Dehydration Behavior of Cellulose Ethers in Aqueous Solution

Jul 18, 2017 - Division of Natural Resources and Eco-materials, Graduate School of Agriculture Science, Tokyo University of Agriculture and Technology...
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Hydration/Dehydration Behavior of Cellulose Ethers in Aqueous Solution Kengo Arai† and Toshiyuki Shikata*,†,‡ †

Department of Symbiotic Science of Environment and Natural Resources, The United Graduate School of Agriculture, and ‡Division of Natural Resources and Eco-materials, Graduate School of Agriculture Science, Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509, Japan ABSTRACT: The hydration numbers of cellulose ethers dissolved into aqueous solution were determined using extremely high-frequency dielectric spectroscopic methods, up to 50 GHz, in which the frequency range of the major dielectric dispersion of free water molecules is clearly observable. Methyl cellulose, hydroxypropylmethyl cellulose, and hydroxyethylmethyl cellulose samples at relatively high degrees of substitution by methyl groups ranged from 1.4 to 1.9, which are an essential reason for water solubility, and additional low molar substitution by hydroxypropyl or hydroxyethyl groups from 0 to 0.25 bearing molar masses (Mw) ranging from 20 × 103 to 300 × 103 was examined in this study. All the samples dissolve well into cold water below room temperature, ca. 25 °C; however, their aqueous solutions show distinct cloud points at high temperatures, such as 38−53 °C, depending not so much on the Mw values but on the substitution condition substantially. The determined hydration numbers per glucopyranose unit (nH) for the cellulose ethers in aqueous solution were more than 12 at 10 °C and were slightly influenced by the substitution condition. The cellulose ethers showed remarkable dehydration behavior with increasing temperature; then, the nH values significantly decreased to the value of ca. 5 almost irrespective of the substitution condition and Mw in the vicinities of their lower critical solution temperatures (LCSTs) controlled by the substitution condition. These observations strongly suggest that the value of nH = 5 near each LCST is the minimum critical quantity for the cellulose ethers to be dissolved into water, and when the value of nH is less than the value at higher temperatures than cloud points, aqueous solutions of cellulose ethers become opaque.

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INTRODUCTION Cellulose, which is constantly generated by many kinds of plants every year, is the most abundant natural organic resource on the globe.1 Natural cellulose is insoluble in most of the usual solvents, including water, due to its highly developed inter- and intramolecular hydrogen bonding between hydroxy groups.2,3 This insolubility of cellulose is quite an important property for its application in the textile industry. However, this insolubility has impeded the application of cellulose to wider industrial applications as a nature-friendly eco-material. To solve such a serious problem, many kinds of chemically modified celluloses have been synthesized from natural cellulose.4 A series of chemically modified celluloses, such as water-soluble nonionic methyl, hydroxypropyl, and hydroxypropylmethyl cellulose ethers, anionic sodium carboxymethyl cellulose ether, and organic solvent-soluble ethyl cellulose ether and cellulose acetate, have been developed by several chemical companies.5,6 When one pays attention to the characteristics of water-soluble methyl and hydroxypropylmethyl cellulose ether samples in aqueous solution, one frequently encounters the complicated solution behavior that the solubilities of the cellulose ethers are highly dependent on temperature. Many synthesized cellulose ethers dissolve into water that is below room temperature, ca. 25 °C; however, in a certain concentration range, they lose solubility and sometimes become turbid gels at higher © XXXX American Chemical Society

temperatures that are markedly dependent on the substitution condition by hydroxypropyl, hydroxyethyl, methyl, and acetate groups.7−10 Shahin et al.11 have discussed temperature and concentration dependencies of the viscoelastic behavior of aqueous hydroxypropylmethyl cellulose solution and proposed the structure that is formed in the system. Arvidson et al.12 have reported the relationship between phase behavior and mechanical properties related to gel formation for aqueous solutions of methyl celluloses possessing various molar masses. Lott et al.13,14 have investigated the structure formed by methyl cellulose in aqueous solution and gel state at temperatures higher than the gel points, and they found a fibril structure in gels using cryogenic transmission electron microscopic techniques and small-angle neutron scattering methods. Moreover, McAllister et al.15,16 have determined the fibril structure formed by methyl cellulose in aqueous solution at temperatures higher than 40 °C during annealing processes for long periods, such as several weeks, and they developed thermodynamics of the formed fibrils and coiled methyl cellulose. Received: April 24, 2017 Revised: July 9, 2017

A

DOI: 10.1021/acs.macromol.7b00848 Macromolecules XXXX, XXX, XXX−XXX

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hydrated state.18−20 Dependencies of the hydration number on the substitution condition and molar mass are fully discussed. The newly determined critical hydration number, which is necessary for the cellulose ether samples to be dissolved into water, is related to the substitution condition. The cooperativity in the hydration/dehydration process is also discussed as a function of the substitution condition of the examined cellulose ethers.

