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Soluble-Insoluble-Soluble Transitions of Aqueous Poly(N-vinylacetamide-co-acrylic acid) Solutions Takeshi Mori,*,†,| Masashi Nakashima,† Yasuhisa Fukuda,† Keiji Minagawa,† Masami Tanaka,‡ and Yasushi Maeda§ Department of Chemical Science and Technology, Faculty of Engineering, The UniVersity of Tokushima, 2-1 Minamijosanjima, Tokushima 770-8506, Japan, Faculty of Pharmaceutical Science, Tokushima Bunri UniVersity, Yamashiro, Tokushima 770-8514, Japan, and Department of Applied Chemistry and Biotechnology, Fukui UniVersity, Fukui 910-8507, Japan ReceiVed January 22, 2006. In Final Form: February 21, 2006 Several poly(N-vinylacetamide-co-acrylic acid)s with various copolymer compositions have been synthesized, and their unique phase-transition behavior in aqueous salt (Na2SO4 or NaCl) solutions was investigated. Copolymers containing more than 51 mol % N-vinylacetamide (NVA) show reentrant soluble-insoluble-soluble transitions with increasing temperature. The soluble-insoluble transition temperature (Tp1) increased linearly with increasing NVA content, whereas the insoluble-soluble transition temperature (Tp2) was almost constant irrespective of the NVA content. Potentiometric titration of the copolymer solutions suggested that the acrylic acid (AA) carboxyl groups form hydrogen bonds with the NVA amide groups even under soluble conditions. Dehydration of the NVA amides and their consequent hydrogen bonding with the AA carboxyl groups during the soluble-insoluble transition process was indicated by FTIR measurements. Addition of salt (Na2SO4 or NaCl) to the aqueous media reduces the solvent quality and enhances the intra- and interchain interactions of the copolymers. Thus, Tp1 was observed to decrease and Tp2 was observed to increase with increasing salt concentration. However, the addition of urea to the media reverses the concentration dependence of Tp1 and Tp2 by disturbing the intra- and interchain interactions of the copolymers.
Introduction Temperature-responsive polymers that show an solubleinsoluble (S-I) transition when heated in aqueous media have received much attention in the fields of tissue engineering,1,2 drug delivery,3-5 separation technology,6,7 and actuators.8,9 The phase diagrams of the S-I transition within the aqueous polymer solutions have a minimum temperature referred to as the lower critical solution temperature (LCST). Several polymers have been shown to exhibit an S-I transition in which their monomeric units commonly have an appropriate hydrophobic-hydrophilic balance.10-14 The hydration around the hydrophobic groups and hydrogen-bonding hydration around hydrophilic or polar groups * To whom correspondence should be addressed. E-mail: moritcm@ mbox.nc.kyushu-u.ac.jp. Tel: +81-92-802-2849. † The University of Tokushima. ‡ Tokushima Bunri University. § Fukui University. | Present address: Department of Applied Chemistry, Kyushu University, 744 Moto-oka Nishi-ku, Fukuoka 819-0395, Japan. (1) Akiyama, Y.; Kikuchi, A.; Yamato, M.; Okano, T. Langmuir 2004, 20, 5506-5511. (2) Ide, T.; Nishida, K.; Yamato, M.; Sumide, T.; Utsumi, M.; Nozaki, T.; Kikuchi, A.; Okano, T.; Tano, Y. Biomaterials 2006, 27, 607-614. (3) Katayama, Y.; Sonoda, T.; Maeda, M. Macromolecules 2001, 34, 85698573. (4) Wei, H.; Zhang, X.-Z.; Zhou, Y.; Cheng, S.-X.; Zhuo, R.-X. Biomaterials 2006, 27, 2028-2034. (5) Huang, X.; Lowe, T. L. Biomacromolecules 2005, 6, 2131-2139. (6) Gonzalez, S. O.; Furyk, S.; Li, C.; Tichy, S. E.; Bergbreiter, D. E.; Simanek, E. E. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 6309-6317. (7) Mori, T.; Umeno, D.; Maeda, M. Biotechnol. Bioeng. 2001, 72, 261-268. (8) Osada, Y.; Kishi, R.; Hasebe, M. J. Polym. Sci., Part C: Polym. Lett. 1987, 25, 481-485. (9) Liu, Z.; Calvert, P. AdV. Mater. 2000, 12, 288-291. (10) Ito, S. Kobunshi Ronbunshu 1989, 46, 437-443. (11) Inomata, H.; Goto, S.; Saito, S. Macromolecules 1990, 23, 4887-4888. (12) Horne, R. A.; Almeida, J. P.; Day, A. F.; Yu, N. T. J. Colloid Interface. Sci. 1971, 35, 77-84. (13) Suwa, K.; Morishita, K.; Kishida, A.; Akashi, M. J. Polym. Sci., Polym. Chem. 1997, 35, 3087-3094.
