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Thermally Induced Association and Dissociation of Methylcellulose in Aqueous Solutions L. Li,*,† H. Shan,† C. Y. Yue,† Y. C. Lam,† K. C. Tam,† and X. Hu‡ School of Mechanical & Production Engineering, School of Materials Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 Received January 9, 2002. In Final Form: June 26, 2002 Aqueous solutions of a methylcellulose, ranging from 0.30 to 2.49 wt %, were studied by means of micro differential scanning calorimetry (micro DSC) and rheology. The effects of polymer concentration on the thermodynamic properties of these solutions were examined through a heating process and a following cooling process at a fixed rate of 1 °C/min. Upon heating, an endothermic peak was observed at about 63 °C, which was independent of polymer concentration. The total energy defined by the endothermic peak area was found to be a linear function of polymer concentration. On the other hand, when samples were cooling from about 90 °C, a broad exothermic peak appeared at about 33 °C, and the peak height and its broadness increased with polymer concentration. A shoulder was observed above the peak temperature of 33 °C, and the shoulder became more prominent with increasing polymer concentration to eventually appear as a second peak at about 40 °C. The thermal analysis results clearly show that the association of methylcellulose molecules in water is thermorevesible but the dissociation occurred at much lower temperatures than the association temperatures. The viscoelastic properties of these solutions correlated excellently with the results obtained from the micro thermal analysis. Thermodynamic mechanisms involved in the association and the dissociation are proposed.
Introduction Cellulose occurs naturally with a high hydrophilicity on its chain structure. Since cellulose can form strong intermolecular hydrogen bonds, however, original cellulose is insoluble in water. When a certain fraction of hydroxyl groups is substituted by hydrophobic groups such as methyl groups or hydroxypropyl groups, intermolecular hydrogen bonds are canceled to result in a water soluble cellulose.1-3 The resultant derivatives are called hydrophobically modified cellulose or water-soluble cellulose. However, a complete substitution of all hydroxyl groups in cellulose with hydrophobic groups (i.e. the degree of substitution is 3) makes resultant cellulose derivatives water-insoluble again. An optimum level of substitution is required for an appropriate water solubility, which is usually between 1.4 and 2.0. In water, there exist three possible intermolecular interactions among cellulose chains or between cellulose and water: (1) hydrogen bonding between unmodified hydroxyl groups on cellulose chains; (2) hydrogen bonding between the hydroxyl groups of cellulose and water molecules; (3) hydrophobic association between hydrophobic groups (i.e. methyl groups for methylcellulose) on cellulose chains which were introduced in the hydrophobic modification process. As the strength of hydrogen bonding or hydrophobic association is dependent on temperature, formation of micellar structures or networks will also be temperature-dependent. For methylcellulose with an appropriate degree of substitution, it is reported that thermal gelation can take place in the temperature range from about 50 to 70 °C mainly due to the hydrophobic association that becomes pronounced at * Correspondence author. Tel: +65-6790 6285. Fax: +65-6791 1859. E-mail:
[email protected]. † School of Mechanical & Production Engineering. ‡ School of Materials Engineering. (1) Guent, J. Thermoreversible Gelation of Polymers and Biopolymers; Academic Press: London, 1992. (2) Sakar, N. J. Appl. Polym. Sci. 1979, 24, 1073. (3) Tanaka, F.; Ishida, M. J. Chem. Soc., Faraday Trans. 1995, 91, 2663.
elevated temperatures.4-8 With the identical mechanism, the thermal gelation of methylcellulose is also explained to be caused by the thermally induced liquid-liquid-phase separation.7 On the other hand, hydrophobically modified cellulose is of thermoreversibility in hydrophobic association. This means that the associative aggregates or junctions formed at elevated temperatures can return to the liquid state again upon cooling through the disassociation of hydrophobic groups. The formation of hydrogen bonding and/or the formation of hydrophobic association on heating or cooling may easily cause a cellulose solution to have a complicated microstructure that varies with environmental and/or processing conditions. At an elevated temperature between about 50 and 70 °C, a hydrophobically modified cellulose gels in water when the cellulose concentration exceeds the critical gelling concentration. Although there has been extensive literature available on thermal gelation of various cellulose derivatives, few scientific studies were carried out on the molecular origin of the gelation behavior.4-12 For example, no reports have been found on the studies of different contributions from hydrogen bonding and hydrophobic association to the formation of microstructures of cellulose in solution. Kobayashi et al. recently reported their studies on thermoreversible gelation of dilute and semidilute (4) Haque, A.; Morris, E. R. Carbohydr. Polym. 1993, 22, 161. (5) Chevillard, C.; Axelos, M. A. V. Colloid Polym. Sci. 1997, 275, 537. (6) Hirrien, M.; Chevillard, C.; Desbrieres, J.; Axelos, M. A. V.; Rinaudo, M. Polymer 1998, 25, 6251. (7) Kobayashi, K.; Huang, C.; Lodge, T. P. Macromolecules 1999, 32, 7070. (8) (a) Desbrieres, J.; Hirrien, M.; Rinaudo, M. Carbohydr. Polym. 1998, 37, 145. (b) Desbrieres, J.; Hirrien, M.; Ross-Murphy, S. B. Polymer 2000, 41, 2451. (9) (a) Sarkar, N. Carbohydr. Polym. 1995, 26, 195. (b) Sarkar, N.; Walker, L. C. Carbohydr. Polym. 1995, 27, 177. (10) Ostravskii, D.; Kjoniksen, A.-L.; Nystrom, B.; Torell, L. M. Macromolecules 1999, 32, 1534. (11) Kjoniksen, A.-L.; Nystrom, B.; Lindman B. Colloids & Surf, A 1999, 149, 347. (12) Badiger, M. V.; Lutz, A.; Wolf, B. A. Polymer 2000, 41, 1377.
