Article pubs.acs.org/Macromolecules
Temperature-Dependent Hydration/Dehydration Behavior of Poly(ethylene oxide)s in Aqueous Solution Toshiyuki Shikata,*,† Misumi Okuzono, and Natsuki Sugimoto Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan ABSTRACT: It has been well-known that aqueous solutions of poly(ethylene oxide) (or poly(ethylene glycol)) (PEO) are clear in a low temperature range but show clouding points due to phase separation behavior at rather high temperatures close to 100 °C slightly dependent on its average molar mass and concentration. We carried out extremely high frequency dielectric relaxation measurements up to 50 GHz over a temperature range from 10 to 70 °C to determine the temperature dependence of a hydration number per ethylene oxide monomer unit (mEO) in PEOs with the average molar masses higher than 3000. The obtained mEO was a constant value of ca. 4 in a temperature range lower than 30 °C and decreased rather gradually and approached ca. 2 at 70 °C. The temperature dependence of mEO, hydration/dehydration behavior, confirmed in this study should be the essential reason for the phase separation behavior observed at high temperatures. The hydration/ dehydration behavior observed in this study was compared with that reported previously using ultrasonic velocity measurement techniques.
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INTRODUCTION In recent years, there has been a considerable increase in interest in poly(ethylene oxide) (poly(ethylene glycol) or poly(oxyethylene) sometimes) (PEO) not only in pure scientific fields but also in practical applications.1−10 For example, PEOs are used as additives in many kinds of foods for its high performance and low toxicity.2 PEOs have been used as a polymer host for solid polymer electrolytes in electrical devises including batteries.3,4 PEOs have also been commonly used as a precipitant for plasmid DNA separation, crystallization procedures of proteins for X-ray diffractions, and so many biological experimental procedures.5 Furthermore, over the past few years polymeric drug delivery systems have been an intensive research field, and PEOs have been paid very much attention as one of polymer sources to make injectable block copolymer hydrogel carriers for sustained release of medicines.6−10 It has been well-known that aqueous solutions of PEOs show unique phase behavior, in which the solutions are clear in a temperature region lower than a certain critical temperature and become turbid and show phase separation in the temperature range higher than critical one. The critical temperature is classified into the lower critical solution temperature (LCST) thermodynamically and slightly dependent on the average molar masses of PEOs; the LCST value for PEO with a sufficiently high molar mass is reported to be of 100 °C.11,12 Such a unique phase behavior showing the LCST has charmed and stimulated many theoreticians, and Matsuyama and Tanaka13 have already proposed a theoretical model that successfully explains the phase behavior semiquantitatively. © 2013 American Chemical Society
According to the model proposed by Matsuyama and Tanaka,13 the hydration number per ethylene oxide unit (mEO) in PEO chains decreases, not in a cooperative manner possessing abrupt, stepwise dehydration behavior at the LCST as found in aqueous solutions of poly(isopropylacrylamide) (PNIPAm),14,15 but rather gradually and approaches to a certain critical value that does not permit for PEO chains to dissolve in water and induces the phase separation.11−13 Although this scenario for the phase separation behavior in aqueous solutions of PEOs is comprehensive, no one has confirmed such the temperature dependence of mEO experimentally. Recently, our group developed a technique to determine hydration numbers of solute molecules dissolved in water using dielectric relaxation measurements performed in an extremely high frequency range up to some tens of gigahertz.16 Because relaxation strength of free water molecules in solutions is precisely evaluable in such a high frequency range, the amount of water molecules hydrated to solute molecules is able to be determined. The value of mEO has been evaluated to be of ca. 4 at room temperature, ca. 25 °C, for PEOs bearing various average molar masses.16 The trend that the mEO for PEOs slightly decreases with decreasing the molar mass was also recognized. The reason for the depression in the mEO value has been attributed to the contribution of the presence of hydroxy groups at their chain terminals. In this study, the dielectric technique to determine the hydration number for solute molecules was extended over a Received: December 22, 2012 Revised: January 30, 2013 Published: February 18, 2013 1956
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wide temperature range from 10 to 70 °C. Then, the temperature dependence of the mEO for PEOs was determined exactly, and the validity of the theoretical prediction including the reduction of mEO was qualitatively revealed. The mEO value determined in this study was compared with that resulted from other techniques, and the reliability of our method was also discussed. Moreover, the origins for the temperature dependence of the mEO values for PEOs were considered on the basis of the temperature dependence of mEO data obtained for small, model oligo-cyclic ethylene oxides, so-called crown ethers such as 15-crown-5 and 18-crown-6.
