic, V.; Gladysz, J. A. Green Chem. 2005, 7, 833-836. (25) Martin, J. W.; Whittle, D. M.; Muir, D. C. G.; Mabury, S. A. EnViron. Sci. Technol. 2004, 38, 5379-5385. (26) Ellis, D. A.; Mabury, S. A.; Martin, J. W.; Muir, D. C. G. Nature 2001, 412, 321-324. (27) Makal, U.; Uslu, N.; Wynne, K. J. Langmuir 2007, 23, 209 -216. (28) Kurt, P.; Wood, L.; Ohman, D. E.; Wynne, K. J. Langmuir 2007, 23, 4719-4723. (29) Makal, U.; Fujiwara, T.; Cooke, R. S.; Wynne, K. J. Langmuir 2005, 21, 10749-10755. (30) Malik, A. A.; Archibald, T. G. U.S. Patent 6,037,483, 2000. (31) Makal, U.; Uilk, J.; Kurt, P.; Cooke, R. S.; Wynne, K. J. Polymer 2005, 46, 2522-2530. (32) Malik, A. A.; Archibald, T. G.; Carlson, R. P.; Wynne, K. J.; Kresge, E. N. U.S. Patent 6,479,623, 2002.

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Table 1. Advancing and Receding Contact Angles for Polymer Surface Modifiers and Modified Polyurethanes dynamic contact angle (deg)a composition

designation

cycle 1 θadv/θrec

cycle 2 θadv/θrec

cycle 3 θadv/θrec

cycle 4 θadv/θrec

cycle 5 θadv/θrec

water cont.b

IPDI-BD(50)/PTMO(2000) IPDI-BD(40)/P[3FOx-3400] IPDI-BD(40)/P[BrOx-2800] IPDI-BD(40)/P[3FOx/BrOx-1:2-3400] IPDI-BD(40)/P[3FOx/BrOx-1:1-4700] IPDI-BD(40)/P[3FOx/BrOx-2:1-4700] 0.5 wt % PU-2/99.5 wt % PU-1 0.5 wt % PU-3/99.5 wt % PU-1 0.5 wt % PU-4/99.5 wt % PU-1 0.5 wt % PU-5/99.5 wt % PU-1 0.5 wt % PU-6/99.5 wt % PU-1

PU-1 PU-2 PU-3 PU-4 PU-5 PU-6 0.5%PU-2 0.5%PU-3 0.5%PU-4 0.5%PU-5 0.5%PU-6

85/55 105/45 102/42 104/34 116/33 108/35 104/41 88/50 100/44 104/40 104/41

82/55 99/45 101/41 102/34 115/32 108/35 104/42 85/51 99/46 103/41 104/42

82/56 98/46 101/41 102/32 116/32 108/35 102/42 83/53 96/48 102/43 103/43

81/56 98/46 101/40 102/32

81/56 98/46 101/40 102/32

108/34 101/43 82/54 94/49 101/43 102/44

108/34 101/43 81/55 93/49 101/44 102/44

no yes no yes no no no no yes no no

a

Experimental error ) (2°. b Indicates whether water contamination was observed in DCA analysis.