10.1021/la020029b CCC: $22.00 © 2002 American Chemical Society Published on Web 08/14/2002
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aqueous solutions of methylcellulose using dynamic light scattering and rheology.7 They found that in a dilute solution (c e 2.5 g/L) at 20 °C there existed only a single relaxation mode with a constant hydrodynamic radius of ca. 38 nm, which was attributed to molecular diffusion of an individual chain in water. On the other hand, two modes of relaxation appeared in the semidilute solutions (20 g/L g c g 5 g/L), where the slow mode was considered to be attributed to the formation of clusters with typical dimension larger than q-1. q is the scattering vector given by (4πn/λ0) sin(θ/2), where the vacuum wavelength of the incident laser beam λ0 was 488 nm. The formation of clusters or other molecular structures in a semidilute solution of methylcellulose at a low temperature is important in affecting gelation mechanisms and network structures formed by a subsequent gelation process upon heating. Kobayashi et al.7 demonstrated that a plateau behavior of dynamic storage modulus G′ was observed at 20 °C in the terminal region of frequency for a 10 g/L solution of methylcelulose. This was explained by the formation of a supermolecular structure with a reversible feature. However, further investigation and discussion on the formation mechanism of the supermolecular structure and details of the structure were not reported by the authors.7 Recent technical advances have led to the development of microcalorimeters such as isothermal titration calorimeters (ITC) and micro differential scanning calorimeters (micro DSC), which can detect small amounts of heat (a few microcalories) absorbed or released under an isothermal titration condition or during a well-controlled heating or cooling process (for micro DSC). A number of applications using micro DSC can be found in the literature.13-15 In this work, we use a micro DSC to determine the thermodynamic properties of aqueous solutions of methylcellulose as a function of polymer concentration, which are involved in the hydrophobic association of methylcellulose in water upon heating or in the dissociation of methylcellulose upon cooling. The thermodynamic results obtained from this work will aid the understanding of the hydrophobic association/dissociation mechanisms and the kinetic factors affecting the formation of junctions for a gel network structure from methycellulose molecules. Experimental Section Materials. A cellulose derivative, methylcellulose (MC), with the trade name of SM4000, made by etherification of pulp cellulose (pulp), was kindly provided by Shinetsu Chemical Co. Ltd., Japan. The original MC was in a form of white fine powder. The manufacture’s specifications indicate that the methylcellulose has an average degree of substitution (DS) of 1.8 and a weightaverage molecular weight of 380 000 determined using light scattering. The polydispersity of molecular weight for this methylcellulose was unknown due to the technical difficulty in determining it using GPC. The viscosity range was reported by the manufacturer to be 4.54 Pa s at 20 °C for a 2 wt % aqueous solution. The material was used as received without further purification. Prior to use, they were vacuum-dried at 55 °C for 24 h and kept in a desiccator at room temperature. The aqueous solutions of MC with various concentrations ranging from 0.30 to about 2.50 wt % were prepared with deionized water from the Millipore water purifier. The weight percentage will be used to represent all the polymer concentrations in this work because (13) Smith, A. L.; Shirazi, H. M. J. Therm. Anal. Callorim. 2000, 59, 171. (14) Winnik, M. A.; Bystryak, S. M.; Chassenieux, C. Langmuir 2000, 16, 4495. (15) Grinberg, V. Y.; Dubovik, A. S.; Kuznetsov, D. V.; Grinberg, N. V.; Grosberg, A. Y.; Tanaka, T. Macromolecules 2000, 33, 8685.
