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Surface Characterization of Biocidal Polyurethane Modifiers Having Poly(3,3-substituted)oxetane Soft Blocks with Alkylammonium Side Chains Pinar Kurt,† Lara J. Gamble,‡ and Kenneth J. Wynne*,† 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 January 21, 2008. ReVised Manuscript ReceiVed February 22, 2008 This paper focuses on surface characterization of P[AB] copolyoxetane soft block polyurethanes having either fluorous (3FOx, -CH2OCH2CF3) or PEG-like (ME2Ox, -CH2(OCH2CH2)2OCH3), A side chains and alkylammonium, B side chains. Physical surface characterization data were analyzed in light of the previously observed order of antimicrobial effectiveness for a set of four surface modifiers. Ample physical evidence for surface concentration of fluorous 2 wt % P[AB]-polyurethane modifiers was obtained from XPS, contact angles, ATR-IR spectroscopy, and TM-AFM. In TM-AFM phase imaging, the most effective biocidal surface modifier, 2 wt % HMDI-BD(30)/ P[(3FOx)(C12)-0.89:0.11]-PU, showed a nanoscale phase-separated structure consisting of 200 nm domains with background features about 10 times smaller. Despite similar surface characterization data, the 2 wt % fluorous C6 analog ranked third in contact biocidal effectiveness. Physical evidence for surface concentration of 2 wt % P[(ME2Ox)(C12)-0.86:0.14]-PU was modest, considering that antimicrobial effectiveness was second only to 2 wt % HMDI-BD(30)/P[(3FOx)(C12)-0.89:0.11]-PU. In this set of surface modifiers, nanoscale morphology is largely driven by the fluorous component, whereas antimicrobial effectiveness is more strongly influenced by alkylammonium chain length. The effect of alkylammonium side chain length on surface concentration and antimicrobial behavior is more pronounced for ME2Ox polyurethanes compared to the 3FOx analogs.
Introduction An efficient way to generate desired surface properties is via polymer surface modifiers (PSMs) that comprise a minority weight percent and concentrate desired functionality without altering bulk properties of the majority polymer.1–6 Polyurethane surface modifiers have been generated with fluorous end groups or soft blocks. Polyoxetane soft blocks with fluorous side chains make conventional polyurethanes water- and oil-resistant.7–11 A base polyurethane modified with 2.5–5 wt % polyurethanes having fluorous end caps conferred hydrophobic behavior, improved biostability, and reduced platelet adhesion.5,12,13
* Corresponding author. E-mail:
[email protected]. † ‡
Virginia Commonwealth University. University of Washington Seattle.
(1) Ward, R. S.; White, K. A.; Hu, C. B. Prog. Biomed. Eng. 1984, 1, 181–200. (2) Ho, T.; Wynne, K. J. Polym. AdV. Technol. 1994, 6, 25–31. (3) Chen, W.; McCarthy, T. J. Macromolecules 1999, 32, 2342–2347. (4) Makal, U.; Wood, L.; Ohman, D. E.; Wynne, K. J. Biomaterials 2006, 27, 1316–1326. (5) 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, 81, 178–185. (6) Grunzinger, S. J.; Kurt, P.; Brunson, K. M.; Wood, L.; Ohman, D. E.; Wynne, K. J. Polymer 2007, 48, 4653–4662. (7) Malik, A. A.; Carlson, R. P. U.S. Patent 5,637,772, 1997. (8) 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. (9) Thomas, R. R.; Anton, D. R.; Graham, W. F.; Darmon, M. J.; Stika, K. M. Macromolecules 1998, 31, 4595–4604. (10) Thomas, R. R.; Ji, Q.; Kim, Y. S.; Lee, J. S.; McGrath, J. E. Polyurethane 2000 Polymer DiVision Abstracts 2000. (11) Kim, Y. S.; Lee, J. S.; Ji, Q.; McGrath, J. E. Polymer 2002, 43, 7161– 7170. (12) Tang, Y. W.; Santerre, J. P.; Labow, R. S.; Taylor, D. G. J. Appl. Polym. Sci. 1996, 62, 1133–1145. (13) Tang, Y. W.; Santerre, J. P.; Labow, R. S.; Taylor, D. G. J. Biomed. Mater. Res. 1997, 35, 371–381.
Beyond hydrophobicity and oleophobicity, fluorous “A” groups have been used to surface-concentrate functional “B” groups. This concept was investigated by Vogl, who attached fluorous tails to UV absorbers.14,15 In a polystyrene matrix, fullerenes were surface-concentrated using fluorous groups.3 Santerre used an “A, B” polymer modifier approach, but with chain ends. These end-group modifiers are derived from mixtures of fluorous alcohols that become Rf “tails” via reaction with isocyanateterminated polyurethanes.5,16 Thus, polyurethanes with a fluorous A end group and a peptide B end group were used to modify polyurethane surfaces, resulting in enhanced cell adhesion.17 Some concern is raised by the mixed fluorous tails that contain eight-carbon Rf moieties, which are known to be perfluorooctanoic acid (PFOA) precursors.18–20 A new approach to surface modification employs P[AB]-soft blocks with short fluorous side chains that act as “chaperones” (14) Vogl, O.; Jaycox, G. D.; Hatada, K. J. Macromol. Sci., Chem. 1990, 27, 1781–1854. (15) Stoeber, L.; Sustic, A.; Simonsick, W. J.; Vogl, O. J. Macromol. Sci., Pure Appl. Chem. 2000, 37, 943–970. (16) McCloskey, C. B.; Yip, C. M.; Santerre, J. P. Macromolecules 2002, 35, 924–933. (17) Ernsting, M. J.; Bonin, G. C.; Yang, M.; Labow, R. S.; Santerre, J. P. Biomaterials 2005, 26, 6536–6546. (18) Ellis, D. A.; Mabury, S. A.; Martin, J. W.; Muir, D. C. G. Nature 2001, 412, 321–324. (19) Giesy, J. P.; Kannan, K. EnViron. Sci. Technol. 2001, 35, 1339–1342. (20) Martin, J. W.; Whittle, D. M.; Muir, D. C. G.; Mabury, S. A. EnViron. Sci. Technol. 2004, 38, 5379–5385.
10.1021/la800203y CCC: $40.75 2008 American Chemical Society Published on Web 04/30/2008
Characterization of Biocidal Polyurethane Modifiers
Figure 1. Schematic of a conventional polyurethane (soft block, solid line) modified with a PSM PU having a copolymer soft block (dashed line) with A and B side chains (one set shown).