of the polymers break upon heating above the phase-transition temperature so that the polymers separate out from aqueous solution. Compared with polymers having S-I transitions in aqueous media, polymers that show inverse transition behavior with increasing temperature, and therefore an insoluble-soluble (IS) transition, are relatively uncommon. Poly(acrylic acid) (PAA),15,16 poly(sulfobetaine),17,18 and poly(6-(acryloxyloxymethyl)uracil)19 are all reported to exhibit an I-S transition temperature. These polymers commonly have a pair of interactive sites that cause the polymers to be insoluble at lower temperatures because of intra- and interchain interactions such as hydrogen bonding and electrostatic attraction. These interactions break at higher temperatures because of intensified molecular motion within the polymer chains, resulting in a hydrated polymer. In contrast to polymers exhibiting an S-I transition, PAA16 and poly(sulfobetaine)17 both have an upper critical solution temperature (UCST). Polymers that exhibit both an LCST and UCST in aqueous media,20-22 such as the nicotine-water system,23 are quite rare. In this case, these polymers can change their water solubility in an soluble-insoluble-soluble (S-I-S) system with increasing solution temperature. This unique solution property will extend (14) Mori, T.; Hamada, M.; Kobayashi, T.; Okamura, H.; Minagawa, K.; Masuda, S.; Tanaka, M. J. Polym. Sci., Part A.: Polym. Chem. 2005, 43, 49424952. (15) Ikegami, A.; Imai, N. J. Polym. Sci. 1962, 56, 133-152. (16) Buscall, R.; Corner, T. Eur. Polym. J. 1982, 18, 967-974. (17) Huglin, M. B.; Radwan, M. A. Polym. Int. 1991, 26, 97-104. (18) Schulz, D. N.; Peiffer, D. G.; Agarwal, P. K.; Larabee, J.; Kaladas, J. J.; Soni, L.; Handwerker, B.; Garner, R. T. Polymer 1986, 27, 1734-1742. (19) Aoki, T.; Nakamura, K.; Sanui, K.; Kikuchi, A.; Okano, T.; Sakurai, Y.; Ogata, N. Polym. J. 1999, 31, 1185-1188. (20) Saeki, S.; Kuwahara, N.; Nakata, M.; Kaneko, M. Polymer 1976, 17, 685-689. (21) Eisenberg, H.; Felsenfeld, G. J. Mol. Biol. 1967, 30, 17-37. (22) Bokias, G.; Staikos, G.; Ilipoulos, I. Polymer 2000, 41, 7399-7405. (23) Barker, J. A.; Fock, W. Discuss. Faraday Soc. 1953, 15, 188-195.
10.1021/la060212v CCC: $33.50 © 2006 American Chemical Society Published on Web 03/24/2006
Soluble-Insoluble-Soluble Transitions Chart 1. Chemical Structure of Poly(N-vinylacetamide-co-acrylic acid)
the application fields of temperature-responsive polymers because it will enable these polymers to respond to temperatures within a limited range. Therefore, it is not only scientifically but also technologically important to find a universal molecular design for polymers with an S-I-S transition. Poly(ethylene oxide) (PEO) was reported to show an S-I-S transition with increasing temperature.20 Although the importance of hydrogen bonding between water and PEO has been reported,20 the mechanism by which the solution behavior occurs at the molecular level is not clear. Moreover, because an I-S transition of PEO appears only at high temperatures (greater than 200 °C) and high pressures, it is difficult to utilize the unique properties of PEO in real applications. However, poly(riboadenylic acid)21 was reported to show S-I-S behavior at atmospheric pressure, where the intrachain stacking interaction among its nucleic acid bases is related to the unique solution behavior. However, because of the complicated molecular structure of poly(riboadenylic acid), this should be recognized as an exceptional example. Recently, Bokias et al. found that an aqueous NaCl solution of a simple copolymer of poly(N-isopropylacrylamide-co-acrylic acid) [poly(NIPAMco-AA)] with a specific copolymer composition shows S-I-S transition behavior at atmospheric pressure.22 The authors indicated that the following two properties will be closely related to this unique solution behavior: (1) intra- and interchain hydrogen bonding between the NIPAM amides and the AA carboxyl groups and (2) the moderate hydrophobicity of the polymer. However, details of the mechanism of the unique phase-transition behavior remain to be explored. We have studied the phase-transition behavior of several copolymers of amide-containing monomers and methyl acrylate (or methyl methacrylate).24-26 Although various copolymer compositions and solution conditions (concentrations of copolymers and additives) have been investigated, all of the copolymer solutions show LCST-type S-I transition behavior rather than S-I-S transition behavior. If the molecular design of the copolymer studied by Bokias