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Figure 1. Frequency, ω, dependencies of real and imaginary parts of electric permittivity, ε′ (open circles) and ε″ (open squares), for aqueous solution of PEO3 at the concentration of cEO = 1.01 M and 25 °C. Broken lines in this figure mean constituent Debye-type relaxation functions to describe the experimental ε′ and ε″ as solid lines via eq 1.
EXPERIMENTAL SECTION
Materials. Poly(ethylene oxide)s bearing the average molar masses of 3000 (PEO3) and 6000 (PEO6) were purchased from Wako Pure Chemical Industries, Ltd. (Osaka), and used without any further purification. Oligo-cyclic ethylene oxides, 15-crown-5 (15C5) and 18crown-6 (18C6), were also purchased from the same company. Highly deionized water with the specific resistance higher than 15 MΩ·cm obtained by an Elix-UV3 system (Millipore-Japan, Tokyo) was used as a solvent for sample solution preparation. The concentration, cEO, of sample solutions ranged from 0.4 to 2.0 M in ethylene oxide monomeric units for PEO3 and PEO6. In the cases of aqueous solutions of oligo-cyclic ethylene oxides, the concentration, c, was varied in a range from 0.3 to 1.2 M. Methods. A dielectric probe kit 8507E equipped with a network analyzer N5230C, ECal module N4693A, and performance probe 05 (Agilent Technologies, Santa Clara, CA) was used for dielectric relaxation measurements over a frequency range from 50 MHz to 50 GHz (3.14 × 108 to 3.14 × 1011 s−1 in angular frequency (ω)). The sample temperature, T, was altered over a range from 10 to 70 °C in accuracy of ±0.1 °C with a temperature controlling unit made of a Peltier devise. A three-point calibration procedure using hexane, 3pentanone, and water as the standard materials was performed prior to dielectric measurements at each measuring temperature. Details for the three-point calibrating procedure used in this studies have been described elsewhere.17,18 Density measurements for all sample solutions were carried out using a digital density meter DMA4500 (Anton Paar, Graz) to determine (average) partial molar volumes of solute molecules (or ethylene oxide units) at each temperature the dielectric relaxation measurements were performed.
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Figure 2. Concentration, cEO, dependencies of relaxation times, τj (a), and strength, εj (b), of constituent Debye-type modes for aqueous solution of PEO3 (open symbols) and PEO6 (closed symbols) at 25 °C.
