Evaluation of Local Gelation Behavior of Aqueous Methylcellulose Solution UsingQuartz Crystal Microbalance+1

Kenji Yamaoka+2, Yoshihisa Fujii+3 and Naoya Torikai

Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu 514-8507, Japan

The physical gelation of an aqueous methylcellulose (MC) solution in response to temperature change was evaluated using a quartz crystalmicrobalance (QCM), which is an extremely sensitive mass balance that measures changes in mass per unit area from nanogram to microgramlevel. Then, the potential use of QCM for interfacial selective viscoelasticity measurements was investigated. The viscosity changesaccompanying gelation were observed as resonance frequency shifts. The gelation temperature determined from the temperature dependence ofthe resonance frequency shifts showed good agreement with the gelation temperatures obtained by visual inclination observation and rheologymeasurements. Furthermore, MC molecules were adsorbed, and the local concentration increased at the interface with hydrophobic quartz unitsdue to the surface properties. We believe that QCM enables the evaluation of interfacial viscoelasticity.[doi:10.2320/matertrans.MT-M2020392]

(Received January 14, 2021; Accepted February 9, 2021; Published March 12, 2021)

Keywords: quartz crystal microbalance, interface, viscoelasticity, physical gel, methylcellulose

1. Introductions

Electronic materials, adhesives, lubricants, selectivelypermeable membranes, and biomaterials exhibit theirfunctionality when in contact with different materials.Therefore, for these polymeric materials to achieve highperformance, the structure and properties at the interfacebetween the polymer and a dissimilar material must beaccurately understood for material design. The polymerinterface is at a significantly different energy state comparedwith the bulk material,1,2) and its structure and properties arenotably different. Presently, spectroscopy using X-rays,neutrons, and sum frequency generation provides a nonde-structive and accurate method to analyze the structures ofmaterial interfaces,3­7) thereby enabling the incorporation ofinterfacial structures in material designs. However, whenanalyzing viscoelasticity, it is extremely difficult toselectively apply and detect micro-strain and force withoutdestroying the material structures near the interface; thus,evaluation methods are limited.

The piezoelectric oscillation of a quartz resonator has beenused as an ultra sensitive mass sensor, utilizing the Sauerbreyrelationship between the resonance frequency and the massper unit area deposited on the crystal.8) This relationshiphas enabled the quartz crystal microbalance (QCM) to bea mainstay of vacuum science. Kanazawa and co-workersdemonstrated that QCM operation in liquids was possible,9)

opening opportunities for QCM to contribute to manyelectrochemical and biological investigations.10) However,the frequency changes depending not only on mass but alsoon viscoelasticity in liquids. Consequently, we focused onthe depth of ultra-small strains and high-frequency vibrationsfrom the probe of a QCM quartz crystal resonator propagatedto a liquid at a distance from the interface. We conceived that

by applying vibrations from the quartz crystal resonator tocreate the strain necessary to evaluate viscoelasticity, it wouldbe possible to selectively evaluate localized regions near theinterface.

Methylcellulose (MC) is a chemically modified cellulosewhere some or all of the hydrophilic hydroxyl groups (OHgroups) at C2, C3, and C6 of the anhydro-¢-glucose ringrepeating unit are replaced with the hydrophobic methoxygroup (CH3O). The chemical structure of MC was showedin Fig. 1. It is produced from cellulose molecules that areisolated and purified from trees; therefore, it is a naturalresource with a low environmental burden. MC withmoderate methoxy group substitution per glucose ring(degree of substitution (DS) of 1.5­2.0) has a nonuniformDS in a chain; thus, it behaves as a water-soluble polymerat low temperatures, reversibly transitioning to a cloudyhydrogel as the temperature increases.11,12) Heyman believedthat the solution-to-gel (sol-gel) transition of MC is caused bydehydration of the molecular chain during heating.13) Katoet al. proposed hydrogen bond and dipole­dipole interactionoperating between molecular chains, as well as hydrophobicinteraction between chain segments with a high DS, ascandidates for reversible physical crosslinking resulting inreversible gelation.14) Kobayashi et al. showed that MC firstundergoes liquid-liquid phase separation forming a polymerdense phase and a dilute phase, followed by the formation ofphysical crosslinking in the polymer dense phase. Thus,gelation occurs in two steps.15) However, much of the initialpath of phase separation is unknown, and many models

Fig. 1 Chemical structure of methylcellulose used in this study.