Li et al. of the convenience and accuracy. Although it is simple to convert wt % to g/L, the interaction between MC and water may cause an error in computing the solution volume by simply summing the component volumes calculated from the component densities and weights. The solution preparation procedure was to disperse the weighed MC powder in hot water (about 70 °C) and shake it well. Since the MC could not be completely dissolved within a short period of time at room temperature, the dispersion was transferred to a refrigerator with a temperature below 10 °C and kept for a minimum period of 48 h prior to measurements. All the solutions obtained were clear and transparent at room temperature (25 °C) or below. Microthermal Analysis. A micro differential scanning calorimeter (VP-DSC microcalorimeter, Microcal Inc.) was used to determine the thermal properties of a cellulose solution during a heating or cooling process. The microcalorimeter is designed for liquid samples, where the sample is injected by a syringe into a 0.516 mL sample cell and the reference cell is filled with pure water or the same solvent used for the sample solution. In this work, we employed the deionized water as the reference. A slow heating or cooling rate of 1 °C/min was employed for all samples to explore the details of the thermal induced association behavior of the cellulose polymer in water. The reason for use of such a low heating or cooling rate was because the hydrophobic association from the relatively stiff cellulose chains was expected to be slow. For the microthermal analysis, the sample underwent heating from room temperature of 25 °C or below to about 85 °C, and subsequently it was immediately cooled from 85 to about 10 °C. After each cycle was completed for the sample, the sample cell was cleaned by a continuous flow of deionized water through a thin plastic tube for more than 1 h. No contamination of the sample cell by the last sample was confirmed by running deionized water before the next sample was injected into the sample cell. If an endothermic or exothermic peak was detected from the sample cell filled with water, the sample cell had to be cleaned again until a no-contamination condition was reached. One of the major limitations of this micro DCS calorimeter is that a highly viscous liquid cannot be injected into the sample cell. Therefore, the concentrations were restricted to be below 2.5 wt % SM4000. All the micro DSC runs were performed at 1 °C/min, except for studying the effects of the heating (or cooling) rate. Turbidity Measurements. Turbidity measurements were performed using a UV/visible spectroscopy system (HewlettPackard 89090A) equipped with a temperature controller. This system allows one to measure the transmittance of a liquid sample in a standard cuvette through a thermal cycle programmed from heating to cooling or vice versa at a desired rate. In this investigation, a visible wavelength of 500 nm was used for the turbidity measurements because the methylcellulose solutions studied did not absorb this wavelength of light. The cloud point was defined as the temperature at which the transmittance is 50% of the overall transmittance. All the measurements were performed at a heating or cooling rate of 1 °C/min. Rheological Measurements. The solution was transferred from a glass bottle to the rheometer (ARES 100FRTN1, Rheometric Scientific). The rheometer was equipped with two sensitive force transducers for torque measurements ranging from 0.004 to 100 g cm. Parallel plates of 25 and 50 mm diameter were used for relatively high viscosity and low viscosity solutions, respectively. The dynamic viscoelastic functions such as the shear storage modulus G′ and loss modulus G′′ were measured as a function of time, temperature, or angular frequency. To prevent dehydration during rheological measurements, a thin layer of low-viscosity silicone oil was placed on the periphery surface of the solution held between the plates. All the dynamic viscoelastic measurements (i.e. temperature sweep) were carried out at an angular frequency of 1 rad/s and low shear strains to ensure the linearity of viscoelasticity.
Results and Discussion Thermal Behavior of MC Aqueous Solutions on Heating and Cooling. A typical example of calorimetric thermograms determined using micro DSC is shown in Figure 1 for a 0.91 wt % solution of SM4000. The relative thermal capacity Cp is employed throughout this report
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Figure 1. Calorimetric thermograms of a 0.91 wt % aqueous solution of SM4000 during a thermal cycle. The heating and cooling rates were 1 °C/min.