The lowest temperature at which homogeneous clear solutions turn into turbid inhomogeneous dispersed systems, i.e., a phase transition, is thermodynamically defined as the lower critical solution temperature (LCST).17 Many scientists believe that the temperature dependence of the hydration number of solute molecules in aqueous solution should be a key parameter in controlling the fundamental physical characteristics related to the whole phase behavior. For both poly(oxyethylene) (POE) and poly(N-isopropylacrylamide) (PNIPAm), which are typical synthetic water-soluble polymers demonstrating the LCST type phase behavior in aqueous solution, the hydration number per monomeric unit has been determined as a function of temperature by using extremely high-frequency dielectric spectroscopic (DS) techniques.18−20 The obtained results have revealed that the temperature dependence of the hydration number is closely related to that of solubility for POE and PNIPAm.18−20 In the case of PNIPAm, the hydration number exhibits a plateau value in a lower temperature region than the room temperature and is followed by a drastic decrease just below the LCST of 32 °C.20 This temperature dependence is successfully explained by a theoretical model proposed by Matsuyama and Tanaka21 that assumes a high cooperativity in the hydration/dehydration process governed by solute molecular characteristics.20 Using temperature-modulated differential scanning calorimetric techniques, endothermic and exothermic processes were separately detected in aqueous PNIPAm solution near its cloud point.22 The measured heat in the endothermic process would be related to the hydration energy in aqueous NIPAm solution.22 On the other hand, the temperature dependence of the hydration number for POE in aqueous solution was much milder than that for PNIPAm, which could be explained by the theory assuming a lower cooperativity in the hydration/ dehydration process than that of PNIPAm.19−21 In the case of the LCST type phase transition behavior frequently observed in aqueous solutions of cellulose ether samples, the hydration number should also be a key parameter determining the temperature dependence of phase behavior uniquely. So far, there have been few experimentally determined and reported studies of the temperature dependence of hydration number of cellulose ethers. Koda et al.23 tried to measure the hydration number of methyl cellulose samples in aqueous solution as a function of temperature using sound velocity measuring techniques. They concluded that the value of the hydration number is reduced with increasing temperature and with the degree of substitution at the same temperature.23 However, they did not discuss the molar mass dependence of the hydration number and the critical hydration number that is necessary for the methyl cellulose sample to be dissolved into water. Such basic information and molecular parameters are essential to understand the whole physicochemical properties of the cellulose ether that possesses higher potential to be used in broader practical applications than they are currently used. In this study, we report the temperature dependence of the hydration numbers of methyl, hydroxypropylmethyl, and hydroxyethylmethyl cellulose samples bearing various substitution conditions and molar masses in aqueous solutions determined using extremely high-frequency DS techniques. Because the dielectric measurement is highly sensitive to a difference in the rates of rotational molecular motions of water molecules, it is able to detect how many water molecules are occupied in a special situation assigned to the hydrated state to solute molecules and is useful to determine the lifetime of the

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EXPERIMENTAL SECTION

Materials. A series of methyl, hydroxypropylmethyl, and hydroxyethylmethyl cellulose samples were kindly supplied by Shin-Etsu Chemical Co. Ltd. (Tokyo). These cellulose ether samples were coded as MC(m:Mw/103), HpMC(hp:m:Mw/103), and HeMC(he:m:Mw/ 103) with numerical quantities, m, hp, he, and Mw, which represent the degree of substitution of the three hydroxy groups per glucopyranose unit by methyl groups, the molar substitution by hydroxypropyl groups, the molar substitution by hydroxyethyl groups, and the weightaverage molar mass, respectively. The chemical structure of cellulose ethers investigated in this study is shown in Scheme 1 in a simple

Scheme 1. Chemical Structure of Cellulose Ethers Investigated in This Study

manner. Characteristics of all the cellulose ether samples examined in this study are summarized in Table 1. Since the hp and he values were less than 0.3, the molar fraction of substituted glucopyranose units by hydroxypropyl or hydroxyethyl groups was less than 0.3 on average. The molar mass distribution indexes given by a ratio of Mw to the number-average molar mass (Mn) were not so sharp and in a range from 2 to 3 in accordance with technical reports provided by the supplying company. Highly deionized water with the specific resistance higher than 18 MΩ·cm, generated by a Direct-Q UV3 (Merck Millipore, Darmstadt), was used as a solvent. The concentrations of cellulose ethers ranged from 0.03 to 0.5 M in glucopyranose units. Methods. Two measuring systems were used to determine the dielectric relaxation behaviors of aqueous solutions of the cellulose ether samples over a frequency range from 1 MHz to 50 GHz. A dielectric probe kit, 8507E, equipped with a network analyzer, N5230C, an ECal module N4693A, and performance probe 05 (Agilent Technologies, Santa Clara, CA), was used for DS measurements over a frequency range from 50 MHz to 50 GHz (3.14 × 108 −3.14 × 1011 s−1 in angular frequency (ω)). The probe, with a diameter of 9.7 mm, was inserted into a glass vial possessing an inner diameter of ca. 16 mm, and the distance from the probe edge to the bottom of the vial was maintained at greater than 3 mm. The volume of tested liquid sample was ca. 2 mL. The real and imaginary parts (ε′ and ε″) of the electric permittivity were automatically calculated from the reflection coefficients measured by the network analyzer via a program supplied by Agilent Technologies. A three-point calibration procedure using n-hexane, 3-pentanone, and water as the standard materials was performed prior to all of the dielectric measurements at each measuring temperature. The details of the three-point calibration procedure used in this study have been described elsewhere.18−20 B

