Reason for the High Solubility of Chemically Modified Poly(vinyl alcohol)

Feb 23, 2015 - monomer (mPVA) shows much higher solubility in water than normal homo poly(vinyl alcohol) (PVA). To clarify the reason for the high sol...
1 downloads 0 Views 999KB Size
Download PDF
Article pubs.acs.org/Macromolecules

Reason for the High Solubility of Chemically Modified Poly(vinyl alcohol)s in Aqueous Solution Kengo Arai,† Misumi Okuzono,‡ and Toshiyuki Shikata*,† †

Division of Natural Resources and Eco-materials, Graduate School of Agriculture, Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509, Japan ‡ Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan ABSTRACT: Partially chemically modified poly(vinyl alcohol) synthesized via random introduction of a diol-type monomer (mPVA) shows much higher solubility in water than normal homo poly(vinyl alcohol) (PVA). To clarify the reason for the high solubility of mPVA, we investigated the temperature dependencies of the hydration numbers per hydroxy group (mOH) for mPVA at various diol-type monomer contents from x = 0 (PVA) to 0.12 using extremely high frequency dielectric relaxation measurements up to 50 GHz over a temperature range from 10 to 70 °C. The values of mOH for mPVA in the temperature range below 20 °C looked constant and increased from 2.4 to 3.4 with increasing x. Above 20 °C, the values of mOH decreased with increasing temperature and approached a constant value close to unity above 70 °C irrespective of x. Although the random introduction of the diol-type monomer into the mPVA backbones effectively increased the mOH values, broad dehydration was commonly observed at approximately 40 °C irrespective of the x value. It is likely that the presence of the diol-type monomers in the polymer backbone considerably disturbs the highly developed sequential intramolecular hydrogen bonding between adjacent hydroxy groups in the normal homo PVA.

■

INTRODUCTION Poly(vinyl alcohol) (PVA) is one of the most famous synthetic water-soluble polymers and has been widely used in many applications due to its highly safe characteristics, including its biodegradable properties.1−3 However, normal homo PVA has some serious problems, which should be improved for much wider practical applications. The most serious problem facing PVA is the low solubility of PVA in water. Temperatures above 90 °C and a long intensive mixing process are needed to dissolve PVA in water at the common pressure condition of 0.1 MPa even at low concentrations.4,5 The gelation tendency of aqueous PVA solutions at moderate to high concentrations is another serious problem for the long-term stability of aqueous PVA systems. In contrast, the gelation phenomenon of PVA is a significant advantage for practical applications.5,6 Moreover, an uncontrollable high degree of crystallinity in concentrated aqueous solution and in the bulk state is another serious problem in normal homo PVA for some practical applications that require perfectly transparent PVA films.4,5,7 Recently, the Nippon Synthetic Chemical Industry Co., Ltd. (Osaka) developed and supplied a series of partially chemically modified PVA by randomly introducing a diol-type monomer, 1-butene3,4-diol, at various mole fractions (x) to precisely control the sequential connection of 1,3-diol-type structures, which has the molecular structure shown in Scheme 1. The chemically modified PVA has demonstrated remarkably improved © XXXX American Chemical Society

Scheme 1. Molecular Structure of mPVA

physicochemical properties, avoiding most of the serious problems facing normal homo PVA in practical applications, such as those listed above, and has been accepted as one of the most improved alternative materials for PVA.8 For example, no special efforts are required to dissolve the chemically modified PVA in water even at room temperature; powder of the chemically modified PVA is simply dropped into water with gentle stirring even at relatively high concentrations. However, the reason that the chemically modified PVA possesses this high solubility in water is not yet fully understood from a physicochemical point of view. Received: December 27, 2014 Revised: February 12, 2015