+1This Paper was Originally Published in Japanese in J. Japan Inst. Met.Mater. 85 (2021) 23­29. Captions of all Figures and Tables are modified.

+2Graduate Student, Mie University+3Corresponding author, E-mail: fujii@chem.mie-u.ac.jp

Materials Transactions, Vol. 62, No. 5 (2021) pp. 647 to 654©2021 The Japan Institute of Metals and Materials

have been proposed. Takeshita et al. and Fairclough et al.proposed that the phase separation of MC is spinodaldecomposition.16,17) On the other hand, Lodge et al.concluded that the process involves nucleation and growthmechanism,18) while Tanaka et al. explained that it wasviscoelastic phase separation.19) Therefore, the phaseseparation of aqueous MC solutions and the detailed gelationmechanism are still unclear.

In this study, QCM was used to examine the physicalgelation behavior of an aqueous methylcellulose (MC)solution that changed in a thermoreversible manner to gainnew insights into MC gelation. Changes in the resonancefrequency of the quartz crystal resonator in the aqueousMC solution and the dissipation rate were evaluated as afunction of temperature, which enabled the gelation behaviorof the aqueous MC solution to be measured. Results werecompared with the bulk gelation behavior obtained viatraditional transition evaluation methods, namely visualinclination observation, light transmittance measurement,and the measurement of rheological properties. The lattermethod is frequently used. In addition, by regulating theelectrode surface properties of the quartz crystal resonator,the interfacial interaction between the quartz crystal resonatorelectrodes and the aqueous MC solution was changed, theimpact of the interface on the gelation of the aqueous MCsolution was evaluated, and the interface selectivity of theviscoelasticity measurement method using the quartz crystalresonator was examined.

2. Quartz Crystal Microbalance

The QCM method is an extremely sensitive weighingmethod that detects changes in mass at the molecular level onthe quartz crystal resonator electrodes through changes inresonance frequency.8) The AT-cut quartz crystal resonatoris a typical quartz crystal resonator comprising an extremelythin quartz crystal cut along AT plane with thin metal filmelectrodes attached to both sides (Fig. 2(a)). Due to theinverse piezoelectric effect of the crystal, when analternating-current (AC) voltage is applied to electrodes,thickness-shear vibration occurs in the direction parallel tothe crystal surface at a certain resonance frequency. Theresonance frequency of the quartz crystal resonator dependson the thickness of the crystal and is typically high (in theorder of 106Hz). In addition, mechanical strain induced bythe quartz crystal resonator has been reported to be extremelysmall, at a sub-nanometer scale.

When the quartz crystal resonator is vibrating at theresonance frequency, it can be represented by the equivalentcircuit in Fig. 2(b). The electrical characteristics of the quartzcrystal resonator change in response to the environmentand the application of mechanical power.20) The QCM canevaluate changes in the mass on the electrode substrate andchanges in the viscoelasticity of a substance adhering to theelectrode substrate from the electrical characteristics of thequartz crystal resonator. Figure 2(c) shows the conductancespectrum of the quartz crystal resonator measured via QCM.The peak frequency is referred to as the resonance frequency( f ), while the half width at half maximum (! ) of the peak isthe dissipation rate due to the viscoelasticity of substance

adhering to the electrodes. Changes in f and ! ("f and "! )are used to evaluate changes in mass and viscoelasticity.

Complex resonance frequency (�f�) is expressed as afunction of "f and "! in the following equation:20)

�f� ¼ �fþ i��� ð1ÞWhen a minute amount of a rigid substance comes in contactwith electrodes of the quartz crystal resonator, the complexresonance frequency changes in proportion to the change inthe mass on the electrodes, which is at the nanogram scale.However, since changes in the dissipation rate are extremelysmall compared with changes in the resonance frequency(«"! « ¹ «"f «), changes in mass on the electrodes andchanges in the resonance frequency are expressed byeq. (2):8)

�f� � �f ¼ �2� n� f02 ��m=Zq ð2Þ

where n represents harmonics, f0 is the basic resonancefrequency of the quartz crystal resonator, "m is the changein mass per unit volume on the quartz crystal resonatorelectrodes, and Zq is the acoustic impedance of AT-cut quartzcrystal (8.8 © 106 kgm¹2 s¹1).

