Polyurethanes Containing Oxetane-Derived Poly(2 ... - ACS Publications

In other cases (PU-2, PU-5), θadv decreased 2−6° while θrec increased 1°. In the absence of water contamination, contact angle hysteresis (Δθ)...
0 downloads 0 Views 423KB Size
Download PDF
Langmuir 2005, 21, 10749-10755

10749

Polyurethanes Containing Oxetane-Derived Poly(2,2-substituted-1,3-propylene oxide) Soft Blocks: Copolymer Effect on Wetting Behavior Umit Makal, Tomoko Fujiwara, Robert S. Cooke, and Kenneth J. Wynne* Department of Chemical and Life Sciences Engineering, Virginia Commonwealth University, Richmond, Virginia 23284-3028 Received May 10, 2005. In Final Form: August 18, 2005 Hydroxy-terminated poly(2,2-substituted-1,3-propylene oxide) telechelics and co-telechelics bearing semifluorinated (R ) -CH2OCH2(CF2)nCF3, n ) 0, 1) and/or bromomethyl pendant groups were synthesized from the corresponding 3,3-substituted oxetanes. The new telechelics were incorporated in polyurethanes (PUs) with isophorone diisocyanate (IPDI) and 1,4-butanediol (BD) as the hard block. Surface properties were evaluated using tapping mode atomic force microscopy (TM-AFM) and dynamic contact angle (DCA) analysis. Interestingly, polyurethanes containing P(3FOx-BrOx) have higher θadv and lower θrec than the homo-telechelic PUs [P(3FOx) ) poly(2-methyl-2-trifluoroethoxymethyl-1,3-propylene oxide; P(BrOx) ) poly(2-methyl-2-bromomethyl-1,3-propylene oxide)]. For IPDI-BD(40)/P(3FOx/BrOxs1:1), θadv (116°) is higher and θrec (32°) is lower (∆θ, 84°) than any other homo- or co-telechelic polyurethane. The unusual wetting behavior for P(FOx/BrOx) polyurethanes is correlated with FOx-BrOx dyad content, and a reversible H-bonding mechanism is proposed to explain the results.

Introduction Incorporating a polymeric surface-modifier (PSM) during coating, foaming, or a similar process is an important method for controlling surface properties.1 Most often, PSMs are employed to change wetting characteristics. Surfaces are made hydrophobic with poly(dimethylsiloxane) (PDMS) PSMs2-7 or both hydrophobic and oleophobic with fluorinated PSMs.8-13 Poly(dimethylsiloxane) PSMs are important in conferring oxidative stability on polyurethanes (PUs) for critical biomaterials applications.14 Our goal is to provide a broader palette of surfacemodifying options such as antimicrobial properties, hydrophilicity, and biocompatibility. In working toward this goal, we have incorporated functional groups into semi* To whom correspondence should be addressed. E-mail: [email protected]. (1) Ward, R. S.; White, K. A.; Hu, C. B. In Biomedical Engineering; Planck, H., Egbers, G., Syre, I., Eds.; Elsevier Science Publishers: Amsterdam, 1984. (2) 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. (3) Tezuka, Y.; Fukushima, A.; Matsui, S.; Imai, K. J. Colloid Interface Sci. 1986, 114, 16-25. (4) Smith, S. D.; DeSimone, J. M.; Huang, H. Y.; York, G. A.; Dwight, D. W.; Wilkes, G. L.; McGrath, J. E. Macromolecules 1992, 25, 25752581. (5) Chen, X.; Lee, H. F.; Gardella, J. A. Macromolecules 1993, 26, 4601-4605. (6) Chen, X.; Gardella, J. A.; Kumler, P. L. Macromolecules 1992, 25, 6631. (7) Ho, T.; Wynne, K. J. Polym. Adv. Technol. 1995, 6, 25-31. (8) Malik, A. A.; Carlson, R. P.; Aerojet General Corporation. U.S. Patent 5,637,772, 1997. (9) Malik, A. A.; Archibald, T. G.; GenCorp. U.S. Patent 6,037,483, 2000. (10) Ho, T.; Malik, A. A.; Wynne, K. J.; McCarthy, T. J.; Zhuang, K. H. Z.; Baum, K.; Honeychuck, R. V. ACS Symp. Ser. 1996, 624, 362376. (11) Kim, Y. S.; Lee, J. S.; Ji, Q.; McGrath, J. E. Polymer 2002, 43, 7161-7170. (12) Thomas, R. R.; Glaspey, D. F.; DuBois, D. C.; Kirchner, J. R.; Anton, D. R.; Lloyd, K. G.; Stika, K. M. Langmuir 2000, 16, 6898-6905. (13) Anton, D. Adv. Mater. 1998, 10, 1197-1205. (14) Wheatley, D. J.; Raco, L.; Bernacca, G. M.; Sim, I.; Belcher, P. R.; Boyd, J. S. Eur. J. Cardio-thor. Sur. 2000, 17, 440-448.

fluorinated telechelics derived from oxetanes. Through subsequent incorporation in a polyurethane, functional telechelics become functional PSMs. This approach to surface concentrate functional groups leverages the tendency of both soft blocks15-17 and fluorinated side chains18,19 to surface concentrate. Semifluorinated groups have been used previously to surface concentrate photostabilizers20 and fullerenes.21 Chaudhury has reported polyurethanes with multiple soft blocks that are hydrophobic (fluorinated, PDMS), hydrophilic (poly(ethylene glycol), PEG) and amphiphilic (PDMS-PEG).22 The presence of the fluorinated soft block acts to concentrate the hydrophilic block near the surface. These amphiphilic polyurethanes undergo reversible hydrophobic-hydrophilic wetting behavior in response to the polarity of the contacting medium.