A

DOI: 10.1021/ma502602r Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules To understand the solubility characteristics of polymeric materials in aqueous media, the determination of hydration numbers for the materials as functions of temperature is important, indispensable information. The hydration number per monomer (or hydroxy group in this study (mOH)) for normal homo PVA was determined to be mOH = 2−2.2 at room temperature, ca. 25 °C, in our previous study.9 This small mOH value observed for PVA strongly reveals the development of intramolecular hydrogen bond formation between sequential monomers. However, isolated hydroxy groups, which never form an intramolecular hydrogen bond because of the long distance between two hydroxy groups due to the molecular structure, such as the hydroxy groups in 1,4-cyclohexanediol, provide a value of mOH = 5 in the temperature range below 20 °C.10 The value of mOH remarkably decreases with increasing temperature and reaches 3 in the temperature range above 60 °C.10 In contrast, hydroxy groups that perfectly form intramolecular hydrogen bonds, such as the six hydroxy groups of myo-inositol, possess only the small value of mOH ∼ 1 independent of temperature.10 These observations provide strong evidence for the reason why the mOH value of hydroxy groups with perfect intramolecular hydrogen bonding is definitely smaller than that of isolated hydroxy groups. Based on the discussion of the temperature dependencies of hydration numbers, the hydration number per hydroxy group for the chemically modified PVA should be remarkably different from that of the normal homo PVA after chemical modification via random introduction of diol-type monomers. Recently, our group developed a technique to determine the hydration numbers of solute molecules dissolved in water using dielectric relaxation measurements performed at extremely high frequency ranges of up to tens of gigahertz.9−12 Because the relaxation strength of free water molecules in solution is precisely evaluable in this high frequency range, the amount of water molecules hydrated to solute molecules is able to be determined. In this study, the dielectric relaxation technique for the determination of the hydration number for solute molecules was used over a wide temperature range from 10 to 70 °C. Then, the temperature dependence of the mOH for some test species as the chemically modified PVA was determined precisely and was compared with those for isolated hydroxy and perfectly intramolecular hydrogen bond forming hydroxy groups. Then, the reason for the high solubility of the chemically modified PVA was quantitatively discussed. Additionally, the origin of the temperature dependence of the mOH values for the chemically modified PVA was considered based on the degree of intramolecular hydrogen bond formation on the backbone of the chemically modified PVA.

■

Table 1. Characteristics of the MPVA Samples Examined in This Study code

xa

DSb

DPc

mPVA470-6 mPVA470-12 mPVA1200-3 mPVA1200-6 mPVA1200-8 PVA1100d

0.06 0.12 0.03 0.06 0.08 0.00

0.99 0.99 0.99 0.99 0.99 0.99

470 470 1200 1200 1200 1100

Content of diol-type monomer (x = n(n + k)−1, cf. Scheme 1), provided by the supplying company. bDegree of saponification, provided by the supplying company. cDegree of polymerization (n + k, determined via inherent viscosity measurements), provided by the supplying company. dHomo poly(vinyl alcohol) sample. a

The concentration, c, of the sample solutions ranged from 0.5 to 2.0 M in average monomeric units for PVA and mPVA. Methods. Two systems were used to measure the dielectric relaxation behaviors of aqueous solutions of sample polymers over a frequency range from 1 MHz to 50 GHz. A dielectric probe kit 8507E equipped with a network analyzer N5230C, ECal module N4693A, and a performance probe 05 (Agilent Technologies, Santa Clara, CA) was used for dielectric relaxation measurements in the high frequency range from 50 MHz to 50 GHz (3.14 × 108 to 3.14 × 1011 s−1 in angular frequency (ω)). A three-point calibration procedure using hexane, 3-pentanone, and water as the standard materials was performed prior to dielectric measurements at each temperature. Details for the three-point calibration procedure used in this study have been described elsewhere.13,14 In this system, the real and imaginary parts (ε′ and ε″) of the electric permittivity were calculated automatically using the software installed in the measuring system. In contrast, in the lower frequency range from 1 MHz to 3 GHz (ω = 6.28 × 106 to 1.88 × 1010 s−1), an RF LCR meter 4287A (Agilent) equipped with a homemade electrode cell with a vacant electric capacity of 0.23 pF was used. The sample temperature, T, was altered over the range from 10 to 70 °C with an accuracy of ±0.1 °C using a temperature controlling unit composed of a Peltier device. Density measurements for all sample solutions were performed using a digital density meter DMA4500 (Anton Paar, Graz) to determine the (average) partial molar volumes of the mPVA and PVA samples at each temperature where the dielectric relaxation measurements were performed.