polyurethane evenly over both sides as the force measured in Wilhelmy plate analysis is proportional to sample perimeter. Uneven coatings give force vs distance curve (fdc) discontinuities that lead to inaccurate contact angles. Solvent was removed by inverting samples at ambient conditions for 24 h and then placing them in an oven overnight at 60 °C under reduced pressure. Typical coating thickness was 0.5 mm. Visually, all of the PU coatings were colorless and transparent with smooth, flat surfaces. The process for a representative coating containing 0.5 wt % PSM PU IPDI-BD(40)/P[3FOx/BrOx-1:1-4700] (PU-5, Table 1) is as follows: 5.4 g of base polyurethane, IPDI-BD(50)/PTMO(2000), PU-1, and 27 mg of IPDI-BD(40)/P[3FOx/BrOx-1:1-4700], (PU-5, Table 1) were dissolved in 28.1 mL of THF, so that the overall composition was 0.5 wt % PSM and 15-20 wt % solid content. Cover glass slides were dip coated from this solution. Solvent was removed at 60 °C in a vacuum oven. Atomic Force Microscopy (AFM). Surface images were obtained using a Digital Instruments (Santa Barbara, CA) dimension mode Nanoscope IIIA. TM-AFM was utilized for topographic and phase contrast images, which were acquired with standard silicon tips. Several setpoint ratios (rsp) were employed, where rsp ) Aexp/Ao ) 0.96-0.70, Aexp is the experimental set point amplitude, and Ao is the free oscillation amplitude. The same polyurethane coatings were used for Wilhelmy plate measurements, AFM imaging, and XPS analysis. X-ray Photoelectron Spectroscopy (XPS). Spectra were taken on a Surface Science Instruments X-probe spectrometer. This instrument has a monochromatized Al KR X-ray beam. Pressure in the analytical chamber during spectral acquisitions was less than 5 × 10-9 Torr. Pass energy for survey spectra (composition) was 150 eV, and pass energy for high-resolution C1s (HRC) scans was 50 eV. The takeoff angle is the angle between the sample normal and the input axis of the energy analyzer. A takeoff angle of 55° was employed to provide elemental composition from a depth of about 5 nm. Wetting Behavior. Dynamic contact angle (DCA) analysis based on the Wilhelmy plate method33 was carried out with a Cahn model 312 Analyzer (Cerritos, CA). The surface tension quantification limit of the instrument is 0.1 dyn/cm. The probe liquid was ∼18.2 MΩ cm deionized water from a Milli-Q (Millipore) system. The surface tension of the probe liquid was checked daily and was typically 72.6 ( 0.5 dyn/cm. Beakers used for DCA analysis were cleaned by soaking in an isopropanol/potassium hydroxide base bath for at least 24 h, rinsed for 30 s with hot tap water and then rinsed another 30 s with Nanopure water. In a typical determination, a coated slide was attached to the electrobalance via a clip, and the stage with the beaker of water was automatically raised and lowered to allow water to impinge upon the slide. By analyzing the resulting force versus distance curves (fdc’s), advancing (θadv) and receding (θrec) contact angles were (33) Wilhelmy, L. Ann. Phys. Chem. Leipzig 1863, 119, 177.

obtained. Unless otherwise noted, the stage speed was 100 µm/s, and the dwell time between advancing and receding test segments was 10 s. If insoluble species diffuse from polymer samples, water surface tension changes and, depending on the extent of contamination, confounds determination of intrinsic contact angles. Contamination is difficult to detect by sessile drop methods34 but is easily discerned by DCA analysis. During a DCA experiment, mass gain or loss during immersion/emersion is recorded by a sensitive electrobalance. The resulting change in mass is proportional to sample perimeter, water surface tension, and contact angle.35 The key to detecting water contamination is to test the water remaining after sample DCA analysis using a clean (flamed) glass cover slip. Successive force distance curves (fdc’s) should be superposable if water contamination has not occurred. If not superposable, the extent of deviations provides a qualitative measure of contamination. One column in Table 1 indicates whether water contamination was observed. When water contamination occurred, the accuracy of the reported contact angles suffered by an additional 1-2° compared to the accuracy noted for non-contaminating samples. In cases of water contamination (e.g., IPDI-BD(40)/P[3FOx-3400]) there is uncertainty concerning the origin of contact angle changes with succeeding cycles. That is, part of the change is due to surface reorganization and part is due to water contamination. Density. The density of 3FOx monomer (1.334 g/cm3) was estimated by weighing a known volume in a syringe.

Results and Discussion In this research the surface modification of a conventional polyurethane by a P[AB] soft block polyurethane was explored (Figure 1). Previously, a Finemann-Ross analysis was used to estimate the reactivity ratios for 3FOx and BrOx as well as copolymer composition.31 The final telechelic dyad composition obtained at complete reaction is not very different from a statistical copolymer. The small compositional perturbation of 0.5-2 wt % P[AB] polyurethane modifier ensures no change in overall polyurethane bulk properties.15 TM-AFM was used to examine changes in surface morphology due to the presence of polymer surface modifier. With a trifluoroethoxy “chaperone” A, evidence was sought from XPS for surface concentration of -CH2Br, which serves as a model functional group B. Wetting behavior via DCA analysis was used to evaluate the consequences of compositional and morphological effects observed by XPS and TM-AFM. Surface Imaging. Surface topology can strongly affect overall wetting characteristics.36-39 Surface roughness enhances surface (34) Uilk, J. M.; Mera, A. E.; Fox, R. B.; Wynne, K. J. Macromolecules 2003, 36, 3689-3694. (35) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley and Sons: New York, 1997. (36) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546-51.