DOI: 10.1021/acs.macromol.7b00848 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Characteristics of Cellulose Ethers Used in This Study sample codes

molar substitution by hydroxypropyl groups

molar substitution by hydroxyethyl groups

degree of substitution by methoxy groups

Mw/103

0 0.25 0.15 0.20 0

0 0 0 0 0.20

1.8 1.9 1.8 1.4 1.5

95, 150 20, 75, 300 75, 215 95 300

3

MC(1.8:Mw/10 ) HpMC(0.25:1.9:Mw/103) HpMC(0.15:1.8:Mw/103) HpMC(0.20:1.4:Mw/103) HeMC(0.20:1.5:Mw/103)

Figure 1. (a) Dielectric spectra (ε′ and ε″ vs ω) for aqueous solutions of HpMC(0.25:1.9:75) at c = 0.37 M and 20 °C. Broken lines represent constituent relaxation modes from j = 1 to 4. The residuals of ε′ − ε1′ − ε∞ and ε″ − ε1″ are also plotted in the same figure with small closed symbols. (b) A Cole−Cole plot for the data shown in (a). A solid semicircle represents single Debye-type presentation for the major dielectric dispersion observed at ω ∼ 1011 s−1. In the lower frequency range from 1 MHz to 3 GHz (ω = 6.28 × 106−1.88 × 1010 s−1), an RF LCR meter 4287A (Agilent) equipped with a homemade electrode cell with a vacant electric capacity of C0 = 0.23 pF was used, which consisted of center and outer electrodes made of gold-covered brass insulated by poly(tetrafluororoethylene). The electrode cell possesses inner and outer conductor diameters of 3.04 and 7.00 mm, respectively, which are the same as those of an impedance matching an APC7 connector with the characteristic resistance of 50 Ω. A short part of the APC7 was used as a counter electrode against the inner conductor electrode with a flat surface. The separation between inner and counter electrodes was 0.5 mm. Open (air), short (0 Ω, a gold plate with the thickness of 0.5 mm), and load (water) calibration procedures were performed prior to sample measurements at each measuring temperature. Dielectric measurements were performed at temperatures ranging from T = 10 °C to the cloud points of each sample solution with the accuracy of ±0.1 °C using a temperature controlling unit made of a Peltier device. The ε′ and ε″ values were calculated using the relationship ε′ = CC0−1 and ε″ = (G − GDC)(C0ω)−1, where C, G, and GDC represent measured electric capacitance, conductance, and direct current conductance of the sample liquid, respectively. The contribution of GDC to the ε″ value was adequately removed in a low ω range as described in the next section. Density measurements for all the sample solutions were performed using a digital density meter, DMA4500 (Anton Paar, Graz), to determine the partial molar volumes of the solute molecules at the same temperature as the performed dielectric measurements.

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4

ε′ =

∑ j=1

4

εj 2

1 + (ωτj)

+ ε∞ ,

ε″ =

∑ j=1

εjωτj 1 + (ωτj)2

(1)

where εj, τj, and ε∞ represent the relaxation strength and time for a mode j and the high-frequency limiting electric permittivity, respectively. The broken lines drawn in the figure represent the constituent Debye-type relaxation mode, εj′ and εj″ (j = 1 to 4 from the fastest relaxation mode due to the shortest relaxation time). To confirm the validity of decomposition into four Debye-type relaxation modes for the spectra, the residuals of ε′ − ε1′ − ε∞ and ε″ − ε1″ are also plotted in the same figure with small closed symbols, which clearly demonstrate the presence of modes j = 2, 3, and 4. The ε″ curve of pure water is identical to a (not shown) curve obtained from ε1″ by multiplying a factor of 1.20. The GDC values for all the measured samples were not so high, but lower than 500 μS. Because ε′ curves were less influenced by the presence of ionic impurities than ε″ curves in a low ω range, the dielectric parameters, εj and τj (j = 3 and 4), were reasonably evaluated from ε′ − ε1′ − ε∞ curves in a low ω range. Then, the GDC values were carefully determined for ε″ − ε1″ curves to precisely reproduce ω dependencies calculated by the evaluated εj and τj (j = 3 and 4) values. The accuracy for curve fit procedure using four relaxation modes in this study was estimated to be 1−2% in uncertainty for relaxation strength. Such a decomposition procedure for dielectric spectra held well in all the aqueous solutions of the examined cellulose ether samples. Various kinds of useful experimental methods have been developed to explore hydration behavior of solute molecules including large biomacromolecules like proteins and DNAs, such as extended depolarized light scattering (EDLS),24−27 nuclear magnetic resonance (NMR),28−30 neutron scattering