Figure 2. Structure of P[AB]-telechelics that become soft blocks for P[AB]-polyurethanes. R: –OCH2CF3, 3FOx or –(OCH2CH2)2OCH3, ME2Ox.
for concentrating B functionality at the surface (Figure 1).21,22 For P[AB], “P” designates the polymerized co-polyoxetane 1 with repeat units having A and B side chains that act in concert to achieve a desired function. The term A or B is used synonymously for the 1,3-propylene oxide repeat that bears the respective side chain. The P[AB]-soft block approach has a combination of features for polymer surface modification: (a) soft blocks are known to be preferentially surface-concentrated,23–26 (b) the comb-like P[AB]-soft block architecture has multiple A and B side chains, which are pseudochain ends and are entropically surface concentrated,27 (c) differing solubility parameters for the P[AB]soft block and other coating constituents enhance phase separation and surface concentration, (d) a low P[AB]-soft block Tg may enhance the B surface function through more facile conjugation with a target receptor, (e) functionality may be optimized by varying the B mole fraction in the P[AB]-soft block, (f) A and B repeat units may synergistically produce a new surface property, (g) fluorous A repeats act as “chaperones” to surface-concentrate B groups, which, in the absence of A, would not be surfaceconcentrated,4 (h) a multiplicity of short, environmentally acceptable Rf A side chains are employed, providing a “green” method for surface-concentrating functionality B, and (i) compositional economy is ensured by using the P[AB]-soft block polyurethane as a minor constituent (e2 wt %) in a blend such that the polymer modifier defines the surface properties and the base polymer defines bulk properties (Figure 1). Compositional economy via a blend approach was discussed thoroughly by Mayes, who modified poly(vinylidene fluoride) membranes for resistance to protein adsorption via surface segregation of the amphiphilic comb polymer poly(methyl methacrylate-copolyoxyethylene methacrylate.28a For neat P[AB]-soft block polyurethanes, considerations (a)-(c) provide a combination of thermodynamic driving forces that act to surface-concentrate the soft block. Of considerable (21) Wynne, K. J.; Makal, U.; Kurt, P.; Gamble, L. Langmuir 2007, 23, 10573– 10580. (22) Kurt, P.; Wood, L.; Ohman, D. E.; Wynne, K. J. Langmuir 2007, 23, 4719–4723. (23) Ratner, B. D.; Cooper, S. L.; Castner, D. G.; Grasel, T. G. J. Biomed. Mater. Res. 1990, 24, 605–620. (24) Tingey, K. G.; Andrade, J. D. Langmuir 1991, 7, 2471–2478. (25) Garrett, J. T.; Runt, J.; Lin, J. S. Macromolecules 2000, 33, 6353–6359. (26) Garrett, J. T.; Siedlecki, C. A.; Runt, J. Macromolecules 2001, 34, 7066– 7070.
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interest is (f, generating a new surface characteristic, which has been observed in three new P[AB]-soft block polyurethanes: contraphilic wetting,28b,,29 a copolymer effect on wetting,30 and nanoscale surface phase separation for a block-P[AB]-soft block.31 With regard to compositional economy (i), fluorous A repeat units act as “chaperones” to surface-concentrate B groups in model 2 wt % P[AB]-soft block polyurethanes.4,21 Furthermore, 2 wt% P[AB]-soft block polyurethane modification concentrates hydantoin B moieties, which after oxidation by hypochlorite (bleach) are biocidal.4,6 Concentrating quaternary B groups has been demonstrated to yield contact antimicrobial coatings whose effectiveness is dependent on the nature of A and B.22 Surface characterization of these 2 wt % P[AB] polyurethanes that contain soft blocks 1 having alkylammonium B side chains along with either fluorous or PEG-like A side chains are the focus of this paper.22 Neat coatings of quaternary ammonium P[AB]-soft block polyurethanes and their 2 wt % blends are investigated using X-ray photoelectron spectroscopy (XPS), attenuated total reflection infrared (ATR-IR) spectroscopy, tapping-mode atomic force microscopy (TM-AFM), and sessile drop and dynamic contact angle (DCA) measurements. Of note is the observation of a new surface morphology for a base polyurethane modified with 2 wt % fluorous-alkylammonium P[AB] polyurethane. Also, although fluorous A is usually essential to surface-concentrate B, the “C12” alkylammonium side chain is shown to be largely “selfchaperoning”.
Experimental Section Materials. Methylene chloride (CH2Cl2), N,N-dimethylformamide (DMF), tetrahydrofuran (THF) and isopropanol (IPA) were obtained from Aldrich and dried by storing over 4 Å molecular sieves. Synthesis. The synthesis of monomers, telechelics, and their polyurethanes (PUs) have been described in detail.32 Alkylammonium telechelics were incorporated into polyurethanes by the soft block first method. P[AB]-soft block molecular weights are close to 6 KDa. The HMDI-BD hard block was 30 wt %. Designations for fluorous polyurethanes are HMDI-BD(30)/P[(3FOx)(C6)-0.89:0.11]PU and HMDI-BD(30)/P[(3FOx)(C12)-0.89:0.11]-PU where HMDIBD represents hard block composition followed by wt % (30); “P” indicates polymerized monomer-in-telechelic followed by ratios of repeat units (m:n); the repeat unit designations “C6” and “C12” represent the 2-alkylammonium butoxymethyl-2-methyl 1,3-propylene oxide segment. Because all compositions have 30 wt % hard block, HMDI-BD(30) is usually omitted from designations. PEGlike polyurethanes33 are designated P[(ME2Ox)(C6)-0.86:0.14]-PU and P[(ME2Ox)(C12)-0.86:0.14]-PU. The base polyurethane was synthesized by a two-step solution polymerization34 using PTMO(1000) as soft block and HMDI-BD as hard block (50 wt %).35 The designation for the base polyurethane (PU) is HMDI-BD(50)/ PTMO(1000). In order to have a reference for some of the surface characterization studies, we prepared a polyurethane with P(3FOx) soft block (Mn ) 3400 g/mol) following the same procedure32 and designated as P[3FOx]-PU. (27) Jalbert, C.; Koberstein, J. T.; Hariharan, A.; Kumar, S. K. Macromolecules 1997, 30, 4481–4490. (28) (a) Hester, J. F.; Banerjee, P.; Mayes, A. M. Macromolecules 1999, 32, 1643–1650. (b) Makal, U.; Wynne, K. J. Langmuir 2005, 21, 3742–3745. (29) Makal, U.; Uslu, N.; Wynne, K. J. Langmuir 2007, 23, 209–216. (30) Makal, U.; Fujiwara, T.; Cooke, R. S.; Wynne, K. J. Langmuir 2005, 21, 10749–10755. (31) Fujiwara, T.; Wynne, K. J. Macromolecules 2004, 37, 8491–8494. (32) Kurt, P.; Wynne, K. J. Macromolecules 2007, 40, 9537–9543. (33) Kurt, P.; Wynne, K. J. Macromolecules 2007, accepted for publication. (34) Grasel, T. G.; Cooper, S. L. Biomaterials 1986, 7, 315–328. (35) Makal, U.; Uilk, J.; Kurt, P.; Cooke, R. S.; Wynne, K. J. Polymer 2005, 46, 2522–2530.
5818 Langmuir, Vol. 24, No. 11, 2008 Coating Process. P[AB]-polyurethanes were dissolved in a THF/ IPA solvent mixture with 15–20 solid wt %. For modifier blends, 2 wt % PSM was codissolved with 98 wt % base PU in THF. The process for a representative coating containing 2 wt % P[AB]polyurethane (2 wt %P[(3FOx)(C12)-0.89:0.11]-PU) is as follows: 4.2g of base polyurethane, HMDI-BD(50)/PTMO(1000), and 85 mg P[(3FOx)(C12)-0.89:0.11]-PU were dissolved in 24.1 mL THF. The overall composition was 2 wt % PSM with 20 wt % solid content in solution. For ATR-IR and TM-AFM, additional compositions with 0.1, 0.5, and 1 wt % PSM were prepared in the same manner. For dynamic contact angle (DCA) analysis, coatings were prepared by dip coating from solutions onto glass coverslips (Corning, 24 × 40 × 1.2 mm3). The polyurethane solution was distributed on both sides to generate even coatings because Wilhelmy plate analysis is sensitive to sample perimeter. Using fairly concentrated solutions, this process was designed to generate ordinary coatings (0.5 to 1 mm) as distinguished from nanofilms. For other modes of surface characterization, polymer solutions were spread on one side of microscope slides. Coverslips (in an inverted, upright position) and microscope slides were left at ambient conditions overnight and in an oven at 60 °C under reduced pressure for 48 h for solvent evaporation. Visually, some neat P[AB]polyurethane coatings were slightly yellow, but 2 wt % modifier blends were colorless. All coatings were transparent with smooth, flat surfaces.
