Langmuir 2007, 23, 11947-11950
11947
Thermoresponsive Characteristics of Fluoroalkyl End-Capped Co-oligomers in Aqueous Solutions and on the Poly(methyl methacrylate) Film Surface Hideo Sawada,*,† Keigo Takahashi,† Masaki Mugisawa,† Takahisa Oya,‡ and Shin-ichi Ogino‡ Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki UniVersity, Bunkyo-cho, Hirosaki 036-8561, Japan, and Kansai Paint Co., Ltd., Hiratsuka, Kanagawa 254-8562 Japan ReceiVed June 11, 2007. In Final Form: October 2, 2007 Fluoroalkyl end-capped N-(1,1-dimethyl-3-oxobutyl)acrylamide - acryloylmorpholine co-oligomers were prepared by the co-oligomerizations of fluoroalkanoyl peroxides with the corresponding monomers. These fluorinated cooligomers exhibited a lower critical solution temperature (LCST) characteristic in aqueous solutions. Of particular interest, a steep time dependence of contact angle values for dodecane was observed from 40 to 60 °C to decrease their values, effectively, on the modified PMMA [poly(methyl methacrylate)] film surface treated with fluorinated co-oligomer possessing the LCST: 36 °C (in water), although such a steep time dependence was not observed from 20 to 30 °C.
Introduction A variety of nonionic polysoaps undergo a thermally induced phase separation in their aqueous solutions when heated above a critical temperature (cloud point), and such polysoaps could be characterized by the lower critical solution temperature related to the hydrophilic-hydrophobic balance.1 Among these nonionic polysoaps, poly(N-isopropylacrylamide)[PNIPAM] is especially well known to exhibit a sharp phase transition in water at 32 °C because it possesses a good balance between hydrophilic and hydrophobic interaction in the polymer.2 In general, a thermally induced phase separation of PNIPAM can be observed above the LCST only in aqueous media because of the interaction between the hydrophilic and hydrophobic characteristics.2 In contrast, there have hitherto been no reports on the LCST behavior in organic media except for a few reports on the ionic liquid system3 by the use of a large variety of polymers, including PNIPAM and its analogues. Therefore, it is of particular interest to develop novel temperature-sensitive polymers that should exhibit the LCST characteristic in organic media. From this point of view, we have very recently found that fluoroalkyl end-capped 2-acrylamido-2-methylpropane sulfonic acid-adamantly acrylate co-oligomeric nanoparticles could exhibit the LCST characteristic in an organic medium (t-butyl alcohol) around 52 °C.4 In this * Corresponding author. E-mail:
[email protected]. Tel: +81172-39-3578. Fax: +81-172-39-3541. † Hirosaki University. ‡ Kansai Paint Co., Ltd. (1) (a) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379-6980. (b) Hirotsu, S.; Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1987, 87, 1392-1395. (c) Tanaka, T.; Fillmore, D.; Sun, S. J. Phys. ReV. Lett. 1980, 45, 1636-1639. (2) (a) Kuckling, D.; Adler, H.-J. P.; Arndt, K.-F.; Ling, L.; Habicher, W. D. Macromol. Chem. Phys. 2000, 201, 273-280. (b) Stoica, F.; Miller, A. F.; Alexander, C.; Saiani, A. Macromol. Symp. 2007, 251, 33-40. (c) Zhang, P.; Liu, Q.; Qing, A.; Shi, J.; Lu, M. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 3312-3320. (d) Duan Q.; Miura, Y.; Narumi, A.; Shen, X.; Sato, S.; Satoh, T.; Kakuchi, T. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 1117-1194. (e) Szczubialka, K.; Loska, R.; Nowakowska, M. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2784-2792. (f) Ju, X.-J.; Chu, L-Y.; Mi, P.; Song, H.; Lee, Y. M. Macromol. Rapid Commun. 2006, 27, 2072-2077. (g) Furyk, S.; Zhang, Y.; Ortiz-acosta, D.; Cremer, P. S.; Bergbreiter, D. E. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 1492-1501. (3) (a) Ueki, T.; Watanabe, M. Langmuir 2007, 23, 988-990. (b) Ueki, T.; Karino, T.; Kobayashi, Y.; Shibayama, M.; Watanabe, M. J. Phys. Chem. B 2007, 111, 4750-4754.