RESULTS AND DISCUSSION Dielectric Behavior for PEOs. Dielectric spectra (frequency, ω, dependencies of real and imaginary parts (ε′ and ε″) of electric permittivity) for an aqueous solution of PEO3 at cEO = 1.01 M and T = 25 °C are shown in Figure 1 as typical examples of the obtained spectra for the PEO aqueous system. Although only a large dielectric dispersion is observed at ω ∼ 1011 s−1 at a glance, the spectra seen in Figure 1 are perfectly decomposed into three dielectric relaxation modes described by Debye-type relaxation modes as given by eq 1 3
ε′ =
∑ j=1
εj 1+
ω 2τj 2
3
+ ε∞ ,
ε″ =
∑ j=1
whereas magnitudes of relaxation strength of each mode were proportional to c EO . These behaviors reveal that the contribution of contact and entanglement between polymer chains sometimes observed in a high concentration region was less effective. Because the value of the shortest relaxation time, τ1, was essentially identical to the dielectric relaxation time, τw (= 8.3 ps), of water molecules in the pure liquid sate at the same temperature as 25 °C, the mode j = 1 is assigned to the rotational mode of free water molecules in solution. The depression in the ε1 in proportion to cEO observed in Figure 2b is related to the evaluation of hydration number described in the next section. The second relaxation mode j = 2 showing the relaxation time of τ2 ∼ 20 ps is assigned to an exchange process of hydrated water molecules to PEO chains by free water molecules. If this assignment is correct, the relaxation strength would be proportional to cEO, since the number of hydrated
εjωτj 1 + ω 2τj 2
(1)
where τj and εj are the dielectric relaxation time and strength of a mode j (= 1, 2, and 3 from the shortest relaxation time). Such a decomposition procedure into three kinds of relaxation modes held well in all the dielectric spectra obtained in this study. The dependencies of τj and εj on the concentration, cEO, for PEO3 and PEO6 at T = 25 °C are shown in Figures 2a and 2b, respectively. All the relaxation times seem to be independent of cEO and the average molar masses in the cEO range examined, 1957
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water molecules responsible for the relaxation strength should be proportional to cEO as observed in Figure 2b. Moreover, many water-soluble substances in spite of polymeric or low mass materials show relaxation modes bearing relaxation times close to 20−30 ps at T = 25 °C in aqueous solution, and these modes have been attributed to the exchange process of hydrated water molecules to solute molecules.16,19,20 Furthermore, some recent computer simulation studies have revealed that the residence time of water molecules hydrated to solute molecules is on the order of 10 ps, which is close to the τ2 value observed in Figure 2a.21−23 Additionally, the fact that dielectric relaxation strength per water molecule hydrated to a solute molecule, which is calculated assuming the second relaxation mode is the exchange process of hydrated water molecules, is not far from that per free water molecule as described later supports this assignment. The slowest relaxation mode possessing a relaxation time of τ3 ∼ 100 ps was attributed to local or segmental relaxation mode of PEO chains. Because an ether group, −H2C−O− CH2−, possesses dipole moment due to a difference between electron negativities of oxygen and carbon atoms, the local or segmental motions of PEO chains are dielectrically detectable irrespective of molar masses and concentration, cEO. According to the results of spin-lattice relaxation time, T1 ∼ 0.73 s, of 13C nuclear magnetic resonance (NMR) measurements carried out in aqueous (D2O) PEO solutions, the average rotational correlation time, τc, of a methylene group, −CH2−, which is essentially the same as that of the ether group was evaluated to be ca. 30 ps at T = 25 °C.11 This value can be easily converted to a time constant, τd, with the same rank of 1 as dielectric relaxation time by multiplying 3. The calculated value of τd ∼ 90 ps reasonably agrees with the τ3.24 Hydration Number and Dynamics for PEOs. The depression observed in ε1 as seen in Figure 2b is explained by two factors: a volumetric effect of solute PEOs and a hydration effect, as described in detail elsewhere.19,20 How much the ε1 value is depressed by the presence of PEOs is quantitatively described by eq 219,20 1 − 10−3VEOc ε1 = − 10−3Vwc EOmEO 10−3V c εW 1 + 2 EO
Figure 3. Concentration, cEO, dependence of a depression ratio of relaxation strength of the mode j = 1 relative to that of pure water, ε1εW−1, for aqueous PEO3 solutions at T = 25 °C.
Figure 4. Dependence of hydration number per ethylene oxide unit, mEO, on temperature, T, for aqueous solution of PEO3, PEO6, 15C5, and 18C6. A solid curve is a guide for eyes.