However, when the quartz crystal resonator is in contactwith a homogeneous Newtonian fluid in a semi-infiniteregion wider than the limit of vibration propagation, thecomplex resonance frequency is proportional to the productof the viscosity and the density of liquid and is expressed bythe following equation:9,20­25)

Fig. 2 (a) Optical image of a quartz oscillator with gold electrodes. (b)Diagram of the equivalent circuit of a quartz oscillator. C0 is thecapacitance of electrode. L1, R1 and C1 are the inductance, resistance, andcapacitance of the AT-cut quartz, respectively. (c) Spectrum of electricalconductance obtained via QCM with the corresponding resonancefrequency ( f ) and dissipation (! ). (d) Schematic representation of aquartz oscillator in a Newtonian liquid. The solid red line representsthe propagation of vibrations damped depending on distance from theinterface (z). u is the displacement field of a shear wave. ¤ is thepenetration depth represented by the analysis depth of the quartz oscillatorin the liquid.20)

K. Yamaoka, Y. Fujii and N. Torikai648

�f�=f0 ¼ ð�1þ iÞ � ð2nf0Þ1=2

� ð©liq � μliqÞ1=2=ð³1=2 � ZqÞ ð3Þ©liq and μliq are the viscosity and density of the liquid,respectively. When �f� is replaced by "f and "! accordingto eq. (1), eq. (3) can be re-organized and expressed as:

j�fj ¼ j�� j ¼ n1=2 � f03=2 � ð©liq � μliqÞ1=2=ð³1=2 � ZqÞ

ð4ÞAs shown in eq. (4), the absolute value of change of theresonance frequency and dissipation rate are equal, moreover,"f and "! are reciprocals of each other. However, thisrelationship does not apply to non-Newtonian fluids.

Furthermore, the vibration amplitude of the quartz crystalresonator attenuates exponentially from the interface. Thus,the distance at which the amplitude is 1/e of vibrationamplitude at the interface is called the viscous invasiveness(¤), which is the analytical depth of the quartz crystalresonator in a liquid (Fig. 2(d)). ¤ is expressed by thefollowing equation:26,27)

¤ ¼ ½©liq=ð³ � f0 � μliqÞ�1=2 ð5ÞWhen the quartz crystal resonator has a basic resonancefrequency of 9MHz, the viscous invasiveness in water isapproximately 190 nm. Therefore, the extremely smallamplitude of the quartz crystal resonator can be directlyapplied to the interface as a stimulant and the viscoelasticityof a microregion near the interface can be measured.

3. Experimental

3.1 Sample and solution preparationWe used Metoloseμ SM-25 provided by Shin-Etsu

Chemical Co., Ltd. as MC with a weight-average molecularweight (Mw) of 5.1 © 104 g/mol, a polydispersity (Mw/Mn)of 1.52, and a DS of 1.8. Vacuum-dried MC powder wasweighed using an electronic balance. An aqueous solutionwith a concentration of 10 times that of the criticalentanglement concentration (C�) was prepared. Here, C� isthe concentration where adjacent polymer chains in thesolution come in contact resulting in entanglement. More-over, it is the concentration where the dilute solutiontransitions to a semi-dilute solution. Since the viscosity ofthe polymer solution increases significantly above C�, it isa key concentration that characterizes the viscosity of apolymer solution. C� is expressed as the inverse of limitingviscosity [©], which represents the coefficient of viscosity permolecule:28)

C� � 1=½©� ð6ÞThe C� of the MC used in the present experiment was0.58mass% in water at 25°C. Viscosity was measured usingan Ubbelohde-type viscometer. When water was directlyadded to the MC powder, only the powder surface becamewet and partially dissolved aggregates formed; thus, weprepared the solution via the hydrothermal method wherewater heated to 70°C or higher was added. The preparedaqueous MC solution was stored overnight at 4°C in arefrigerator before use.