Previously, we reported amphiphilic poly(1,3-propylene oxide) co-telechelics with hydrophobic semifluorinated and (15) Garrett, J. T.; Siedlecki, C. A.; Runt, J. Macromolecules 2001, 34, 7066-7070. (16) Grasel, T. G.; Cooper, S. L. Biomaterials 1986, 7, 315-328. (17) Tingey, K. G.; Andrade, J. D. Langmuir 1991, 7, 2471-2478. (18) Iyengar, D. R.; Perutz, S. M.; Dai, C. A.; Ober, C. K.; Kramer, E. J. Macromolecules 1996, 29, 1229-1234. (19) Li, X.; Andruzzi, L.; Chiellini, E.; Galli, G.; Ober, C. K.; Hexemer, A.; Kramer, E. J.; Fischer, D. A. Macromolecules 2002, 35, 8078-8087. (20) Vogl, O.; Jaycox, G. D.; Hatada, K. J. Macromol. Sci. Chem. 1990, 27, 1781-1854. (21) Chen, W.; McCarthy, T. J. Macromolecules 1999, 32, 23422347. (22) Vaidya, A.; Chaudhury, M. K. J. Colloid Interface Sci. 2002, 249, 235-245.

10.1021/la051245y CCC: $30.25 © 2005 American Chemical Society Published on Web 10/08/2005

10750

Langmuir, Vol. 21, No. 23, 2005

hydrophilic alkyl ether pendant groups.23 The contrasting nanoscale surface phase separation and wetting behavior of polyurethanes incorporating telechelics with random or block sequences of semifluorinated and alkyl ether pendant groups was reported.51 To obtain surface-active telechelics bearing reactive groups, we have prepared cotelechelics 1 containing semifluorinated and bromomethyl groups.24 Using the new P(FOx/BrOx) telechelics, where “P” designates the telechelic-incorporated monomer, polyurethanes were prepared employing isophorone diisocyanate (IPDI)/1,4-butanediol (BD) hard blocks. In the present paper, we report wetting behavior for IPDI-BD/ P(FOx/BrOx) polyurethanes via dynamic contact angle (DCA) analysis. Interestingly, a copolymer effect was found whereby copolymer-telechelic polyurethanes have higher θadv and lower θrec than the homo-telechelic PUs. The unexpected wetting behavior prompted an investigation of surface topology (roughness) and morphology of these and related polyurethanes by tapping mode atomic force microscopy (TM-AFM). Separately, we report a “reactionon-polymer” approach for substituting -CH2-Br and preparing -CH2-5,5-dimethylhydantoin soft block substituted polyurethanes that have unusual wetting behavior. The latter coatings are hydrophilic when dry and hydrophobic when wet.25 The 5,5-dimethylhydantoin substituted polyurethane coatings are precursors to antimicrobial PSMs.26 Experimental Section Materials. 3-(2,2,2-Trifluoroethoxymethyl)-3-methyloxetane (3FOx), 3-(2,2,3,3,3-penta-fluoropropoxymethyl)-3-methyloxetane (5FOx), and 3-bromomethyl-3-methyloxetane (BrOx) were synthesized following published procedures9 or were generously provided by Gencorp Aerojet (Sacramento, CA) or OMNOVA Solutions (Akron, OH). Monomers were distilled under vacuum before use: 3FOx and 5FOx at close to 100 °C/5 mmHg and BrOx at 85 °C/5 mmHg. Boron trifluoride dietherate (BF3O(C2H5)2) was used as received. Methylene chloride, tetrahydrofuran (THF), dimethylformamide (DMF), and methanol were either used as received or dried and stored over 4 Å molecular sieves. Isophorone diisocyanate (IPDI, 98%), 4,4-methylenebis(phenyl isocyanate) (MDI, 98%), poly(tetramethylene oxide) (PTMO-2000), and dibutyltin dilaurate catalyst (T-12) were obtained from Aldrich. 1,4-Butanediol (BD) was purchased from Acros and used as received. Synthesis. The synthesis and characterization of the new P(FOx/BrOx) telechelics and IPDI-BD/P(FOx/BrOx) polyurethanes have been reported in detail.24 IPDI-BD/P(FOx/BrOx) polyurethanes were typically purified by reprecipitation (THF solutions into deionized water/methanol). Without adequate purification, coatings can contaminate the water surface, thereby changing the surface tension (and contact angle). Such contamination is difficult to detect by sessile drop methods.27 Samples were prepared for DCA analysis by dip coating from THF solutions onto glass cover slips (Corning, 24 × 40 × 0.5 mm). Care was taken to distribute the 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. The samples were placed in an inverted, upright position at ambient conditions under an inverted beaker (to slow solvent evaporation) for 24 h and then in an oven overnight at 60 °C under reduced pressure. By visual inspection, all PU coatings were colorless and transparent with smooth, flat surfaces. (23) Fujiwara, T.; Makal, U.; Wynne, K. J. Macromolecules 2003, 36, 9383-9389. (24) Makal, U.; Uilk, J.; Kurt, P.; Cooke, R. S.; Wynne, K. J. Polymer 2005, 46, 2522-2530. (25) Makal, U.; Wynne, K. J. Langmuir 2005, 21, 3742-3745. (26) Wynne, K. J.; Makal, U.; Fujiwara, T.; Ohman, D.; Wood, L. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 2004, 45, 100. (27) Uilk, J. M.; Mera, A. E.; Fox, R. B.; Wynne, K. J. Macromolecules 2003, 36, 3689-3694.