et al. has generality, our copolymers will also show S-I-S transition behavior if AA is used instead of methyl acrylate. Therefore, we synthesized random copolymers between AA and N-vinylacetamide (NVA) (Chart 1) and examined their solution behavior with varying copolymer composition and solution conditions. Here, we report the results of these experiments and highlight a detailed study on the mechanism involving the unique solution properties of the copolymers, with a particular focus on the role of intramolecular hydrogen bonding. Experimental Section Materials. A 35 wt % aqueous solution of PAA (Mw: ca. 250 000) was purchased from Aldrich. N-Vinylacetamide (Aldrich) was recrystallized from benzene/cyclohexane. Acrylic acid (Kanto Kagaku) was purified by distillation. 2,2′-Azobisisobutyronitrile (AIBN) (Kanto Kagaku) was recrystalized from methanol. Anhydrous (24) Okamura, H.; Morihara, Y.; Masuda, S.; Minagawa, K.; Mori, T.; Tanaka, M. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 1945-1951. (25) Okamura, H.; Masuda, S.; Minagawa, K.; Mori, T.; Tanaka, M. Eur. Polym. J. 2002, 38, 639-644. (26) Mori, T.; Fukuda, Y.; Okamura, H.; Minagawa, K.; Masuda, S.; Tanaka, M. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 2651-2658.
Langmuir, Vol. 22, No. 9, 2006 4337 N,N-dimethylformamide (DMF) (Wako) and other reagents were used without purification. Syntheses of Copolymers. The monomers (total concentration: 3.0 M) and an AIBN initiator (30 mM) were dissolved in DMF and placed into a round-bottomed flask. The mixture was cooled in a cooling bath and degassed. After repeating the cooling-degassing cycle three times, the mixture was sealed in the flask under high vacuum. Polymerization was conducted for 4 h at 60 °C in a thermostatic bath. The resulting mixture was diluted with methanol, and the solution was poured into a large excess of diethyl ether. The reprecipitation procedure was repeated, and the obtained copolymer was dried under reduced pressure. The copolymer composition was determined by 1H NMR. Measurement of the Phase-Transition Temperature. The copolymer was dissolved in deionized water to obtain a stock solution (3.0 × 102 g/L). The measurement solutions were prepared from the stock solution, salt (NaCl and Na2SO4) solutions, and NaOH (or HCl) solution. The transmittance curve of each solution was recorded at 500 nm using a spectrophotometer (U-3210, Hitachi), equipped with a temperature controller (SPR-10, Shimadzu), operating with a heating and cooling rate of 1.0 °C/min. Molecular Weight Determination. The molecular weights (Mw) of the polymers were determined by static light scattering using a DLS-7000 instrument (Otsuka Electronics) equipped with an Ar ion laser (488 nm, 75 mW) light source at 25 °C. The measurement solutions were prepared by diluting the stock solutions with deionized water to a concentration of 0.50 g/L. The solutions were filtered through a 0.22 µm Milex GV filter (Millipore). The dn/dC values of the copolymers were measured using a DRM-1021 instrument (Otsuka Electronics) at 488 nm. Titration. A potentiometric titration was performed using an automatic titrator (AUT-501, TOA DKK). The polymer was dissolved in a 0.1 N KCl aqueous solution to a final concentration of 0.01 N (in AA unit concentration). Titrations were run in a cleaned thermostated (25 °C) beaker fitted with a stirrer, a pH electrode, and a nitrogen purge line. A 0.01 N NaOH solution containing 0.1 N KCl was used as a titrant. The pKa values of the polymers were determined from Henderson-Hasselbach plots.27 FTIR Measurements. Details of the method for conducting FTIR measurements have been described in a previous paper.28 The copolymer was dissolved in D2O (containing NaCl) to a concentration of 2.0 × 102 g/L, and the solution pH adjusted using DCl. The copolymer solution (10 µL) was placed between two CaF2 windows at a path length of 10 µm. The IR cell was attached to a metal cell holder, and the temperature was controlled using a circulating water bath. The background spectrum of the first measurement cycle was obtained with a sample solution equilibrated at the starting temperature (28 °C). IR spectra were then collected continuously at a certain temperature while heating the IR cell at a rate of ca. 1 °C/min. Other Measurements. Optical images of the polymer solutions at temperatures above and below the phase-transition temperature were recorded using a Nikon E600 microscope equipped with a Nikon Coolpix 995 digital camera. 1H NMR analysis was conducted using a JEOL GX400 spectrometer.