because of the nature of DSC where the relative heat between the sample and the reference is always measured. In the heating process, the sharp endothermic peak is observed at 62.6 °C, while the broad exothermic peak appears at 32.5 °C in the subsequent cooling process. In Figure 1, the difference in the peak temperature between the heating and the cooling processes is 30 °C, which is large. The second remarkable feature is that the exothermic peak in the cooling process is much broader than the endothermic peak in the heating process, and it is also associated with a shoulder or a secondary peak at the high-temperature side. The thermoreversibility was confirmed by conducting three complete cycles at fixed heating and cooling rates of 1 °C/min for the 0.91 wt % of SM4000. Each cycle was programmed from 10 to 90 °C for heating and 90 to 10 °C for cooling. This was repeated for the next two cycles. The three heating thermograms and the three cooling ones are plotted together in Figure 2a,b, respectively, and the cycle orders are indicated with the numerals. For ease of visualization, all the thermogram curves for the second and third runs were vertically shifted by arbitrary amounts to prevent the curves from overlapping. It is evidenced that all three thermal cycles resulted in the same thermograms. If the calorimetric curves 2 and 3 in Figure 2 are to superpose on curves 1, they overlap perfectly. The conclusion here is that the methylcellulose aqueous solutions are completely thermoreversible. Figures 3 and 4 show the effects of polymer concentration on the heating and cooling thermograms, respectively. In Figure 3, the endothermic peak increases in height with polymer concentration but the peak temperature remains almost the same at 63 °C. In addition, the endothermic peak, although it remains relatively narrow, becomes broader with increasing MC concentration. However, the MC solutions in the cooling process (Figure 4) behaved completely differently from those in the heating process. In the cooling process, the broad exothermic peaks are observed at the peak temperature of about 33 °C, and the peak height and its broadness increased with polymer concentration. A shoulder was observed above the peak temperature of 33 °C, and the shoulder became more prominent with increasing polymer concentration to eventually appear as a secondary peak at about 40 °C. In the cooling process, the dissociation of the associated structures of methylcellulose in water, which were formed in the heating process, appears to have to overcome two energy barriers: a primary barrier at about 32 °C and a secondary one in the broad range from about 70 to 35 °C.
Figure 2. Calorimetric thermograms of a 0.91 wt % aqueous solution of SM4000 during three thermal cycles at the same heating and cooling rates of 1 °C/min: (a) heating from the lowest to the highest temperatures; (b) cooling from the highest to the lowest temperatures.
Figure 3. Calorimetric thermograms of aqueous solutions of SM4000 with various concentrations on heating from about 15 °C at a heating rate of 1 °C/min.
In summary, the two interesting features for these exothermic peaks are (1) the height of the primary peak increases with MC concentration but the peak position remains the same and (2) the secondary peak becomes more pronounced with increasing MC concentration. For example, the highest concentration of 2.49 wt % gives the most distinct secondary peak at about 40 °C. The
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Table 1. Peak Temperatures of the Calorimetric Thermograms during Heating and Cooling Processes and Onset (T1) and Offset (T2) Temperatures of the Peaks on Heating for Aqueous Solutions of SM4000 c, wt % peak on heating, °C T1, onset of peak, °C T2, offset of peak, °C peak on cooling, °C
0.30
0.56
0.91
1.53
1.83
2.19
2.49
64.5 59.6 69.7 33.3
63.2 59.1 69.7 32.7
62.6 58.7 69.7 32.5
64.2 55.6 71.8 32.2
63.8 54.0 71.8 31.5
62.3 51.2 71.9 31.4
61.3 50.9 71.9 31.3
Table 2. Heat Absorbed on Heating, Heat Released on Cooling, and Entropy Change on Heating for Aqueous Solutions of SM4000 c, wt % heat absorbed, kJ/L heat released, kJ/L entropy change on heating, J/(K L)
0.30
0.56
0.91
1.53
1.83
2.19
2.49
0.0394 0.0549 0.0839
0.0726 0.103 0.199
0.116 0.160 0.308
0.198 0.280 0.537
0.242 0.318 0.643
0.277 0.345 0.649
0.316 0.458 0.814
Figure 4. Calorimetric thermograms of aqueous solutions of SM4000 with various concentrations on cooling at a cooling rate of 1 °C/min.
characteristic temperatures (i.e. peak, onset, and offset temperatures of the peaks) of the endothermic and exothermic peaks are summarized in Table 1, where the onset and offset temperatures for the exothermic peaks are not given because of the difficulty in defining the onset temperature due to the peak broadness. Endothermic versus Exothermic Heats. From Figures 1-4, we know that the endothermic and exothermic heats are observed on heating and cooling, respectively. For simplicity, we defined the peak area as the total endothermic or exothermic heat [in joules (J)], normalized by the sample volume of 0.516 mL. Knowing the thermal capacity Cp(T), where T is temperature, the entropy change ∆S can be directly calculated using
∆S )
∫TT
2
1
(Cp/T) dT
(1)
The entropy changes on heating, together with the endothermic and exothermic heats for all the concentrations of MC, are given in Table 2. All the values of ∆S are positive, indicating that the hydrophobic association on heating is an entropy-driven process. The endothermic and exothermic heats are plotted in Figure 5a,b, respectively. With the only minor deviation, Figure 5 indicates that both endothermic and exothermic heats are linear functions of concentration. Therefore, it appears that, in the range of MC concentrations studied, the endothermic heat on heating or the exothermic heat on cooling is only a
Figure 5. (a) Heat absorbed on heating as a function of SM4000 concentration. (b) Heat released on cooling as a function of SM4000 concentration.
