Probing Glass Transitions in Thin and Ultrathin Polystyrene Films byStick−Slip Behavior during Dynamic Wetting of Liquid Droplets onTheir SurfacesBiao Zuo, Chao Qian, Donghuan Yan, Yingjun Liu, Wanglong Liu, Hao Fan, Houkuan Tian,and Xinping Wang*

Department of Chemistry, Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Education Ministry,Zhejiang Sci-Tech University, Hangzhou 310018, China

*S Supporting Information

ABSTRACT: A novel method was developed to detect the glasstransition of thin and ultrathin polystyrene (PS) films by correlatingthe relationships between the temperature-dependent viscoelasticity ofthe PS films and stick−slip behavior on their surfaces during dynamicwetting of glycerol or oligo-poly(ethylene glycol) droplets. The peaktemperature (Tjm) obtained from the jumping angle−film temperaturecurve, in which the jumping angle Δθ was employed to scale thestick−slip behavior, was nearly identical to the corresponding Tg (orTα) of the PS film. This was confirmed by dynamic mechanical analysis(DMA) and differential scanning calorimetry (DSC). The change of the measured Tjm with film thickness and substratechemistry (SiO2−Si and H−Si) further confirmed that the developed method is very sensitive for detecting the dynamics ofultrathin polymer films.

■ INTRODUCTION

With the recent advances in nanotechnology, increasing interestin thin film coatings and nanodevices of polymer materialsexacerbates our concerns on the precise understanding of thephysical properties and performances of nanodevices related tothe segmental dynamics of polymers under confinement.1−5

The glass transition temperature (Tg) is defined as thetemperature at which the dynamics of molecules dramaticallyslow down6,7 and the molecular liquid falls out of equilibrium,8

manifested as a dramatic increase of the mechanical modulus,an increase in relaxation times, and also in the rise of viscosityby many orders of magnitude.6−8 It is a key parameter forcharacterizing the mobility of polymer chains and determinesthe temperature range for application of many materials. Theglass transition dynamics in thin or ultrathin polymer films(thickness comparable to the radius of gyration of chains) is afundamental as well as a pressing problem in polymercondensed matter physics.9−12 Because of the finite sizeeffects,13−16 surface and interface effects,17−21 and otherimpacts,22−25 the glass transition dynamics of nanometers-thick polymer films significantly deviates from that in bulk. Theglass transition phenomenon in nanostructured systems is arelatively new topic and object of intense scientific debate.Despite two decades of effort since 1994 with the first report

by Keddie of the striking decrease of Tg for polymer thinfilms,26 how the glass transition dynamics of ultrathin polymerfilms differ from those in bulk remains unclear and has becomean intellectual challenge, partly due to the inherent measure-ment difficulties of the dynamics at nanoscales. Four types of

accepted methods have been developed to measure the Tg ofpolymer thin films, which are based on (1) the discontiguouschanges of volume, thermal expansion, or density,20,22,23,26−32

(2) nanorheological properties,33−36 (3) the specific heat of thepolymer,37,38 and (4) the segmental dynamics39−41 at the glasstransition region. Some additional techniques, such as thermalprobe,42 Raman43 and infrared spectroscopy,44 sum frequencygenerational spectroscopy,45 and birefringence46 are alsosensitive to the glass transition of polymer films.Although considerable approaches have been developed for

the Tg measurements of thin films, much controversy remainsdue to apparently contrasting and conflicting results whichexhibit variously an increase,21,29,47,48 decrease,22,23,26,31,33−35 orno change of Tg

36−39,45 with the thickness of thin films, asobtained by various techniques. The lack of consensus hasprovided a strong motivation for development of alternativetechniques.9 The development of accepted models to describethe glass transition dynamics in confined geometries will alsorequire continued efforts at developing new experimentalapproaches and insightful mechanisms. Such new experimentalapproaches and mechanisms are of significance, both in termsof unveiling the nature of glass transition dynamics and forproviding effective ways for resolution of the controversies inunderstanding the glass transition dynamics of confined films.