Results and Discussion S-I-S Transition of Copolymers. Copolymers with various compositions were synthesized by radical copolymerization, and their molecular parameters are summarized in Table 1. All of the synthesized copolymers failed to show a phase transition in aqueous media in the absence of salt (at any pH). In the presence of salt, however, the copolymers containing more than 51 mol % NVA showed a reentrant S-I-S transition with increasing temperature under acidic conditions. Figure 1 shows transmittance curves corresponding to the heating and cooling processes (27) Katchalsky, A.; Spitnik, P. J. Polym. Sci. 1947, 2, 432-446. (28) Maeda, Y.; Nakamura, T.; Ikeda, I. Macromolecules 2001, 34, 13911399.
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Figure 2. Optical microscopy image of the phase-separated NA51 solution at ca. 50 °C (polymer concentration ) 10 g/L, pH 2.6, [Na2SO4] ) 0.50 M). Figure 1. Transmittance curves for the heating and cooling processes of NA51 and the determination of transition temperatures (heating/ cooling rate ) 1.0 °C/min, polymer concentration ) 10 g/L, pH 2.6, [Na2SO4] ) 0.50 M). Table 1. Characterization of Copolymers sample
NVA content (mol %)
dn/dC (mL/g)
Mw
NA39 NA51 NA56 NA62 NA72 NA83 NA92 PNVA
39 51 56 62 72 83 92 100
0.153 0.154 0.154 0.157 0.158 0.163 0.165 0.168
3.4 × 104 1.6 × 104 3.2 × 104 2.2 × 104 1.5 × 104 4.3 × 104 5.0 × 104 8.8 × 104
involving NA51 under the following solution conditions: polymer concentration ) 10 g/L, pH 2.6, and [Na2SO4] ) 0.50 M. The S-I (Tp1) and I-S (Tp2) transition temperatures were determined from the heating and cooling process curves, respectively, as indicated in Figure 1. These temperatures were defined as the temperatures at which transmittance starts to decrease. In the present case, Tp1 and Tp2 were determined to be 35 and 90 °C, respectively. All of the copolymers except for NA39 showed an S-I-S transition under the same solution conditions as shown in Figure 1. Although NA39, which has the smallest NVA content of the copolymers studied, also showed an S-I transition at 7.5 °C under the same solution conditions, only a partial solubilization was observed at the I-S transition (ca. 70 °C), i.e., the transmittance of the solution recovered was 40% (data not shown). The S-I-S transition behavior was also observed for these copolymers when NaCl was used instead of Na2SO4. For example, when the NaCl concentration is 1.35 M (NA51 concentration ) 10 g/L, pH 2.6), Tp1 and Tp2 are 58 and 91 °C, respectively. The phase-separated state of NA51 was observed using optical microscopy (Figure 2). The liquid coacervate droplets were observed at ca. 50 °C. These droplets coalesced into the bulk layer within 1 h. Thus, the phase separation of the copolymer is not the quasi-liquid-solid type observed in polyNIPAM but the liquid-liquid separation type observed in many thermosensitive copolymers. Because the latter copolymers contain strongly hydrophilic groups, these remain hydrated even after dehydration of the hydrophobic groups.29-31 PNVA and PAA are soluble over the whole temperature range (0-100 °C) under the same solution conditions as shown in (29) Yin, X.; Sto¨ver, H. D. H. Macromolecules 2005, 38, 2109-2115. (30) Miyazaki, H.; Kataoka, K. Polymer 1996, 37, 681-685. (31) Yamamoto, K.; Serizawa, T.; Akashi, M. Macromol. Chem. Phys. 2003, 204, 1027-1033.