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hydrophobicity or hydrophilicity depending on the initial equilibrium contact angle of the substrate with water.40,41 For the compositions reported herein, surface topology was investigated by TM-AFM.29 Root-mean-square roughness (Rq) for 2 µm × 2 µm areas was on the order of 4-10 nm. It is well-known that the surface roughness on this scale does not significantly affect the wetting behavior.42 TM-AFM in phase contrast mode is valuable for imaging the spatial variation of near-surface morphology in soft materials such as block copolymers and nanocomposites. In this mode, the phase shift of the tip oscillation associated with proximal interaction with differing morphological domains is measured.43 More interaction occurs with soft domains compared to those with hard domains, thus resulting in differing phase shifts. The scales of AFM phase images are set so that the harder phase induces a higher phase offset and appears lighter, whereas the softer phase appears darker.43 Phase images for segmented polyurethanes and polyurethane ureas reflect the presence of near-surface hard- and soft-block domains.19,44-46 If the bulk has a well phase-separated morphology, the surface is also phase separated with the soft block occupying the surface at the air/polymer interface.20 This is evidenced by a nearly featureless phase image at high setpoint ratios, that is rsp ) Aexp/Ao> 0.9, where Aexp is the experimental tip oscillation amplitude and Ao is the free oscillation amplitude. If the proximal interaction with the sample is increased by decreasing the setpoint ratio (harder tapping) near surface hard domains are resolved. The objective of the TM-AFM study was to investigate whether evidence could be obtained for the presence of the PSM at the surface. IPDI-BD(40)/P[3FOx/BrOx-1:1-4700], PU-5, was chosen as a representative polymer surface modifier. TM-AFM images of base polyurethane IPDI-BD(50)/PTMO, PU-1, IPDIBD(40)/P[3FOx/BrOx-1:1-4700], and modified coatings containing 0.5, 1.0, and 2.0 wt % IPDI-BD(40)/P[3FOx/BrOx-1: 1-4700] are shown in Figure 2. Soft tapping (rsp ) 0.96-0.91) and hard tapping (rsp) 0.70-0.73) were employed for imaging 2 µm × 2 µm areas. Previously, DSC analysis showed that IPDI-BD(40)/P[3FOx/ BrOx-1:1-4700] is well phase separated, with 90% of the soft block in the phase-separated soft block domain.31 TM-AFM images for IPDI-BD(40)/P[3FOx/BrOx-1:1-4700] are shown in Figure 2 A. The image on the left is for soft tapping (rsp ) 0.93), whereas that on the right is for hard tapping (rsp ) 0.70). The featureless phase image at soft tapping supports the notion that the surface is dominated by the low Tg soft block. Harder tapping reveals the presence of near-surface hard block as was observed by Runt for a well phase-separated MDI/ED/PTMO urethane-urea.20 The phase image for the base polyurethane IPDI-BD(50)/ PTMO(2000) shows the presence of hard block at both soft and hard tapping (Figure 2E). The presence of hard block even at soft (37) Youngblood, J. P.; McCarthy, T. J. Macromolecules 1999, 32, 68006806. (38) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 13771380. (39) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395-3399. (40) Wenzel, R. N. J. Ind. Eng. Chem. (Washington, D.C.) 1936, 28, 988-94. (41) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 357-360. (42) Nakae, H.; Inui, R.; Hirata, Y.; Saito, H. Acta Mater. 1998, 46, 23132318. (43) Magonov, S. N.; Elings, V.; Whangbo, M. H. Surf. Sci. 1997, 375, L385L391. (44) Sauer, B. B.; McLean, R. S. Macromolecules 1997, 30, 8314-8317. (45) Garrett, J. T.; Lin, J. S.; Runt, J. Macromolecules 2002, 35, 161-168. (46) Aneja, A.; Wilkes, G. L. Polymer 2003, 44, 7221-7228.