function of MC concentration. If one normalizes the endothermic or exothermic heats by the MC concentration, a constant value results. This implies that the endothermic heat (or exothermic heat) involved in the thermodynamic change for the hydrophobic association on heating (or in the thermodynamic change for the hydrophobic dissociation on cooling) is determined only by the amount of methylcellulose in the solution. Phase Diagram for the Sol-Gel Transition. From the onset and offset temperatures listed in Table 1, one can directly make a phase diagram. The obtained phase diagram for the heating process is shown in Figure 6, where the heating rate was 1 °C/min. It should be noted here that this kind of phase diagram does not indicate the
Methylcellulose in Aqueous Solutions
Figure 6. Phase diagram for a SM4000 aqueous system at a heating rate of 1 °C/min.
phase behavior in the thermodynamically equilibrium state, but it is dependent on the heating rate. Under a fixed heating rate, however, this phase diagram is useful in describing the kinetic phase behavior for any MC concentration in the range. Below the onset temperature line, it is the solution (sol) region where there would be MC aggregates or supermolecular structures7 but there is no detectable heat by micro DSC. A gel or a microgel would be formed only when the temperature is above the onset temperature (the lower line). In the temperature region between the onset and offset temperature lines, indicated by “Sol-Gel”, a MC solution undergoes the sol-gel transition at the give heating rate. Thus, at any temperatures below the upper line but above the lower line, gelation is possible. Above the upper temperature line, gelation is complete for a given concentration of MC. When the temperature is above the upper line but below about 80 °C, a MC solution will not precipitate because of its kinetic nature. However, according to Takahashi et al.,16 precipitation might begin above 80 °C; thus, a dotted line in Figure 6 shows this possibility. As demonstrated in our measurement of the dynamic viscoelastic properties (as discussed in a later section; see Figure 9) for a 1.8 wt % MC solution through a heating process at the same heating rate of 1 °C/min up to about 78 °C, which was above the offset temperature of 71.8 °C for this MC concentration, there was no decrease in the dynamic modulus G′ in the temperature range from 71.8 to 78 °C. This shows that there is no precipitation of MC at least until 78 °C at the given heating rate. The second point to be emphasized here is that when a MC concentration is within the “gel” region, it does not necessarily mean that the solution will yield a solidlike gel if the MC concentration is below the sol-gel transition concentration.17,18 Instead of the ordinary image of a gel, a microgel could be formed for MC concentrations below the sol-gel transition concentration. Under this condition, the MC solution behaves like a solution and not a semisolid gel, but there are microdomains consisting of microgels in the solution. This consideration for the molecular (16) (a) Takahashi, M.; Shimazaki, M. J. Polym. Sci. Polym. Phys. 2001, 39, 943. (b) Takahashi, M.; Shimazaki, M.; Yamamoto, J. Kobunshi Ronbunshu 1998, 55, 269. (17) (a) Li, L.; Aoki, Y. Macromolecules 1997, 30, 7835. (b) Li, L.; Uchida, H.; Aoki, Y.; Yao, M. L. Macromolecules 1997, 30, 7842. (c) Li, L.; Aoki, Y. Macromolecules 1998, 31, 740. (18) Li, L.; Thangamathesvaran, P. M.; Yue, C. Y.; Tam, K. C.; Hu, X.; Lam, Y. C. Langmuir 2001, 17, 8062.
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Figure 7. Transmittance as a function of temperature during a thermal cycle from heating to cooling at 1 °C/min for a 1 wt % aqueous solution of SM4000. The wavelength for the transmittance measurement was 500 nm.