3.2 Visual inclination observationWe visually observed the gelation behavior of the bulk

aqueous MC solution. The solution was heated from 10°Cat a rate of 1°C/min. At pre-determined temperatures, thescrew-cap vial containing the solution was tilted 90° tovisually observe if there was a change in state and fluidity.Subsequently, the solution was cooled to 10°C at the samerate and the change from gel to solution was visuallyobserved. The temperature of the solution was recorded usinga thermocouple thermometer. When tilting the screw-cap vial,a solution that flowed under its own weight was defined as“sol” and a solution that did not flow was defined as “gel”.The temperature at which fluidity was lost was defined as thegelation temperature (Tgel). Each experiment was performedfive times and the average value was used.

3.3 Light transmittance measurementsA spectrophotometer (V-650, JASCO Corporation) was

used to evaluate the temperature dependence of transmittanceto assess the phase separation behavior that induces thegelation of the aqueous MC solution. An aqueous MCsolution with concentration of 10 C� was placed in a quartzcell with an optical path length of 1 cm. The cell was sealedwith a rubber stopper to avoid the evaporation of waterduring heating. An aluminum heating block was used toincrease the temperature of the solution from 20 to 70°C ata rate of 1°C/min. The transmittance of light with awavelength of 380­780 nm was measured every 5°C, aswell as every 2°C between 40 and 60°C in the vicinity of thegelation temperature. Subsequently, the MC gel that washeated to 70°C was cooled at a rate of 1°C/min, and thetransmittance of light with a wavelength of 380­780 nm wasmeasured every 5°C. This measurement was performed every2°C between 40 and 20°C in the vicinity of the temperaturewhere the gel returned to sol.

3.4 Rheology measurementsA rheometer (MCR302, Anton Paar GmbH) was used to

evaluate changes in viscoelasticity associated with thegelation of the bulk aqueous MC solution. We pouredapproximately 20mL of the solution into the cup of a coaxialcylindrical jig, and after moving the rotor (inner cylinder) tothe measurement position, a sample from the upper part ofthe rotor was removed using a pipette (trimming) to improvethe reproducibility of the experimental data. The upper part ofthe sample was sealed with silicon oil with viscosity of 10 cS(Shin-Etsu Chemical Co., Ltd.). The provided lid for theprevention of solvent evaporation was applied from the topof the jig to minimize changes in concentration throughsolvent evaporation during measurement. The resonancefrequency was set at 1Hz and strain was fixed at 1%, whichis the linear range. The storage modulus (GA) and the lossmodulus (GAA) were measured in 1°C increments. Temper-ature was regulated via a Peltier temperature control system(C-PTD200, Anton Paar GmbH) and was increased from 10to 70°C at a rate of 1°C/min. Subsequently, the aqueous MCsolution was cooled down to 10°C at the same rate and thetemperature dependence of the moduli during gel-to-soltransition was evaluated.

Evaluation of Local Gelation Behavior of Aqueous Methylcellulose Solution Using Quartz Crystal Microbalance 649

3.5 QCM measurementsFigure 3 illustrates a schematic of the experimental device.

The quartz crystal resonator had a basic resonance frequencyof 9MHz and gold (Au) electrodes. The surface of theelectrodes was ultrasonically cleaned for 15min in ethanol.The quartz crystal resonator with a Teflon dip-type cell,which allows for measurement in liquids, was immersed inthe aqueous MC solution. The temperature of the solutionwas regulated using an aluminum heating block and washeated from 10 to 70°C at a heating rate of 1°C/min. Thetemperature of the solution near the quartz crystal resonatorwas recorded using a thermocouple thermometer. "f and "!

were measured via a quartz crystal microbalance measure-ment system, QCM922A (SEIKO EG&G Co., Ltd.). The MCgel heated to 70°C was cooled to 10°C at a rate of 1°C/min,and the temperature dependence of "f and "! duringtransition from gel to sol was evaluated.

In addition to the Au electrode quartz crystal resonator, weused a silica (SiO2) electrode as the hydrophilic surface. Thenatural oxide layer (Si­OH group) at the outermost surfaceof the silicon (Si) electrode quartz crystal resonator washydrophobized (Si­H groups) using a 1% of hydrofluoric acidaqueous solution. We examined the impact on gelation of theinteraction at the interface between these three electrodes andthe aqueous MC solution with a concentration of 10C�.