RESULTS AND DISCUSSION

Dielectric Behavior. Dielectric spectra (ε′ and ε″ vs ω) for the aqueous solution of HpMC(0.25:1.9:75) at c = 0.37 M and 25 °C are shown in Figure 1a as typical examples. The dependencies of ε′ and ε″ on ω were well described with the summation of four Debye-type relaxations as follows: C

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Figure 2. Concentration dependencies of dielectric relaxation times (a) and strength (b) for each mode observed in aqueous solution of HpMC(0.25:1.9:75) shown in Figure 1.

the rotational relaxation time, τw, of water molecules in the pure liquid state as shown in Figure 2a. Then, we can easily attribute the mode j = 1 to the rotation relaxation mode of free water molecules in the aqueous solution of HpMC(0.25:1.9:75). The value of ε1 substantially decreased with increasing concentration, c, as shown in Figure 2b. The presence of solute HpMC(0.25:1.9:75) molecules accompanying a certain volume fraction in the aqueous solution and the effect of hydration behavior substantially reduced the amount of free water molecules in the solution. In a later section, we will quantify the number of water molecules hydrated to a glucopyranose unit of the HpMC(0.25:1.9:75) molecules (hydration number, nH) from the c dependence of ε1. The second relaxation mode j = 2 possesses the relaxation time τ2 ∼ 2.3τ1 and the strength ε2, which is proportional to c as shown in Figure 2a,b. According to the previous reports for water-soluble materials,18−20,27,33−35 MD simulation studies,36−38 and dielectric and EDLS studies,27 the mode j = 2 is attributed to the exchange process for hydrated water molecules by free water molecules in the solution, and the relaxation time, τ2, would be related to the lifetime of the hydration state of HpMC(0.25:1.9:75) molecules. The ratio of τ2/τ1 has been called a retardation ratio (or slowdown factor). Although the original definition of the retardation ratio is τ2/τw,24−27,31 the evaluated value of τ2/τ1 is essentially identical to that of τ2/τw in this study. The reason for this fact is that the determined τ1 value was close to that of τw (cf. Figure 2a). Since the value of τ2/τ1 ∼ 2.3 seen in Figure 2a and also in other aqueous cellulose ether systems examined in this study well corresponds to that reported for aqueous solutions of glucose, fructose, sucrose, and maltose determined using EDLS and also DS techniques,27,33−35 the characteristics of hydration behavior observed for the cellulose ethers in aqueous solution are not so different from that for the mono- and disaccharide molecules. The fact that the value of nHc(εw/55.6), which is the calculated dielectric relaxation strength for hydrated water molecules assuming the same relaxation strength as free water molecules, is close to the value of ε2 strongly suggests the validity of this assignment. The minor third and fourth relaxation modes, j = 3 and 4, showed the relaxation times of τ3 ∼ 220 ps and τ4 ∼ 3 ns, respectively. The relaxation magnitudes of these modes, ε3 and ε4, are much smaller than ε2 but are proportional to c as ε2 (Figure 2a,b). Since HpMC(0.25:1.9:75) molecules bear a few kinds of polar groups bearing finite dipole moments, such as hydroxy, methoxy, and ether groups (−OH, −OCH3, and −CH2OCH2−), the rotational motions of these polar groups

(NS),31 time-resolved fluorescence decay,32 dielectric spectroscopy, DS,18−20,27,33−35 and molecular dynamics simulation (MD)36−38 techniques. The DS technique is one of the most useful methods for the purpose because it directly probe dynamics of water molecules sensitively.18−20,27,33−35 In the case of dielectric measurements carried out in an extremely high frequency range up to 50 GHz, it has been well-known that dielectric spectra of the collective rotational relaxation process for water molecules are well described with a single Debye-type mode at the relaxation time of τw = 9.0 ps (at T = 20 °C) in pure liquid water.39 Moreover, dielectric spectra for aqueous solutions of various kinds of solutes were perfectly described assuming the presence of additional Debye-type relaxation mode(s) assigned to the rotational process of hydrated water molecules (and that of dipoles attached to solute molecules) in aqueous systems (cf. eq 1).18−20,27,33−35 Then, we employed the summation of Debye-type relaxation modes given by eq 1 to describe the dielectric spectra obtained in this study. It has been well-known that the Cole−Cole40 and Cole−Davidson41 models are also useful to express dielectric behaviors showing nonsingle Debye-type dielectric spectra. Figure 1b represents a so-called Cole−Cole plot for the data shown in Figure 1a together with the best fit single Debye-type behavior represented by a solid semicircle for major dielectric dispersion observed at ω ∼ 1011 s−1. Small but non-negligible deviation from the sloid circle is recognized in data points in a lower frequency side, which corresponds to the additional minor contribution described by the modes j = 3 and 4 using eq 1. Because the agreement between the solid semicircle and data points in the major dispersion portion seems rather well, the Cole−Cole or Cole−Davidson model is not the best way to express such a slight nonsingle Debye-type behavior observed in this study. Then, we did not employ the models to analyze dielectric behavior observed in this study. However, it is worthy to note that the Cole−Davidson model is also quite useful to express the broad frequency dependence of susceptibility of aqueous solutions determined by spectroscopic methods other than the DS method to explore hydration behavior, such as EDLS24−27 and NS31 techniques. For example, in the procedure of NS data analysis, the Cole−Davidson model has been successfully employed as a useful tool to demonstrate the contribution of both translational and rotational diffusion modes of water molecules.31 The concentration dependencies of the dielectric relaxation times and strength for each mode observed in the aqueous solution of HpMC(0.25:1.9:75) are shown in Figures 2a and 2b. The fastest relaxation time, τ1, was identical to the value of D