Makal et al. Atomic Force Microscopy (AFM). Polyurethane surface images were obtained using a Digital Instruments (Santa Barbara, CA) nanoscope IIIA with a multimode head. TM-AFM was utilized for topographic and phase-contrast images that were acquired with standard silicon tips. Several tapping forces were employed, Asp/Ao ) 0.96-0.70, where Asp/Ao ) set point amplitude/free amplitude of oscillation. The same polyurethane coatings prepared for Wilhelmy plate measurements were used for AFM imaging. Wetting Behavior. Dynamic contact angle (DCA) analysis based on the Wilhelmy plate method28 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 MΩ‚cm deionized water from a Barnstead (Dubuque, IA) Nanopure 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 a 2-propanol/potassium hydroxide base bath for at least 24 h, rinsing for 30 s with hot tap water, and then rinsing 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 obtained. Unless otherwise noted, the stage speed was 100 µm/s and dwell times between advancing and receding test segments were 10 s. Other stage speeds (150 µm/s and 200 µm/s) were tried using three representative coatings (PU-4, 5, and 8). Identical values were obtained. As noted above, contamination of the water surface by species diffusing from the coating being examined often invalidates the determination of intrinsic contact angles. Indeed, for applications where leached species are unwanted, such as for biomedical materials, sensors, and microfabrication processes, the DCA method is an excellent way to ascertain whether a coating or molded object contaminates water. Surface-contamination effects are common in water due to insolubility and/or surface activity of organic contaminants. Fortunately, surface contamination is readily detected in the DCA experiment by (1) an irreversible change in the fdc coupled with (2) a test of the water used for the DCA experiment with a clean (flamed) glass cover slip. If the water surface is contaminated, a deviation from the normally linear advancing and receding fdc’s for water is readily apparent.

Results and Discussion In designated compositions, such as IPDI-BD(40)/ P(3FOx/BrOxs1:1), the hard block composition is followed by hard block weight percent in parentheses. Next is the soft block composition followed by the soft block segment mole ratio. The Mn values for soft blocks are shown in Table 1 following the polyurethane composition in parentheses. Chemical and/or topological heterogeneities have effects on contact angles.29 Increased surface heterogeneity leads to higher θadv and lower θrec.30,31 Before discussing DCA analysis, sample topology is considered. Surface Imaging. Near-Surface Phase Separation. Tapping mode atomic force microscopy (TM-AFM) was used to assess surface morphology and topology. Imaging was carried out as a function of tapping force.15 It is assumed that lighter areas correspond to near-surface hard block regions of higher modulus.15,32 Discussion of surface imaging and wetting behavior focuses on the three polyurethanes shown in Figure 1. IPDI-BD(40)/P(3FOx/ (28) Wilhelmy, L. Ann. Phys. Chem. (Leipzig) 1863, 119, 177. (29) Cassie, A. B. D. Discuss. Faraday Soc. 1948, 3, 11-16. (30) Johnson, R. E., Jr.; Dettre, R. H. In Surfactant Science Series; Berg, J. C., Ed.; Marcel Dekker: New York, 1993; Vol. 49, pp 1-73. (31) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395-3399. (32) Magonov, S. N.; Elings, V.; Whangbo, M. H. Surf. Sci. 1997, 375, L385-L391.