way, fluoroalkyl end-capped oligomers are attractive functional materials because they exhibit a variety of unique properties such as high solubility, surface-active properties, biological activities, and nanometer size-controlled self-assembled molecular aggregates including the above-mentioned LCST characteristic in organic media that cannot be achieved by the corresponding non-fluorinated and randomly fluoroalkylated ones.5 In particular, we can easily prepare surface-modified traditional organic polymers by the use of fluoroalkyl end-capped oligomers in which these fluorinated oligomers should be arranged regularly above the modified polymer surface to exhibit good oleophobicity imparted by fluorine.5 On the modified polymer film surface treated with these oligomers, it is strongly expected that oleophilic-oleophilic interaction between the oleophilic moieties in co-oligomers and oleophilic compounds such as dodecane should cause the flip-flop motion between the fluoroalkyl groups and oleophilic moieties in co-oligomers on the surface to exhibit the LCST-like characteristic in air through the contact angle measurements of dodecane. In this letter, we demonstrate the first LCST-like behavior on the modified PMMA [poly(methyl methacrylate)] film surface treated with fluoroalkyl end-capped N-(1,1-dimethyl-3-oxobutyl)acrylamide-acryloylmorpholine cooligomers, including their LCST characteristics in water.
Results and Discussion Fluoroalkyl end-capped N-(1,1-dimethyl-3-oxobutyl)acrylamide-acryloylmorpholine co-oligomers [RF-(DOBAA)x(ACMO)y-RF] were prepared by the reactions of fluoroalkanoyl peroxides with the corresponding monomers in 73-88% isolated yields. The reaction scheme and the molecular weights including co-oligomerization ratios are illustrated in Scheme 1. We have tested these RF-(DOBAA)x-(ACMO)y-RF co-oligomers in Scheme 1 for their solubility in a variety of solvents including water. These fluorinated co-oligomers were found to exhibit an amphiphilic characteristic, and these co-oligomers have good solubility not only in water but also in traditional organic solvents such as tetrahydrofuran (THF), 1,2-dichloro(4) Mugisawa, M.; Ohnishi, K.; Sawada, H. Langmuir 2007, 23, 5848-5851. (5) Sawada, H. Prog. Polym. Sci. 2007, 32, 509-533.
10.1021/la701712a CCC: $37.00 © 2007 American Chemical Society Published on Web 10/23/2007
11948 Langmuir, Vol. 23, No. 24, 2007 Scheme 1. Preparation of RF-(DOBAA)x-(ACMO)y-RF Co-oligomers
Letters Table 1. Size and LCSTs of Fluorinated Molecular Aggregates Formed by RF-(DOBAA)x-(ACMO)y-RF Co-oligomers and RF-(ACMO)n-RF Homo-oligomers in Aqueous Solutions as Determined by Dynamic Light Scattering Measurements from 20 to 60 °C size of fluorinated moleculer aggregates [nm] fluorinated oligomer
20 °C
30 °C
40 °C
50 °C
60 °C
LCST (°C)e
RF-(ACMO)n-RFa,b RF-(DOBAA)x-(ACMO)y-RF (x/y ) 11:89)a RF-(DOBAA)x-(ACMO)y-RF (x/y ) 14:86)a RF-(DOBAA)x-(ACMO)y-RF (x/y ) 30/70)a
11.5 10.9
11.3 11.3
12.9 11.7
11.3 11.6
11.8 12.0
>80 >70
12.0
11.2
11.2
11.1
11.7
67
10.8
10.9
11.0
12.0
118.8
58
RF-(ACMO)n-RFc,d RF-(DOBAA)x-(ACMO)y-RF (x/y ) 12:88)d
10.8 11.8
10.8 11.4
11.2 78.2
123.2 78.7
210.0 80.7
50 36
a RF ) CF(CF3)OCF2CF(CF3)OC3F7. b Mn ) 8280; see ref 6. c Mn ) 8310; see ref 6. d RF ) CF(CF3)OC3F7. e Defined by the temperature where the transmittance was 50%.