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tapping is consistent with a lower degree of bulk phase separation observed by DSC studies. For this polyurethane, only 62% of the soft block is in the soft block domain.31 From DSC and TM-AFM IPDI-BD(40)/P[3FOx/BrOx-1:14700] is well phase separated compared to IPDI-BD(50)/PTMO(2000). The average degree of polymerization for P[3FOx/BrOx1:1-4700] and PTMO(2000) is similar (33 and 28, respectively). Hence, the high degree of bulk phase separation for IPDI-BD(40)/P[3FOx/BrOx-1:1-4700] is due to limited miscibility of the fluorous soft block with the hard block and multiple low surface free energy side chains that act as pseudochain ends.23 For coatings of the base polyurethane to which increasing weight fractions of polymer surface modifier have been added, compositionally dependent changes are observed for the nearsurface morphology employing light tapping (Figure 2, B-1 to D-1). For 2%PU-5 and 1%PU-5 the phase image is similar to that of IPDI-BD(40)/P[3FOx/BrOx-1:1-4700]. For these compositions the P[3FOx/BrOx-1:1-4700] soft block is surface concentrated. Decreasing the modifier content further (Figure 2, D-1) yields a near-surface morphology similar to that of base polyurethane PU-1. The transition from PU-5-like to PU-1-like surface structure occurs between 1% and 0.5% PSM. TM-AFM phase contrast images at lower setpoint ratios probe near-surface morphology at greater depth.20,46 Phase images at harder tapping (Figure 2, B-2 to D-2, Asp/Ao ) 0.70-0.73) provide complementary information about near-surface hard block. For 2%PU-5 and 1%PU-5 (Figure 2, B-2, B-3) the light-colored domains characteristic of near-surface hard block domains are evident. The corresponding image for 0.5%PU-5 shows a surface morphology that is virtually indistinguishable from that of the base polyurethane PU-1. In summary, TM-AFM results at soft tapping provide evidence for the surface concentration of the P[3FOx/BrOx-1:1-4700] soft block at 1 and 2 wt % modifier. At harder tapping, phase contrast images for PU-5 modified PU-1 at 2 wt % and 1 wt % reveal a complex subsurface phase-separated morphology not observed for the PU-5 alone under comparable imaging conditions. X-ray Photoelectron Spectroscopy. Table 2 contains F and Br atom %’s for the P[3FOx/BrOx-m:n] polyurethanes and base polyurethane compositions modified with IPDI-BD(40)/P[3FOx/ BrOx-1:1-4700]. The C, N, and O surface atomic compositions, which are relatively insensitive to polyurethane blend compositions, are not shown but are provided in Supporting Information (Table 1S). Compositions employing IPDI-BD(40)/P[3FOx/ BrOx-1:1-4700] were examined at 1 wt % and 0.5 wt % levels (Table 2). The observed F and Br atom %’s are similar for both compositions. All other compositions were examined with 0.5 wt % polymer surface modifier. IPDI-BD(40)/P[3FOx/BrOx-1:1-4700]. The experimental atom % F (17.7%) for PU-5 alone (Table 2) may be compared with the bulk value (9.1%) and that for the P[3FOx/BrOx-1:1] soft block (16.8%). The good agreement of atom % F with that calculated for the soft block alone provides evidence for surface concentration of the soft block, as observed for fluorous or poly(dimethylsiloxane) surface modifiers.2,9-13,47-49 The -CF3 groups at the termini of the 3FOx side chains act as pseudochain ends, driving surface concentration.50 (47) Ho, T.; Wynne, K. J. Polym. AdV. Technol. 1994, 6, 25-31. (48) Ratner, B. D.; Yoon, S. C.; Kaul, A.; Rahman, R. In Polyurethanes in Biomedical Engineering II; Planck, H., Syre, I., Dauner, M., Egbers, G., Eds.; Elsevier: New York, 1986; Vol. 3, pp 213-229. (49) Tezuka, Y.; Fukushima, A.; Matsui, S.; Imai, K. J. Colloid Interface Sci. 1986, 114, 16-25. (50) Koberstein, J. T.; O’Rourke-Muisener, P. A. V.; Kumar, S. Macromolecules 2003, 36, 771-781.

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Figure 2. TM-AFM images, 2 µm × 2 µm (left, soft tapping; right, hard tapping): (A) PU-5, A-1, rsp ) 0.93, A-2, rsp ) 0.70; (B) 2.0%PU-5, B-1, rsp ) 0.91, B-2, rsp ) 0.71; (C) 1.0%PU-5, C-1, rsp ) 0.90, C-2, rsp ) 0.70; (D) 0.5%PU-5, D-1, rsp ) 0.91, D-2, rsp ) 0.71; (E) PU-1, E-1, rsp ) 0.95, E-2, rsp ) 0.70. Height images, z ) 15 nm, phase images, z ) 20°.

The observed Br atom % (2.9%) is closer to that calculated for the bulk (2.5%) than for the soft block alone (4.7%). The relatively low value for Br atom % suggests -CH2Br groups may be largely subsurface as depicted in Figure 3. Because photoelectron emission is an exponential function of depth, the partitioning depicted would result in a considerable decrease in the observed Br atom %. Thus, fluorinated moieties act as chaperones for concentrating -CH2Br groups near the surface, but CF3CH2OCH2- groups preferentially occupy the outermost surface. IPDIBD(40)/P[BrOx]. For this polyurethane we have previously shown by DSC that ∼90% of the P[BrOx] soft block is in the soft-block phase.29 Retention of the low telechelic Tg in the polyurethane soft block is characteristic of well phaseseparated polyurethanes such as IPDI-BD(40)/P[3FOx-3400] (PU-2)29 and conventional PTMO polyurethanes with MDI-BDbased hard blocks.51 (51) Velankar, S.; Cooper, S. L. Macromolecules 1998, 31, 9181-9192.