Copolymer Effect on Polyurethane Wetting Behavior

Langmuir, Vol. 21, No. 23, 2005 10751

Table 1. Advancing and Receding Contact Angles for Homo- and Co-telechelic PUs composition

designation PU

cycle 1 θadv/θrec

cycle 2 θadv/θrec

cycle 3 θadv/θrec

cycle 4 θadv/θrec

cycle 5 θadv/θrec

IPDI-BD(50)/PTMO(2000) IPDI-BD(40)/P(BrOx)(2800) IPDI-BD(40)/P(3FOx)(3400) IPDI-BD(40)/P(3FOx/BrOxs2:1)(4700) IPDI-BD(40)/P(3FOx/BrOxs1:1)(4700) IPDI-BD(40)/P(3FOx/BrOxs1:2)(3400) IPDI-BD(40)/P(5FOx/BrOxs2:1)(2500) IPDI-BD(40)/P(5FOx/BrOxs1:1)(4100) IPDI-BD(40)/P(5FOx/BrOxs1:2)(3400) MDI-BD(15)/P(3FOx/BrOxs1:1)(4700)

base PU PU-1 PU-2 PU-3 PU-4 PU-5 PU-6 PU-7 PU-8 PU-9

85/55 102/42 105/45 108/35 116/33 104/34 109/38 109/35 107/36 113/38

82/55 101/41 99/45 108/35 115/32 102/34 108/38 109/35 106/36 113/38

82/56 101/41 98/46 108/35 116/32 102/34 108/38 109/35 106/36 113/38

81/56 101/40 98/46 108/34

81/56 101/40 98/46 108/34

102/34 108/38 109/35 106/36 113/38

102/34 108/38 109/35 106/36 113/38

BrOxs1:1) was studied in detail because the wetting behavior was unusual, as described below. For comparison with a conventional polyurethane, TM-AFM images for IPDI-BD(50)/PTMO were also obtained. TM-AFM imaging of MDI-BD(15)/P(3FOx/BrOxs1:1) provided an assessment of changing hard block. This is an imperfect comparison because of the differing hard segment contents. The latter comes about because much smaller percentages of MDI hard block yield comparable mechanical properties. Nevertheless, the comparison gives useful information for interpretation of the differing wetting behavior of MDI and IPDI polyurethanes.

Figure 1. Structure and compositions (wt %) of polyurethanes: (A) IPDI-BD(50)/PTMO; (B) IPDI-BD(40)/P(3FOx/ BrOxs1:1); and (C) MDI-BD(15)/P(3FOx/BrOxs1:1).

Figure 2 shows TM-AFM phase images (2 µm × 2 µm) at two representative setpoint ratios for the three polyurethanes noted above (Figure 1). Even at low tapping force (Asp/Ao ) 0.96), surface phase separation is discernible for IPDI-BD(40)/P(3FOx/BrOxs1:1) (Figure 2B-1), reflecting the presence of a near-surface hard segment. With increasing tapping force (Figure 2B-2), the phaseseparated near-surface structure becomes more evident.11,15,33-35 The IPDI-BD(50)/PTMO surface (Figure 2A) also shows both hard and soft phases, even at low (33) Sauer, B. B.; McLean R. S. Macromolecules 1997, 30, 83148317. (34) Garrett, J. T.; Runt, J.; Lin, J. S. Macromolecules 2000, 33, 63536359. (35) Karbach, A.; Drechsler, D. Surf. Interface Anal. 1999, 27, 401409.

water con. no no yes no no yes no yes yes no

tapping force. As the tapping force is increased, phase separation becomes even clearer (Figure 2A-2). TM-AFM on MDI-BD(15)/P(3FOx/BrOxs1:1) with 15% hard segment content gave a nearly featureless phasecontrast image with a light tapping force (Asp/Ao ) 0.96, Figure 2C-1). Harder tapping (Asp/Ao ) 0.70, Figure 2C-2) resulted in clear imaging near-surface hard block, paralleling the results of Garrett et al. on MDI-ED(22)/PTMO.15 These results offer compelling evidence that the soft block is the dominant constituent of the surface. Well-ordered hydrogen bonding by the MDI-ED hard block results in good bulk and surface phase separation.34,36,37 Thus, excellent mechanical properties are obtained at low MDI-ED weight fractions. Using differential scanning calorimetry (DSC) and the Fox equation analysis, we have shown that IPDI-BD(40)/P(3FOx/ BrOxs1:1) and MDI-BD(15)/P(3FOx/BrOxs1:1) are also well phase separated.24 The amount of pure soft block in the soft segment domain was estimated at 88 and 93%, respectively. Despite good bulk phase separation, the differences between soft and hard tapping images, while apparent, are not as clearly marked for IPDI-BD(40)/ P(3FOx/BrOxs1:1) (Figure 2B) as for MDI-BD(15)/ P(3Fox/BrOxs1:1) (Figure 2C). It is noted that the phase angle (20°) was optimized to obtain Figure 2B. If larger (e.g., 60°) phase angles were utilized, it was possible to magnify apparent differences between soft and hard tapping images. The observation of near-surface hard block for IPDIBD(50)/PTMO and IPDI-BD(40)/P(3FOx/BrOxs1:1) is attributed to a high hard block weight fraction (50 and 40%, respectively). This arises because IPDI-BD hard blocks form poorly ordered hydrogen-bonded domains38 and high hard block content is necessary to achieve mechanical properties equivalent to MDI hard block systems. In contrast, the TM-AFM images of MDI-BD(15)/P(3FOx/BrOxs1:1) PU is featureless at soft tapping (Figure 2C-1), and the near-surface phase separation becomes apparent only with higher tapping force (Figure 2C-2). This result is similar to the TM-AFM results of Garrett et al. on MDI-ED(22)/PTMO.34 In summary, while TM-AFM results for IPDI-BD(40)/P(3FOx/BrOxs1:1) are consistent with the tendency of soft blocks to preferentially occupy the air-polymer interface, the hard block has nearsurface prominence due to the high weight fraction. Surface Area/Roughness. Surface roughness and/or chemical heterogeneity can strongly influence wetting behavior.29,31,39,40 Surface roughness typically leads to (36) Lamba, N. M. K.; Woodhouse, K. A.; Cooper, S. L. Polyurethanes in Biomedical Applications; CRC Press: Boca Raton, FL, 1998. (37) Saiani, A.; Daunch, W. A.; Verbeke, H.; Leenslag, J. W.; Higgins, J. S. Macromolecules 2001, 34, 9059-9068. (38) Velankar, S.; Cooper, S. L. Macromolecules 1998, 31, 91819192. (39) Youngblood, J. P.; McCarthy, T. J. Macromolecules 1999, 32, 6800-6806.

