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Langmuir 2000, 16, 2042-2044
Thermodynamic Studies on Coil-Globule Transitions of Poly(N-vinylisobutyramide-co-vinylamine) in Aqueous Solutions
In this paper, we extended this line of study to see the effect of cations on the P-T plane (10-50 °C/0.1-400 MPa) and examined the effects of cationic comonomers on the calorimetric properties of these polymers at atmospheric pressure.
S. Kunugi,* T. Tada, and Y. Yamazaki Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606-8585, Japan K. Yamamoto and M. Akashi Department of Applied Chemistry and Chemical Engineering, Kagoshima University, Koorimoto, Kagoshima 890-0065, Japan Received August 2, 1999. In Final Form: October 13, 1999
Poly(N-vinylisobutyramide) (PNVIBA),1,2 is one of the so-called thermoresponsive synthetic polymers3 that carry both hydrogen-bonding and hydrophobic properties, which causes changes in their molecular level states (coil-tocollapse) in an aqueous solution. Compared with the wellknown polymer, poly(N-isopropylacrylamide) (PNIPAM), PNVIBA solutions show a much sharper transition1,2 in solution and much greater swelling and shrinking in gel forms.4 This type of transition is caused not only by changes in temperature but also by alternation in other physical or physicochemical factors, such as pressure, type of solvent, and ionic additives. This behavior has attracted considerable attentions in both applicational fields and basic science,5,6 especially with relation to the coil-globule transition of biopolymers.7,8 We have focused attention on the pressure-temperature behaviors of this type of polymer 9-13 and reported that aqueous solutions of PNVIBA exhibit a characteristic pressure response creating an apparent extremum of transition in the P-T diagrams9,10 and that the P-T transition was strongly influenced by ion concentration.11-13 In the case of PNIPAM, the clouding temperature and pressure became highly pH dependent with the introduction of an anionic comonomer (pentenoic acid).12,13 * To whom correspondence should be addressed. Tel.: +81-75724-7836. Fax: +81-75-724-7710/7800. E-mail:
[email protected]. (1) Akashi, M.; Nakano, S.; Kishida, A. J. Polym. Sci., Part A: Polym. Chem. Ed. 1996, 34, 301. (2) Suwa, K.; Wada, Y.; Kikunaga, Y.; Morishita, K.; Kishida, A.; Akashi, M. J. Polym. Sci., Part A: Polym. Chem. Ed. 1997, 35, 1763. (3) Heskins, M.; Guillet, J. E. J. Macromol. Sci., Polym. Phys. Ed. 1968, 1441. (4) Suwa, K.; Morishita, K.; Kishida, A.; Akashi, M. J. Polym. Sci., Part A: Polym. Chem. Ed. 1997, 35, 3377. (5) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163. (6) Shibayama, M.; Tanaka, T. Adv. Polym. Sci. 1993, 109, 1. (7) Tiktopulo, E. I.; Bychkova, V. E.; Ricka, J.; Ptitsyn, O. B. Macromolecules 1994, 27, 2879. (8) Tiktopulo, E. I.; Uversky, V. N.; Lushchik, B. B.; Klenin, S. I.; Bychkova, V. E.; Ptitsyn, O. B. Macromolecules 1995, 28, 7518. (9) Kunugi, S.; Takano, K.; Tanaka, N.; Suwa, K.; Akashi, M. Macromolecules 1997, 30, 4499. (10) Kunugi, S.; Takano, K.; Tanaka, N.; Suwa, K.; Akashi, M. In High-Pressure Research in the Biosciences and Biotechnology; Heremans, K., Ed.; Leuven University Press: Leuven, 1997; pp 59-62. (11) Suwa, K.; Yamamoto, K.; Akashi, M.; Takano, K.; Tanaka, N.; Kunugi, S. Colloid Polym. Sci. 1998, 276, 529. (12) Kunugi, S.; Takano, K.; Tanaka, N.; Akashi, M. In High-Pressure Food Science, Bioscience and Chemistry; Isaacs, N. S., Ed.; The Royal Society of Chemistry: London, 1998; pp 61-66. (13) Kunugi, S.; Yamazaki, Y.; Takano, K.; Tanaka, N. Langmuir 1999, 15, 4056.