Table 2. Experimental (XPS/55° take off angle) and Calculated atom %’s for Fluorine and Bromine for 100% and 0.5 wt % PSM Polyurethane Coatings atom %, bulk observed sample

F

Br

atom % soft segment only

calculated F

PU-2 18.1 n/da 15.4 PU-3 1.8 7.0 0.0 PU-4 n/e n/e 5.9 PU-5 17.7 2.9 9.1 PU-6 n/eb n/eb 11.8 0.5%PU-2 17.9 n/da 0.07 0.5%PU-3 1.2 0.1 0.0 0.5%PU-4 12.2 1.8 0.03 0.5%PU-5 18.2 2.2 0.04 1.0%PU-5 18.0 2.4 0.08 0.5%PU-6 18.4 1.4 0.05

calculated

Br

F

Br

0.0 6.3 3.3 2.5 1.8 0.0 0.03 0.02 0.01 0.02 0.01

25.1 0.0 12.6 16.8 19.7 25.1c 0.0c 12.6 c 16.8 c 16.8 c 19.7 c

0.0 14.3 7.1 4.7 3.0 0.0 c 14.3 c 7.1 c 4.7 c 4.7 c 3.0 c

a n/d: Not detected. b n/e: Not examined. c atom %’s for soft block alone.

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calculation52 of rms end-to-end distance (2.2 nm) for P[3FOx/ BrOx-1:1-4700] is provided in the Supporting Information. A combination of bulky side groups and the neglect of restrictions on bond rotation makes it likely that this estimate is low by a factor of ∼2.52 Thus, atomic composition determined by XPS is strongly influenced by even a “single layer” of soft-block domain. Figure 3. Depiction of near surface partitioning of 3FOx and BrOx side chains.

As for neat IPDI-BD(40)/P[3FOx/BrOx-1:1-4700], the observed atom % Br for 0.5%PU-5 is about half (2.2%) that calculated for the soft block alone (4.7%). While the surface concentration of Br shows the chaperone effect of fluorous side chains, an analogous partitioning of the -CH2Br groups for 0.5%PU-5 (Figure 3) is proposed to explain the low atom % Br compared to the soft-block-alone value. Similar polymer surface modifier concentration is observed for 0.5 wt%PU-4 and 0.5%PU-6. For 0.5%PU-4 where the segment ratio of 3FOx to BrOx is 1:2, the atom % Br is only 25% of the soft-block-alone value. This suggests that diads and triads of segments containing -CH2Br side chains may allow even further “submersion” of Br from the surface. The atom % F for 0.5%PU-4 (12.2%) is close to the calculated soft block value (12.6%).

Figure 4. TM-AFM phase images for IPDI-BD(40)/P[BrOx-2800], PU-3: (A) Asp/Ao ) 0.95; (B) Asp/Ao ) 0.70; z ) 20°; image size: 1 × 1 µm.

Phase contrast TM-AFM at soft and hard tapping for IPDIBD(40)/P[BrOx-2800] is shown in Figure 4. At soft tapping, Asp/Ao ) 0.95, the image is featureless, but at harder tapping, Asp/Ao ) 0.70, the familiar near-surface hard-block morphology is observed. These results confirm the expectation that well phaseseparated polyurethanes have well phase-separated surfaces with surface-concentrated soft block. By XPS (Table 2) the observed Br atom % (7.0%) for PU-3 is somewhat higher than the calculated bulk value (6.3%) but only about half the value for the soft block alone (14.3%). This slight surface enrichment of Br for PU-3 suggests the methyl groups at the 3-position preferentially occupy the air- and vacuumpolymer interface vis-a`-vis -CH2Br. A model illustrating this surface structure is shown in Figure 5A wherein the more polar -CH2Br groups are again partitioned to a subsurface regime in air or vacuum. The proposed rearrangement in water (Figure 5B) is discussed in the next section on wetting behavior. Polymer Surface Modifiers (0.5 wt %). The calculated bulk percentages for F and Br based on uniform distribution of 0.5 wt % polymer surface modifier are so low that neither would be detectable (Table 2). However, XPS results for 0.5%PU-5 (Table 2) show characteristic atom % F enrichment. The observed atom % F (18.2%) for 0.5%PU-5 is somewhat higher than that calculated for the PU-5 soft block alone (16.8%). The XPS results for 0.5%PU-5 may be compared with the phase image shown in Figure 2, D-1, Asp/Ao ) 0.91. TM-AFM for 0.5%PU-5 at light tapping shows the presence of near-surface hard domain and a morphology almost identical to that of the base polymer (Figure 2, D-1, Asp/Ao ) 0.91). Thus, TM-AFM does not provide evidence of surface concentration of the polymer surface modifier. However, at the 5-nm depth resolution of XPS, the surface of 0.5%PU-5 is indistinguishable from that of PU-5 alone. These results are a reminder that TM-AFM is an imaging method relying on mechanical/physical interactions and that for complex systems images must be interpreted with caution. XPS provides direct physical chemical analysis of the surface with depth sensitivity at length scales comparable to the root-meansquare (rms) end-to-end distance of soft blocks (2-5 nm). A