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water molecule and a hydration site of HpMC(0.25:1.9:75) molecules are slightly greater than that between water molecules in pure liquid state. On the other hand, the slope of a dotted line for τ2 data above 30 °C (below 3.3 × 10−3 K−1) provides the activation energy of 12 kJ mol−1. This activation energy for the mode j = 2 lower than that for the mode j = 1 suggests that interaction energies between a hydrated water molecule and a hydration site become substantially lower than that between water molecules in pure liquid state above 30 °C. The reduction in the interaction energy between a hydrated water molecule and a hydration site found above 30 °C would be related to dehydration behavior discussed in a later section. Similar temperature dependencies for the τ1 and τ2 values were also obtained for other cellulose ether samples examined in this study. Temperature dependence of the retardation ratio, τ2/τ1, seems not so strong, and the τ2/τ1 value is kept at 2.3−2.5 in the temperature range examined. This retardation ratio reasonably agrees with the values previously reported for hydrated water molecules in aqueous solutions of mono- and disaccharides determined at room temperature using EDLS and DS techniques.27,33−35 Since the temperature dependence of the retardation ratio has not been reported for aqueous saccharide systems so far, there are no data to be compared with that of the τ2/τ1 value obtained in this study. Perticaroli et al.31 determined the retardation ratio of hydrated water molecules at two different temperatures, 7 and 30 °C, in aqueous solution of green fluorescent protein using NS techniques. They reported that the retardation ratio is not influenced by temperature, and the value is ranged from 3 to 7 depending on the scattering vector.31 In general, it is known that ordinary DS measurements used in this study do not provide information on the length scale dependence as (inelastic) NS techniques which accumulate data as functions of energy (or frequency) and momentum transfer (scattering vector) simultaneously. Moreover, it is concluded that the retardation ratio of ranged from 3 to 7 is consist with the value determined for hydration water molecules in aqueous systems of other proteins like lysozyme and constituent amino acid oligomers determined using various techniques such as EDLS,27 DS,18,27 NMR28,29 techniques, and MD simulations.37,38 It is likely that amide groups constructing oligo peptides and proteins possess substantially greater retardation ratios for hydrated water molecuels than ether and hydroxy groups forming polysaccharides. In the cases of DS and EDLS techniques, translational motions of water molecules do not cause relaxations but do rotational motions. Because NS data for pure liquid water revealed that the translational relaxation time of water molecules strongly depends on the scattering vector, whereas the rotational relaxation time for water molecules is kept at the same quantity, ca. 2.5 ps,31 irrespective of the scattering vector, the relaxation time determined using DS and EDLS techniques should be compared with the rotational relaxation time or the average relaxation time determined at high scattering vectors such as 15−20 nm−1 using the NS measurements. It should be noted that the rank of rotational dynamics is l = 1 for the DS measurements and l = 2 for the EDLS and NS ones.27,31 Then, the rotational relaxation time for water molecules determined by the DS method are 3 times as long as that by the EDLS and NS methods theoretically. However, the retardation ratios evaluated from the DS, EDLS, and NS techniques are able to be compared with one another directly.

attached to glucopyranose units would be the reason for the dielectric relaxation processes. The rotational relaxation time of a hydroxy group would be governed by the lifetime of intramolecular hydrogen bonding between two hydroxy groups, the hydroxy group and a methoxy group, and the hydroxy group and an ether group. The rotational relaxation time of a methoxy group is also controlled by the lifetime of intramolecular hydrogen bonding between the methoxy group and a hydroxy group. Moreover, native water-soluble dextran without methoxy groups shows a distinct mode j = 3 and a negligibly small mode j = 4. Consequently, the rotational motions of methoxy groups occur after the completion of the hydroxy groups that are hydrogen-bonded to them. Then, τ3 and τ4 would be assigned to the rotational relaxation times of hydroxy and methoxy groups, respectively. The motions of ether groups included in glucopyranose units and glycosidic bonds seem to be related to molecular motions of cellulose ether main chains that are much slower than the mode j = 4. If the cellulose ethers possess a characteristic configuration in aqueous solution such as a rodlike shape as proposed previously,42 the motions of the ether groups would be detectable in a similar way to the rotational relaxation mode of rodlike particles of cellulose ethers. The main aim of this study is a basic argument about the hydration/dehydration behavior of the cellulose ethers in aqueous solution. Then, we did not expand the frequency range of dielectric measurements lower than 1 MHz. Figure 3 shows dependencies of dielectric relaxation times, τ1 and τ2, and the retardation ratio, τ2/τ1, on the reciprocal