Figure 3. Transmittance curves for the heating and cooling processes involving the 1/1 PAA/PNVA (monomeric unit ratio) mixture (heating/cooling rate ) 1.0 °C/min, total polymer concentration ) 10 g/L, pH 2.6, [Na2SO4] ) 0.50 M).
Figure 1 (polymer concentration ) 10 g/L, pH 2.6, [Na2SO4] ) 0.50 M). However, a 1/1 mixture of PNVA and PAA (monomeric unit mixing ratio) exhibited an I-S transition from 80-100 °C (Figure 3). This phenomenon is probably due to the hydrogenbonded complex formation between the amide and carboxylic acid groups of the two polymers as well as the dissociation of the resulting complex at high temperature because PAA is known to form hydrogen-bonded complexes with proton-accepting polymers such as PEO,32 polyacrylamide,33,34 poly(N-alkylacrylamide)s,35,36 and poly(N,N-dimethylacrylamide).37 Because the S-I-S transition was not observed in the homopolymer mixed system (PNVA/PAA), it is concluded that the copolymerization of these two monomers is essential for the unique phase-transition behavior. Figure 4 shows the polymer concentration dependence of both transition temperatures of NA51. These transition temperatures show opposite polymer concentration dependences. Here, Tp1 (32) Maltesh, C.; Somasundaran, P.; Pradip; Kulkarni, R. A.; Gundiah, S. Macromolecules 1991, 24, 5775-5778. (33) Garces, F. O.; Sivadasan, K.; Somasundaran, P. Turro, N. J. Macromolecules 1994, 27, 272-278. (34) Ilmain, F.; Tanaka, T.; Kokufuta, E. Nature 1991, 349, 400-401. (35) Chen, G.; Hoffman, A. S. Macromol. Rapid Commun. 1995, 16, 175182. (36) Kokufuta, E.; Tanaka, T.; Ito, S.; Hirasa, O, Fujishige, S. Yamauchi, A. Phase Trans. 1993, 44, 217-225. (37) Aoki, T.; Kawashima, M.; Katono, H.; Sanui, K.; Ogata, N.; Okano, T.; Sakurai, Y. Macromolecules 1994, 27, 947-952.
Soluble-Insoluble-Soluble Transitions
Figure 4. Polymer concentration dependence on the transition temperatures of NA51 (pH 2.6, [Na2SO4] ) 0.50 M).
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Figure 6. pH dependence of transition temperatures for NA51 (squares) and NA62 (circles). Tp1 and Tp2 are closed and open symbols, respectively (polymer concentration ) 10 g/L, [Na2SO4] ) 0.50 M). Table 2. Results of Potentiometric Titration of Copolymers
Figure 5. NVA content dependence of transition temperatures in copolymers (polymer concentration ) 10 g/L, pH 2.6, [Na2SO4] ) 0.53 M).
was observed to decrease with increasing polymer concentration, becoming almost constant above 10 g/L, whereas Tp2 increased with increasing polymer concentration and exceeded 100 °C above 20 g/L. Both LCST and UCST, which are a minimum and a maximum temperature in a phase diagram, respectively, were not observed in the examined concentration range. Figure 5 shows the dependence of Tp1 and Tp2 on the NVA content of the copolymers, ranging from 39 to 92 mol %. Here, Tp2 was practically independent of the NVA content, whereas Tp1 increased with increasing NVA content. As for the S-I transition, the increase in the transition temperature is also reported to occur for poly(NIPAM-co-AA) with increasing NIPAM content (50-90 mol %) under acidic conditions.38 In copolymer gels of N-n-propylacrylamide (NNPAM) and AA, the increase in the volume phase-transition temperature is also observed with increasing NNPAM content (67.7-100 mol %).36 These transition temperature dependences on the content of the amide-containing monomers are probably caused by a similar mechanism. It is surprising that NA92, which has the smallest AA content, exhibits both Tp1 and Tp2, whereas PNVA exhibits neither. This fact also demonstrates the importance of copolymerization for the unique transition behavior. (38) Yoo, M. K.; Sung, Y. K.; Lee, Y. M.; Cho, C. S. Polymer 1998, 39, 3703-3708.
