論文の公開元へStructure and dynamics of chemically modified cellulose ethers in aqueous solution
概要
Cellulose is the most abundant natural organic resource on the globe, and annual production of cellulose in nature is estimated to be 1014 – 1015 kg.1 Cellulose is a high-molecular weight linea polysaccharide which consists of D-glucopyranose unit via β-(1,4) glycosidic linkage.
The repeating units do not lie in a plane structure, rather they possess a chair conformation with succeeding glucose residues rotated by 180º on the molecular axis and possessing hydroxy groups in an equatorial position.2 Cellulose fibers are bundles of microfibrils, and the cellulose molecules are always biosynthesized in the form of nanosized fibrils. The cellulose fibers work as the load-bearing constituents in plants.3 The major industrial applications of cellulose are paper and textile ones. In addition, cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs) have been used as new materials due to their unique properties such as high mechanical characteristics and low density.4,5 However, native cellulose is insoluble into most usual solvents, including water, due to highly developed inter- and intramolecular hydrogen bonding between its hydrophilic groups.6,7 This insolubility has highly limited industrial applications of cellulose as a nature-friendly eco-material.
A series of chemically modified celluloses, such as water-soluble nonionic methyl, hydroxypropyl, hydroxyethyl and hydroxypropylmethyl cellulose ethers, anionic sodium carboxymethyl cellulose ether, and organic solvent-soluble ethyl and cyanoethyl cellulose ethers, and cellulose nitrate and cellulose acetate, have been developed by several chemical companies.3 These chemically modified cellulose samples exhibit useful properties, such as viscosity thickening, surfactant activity, film formation, adhesion, and so on. Then, they have been widely used in various areas such as pharmaceutical productions, foods, construction agents, cosmetics, oil drilling, and paint.3,8 In spite of their wide application range, the reasons why these chemically modified cellulose samples can be dissolved in solvent have not been fully understood so far. Moreover, when one pays attention to the characteristics of aqueous solutions of non-ionic water-soluble cellulose ether samples (WSCEs), such as methyl, hydroxypropyl, hydroxyethyl, hydroxypropylmethyl and hydroxyethylmethyl cellulose ethers, illustrated in Scheme 1.1, one frequently encounters the complicated solution behavior that the solubilities of these cellulose ethers are highly dependent on temperature. Although the WSCEs well dissolve into water below room temperature, ca. 25 ºC, most of them lose their high solubility and sometimes become turbid gels at high temperatures, i.e., they have lower critical solution temperatures (LCST), which are strongly dependent on their substitution conditions by hydroxypropyl, hydroxyethyl and methyl groups.9–11 However, the mechanism of losing water solubility for WSCEs with increasing temperature have not been clarified. Understanding the water solubility of WSCEs is very important to bring out their higher potential to be used in broader practical applications rather than they are currently used.
Scheme 1.1 Chemical structure of water-soluble cellulose ether samples
R = -H, -CH3, -CH2CH(OH)CH3 or -CH2CH2OH
The water solubility of materials is one of the important fundamental properties. Water soluble compounds are usually called hydrophilic compounds. The essential reason for water solubility of the hydrophilic compounds is considered to be the hydration of hydrophilic groups in solute molecules and water molecules, resulted from the formation of intermolecular hydrogen bonding between the hydrophilic groups and water molecules. However, the relation between hydration behavior and water solubility of the hydrophilic compounds has not been fully understood. Then, many studies have been carried out to explore the hydration behavior of various hydrophilic compounds in order to clarify the reasons why hydrophilic compounds are dissolved into water using various methods, such as extended depolarized light scattering (EDLS),12–16 nuclear magnetic resonance (NMR),17–19 neutron scattering (NS),20 time-resolved fluorescence decay,21 dielectric spectroscopy (DS)15,22–27 and molecular dynamics simulation (MD)28–31 techniques. These studies have revealed that the dynamics of hydrated water molecules in aqueous systems is affected by the solute molecules. In discussion on the water solubility of materials, many scientists believe that the hydration number for solute molecules in aqueous solution is a key parameter.
The dielectric spectroscopy measurement is one of the most useful methods to determine the amount of water molecules hydrated to solute molecules because the measurement is highly sensitive to a difference in the rates of rotational molecular motions of water molecules. The hydration numbers of hydrophilic chemical groups, such as ether,22 hydroxy,32 ester,33 carbonyl,33 cyano34 and nitro34 groups, have been determined by using the dielectric techniques to discuss the relationship between hydration number and their water solubility. Moreover, not only the hydration number of one hydrophilic group but also that of the complicated compounds which bear multiple hydrophilic groups would give important and indispensable information to understand the water solubility. This is because the solubility of the complicated compounds cannot be determined simply by the number of hydrophilic groups of them. Each hydrophilic group of the complicated compounds does not effectively demonstrate their potential hydration ability in water, because they would form inter- and intramolecular hydrogen bonding between themselves. For example, although glucose is the constituent monomer of cellulose and possesses high water solubility, native cellulose is completely insoluble in water. On the other hand, native dextran and pullulan which consist of α-1,6-D-glucopyranose unit and α-1,6-linked maltotriose molecule, respectively, well dissolve in water.35
Moreover, the water soluble cellulose ethers, WSCEs, are soluble in water below room temperature, whereas many kinds of them lose solubility at high temperatures. Ono et al.23 have determined the temperature dependence of hydration numbers for poly(N-isopropylacrylamide) (PNIPAm) which is typical synthetic water-soluble polymer showing the LCST type phase behavior in aqueous solution, and clarified the temperature dependent hydration numbers are closely related to its solubility. Then, the dependence of hydration number for the WSCEs on temperature would have relation to their solubility in water and provide an important information to understand the water solubility of hydrophilic compounds.