4. Results and Discussion

4.1 Visual inclination observation of the gelation behav-ior of the aqueous MC solution

Figure 4(a) shows photographs of the state changeassociated with increased temperature of the aqueous MCsolution with a concentration of 10C�. At lower temperatures,MC dissolved in water forming a clear and colorless aqueoussolution. However, as the temperature increased, the solutionbecame cloudy due to the change in the solubility of theMC molecules in water. Methoxy groups within the MCmolecules dehydrated as the temperature increased.29) Aschain segments with numerous hydrophobic methoxy groupsaggregated through hydrophobic interaction, phase separa-tion into a polymer dense phase and a dilute phaseoccurred,17) resulting in the clouding of the aqueous solution.As clouding progressed, the viscosity of the solution

increased. As the temperature continued to increase, at aspecific temperature, the solution completely lost its fluidityand changed to a gel. Within the polymer dense phase,physical crosslinking occurred leading to aggregation. Thehydrophobic parts of the MC acted as crosslinking points,leading to the reversible formation of a network structure.The temperature at which the fluidity of the solution wascompletely lost was 50.9 « 0.9°C, which was set as thevisual Tgel. During cooling (Fig. 4(b)), the solution clearedwith decreasing temperature and fluidity re-appeared atapproximately 30°C, which was lower than that in case ofthe Tgel obtained during heating. Thus, hysteresis wasobserved in the gelation behavior of the aqueous MCsolution.

4.2 Coarsening of the aggregate structure associatedwith gelation

Figure 5(a) shows the temperature dependence of trans-mittance during heating measured in the wavelength bandof 380­780 nm. When the aqueous methylcellulose solutionwas clear and colorless, transmittance was almost 100%.However, transmittance at 380 nm was lower at approx-imately 80% because the MC molecules absorb light near210 nm within the ultraviolet region. When heated, trans-mittance rapidly decreased at approximately 35­40°C. Thetemperature at which transmittance began to decrease shiftedtoward higher temperatures as the wavelength of the lightincreased. We believe this was due to the size of aggregatesconsisting of MC molecules. When the temperature of theaqueous MC solution was low (20­30°C), the molecules

Fig. 3 Schematic illustration of QCM measurement equipment.

Fig. 4 Optical images of the aqueous methylcellulose solution at varioustemperatures during (a) heating and (b) cooling.

K. Yamaoka, Y. Fujii and N. Torikai650

dissolved in water and minimal aggregation of the moleculesoccurred.30) Therefore, most light passed through the aqueousMC solution without scattering. However, since light withshort wavelengths was scattered by the MC molecules, short-wavelength transmittance was reduced even at the lowtemperatures. As the temperature of the solution increased,MC molecules aggregated. As the size of the aggregatesincreased, initially only short-wavelength light was scattered,reducing transmittance. As the size of aggregates furtherincreased, even longer wavelength light was scattered Attemperatures of 60°C or higher, transmittance of allwavelengths reduced to 0% and visual observation confirmedcomplete clouding of the MC gel.

The temperature dependence of transmittance duringcooling (Fig. 5(b)) displayed a different behavior from thatduring heating. At all wavelengths, transmittance rapidlyincreased at temperatures, above which transmittance rapidlydecreased during heating, i.e., 20°C. This confirmed hystere-sis and the thermal reversibility of the gelation of aqueousMC solutions with respect to the temperature dependence oftransmittance. In addition, since the transmittance of longwavelengths gradually increased with cooling, it is assumedthat the size of the aggregates in the MC molecular chaingradually decreased during cooling.

