Letter pubs.acs.org/Langmuir
Phase Behavior of Aqueous Solutions of Copolymers of N,N′Diisopropylfumaramide and N-Isopropylacrylamide: Effect of the Density of Side Chains Akihito Hashidzume,*,† Akiko Matsumoto,† Takeshi Mori,‡,§ Toshiyuki Shikata,† and Takahiro Sato† †
Department of Macromolecular Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan ‡ Department of Applied Chemistry and §Center for Future Chemistry, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *
ABSTRACT: This letter describes the phase behavior of aqueous solutions of an N-isopropylacrylamide (NIPAM) homopolymer and copolymers of N,N′-diisopropylfumaramide (DIPFAM) and NIPAM as studied by transmittance measurements, infrared spectroscopy, and differential scanning calorimetry to reveal the effect of the density of Nisopropylamide side chain upon the phase behavior. The clouding-point and clearing-point temperatures decreased with increasing the mole fraction of DIPFAM (xD). It was noteworthy that only an extra side chain per ca. 7 NIPAM units had a remarkable effect on the phase behavior; the interactions between side chains were stronger, the intrapolymer contraction was less favorable, and the cooperativity of phase transition was lower at xD = 0.15 presumably because of the steric hindrance of dense side chains.
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behavior. PNIPAM carries N-isopropyl side chains on every two carbons in the polymer backbone. A question arising here is how the phase behavior of aqueous solutions alters when Nisopropylamide side chains are attached more densely onto the polymer backbone. To answer this question, we have investigated the phase behavior of aqueous solutions of copolymers of N,N′-diisopropylfumaramide (DIPFAM) and NIPAM (Scheme 1, in which xD denotes the mole fraction of DIPFAM in the polymer) by transmittance measurements, IR spectroscopy, and DSC. Three polymers were prepared in this study as listed in Table S1 in the Supporting Information. One was a NIPAM homopolymer (xD = 0, D0) and the other two were DIPFAM/NIPAM copolymers of xD = 0.06 and 0.15 (D6 and D15, respectively). Weight average molecular weights Mw were determined to be 4.6 × 104, 3.0 × 104, and 2.0 × 104 for
n the past decade, stimuli-responsive polymers have been attracting considerable interest from a number of researchers as smart soft materials because they are promising in a variety of applications in materials science.1 Among stimuli-responsive polymers, poly(N-isopropylacrylamide) (PNIPAM) is the most examined because its aqueous solutions undergo a phase transition of the type of lower critical solution temperature (LCST) at about the body temperature (ca. 32 °C).2 This phenomenon has been investigated by a number of research groups using various characterization techniques, including turbidimetry,3−6 viscometry,4,7,8 light scattering,4,7−9 differential scanning calorimetry (DSC),3,5,7,10,11 infrared (IR) spectroscopy,12−14 fluorescence spectroscopy,5,15−17 and dielectric relaxation.18,19 These studies have elucidated that PNIPAM chains undergo cooperative dehydration and rehydration around the clouding and clearing points, respectively. Since PNIPAM possesses a hydrophilic amide group and a weak hydrophobic isopropyl group in the side chain, it is likely that the balance between the hydrophilic amide group and the weak hydrophobic isopropyl group is crucial in the LCST-type phase © 2012 American Chemical Society
Received: March 5, 2012 Revised: March 15, 2012 Published: March 16, 2012 5522
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the aqueous D0 solution (the solid line in Figure 1a), the transmittance commences to decrease at ca. 28.0 °C and levels off at temperatures > 29.5 °C. The temperature width ΔTtr of the transmittance change is ca. 1.5 °C. In the cooling process, the transmittance starts to increase at ca. 28.0 °C and levels off at temperatures > 26.7 °C (ΔTtr ≈ 1.3 °C). We define the clouding-point and clearing-point temperatures (Tcloud and Tclear) as the temperatures where transmittance commences to decrease and levels off, respectively. It should be noted here that Tcloud and Tclear are lower for the aqueous solution of our D0 sample compared with those for PNIPAM reported previously.3−6 It is known that the clouding-point temperature of aqueous PNIPAM solutions depends on the polymerization condition of the PNIPAM sample, which