Figure 3. Temperature dependencies of dielectric relaxation times, τ1 and τ2, and the retardation ratio, τ2/τ1, for aqueous solution of HpMC(0.25:1.9:75) at c = 0.1 M.

temperature, T −1, for aqueous solution of HpMC(0.25:1.9:75) at c = 0.1 M as typical examples. The slope of τ1 data for the mode j = 1 in Figure 3 provides the activation energy evaluated to be 19 kJ mol−1. The fact that this value is the same as that of the rotational relaxation time, τw, of pure liquid water confirms the validity of the assignment for the mode j = 1. The data of τ2 in Figure 3 do not make a simple straight line as observed for the τ1 data but seem to alter their slopes gradually with increasing temperature. Here, we assume the presence of a break point in the slope of τ2 data at T = 30 °C to make argument simple. The slope of a solid line for τ2 data below 30 °C (above 3.3 × 10−3 K−1) drawn in Figure 3 provides the activation energy of 21 kJ mol−1. This activation energy for the mode j = 2, which is slightly greater than that for the mode j = 1, suggests that the interaction energy between a hydrated E

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HpMC(0.25:1.9:75) at 20 °C. The value of the partial molar volume, Vm, of the average repeating unit of HpMC(0.25:1.9:75) in aqueous solution at 20 °C was determined to be 151.8 cm3 mol−1 from density data and slightly increased with increasing temperature, T. The experimental ε1εw data points are far from a theoretically predicted line assuming no hydration effect, i.e., nH = 0, whereas the agreement between the ε1εw data and a line obtained assuming nH = 13 is perfect over the entire c range examined. Then, we conclude that the hydration number, nH, for HpMC(0.25:1.9:75) at 20 °C is 13. Such a procedure to determine the nH value was performed successfully from 10 °C to a temperature just below cloud points for all the aqueous solutions of cellulose ether samples that were examined. The temperature, T, dependence of nH for aqueous HpMC(0.25:1.9:75) solutions obtained at c values ranging from 0.11 to 0.37 M is shown in Figure 5a. Agreement between the nH values determined from sample solutions at different concentrations looks perfect at each temperature. Then, one might conclude that the nH value is precisely determined and has quite weak c dependence in the range examined. Moreover, nH maintains the value of ca. 14 at 10 °C and then markedly decreases with increasing temperature down to 5 at 53 °C. This pronounced dehydration behavior with increasing temperature corresponds well to the presence of a cloud point, which was observed at 53 °C irrespective of the c values, i.e., the LCST for aqueous ChMC(0.25:1.9:75) solution as shown in Figure 5a. Figure 5b shows the temperature, T, dependence of the hydration number, nH, for aqueous solutions of HpMC(0.25:1.9:Mw/103) with various molar masses, Mw. This figure reveals that a difference in the Mw values does not affect the nH data over the T range up to a cloud point (LCST) of 53 °C indicated with an arrow, which was also independent of the Mw values in the examined range. We might conclude that the nH is independent of the values of c and Mw in the ranges examined. The temperature dependence of nH in the dehydration behavior shown in Figure 5 is not as sharp as that observed in aqueous PNIPAm solution,20 but it is similar to the slow behavior observed in aqueous POE solution.19 Thus, the cooperativity in the hydration/dehydration process for cellulose ethers in aqueous solution is not as high as in the case of PNIPAm. Similar c and Mw dependencies of nH are observed also in aqueous solutions of other HpMC samples investigated in this study, bearing slightly different substitution conditions and similar LCST values, ca. 53 °C, as shown in a later section.

Hydration Number. The c dependence of ε1 allows us to determine the hydration number, nH, per glucopyranose unit using eq 2, as follows:18−20 ε1 1 − 10−3Vmc = − 10−3VwcnH εW 1 + 10−3Vmc /2

(2)

where Vm and Vw represent the partial molar volumes of a glucopyranose unit and water molecule, respectively, at each measured temperature in the units of cm3 mol−1. The value of Vm was calculated from the density data of examined aqueous sample solution, and precisely determined values for Vw are available in the literature. The first term of eq 2 represents the contribution of the volume excluded by the presence of solute (organic) molecules, which have much lower electric permittivities, ca. 2−3, than that of water molecules, εw, to the relaxation strength of free water in the tested aqueous solution.39 The second term represents the contribution of the hydration effect to the solute molecules. Because the hydrated water molecules reside in hydration sites on solute molecules for a while, the hydration lifetime and the rates of molecular motions for hydrated water molecules are substantially reduced. Then, the dielectric relaxation strength of hydrated water molecules should be subtracted in the manner of the second term of eq 2. Figure 4 shows a typical experimental relationship between the values of ε 1 ε w and c for aqueous solutions of

Figure 4. Relationship between the values of ε1εw and the concentration, c, for aqueous solutions of HpMC(0.25:1.9:75) at 20 °C.