Characterization X-Ray Photoelectron Spectroscopy (XPS). A Surface Science Instruments X-probe spectrometer with a monochromatized Al KR X-ray beam was used. Pressure in the analytical chamber during spectral acquisitions was less than 5 × 10-9 Torr. The X-ray spot size for these acquisitions was on the order of 800 µm. The pass energy for survey spectra (composition) was 150 eV, and the pass energy for high-resolution C1s (HRC) scans was 50 eV. A takeoff angle (the angle between the sample normal and the input axis of the energy analyzer) of 55° was used to determine surface composition at a depth of 50 Å. Attenuated Total Reflection Infrared (ATR-IR) Spectroscopy. ATR-IR spectra of polyurethane film surfaces were obtained by using a Nicolet 400 FT-IR with a Thunderdome attachment. A background spectrum was taken before each scan. Coated microscope slides were placed on the Ge crystal, and 128 scans from 500 to 4000 cm-1 were taken to obtain vibrational modes to a depth of about 0.5µm. Spectra were analyzed using Omnic software. Atomic Force Microscopy (AFM). Morphological analyses of polyurethane surfaces were carried out using a Dimension3100 (Digital Instruments, CA) atomic force microscope with a NanoScope V controller. Imaging was performed in tapping mode using a microfabricated silicon cantilever (40 N/m, Veeco, Santa Barbara, CA) in air. The tapping force was increased from soft to hard by decreasing the setpoint ratio rsp or Aexp/Ao, where Ao is the free oscillation amplitude and Aexp is the experimental oscillation amplitude. Images were analyzed by using NanoScope v710r1 software. Water Uptake. Microscope slides coated with base and neat P[AB]-polyurethanes were weighted and immersed completely in 120 mL Nanopure water (∼18.2 MΩ, Milli-Q (Millipore). After 24 h, the slides were wiped gently with Kimwipe and weighed. The difference in mass was recorded. Wetting Behavior. Static contact angles were obtained using a Ramé-Hart goniometer equipped with an LCD camera. Deionized water (∼18.2 MΩ) was used as a probe liquid. A 2 µL drop was placed on the coating surface, and the image was captured immediately and after 5 min to study surface reorganization. Captured images were analyzed, and contact angles were measured using Dropview image software version 1.4.11.
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Average values for 30 observations were reported (2 samples, 3 drop sites, 5 readings each). Dynamic contact angles (DCA) were obtained using a Cahn Model 312 Analyzer (Cerritos, CA). DCA measurements are based on the Wilhelmy plate method.36 The surface tension of the probe liquid (Nanopure water) was checked before each measurement and was typically 72.6 ( 1 dyn/cm. Beakers used for DCA analysis were cleaned by soaking in an isopropanol/ potassium hydroxide base bath for at least 24 h, rinsing with Nanopure water and treated with a gas/oxygen flame. A coated slide was attached to the electrobalance via a clip. The stage with the beaker of water was automatically raised and lowered with a speed of 100 µm/s. Resulting force versus distance curves (fdc’s) are used to calculate advancing (θadv) and receding (θrec) contact angles. The dwell time between the advancing and receding test segments was 1 s. Five cycles in succession were obtained to study any change in wetting behavior on exposure to water.
Results and Discussion The goal of preparing P[AB]-soft block polyurethanes with alkylammonium B side chains was to concentrate cationic moieties at the surface of conventional polyurethanes (Figure 1). To understand surface concentration and correlate with function, the present paper is aimed at surface characterization of a conventional polyurethane modified with 2 wt % P[AB]-soft block polyurethane. The contrasting characteristics of A-soft block co-segments (fluorous or PEG-like) are chosen to investigate synergetic and/or complementary interactions affecting surface concentration of quaternary ammonium B-groups. The effect of alkyl chain length is investigated by comparing C6 and C12 compositions with the same A co-repeat. To provide information on atomic composition, morphology, and dynamic wetting properties, surface concentration of the P[AB]-polyurethane soft block is studied via ATR-IR spectroscopy, XPS with 55° takeoff angle, TM-AFM imaging, and contact angle measurements. ATR-IR Spectroscopy. Given a coating thickness of 0.5–1 mm, the maximum thickness of a 2 wt % phase-separated domain is about 10 µm. With this information, ATR-IR spectroscopy (0.5–1 µm depth) was employed for qualitative analysis in detecting vibrational modes from surface-concentrated species. To assess modifier surface concentration, we first obtained spectra for neat P[(3FOx)(C12)-0.89:0.11] and P[(ME2Ox)(C12)-0.86:0.14] polyurethanes and base polyurethane. The ATR-IR spectrum for the base polyurethane was subtracted from P[(3FOx)(C12)-0.89:0.11] and P[(ME2Ox)(C12)-0.86: 0.14] polyurethanes to discern separate absorptions (Figure 1S). C-F stretching modes at 1170 and 1280 cm-1 are clearly observed for P[(3FOx)(C12)-0.89:0.11]-PU. P[(ME2Ox)(C12)0.86:0.14]-PU has only modest differences from the base polyurethane with peaks at 1090 and 1150 cm-1 due to additional C-O-C bands for ME2Ox side chains. As suggested by the above results, an examination of 2 wt % P[(ME2Ox)(C12)-0.86: 0.14] polyurethane showed negligible residual absorptions when the base polyurethane was subtracted. ATR-IR spectra of fluorous PSM blends were used to determine the ATR-IR detection limit for surface concentration. Spectra for 0.1, 0.5, 1, and 2 wt % P[(3FOx)(C12)-0.89:0.11]-PU are shown in Figure 2S. For the 2 wt % composition, C-F bands at 1170 and 1280 cm-1 are barely observable compared to neat P[AB]-polyurethane. At 1 wt % C-F and below, absorptions fade into the baseline. A similar situation is seen for 2 wt % (36) Wilhelmy, L. Ann. Phys. Chem. Leipzig 1863, 119, 177.
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Table 1. Calculated Atomic Compositions and Those Determined by XPS for Neat, 2 and 1 wt % P[AB]-polyurethanes atom % PSM composition
PSM (wt %)
P[(3FOx)(C6)-0.89:0.11] 100 2 P[(3FOx)(C12)-0.89:0.11]
P[(ME2Ox)(C6)-0.86:0.14]
P[(ME2Ox)(C12)-0.86:0.14]
a
Experimental error: (0.1.
b
100 2 – – 100 2 – – 100 2 1
calc/ obs calc calc obs obs calc calc obs obs calc calc obs obs calc calc obs obs obs
PU soft block PU soft block PU soft block PU soft block
C
O
N
Bra
F
66.8 62.4 59.9 61.5 68.1 64.2 57.7 60.6 74.2 73.3 63.3 65.7 75.2 74.7 67.4 66.8 68.4
15.4 15.4 16.6 16.0 14.9 14.6 16.4 16.6 22.0 24.8 21.8 19.4 21.1 23.5 21.5 19.0 20.5
3.2 0.8 1.3 1.3 3.2 0.8 1.3 1.2 3.2 0.9 1.0 2.0 3.1 0.9 1.6 1.7 1.8
0.6 0.8 0.4 0.2 0.5 0.8 0.3 0.2 0.7 0.9 0.2 0.0 0.6 0.9 0.3 0.1c 0.0
14.1 20.6 20.9 21.0 13.2 19.6 21.9 18.7 0.0 0.0 7.2b 5.9b 0.0 0.0 5.5 b 8.9b 4.8b
Observation due to fluorocarbon contamination. c Standard deviation: 0.0.