sample
copolymer conc (g/L)
initial pH
Rinit × 102
pKa
PAA NA62 NA92
0.722 2.11 10.5
3.44 3.61 3.72
3.63 2.45 1.91
5.59 5.42 5.36
Effect of pH on Phase Transition. Figure 6 shows the pH dependences of the Tp1 and Tp2 temperatures for NA51 and NA62 in which the two plots show a similar trend. Both Tp1 and Tp2 were almost constant below pH ∼2.2, whereas above this pH Tp1 showed a slight increase and Tp2 showed a steep decrease with increasing pH. The phase-transition behavior disappeared above pH 3. The pH-dependent changes in the transition temperatures indicate that the protonation of the AA carboxyl group is necessary to account for the unique phase-transition behavior of the copolymer. Thus, the hydrogen bonding between the NVA amide and the AA carboxyl group will be a key factor in determining the phase-transition behavior. We previously reported on the phase-transition behavior of poly(NVA-co-methyl acrylate)s with various copolymer compositions.26 All of the copolymers studied showed only LCST-type S-I transition behavior, providing evidence that the carboxylic acid protons in the poly(NVA-coAA)s play a crucial role in their unique phase-transition behavior. The pKa values of the copolymers and PAA were determined by potentiometric titration and are summarized in Table 2. All of these polymers were soluble during the titration. For the titration measurements, the polymer concentration was adjusted to 0.01 N in AA unit concentration. The “initial pH” in Table 2 corresponds to the pH of the polymer solutions obtained by dissolving the polymers in a KCl-containing aqueous solution. The pKa of the polymers was found to decrease with increasing NVA content. This tendency can be explained by the so-called “polyelectrolyte effect”. With increasing AA content, the distance between the adjacent AA units in the copolymers becomes shorter such that the removal of protons from the AA units becomes more difficult because of the strong propensity for proton capture by the additional carboxylate groups and the intensified electrostatic repulsion among the dissociated AA units.39 This same polymer effect on the pKa value was also observed in aqueous solutions of poly(N-vinyl-2-pyrrolidone-co-AA).39 Despite the higher dissociation ability of copolymers with higher NVA content, the initial degree of dissociation (Rinit), calculated (39) Dima, M.; Scondac, I.; Roman, A. ReV. Roum. Chim. 1966, 11, 965-973.
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Figure 7. (a) IR and (b) differential IR spectra of an NA56 solution at various temperatures and (c) the temperature dependence of ∆∆A for the amide II band (polymer concentration ) 2.0 × 102 g/L, pH 2.6, [Na2SO4] ) 0.50 M). The arrow in c indicates the Tp1 values of the NA56 solution under the measuring conditions.
from the initial pH, showed opposite results; Rinit decreased with increasing NVA content. This means that the dissociation of the AA unit is more suppressed in copolymers with higher NVA content under the initial conditions at which almost all of the AA units are protonated. The suppression of this dissociation is probably brought about by hydrogen bond formation between the AA carboxyl group and the NVA amide because hydrogenbonded carboxylic acids should be difficult to dissociate. Suppressed dissociation of the carboxyl group by hydrogen bonding was also observed in poly(NIPAM-co-AA),22 and more commonly in polymer complexes comprising polyacids and proton-accepting polymers.40 FT-IR Observation of the Phase Transition. FTIR measurements were performed on the copolymers in an attempt to prove the existence of hydrogen bonding between the NVA and AA units. Figure 7a shows the temperature-dependent changes in the FTIR spectra of NA56. The assignment of each band is as follows: carboxyl group CdO stretch (1700 cm-1), amide I (1625 cm-1), amide II (1480 cm-1), and C-H deformation (1445 cm-1). D2O was used as a solvent instead of H2O to prevent the O-H bending band of H2O (∼1640 cm-1) from overlapping with the amide I band of the copolymer. The amide I band mainly results from the CdO stretching vibration, whereas the amide II band is a combination of N-D bending and C-N stretching
vibrations.41 The CdO stretching vibrations of the carboxylic acid ν(COOD) and carboxylate ν(COO-) of the AA unit are reported to appear at 1705 and 1560 cm-1, respectively.42 Because the latter band was not observed in the FTIR spectra, all of the carboxyl groups were considered to be protonated under these solution conditions. The intensity of the three bandssν(COOD), amide I, and IIsdecreased with increasing temperature. No shifting was observed in the former two bands during the temperature change, whereas the amide II band showed a red shift with increasing temperature. The differential IR (∆A) spectra of the amide II band, obtained by subtraction of the IR spectrum at 29.4 °C (below Tp1) from those spectra above this temperature, are shown in Figure 7b. The technique of IR difference spectroscopy is a convenient method for visualizing structural changes in polymers. The IR difference peaks observed at 1485 cm-1 (negative) and 1458 cm-1 (positive) are due to a red shift in the amide II band. To show the progress of the phase transition, the difference between ∆A at the positive and negative peaks (∆∆A) is plotted against temperature (Figure 7c). The ∆∆A value was almost constant below Tp1 (34 °C) but was observed to increase above this temperature. This indicates that the hydration state of the NVA amide changed at Tp1. A similar red shift in the amide II band is observed for the S-I transition in aqueous poly(N-alkylacrylamide) solutions and is attributed to the
(40) Tsuchida, E.; Abe, K. Interactions between Macromolecules in Solution and Intermacromolecular Complexes; Advances in Polymer Science; SpringerVerlag: Berlin, 1982; Vol. 45.