structure is meaningful because of the thermal properties given in Figures 4-6. In the MC concentration range studied, it may be considered that any concentration is able to form a gel or a microgel, depending on MC concentration, from a thermodynamic perspective due to the similarity in their thermodynamic properties. To further confirm the physical state of the MC solutions, which may vary as a function of temperature, the turbidity measurements were carried out through the same thermal cycle as that of micro DSC. A representative example for the turbidity hysteresis is given in Figure 7for a 1 wt % SM4000 solution at a constant scanning rate of 1 °C/min. The result shows excellent correlation with thermal properties. In the heating process, the transmittance begins to decrease at about 50 °C, falls to 50% at 62 °C (close to the peak temperature of endotherm), and becomes completely turbid at about 74 °C. In the subsequent cooling process, the turbidity remains until about 50 °C and the transmittance begins to increase as the temperature is lowered further. The 50% transmittance is achieved at the temperature of 33 °C, which is consistent with the peak temperature (Figure 4) of the exotherm determined by micro DSC. These results also indicate that the MC solutions become turbid on heating but no precipitation occurs until at least 75 °C. If the precipitation of MC takes place due to the phase separation between MC and water at high temperatures, the solution will turn clear again. Effect of Heating (or Cooling) Rate. The hydrophobic association (or dissociation) of methylcellulose in water is expected to be dependent on heating (or cooling) rate. Figure 8 shows an example of the effect of heating and cooling rates on the thermal association and dissociation for a 0.91 wt % aqueous solution of SM4000, where two rates (1 and 1/6 °C) were applied. In the heating process, the lower rate of heating resulted in a lower peak temperature of about 60.7 °C as compared to 63.2 °C at the higher rate of heating. However, the whole pattern of the endothermic peak did not change significantly with the heating rate. This result is consistent with the typical viscoelaticity19 that the molecular association of MC is expected to be a rate-dependent process. However, in the cooling process, the primary peak temperature remained almost the same when the cooling (19) Mark, J. E.; Eisenberg, A.; Graessley, W. W.; Mandelkern, L.; Koenig, J. L. Physical Properties of Polymers; American Chemical Society: Washington, DC, 1981.
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Figure 8. Calorimetric thermograms of a 0.91 wt % aqueous solution of SM4000 at two rates of heating and cooling (1 and 1/6 °C/min as indicated).
Figure 9. Storage modulus G′ (filled circles) and loss modulus G′′ (empty circles) as a function of temperature in a heating and a cooling processes for a 1.80 wt % SM4000 solution. A shear strain amplitude of 3% and an angular frequency of 1 rad/s were applied. The heating and cooling rates were about 1 °C/min.
rate was varied from 1 to 1/6 °C, but the secondary peak (the shoulder) appeared more clearly at the lower rate of cooling than at the higher rate. The independence of the primary dissociation on cooling rate may suggest that when the low-temperature condition is reached for dissociation, it is too fast to be affected by the cooling rate change from 1 to 1/6 °C/min. This consideration may help to elucidate the dissociation mechanism. Viscoelatic Behavior on Heating and Cooling. To correlate the thermal properties with the viscoelastic behavior of MC solutions, dynamic viscoelastic measurements were carried out using the same rates of heating and cooling in the similar range of temperature. An example is given in Figure 9 for a 1.80 wt % MC solution, where the heating and cooling rates were about 1 °C/min and an angular frequency of 1 rad/s was used. As shown in Figure 9, in the heating process from 20 to about 78 °C, there are at least three distinct regions. The first region is before the crossover of storage modulus G′ and loss modulus G′′, where G′ is below G′′ showing the common viscoelastic behavior of a liquid. G′ crosses over G′′ at 31.5 °C. Beyond 31.5 °C, G′ gradually increases until about 58.5 °C. In this second region, while G′ gradually increases with temperature, G′′ decreases
Li et al.
slightly until about 56 °C. Since G′ is always higher than G′′ in this region, one can consider that a weak but elastic structure of MC is being formed in water with temperature. However, on the basis of the microcalorimetric results shown in Figures 1-5, if one considers that the hydrophobic association is the only driving force for the formation of a gel or a microgel, the second region would not be responsible for gelation because there is no endothermic peak in this range. G′ speedily increases from 58.5 °C, passes a transition at about 63 °C, and eventually reaches the plateau at about 70 °C. Traditionally, the crossover of G′ and G′′ is used as an indication of the sol-gel transition point.20 This method is simple and convenient, but the gel point defined by this method is usually dependent on frequency used in the measurement. Winter’s group21-23 defined the sol-gel transition as the point at which both G′ and G′′ scale with ωn and the ratio of G′′ to G′ (i.e. tangent δ) is independent of frequency ω. In other words, at the sol-gel transition with the frequency independence, G′ must be parallel to G′′. For the methylcellulose (SM4000) aqueous solutions, however, we have confirmed that this condition does not exist for satisfying the requirement in applying Winnter’s scaling law at the sol-gel transition.18 However, this does not seem reasonable to employ the traditional definition (i.e. the crossover of G′ and G′′) for the sol-gel transition for the case in Figure 9, for the following two reasons: (1) In the temperature range from 31.5 to 58.5 °C, G′ values