10752

Langmuir, Vol. 21, No. 23, 2005

Makal et al.

Figure 2. TM-AFM images of A, IPDI-BD(50)/PTMO, A-1, Asp/Ao ) 0.95, A-2, Asp/Ao ) 0.70; B, IPDI-BD(40)/P(3FOx/BrOxs1:1) PU, B-1, Asp/Ao ) 0.96, B-2, Asp/Ao ) 0.70; C, MDI-BD(15)/P(3FOx/BrOxs1:1) PU, C-1, Asp/Ao ) 0.96, C-2, Asp/Ao ) 0.70. Height images at z ) 15 nm, and phase images at z ) 20°.

higher advancing and lower receding contact angles.41 To evaluate how surface roughness varied with interrogated surface area, a series of TM-AFM images were collected at different scan sizes. Figure 3 shows TM-AFM 2D and 3D height and phase images of the same PU with scan sizes varying from 10 000 µm2 to 0.3 µm2. The 3D height images are also provided for visualizing surface topology. The root-mean-square (Rq) roughness ranges from 1.46 nm (10 000 µm2) to 0.28 nm (0.3 µm2). There is an interesting change from the largest to smallest areas. The 10 000 µm2 image (Figure 3C) has the highest Rq roughness (1.46 nm), resulting in considerable contrast in the 2D height image. Nanoroughness appears in the 2D height image as light points in false color images. At this scale, the phase image is poorly resolved. Interestingly, in the smallest area scan (556 × 556 nm, 0.3 µm2), it is the phase image that clearly resolves near-surface phase separation, while the 2D image is featureless. The origin of the nanoroughness (1.46 nm/0.01 mm2) in IPDI-BD(40)/ P(3FOx/BrOxs1:1) is not clear but may originate from differential solvent evaporation during the dip-coating process. In summary, our AFM results are consistent with the presence of the soft block at the surface, but because of the high weight fraction of IPDI-BD, this phase is also prominent in the near-surface region. Solvent casting for IPDI-BD(40)/P(3FOx/BrOxs1:1) provides exceptionally smooth coatings with surface roughnesses comparable to those obtained for spin-coated or grafted nanofilms.42-44 (40) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377-1380. (41) Sun, T. L.; Wang, G. J.; Feng, L.; Liu, B. Q.; Ma, Y. M.; Jiang, L.; Zhu, D. B. Angew. Chem., Int. Ed. 2004, 43, 357-360. (42) Julthongpiput, D.; Lin, Y. H.; Teng, J.; Zubarev, E. R.; Tsukruk, V. V. Langmuir 2003, 19, 7832-7836.

For example, the Rq for a well-defined polyimide film was 0.5 nm for a 25 µm2 area.45 Wetting Behavior. Polyurethane wetting behavior was determined by the Wilhelmy plate method using a dynamic contact angle analyzer (DCA).28,46,47 The Wilhelmy plate experiment has been discussed in connection with the measurement of intrinsic contact angles for model PDMS networks.27 Several advantages for the Wilhelmy plate method have been noted.48 A large polymer surface is interrogated in the DCA experiment compared to the sessile drop method.27 Another feature is facile testing of water surface tension after sample analysis.27 This important control is performed by using a glass cover slip (flamed to remove organic contaminants) to analyze water surface tension before and after evaluating a polymer coating. A decrease in post-use water surface tension indicates water surface contamination by surface-active materials such as oils and/or amphiphiles leached from the coating. The ready qualitative detection of water insoluble leached species is of particular importance to biomedical, electronic, and space applications. In the present work, glass cover slips were dip coated using THF solutions followed by solvent removal in vacuo. (43) Lemieux, M.; Usov, D.; Minko, S.; Stamm, M.; Shulha, H.; Tsukruk, V. V. Macromolecules 2003, 36, 7244-7255. (44) Minko, S.; Patil, S.; Datsyuk, V.; Simon, F.; Eichhorn, K. J.; Motornov, M.; Usov, D.; Tokarev, I.; Stamm, M. Langmuir 2002, 18, 289-296. (45) Chae, B.; Kim, S. B.; Lee, S. W.; Kim, S. I.; Choi, W.; Lee, B.; Ree, M.; Lee, K. H.; Jung, J. C. Macromolecules 2002, 35, 10119-10130. (46) Hogt, A. H.; Gregonis, D. E.; Andrade, J. D.; Kim, S. W.; Dankert, J.; Feijen, J. J. Colloid Interface Sci. 1985, 106, 289-298. (47) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley and Sons: New York, 1997. (48) Lander, L. M.; Siewerski, L. M.; Brittain, W. J.; Vogler, E. A. Langmuir 1993, 9, 2237.