Materials and Methods Homopolymers of PNVIBA were synthesized as described previously2 and fractionated chromatographically if necessary. Their molecular weights and molecular weight distributions were determined by gel permeation chromatography [GPC; Shodex AD-80M/S or TSK gel super-H 3000 and 4000 columns with a dimethylformamide (DMF) solution (containing 10 mM LiBr)]. Mn values were based on commercial poly(ethylene glycol) PEGpoly(ethylene oxide) (PEO) standards. The GPC equipment was a Toso HLC-8120 GPC or Shimadzu LC10A system with an RI detector. PNVIBA: Mn ) 66 kDa and Mw/Mn ) 1.6 or Mn ) 1.4 kDa and Mw/Mn ) 3.3. These molecular weights are above the minimum domain size for PNIPAM to show an all-transition or nontransition.7,8 Copolymers of NVIBA and vinylamine (VAm) were prepared by the acid hydrolysis of pNVIBA-co-N-vinylformamide (NVF)14 which was synthesized by radical polymerization, and the extent of selective hydrolysis (to practically 100%) of NVF to VAm was monitored by NMR. The copolymer composition was determined by the level of pNVIBA-co-NVF by NMR as well as by GPC fractionation. The hydrolyzed copolymer was neutralized, dialyzed, and lyophilized. In the present study, four copolymers were prepared and their NVIBA/VAm contents, molecular weights, and Mw/Mn were (92/8, 20 kDa, 2.0), (82/18, 34 kDa, 2.8), (74/26, 33 kDa, 2.7), and (61/39, 31 kDa, 2.5), respectively. The cloud points of the aqueous solutions of these polymers (normally 0.1 w/v %) were determined by observing light transmission. A high-pressure optical cell, with two sapphire windows15 (Teramecs Co., Kyoto, Japan), was placed between the light source (Xe lump) and the monochromator/photomultiplier (Otsuka Electronics Co., Hirakata, Japan) via optical fibers. Apparent transmittance at 500 nm was recorded while changing either the temperature at a constant pressure or the pressure at a constant temperature. The temperature of the cell was controlled by a Peltier-type thermoregulater and detected by a Pt resistance thermometer. The extraneous pressure was applied by a high-pressure hand pump equipped with an intensifier (ratio 8.5:1) (Teramecs Co.), and the pressure medium was deionized water.15 The pressure was measured by a Bourdon tube-type pressure gauge. The transition temperature or pressure was determined as the peak of the first derivative of the transmittance-temperature (or pressure) curve. DSC measurements were performed by a high-sensitivity Provo, DSC meter, Nano-DSC model 5100 (Calorimetry Science Co., UT). About 1 mL of a 0.1% aqueous solution of polymer was introduced to the sample tube of the apparatus, and the temperature scanning rate was usually 1 °C/min. The reference solution was the equilibrium solution used in the overnight dialysis of each sample. The pH of the solution was adjusted by adding an appropriate amount of a KOH or HCl solution before dialysis, and the pH value of the sample solution was recorded after the DSC measurement.
Results and Discussion Pressure-Temperature Dependence of the Transition. The cloud points of aqueous solutions of copolyNVIBA containing 8 (monomer mol) % VAm (pNVIBA92co-VAm8), equilibrated at various pH values, were measured by scanning temperature under various con(14) Yamamoto, T.; Serizawa, T.; Muraoka, Y.; Akashi, M. Submitted for publication. (15) Morishita, M.; Tanaka, T.; Kawai, S. In High-Pressure Bioscience and Technology; Suzuki, A., Hayashi, R., Eds.; San-ei Pub. Co.: Kyoto, Japan, 1997; pp 187-192.
10.1021/la991041f CCC: $19.00 © 2000 American Chemical Society Published on Web 12/22/1999
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Langmuir, Vol. 16, No. 4, 2000 2043
Figure 2. DSC thermogram of pNVIBA92-co-VAm 8 at various pH’s. 0.1 w/v % of copolymers. Temperature scanning was at 1 °/min. Figure 1. P-T diagrams for the cloud point of pNVIBA92co-VAm8 in aqueous solutions: (O) pH ) 13; (4) pH ) 12; (0) pH ) 11. 0.1 w/v %.
stant pressures or scanning pressure at various constant temperatures. The results are shown in Figure 1 in the P-T plane. At pH 13, the obtained curve was more or less similar to that for the homoPNVIBA solution; the overall shape was circular or ellipsoidal, and the extremum was observed at about 50 MPa, above which the transition temperature decreased with increasing pressure. With decreasing pH, the transition contour seemed to shift upward and to the right and the transition temperature at each pressure became higher or the transition pressure at each temperature became larger. The pKa (or halfneutralization pH) of small alkylamines is about 10.6, and therefore decreases in pH within this range gradually produce cationic (quaternary ammonium) groups on the VAm moiety. These results are comparable to the P-T dependence of the cloud point for the copolymers of NIPAM carboxylate comonomers such as pentenoic acid.12,13 The transitions of thermoresponsive polymers in aqueous solutions occur by dehydration and the strengthening of hydrophobic interactions among side chains. The observed properties of ionic copolymers can be explained primarily by considering the electrostatic repulsive forces in the dehydration process, the strengthening of the hydration around side chains through ion-dipole interactions, and the weakening of hydrophobic interactions among the side chains.16-19 In the case of carboxylic acid copolymers, pressure will affect the acid dissociation process to a greater extent, because it is known that (16) Chen, G.; Hoffman, A. S. Makromol. Chem. Phys. 1995, 196, 1251. (17) Seida, Y.; Nakano, Y. J. Chem. Eng. Soc. Jpn. 1993, 26, 328. (18) Shibayama, M.; Ikkai, F.; Inamoto, S.; Nomura, S.; Han, C. C. J. Chem. Phys. 1996, 105, 4358. (19) Kawasaki, H.; Sasaki, S.; Maeda, H. J. Phys. Chem. 1997, 101, 5089.