P[(3FOx)(C6)-0.89:0.11]-PU (Figure 3S), but the anomalously strong 1280 cm-1 band is unexplained. ATR-IR spectra (Figures 2S-3S) provided only marginal evidence for surface concentration of fluorous modifiers. The most reliable evidence was for the 1170 cm-1 C-F absorption, but only at 2 wt %. X-ray Photoelectron Spectroscopy. P[AB] copolyoxetanes have Mn close to 6 KDa.32 Using the valence angle model, we find that a single chain of molecular weight 6000 has a rootmean-square end-to-end distance of 2–3 nm.37 This is a poor approximation because of the presence of long ME2Ox (9 atoms) and alkylammonium (19 atoms) side chains (Figure 4S) similar to graft architectures. Nevertheless, with a surface-concentrated P[AB] soft block, the depth penetration of XPS is a good match for soft block “thickness” of 2–3 nm. Atomic percentages (atom %) for neat P[AB]-polyurethane surfaces are listed in Table 1. Si contamination (0.9 –6.5 atom %) was observed in all coatings. Contamination from F (5–8 atom %) was detected in ME2Ox compositions. Renormalized data are provided in Table 1S correcting ME2Ox compositions for Si and F contamination and 3FOx compositions for Si contamination. Renormalized results for ME2Ox compositions result in the C atom % (73.3% for C6, 74.2% for C12 series) being close to that for soft block alone (73.3% (C6), 74.7% (C12)). However, the calculated C atom % for soft blocks and polyurethanes are similar precluding insight into soft block surface concentration based on C atom %. In the discussion below, data from Table 1 is used because the values for renormalized atom percents do not significantly affect the conclusions. The surface atomic composition of P[(3FOx)(C12)-0.89:0.11]polyurethane and the 2 wt % P[(3FOx)(C12)-0.89:0.11]-PU modified polyurethane are shown in Table 1. The atom % F (21.9) for neat P[(3FOx)(C12)-0.89:0.11]-PU compares favorably to the soft block value of 19.6 atom %. The agreement of atom % F with that for the soft block alone attests to the surface concentration of 3FOx side chains as observed previously for fluorous soft block systems.2,7–11,38–40 More importantly, the (37) Chanda, M. AdVanced Polymer Chemistry; Marcel Dekker: New York, 2000. (38) 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. (39) Tezuka, Y.; Fukushima, A.; Matsui, S.; Imai, K. J. Colloid Interface Sci. 1986, 114, 16–25. (40) Thanawala, S. K.; Chaudhury, M. K. Langmuir 2000, 16, 1256–1260.
atom % F (18.7) for 2 wt % P[(3FOx)(C12)-0.89:0.11]-PU is also close to that for the soft block alone (19.6 atom %). The atom % Br for the neat P[(3FOx)(C12)-0.89:0.11]-PU is 0.3 atom % versus 0.8 calculated for the soft block alone. The low atom % Br may be due to Br being “buried” beneath the 19 atom C12 side chain. “Immersion” of the polar quaternary salt [B+]Br- site in the vacuum environment may be facilitated by the low soft block Tg. Photoelectron emission exponentially decreases as function of depth, which negatively amplifies any “immersion” tendency. For 2 wt % P[(3FOx)(C6)-0.89:0.11]-PU, the atom % Br is 0.2 versus 0.8 atom % calculated for the soft block. Again, atom % F gives evidence of P[AB]-soft block surface concentration, but the low atom % Br parallels the observation for the neat modifier, suggesting a parallel near-surface immersion of Brin the vacuum environment. For nonfluorous P[(ME2Ox)(C12)-0.86:0.14]-PU, the calculated atom % Br for soft block alone is 0.9, which may be compared to 0.3 and 0.1 atom % Br for neat and 2 wt % P[(ME2Ox)(C12)-0.86:0.14]-PU, respectively. These data provide evidence for [B+]Br- surface concentration and confirm “self-chaperoning” for C12. The low atom % Br compared to that calculated for the P[AB]-soft block is once again attributed to the near-surface “immersion” of Br- in vacuo. In addition, the low atom % Br for 2 wt % P[(ME2Ox)(C12)0.86:0.14]-PU is likely due to low modifier surface concentration for this nonfluorous modifier. Br is not detected for 1 wt % P[(ME2Ox)(C12)-0.86:0.14]-PU, indicating minimal near-surface concentration. For P[(ME2Ox)(C6)-0.86:0.14]-PU, 0.2 atom % Br was observed versus a calculated value of 0.7 atom % for the P[AB]soft block. Br was not detected for 2 wt % coatings. This indicates that PEG-like chains with shorter alkyl ammonium side chains have a diminished tendency to surface concentrate. Interestingly, this observation is in agreement with the poorer biocidal effectiveness of 2 wt % P[(ME2Ox)(C6)-0.86:0.14]-PU compared to the C12 analog.33 In all P[AB]-soft block polyurethanes, the calculated soft block value for N atom % is 0.8–0.9 compared to 3.1–3.2 atom % for the polyurethane. Thus, the P[AB]-soft blocks have ∼3.5 times lower atom % N than the P[AB]-polyurethanes. For the neat and 2 wt % 3FOx-C6 and -C12 polyurethanes, the observed 1.2–1.3 N atom % is consistently close to soft block value (0.8 atom %) and much lower than that calculated for the P[AB]-polyurethanes
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Figure 3. TM-AFM phase images, 1 x 1 µm2, A series, P[(3FOx)(C12)-0.89:0.11]-PU; B series, P[(ME2Ox)(C12)-0.86:0.14]-PU: A-1, B-1 rsp ) 0.97; A-2, B-2 rsp ) 0.95; A-3, B-3, rsp ) 0.90; A-4, B-4 rsp ) 0.80. z ) 20°.