ethane, t-BuOH, i-PrOH, EtOH, MeOH, and dimethyl sulfoxide (DMSO). Fluorinated co-oligomers in Scheme 1 afforded welldispersed transparent colorless solutions in these solvents. We have measured the size of these co-oligomeric aggregates in these solvents by dynamic light scattering (DLS) measurements at 20 °C, and the results are shown in Figure 1. As shown in Figure 1, the sizes of RF-(DOBAA)x-(ACMO)yRF co-oligomeric aggregates were 10-600 nm, and the sizes of these fluorinated co-oligomeric aggregates were also very sensitive to the dielectric constants. The size of these aggregates could afford the increase in aggregate size in moderate solvent dielectric constants ( ) 18-33) such as i-PrOH, EtOH, and MeOH; however, higher or relatively lower dielectric constant solvents such as water and t-BuOH greatly decreased in size. In this way, our present RF-(DOBAA)x-(ACMO)y-RF cooligomers were demonstrated to be an interesting polymeric material for the preparation of nanometer-size controlled cooligomeric aggregates in a large variety of solvents. Interestingly,
the size of these fluorinated co-oligomeric aggregates is very sensitive to solvent changes. Thus, these fluorinated co-oligomeric aggregates are also expected to become sensitive to temperature changes. We measured the sizes of these fluorinated co-oligomeric aggregates by DLS measurements in water from 20 to 60 °C, and the results are shown in Table 1. As shown in Table 1, the size of some fluorinated oligomers was found to increase effectively with increasing temperature from 40 to 60 °C, indicating that fluorinated oligomeric aggregates could cause a thermally induced phase transition in water. We have found that aqueous solutions of fluorinated oligomeric aggregates showed a cloud point on heating. The lower critical solution temperatures (LCSTs) of aqueous solutions of 4 g/dm3 fluorinated co-oligomeric aggregates, of which concentration corresponds to the formation of self-assembled fluorinated aggregates, were measured, and the results are shown in Figure 2. As shown in Figure 2, a phase separation in each fluorinated co-oligomeric aggregates occurred around 50-80 °C, where the
Figure 1. Relationship between the size of fluorinated molecular aggregates formed by RF-(DOBAA)x-(ACMO)y-RF co-oligomers in a variety of solvents and the dielectric constant () of solvents at 20 °C: (a) RF ) CF(CF3)OCF2CF(CF3)OC3F7 and (b) determined by dynamic light scattering measurements.
Letters
Langmuir, Vol. 23, No. 24, 2007 11949 Table 2. Time Dependence of Contact Angle Values for Dodecane and Decrease Rates (k)a of the Contact Angle Values for Dodecane on the Modified PMMA Films Treated with RF-(DOBAA)x-(ACMO)y-RF Co-oligomerb (36 °C LCST (in Water); RF ) CF(CF3)OC3F7) from 20 to 60 °C contact angle (deg)
Figure 2. Temperature dependence of transmittance at 500 nm for aqueous solutions of 4 g/dm3 RF-(DOBAA)x-(ACMO)y-RF cooligomers (RF ) CF(CF3)OCF2CF(CF3)OC3F7).
Figure 3. Temperature dependence of transmittance at 500 nm for aqueous solutions of 4 g/dm3 RF-(DOBAA)x-(ACMO)y-RF cooligomers.
solubility of these co-oligomeric aggregates changed sharply. The LCSTs were found to decrease effectively with the increasing content of DOBAA segments in co-oligomers from 11 to 30%, indicating that the hydrophilic interaction between amphiphilic ACMO segments in oligomers and water should be essential for the preparation of transparent solutions below the LCSTs. In addition, we could observe the higher LCST (from 36 to 67 °C) with the increasing length of end-capped fluoroalkyl segments in the co-oligomers possessing similar co-oligomerizaton ratios (Figure 3). This finding suggests that hydrophobichydrophobic interaction between the DOBAA segments in cooligomers should decrease because of the presence of longer fluoroalkyl groups, which exhibit a stronger hydrophobic characteristic in aqueous solutions. More interestingly, DLS measurements indicate a dramatic increase in fluorinated oligomeric aggregate size around the LCSTs (58, 50, and 36 °C; see Table 1) from 10 nm to 78-123 nm. This finding suggests that the oleophilic-oleophilic interactions between the oleophilic segments in oligomeric aggregates in aqueous solutions should provide the architecture of new selfassemblies derived from fluorinated oligomeric aggregates. Previously, we reported that a variety of fluoroalkyl endcapped oligomers were applicable of traditional organic polymers such as poly(methyl methacrylate) (PMMA) whose surfaces were modified to exhibit a good oleophobic characteristic.7 In addition, it has been already verified that these fluorinated oligomers could be arranged regularly on the modified PMMA film surface by (6) Fluoroalkyl-end-capped acryloylmorpholine homo-oligomers were prepared according to our previously reported method: Sawada, H.; Kawase, T.; Ikematsu, Ishii, Y.; Oue, M.; Hayakawa, Y. Chem. Commun. 1996, 179-180. (7) Sawada, H.; Yanagida, K.; Inaba, Y.; Sugiya, M.; Kawase, T.; Tomita, T. Eur. Polym. J. 2001, 37, 1433-1439.