Copolymer Effect on Polyurethane Wetting Behavior

Langmuir, Vol. 21, No. 23, 2005 10753

Figure 3. TM-AFM height (3D and 2D) and phase images of IPDI-BD/P(3FOx/BrOxs1:1) PU. The phase scale is 25°, 3D height areas: (A) 556 nm × 556 nm × 15 nm; (B) 10 µm × 10 µm × 15 nm; (C) 100 µm × 100 µm × 15 nm. RMS roughness: (A) 0.28 nm; (B) 0.62 nm; (C) 1.46 nm. 2D height and phase image sizes: (A) 556 nm × 556 nm; (B) 10 µm × 10 µm; (C) 100 µm × 100 µm.

Dip coating rather than spin coating was used because Wilhelmy plate analysis requires complete coverage. Careful processing is required to avoid surface inhomogenities affecting wetting behavior (see Experimental Section). As noted above, TM-AFM showed Rq’s (0.3-1.5 nm) similar to typical values for spin coating.42-44 Advancing (θadv) and receding (θrec) contact angles are shown in Table 1. Polyurethanes were reprecipitated at least once into water/methanol mixtures, and in most cases, little or no water contamination was observed. The presence or absence of contamination for each coating according to the procedure described above is noted in Table 1. For some cases where contamination was detected (PU-7, PU-8), there was no measurable change in contact angles over five cycles. In other cases (PU-2, PU-5), θadv decreased 2-6° while θrec increased 1°. In the absence of water contamination, contact angle hysteresis (∆θ) reflects rapid, enthalpically driven surface reorganization.49 Gradual changes in contact angles due to water contamination are irreversible when multiple runs are performed using the same interrogation water. Gradual changes in contact angles due to surface hydration and/or reorganization are reversible if the coating is dried. Solvent casting of IPDI-BD(40)/P(3FOx/BrOxs1:1) provides exceptionally smooth coatings with surface roughnesses comparable to those obtained for nanofilms.44 The Rq roughness values for the polymers reported here were not greater than 13 nm. (TM-AFM). Topological (49) Uilk, J.; Mera, A. E.; Fox, R. B.; Wynne, K. J. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 2002, 43, 645.

surface roughness at this scale does not significantly affect the wetting behavior.50 However, we have shown in one case that surface chemical compositional changes on the nanoscale (ran vs block soft blocks) affect wetting behavior.51 Wetting behavior of a conventional polyurethane, IPDIBD(50)/PTMO(2000), is discussed before considering P(FOx-BrOx) homo- and co-telechelic PUs. Water surface contamination by IPDI-BD(50)/PTMO was eliminated after reprecipitation and extraction of the polymer in methanol/water for 36 h. Initial contact angles are θadv ) 85°, θrec ) 56°, and ∆θ ) 29° (Table 1), but surface reorganization/hydration changes wetting behavior to θadv ) 81°, θrec ) 56°, and ∆θ ) 25° over the course of ∼12 min immersion (four DCA cycles). For IPDI-BD(50)/PTMO, θadv is somewhat lower and θrec is higher than that for MDI-BD/PTMO PUs. Typical contact angles for MDI PUs are those reported by Herbert et al., θadv ) 88° and θrec ) 48°.52 Considering bulk phase mixing inferred from DSC24 and surface heterogeneity for IPDI-BD(50)/PTMO (Figure 2A), it is likely that surface phase mixing (chemical heterogeneity) accounts for the different wetting behavior compared to MDI materials. That is, the presence of hard block at or near the surface increases hydrophilic character (50) Nakae, H.; Inui, R.; Hirata, Y.; Saito, H. Acta Mater. 1998, 46, 2313-2318. (51) Fujiwara, T.; Wynne, K. J. Macromolecules 2004, 37, 84918494. (52) Herbert, C. B.; Hernandez, A. M.; Hubbell, J. A. Biotechnol. Bioeng. 1996, 52, 81-88.

10754

Langmuir, Vol. 21, No. 23, 2005

Makal et al.

Figure 4. DCA analysis of an IPDI-BD(40)/P(3FOx/BrOxs 1:1) coating (PU-4).

Figure 6. Contact angles for PU-3, -4, and -5 correlated with 3FOx-BrOx dyad content: navy diamond, θadv; pink square, θrec; black triangle, mol % 3FOx-BrOx dyad.