In contrast to polymer surface modifiers containing the 3FOx side chain, bromine is barely detected for 0.5%PU-3 (0.1 atom % Br) wherein the polymer surface modifier is IPDI-BD(40)/ P[BrOx-2800]. These data provide a negative control that emphasizes the importance of the fluorous side chains in bringing the -CH2Br groups to the surface. Wetting Behavior. The Wilhelmy plate method was used to determine wetting behavior.33,35,53 Several advantages for the Wilhelmy plate over sessile drop have been noted.34,54 In the present work, the surface tension of the interrogating water was tested before and after sample analysis. The force distance curve (flamed glass slide) after sample analysis is identical to that before sample analysis if water contamination has not occurred. Thus, data for after-sample measurement distinguishes between thermodynamically driven surface reorganization and cycle-tocycle changes in force distance curves due to water contamination.29 Each coating was evaluated for at least three cycles. Advancing (θadv) and receding (θrec) contact angles are shown in Table 1. Polyurethane Wetting. The initial force distance curves (fdc’s) for IPDI-BD(50)/PTMO(2000) gave θadv ) 85°, θrec ) 56°, and ∆θ ) 29° (Table 1). Due to a combination of surface reorganization and surface hydration, initial contact angles changed to θadv ) 81°, θrec ) 56°, and ∆θ ) 25° (Table 1) over four consecutive immersion/emersion cycles.29 An identical result was obtained after drying the coating. For PU-2, IPDI-BD(40)/P[3FOx-3400], θadv ) 105° and ∆θ ) 60°. A 6-7° change in θadv was observed after several fdc cycles (Table 1). This was partly (∼2-3°) due to the water contamination that was not eliminated despite several reprecipitations. The high contact angle hysteresis for polyurethanes containing semifluorinated groups reflects the presence of a surface soft block that rapidly undergoes enthalpically driven reorganization. Hydrogen bonding of water to the -CH2 group adjacent to -CF3 was previously proposed to account for this (52) Chanda, M. AdVanced Polymer Chemistry; Marcel Dekker: New York, 2000. (53) Hogt, A. H.; Gregonis, D. E.; Andrade, J. D.; Kim, S. W.; Dankert, J.; Feijen, J. J. Colloid Interface Sci. 1985, 106, 289-298. (54) Lander, L. M.; Siewerski, L. M.; Brittain, W. J.; Vogler, E. A. Langmuir 1993, 9, 2237.

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Figure 5. Depiction of near surface structure of the P[BrOx] soft block: (A) air/vacuum and (B) water.

Figure 6. Advancing contact angles (fdc-1) for PU-1 modified with varying amounts of surface modifier PU-5 ([), and PU-3 (red 9).