provides different stereoregularity,20−23 chain end,24 and degree of branching.25 Since our D0 sample, prepared by radical polymerization in DMSO, is practically atactic and contains a fragment of the conventional initiator, 2-isobutyronitrile moiety, it is likely that the D0 sample possesses a degree of branching different from those of PNIPAM samples previously studied. Furthermore, ΔTtr upon heating and cooling are wider for the aqueous solution of D0 presumably because of the different branching structure or the relatively wide molecular weight distribution (Mw/Mn = 2.4). In the heating process for the aqueous solutions of D6 and D15, the transmittance commences to decrease at ca. 27.8 and 26.1 °C, respectively. In the cooling process for the aqueous solutions of D6 and D15, the transmittance levels off at ca. 26.5 and 25.0 °C, respectively. The temperature widths ΔTtr of the transmittance change are ca. 1.5 and 1.7 °C for aqueous D6 and D15 solutions in the heating process, and 1.3 and 2.0 °C in the cooling process, respectively. For the D15 solution, ΔTtr in the cooling process is slightly wider than the other solutions. As shown in Supporting Information Figure S1, macroscopic precipitates appear in the phase-separating solution of D15 sample with the concentration 1.0 wt % at 35 °C, while no precipitation takes place in the phase-separating turbid solutions of the other samples. Thus, grains of the concentrated phase in the D15 solution are larger and difficult to resolve upon cooling. For the aqueous solutions of D6 and D15, Tcloud and Tclear were also estimated in the same manner as for the D0 solution. The Tcloud and Tclear values were plotted in Figure 2 against xD. The solubility to water becomes poor with increasing xD, that is, the density of side chains. The phase behavior of aqueous solutions of the polymers was also investigated by IR spectroscopy. For the IR measurements, D2O was used as the solvent to avoid the overlapping between
Scheme 1. Chemical Structure of DIPFAM/NIPAM Copolymer
D0, D6, and D15, respectively, by SEC-MALS. Since the Mw values were considerably large, the effect of the chain end on the phase behavior could not be important. 1H NMR spectra measured in DMSO-d6 at 120 °C indicated that the tacticities of NIPAM units were almost the same and practically atactic independent of xD (data not shown). On the basis of the reactivity ratios of DIPAM and NIPAM (0.01 ± 0.02 and 10.0 ± 0.8, respectively), it is likely that the DIPFAM/NIPAM copolymers contain almost no DIPFAM diad (see also the Supporting Information). The phase behavior of aqueous solutions of D0, D6, and D15 was first investigated by transmittance measurements. Figure 1 exhibits a typical example of transmittance data measured at 570 nm for 1.0 wt % aqueous solutions of the polymers with heating and cooling at 0.10 °C min−1. In the heating process for
Figure 2. Clouding-point and clearing-point temperatures (Tcloud (●) and Tclear (■), respectively) as a function of xD for the 1.0 wt % aqueous solutions of the polymer samples examined.
Figure 1. Transmittance data for 1.0 wt % aqueous solutions of D0 (a), D6 (b), and D15 (c) monitored at 570 nm with heating (solid line) and cooling (broken line) at 0.10 °C min−1. 5523
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O−H bending and the amide I band.14 IR spectra were recorded for 8.0 wt % solutions of D0, D6, and D15 in D2O with heating in the range 20.6−44.5 at 0.50 °C min−1. In the spectra for the 8.0 wt % aqueous D0 solution measured at 20.6 °C (Figure S2 in the Supporting Information), absorption bands at ca. 1620 and 1470 cm−1 are ascribable to the amide I and II bands, which correspond to the stretching vibration of CO bond and the deformation of N−H bond, respectively. The spectrum at 20.6 °C also contains an absorption band assignable to the asymmetric C−H stretching at ca. 2980 cm−1. At 44.5 °C, the absorption band due to the amide I band is broader with a shoulder at ca. 1650 cm−1, whereas the absorption bands due to the amide II band and the asymmetric C−H stretching are seen at smaller wavenumbers. These observations are indicative of a difference in the hydration state of N-isopropylamide side chains at 20.6 and 44.5 °C. IR spectra for the solutions of D6 and D15 also exhibit the same differences at 20.6 and 44.5 °C (Figures S3 and S4 in the Supporting Information). According to the procedure of