dodecane
temp (°C)
0 min
5 min
10 min
15 min
20 min
25 min
30 min
20 30
42 40
42 40
42 39
41 39
41 39
41 39
41 39
40 50 60
39 36 36
38 35 31
37 31 25
36 27 21
35 25 21c
34 23 21c
33 21 21c
k × 102 (min-1)
0.73 1.58 4.17
a Decrease rate: ln(CA0/CAt)t) ) kt; CA ) contact angle value of dodecane; CAo indicates the initial value (0 min). b The concentration of co-oligomer based on PMMA is 1% (m/m). c These data were not included in the decrease rates.
the use of the X-ray photoelectron spectroscopy (XPS) technique.8 Thus, our present fluoroalkyl end-capped DOBAA-ACMO cooligomer possessing an LCST of 36 °C (in water) should be arranged regularly above the modified PMMA film surface. In fact, we succeeded in preparing the modified PMMA film treated with this fluorinated co-oligomer (1% (m/m) based on PMMA; film thickness, 203 µm). The contact angle value for dodecane (42°) on the modified PMMA film at 20 °C exhibits good oleophobicity imparted by fluorine on the surface, although the contact angle value for dodecane on the reverse side is 0° in this film. A steep time dependence of contact angle values for dodecane was not observed in this co-oligomer over 30 min at this temperature (Table 2). It is suggested that oleophilic-oleophilic interaction between the DOBAA segments in co-oligomer and dodecane should more easily cause the flip-flop motion between the fluoroalkyl groups and oleophilic moieties in co-oligomers upon raising the temperature at the interface between the modified PMMA film and the air. Thus, it would become possible to observe the decrease in the contact angle values for dodecane through the oleophilicoleophilic interaction between dodecane and oleophilic DOBAA segments in co-oligomer exhibiting an LCST-like characteristic in air upon raising the temperature at the film surface. We have measured the contact angle values for dodecane on the modified PMMA film surface treated with fluorinated cooligomer possessing an LCST of 36 °C (in water) from 30 to 60 °C, and the results are shown in Table 2. Interestingly, a steep time dependence of contact angle values for dodecane was observed in this co-oligomer from 40 to 60 °C over 30 min, although we could not observe such a steep time dependence of contact angles for dodecane at 30 °C as in Table 2. More interestingly, the decreasing ratios of the contact angle values for dodecane were found to increase effectively upon raising the temperature from 40 to 60 °C. We tried to determine first-order rate constants (k) for the decrease in contact angle values for dodecane at these temperatures, and the results are shown in Table 2. The decrease ratio was found to follow the first-order equation to increase from k ) 0.73 × 10-2 to 4.17 × 10-2 (min-1) upon increasing the temperatures from 40 to 60 °C, respectively, as shown in Table 2. This finding suggests that the oleophilicoleophilic interaction between the oleophilic moieties in the cooligomer and dodecane should cause the flip-flop motion between the fluoroalkyl groups and that the oleophilic moieties in the (8) Sawada, H.; Sasaki, A.; Sasazawa, K.; Kawase, T.; Ueno, K.; Hamazaki, K. Colloid Polym. Sci. 2005, 283, 583-586.