Figure 5. DCA analysis of an MDI-BD(15)/P(3FOx/BrOxs 1:1) coating (PU-9).

by virtue of hard block urethane groups occupying a significant surface-area fraction. Surface phase mixing is also consistent with changes in contact angles for successive cycles. This is attributed to hydration of the nearsurface hard segments. Drying the coatings restores the originally observed θadv. IPDI-BD(40)/P(3FOx) exhibits high θadv and high contact angle hysteresis (∆θ) characteristic of semifluorinated side chains (θadv ) 105° and θrec ) 45°). θadv is not as high as the 123° value observed for longer semifluorinated side chains that form liquid crystal (LC) phases.53 IPDI-BD(40)/P(3FOx) shows a change in θadv for successive cycles. This is partly due to water contamination that could not be eliminated despite attempts at purification. It is interesting that PU-1, IPDI-BD(40)/ P(BrOx), containing the P(BrOx) homo-telechelic, also has a high θadv (101°) coupled with a rather low θrec (41°), 3° lower than that for IPDI-BD(40)/P(3FOx) PU. The θadv for PU-1 reflects the nonpolar nature of the C-Br bond. The 41° value for θrec indicates that the surface rearranges and that a local, strongly hydrogen-bonded structure is formed on immersion. Such surface reorganization has long been recognized.17 Interestingly, all co-telechelic PUs have higher θadv and lower θrec than the parent homo-telechelic PUs (Table 1). The wetting behavior of PU-4, IPDI-BD(40)/P(3FOx/ BrOxs1:1), is unique among the PU co-telechelics (Figure 4, θadv ) 116° and θrec ) 32°). These values are constant over three cycles (superimposed in Figure 4, no water contamination). PU-9, MDI-BD(15)/P(3FOx/BrOxs1:1), containing the same co-telechelic but the MDI-BD hard block at a lower weight percent, shows similar behavior as PU-4 (Figure 5, θadv ) 113° and θrec ) 38°). Although the FOx/BrOx soft blocks in IPDI-BD/P(FOx/ BrOx) compositions have glass transition temperatures from -25 to -37 °C, we were concerned that an artifact associated with platform speed might have led to unusual results for PU-4 and PU-9. The default immersion and withdrawal stage speeds for our DCA experiments are 100 µ/s. Results for increased stage speeds (150 and 220 µ/s) for PU-4, PU-5, and PU-8 were the same within the experimental error. Thus, cycling between high θadv and (53) Wang, J. G.; Mao, G. P.; Ober, C. K.; Kramer, E. J. Macromolecules 1997, 30, 1906-1914.

low θrec is rapid compared to the time scale of the DCA experiment and indicates that a facile surface reorganization occurs in water. The high and stable contact angle hysteresis (84°, PU4; 75°, PU-9) is notable for topologically smooth surfaces (vida supra). Polymers with θadv that exceed 116° are uncommon. These include perfluorinated polymers such as poly(tetrafluoroethylene) (PTFE),54 acrylates containing fluorinated side chains that form LC53 or ordered phases,55 and poly(dimethylsiloxane) networks.27 Unlike perfluorinated polymers such as PTFE, polymers with semifluorinated side chains such as acrylates and methacrylates are often characterized by high ∆θ.55 One semifluorinated acrylate prepared from CH2dCHCOOCH2CF3 was reported by Katano with θadv/θrec of 130/22° or 133/17° depending on processing.55 Use of semifluorinated (trialkoxy)silanes in condensation cure gives rise to PDMS hybrids with high ∆θ, e.g., “FTEOS-12x”, θadv ) 135°, θrec ) 56°, ∆θ ≈ 80°.53,56 If the semifluorinated group forms an LC phase, θadv is high (∼120°) and reorganization is thermodynamically unfavorable, resulting in very low ∆θ.55,57 Figure 6 shows contact angles for PU-3, -4, and -5 with 3FOx-BrOx dyad ratios predicted using data from a Fineman-Ross analysis.24,58,59 For PU-4 containing P(3FOx/BrOxs1:1), the fraction of 3FOx-BrOx dyads is maximized in the 1:1 telechelic composition. At this composition, maximum θadv (116°) and minimum θrec (32°) are observed. For understanding a proposed model for correlating copolymer contact angles, the wetting behavior of the 3FOx homotelechelic polyurethane is first considered [IPDIBD/P(3FOx), PU-2 (θadv ) 105°, θrec ) 46°)]. We suggest that the inter/intramolecular hydrogen bonding of the (54) Boker, A.; Reihs, K.; Wang, J. G.; Stadler, R.; Ober, C. K. Macromolecules 2000, 33, 1310-1320. (55) Katano, Y.; Tomono, H.; Nakajima, T. Macromolecules 1994, 27, 2342-2344. (56) Wynne, K. J.; Ho, T.; Johnston, E. E.; Myers, S. A. Appl. Organomet. Chem. 1998, 12, 763-770. (57) Xiang, M. L.; Li, X. F.; Ober, C. K.; Char, K.; Genzer, J.; Sivaniah, E.; Kramer, E. J.; Fischer, D. A. Macromolecules 2000, 33, 6106-6119. (58) Hagiopol, C. CopolymerizationsTowards a systematic approach; Kluwer-Academic/Plenum: New York, 1999. (59) The mole fraction of the mixed dyad at 95 mol % conversion was estimated by numerical integration of the four propagation reaction rate expressions using Euler’s method implemented in an Excel spreadsheet. The reactivity ratios were determined experimentally as previously described (ref 24). A simplifying assumption was made that the monomer charge was added in one portion, rather than in increments. The linear least squares were calculated by using the software included in ref 58.