carboxylic acids such as acetic acid show large and negative ∆V of acid dissociation (-11 mL/mol for acetic acid at 25 °C) while amino groups (their conjugated acid forms) show a positive ∆V of dissociation, though much smaller in absolute value.20 In the former, the dissociation (anion formation) is facilitated under high pressure (i.e., the shifts of transition curves in carboxylate copolymers are enhanced under higher pressure), but the degree of ionization (protonation) is slightly depressed under high pressure in the latter. The ∆H of acid dissociation of carboxylic acids are relatively small and sometimes negative, while that of protonated amines is large and positive, and hence the effects of temperature on the ionization of these groups (and also of polymers carrying them) are also contrastive. These differences are one important factor which determines the sensitivity to pH effects under elevated pressure and temperature. Calorimetric Properties at Atmospheric Pressure. Figure 2a shows the thermogram of pNVIBA92-co-VAm8 on DSC at several pH values. At pH 13, an endothermic peak similar to the homopolyNVIBA was observed, though peak position is somewhat higher and peak height is slightly lower than those in the latter. With decreasing pH, the peak shifted to a higher temperature and it became broader. At pH 11, a minor peak was observed on the lower temperature side. The peak position (Tp; approximately equal to the transition temperature, Ttr), the enthalpy change (∆Htr; polymermole basis), as calculated by integrating the peaks, and the ∆Cp values are shown in Table 1. With an increased portion of ionic residues in the copolymer caused by decreasing the pH for the amine polymer, the transition temperature (Ttr) shifted to higher values and ∆Htr became smaller. These changes in thermodynamic parameters can similarly be explained by charge production in the side chains. Charge production creates electrostatic repulsive forces in the dehydration process and strengthens the hydration process, which in turn hinders the collapse of the polymer chains. The latter contribution is achieved also by introducing the noncationized amino groups into the main chain. The effect of the vinylamine content on Ttr of pNVIBA-co-VAm was measured at pH 13, where the amino groups are practically uncharged, and the results are shown in Table 1. With an (20) Kunugi, S. Prog. Polym. Sci. 1993, 18, 805.
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Notes
Table 1. Thermodynamic Parameters of the Transition of pNVIBA-co-VAm in an Aqueous Solution, As Determined by DSC Measurementa copolymer
pH
Ttr (°C)
∆Htr (kJ/mol)
∆Cp [J/(mol/K)]
∆HvH (kJ/mol)
∆Htr/∆HvH
pNVIBA92-co-VAm8 pNVIBA92-co-VAm8 pNVIBA92-co-VAm8 pNVIBA82-co-VAm18 pNVIBA74-co-VAm26 pNVIBA61-co-VAm39 homopolyNVIBA
13 11 10 13 13 13 13
47.2 54.6 67.9 52.7 61.7 73.6 40.1
1450 617 566 1461 759 395 7243
-16 -40 -21 -11 -9 -2 -83
906 371 137 399 294 175 4276
1.6 1.7 4.2 3.7 2.6 2.3 1.7
a 0.1% aqueous solution of polymer. Temperature scanning rate was 1 °C/min. The reference solution was the equilibrium solution used in the overnight dialysis of each sample. The pH of the solution was adjusted by adding an appropriate amount of HCl or KOH solution before dialysis, and the pH value of the sample solution was recorded after the DSC measurement.
increase in the VAm content, Ttr shifted to a higher temperature and ∆Htr became smaller. From these thermograms, approximate van’t Hoff enthalpy changes (∆HvH) were calculated, assuming that the transition is in two states and that Tp = Ttr, by using an equation ∆HvH ) 4RTp2 (Cpmax/∆hcal), where Cpmax and ∆hcal are the g-basis excess heat capacity at the transition peak and the g-basis enthalpy change of the transition, respectively. The calculated values are listed in Table 1, as well as the ratios of ∆Htr/∆HvH.21,22 This parameter is (21) Privalov, P. L. Adv. Protein Chem. 1979, 33, 167.
related to the number of cooperative domains in one polymer molecule required to show structural transition. A smaller number means the collapse domain is larger. Charge production on the copolymers resulted in an increase in this parameter, which indicates that the cooperative domain size becomes smaller, because of division of the domain due to interference by charged groups. LA991041F (22) Privalov, P. L. Adv. Protein Chem. 1982, 35, 1.