■

RESULTS AND DISCUSSION Dielectric Behavior. Dielectric spectra (frequency, ω, dependencies of real and imaginary parts (ε′ and ε″) of electric permittivity) for an aqueous solution of mPVA1200-6 at c = 1.87 M in average monomer units (8.00 wt %) and T = 25 °C are shown in Figure 1 as typical examples of the resulting spectra for mPVA in aqueous solutions. Although only the dielectric dispersion at ω ∼ 1011 s−1 is large, as shown in Figure 1, the spectra are perfectly decomposed into three dielectric relaxation modes described by the Debye-type relaxation modes given by eq 1

EXPERIMENTAL SECTION

Materials. A series of chemically modified PVA samples (mPVA) with various degrees of polymerization (DP) and mole fractions of diol-type monomers (x = n(n + k)−1, cf. Scheme 1) with a degree of saponification (DS) close to unity were kindly synthesized and supplied by the Nippon Synthetic Chemical Industries Co. Ltd. (Osaka) as test species for basic research. The molecular characteristics of the used mPVA are summarized in Table 1. A PVA sample (PVA1100) was kindly supplied by the same company. The molecular characteristics of PVA1100 are also summarized in Table 1. The supplied polymeric samples were used without any further purification. Highly deionized water with specific resistance higher than 18 MΩ cm, obtained using a Direct-Q 3UV system (Merck-Millipore, Darmstadt), was used as a solvent for sample preparation.

3

ε′ =

∑ j=1

εj 1+

ω 2τj 2

3

+ ε∞ ,

ε″ =

∑ j=1

εjωτj 1 + ω 2τj 2

(1)

where τj and εj are the dielectric relaxation time and strength of mode j (= 1, 2, and 3 from the shortest relaxation time). This decomposition procedure into the three types of relaxation modes agreed well for all the spectra obtained in this study, including the aqueous PVA solution. The dependencies of τj and εj on the concentration, c, of mPVA1200-6 at T = 25 °C are shown in Figures 2a and 2b, B

DOI: 10.1021/ma502602r Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

mPVA1200-6, and these modes have been attributed to the exchange process of hydrated water molecules to solute molecules.9−12,15 Additionally, 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.16−18 Recent depolarized Raman scattering experiments have also clearly revealed that a relaxation mode observed at a frequency slightly lower than that of free water molecules is attributed to an exchange mode of water molecules hydrated to solute molecules with free water molecules.19 The slowest relaxation mode with a relaxation time of τ3 ∼ 500 ps was attributed to the local relaxation modes of polymer chains of mPVA1200-6. Because a hydroxy group (−OH) possesses a finite dipole moment, a rotational mode of the hydroxy groups is dielectrically active in both the PVA and mPVA. The rotation of hydroxy groups attached to polymer chains would be allowed when intramolecular hydrogen bonds disappear due to the lifetime. Then, the local or segmental motions of polymer chains, e.g., motions of methylene and methine groups (−CH2− and −CH−), would be possible in a time region longer than the lifetime of the intramolecular hydrogen bonds. According to the determined longitudinal relaxation time, T1 ∼ 0.4 s, of 13C nuclear magnetic resonance (NMR) measurements performed in aqueous (D2O) PVA solutions, the average rotational correlation time (τc) of a methine group, −CH2−, was evaluated to be ca. 200 ps at T = 25 °C.9 This τc value can be converted to a characteristic time constant (τd) with the same rank of 1 as the dielectric relaxation time by multiplying by 3. The calculated value of τd ∼ 600 ps agrees with τ3.9 These considerations support the validity of the assignment for the slowest mode to local or segmental motions of polymer chains of PVA and mPVA. Temperature Dependence of the Hydration Numbers. The decrease in the ε1 value clearly observed in Figure 2b is explained by two factors: a volumetric effect of the solutes and a hydration effect, as described elsewhere in detail.9−12 The amount that the ε1 value is depressed by the presence of mPVA is quantitatively described by eq 29−12