Figure 5. (a) Temperature, T, dependence of nH for aqueous HpMC(0.25:1.9:75) solutions at several c values ranged from 0.11 to 0.37 M. The arrow in this figure represents the lowest cloud point observed in a high concentration side, i.e., LCST. (b) T dependence of nH for aqueous solutions of HpMC(0.25:1.9:Mw/103) with various molar masses, Mw. The LCST value shown with an arrow does not depend on Mw. F

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Macromolecules However, it has been reported that in many cases for synthetic water-soluble polymers the LSCT value substantially depends on the molar mass when the molar mass is lower than a certain critical value, depending on the molecular structure of the tested polymers.43,44 Distinct molar mass and concentration dependencies of the LCST values were also reported in aqueous solution of elastin-like polyptide.45 The dependence of the nH value on temperature, T, shown in Figure 6 demonstrates the hydration/dehydration behavior for

Figure 7. Dependencies of the nH values on T for each cellulose ether sample examined in this study. Solid arrows in this figure represent the LCST values for each cellulose ether sample in aqueous solution. A dotted arrow represents a gelation point without turning into the turbid state for aqueous HeMC(0.2:1.5:300) solution at 0.1 M.

ypropyl groups effectively increases the LCST values. Although the substitution of hydroxy groups of cellulose by hydroxypropyl groups always reduces the number of native hydroxy groups (strongly) forming intra- and intermolecular hydrogen bonds, the total number of hydroxy groups is always kept at the original value. Then, the hydrophilicity or solubility of the cellulose ethers would be effectively increased by an additional substitution by hydroxypropyl groups in the examined range. However, the nH values observed just below the LCST values are found to be ca. 5, irrespective of the molar substitution up to 0.25 by hydroxypropyl groups for each HpMC sample. Consequently, one might conclude that the nH value of ca. 5 is the critical hydration number for the MC and HpMC samples examined in this study that is necessary to be dissolved into water. The LCST value for the aqueous solution of HpMC(0.2:1.4:95) was 53 °C; however, HeMC(0.2:1.4:300), which bears the identical degree of substitution and molar substitution, did not show a cloud point in aqueous solution at c = 0.1 M, but the solution turned into a transparent gel above 66 °C as shown by a dotted arrow in Figure 7. Aqueous solutions of HeMC(0.2:1.4:300) at concentrations higher than 0.1 M showed pronounced viscoelastic behaviors in a low temperature range and turned into transparent strong gels also approximately 66 °C without showing cloud behavior. This observation strongly suggests that the substitution by hydroxyethyl (−CH2CH2OH) groups has a higher ability to expand the temperature range of water solubility much wider than that of hydroxypropyl (−CH2CH(OH)CH3) groups. Moreover, because the nH value for HeMC(0.2:1.4:300) is markedly higher than that of HpMC(0.2:1.4:95) in a temperature range higher than 30 °C, the essential reason for the higher ability to provide the wider water-soluble range given by hydroxyethyl groups is keeping the hydration number, nH, higher. The presence of a methyl group attached to the substitutive group seems to reduce the hydration number, nH, effectively. However, the nH value observed just below the LCST for aqueous solutions of HpMC(0.2:1.4:95) was ca. 5, and the nH value for HeMC(0.2:1.4:300) below the gelation point was ca. 6. These quantities were not so much influenced by a difference in the substitution groups. Consequently, it is likely that the quantity of ca. 5 is the critical nH value for series of cellulose ether samples examined in this study, which is necessary for the samples to dissolve into water.

Figure 6. Dependencies of the nH values on T for aqueous solutions of MC(1.8:95) at 0.32 M and that of MC(1.8:150) at 0.16 M. A solid arrow in this figure represents the LCST value for MC(1.8:95) and MC(1.8:150) independent of Mw values, and a broken arrow represents a cloud point temperature for MC(1.8:150) observed at a dilute condition of c = 0.16 M.