11950 Langmuir, Vol. 23, No. 24, 2007
co-oligomer on the modified film surface should exhibit the LCST-like characteristic toward air. As shown in Table 1 and Figure 3, this fluorinated co-oligomer could have an especially strong hydrophobic-hydrophobic interaction between the hydrophobic DOBAA segments in co-oligomers in aqueous solutions above the LCST (36 °C). Therefore, at the interface between the modified PMMA film surface and the air, a similar oleophilic-oleophilic interaction between the hydrophobic DOBAA segments in the co-oligomer and dodecane should become stronger in air with the increase in temperature from 30 to 40 °C, as should the LCST characteristic in the aqueous system. In conclusion, we have prepared a variety of fluoroalkyl endcapped DOBAA-ACMO co-oligomers by the reactions of fluoroalkanoyl peroxides with the corresponding monomers. These fluorinated co-oligomers were found to exhibit the LCST characteristic in aqueous solutions, and the LCSTs were found to decrease upon increasing the co-oligomerization ratios of hydrophobic DOBAA segments in co-oligomers. In addition, the shorter fluoroalkyl end-capped co-oligomers exhibited the lower LCST. Interestingly, the modified PMMA film surface treated with fluorinated co-oligomer was found to exhibit the LCST-like characteristic in air. The oleophilic-oleophilic interaction between the oleophilic DOBAA segments in the cooligomer and dodecane could increase more effectively upon raising the temperature at the film surface to change the surface activity from oleophobic to oleophilic character on the surface through the flip-flop motions between the end-capped fluoroalkyl segments and the oleophilic segments in the co-oligomer. The increase in such oleophilicity upon raising the temperature at the film surface would in part be correlated to the decrease in the LCSTs in water upon increasing the content of oleophilic DOBAA moieties in co-oligomers. We believe that this interesting LCSTlike characteristic in air on the modified polymer surface is the first example. Further studies are actively in progress. Experimental Section NMR spectra and Fourier-transform infrared (FTIR) spectra were measured using a JEOL JNM-400 (400 MHz) FTNMR system (Tokyo, Japan) and a Shimadzu FTIR-8400 FTIR spectrophotometer (Kyoto, Japan), respectively. Dynamic light-scattering (DLS) measurements were made using an Otsuka Electronics DLS-7000 HL (Tokyo, Japan). Ultraviolet-visible (UV-vis) spectra were measured using Shimadzu UV-1600 UV-vis spectrophotometer (Kyoto, Japan). A variety of fluoroalkyl end-capped DOBAA-ACMO co-oligomers in Scheme 1 were prepared by the reactions of fluoroalkanoyl peroxides with DOBAA and ACMO in AK-225 (1:1 mixed solvents of 1,1-dichloro-2,2,3,3,3-pentafluoropropane/1,3dichloro-1,2,2,3,3-pentafluoropropane) according to our previously
Letters reported method.9 The fluorine contents (%) and the end-capped RF contents (%) in co-oligomers illustrated in Scheme 1 are summarized as follows.
run 1 run 2 run 3 run 4
fluorine content (%)
end-capped RF content (%)
5.0 4.9 3.9 2.3
14.0 13.6 11.0 6.3
The fluorinated co-oligomers thus obtained exhibited the following FTIR spectral characteristics (ν/cm-1): run 1: 1717, 1636 (CdO), 1304 (CF3), 1242 (CF2); run 2: 1717, 1636 (CdO), 1304 (CF3), 1242 (CF2); run 3: 1717, 1636 (CdO), 1303 (CF3), 1242 (CF2); run 4: 1717, 1636 (CdO), 1305 (CF3), 1239 (CF2). A significant decrease in the surface tension of water was observed in fluorinated cooligomers in Scheme 1, and these co-oligomers exhibited a clear break point resembling a cmc in water of around 0.1-2 g/dm3. The LCSTs of RF-(DOBAA)x-(ACMO)y-RF co-oligomers in water were measured by a turbidity method. A UV-vis spectrophotometer equipped with a temperature controller was used to trace the phase transition by monitoring the transmittance of the light at a wavelength of 500 nm as a function of temperature. The concentration of the co-oligomer solutions used was 4 g/dm3, and the temperature of the aqueous solutions was raised (heating ratio, 1 °C/min) from 20 to 80 °C. The LCST was defined as the temperature where the transmittance was 50%. The modified PMMA film was prepared by casting the 1,2dichloroethane solution (10 mL) of PMMA (0.99 g) and the 1,2dichloroethane solution (10 mL) containing RF-(DOBAA)x-(ACMO)yRF (10 mg) on glass plates. The solvent was evaporated at room temperature, and the film that formed was peeled off and dried at 50 °C for 24 h under vacuum to afford the modified PMMA film (film thickness, 203 µm). The contact angle values for dodecane were measured with a Drop Master 300 (Kyowa Interface Science Co.) by depositing a drop of dodecane (2 µL) on the modified PMMA film (10 mm × 10 mm; film thickness, 203 µm) surface, which was left in the box (55 mm × 98 mm × 26 mm) and equipped with a temperature controller after the preincubation of the film left in this box at each temperature (20-60 °C) for 1 h.
Acknowledgment. Thanks are due to Kohjin Co., Ltd., Tokyo, Japan, and Kyowa Hakko Kogyou Co., Ltd., Tokyo, Japan, for supplying the ACMO and DOBAA, respectively. LA701712A (9) Sawada, H.; Yoshino, Y.; Ikematsu, Y.; Kawae, T. Eur. Polym. J. 2000, 36, 231-240.