This hydration/dehydration behavior seems to be independent of the average molar mass. It has been well-known that aqueous PEOs solutions have clouding points and show phase separation behavior at a certain critical temperature slightly dependent on the average molar mass.11,12 Further increasing temperature beyond 70 °C will accelerate dehydration and decreases mEO down to certain critical values for PEOs not to dissolve into water. The temperature at which the mEO reaches the critical value for insolubility in water should be identical to the clouding point. The critical mEO value for insolubility in water should be a function of the average molar mass and would be a quite small value or diminishes when the average molar mass of PEO decreases. More than two decays ago, Matsuyama and Tanaka13 theoretically dealt with the unique phase behavior of aqueous solutions PEOs, and successfully developed a model that was able to predicts the phase behavior showing LCST, a closedloop type, and also hourglass shape coexistence regions in temperature vs composition phase diagrams choosing thermodynamic parameters. In their model, a Flory−Huggins type lattice model26 was assumed for the expression of the total free energy of the system and also assumed that PEO chain segments were able to make 1:1 complexes with water molecules. The complexes are hydrated parts of PEO chains and dissolve in a low temperature range and dehydrate with increasing temperature. Consequently, aqueous PEO solutions show phase separation due to insolubility of PEO chains at a critical temperature identical to LCST.13 Because the full hydration number per segment is assumed to be unity in the model for simplicity, direct comparison between the experimental mEO data obtained in this study and the theory is impossible. However, the hydration/dehydration behavior depending on temperature for ethylene oxide unit of PEO chains obtained in this study as seen in Figure 4 is qualitatively
(2)
where εW is the dielectric relaxation strength of pure water, VEO and VW are the average partial molar volume of the ethylene oxide unit and that of water molecules, and mEO is the hydration number per ethylene oxide unit. The first term of eq 2 represents the contribution of the volumetric effect of the solute, PEOs, and the second one the hydration effect. Figure 3 shows the concentration, cEO, dependence of a depression ratio, ε1εW−1, for aqueous PEO3 solutions at T = 25 °C as a typical example. If one assumes that there is no hydration effect, the data should obey a line calculated via eq 2 assuming mEO = 0. However, the obtained data well followed a line calculated assuming mEO = 4 irrespective of cEO. Consequently, we may conclude the hydration number, mEO, is of ca. 4 at the temperature. This value of 4 has been also confirmed by another group using dielectric techniques.25 According to the same procedure, the temperature dependence of mEO was obtained for PEOs as shown in Figure 4. The mEO seemed to be a constant value of 4 in a temperature range lower than 30 °C, whereas it decreased gradually with increasing temperature and reached a value of ca. 2 at the highest measured temperature of 70 °C as seen in Figure 4. 1958
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explained by the theory. Consequently, the scenario for the phase separation with increasing temperature is essentially confirmed by the model. Temperature dependencies of relaxation times, τj, provide information about differences in dynamics for each relaxation mode, j. Figure 5 shows so-called Arrhenius plots of relaxation
Figure 6. Relationship between the relative dielectric strength, ε2(cEOmEOε1VW)−1, and T for aqueous solutions of PEO3, PEO6, 15C5, and 18C6.
were obtained for the relative dielectric strength, ε2(cEOmEOε1VW)−1, irrespective of temperatures. This observation suggests that hydrated water molecules still keep dielectric relaxation strength not so different from that of free water molecules in spite of their restricted situation in the hydration sites. Such a relationship ε2(cEOmEOε1VW)−1 ∼ 1 would roughly hold in many aqueous solution including solutes not only synthetic polymers like PEOs but also biopolymers like proteins.34,35 Dielectric and Hydration Behavior for Model Cyclic Compounds. To understand the hydration/dehydration behavior observed in aqueous PEO solutions, we carried out dielectric measurements on aqueous solutions of some model cyclic compounds: oligo-cyclic ethylene oxides such as 15C5 and 18C6. These model cyclic compounds are made from ethylene oxide monomer units and possess cyclic shapes with different unit sizes without terminal chain ends of hydroxyl groups. Thus, these are convenient low mass substances to probe hydration/dehydration behavior of only ethylene oxide units in the pure state without the contribution of chain ends. Dielectric spectra for aqueous 15C5 solution at c = 0.71 M and 25 °C are shown in Figure 7 as typical examples for
Figure 5. Dependencies of logarithmic τj on the reciprocal temperature, T−1, for aqueous solutions of PEO3, PEO6, 15C5, and 18C6.