Figure 1. Frequency, ω, dependence of the real and imaginary parts of the electric permittivity, ε′ (open circles) and ε″ (closed squares), for aqueous solutions of mPVA1200-6 at a concentration of c = 1.87 M in average monomer units (8.0 wt %) and 20 °C. The broken lines in this figure indicate the constituent Debye-type relaxation functions calculated using eq 1 that describe the experimental ε′ and ε″, which are shown as solid lines.

respectively. All the relaxation times seem to be independent of c, whereas the magnitudes of the relaxation strength of each mode, εj, look almost proportional to the c value. These observations reveal that the contribution of the contact and entanglement between polymer chains is less effective in the c range examined. Because the value of the shortest relaxation time, τ1, was identical to the dielectric relaxation time, τw (= 9.5 ps), of water molecules in the pure liquid state at 20 °C, the mode j = 1 is simply assigned to the rotational mode of free water molecules in the aqueous solution. The depression in ε1 approximately proportional to c, as shown in Figure 2b, is related to the evaluation of the hydration number described in a later section. The second relaxation mode j = 2 shows a relaxation time of τ2 ∼ 27 ps, which is assigned to an exchange process of hydrated water molecules in the polymer chain of mPVA1200-6 with free water molecules. The validity of this assignment is supported by the fact that the relaxation strength, ε2, is proportional to c. The number of hydrated water molecules, which are responsible for the relaxation strength, ε2, should be proportional to c, as observed in Figure 2b. The dielectric relaxation strength per water molecule hydrated to a solute molecule, which is calculated to be 1.26 × 103 cm3 mol−1 assuming that the second relaxation mode is the exchange process of hydrated water molecules, agreed with the value per water molecule of 1.35 × 103 cm3 mol−1 in the pure liquid state. Many water-soluble substances, even polymeric or low molar mass compounds, show relaxation modes with characteristic times of 20−30 ps at T = 25 °C in aqueous solution, similar to

1 − 10−3Vmonc ε1 = − 10−3Vwcmmon 10−3Vmonc εW 1+ 2

(2)

where εW is the dielectric relaxation strength of pure water, Vmon and Vw are the average partial molar volume of a monomer unit and that of water molecules, and mmon is the hydration number per average monomer unit. The first term of eq 2 represents the contribution of the volumetric effect of the

Figure 2. Concentration, c, dependencies of the relaxation times, τj (a), and strengths, εj (b), of the constituent Debye-type modes for aqueous solutions of mPVA1200-6 at T = 20 °C. C

DOI: 10.1021/ma502602r Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

temperature dependence of mOH does not depend on DP but only on the x value. The temperature dependencies of the mOH values determined in the range of c < 2.0 M are shown in Figure 4b for the mPVA samples with different x values together with the data of PVA1100. For all the mPVA samples, the mOH values seem to be constant values in the temperature range below 20 °C, whereas they decreased substantially with increasing temperature, i.e., dehydration behavior, and reached other constant values in the temperature range above 60 °C, as observed in Figure 4b. It is likely that this hydration/dehydration behavior commonly observed at approximately 40 °C is independent of the DP values and also the diol-type monomer content, x. However, the constant values obtained in the low- and hightemperature regions (mOHL and mOHH) and the difference between the two constants (ΔmOH = mOHL − mOHH) depend considerably on x. The mOH value for normal homo PVA was measured at 25 °C in our previous study.9 The dependence of mOH on T determined in this study reasonably coincided with the value at this temperature reported previously. Figure 5 shows the dependencies of the mOHL determined at 10 °C and mOHH at 70 °C on the diol-type monomer content,

solute, i.e., mPVA, without the hydration effect, and the second one represents the hydration effect of the solute. When the solute molecules exist in aqueous solution at a volume fraction given by 10−3Vmonc, the depression of ε1 relative to εw is approximately given by the first term of eq 2.9−12 Because organic solute molecules like mPVA have much lower dielectric constants than that of water, ca. 80, the presence of solutes is approximated to be the vacuum possessing the same volume fraction; then the first term of eq 2 is obtained. If there exist hydrated water molecules to solutes in aqueous solution, which no longer behave as free water molecules in the bulk state possessing the relaxation time of τw, the dielectric contribution of the hydrated water molecules at the concentration of cmmon should be subtracted as the second term in eq 2. Figure 3 demonstrates the concentration, c, dependence of the depression ratio, ε1εW−1, for aqueous mPVA1200-6