Copolymer Effect on Polyurethane Wetting Behavior

Figure 7. Free A and hydrogen-bonded B 3FOx surface groups. A has a higher surface fraction of CF3 moieties.

Figure 8. Steric blocking of CF3CH2 hydrogen bonding by Br.

CF3CH2 hydrogens to main-chain ether oxygens (Figure 7B) disrupts the CF3 group orientation and lowers the surface fraction of unimpeded CF3 groups (Figure 7A) resulting in lower θadv. A proposed model for the higher advancing contact angles for polyurethanes containing P(3FOx-BrOx) copolymer soft blocks is shown in Figure 8. Here, the nonpolar BrOx group sterically hinders access of CF3CH2 hydrogens to ether oxygens, resulting in a higher surface fraction of unimpeded CF3 groups and higher θadv. The BrOx steric hindrance of ether-CF3CH2 hydrogen bonding results in increased access of liquid water to CF3CH2 and a lower receding contact angle. The low θrec for PU-4 indicates a strong hydrogenbonding interaction between water and the coating surface. While strong, this interaction must be highly surface localized, perhaps to the outermost 0.5 nm. The highly localized nature of the water-soft block interaction is required by the perfectly repeatable force distance curves shown in Figures 4 and 5 (∼2.5 min to remove and reimmerse the coated slide). If water hydrogen-bonding interactions were even partly nonlabile, resulting in retained near-surface water, successive cycles would reflect different wetting behavior compared to cycle 1. Nonlabile behavior was noted above for the conventional PTMO PU, IPDI-BD(50)/PTMO. For the latter, nonlabile water, being bound more strongly results in changes in wetting behavior in successive cycles. Nonlabile behavior is characterized by a return to the original wetting behavior after drying. To gain information about longer term immersion, IPDI-BD(50)/PTMO was left overnight in water. The coating became white, indicating hydration. After 14 days, the mass increased 2.63%. Oven drying restored optical clarity and the original wetting behavior. In contrast, a representative co-telechelic polyurethane, PU-4 with the P(3FOx/BrOxs1:1) soft block, was immersed in water

Langmuir, Vol. 21, No. 23, 2005 10755

overnight and remained optically transparent. No mass gain could be detected. The differing long-term results for the base PU and PU-4 serve to emphasize the differing extents of interaction with water. The compositionally dependent wetting behavior for P(3FOx-BrOx) polyurethanes is not seen for the P(5FOx) series. All P(5FOx/BrOx) PUs have high θadv (108° ( 1°) and high ∆θ (73°). A labile wetting mechanism paralleling that for PU-4 must also occur for P(5FOx/BrOx) PUs. As yet, we have not been able to prepare a P(5FOx) polyurethane, but the lack of compositional dependence of wetting behavior of the P(5FOx/BrOx) PU series suggests the presence of BrOx segmers is less influential in determining wetting behavior with the longer semifluorinated side chain. Conclusions TM-AFM results on polyurethanes containing P(FOx/ BrOx) soft blocks are consistent with the presence of the soft block at the surface, but because of the high weight fraction of IPDI-BD, this phase is also prominent in the near-surface region. The wetting behavior of the IPDIBD/P(3FOx/BrOx) series is compositionally dependent. All co-telechelic PUs have higher θadv and lower θrec than the parent homo-telechelic PUs. PU-4, IPDI-BD(40)/ P(3FOx/BrOxs1:1), is unique with θadv (116°) and low θrec (32°). This result is correlated with higher FOx-BrOx dyad content, and a reversible H-bonding mechanism is proposed to explain the high ∆θ. In view of a previously determined order of phase separation [IPDI-BD(50)/ PTMO , PU-1, PU-2 < co-telechelic PUs], BrOx steric inhibition of H-bonding (Figure 8) may play a role in minimizing phase mixing for co-telechelic PUs as well as contributing to the unusual wetting behavior. Characterization of bulk and surface properties provided interesting insight into phase separation and surface wetting behavior seemingly connected by the common denominator of FOx-BrOx dyad H-bonding interactions in the soft block. However, the primary purpose for preparation and characterization of the co-telechelic PUs reported herein was to add to our palette of PSMs by providing a reactive functionality (-CH2Br). Extension of this research using a reaction-on-polymer approach to create new biocidal PSMs has recently been successful.60 Acknowledgment. The authors are grateful to the National Science Foundation for support (DMR-0207560). The authors thank Dr. Steven Aubuchon, TA Instruments, for thermal analysis technical advice and equipment. LA051245Y (60) Makal, U.; Wood, L.; Ohman, D.; Wynne, K. J. Biomaterials 2005, in press.