aqueous solutions of MC(1.8:95) and MC(1.8:150). Because differences between the nH data for the two MC samples at each temperature were negligibly small, essentially, the T dependence of nH is not affected by a difference in the Mw values over the range examined as in the case of HpMC samples described above. The value of a cloud point temperature was more strongly dependent on the concentration in the case of the aqueous solutions of the MC samples than that of HpMC samples, as previously reported by Arvinson et al.12 A broken arrow in Figure 6 indicates the cloud point for the aqueous MC(1.8:150) solution at a dilute condition of c = 0.16 M. With increasing c value, the cloud point for aqueous MC(1.8:150) solution decreased and approached the LCST at 38 °C, as shown by a solid arrow in the condition of c > 0.3 M. Since this quantity is identical to the LCST for aqueous MC(1.8:95) solution, the LSCT is independent of the Mw values for the MC samples examined in this study. It seems that the nH value just below the LCST is close to 5, as in the case of the aqueous solution of HpMC(0.25:1.9:Mw/103) samples shown in Figure 5. Figure 7 demonstrates the T dependence of the nH data for all the samples in the aqueous solution examined in this study, including that for HpMC(0.25:1.9:Mw/103) and MC(1.8:Mw/ 103) shown in Figures 5 and 6, respectively. The data shown in this figure are averages determined from data obtained at a variety of concentrations. As the molar substitution by hydroxypropyl groups is increased slightly from 0 to 0.25 for cellulose ether samples at similar degrees of substitution by methyl groups such as (1.4) 1.8−1.9 and similar molar masses as MC(1.8:95), HpMC(0.15:1.8:75), HpMC(0.20:1.4:95), and HpMC(0.25:1.9:75), the LCST value increases substantially from 38 up to 53 °C (by 15 °C). These observations suggest that an additional small amount of substitution by hydroxG

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hydroxyethyl groups in increasing the hydration number is stronger than that of the hydroxypropyl group. The critical hydration number for these cellulose ether samples necessary to obtain solubility into water are evaluated to be ca. 5 because the hydration numbers observed just below their lower critical solution temperatures are close to 5 irrespective of molar masses and substitution conditions for the cellulose ether samples examined in this study.

Because the structure of cellulose ether samples dissolved into water has not been precisely determined, their surface areas have not been evaluated so far. Then, we are not able to estimate the number of hydration shells at present. However, we speculate that there are at least two hydration layers on the surface of cellulose ethers as discussed previously in aqueous systems of poly(oxyethylene) (POE).19 In the first hydration layer, hydrated water molecules form hydrogen bonds to hydroxy and/or ether groups of cellulose ethers directly. Hydrated water molecules in the second hydration layer form hydrogen bonds to hydrated water molecules in the first hydration layer. Dehydration behavior observed in this study with increasing temperature mainly would occur in hydrated water molecules belonging to the second hydration layer. When one compares the nH values observed at 10 °C for the cellulose ethers examined in this study, the values increase in the order of the summation of the degree of substitution by methyl groups and the molar substitution by hydroxypropyl or hydroxyethyl groups, i.e., the total substitution. This observation reveals that the total substitution controls the characteristic hydration number to the fully hydrated state, which is reached by lowering the temperature down to 10 °C, irrespective of the kinds of substitution groups. It is well-known that commercially available hydroxyethyl cellulose (HeC) bearing molar substitution higher than 1.7 demonstrates much higher solubility into water than MC, HpMC, and HeMC samples, which have the degrees of substitution by methyl groups higher than 1.4 responsible for high water solubility as investigated in this study and no cloud (and gel) points in a temperature range lower than the boiling point.9,46,47 However, hydroxypropyl cellulose (HpC) molecules bearing a value of molar substitution higher than 2.7 demonstrate water solubility that is not so different from MC, HpMC, and HeMC samples examined in this study and exhibit clear cloud points at temperatures lower than 48 °C in aqueous solutions depending on the substitution conditions.48 These facts mean that the presence of a methyl group in the substituting groups is a decisive factor in the solubility into water. A project is now in progress to explore the reason for a difference in solubility into water and the presence of cloud points between HeC and HpC on the basis of experimental hydration numbers, nH, obtained by using extremely highfrequency DS techniques.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.S.). ORCID

Toshiyuki Shikata: 0000-0001-6846-4985 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI Grantin-Aid for Scientific Research (B) Number 26288055. All the cellulose ether samples were kindly supplied by Shin-Etsu Chemical Co. Ltd. (Tokyo), and this work was partially supported by the same company.

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REFERENCES

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CONCLUDING REMARKS Commercially available methyl cellulose, hydroxypropyl cellulose, and hydroxyethylmethyl cellulose samples that have degrees of substitution by methyl groups higher than 1.4 are responsible for high water solubility, and additional small amounts of molar substitution by hydroxypropyl or hydroxyethyl groups less than 0.25 exhibit remarkable dehydration behavior in aqueous solutions with increasing temperature. Hydration numbers per glucopyranose unit for these cellulose ether samples in aqueous solutions were precisely determined using high-frequency DS techniques as functions of temperature below their critical lower solution temperatures. The characteristic hydration numbers of these cellulose ether samples on the low-temperature side are governed by the total quantities of the degree of substitution by methyl groups and the molar substitution by hydroxypropyl or hydroxyethyl groups. Additional substitution by hydroxypropyl or hydroxyethyl groups is quite effective to extend solubility into water, i.e., to increase the cloud point temperature. The ability of the H

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DOI: 10.1021/acs.macromol.7b00848 Macromolecules XXXX, XXX, XXX−XXX