(3.2 atom %). XPS values for the neat and 2 wt % ME2Ox containing soft block compositions are mostly near 2 atom % compared to the calculated values for the soft block (0.9 atom %). The trend is consistent with poorer phase separation for ME2Ox-based soft block compositions and phase mixing with the base polyurethane for the 2 wt % composition. However, low atom percents coupled with the presence of contaminating elements (F, Si), both of which affect accuracy, preclude a strong correlation. In summary, N atom % provides additional evidence for P[AB]soft block surface concentration at a length scale commensurate with the root-mean-square end-to-end soft block distance (ca. 2 nm). A similar XPS trend for depressed N atom % was observed by Cooper,34 Ratner,23 and Hercules41 providing evidence for surface concentration of conventional polyether soft blocks. AFM Surface Imaging. Compositional or topological heterogeneity can increase advancing contact angles of hydrophobic surfaces while making hydrophilic surfaces more hydrophilic.42,43 The P[AB]-PU coatings were visibly smooth but were studied by TM-AFM (1 × 1 µm2) to gain insight into nanoscale topology, compositional heterogeneity, and near-surface morphology. All coatings gave roughness values (Rq) of less than 2 nm, indicating extremely smooth surfaces. Roughness alone at such levels does not significantly affect wetting behavior.44 Near-surface phase separation in polyurethanes has been the subject of several TM-AFM studies.25,45–47 If soft and hard blocks are well phase separated, then at light tapping (rsp > 0.95) only the surface soft domain interacts with the AFM tip, resulting in a featureless phase image.26 If the proximal interaction with the sample is increased by decreasing the setpoint ratio, that is, tapping harder, then near-surface hard blocks are observed in artificial (41) Graham, S. W.; Hercules, D. M. J. Biomed. Mater. Res. 1981, 15, 465– 77. (42) Wenzel, R. N. J. Ind. Eng. Chem. (Washington, D.C.) 1936, 28, 988–94. (43) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 357–360. (44) Nakae, H.; Inui, R.; Hirata, Y.; Saito, H. Acta Mater. 1998, 46, 2313– 2318. (45) Sauer, B. B.; McLean, R. S. Macromolecules 1997, 30, 8314–8317. (46) Garrett, J. T.; Lin, J. S.; Runt, J. Macromolecules 2002, 35, 161–168. (47) Aneja, A.; Wilkes, G. L. Polymer 2003, 44, 7221–7228.
color images as yellow domains in a brown background.48 If the polymer is poorly phase separated, then hard domains are resolved even at light tapping. Figure 3 shows phase images for P[(3FOx)(C12)-0.89:0.11]PU and P[(ME2Ox)(C12)-0.86:0.14]-PU at rsp values ranging from 0.80 to 0.97. The featureless phase image at lightest tapping (rsp, 0.97; 0.95) for P[(3FOx)(C12)-0.89:0.11]-PU confirms good near-surface phase separation with the amorphous soft block surface concentrated. DSC analyses previously showed little change in fluorous telechelic Tg compared to the polyurethane soft block.33 A Fox analysis indicated that phase separation was minimal (96–99% pure soft block in soft domain). These results confirm that good bulk-phase separation portends good surfacephase separation.26 The images in Figure 3 were obtained with a Nanoscope V controller while a Nanoscope IIIa was used previously.21,30 After collecting images for several months with the Nanoscope V, we observed that the effects of changing the setpoint ratio are much more clearly delineated. This is due to increased bandwidth for amplitude and phase signal detection and the integrated lock-in amplifier, which provides a linear and accurate phase measurement in the Nanoscope V. Thus, for P[(3FOx)(C12)-0.89:0.11]PU there is a sharply delineated change in phase image for a setpoint ratio of 0.95 compared to 0.90 (Figure 3). Such changes were observed gradually with the Nanoscope IIIa. Also, for a given composition, imaging near-surface hard block domains (Figure 3-A-3) occurred at lower setpoint ratios for the Nanoscope IIIa.21,30 The phase images for P[(ME2Ox)(C12)-0.86:0.14]-PU show a clear change from the nearly featureless image (rsp ) 0.97) to one delineating near-surface hard block domains (rsp ) 0.95). The appearance of near-surface hard block domains at a higher rsp compared to P[(3FOx)(C12)-0.89:0.11]-PU (rsp ) 0.90) indicates greater near-surface phase mixing for P[(ME2Ox)(C12)0.86:0.14]-PU compared to the fluorous analog. This observation correlates with DSC that showed a Tg 26 °C higher for P[(ME2Ox)(C12)-0.86:0.14]-PU compared to the telechelic –59 °C. An analysis using the Fox equation indicated 73–75% soft (48) Magonov, S. N.; Elings, V.; Whangbo, M. H. Surf. Sci. 1997, 375, L385–L391.
Characterization of Biocidal Polyurethane Modifiers
block in the soft block domain.33 The correlation of bulk phase separation as a predictor of the onset of near-surface phase separation measured by setpoint ratio is once again clear. For P[(3FOx)(C12)-0.89:0.11]-PU and P[(ME2Ox)(C12)-0.86: 0.14]-PU phase images are shown in Figure 3; the z phase angle range was 20°. For both P[AB]- polyurethanes, the signal saturates at a setpoint ratio of 0.8 resulting in pink colored areas. Saturation is particularly evident for P[(ME2Ox)(C12)-0.86:0.14]-PU at rsp ) 0.80. By comparison, saturation was reached at rsp ) 0.6–0.7 for the Nanoscope IIIa. An objective of TM-AFM imaging was assessment of morphological consequences of P[AB]-polyurethane modifier surface concentration. For this purpose, phase images of P[(3FOx)(C12)-0.89:0.11]-PU and its blends (0.1–2 wt %) at light tapping (rsp ) 0.95 and 0.97) are compared to base polyurethane HMDI-BD(50)/PTMO(1000) in Figure 4. Previously, it was reported that HMDI-BD(50)/PTMO(1000) was partially phase-mixed.49 The change in phase images from featureless (rsp ) 0.97) to a characteristic mixed-phase image (rsp ) 0.95) for HMDI-BD(50)/PTMO(1000) attests to nearsurface phase mixing. In contrast, featureless phase images are obtained for well phase separated P[(3FOx)(C12)-0.89:0.11]PU. This differing near-surface phase separation parallels the percentages of soft block in the soft block phase for the fluorous (96–99%) and base (78%) polyurethanes.49 At the lowest concentration P[(3FOx)(C12)-0.89:0.11]-PU modifier (0.1 wt %), the rsp ) 0.97 phase image is characterized by scattered 10–15 nm scale features. Even at this low wt %, a phase-separated nanoscale morphology is evident. For the rsp ) 0.95 image, the near-surface base polyurethane hard block domains dominate the image and the modifier features are obscured. At 0.5 wt % and rsp ) 0.97 the surface density of 10–15 nm features increases. At rsp ) 0.95 the morphology appears similar to that for the 0.1 wt % composition, with the near-surface hard block obscuring the smaller modifier features. The phase image for 1 wt % P[(3FOx)(C12)-0.89:0.11]-PU is very complex. The images are quite different compared to lower (0.5 wt %) and higher (2 wt %) modifier compositions. The images at rsp ) 0.97 and 0.95 are virtually identical. A mix of different nanoscale features is seen ranging from 10 nm (characteristic of 0.5 wt %) to 200 nm features similar to the 2 wt % image discussed below. Thus, the 1 wt % composition has characteristics of a transitional morphology. At 2 wt %, the phase image at rsp ) 0.95 shows 200 nm domains along with a background of features about 10 times smaller. The images thus reflect at least two different length scales for nanoscale phase separation. This morphology is also seen more faintly in the rsp ) 0.97 image. One model to explain these results is that the 200 nm features are rich in fluorous modifier (as suggested by Figure 5S). The 200 nm features may result from an aggregation of the smaller 10–20 nm domains. As noted above, the composition at which the transition from smaller to larger nanoaggregates occurs between 1 and 2 wt %. Aggregation of block copolymers is well-known to result in the formation of micelles that are typically imaged after adsorption on substrates such as glass by techniques including AFM.50 Aggregate size varies considerably from 20 nm for uncharged block copolymer micelles51 to 200 nm for block ionomer complexes.52 The observation of 200 nm aggregates for 2 wt % P[(3FOx)(C12)-0.89:0.11]-PU will be the subject of further study. (49) Brunson, K. M. M.S. Thesis, Virginia Commonwealth University, 2006. (50) Zhu, J.; Hanley, S.; Eisenberg, A.; Lennox, R. B. Makromol. Chem., Macromol. Symp. 1992, 53, 211–20. (51) Webber, G. B.; Wanless, E. J.; Buetuen, V.; Armes, S. P.; Biggs, S. Nano Lett. 2002, 2, 1307–1313.