Maeda and co-workers,14,26,27 differential spectra were obtained by subtraction of the reference spectrum recorded at 20.6 °C from spectra at varying temperatures to see details of differences in spectra. In the differential spectra, the differences in absorbance (ΔΔA) between the maxima and minima were calculated for the asymmetric C−H stretching and amide I and II bands. Temperature dependencies of ΔΔA for the three bands of all the polymer samples shown in Figure 3 are similar to those of PNIPAM reported previously,14 indicating that D0, D6, and D15 undergo the same dehydration as those for PNIPAM reported previously. Supporting Information Figure S5 shows the temperature dependency of ΔΔA for the amide II band of the three polymers in the heating process in a restricted temperature range from 24 to 32 °C. For all the samples, ΔΔA starts to increase at temperatures almost the same as Tcloud determined by the transmittance measurements. Although not shown here, the temperature dependencies of ΔΔA for the asymmetric C−H stretching and amide I bands also correlate to Tcloud, which indicates that phase separations of aqueous solutions of D0, D6, and D15 samples are induced by the dehydration of the polymer chains. As can be seen in Figure S6a in the Supporting Information, the amide I band can be deconvoluted into two components at ca. 1623 and 1648 cm−1 with Gaussian curves. For all the IR spectra recorded, the amide I bands were deconvoluted, fixing the width of Gaussian curve for the component at ca. 1623 nm−1. The fractions of the component at the larger wavenumber ( f) were calculated and plotted against temperature (Figure S6b in the Supporting Information). The D0 and D6 solutions indicate almost the same temperature dependencies and values of f, whereas the D15 solution shows larger f values in the whole temperature range. These observations indicate that more fractions of CO groups are used for intrapolymer or interpolymer hydrogen bonding in the case of the D15 solution, indicative of stronger interactions of dense side chains. The phase behavior of aqueous solutions of D0, D6, and D15 was further investigated in detail by DSC. Figure 4 displays the temperature dependencies of heat capacity (Cp) for 1.0 wt % aqueous solutions of D0, D6, and D15 measured with heating and cooling at 0.50 °C min−1. In this figure, the solid and broken curves correspond to the heating and cooling processes, respectively. As can be seen in Figure 4a, the DSC curve for the heating process of the aqueous D0 solution contains an
Figure 3. ΔΔA as a function of temperature for the asymmetric C−H stretching (solid line), amide I (broken line), and amide II bands (one dot chain line) for 8.0 wt % solutions of D0 (a), D6 (b), and D15 (c) in D2O with heating at 0.50 °C min−1.
endothermic signal, which starts at ca. Tcloud, with a maximum at 30.3 °C ascribable to dehydration. The DSC curve for the cooling process of the aqueous D0 solution indicates an exothermic signal, which ends at ca. Tclear, with a sharp minimum at 28.8 °C ascribable to rehydration. These observations confirm that the dehydration of D0 causes the phase separation of the aqueous solution. The exothermic signal in the cooling process is remarkably asymmetric, indicating that the phase transition includes both interpolymer aggregation and intrapolymer contraction.28,29 This was confirmed by the DSC curve for a 0.1 wt % aqueous D0 solution in the cooling process, which exhibited a bimodal exothermic signal with a more considerable component at a higher temperature (Figure S7a in the Supporting Information). The DSC signals for the 1.0 wt % aqueous D6 solution are slightly broader but practically the same as those for the D0 solution (Figure 4b). These observations also indicate that 6 mol % DIPFAM units in the copolymer have no significant effects on the phase behavior of aqueous solutions. On the other hand, DSC signals for the 1.0 wt % D15 solution are markedly broader. In the heating process, the endothermic signal starts at ca. Tcloud and shows a maximum at 30.3 °C. The endothermic signal contains a shoulder at ca. 28 °C, indicative of a multistep phase transition of the aqueous D15 solution. The DSC curve obtained at a lower concentration (0.1 wt %) in 5524
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Table 1. Thermodynamic Parameters for the Phase Transition of Aqueous Solutions of D0, D6, and D15a polymer