times, logarithmic value of τj vs T−1, for PEOs. Slopes evaluated from this kind of plot provide the apparent activation energies for each τj. The activation energy, E1*, for the relaxation mode j = 1 was evaluated to be 19 kJ mol−1, which is identical to that of rotational relaxation time of pure water molecules, EW*. Then, there is no doubts for the assignment of the mode j = 1. In the case of τ2, the plot seen in this figure does not seem to be a straight line to provide a single activation energy in the T range examined but has a break point at T−1 ∼ 0.0032 K−1. This observation reveals that the relaxation mechanism of the mode j = 2 alters from a mechanism possessing E2* (∼E1*) to other ones with lower activation energies with increasing T. When one simply plots the τ2 data as a function of temperature, T, a break point is obviously recognized at ca. 27 °C. These strongly suggest there is a difference between the exchanging mechanism of hydrated water molecules to PEOs in a low temperature range possessing mEO = 4 and that in a higher temperature range with mEO < 3. The slowest relaxation mode j = 3 showed an activation energy, E3*, close to the value of E1* and EW* as seen in Figure 5. The relaxation time of local or segment motions of polymer chain molecules is controlled by sizes of segments, force constants connecting segments, and viscosities of solvents.21 Since the temperature range covered in this study was not so wide, the temperature dependence of segmental relaxation time should be mainly controlled by that of viscosity of the solvent, water. The reason for this rough estimation is that the relaxation time of a local segment mode, τ2, is proportional to the product of the cube of segment size, the viscosity of water, and the reciprocal of the absolute temperature, and the most varying factor in these in a restricted temperature range is the viscosity of water.27,28 Of course, it has been known that the activation energy of the viscosity, Eη*, is identical to EW*. Dielectric relaxation strength per hydrated water molecule reduced at 1 M is calculated to be ε2(cEOmEO)−1, and that per free water molecule reduced at 1 M is ε1VW. The ratio of ε2(cEOmEO)−1 to ε1VW, ε2(cEOmEOε1VW)−1, implies relative dielectric strength between hydrated and free water molecules. The relationship between the relative dielectric strength, ε2(cEOmEOε1VW)−1, and T for aqueous solutions of PEOs is shown in Figure 6. Almost constant values not so far from unity
Figure 7. Dependencies of ε′ (open circles) and ε″ (open squares) on ω for aqueous solution of 15C5 at c = 0.71 M and 25 °C. Broken lines in this figure mean constituent Debye-type relaxation functions to describe the experimental ε′ and ε″ as solid lines via eq 1.