(41) Susi, H. Methods Enzymol. 1972, 26, 445-472. (42) Maeda, Y.; Yamamoto, H.; Ikeda, I. Colloid Polym. Sci. 2004, 282, 12681273.
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Figure 8. Effect of urea on the phase-transition behavior of NA51 (heating rate ) 1.0 °C/min, polymer concentration ) 10 g/L, pH 2.6, [Na2SO4] ) 0.50 M). Urea concentrations are 0 (O), 0.50 (0), and 1.0 M ()).
dehydration of the amide and the consequent formation of weak intra- and interchain hydrogen bonding between the amides.43 Phenomena occurring around the Tp1 of the copolymers studied here are probably similar to those occurring in the poly(Nalkylacrylamide) solutions; the cooperative dehydration of the NVA amide groups occurs at Tp1 such that inter- and intrachain hydrogen bonding occurs in the copolymer chains. Here, the most likely hydrogen-bonding partnership is between the NVA amide and AA carboxyl groups because hydrogen bonding between these groups has been deemed to be advantageous in other reported systems.22,33-36 The change in the hydration state of the copolymer at Tp2 (ca. 90 °C) was difficult to observe because of the high temperature. The absence of a shift in ν(COOD) during the phase transition is probably due to the insensitivity of this band to changes in its hydrogen bonding partners (from D2O to the NVA amide).42 Effect of Urea on Phase Transition. To reveal the significance of copolymer intra- and interchain hydrogen bonding during the unique phase-transition behavior, we examined the effect of adding urea. Urea is known to break hydrogen bonds as well as disrupt hydrophobic interactions. Thus, by hydrogen bonding with the copolymer, the urea molecules will compete with the intra- and interchain hydrogen bonding in the copolymers. As shown in Figure 8, the addition of 0.5 M urea to the copolymer solution caused an increase in Tp1 and a decrease in Tp2. As for the I-S transition, the decrease in the transition temperature with increasing urea concentration was also observed in poly(sulfobetaine)18 and poly(6-(acryloxyloxymethyl)uracil).19 Further addition of urea (1.0 M) to the copolymer solution extinguished the phase-transition behavior. These phenomena were caused by the interference of intra- and interchain hydrogen bonding by urea. Therefore, we have concluded that this hydrogen bonding is essential for the demixing of the copolymer. Mechanism of the S-I-S Transition. From the above discussion, we speculate that the mechanism associated with the S-I-S transition in the copolymers occurs as follows. Below Tp1, a small portion of the NVA amide groups form hydrogen bonds with the AA carboxyl groups, and the remainder are hydrated. With increasing solution temperature, the stability of the hydration around the amide groups decreases. During the first transition temperature Tp1, the cooperative dehydration of the NVA amide groups occurs, accompanied by the simultaneous (43) Maeda, Y.; Higuchi, T.; Ikeda, I. Langmuir 2000, 16, 7503-7509.