observation.27,55 Hydrogen bonding to ether oxygens (side chain, main chain) may also occur. The high θadv (102°) for PU-3, IPDI-BD(40)/P[BrOx-2800], coupled with the low atom % Br determined by XPS is explained by the predominance of surface methyl groups (Figure 5A). The advancing contact angle is similar to that observed for isotactic polypropylene (104°).38 Considering the relatively nonpolar nature of the P[BrOx] soft block, the low receding contact angle (42°) is of note. The low value for θrec must be associated with a rapid surface reorganization facilitated by the low Tg of the P[BrOx] soft block (-24 °C). A model for this reorganization is proposed in Figure 5, which suggests hydrogen bonding of water to the oxygen in the main-chain oxetane. The order of receding contact angles PTMO (52°) > BrOx (42°) > PEO (32°)22 may reflect the decreasing ratio of CH2/O in the repeat unit of this series. Wetting behavior for the IPDI-BD(40)/P[3FOx/BrOx-m:n] polyurethanes is summarized in Table 1 and Figure 2S. The θrec for the IPDI-BD(40)/P[3FOx/BrOx-m:n] polyurethanes is nearly constant (33 ( 1°), but advancing contact angles vary by 1214°. θadv (116°) for PU-5, IPDI-BD(40)/P[3FOx/BrOx-1:14700], is unique.29 The force distance curves (three cycles) for PU-5 are superposable, indicating a rapid and reversible surface reorganization (Figure 7B). A θadv of 116° is noteworthy for a topologically smooth surface. Previously, a model was proposed by which nonpolar BrOx groups sterically hindered access of -CH2CF3 to ether oxygens, resulting in a higher surface fraction of unimpeded -CF3 groups and higher θadv.29 Considering XPS results that show consistently low Br, this model is revised (Figure 1S) to indicate that the methyl group, rather than the -CH2Br group, sterically hinders access. The postulate remains that steric hindrance of ether/-CH2CF3 hydrogen bonding results in increased access of liquid water to -CH2CF3 and a lower receding contact angle. The overall consequence is a very high contact angle hysteresis (∆θ ) θadv - θrec ) 84°).29 PSM-Modified PU-1. The surface activity of the polyurethane polymer surface modifiers was examined by contact angle measurements (Table 1). IPDI-BD(40)/P[3FOx/BrOx-1:14700], PU-5, was selected for evaluation of initial contact angle (55) Johnston, E.; Bullock, S.; Uilk, J.; Gatenholm, P.; Wynne, K. J. Macromolecules 1999, 32, 8173-8182.

Figure 7. Force vs distance curves (fdc’s) and dynamic contact angles for A, IPDI-BD(50)/PTMO(2000), PU-1; B, IPDI-BD(40)/ P[3FOx/BrOx-1:1-4700], PU-5; and C, 0.5%PU-5.

as a function of concentration (Figure 6). The threshold for maximum hydrophobic behavior is between 0.5 and 1 wt % PSM. For the 0.5%PU-5 surface, θadv (104°) was only 1° lower than the coatings with 1 wt % PU-5 (105°). For the remaining study of wetting behavior, 0.5 wt % PSM was used (Table 1). Figure 7 shows force-distance curves and contact angles for each immersion/emersion cycle for PU-1, PU-5, and 0.5%PU-5 coatings. The effect of surface concentration of surface-modifier PU-5 on hydrophobicity was expected from XPS and TM-AFM studies discussed above. Figure 7A for PU-1 shows the increase in mass on initial contact with water that is characteristic of a hydrophilic surface (θadv < 90°). The force-distance curve for IPDI-BD(40)/P[3FOx/BrOx-1:1-4700] shows the apparent mass loss due to a hydrophobic surface (Figure 7B). The modified surface is also characterized by an initial loss in apparent mass. The θadv for 0.5%PU-5 coating (104°) was much greater than θadv for PU-1 (85°) but 12° less than PU-5 (116°). Thus, the unusually high water contact angle for the pure PSM is not retained in the modified coating under our processing conditions. Instead, θadv is close to that for IPDI-BD(40)/P[3FOx] (Table 1). Thus, the surface structure proposed for pure PU-5 (Figure 1S) is disrupted in the modified composition. The remainder of the semifluorinated 0.5 wt % polymer surfacemodified IPDI-BD(50)/PTMO(2000) coatings had initial θadv values within ∼4° of the corresponding IPDI-BD(40)/P[3FOx/ BrOx-m:n] surfaces (Table 1). The advancing contact angle for PU-1 typically increased more than 17° after adding 0.5 wt % IPDI-BD(40)/P[3FOx/BrOx-m:n], reflecting fluorous surface modifier enrichment.