aqueous solutions of the model cyclic compounds. The ω dependencies of ε′ and ε″ were quite similar to those of PEO solutions as seen in Figure 1. The decomposition procedure for dielectric spectra with three Debye-type relaxation modes also held in the aqueous model cyclic compounds as well as in the aqueous PEO systems. Except for the slowest relaxation mode j = 3, the assignments for relaxation modes, j = 1 and 2, given to the solutions of PEOs above held in solutions of model cyclic compounds, since the concentration dependencies of relation times and strength for the systems were essentially the same as those of PEO systems. However, the slowest relaxation time, τ3, 1959
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hydrodynamic radius of PEO chains in D2O solution evaluated from self-diffusion coefficients determined by using NMR techniques dramatically shrank with increasing temperature above 30 °C in relation with dehydration behavior. The reduction of hydrodynamic radii for polymeric molecules with increasing temperature was also confirmed and was related to the dehydration behavior of solute molecules in aqueous solutions of collagen model polypeptides.34,35 Miyazaki et al.36 reported the hydration number per ethylene oxide unit, mEO, to be 1.9−2.0 for the model cyclic compounds 18C6, 15C5, and 12C4, at 20 °C, based on the results of the most compact hydration structure determination procedure via ultrasonic velocity measurements. They also dealt with the mEO values for short linear oligo-ethylene oxides and manifested that mEO ∼ 2.0 irrespective of chain length.37 Then, they claimed that the mEO value of 2 is independent of the shape of PEO chains.36 Unfortunately, they did not discuss the temperature dependence of mEO and dehydration behavior for short PEOs. The fact that the mEO value for PEOs in a low temperature region evaluated by dielectric techniques in this study is about twice as large as that obtained by ultrasonic velocity measurements suggests that the physical meanings of hydration (number) is slightly different between the two methods. Molecular dynamic simulation38 and Monte Carlo study39 also elucidated that mEO is close to 4 for 18C6. Furthermore, nearinfrared spectroscopic study40 revealed that the value of mEO is not so different from 4 for 18C6 in dilute aqueous solution. According to our previous study,16 an ethylene oxide unit of PEO chains has two (primary) hydration sites, in which water molecules directly hydrated an oxygen atom of the EO unit as schematically shown in Figure 8. Additional two water
obtained for the aqueous model cyclic compound solutions were slightly shorter than that in the PEO system. The reason for this discrepancy in the values of τ3 should be that the numbers of ethylene oxide units in the model cyclic compounds are too small for the ethylene oxide units to behave as usual segments in their small cyclic molecular shape. Since sufficiently long linear PEOs possess Kuhn length of ca. 0.8 nm in aqueous solution, the number of ethylene oxide monomeric units (if possessing a trans−trans−gauche conformation with length of ca. 0.28 nm on average as in crystalline sate) in the segment should be more than two.29,30 Alessi et al.31 have claimed that PEO chains do not possess a trans−trans−gauche conformation, but a random coil conformation in aqueous solution. In the latter case, the number of ethylene oxide units in the segment is more than the former case. On the other hand, in the case of model cyclic compounds, rates of overall free rotations of the molecules would be mainly observed due to their small sizes around the frequency near local or segmental motion of long linear PEOs. From the depression ratio in the dielectric relaxation strength of the first mode, ε1εW−1, the hydration number per ethylene oxide unit, mEO, for the model cyclic compounds in aqueous solution was evaluated via eq 2 as well as in the PEO system. Temperature, T, dependence of mEO for aqueous solution of model cyclic compounds is also plotted in Figure 4. The value of mEO for the model cyclic compounds did not show any ring size dependence. Moreover, T dependence of mEO for the model compounds completely coincided with that of