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formation of intra- and interchain hydrogen bonds between the NVA amide and AA carboxyl groups. With further increases in solution temperature, the intra- and interchain hydrogen bonding cooperatively dissociates at Tp2 because of the activated motion of the copolymer chains. The increase in Tp1 with increasing NVA content (observed in Figure 5) is attributable to the hydrogen bond formation between the NVA and AA units. The intra- and intermolecular hydrogen bonding reduce the hydration around the amide and carboxyl groups of the copolymers. More hydrogen bonding occurs when the contents of both monomers in the copolymer become comparable. Thus, the transition temperature Tp1 is observed to increase with increasing NVA content in the examined range (39-92 mol %). The Tp2 temperature decrease with increasing pH in the pH range of 2.2-3.0 (Figure 6) is due to the weakening of the hydrogen bonding between NVA and AA on account of the dissociation of the AA carboxylic acid groups in this pH range. Although the degree of dissociation of the AA units is expected to be low in this pH range, Tp2 was still observed to decrease with increasing pH, and the unique transition behavior consequently subsided above pH 3. However, the S-I-S transition in poly(NIPAM-co-AA) occurs even at pH 3.22 Thus, the unique transition behavior of the copolymers studied here is more sensitive to the degree of dissociation of the AA unit. Because the copolymerizability of NVA with AA is not very high, the obtained copolymer has a somewhat blocklike structure rather than a random distribution of comonomers. Thus, it is important to consider the effect of the comonomer distribution along the polymer backbone on the S-I-S transition, although we cannot mention this in detail from the present data. The mixture of homopolymers shows only the I-S transition and is not soluble at lower temperatures (Figure 3). This result will be explained as follows: the hydrogen bonding among polymer segments is stronger in the homopolymer mixture than in the copolymer because of a sequence of the same kind monomer along the backbone so that the homopolymer mixture is insoluble even at low temperatures at which polymer hydration is more advantageous than polymer-polymer hydrogen bonding. From this discussion, it is speculated that a perfect block copolymer does not show an S-I-S transition because of similar strong hydrogen bonding among both blocks and a more random distribution of comonomers along the backbone is important for the S-I-S transition behavior. Effect of Salt on Phase Transition. Finally, the effect of salt concentration on the unique phase transition behavior of the copolymers was investigated. Figure 9 shows the Na2SO4 concentration dependence on the transition temperatures of the copolymers with various NVA content. The Tp1 temperatures for all of the copolymers studied showed a linear decrease with increasing salt concentration, where the slopes are roughly constant, independent of the copolymer composition. This salt concentration dependence of Tp1 is similar to that expressed by the polymers exhibiting a LCST,14,44,45 a phenomenon known as salting out. In contrast, the Tp2 temperature revealed a slight increase with increasing salt concentration and exceeded 100 °C above 0.6 M. A similar positive proportion of a I-S transition temperature to a salt concentration was reported for PAA.15 The S-I-S transition of the copolymers could be observed within a relatively narrow Na2SO4 concentration range. The temperature range of the phase-separated state becomes wider with increasing salt concentration. (44) Schild, H. G.; Tirrell, D. A. J. Phys. Chem. 1990, 94, 4352-4356. (45) Freitag, R.; Garret-Flaudy, F. Langmuir 2002, 18, 3434-3440.
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Conclusions
Figure 9. Na2SO4 concentration dependence on the transition temperatures of copolymers (polymer concentration ) 10 g/L, pH 2.6).
The contrasting linear dependences of Tp1 and Tp2 on salt concentration were also observed when NaCl was used instead of Na2SO4 (data not shown). The salt concentration dependence of the transition temperatures can be qualitatively explained as follows. The addition of ions decreases the number of waters of hydration around the copolymer because of the competing hydration of the ions. Consequently, as the solvent quality decreases, intra- and interchain interactions such as hydrogen bonding and hydrophobic interactions become enhanced. Thus, the S-I and I-S transitions occur at lower and higher temperatures, respectively, with increasing Na2SO4 concentration.
We succeeded in showing the reentrant S-I-S transition in various copolymers of poly(NVA-co-AA) in salt-containing aqueous media. The homopolymer (PNVA and PAA) mixture did not show this behavior. From potentiometric titrations of the copolymers, it was suggested that the carboxyl groups of the AA units form hydrogen bonds with the amide groups of NVA, even under soluble conditions. The dehydration of the amides and the consequent formation of hydrogen bonds at the S-I transition were indicated by FTIR measurements. Thus, it is concluded that introducing (1) a hydrogen-bonding pair and (2) a polymer with moderate hydrophobicity affords a universal design for polymers with an S-I-S transition in aqueous solutions. As such, the two conditions under which a polymer shows this unique solution behavior is the sum of each common condition of the polymers exhibiting a LCST and UCST. We expect that many copolymer systems exhibiting an S-I-S transition will be obtained by copolymerization based on this molecular design. Compared with traditional thermosensitive polymers exhibiting only one transition temperature, the copolymers described here can respond to temperature in a limited range. Therefore, these copolymers will extend the fields of application of temperatureresponsive polymers. Acknowledgment. We thank Professor M. Kaneshina for valuable discussions and Dr. T. Hirano and Dr. A. Hashizume for their critical reading of the manuscript. LA060212V