concentration/ wt %
Tp/ °Cb
ΔT1/2/ °Cc
ΔH / kJ mol−1d
ΔCp / kJ (°C mol)−1e
D0 D0 D6 D6 D15 D15
1.0 0.1 1.0 0.1 1.0 0.1
30.3 31.1 30.3 31.1 30.3 32.3
3.11 3.63 3.63 4.31 6.21 7.05
4.64 3.89 5.90 5.19 5.94 5.56
1.1 0.79 1.3 0.96 0.84 0.67
Determined by DSC data measured at 0.50 °C min−1. bPhase transition temperature determined from the endothermic peak. c Maximum half-width of endothermic peak. dCalorimetric enthalpy. e Endothermic peak hight. a
noted that Tp determined for the cooling process of DSC measurements decreases from 28.8 to 26.2 °C with increasing xD from 0 to 0.15, consistent with Tclear.) Tp is dependent strongly on the polymer concentration because the present phase transition contains both interpolymer aggregation and intrapolymer contraction. At a lower concentration, Tp for the heating process increases from 31.1 to 32.3 °C with increasing xD, indicating that the intrapolymer contraction is less favorable at higher xD presumably because of the steric hindrance of dense side chains around DIPFAM units. The half width (ΔT1/2) of the endothermic signal increases with xD, indicating that the phase transition occurs in a wider temperature range, that is, a less cooperative phase transition, at higher xD. It is likely that the composition distribution or the steric hindrance around DIPFAM units decreases the cooperativity of phase transition. The change in enthalpy per monomer unit (ΔH) increases with xD. This is because the number of hydrated water molecules per monomer unit increases with xD (i.e., the density of side chains), resulting in an increase in the number of water molecules which dehydrate. This was confirmed by dielectric relaxation measurements, by which the numbers of hydrated water molecules per monomer unit were estimated to be 13, 15, and 18 for D0, D6, and D15, respectively (see the Supporting Information). In conclusion, the phase behavior of aqueous solutions of D0, D6, and D15 was studied by transmittance measurements, IR spectroscopy, and DSC to understand the effect of the density of N-isopropylamide side chains on the phase behavior. The clouding and clearing points decreased from 28.7 to 26.9 °C and from 27.1 to 24.1 °C, respectively, with increasing xD from 0 to 0.15 for 1.0 wt % aqueous solutions. This may be because the hydrated state of DIPFAM units is less stable because of the steric hindrance of the dense side chains. IR and DSC studies indicated that 15 mol % DIPFAM units showed a remarkable effect on the phase behavior whereas 6 mol % DIPFAM units did not, indicating that an extra side chain per ca. 7 NIPAM units was important. In the case of D15, the interactions between side chains were stronger, intrapolymer contraction was less favorable, and the cooperativity of phase transition was lower presumably because of the steric hindrance of dense side chains.
Figure 4. DSC curves for 1.0 wt % aqueous solutions of D0 (a), D6 (b), and D15 (c) with heating (solid line) and cooling (broken line) at 0.50 °C min−1.
the heating process also exhibits a multimodal endothermic signal as can be seen in Supporting Information Figure S7c. At present, we cannot assign each mode, but the multistep phase transition may be due to the interpolymer and intrapolymer events, or the composition distribution. In the cooling process for the aqueous D15 solution, Cp changes gradually and even below Tclear (= 25.0 °C) it keeps decreasing. This indicates that the clearing point is strongly dependent on the cooling rate for the 1.0 wt % aqueous D15 solution because the cooling rate for DSC measurements (0.5 °C min−1) was faster than that for the transmission measurements (0.1 °C min−1). Furthermore, DSC signals measured for the aqueous D15 solution were dependent on the number of scan times (see Figure S8 in the Supporting Information). (Figure 4c shows the first heating and cooling scans.) These observations indicate that once the aqueous D15 solution undergoes the phase separation at higher temperatures in a heating process, it takes a longer time that the grains of precipitate are redissolved in the following cooling process, indicative of strong interactions between the dense side chains in the phase-separated state. As can be seen in Figure S9 in the Supporting Information, the difference in baselines at lower and higher temperatures in the heating process increases with xD, indicating that less hydrophobic residues are exposed to the bulk water phase for a polymer of higher xD in the phaseseparated state.30 This observation is also indicative of the stronger interactions of the dense side chains. Table 1 lists thermodynamic parameters determined from the DSC data for the heating processes. The phase transition temperatures (Tp) determined from the maxima of endothermic signals coincide at 30.3 °C independent of xD. (It should be