On the other hand, dynamics and conformation of solute molecules in aqueous solution are considerably affected by water molecules. Thus, in order to fully understand the hydration behavior in aqueous solution, it is important to clarify the dynamics of solute molecules in aqueous solution. The dynamics of polymer molecules in solution can be observed by dynamic mechanical analysis (DMA) measurements using rheometers. The DMA measurements can precisely detect the mechanical behavior of tested solution samples in a frequency range from 10-2 to 102 s-1 which are much lower than the frequency range for the dynamics of water molecules. Then, many studies have discussed the dynamics, phase separation and gelation behavior for solutions of polymeric compounds by using DMA measurements.
Briscoe et al.36 have investigated the rheological properties of aqueous poly(vinyl alcohol), PVA, solutions and the effects of the degree of polymer hydrolysis, temperature, pressure and addition of electrolytes on rheological behavior of the solutions. They have concluded that these conditions alter the contribution of two types of hydrogen bonding, between the polymer chains and water molecules, and inter- and intra- polymer chains, in aqueous solutions, and the contribution determines the rheological properties of the solution.
Lessard et al.37 have investigated the phase separation of aqueous poly(N,N-
diethylacrylamide) (PDEA) solution by using rheological and dynamic light scattering techniques. They reported that the aqueous systems show a coil-globule transition with increasing temperature before phase separation and finally demonstrated an aggregation phenomenon at a higher temperature.
Zhang et al.38 have determined the molecular weight dependent the gelation concentration point, cgel, for aqueous solutions of lentinan which consists of a β-1,3-D-glucan with a side chain of β-1,6-D-glucan and possesses triple helical structure, by using rheological measurements. They
reported that obtained cgel for the aqueous solution of lentinan with a high molecular weight was much lower than other many polymer solutions because of the high stiffness of triple helical structure and strong intra- and intermolecular interactions between lentinan molecules.
Although it is important to investigate the phase separation and gelation behavior of the polymer solutions directly, other fundamental information on solute polymer molecules, such as the conformation of the polymer molecules in the solution, is also indispensable to fully understand the complicated behavior. DMA measurements are one of the useful methods to explore the conformation of solute molecules. Doi and Edwards39 have proposed famous theoretical models for polymer systems that predict the viscoelastic properties of the polymer systems dependent on conformation of polymer molecules. Ferry40 has experimentally investigated the viscoelastic behavior of polymer systems in detail and discussed the dependence of the viscoelastic properties on many variables, such as temperature, molecular weight and concentration.
In my doctoral thesis, the hydration behavior of water soluble cellulose ether samples (WSCEs), such as methyl cellulose (MC), hydroxypropylmethyl cellulose (HpMC), hydroxyethylmethyl cellulose (HeMC), hydroxypropyl cellulose (HpC) and hydroxyethyl cellulose (HeC), in aqueous solution will be discussed from two different perspectives. The first one is the dynamics of water molecules in aqueous WSCEs solutions observed using dielectric spectroscopy measurements. The second one is the dynamics of solute cellulose molecules dissolved in aqueous
systems using dynamic viscoelastic techniques.
In Chapter 2, the temperature dependence of hydration numbers for HeC, MC, HpMC, HeMC, HpC samples with various substitution conditions and molecular weights in aqueous solution is determined by using extremely high-frequency dielectric spectroscopy techniques. The dependence of hydration numbers on the substitution conditions and molecular weight will be fully discussed.
In Chapter 3, the temperature dependence of viscoelastic behavior for aqueous solutions of MC and HpMC samples is investigated to understand the gelation mechanism of these solutions from the rheological point of view. The differences of temperature dependent viscoelasticity between aqueous solutions of MC and HpMC are discussed in relation to the hydration numbers of them. The concentration and molecular weight dependence of viscoelastic behavior for aqueous HpMC solution is investigated at a low temperature. The obtained results will be fully discussed to clarify the conformation of HpMC molecules in aqueous solution.
In Chapter 4, the temperature dependence of viscoelastic behavior for aqueous solutions of HpC and HeC samples is investigated to understand the gelation mechanism of the aqueous HpC system and confirm whether the HeC molecules are perfectly dissolved in water over a wide temperature range by using dynamic viscoelastic measurements. Moreover, I investigated the concentration and molecular weight dependence of viscoelastic behavior for aqueous solutions of HeC with a wide molecular weight range at a low temperature. The obtained results provide the information on the conformation of HeC molecules in aqueous solution. In addition, the differences of the viscoelastic behavior between aqueous solutions of HeC and HpMC at a low temperature will be discussed.
In Chapter 5, I summarize the results and discussion described in Chapter 2 to 4 and conclude the hydration behavior of aqueous WSCEs systems.
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