Figure 3. Dependence of the depression ratio of the relaxation strength of mode j = 1 relative to that of pure water, ε1εW−1, on c for aqueous mPVA1200-6 solutions at T = 20 °C.

solutions at T = 20 °C as a typical example. If there is no hydration effect, the data should obey a line calculated via eq 2 assuming that mmon = 0. However, the resulting data followed the calculated line assuming that mmon = 3 irrespective of c. Consequently, we can conclude the relationship mmon ∼ 3 at that temperature. According to the same procedure, the values of mmon were determined as functions of temperature ranging from 10 to 70 °C for all the mPVA and PVA samples examined. The values of mmon were easily converted to hydration numbers per hydroxy group (mOH = mmon(1 + x)−1) for each sample. The temperature dependencies of the mOH values for two mPVA samples, mPVA1200-6 and mPVA470-6, with an identical x value of 0.06 but different DP values determined in the range of c < 2.0 M are shown in Figure 4a. Because the agreement between the mOH values for the two mPVA samples is perfect, as shown in this figure, it might be concluded that the

Figure 5. Dependencies of mOHL (at 10 °C) and mOHH (at 70 °C) on the diol-type monomer content, x, in mPVA. A broken line implies the limiting value of mOHH, which is approached at temperatures sufficiently above 70 °C.

x, for mPVA and also shows the values of normal homo PVA, i.e., x = 0. Although we could not perform dielectric relaxation experiments above 70 °C, the mOH values for mPVA samples with x = 0.06−0.12 seem to approach unity at temperatures higher than 70 °C. Because the essential difference between the hydration behaviors of mPVA and PVA is the value of mOHL, this difference controls the solubility behaviors of the polymers in water. At higher mOHL values, the solubility in water is higher. Moreover, the x value clearly governs the mOHL value of mPVA,

Figure 4. (a) Dependence of the hydration number per hydroxy group, mOH, on temperature, T, for mPVA470-6 and mPVA1200-6 with the same x value of 0.06 in aqueous solution. (b) Dependence of mOH on T for aqueous solutions of mPVA. The solid curves are guides for the eyes. D

DOI: 10.1021/ma502602r Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 6. Schematic depiction of the differences in the hydration state of PVA and mPVA with diol-type monomers in aqueous solutions. The light blue and purple broken lines indicate hydrogen bonds formed intramolecularly and between a hydrated water molecule and OH group of the polymer, respectively.

Consequently, a diol-type monomer, 1-butene-3,4-diol, used in mPVA is not the only monomer that can improve the solubility of PVA, but it is one of the most useful monomers that can simultaneously improve other problems in many practical applications despite the low solubility in water: a strong tendency of gelation of aqueous solution and a high degree of crystallinity. To claim that our interpretation for the improved water solubility of PVA by random introducing of the diol-type monomer disturbing hydrogen bond formation is not a particular case, but a general one for poorly water-soluble polymers due to inter- and/or intramolecular hydrogen bond formation between hydroxy groups, we state examples of improved water solubility of cellulose realized by several chemical modification techniques. As is well-known, natural cellulose is insoluble in water due to strong inter- and/or intramolecular hydrogen bond formation between hydroxy groups attached to repeating β-D-glucose rings. However, chemical modification to produce hydroxypropyl cellulose, in which some hydroxy groups of β-D-glucose rings are converted to be hydroxypropoxy groups, considerably increases water solubility of cellulose.20 Moreover, acetylation of cellulose, in which some hydroxy groups are converted to be acetate groups, highly increases water solubility of cellulose at a certain degree of acetylation.20,21 Furthermore, methylation of cellulose is well-known as a simple method to increase the water solubility of cellulose.20 In these chemical processes on cellulose, random inserting of hydroxypropoxy, acetate, and methoxy groups to βD-glucose rings effectively disturbs the regularly developed inter- and/or intramolecular hydrogen bond formation between hydroxy groups in natural cellulose and leads to highly improved water solubility.