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The wetting behavior over five DCA cycles was measured to evaluate the stability of the surface modification (Table 1). Inspection of these data shows that retention of high θadv values is correlated with PSMs containing higher ratios of semifluorinated groups. The poorest retention of initial wetting behavior was observed for 0.5%PU-4 (0.5%IPDI-BD(40)/P[3FOx/BrOx1:2-3400], θadv1, 100° to θadv5, 93°). Consistent with XPS, that showed negligible atom % Br for 0.5%PU-3, IPDI-BD(40)/ P[BrOx-2800], is not surface active at these concentrations and processing conditions. This makes clear the importance of the fluorous chaperone in surface concentrating -CH2Br groups. Soft Block Solubility Parameters. While it is clear that a fluorous chaperone is necessary to surface concentrate -CH2Br groups, the reason for lack of surface affinity for -CH2Br has not been addressed. To explore this point, the solubility parameters of PTMO, P[BrOx], and P[3FOx] soft blocks were examined. Solubility parameters may be estimated from group molar attraction constants according to eq 1

δ)

F M

i

∑1 Fi

(1)

where δ is the solubility parameter, F is density, M is molecular weight of the repeat unit, and ∑Fi is the sum of the component molar attraction constants for the repeat unit.52,56 For P[BrOx] and P[3FOx] the telechelic densities were approximated by those of the respective monomers, 3-methyl3-bromomethyloxetane (1.442)57 and 3-methyl-3-trifluoromethoxymethyloxetane (1.334, Experimental Section). Calculations are provided in the Supporting Information. The resulting solubility parameters are P[BrOx], 8.3, and P[3FOx], 7.1. The solubility parameter for PTMO (8.6) was calculated by the same method using the amorphous density. [Bowman, 1969, #1905] The literature value for PTMO is 8.2.58 Thus, the solubility parameters for P[BrOx] and PTMO are fairly close. The fact that P[BrOx] does not surface concentrate in 0.5%PU-3 may be explained by the solubility of the two amorphous soft block domains. Solubility also apparently disrupts the surface structure that leads to the copolymer effect on wetting behavior for 0.5% IPDI-BD(40)/P[3FOx/BrOx-1:1-4700]. Another contributing factor impeding surface concentration is that -CH2Br is a short side chain that does not adequately mimic an end group.23 (56) Rudin, A. The Elements of Polymer Science and Engineering; Academic Press: Orlando, 1982. (57) Product Data Safety Sheet; Ameribrom: Fort Lee, NJ, 2000. (58) Hwang, K. K. S.; Lin, S. B.; Tasy, S. Y.; Cooper, S. L. Polymer 1984, 25, 947-55.

By comparison, P[3FOx] has a low solubility parameter expected for a fluorous amorphous polymer. In addition, the 3-methyl-3-trifluoromethoxymethyloxetane has a modest projection from the main chain that makes it more like an end group.

Conclusion Polyurethanes with P[3FOx/BrOx-m:n] homo- and cotelechelic soft blocks having pendant semifluorinated and bromomethyl groups were prepared and used as surface modifiers for a conventional PTMO polyurethane. A combination of XPS, TM-AFM, and DCA studies showed that these fluorous polymer surface modifiers confer surface properties similar to those of the parent at 0.5-1.0 wt %. The retention of initial wetting behavior in water was enhanced with higher 3FOx/BrOx ratios that correspond to increasing fluorous character. Polymer surface modifiers ideally confer the surface property associated with the modifier while retaining the bulk properties of the majority polymer. In this regard, the copolymer effect for IPDI-BD(40)/P[3FOx/BrOx-1:1-4700], PU-5, that resulted in a higher than expected θadv (116°) is not observed for 0.5%PU-5. Furthermore, IPDI-BD(40)/P[BrOx-2800], which has a high θadv (102°), is not surface concentrated as wetting behavior for 0.5%PU-3 is similar to the bulk polyurethane, and only trace atom % Br is detected by XPS. In summary, 3FOx repeat units are surface concentrated by virtue of low surface energy, immiscibility, and side-chain extensions that mimic end groups. The high surface fluorine content of P[3FOx/BrOx] modified polyurethane is not matched by high surface bromine content, suggesting that the BrOx component is miscible in the polyurethane base polymer. The calculated solubility parameters for PTMO and P[BrOx], which are similar, support the notion of BrOx miscibility with the base polyurethane. The combination of miscibility of BrOx repeat units and lack of an end-group-like architecture minimizes BrOx surface concentration in the chosen bulk polyurethane. Acknowledgment. We are grateful to the National Science Foundation for support (DMR-0207560). L.G. acknowledges support from NESAC/BIO (NIH Grant No. EB-002027). We thank Dr. Stephan Golledge for preliminary XPS data. Supporting Information Available: C, N, and O surface atomic compositions; solubility parameters calculated for P[BrOx], P[3FOx], and PTMO; and calculation of rms end-to-end distance for P[3FOx/ BrOx-1:1-4700]. This material is available free of charge via the Internet at http://pubs.acs.org. LA701684A