Langmuir, Vol. 24, No. 11, 2008 5821
Figure 4. TM-AFM phase images (1 × 1 µm2) for neat P[(3FOx)(C12)0.89:0.11] polyurethane (100%), HMDI-BD(50)/PTMO(1000) (base), and 0.1–2% P[(3FOx)(C12)-0.89:0.11] modified base polyurethane at rsp ) 0.97 and 0.95; z ) 20°.
A somewhat different morphology is observed for 2 wt % P[(3FOx)(C6)-0.89:0.11]-PU compositions (Figure 6S). Most of the features observed are on the order of 20 nm, but portions of larger-scale aggregates (250 nm) are observed. Thus, although the fluorous side chains appear to drive nanoscale phase separation, alkylammonium chain length plays a contributing
5822 Langmuir, Vol. 24, No. 11, 2008
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Figure 5. Phase images of P[(ME2Ox)(C12)-0.86:0.14]-PU (100%), 2 wt % P[(ME2Ox)(C12)-0.86:0.14]-PU modified base polyurethane and base polyurethane; rsp ) 0.95; z ) 20°. Table 2. Water Uptake (24 Hr) for Neat Polyurethanes and Static Contact Angles by Goniometry Using a 2 µL Drop static contact angles (°) 100% PSM coating
2% PSM coating
compositions
water uptake (wt %)
θ0 min
θ5 min
θ0 min
θ5 min
HMDI-BD(50)/PTMO-1000 P[(3FOx)(C6)-0.89:0.11]-PU P[(3FOx)(C12)-0.89:0.11]-PU P[(ME2Ox)(C6)-0.86:0.14]-PU P[(ME2Ox)(C12)-0.86:0.14]-PU
6.1 ( 2.6 2.7 ( 0.6 5.1 ( 0.5 9.1 ( 0.3 7.7 ( 2.2
89 89 90 66 65
85 88 85 60 61
n/a 89 89 87 83
n/a 88 87 84 78
role. All fluorous coatings are optically transparent, giving no evidence of microscale phase separation. Although the near-surface morphology for 2 wt % P[(3FOx)(C6)0.89:0.11]-PU and 2 wt % P[(3FOx)(C12)-0.89:0.11]-PU compositions are closely related, antimicrobial effectiveness is not the same.22 The 2 wt % P[(3FOx)(C12)-0.89:0.11]-PU coating effected 100% kill of sprayed-on Gram-negative P. aeruginosa and E. coli and Gram-positive S. aureus, a 3.6–4.4 log reduction in 30 min. Under comparable test conditions, the 2 wt % P[(3FOx)(C6)-0.89:0.11]-PU coating effected a 98–99% kill with 1.7 – 1.9 log reduction for the same bacteria strains. Thus, although nanoscale morphology is largely driven by the fluorous component, the antimicrobial effectiveness is more strongly influenced by alkylammonium chain length. TM-AFM images (rsp ) 0.95) for neat P[(ME2Ox)(C12)0.86:0.14]-PU, base polyurethane, and 2 wt % P[(ME2Ox)(C12)0.86:0.14]-PU are shown in Figure 5. Near-surface nanoscale morphologies for the base and neat polyurethane modifier are similar. For 2 wt % P[(ME2Ox)(C12)-0.86:0.14]-PU, a different phase-separated morphology is seen, which is characterized by scattered 10 nm domains against a background of near-surface phase-separated structures. Morphological evidence for modifier surface concentration is less evident than that observed for the fluorous analog. Thus, TM-AFM is of limited utility in providing information on P[(ME2Ox)(C12)-0.86:0.14]-PU modifier surface concentration. Evidence for the latter comes from XPS (vida supra) and contact antimicrobial effectiveness.22 Wetting Behavior. Cationic functionality is conventionally introduced into hard blocks using a chain extender that generates polyurethane emulsions for “water-borne” coatings.53–55 Using this chain extender approach, Cooper introduced pyridinium groups into linear polyurethanes and evaluated biocidal behavior.56 Depending on the percent alkylpyridinium content, water uptake of 22 to 49 wt % occurred after 48 h. Even the semiflu-
orinated alkylpyridinium polyurethane absorbed 23–31 wt % water. Before assessing wetting behavior, water uptake of HMDIBD(50)/PTMO(1000) and the P[AB]-soft block polyurethanes was investigated by immersion in water for 24 h (Table 2). Water uptake for the base polyurethane is 6.1 wt %, which may be compared with 6.7 wt % for nonquaternized “base” MDI-BIN (85)/PTMO [PTMO/MDI/BIN(1:4:3)].56 The water uptake for P[AB]-soft block polyurethanes is 2.7–5.1 wt % for those with 3FOx-quaternary soft blocks and 7.7–9.1 wt % for the ME2Oxquaternary soft block analogs. Water uptake for the P[AB]-soft block polyurethanes is thus comparable to the HMDI-BD(50)/ PTMO(1000) base polyurethane. The higher water uptake for quaternized BIN polyurethanes (22–49%) compared to P[AB]polyurethanes correlates with much higher quaternary weight fractions for the former. Water uptake for polyurethanes obviously affects wetting kinetics. As the area fraction of physisorbed water increases with time, advancing contact angles must decrease. Such physisorption may synergistically cause migration of near-surface hard blocks to the water-polymer interface. As a part of surface reorganization, this explains why contact angle hysteresis often decreases as a function of exposure time.57 Wetting behavior of modified polyurethane coatings was analyzed by sessile drop and dynamic contact angle (DCA) measurements. The sessile drop method is widely used, but the small surface area interrogated, potential dependence on drop volume, and difficulty in assessing contamination of water by the sample usually leads to results with unknown precision and accuracy.58–60 DCA measurements by the Wilhelmy plate method36 integrates wetting behavior over the entire surface of the coated slide and provides information about contact-angle hysteresis and contamination (if any) of the interrogating liquid.58 With regard to
(52) Solomatin, S. V.; Bronich, T. K.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Langmuir 2007, 23, 2838–2842. (53) Kim, B. K. Colloid Polym. Sci. 1996, 274, 599–611. (54) Nagotkar, S. A.; Shenoy, M. A.; Jagtap, R. N. Paintindia 2000, 115–116. (55) Czech, Z.; Kocmierowska, M. Coating 2005, 38, 475–479. (56) Grapski, J. A.; Cooper, S. L. Biomaterials 2001, 22, 2239–2246.
(57) Pike, J. K.; Ho, T.; Wynne, K. J. Chem. Mater. 1996, 8, 856–860. (58) Uilk, J.; Johnston, E. E.; Bullock, S.; Wynne, K. J. Macromol. Chem. Phys. 2002, 203, 1506–1511. (59) Vafaei, S.; Podowski, M. Z. AdV. Colloid Interface Sci. 2005, 113, 133– 146. (60) Zielecka, M. Polimery (Warsaw, Poland) 2004, 49, 327–332.