PEOs. These observations suggest that the dependence of mEO on T found in Figure 4 is a characteristic feature of the ethylene oxide unit irrespective of the degree of polymerization and a difference in the shape of molecules, linear or cyclic chains. Temperature dependencies of relaxation times, τj (j = 1−3), for aqueous solutions of model cyclic compounds are also plotted in Figure 5. The τ1 data for the solutions agree well those for PEO system. Moreover, including the presence of a break point at T−1 ∼ 0.0032 K−1, T dependences of τ2 for both the systems agree well. Then, we may conclude that the relaxation mechanisms for the modes j = 1 and 2 assigned to the rotational relaxation of free water molecules and the exchange process of hydrated water molecules by bulk water are completely the same in both the PEO and model cyclic compound systems. In the case of the slowest relaxation mode j = 3, the values of τ3 for the model cyclic compounds are slightly dependent on the size of rings, whereas slopes of the plots indicating magnitudes of activation energies of the mode j = 3 seem to be close to each other, including PEO solutions. Comparison of Hydration Numbers. It has been known that adiabatic compressibility determination of aqueous solutions via ultrasonic velocity measurements also provides hydration numbers of solute molecules on the basis of assumption that the compressibility of hydrated portions on solute molecules dissolved in the tested solution is zero.32 Magazù32,33 investigated the temperature dependence of hydration number per PEO chain for several kinds of PEOs with molar masses from 200 to 3000 via ultrasonic velocity measurements. The value of mEO converted from his data for PEOs with higher molar masses was of 2.4 at 10 °C and gradually decreased to 2.0 at 80 °C. Although the value of 2.4 at 10 °C does not agree with 4 obtained in this study, the dehydration behavior with increasing temperature was confirmed. In the same work,32 Magazù also demonstrated that
Figure 8. Schematic depiction of a hydrated ethylene oxide unit of PEO chains at the hydration number of mEO = 4 in a temperature region lower than 30 °C.11
molecules secondly hydrate to the directly hydrated two water molecules to the EO unit. Taking account of this hydration idea, it is likely that the ultrasonic velocity measurements technique is less sensitive to the presence of the secondary hydrated water molecules. The binding energy of the secondary hydrated water molecules would be slightly lower than that of primary hydrated water molecules. Then, the secondary hydrated water molecules would dehydrate more easily than the primary ones at higher temperatures. The dependence of the relative dielectric strength of water molecules hydrated to model cyclic compounds to that of free water molecules, ε2(cEOmEOε1VW)−1, on temperature, T, is also plotted in Figure 6. Almost flat T dependencies were recognized in the relative dielectric strength as well as in the case of aqueous PEO solutions. Moreover, agreement between the relative dielectric strength data for PEOs and model cyclic 1960
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(8) Yu, L.; Chang, G. T.; Zhang, H.; Ding, J. D. J. Polym. Sci., Part A 2007, 45, 1122−1133. (9) Yu, L.; Chang, G. T.; Zhang, H.; Ding, J. D. Int. J. Pharm. 2008, 348, 95−106. (10) Zhang, H.; Yu, L.; Ding, J. Macromolecules 2008, 41, 6493− 6499. (11) Saeki, S.; Kuwahara, N.; Nakata, M.; Kaneko, M. Polymer 1976, 17, 685−689. (12) Bailey, F. E.; Koleske, J. V. Poly(Ethylene Oxide); Academic Press: New York, 1976. (13) Matsuyama, A.; Tanaka, F. Phys. Rev. Lett. 1990, 65, 341−344. (14) Okada, Y.; Tanaka, F. Macromolecules 2005, 38, 4465−4471. (15) Ono, Y.; Shikata, T. J. Am. Chem. Soc. 2006, 128, 10030−10031. (16) Shikata, T.; Takahashi, R.; Sakamoto, A. J. Phys. Chem. B 2006, 110, 8941−8945. (17) Shikata, T.; Sugimoto, N. Phys. Chem. Chem. Phys. 2011, 13, 46542−16547. (18) Shikata, T.; Sugimoto, N.; Sakai, U.; Watanabe, J. J. Phys. Chem. B 2012, 116, 12605−12613. (19) Satokawa, Y.; Shikata, T. Macromolecules 2008, 41, 2908−2913. (20) Shikata, T.; Takahashi, R.; Satokawa, Y. J. Phys. Chem. B 2007, 111, 12239−12247. (21) Tasaki, K. J. Am. Chem. Soc. 1996, 118, 