as observed in Figure 5. This discovery strongly indicates that the random introduction of a diol-type monomer into the PVA chains effectively disturbs the formation of intramolecular hydrogen bonds and increases the hydration numbers, mOHL. The reason for the depression in the formation of intramolecular hydrogen bonds with increasing x is that isolated hydroxy groups without the formation of intramolecular hydrogen bonds, which can be found in diol-type molecules, such as 1,4-cyclohexanediol, possess the highest hydration number of mOHL = 5 (and mOHH = 3). In contrast, hydroxy groups that form sequential intramolecular hydrogen bonds perfectly, such as those of myo-inositol, exhibit the smallest hydration number of mOHL (= mOHH) = 1 irrespective of the temperature.10 The hydration behavior of mPVA in aqueous solution is intermediate between that of the isolated and perfect sequential intramolecular hydrogen bonding hydroxy groups. The random introduction of a diol-type monomer quite effectively alters the degree of sequential intramolecular hydrogen bond formation, as schematically depicted in Figure 6. Although the hydration number of the terminal OH groups of intramolecular hydrogen bonding sequences, which donate protons to adjacent OH groups and possess two lone pair electrons, is thought to be ca. 4 at a low temperature, that at a high temperature should be ca. 2.10 The second hydration layer would disappear due to the dehydration effect. It is possible that partial chemical modification of PVA by the random introduction of monomers other than the diol-type monomer used in mPVA effectively disturbs the formation of sequential intramolecular hydrogen bonds and improves the low solubility in water by increasing the hydration number. PVA samples bearing DS values lower than 0.85 demonstrate much higher solubilities in aqueous solutions than the normal homo PVA in the low temperature range.4,5 Because PVA with a low DS value is a (nonrandom) copolymer of vinyl alcohol and vinyl acetate, the presence of vinyl acetate monomers effectively disturbs the sequential intramolecular hydrogen bond formation of hydroxy groups. Aqueous solutions of PVA samples with a DS lower than 0.85 sometimes demonstrate phase separation, i.e., clouding points at relatively low temperatures, ca. 50 °C, due to the hydrophobicity introduced by the presence of vinyl acetate monomers. In contrast, aqueous solutions of mPVA samples never showed phase separation even at 70 °C.

■

CONCLUSIONS Partially chemically modified poly(vinyl alcohol) synthesized via the random introduction of a diol-type monomer, 1-butene3,4-diol, abbreviated to be mPVA improves some characteristics of PVA in aqueous solution and the bulk state. The reason for the improved solubility of mPVA to values much higher than normal homo PVA is the effective disturbance of sequential intramolecular hydrogen bond formation along the polymer backbone due to the presence of the diol-type monomer. The disturbance of sequential intramolecular hydrogen bond E

DOI: 10.1021/ma502602r Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

(15) Shikata, T.; Takahashi, R.; Satokawa, Y. Hydration and Dynamic Behavior of Cyclodextrins in Aqueous Solution. J. Phys. Chem. B 2007, 111, 12239−12247. (16) Tasaki, K. Poly(oxyethylene)−Water Interactions: A Molecular Dynamics Study. J. Am. Chem. Soc. 1996, 118, 8459−8469. (17) Borodin, O.; Bedrov, D.; Smith, G. D. Molecular Dynamics Simulation Study of Dielectric Relaxation in Aqueous Poly(ethylene oxide) Solutions. Macromolecules 2002, 35, 2410−2412. (18) Deshmukh, S. A.; Sankaranarayanan, S. K. R. S.; Suthar, K.; Mancini, D. C. Role of solvation dynamics and local ordering of water in inducing conformational transitions in Poly(N-isopropylacrylamide) oligomers through the LCST. J. Phys. Chem. B 2012, 116, 2651−2663. (19) Perticaroli, S.; Nakanishi, M.; Pashkovski, E.; Sokolov, A. P. Dynamics of Hydration Water in Sugars and Peptides Solutions. J. Phys. Chem. B 2013, 117, 7729−7736. (20) Feller, R. L.; Wilt, M. Evaluation of Cellulose Ethers for Conservation; Getty Cons. Inst.: Los Angeles, 1990; Chapter 2. (21) Buchanan, C. M.; Edgar, K. J.; Wilson, A. K. Preparation and Characterization of Cellulose Monoacetate: The relationship between Structure and Water Solubility. Macromolecules 2013, 20, 193−198.

formation effectively leads to an increase in the hydration numbers of the hydroxy groups on the polymer backbone and, consequently, results in the improved solubility of mPVA in aqueous solutions.