4.3 Moduli changes associated with the gelation of theaqueous MC solution

Figure 6 shows the temperature dependence of the storage

modulus (GA) and loss modulus (GAA) of the aqueous MCsolution with a concentration of 10C�. At lower temperatures,GAA (viscosity component) was larger than GA (elasticitycomponent), indicating that the aqueous MC solution was inthe sol state. The gradual decrease in the moduli between 10and 40°C was caused by the increased thermal activity ofmolecules with increasing temperature that led to decreasingintermolecular interaction, which in turn lowered the solutionviscosity.31) Above approximately 40°C, all moduli rapidlyincreased. At higher temperatures, GAwas larger than GAA andthe aqueous MC solution transitioned to the gel state. Thus,we defined the temperature at which GA and GAA reversed asthe “rheometer Tgel”. The rheometer Tgel of the aqueous MCsolution with a concentration of 10C� was 50.4°C. On theother hand, during the cooling of the MC gel, GA and GAA bothdisplayed constant values down to 40°C, followed by a rapiddecrease from approximately 35°C. The relative values of GAand GAA reversed at 25°C. The moduli of the aqueous MCsolution followed different paths during heating and cooling,thus displaying hysteresis, which was attributed to thegelation of MC being an entropy-driven reaction.32) Tohydrate the dehydrated MC molecules, entropy must belowered to change water molecules from a random state to arelatively ordered state. To produce the required energy state,the aqueous solution must be cooled. Therefore, the networkstructure of the MC molecular chain was maintained at alower temperature, leading to observation of hysteresis. Aftercooling to below 15°C, the values of the moduli were similarto those before heating. This indicates that the gelation ofthe aqueous MC solution is thermally reversible.

4.4 Investigation of gelation behavior of the aqueousMC solution via QCM

We used a quartz crystal resonator with Au electrodes tomeasure the temperature dependence of changes in resonancefrequency ("f ) and dissipation rate ("! ) associated with thegelation of the aqueous MC solution with a concentration of10C�. The results are shown in Fig. 7. "f and "! weredependent on the solution viscosity. The gradual increase in"f (decrease in "! ) between 10 and 40°C was caused by adecrease in the solution viscosity associated with increasing

Fig. 5 Temperature dependence of the transmittance of the aqueousmethylcellulose solution during (a) heating and (b) cooling.

Fig. 6 Temperature dependence of the storage modulus (GA) and lossmodulus (GAA) of the aqueous methylcellulose solution during heating andcooling.

Evaluation of Local Gelation Behavior of Aqueous Methylcellulose Solution Using Quartz Crystal Microbalance 651

temperature, similar to the gradual decrease in moduliobserved during the measurement of rheological proper-ties.33) "f decreased rapidly (increase in "! ) at temperaturesabove 45°C because the gelation of the aqueous MC solutionrapidly increased the solution viscosity. Subsequently, at60°C and higher, the gelation of the aqueous MC solutionwas complete; therefore, "f and "! displayed constantvalues. As such, "f and "! changed due to the gelationof the aqueous MC solution. Therefore, we defined theinflection point where "f rapidly decreased as the “QCMTgel”. The QCM Tgel of the aqueous MC solution with theconcentration of 10C� was 50.4 « 0.5°C. It is listed inTable 1 along with Tgel obtained from the measurement ofrheological properties. The Tgel values obtained via thedifferent measurement methods were consistent. When the

MC gel was cooled from 70 to 10°C, "f and "! did notchange until near 40°C, displaying constant values. Fromapproximately 35°C, "f rapidly increased ("! decreased)to a value similar to the pre-heating value at 20°C and below.Hysteresis and thermal reversibility of the aqueous MCsolution observed during rheological measurements werealso observed as changes in "f and "! during the QCMmeasurements, empirically demonstrating that QCM can beused to evaluate the gelation of the aqueous MC solution.

4.5 Effect of the surface properties of the quartz crystalresonator

Figure 8 shows the temperature dependence of "f and "!

measured via quartz crystal resonators with three differentelectrodes, namely Au, hydrophilic SiO2, and hydrophobicSi. There was no notable difference in the temperaturedependence of "f and "! for the aqueous MC solutionwhen using the Au and SiO2 electrodes. However, whenusing the hydrophobic Si electrode, the resonance frequencywas approximately 1000Hz lower than that measured withthe Au and SiO2 electrodes, while the dissipation rate wasapproximately 500Hz higher, indicating that the viscosityof the solution was high near the interface. Since the changein the resonance frequency was greater than the change inthe dissipation rate, it was assumed that MC molecularchains were adsorbed onto the electrode thereby increasingthe viscosity. In addition, the Tgel values obtained from

Fig. 7 Temperature dependence of (a) the resonance frequency shifts and(b) the dissipation shift of the aqueous methylcellulose solution duringheating and cooling.

Table 1 Gelation temperature of an aqueous methylcellulose solutiondetermined by visual observation, rheometer measurements and QCMevaluation.