8459−8469. (22) Borodin, O.; Bedrov, D.; Smith, G. D. Macromolecules 2002, 35, 2410−2412. (23) Deshmukh, S. A.; Sankaranarayanan, S. K. R. S.; Suthar, K.; Mancini, D. C. J. Phys. Chem. B 2012, 116, 2651−2663. (24) Böttcher, C. J. F.; Bordewijk, P. Theory of Electric Polarization, 2nd ed.; Elsevier: Amsterdam, 1978; Vol. 2. (25) Sato, T.; Sakai, H.; Sou, K.; Buchner, R.; Tsuchida, E. J. Phys. Chem. B 2007, 111, 1396−1401. (26) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953; Chapters 12 and 13. (27) Rouse, P. E. J. Chem. Phys. 1953, 21, 1272−1280. (28) Ferry, D. J. Viscoelastic Properties of Polymers, 3rd ed.; Wiley: New York, 1980; Chapter 9. (29) Kienberger, F.; Pastushenko, V. P.; Kada, G.; Gruber, H. I.; Riener, C.; Schindler, H.; Hinterdorfer, P. Single Mol. 2000, 1, 123− 128. (30) Oesterhelt, F.; Rief, M.; Gaub, H. E. New J. Phys. 1999, 1, 6.1− 6.11. (31) Alessi, M. L.; Norman, A. I.; Knowlton, S. E.; Ho, D. L.; Greer, S. C. Macromolecules 2005, 38, 9333−9340. (32) Magazù, S. J. Mol. Struct. 2000, 523, 47−59. (33) Faraone, A.; Magazù, S.; Maisano, G.; Migliardo, P.; Tettamanti, E. J. Chem. Phys. 1999, 110, 1801−1806. (34) Shikata, T.; Minakawa, A.; Okuyama, K. J. Phys. Chem. B 2009, 113, 14504−14512. (35) Shikata, T.; Yoshida, N.; Okuyama, K. J. Phys. Chem. Lett. 2010, 1, 412−416. (36) Miyazaki, Y.; Matsuura, H. Bull. Chem. Soc. Jpn. 1991, 64, 288− 290. (37) Matsuura, H.; Fukuhara, K. Bull. Chem. Soc. Jpn. 1986, 59, 763− 768. (38) Kowall, T.; Geiger, A. J. Phys. Chem. 1994, 98, 6216−6224. (39) Patil, K. J.; Pawar, R. B.; Gokavi, G. S. J. Mol. Liq. 1998, 75, 143−148. (40) Patil, K. J.; Pawar, R. B. J. Phys. Chem. B 1999, 103, 2256−2261. (41) Boucher, E. A.; Hines, P. M. J. Polym. Sci., Polym. Phys. Ed. 1978, 16, 501−511. (42) Briscoe, B.; Luckham, P.; Zhu, S. Macromolecules 1996, 29, 6208−6211. (43) Briscoe, B.; Luckham, P.; Zhu, L. Polymer 2000, 41, 5851−5860.
compounds strongly suggests that there is no differences in the mechanisms of mode j = 2, which has been assigned to the exchange process of hydrated water molecules by the free water ones, for PEOs and model compounds in aqueous solution. In the near future, we should develop a method to determine hydration numbers of solutes in aqueous solution under the presence of buffer agents and/or simple salts using the dielectric relaxation technique because clouding points and other physicochemical properties of aqueous solutions of some compounds highly depend on the pH value and the concentration of added salts.41−43 Hydration numbers of some solutes are possibly influenced by the presence of ionic species.
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CONCLUSIONS We carried out dielectric relaxation measurements up to 50 GHz over a temperature range from 10 to 70 °C on aqueous poly(ethylene oxide) solutions to determine the temperature dependence of a hydration number per ethylene oxide monomer unit (mEO). The obtained mEO was a constant value of ca. 4 in a temperature range lower than 30 °C and decreased rather gradually and approached to ca. 2 at 70 °C. Such a temperature-dependent hydration/dehydration behavior newly confirmed in this study should be the essential reason for the phase separation behavior observed at higher temperatures higher than 100 °C. Because the same hydration/dehydration behavior as observed in aqueous solutions of poly(ethylene oxide)s was also confirmed in aqueous solutions of crown ethers like 15crown-5 and 18-crown-6 composed from only ethylene oxide units in a circular shape, the hydration/dehydration features are characteristics of the ethylene oxide monomeric unit in spite of the degree of polymerization and the shape of molecules
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address †
Division of Natural Resources and Echo-materials, Graduate School of Agriculture, Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509, Japan. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS T.S. acknowledges DIC Corporation (Tokyo) for their kind financial support of this study.
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REFERENCES
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dx.doi.org/10.1021/ma3026282 | Macromolecules 2013, 46, 1956−1961