■

AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS T.S. acknowledges the Nippon Synthetic Chemical Industry Co., Ltd. (Osaka), for their kind financial support and test sample supplies for this study. T.S. is also indebted to Mr. Mitsuo Shibutani of the Nippon Synthetic Chemical Industry Co., Ltd., for his kind instructive suggestions and fruitful discussion in this study. This work was partially supported by JSPS Grant-in-Aid for Scientific Research (B) Number 26288055.

■

REFERENCES

(1) Molyneux, P. Water-Soluble Synthetic Polymers: Properties and Behavior; CRC Press: Boca Raton, FL, 1983; Vol. 1. (2) Baker, M. I.; Walsh, S. P.; Schwartz, Z.; Boyan, B. D. A review of polyvinyl alcohol and its uses in cartilage and orthopedic applications. J. Biomed. Mater. Res., Part B 2012, 5, 1451−1457. (3) Sheftel, V. O. Indirect Food Additives and Polymers: Migration and Toxicology; Lewis Publishers: Boca Raton, FL, 2000; pp 1114−1116. (4) Hassan, C. M.; Trakampan, P.; Peppas, N. A. Water Solubility Characteristics of Poly(vinyl alcohol) and Gels Prepared by Freezing/ Thawing Processes. In Water Soluble Polymers, Solution Properties and Applications; Amjad, Z., Ed.; Springer: New York, 2002; pp 31−40. (5) Hassan, C. M.; Peppas, N. A. Structure and Applications of Poly(vinyl alcohol) Hydrogels Produced by Conventional Crosslinking or by Freezing/Thawing Methods. Adv. Polym. Sci. 2000, 153, 37−65. (6) Peppas, N. A.; Mongia, N. K. Ultrapure poly(vinyl alcohol) hydrogels with mucoadhesive drug delivery characteristics. Eur. J. Pharm. Biopharm. 1997, 43, 51−58. (7) Gupta, S.; Goswami, S.; Sinha, A. A combined effect of freeze– thaw cycles and polymer concentration on the structure and mechanical properties of transparent PVA gels. Biomed. Mater. 2012, 7 (015006), 1−8. (8) http://www.nichigo.co.jp/english/product/index07.html. (9) Satokawa, Y.; Shikata, T. Hydration Structure and Dynamic Behavior of Poly(vinyl alcohol)s in Aqueous Solution. Macromolecules 2008, 41, 2908−2913. (10) Shikata, T.; Okuzono, M. Hydration/Dehydration Behavior of Polyalcoholic Compounds Governed by Development of Intramolecular Hydrogen Bonds. J. Phys. Chem. B 2013, 117, 2782−2788. (11) 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. (12) Shikata, T.; Takahashi, R.; Sakamoto, A. Hydration of Poly(ethylene oxide)s in Aqueous Solution As Studied by Dielectric Relaxation Measurements. J. Phys. Chem. B 2006, 110, 8941−8945. (13) Shikata, T.; Sugimoto, N. Reconsideration of the anomalous dielectric behavior of dimethyl sulfoxide in the pure liquid state. Phys. Chem. Chem. Phys. 2011, 13, 46542−16547. (14) Shikata, T.; Sugimoto, N.; Sakai, U.; Watanabe, J. Dielectric Behaviors of Typical Benzene Monosubstitutes, Bromobenzene and Benzonitrile. J. Phys. Chem. B 2012, 116, 12605−12613. F

DOI: 10.1021/ma502602r Macromolecules XXXX, XXX, XXX−XXX