Fig. 8 Temperature dependence of (a) the resonance frequency shifts and(b) the dissipation shifts of the aqueous methylcellulose solution with Au(yellow circles), SiO2 (gray squares) and Si (blue triangles) electrodes.

K. Yamaoka, Y. Fujii and N. Torikai652

temperature dependence of "f for each electrode aresummarized in Table 2. The lowest value was observed incase of the Si electrode. It was implied that at the interfacewith the Si electrode substrate, the local MC concentrationwas higher than at the Au and SiO2 electrode interfaces.

To determine the reason for the differences in Tgel for thedifferent electrodes, we evaluated the surface free energy (£)and root mean square (RMS) roughness of each electrodesurface. £ was calculated from the contact angle of theelectrode surface to water and diiodomethane, while RMSroughness was evaluated via atomic force microscopy of theelectrode surface (Table 2).

The SiO2 and the Si electrodes that had been hydro-phobized with hydrofluoric acid displayed similar £ valuesthat were larger than that of the Au electrode. £ of the Auelectrode was close to the theoretical value;34,35) however, theSi and SiO2 electrodes deviated from the hydrophilic andhydrophobic behavior observed for a typical Si substratesurface. In case of the Si electrode, we believe that this wasdue to the extremely unstable nature of the Si­H group thatcovers the outermost surface of the dehydrated Si electrode,along with the impact of being oxidized even in ambientatmosphere. For the SiO2 electrode, the deviation was dueto the inadequate acidification of the outermost surface ofthe electrode because of the structure of the quartz crystalresonator. In addition, MC has both hydrophilic hydroxylgroups and hydrophobic methoxy groups, thus displayingamphiphilicity. Therefore, adhesion occurred on all surfacesindependent of the hydrophilic or hydrophobic nature of theelectrode resulting in no significant difference in £. The aboveresults are expected since surface free energy exhibitsextremely short distance interaction compared with the depthof analysis of the quartz crystal resonator.

The RMS roughness was approximately 0.8 nm for the Auand SiO2 electrodes, but notably lower than 1.8 nm for thehydrophobized Si electrode surface. Since the RMS rough-ness of the Si electrode surface prior to hydrophobizationby hydrofluoric acid was 0.76 nm, the hydrofluoric acidtreatment likely increased the surface roughness of the Sielectrode. Consequently, the surface area of the Si electrodewith hydrophobization treatment increased and more MCmolecules adhered to the interface than in case of otherelectrodes. (This was reflected by the change in "f.) Thus, itcan be concluded that an increase in the surface area ofelectrodes resulted in a higher local concentration of theaqueous MC solution near the interface, leading to a decreasein Tgel.

5. Conclusions

We successfully observed changes in solution viscosityassociated with the gelation of an aqueous MC solution withthe concentration of 10C� as changes in resonance frequencyby via QCM and thereby determined the gelation temper-ature. Similar to the rheological behavior, the hysteresis andthermal reversibility of the aqueous MC solution weresuccessfully demonstrated using the temperature dependenceof "f and "!. In addition, the temperature dependence of "fand "! associated with the gelation of the solution usingthree different electrodes was investigated. The measure-ments confirmed an increase in the adsorption of MCmolecules onto the increased surface area of the quartzcrystal resonator electrodes and an associated decrease in Tgel,indicating that QCM can measure viscoelasticity near theinterface.

Acknowledgement

This work was supported by JSPS KAKENHI GrantNumbers JP19H05720 and JP16K05926. Part of this studyutilized the Alumni Association research fund of the Facultyof Engineering at Mie University. In addition, the measure-ment of rheological properties was performed at NationalInstitute for Materials Science (NIMS) supported by NIMSJoint Research Hub Program. We would like to extend ourmost sincere appreciation to the NIMS Data-driven PolymerDesign Group Leader, Dr. Masanobu Naito, for providing anopportunity for measurement.

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Table 2 Gelation temperature (Tgel) of an aqueous methylcellulose solutionvia a quartz oscillator with various electrodes. £ is the surface free energyand RMS is the root mean square of the surface roughness of theelectrodes.

Evaluation of Local Gelation Behavior of Aqueous Methylcellulose Solution Using Quartz Crystal Microbalance 653

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