were below a few tens of pascals and the solution did not behave like a gel. (2) The crossover temperature of 31.5 °C is far below the onset temperature (54 °C) of the endothermic peak as shown in Figure 3 or 6. The most significant finding here is that there is an abrupt increase of G′ in the vicinity of 63 °C, which correlates excellently with the endothermic peak observed by the micro DSC measurements (Figure 3). Therefore, it would be more appropriate to use 63 °C to define the sol-gel transition rather than the crossover point of G′ and G′′. We intend to develop this concept further to result in a new definition of the sol-gel transition for thermoreversible gels such as hydrophobically modified cellulose. Starting from the plateau, the cooling process was conducted at a similar cooling rate of 1 °C/min. In contrast to the sharp increase of G′ in the heating process in the temperature range from 58.5 to 70 °C, the gradual decrease in G′ with temperature in the cooling process shows an outstanding deviation from the heating curve. This clearly indicates that the thermally induced hydrophobic dissociation is not an exact reversal of the hydrophobic association occurring in the heating process. Interestingly, the crossover of G′ and G′′ is observed again at a temperature near 31.5 °C as found in the heating process. Before this crossover point, G′ rapidly decreases from the plateau by passing the transition temperature near 33 °C. Thus, these viscoelastic properties have correlated excellently with the results obtained from the microthermal analysis. Association and Dissociation of MC in Water. From the experimental results, it is known that the heating process is endothermic (∆H > 0). Therefore, to obtain a negative ∆G (∆G < 0), the entropy change ∆S must be positive. If only the methylcellulose molecules are considered, then the formation of gel networks upon heating should be an entropy decreasing process (∆S < 0) because (20) Ferry, J. D. Viscoelastic Properties of Polymers, 3rd ed.; John Wiley & Sons: New York, 1980. (21) Chambon, F.; Winter, H. H. Polym. Bull. 1985, 13, 499. (22) Winter, H. H.; Chambon, F. J. Rheol. 1986, 30, 367. (23) Chambon, F.; Winter, H. H. J. Rheol. 1987, 31, 683.
Methylcellulose in Aqueous Solutions
the polymer molecules fall into a more ordered structure in the gel state from a disordered state in solution. However, the experimental results cannot be explained by this mechanism and the role of water molecules must be taken into consideration. At low temperatures, MC is soluble in water where water molecules surround each individual methylcellulose chain. While the water solubility of MC has been interpreted to be due to the cancellation of intermolecular hydrogen bonding between MC chains by introducing the hydrophobic methyl groups,1-3 the cagelike structures are often used by many researchers to explain the solubility and thermodynamic behavior of a solute in solution. For example, Grunwald and Steel24 employed the concept of solvent reorganization to depict the thermodynamics of weakly associating systems especially aqueous solutions where the hydrogen bonding of a solute with water or the role of water is important. It was considered that the establishment of cage environments dominates the solvation of a solute and when the free energy change ∆Genv for the establishment of the cage environments reaches equilibrium, the thermodynamic enthalpy-entropy compensation is resulted.24 In other words, in the equilibrium state, the enthalpy (Henv) released (or consumed) is to compensate the entropy (Senv) by establishing (or destroying) the cage structures. Therefore, the dissolution, association, or precipitation of a solute must be accompanied by the necessary changes (i.e. formation or destruction) of cage structures. For a long time, the role of water has been recognized to be vital in biological systems where water can solvate polar molecules such as proteins or DNA and hence weaken intermolecular ionic and hydrogen bonds.25 The formation of cagelike structures for methylcellulose to become water soluble has been extensively accepted by many researchers, for example, Haque et al.,4 Sarkar,9 and Kobayashi et al.7 When water molecules form hydrogen bonds along MC chains and/or cagelike structures surrounding the methyl groups, these water molecules in the solution will not be free and random, and they will have a certain degree of order. Heating of a MC solution will cause the destruction of the cage structures of water and the exposure of the hydrophobic regions of MC, leading to the formation of hydrophobic aggregates of MC. This is the so-called hydrophobic association. This hydrophobic association is also considered to be a morphological change from the MC coils, where there might be mostly intramolecular interactions and which could be expanding by heating, to the mostly intermolecular hydrophobic association. Compared to the energy needed in the destruction of the cage structures, the energy for the formation of hydrophobic aggregates is much lower. As a result, the total detectable endothermic heat in the heating process is considered to be mainly attributed to the destruction of the cage structures formed from water molecules. As the temperature increases, the number and the sizes (i.e. the aggregation number) of hydrophobic aggregates increase and a gel will eventually begin to form when a percolation of the polymer chains connecting the hydrophobic aggregates is achieved across the solution volume. From the results (Figures 3 and 9) obtained by micro DSC and rheology, one could consider that the destruction of caged water structures occurs suddenly when a sufficient level of energy is reached. At almost the same time, the hydrophobic association takes place in the same range (24) Grunwald, E.; Steel, C. J. Am. Chem. Soc. 1995, 117, 5687. (25) Stryer, L. Biochemistry, 4th ed.; W. H. Freeman and Co.: New York, 1995.