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Table 3. Dynamic Contact Angles for Base, P[AB] Polyurethanes and 2 wt % P[AB] Polyurethanesa contact angles (deg) cycle 1 compositions HMDI-BD(50)/PTMO(1000) HMDI-BD(30)/P(3FOx) P[(3FOx)(C6)-0.89:0.11]-PU P[(3FOx)(C12)-0.89:0.11]-PU P[(ME2Ox)(C6)-0.86:0.14]-PU P[(ME2Ox)(C12)-0.86:0.14]-PU a
cycle 2
cycle 3
cycle 4
cycle 5
PSM (wt %)
θadv
θrec
θadv
θrec
θadv
θrec
θadv
θrec
θadv
θrec
100 100 2 100 2 100 2 100 2
90 107 94 92 100 99 65 85 80 94
47 55 48 48 49 49 56 48 55 54
89 106 96 91 100 99 67 84 78 81
50 54 48 48 49 49 56 49 55 53
86 105 95 92 98 98 65 81 78 79
52 53 48 48 50 50 57 49 55 53
83 105 94 91 98 97 n/a 80 78 79
52 53 50 48 50 50 56 49 56 53
82 105 93 91 97 97 n/a 80 78 78
50 53 50 48 50 50 56 49 56 53
Advancing (θadv) and receding (θrec) contact angles are reported for five consecutive cycles.
the latter, the surface tension of water remaining after each analysis was determined using a flamed glass slide. Despite a minimum of two reprecipitations for the alkylammonium P[AB]-soft block polyurethanes, post-analysis water surface tension typically decreased slightly. From force–distance curves before and after analysis for P[(3FOx)(C12)-0.89:0.11]-PU, a decrease of 3 dyn/ cm is observed. The maximum change in θadv due to this level of contamination is 1°. Two weight percent P[AB]-polyurethane coatings showed less water contamination corresponding to a maximum impact on θadv of 0.5°. Regardless of method, on exposure to water θadv for all neat and 2 wt % modified coatings usually decreases by a few degrees (Tables 2 and 3). An exception is P[(ME2Ox)(C12)-0.86:0.14]PU, where θadv measured by DCA changes by 16° from 94° to 78° over five cycles (vida infra). Receding contact angles (DCA) are little changed over five cycles. Where contact angle changes were observed, the original values for “dry” coatings (ambient humidity) were restored after 5–10 min in air. The restoration of wetting behavior is in accord with the low glass transition temperatures of the surface-concentrated soft block domain. Base-Polyurethane Wetting. The sessile drop contact angle for HMDI-BD(50)/PTMO(1000) is 89° initially, dropping to 85° over the course of 5 min (Table 2).49 By DCA (Table 3) θadv-1 ) 90° and θrec-1 ) 47°, showing characteristically high contact-angle hysteresis (θadv-1 - θrec-1 ) θ∆ ) 43°).24 After 5 DCA cycles, the dynamic contact angles change to θadv-5 ) 82° and θrec-5 ) 50° (θ∆ ) 32°). From these results, it is evident that sessile drop contact angles track DCA advancing contact angles fairly closely. P[(3FOx)(C6/C12)-0.89:0.11]-polyurethane Wetting. Initial static contact angles for the fluorous polyurethanes are C6 or C12, θ0 min ≈ 89°. Decreases of 1° (for C6) and 5° (for C12) are observed after 5 min. Sessile drop images for P[3FOx]-PU (105°) and P[(3FOx)(C6)-0.89:0.11] (89°) polyurethanes are compared in Figure 7S. Compared to HMDI-BD(30)/P[3FOx] (107°),21 the dynamic θadv-1 was reduced by 13° for the P[(3FOx)(C6)] polyurethane but only 7° for the C12 analog (Table 3). The receding contact angle for P[(3FOx)(C6)/(C12)] polyurethanes is 49 ( 1° over five DCA cycles. By comparison with θrec54° for HMDI-BD(30)P[3FOx], the alkylammonium side chains increase wettability by decreasing both θadv and θrec. Static contact angles also attest to this trend. P[(3FOx)(C6)/(C12)] Modified Coatings. With compositional economy (i) in mind, the wetting behavior of modified coatings was examined for comparison with neat P[AB]-soft block polyurethanes. Wetting behavior by the sessile drop method for 2 wt % fluorous modified coatings was virtually identical to the neat modifier (Table 2). Dynamic contact angles for 2 wt % P[(3FOx)(C6)-0.89:0.11]-PU were θadv-1 ) 92° and θrec-1 ) 48°.
The C12 analog values were θadv-1 ) 99° and θrec-1 ) 49°. The virtually identical contact angles of neat and 2 wt % coatings attest to the surface concentration of the fluorous alkylammonium surface modifiers. In a manner similar to the neat coatings, the presence of longer alkylammonium side chains results in more hydrophobic surfaces. This supports evidence for soft block surface concentration from XPS and ATR-IR spectroscopy. Advancing dynamic contact angles for the fluorous systems are consistently higher than the sessile drop values. The dynamic θadv for the P[(3FOx)(C12)] polyurethane and the 2 wt % modified base polyurethane are 10° higher than the sessile drop values. This result and that discussed below for the ME2Ox systems suggest that DCA measurements are more responsive to subtle changes in wetting in the range investigated. Neat ME2Ox Polyurethanes. Sessile drop measurements (Table 2) show that P[(ME2Ox)(C6)-0.86:0.14] polyurethane is hydrophilic with θ0 min ≈ 66° and θ5 min ≈ 60°. The dynamic contact angle data are in agreement with θadv ) 65° and θrec ) 56° (Table 4). These values are little changed after three DCA cycles. The contact angle hysteresis is very low (θ∆ ) 9°). By the sessile drop method, θ0 min for the P[(ME2Ox)(C12)0.86:0.14]-polyurethane is 65°; the contact angle decreases to 61° after 5 min. In contrast, by DCA measurement, θadv ) 80° for P[(ME2Ox)(C12)-0.86:0.14]-PU. The 15° higher DCA θadv for P[(ME2Ox)(C12)-0.86:0.14]-PU compared to P[(ME2Ox)(C6)0.86:0.14]-PU can be rationalized as the effect of the long, hydrophobic C12 moiety, but the effect seems disproportionately large considering the low mol % quaternary side chain. The reason for the difference in sessile drop (65°) and DCA (θadv-1 ) 80°) values for P[(ME2Ox)(C12)-0.86:0.14]-PU is not clear. Receding contact angles were the same (56° ( 1) for P[(ME2Ox)(C6)-0.86:0.14]-PU and P[(ME2Ox)(C12)-0.86: 0.14]-PU. These values are close to the 55° value for ME2Ox homo telechelic polyurethanes.61 Thus, once the surface is wetted, the presence of physisorbed water, charge, and polar groups results in a more hydrophilic surface. Interestingly, the receding contact angles for P[(ME2Ox)(C6)-0.86:0.14]-PU (and C12) are about 7° higher than the 3FOx analogs. Water hydrogen bonding to the -OCH2-CF3 hydrogens has previously been proposed to explain the large contact angle hysteresis for fluorous systems.4,58 P[(ME2Ox)(C6)/(C12)] Modified Coatings. Sessile drop measurements for 2 wt % P[(ME2Ox)(C6)-0.86:0.14] polyurethane (θ0 min, 87°; θ5 min, 84°) shows a wetting behavior similar to that of the base polyurethane (θ0 min, 89°, θ5 min, 85°). Dynamic contact angles for the 2 wt % P[(ME2Ox)(C6)-0.86:0.14]-PU are θadv-1 ) 85 and θrec-1 ) 48° (Table 3). By comparison with (61) Fujiwara, T.; Taskent, H.; Gamble, L.; Wynne, K. J. Manuscript in preparation, 2008.