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ASSOCIATED CONTENT
S Supporting Information *
Experimental section, copolymerization of DIPFAM and NIPAM, the number of water molecules hydrated per monomer unit, and additional experimental results. This 5525
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(16) Winnik, F. M. Quenching of fluorescence from pyrene-labeled poly(N-isopropylacrylamide) solutions heated above their lower critical solution temperature. Macromolecules 1990, 23, 1647−1649. (17) Winnik, F. M. Phase transition of aqueous poly(Nisopropylacrylamide) solutions: A study by non-radiative energy transfer. Polymer 1990, 31, 2125−2134. (18) Ono, Y.; Shikata, T. Hydration and dynamic behavior of poly(N-isopropylacrylamide)s in aqueous solution: A sharp phase transition at the lower critical solution temperature. J. Am. Chem. Soc. 2006, 128, 10030−10031. (19) Ono, Y.; Shikata, T. Contrary hydration behavior of Nisopropylacrylamide to its polymer, P(NIPAm), with a lower critical solution temperature. J. Phys. Chem. B 2007, 111, 1511−1513. (20) Ray, B.; Okamoto, Y.; Kamigaito, M.; Sawamoto, M.; Seno, K.-i.; Kanaoka, S.; Aoshima, S. Effect of tacticity of poly(N-isopropylacrylamide) on the phase separation temperature of its aqueous solutions. Polym. J. (Tokyo, Jpn.) 2005, 37, 234−237. (21) Ito, M.; Ishizone, T. Living anionic polymerization of Nmethoxymethyl-N-isopropylacrylamide: Synthesis of well-defined poly(N-isopropylacrylamide) having various stereoregularity. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 4832−4845. (22) Hirano, T.; Kamikubo, T.; Okumura, Y.; Bando, Y.; Yamaoka, R.; Mori, T.; Ute, K. Heterotactic-specific radical polymerization of Nisopropylacrylamide and phase transition behavior of aqueous solution of heterotactic poly(N-isopropylacrylamide). J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 2539−2550. (23) Katsumoto, Y.; Kubosaki, N. Tacticity effects on the phase diagram for poly(N-isopropylacrylamide) in water. Macromolecules 2008, 41, 5955−5956. (24) Ise, T.; Nagaoka, K.; Osa, M.; Yoshizaki, T. Cloud points in aqueous solutions of poly(N-isopropylacrylamide) synthesized by aqueous redox polymerization. Polym. J. (Tokyo, Jpn.) 2011, 43, 164− 170. (25) Kawaguchi, T.; Kojima, Y.; Osa, M.; Yoshizaki, T. Cloud points in aqueous poly(N-isopropylacrylamide) solutions. Polym. J. (Tokyo, Jpn.) 2008, 40, 455−459. (26) Mori, T.; Hirano, T.; Maruyama, A.; Katayama, Y.; Niidome, T.; Bando, Y.; Ute, K.; Takaku, S.; Maeda, Y. Syndiotactic poly(N-npropylacrylamide) shows highly cooperative phase transition. Langmuir 2009, 25, 48−50. (27) Mori, T.; Beppu, S.; Berber, M. R.; Mori, H.; Makimura, T.; Tsukamoto, A.; Minagawa, K.; Hirano, T.; Tanaka, M.; Niidome, T.; Katayama, Y.; Hirano, T.; Maeda, Y. Design of temperature-responsive polymers with enhanced hysteresis: α,α-Disubstituted vinyl polymers. Langmuir 2010, 26, 9224−9232. (28) Ding, Y.; Zhang, G. Microcalorimetric investigation on association and dissolution of poly(N-isopropylacrylamide) chains in semidilute solutions. Macromolecules 2006, 39, 9654−9657. (29) Ding, Y.; Ye, X.; Zhang, G. Can coil-to-globule transition of a single chain be treated as a phase transition? J. Phys. Chem. B 2008, 112, 8496−8498. (30) Tiktopulo, E. I.; Uversky, V. N.; Lushchik, V. B.; Klenin, S. I.; Bychkova, V. E.; Ptitsyn, O. B. Domain” coil-globule transition in homopolymers. Macromolecules 1995, 28, 7519−7524.
material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to Mr. Seiji Adachi, Department of Chemistry, Graduate School of Science, Osaka University, for 1 H and HSQC NMR measurements on an Agilent 600 NMR spectrometer, and to Professor Atsushi Maruyama, Kyushu University, for the use of DSC instrument. This work was partly supported by Grant-in-Aid for Scientific Research No. 23350055 and 23550137 from the Japan Society for the Promotion of Science.
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REFERENCES
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