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of temperature, eventually leading to gelation. As a result, the only single endothermic peak and the abrupt increase in G′ (i.e. gelation) are observed in the narrow temperature range. The endothermic energy is consumed to break the cage structures formed between the methyl groups of methylcellulose and water molecules. Thus, the hydrophobic association causes the water molecules changing from the ordered state in solution to the disordered state so that the heating process is an entropy-increasing process (∆S > 0). In contrast to the heating process for the hydrophobic association and gelation, the cooling process is exothermic. This is because heat will be released when the cage structures are formed again between water molecules and methyl groups of MC. As a reverse process of association, the entropy change for dissociation or degelation must be negative. Here it is noted that the most outstanding difference between the heating and cooling is the temperature at which the peak occurs. The peak temperature for heating occurs about 63 °C, whereas the peak temperature for cooling is about 33 °C that is defined as the primary peak, independently of MC concentration. The temperature difference is 30 °C between the heating and cooling peaks. There is also a secondary peak, observed only in the cooling process. This secondary peak becomes more notable for higher concentrations of MC. We consider that the difference between the peak temperatures is due to the kinetics of association and dissociation of MC in water. A delay seems to be involved in the dissociation of the gel network formed in the heating process. Thermodynamically, one may consider that there are two different reaction constants for the association and dissociation, respectively. The double exothermic peaks on the cooling curve suggest that the dissociation or degelation process occurs in two steps. The following mechanism is proposed for the two-step dissociation (or degelation), which may provide readers with a first-version image of MC dissociation. Upon cooling, a MC gel formed at high temperatures tends to undergo a progressive transition to the sol state. The driving force for this gel-sol transition is the negative free energy change (i.e. ∆G < 0). To get ∆G < 0 with a negative ∆S, ∆H has to be negative. The secondary exothermic peak appears at about 40 °C, which is near the temperature from which G′ starts to drop abruptly from the G′ plateau. Below 40 °C, G′ withstands the high values (∼ 103 Pa), indicating the existence of the gel network. However, 40 °C can be considered as a critical temperature at which the gel network has reached the verge of the disruption of the overall gel network. Now we attempt to find the mechanism for the secondary peak. For a MC gel to reach the verge of the disruption of the overall gel network, the network structure has to be gradually weakened through the cooling process. The weakening of the network may be achieved by two factors: (1) decreasing the hydrophobic association strength by reducing temperature; (2) surrounding the hydrophobic junctions of the gel network by water molecules. Factor 2 may require the water molecules to form relatively ordered structures surrounding the junctions, contributing to the secondary exothermic heat. The primary exothermic peak at 33 °C corresponds to the sharp drop in G′ (see Figure 9); this step can be considered as the massive destruction of the whole gel network. Below 33 °C, all the hydrophobic junctions are disintegrated so that MC chains become free again. Simultaneously, the arrangement of water molecules for the re-formation of the cage structures takes place so that
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the large exothermic heat is required. As a result, the system returns to the original liquid state when the cooling temperature is below the offset temperature of the exothermic peak for each MC concentration. As discovered in Figure 2, the MC solutions can return to exactly the same original state after a complete thermal cycle. Conclusions The thermally induced association and dissociation of a methylcellulose (MC) in water have been investigated in detail as a function of MC concentration by micro DSC, rheological, and turbidity measurements. A MC aqueous solution absorbs heat on heating while it releases heat on cooling. A completely thermoreversible behavior has been observed for all the concentrations of MC studied, which covered the range from 0.30 to 2.49 wt %. Both the endothermic heats on heating and exothermic heats on cooling were linear functions of MC concentration, showing that the thermodynamic changes are dependent simply and only on the MC amount in water. On heating, the MC solutions showed a consistent endothermic peak at about 63 °C, which was independent of the MC concentration. However, on cooling, the broad exothermic peak was observed with a primary peak of 33 °C and a secondary peak of about 40 °C. Thus, through
Li et al.
a heating or cooling process at a given heating or cooling rate, one could directly determine a phase diagram for the gelation or degelation. The thermodynamic mechanism involved in the hydrophobic association and dissociation has been explained by the entropy-driven process that was mainly controlled by the thermally induced destruction of hydrogen bonds formed between water molecules and MC chains. The rheological properties of MC solutions during a thermal cycle have correlated excellently with the thermal properties, and the rheological results have proved that the gelation of MC in water takes place in the vicinity of 63 °C but not at the crossover point (about 32 °C) of G′ and G′′. Therefore, the crossover point of G′ and G′′ was found not to be a satisfactory method for the definition of the sol-gel transition, especially for gels formed from MC aqueous solutions. Acknowledgment. This work was supported by a research grant from Nanyang Technological University, Singapore. The authors thank S. Dai, C. Wang, and P. Zheng for their assistance in the micro DSC and transmittance measurements. LA020029B