5824 Langmuir, Vol. 24, No. 11, 2008
the θadv for the base polyurethane (89°), the modifier makes the surface slightly more hydrophilic. The θadv for 2 wt % P[(ME2Ox)(C6)-0.86:0.14]-PU decreased to 80° over the course of five cycles (Table 3), while the corresponding value for the base polyurethane is 82°. The 5° decrease in the first DCA θadv cycle is tenuous evidence for surface concentration of the P[(ME2Ox)(C6)-0.86:0.14]-polyurethane modifier. Sessile drop measurements for 2 wt % P[(ME2Ox)(C12)0.86:0.14] polyurethane (θ0 min, 83°; θ5 min, 78°) suggest more hydrophilic wetting behavior than the base polyurethane (θ0 min, 89°; θ5 min, 85°). The DCA θadv-1 (94°) for 2 wt % P[(ME2Ox)(C12)0.86:0.14]-PU is initially higher than both the base polyurethane (90°) and neat modifier (80°) but drops to 81° in the second cycle. The wetting behavior of 2 wt % P[(ME2Ox)(C12)-0.86: 0.14]-PU is similar to the neat modifier after the second cycle θadv-5 ) 78°. The 13° drop (θadv-1 - θadv-2) for the C12 modifier compared to a negligible change in θadv for the C6 analog must be a result of the higher surface concentration of the (ME2Ox)(C12) soft block.
Conclusions Previously, antimicrobial testing results were reported for neat and 2 wt % P[AB]-polyurethane modified coatings.22 The neat P[AB]-polyurethanes were efficient antimicrobial coatings that effected virtually 100% kill against a spray-on challenge of >107 cfu Gram +/- bacteria, while the order of effectiveness for the P[AB]-polyurethanes as 2 wt % surface modifiers was P[(3FOx)(C12)] > P[(ME2Ox)(C12)] > P[(3FOx)(C6)] >> P[(ME2Ox)(C6)]. Only 2 wt % P[(3FOx)(C12)-0.89:0.11] fulfilled criterion (i) by displaying antimicrobial effectiveness indistinguishable from the neat modifier. It was surprising that 2 wt % P[(ME2Ox)(C12)-0.86:0.14]-PU was almost as effective as the fluorous analog. In this summary, the following question is considered: “can the observed order of antimicrobial effectiveness be rationalized from surface characterization?” Starting with 2 wt % P[(ME2Ox)(C6)-0.86:0.14]-PU, there was only tenuous evidence for surface concentration. Br was not detected by XPS, and no significant morphological change was observed by TM-AFM. The wetting behavior for 2 wt % P[(ME2Ox)(C6)-0.86:0.14]-PU by sessile drop measurements was very similar to the base polyurethane. Relative to the base polyurethane, DCA measurements showed a 5° lower θadv and a decrease in θadv to a final value 2° lower than the base polymer. In terms of biocidal testing, 2 wt % P [(ME2Ox)(C6)-0.86:0.14]PU turned in the poorest performance with, 60% kill (less than 0.5 log reduction) in 30 min against 107 cfu sprayed on Gram +/- bacteria. In view of physical surface characterization that provides scant evidence for surface concentration, it is interesting that antimicrobial behavior was observed at all. A stronger case can be made for 2 wt % P[(ME2Ox)(C12)0.86:0.14]-PU because Br was detectable (0.1 atom %), although a factor of 9 lower than the atom % calculated for the soft block alone. Though the mechanism is obscure, the 16° drop in θadv over five DCA cycles for 2 wt % P[(ME2Ox)(C12)-0.86:0.14]PU provides evidence for surface concentration. TM-AFM phase imaging indicates that there is an altered surface morphology
Kurt et al.
(Figure 5). On the basis of this evidence from surface characterization, it is interesting that the surface concentration of 2 wt % P[(ME2Ox)(C12)-0.86:0.14] is sufficient to effect biocidal activity almost as great as P[(3FOx)(C12)-0.89:0.11].22 The longer C12 B side chain is evidently important in surface concentration as a pseudo chain end that entropically enhances surface concentration (b). Ample physical evidence was obtained for surface concentration of fluorous 2 wt % P[AB]-polyurethane modifiers. XPS, contact angles, ATR-IR spectroscopy, and TM-AFM attest to the presence of a modified surface. Despite this strong physical evidence, by criterion (i), 2 wt % P[(3FOx)(C6)-0.89:0.11] was a less-effective biocidal surface modifier than 2 wt % P[(ME2Ox)(C12)-0.86:0.14].22 2 wt % P[(3FOx)(C6)-0.89:0.11] effected a 1.7–1.9 log kill against Gram +/- bacteria, whereas the range for 2 wt % P[(ME2Ox)(C12)-0.86:0.14] was 1.9–4.3. Ignoring the A group, the trend follows the well-known effectiveness for quats with longer alkyl chain lengths. It might be posited that the observed nanoscale phase separation that we ascribe to aggregation of fluorous species works against biocidal behavior. However, if the latter were true, then 2 wt % P[(3FOx)(C12)0.89:0.11] should be even less effective because TM-AFM shows clearer surface nanoscale phase separation compared to the C6 analog. This is not the case at all because 2 wt % P[(3FOx)(C12)0.89:0.11] is the best antimicrobial surface modifier. Whatever the underlying reason, despite clear evidence of surface concentration due to the fluorous chaperone, 2 wt % P[(3FOx)(C6)0.89:0.11] falls into third position in terms of function, that is, biocidal effectiveness in this series. Physical evidence for 2 wt % P[(3FOx)(C12)-0.89:0.11] surface concentration was uniquely strong. Both XPS and ATR-IR spectra gave evidence of surface concentration. TM-AFM phase imaging for the 2 wt % P[AB]-polyurethane was distinct from either parent and consisted of a nanoscale phase-separated structure (Figure 4). The 2 wt % P[(3FOx)(C12)-0.89:0.11]-PU coating affected 100% kill of sprayed-on Gram-negative P. aeruginosa and E. coli and Gram-positive S. aureus, resulting in a 3.6–4.4 log reduction in 30 min. Thus, although the nanoscale morphology is largely driven by the fluorous component, the antimicrobial effectiveness is more strongly influenced by alkylammonium chain length. Overall, the effect of alkylammonium side chain length on surface concentration and antimicrobial behavior is more pronounced for ME2Ox polyurethanes compared to the 3FOx analogs. Acknowledgement:We thank the National Science Foundation (grant DMR-0207560) for support of this research. The surface analysis experiments done at NESAC/BIO were supported by NIH grant EB-002027 from NIBIB. We also thank John Thornton (Veeco) for critical discussions of TM-AFM images acquired with Nanoscope IIIa and V controllers. Supporting Information Available: ATR-IR spectra, model of P[(ME2Ox)(C12)-0.89:0.11], suggested model of micelle-like features, AFM phase images of polyurethane, and sessile drop contact angles. This information is available free of charge via the Internet at http://pubs.acs.org. LA800203Y