Received: November 17, 2012Revised: January 21, 2013Published: February 22, 2013

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The wetting dynamics of a solid surface by a liquid greatlydepends on the corresponding mechanical properties of thesolid film.49−55 Carre and Shanahan demonstrated that thespreading of a liquid on a soft solid was much slower than thaton a rigid solid.50−52 Long et al.56,57 simply modeled thecorrelation between the dynamic wetting properties of a liquidwith the viscosity and relaxation time of rubber films and foundthat the spreading speed of a liquid can be controlled by theviscoelasticity of the polymer films. Therefore, it can be inferredthat observation of the dynamic wetting of liquid droplets onpolymer films may be one potential way for detecting theviscoelasticity and glass transition of polymers.Automated axisymmetric drop shape analysis-profiling

(ADSA-P) has been shown to be a very powerful method forstudying the dynamic wetting on a surface by advancing andreceding contact angle measurements.58,59 A phenomenontermed “stick−slip” may occasionally be observed during theevolution of a drop in advancing contact angle measurements,in which the wetting front remains static for most of the timebut from time to time moves quite abruptly.58,60,61 Our groupreported previously that the stick−slip patterns occurringduring water drop spreading appear to be strongly dependenton the surface viscoelasticity of poly(styrene-b-isoprene-b-styrene) triblock copolymer films, in which there is a linearrelationship between jumping angle (Δθ) from stick−slippatterns and the surface modulus of the film.61 Theviscoelasticity of polymers is temperature dependent, the elasticmodulus shows a sharp reduction, and the viscoelasticdissipation of polymers reaches its highest in the glass-to-rubbery transition region.33 This change in polymer viscoelas-ticity by temperature variance would bring about largeperturbations of the dynamic wetting behavior, e.g., stick−slip, of a liquid.The purpose of this study is to directly detect the glass

transition of polystyrene (PS) thin films on the basis oftemperature-dependent dynamic wetting of a liquid. To thisend, we first measured the temperature-dependent dynamicwetting behavior of liquid on a PS thick film (around 600 nmthick) and verified that dynamic wetting is a feasible way todetect the glass transition of polymers. In the second part ofthis study, this approach was utilized to measure the glasstransition of PS thin and ultrathin films with thickness down to4.5 nm, and the effect of substrate/PS interfacial interactions onthe glass transition of PS ultrathin films was also investigated.

■ EXPERIMENTAL SECTIONMaterials. Atactic-polystyrene (PS) (Mw = 56 530 g/mol, PDI =

1.08) was synthesized by atom transfer radical polymerization(ATRP), and the other five monodispersed atactic-PS polymers withmolecular weights ranging from 3.7 to 815 kg/mol (PDI = 1.02−1.04)were purchased from Showa Denko K.K. in Japan (Shodex Standard).The oligo-poly(ethylene glycol) (PEO) with molecular weight of 400and glycerol were purchased from Aldrich Chemical Co. and used astest liquids for dynamic wetting measurements. We used silicon (100)wafers (Aldrich Co.), diced into 3.0 × 3.0 cm2 pieces, as substrates.According to the reported method,62 if SiO2−Si substrates are desired,the silicon wafer is submerged in a piranha solution consisting ofH2SO4:H2O2 (3:1) preheated to 363 K for 30 min, then rinsedthoroughly in excessive deionized water, and dried with nitrogen. Thisprocess removes any organic contaminants, leaving the silicon surfaceas a native oxide layer covered with the Si−OH groups with watercontact angle less than 10°. Unless otherwise stated, the native oxide-covered silicon wafer (SiO2−Si) was used as the supporting substrateof the PS films. If hydrogen-passivated silicon (H−Si) is desired, asoutlined in a reported method,29 we submerged the silicon wafer,

precleaned by piranha solution as above, in a 4% aqueous solution ofHF for 5 min and then rinsed and dried them as described above. Thisprocedure removes the native oxide layer and leaves the silicon surfaceterminated with Si−H groups with water contact angle of 72 ± 4°.

Since the advancing contact angles on PS films were measured attemperatures ranging from 303 to 453 K, the oligo-poly(ethyleneglycol) (PEO) withMn of 400 and glycerol with high boiling point andlower vapor pressure were chosen as test liquids for advancing contactangle measurements. The physical parameters of these two liquids aregiven in Table 1. The boiling points of PEO and glycerol are well

above the glass transition temperature (Tg ≤ 378 K) of PS film. PS isinsoluble in PEO and glycerol at room temperature and attemperatures as high as 403 K over extended periods of time, asconfirmed by both the appropriate solubility parameters anddissolution experiments. In addition, PS film surfaces were not swollenby PEO and glycerol in the time scale of advancing contact anglemeasurements (∼3 min) even if the PS film was heated to 400 K.

Film Formation. The PS films were prepared by the spin-coatingmethod, from toluene solution onto the clean substrates. Thethickness of the PS films was controlled only by varying theconcentration of the PS solution, while keeping the spin speedconstant at 2000 rpm/min. The thickness of the PS films wasmeasured by spectroscopic ellipsometry (M-50, JASCO Co., Ltd.). Allthe samples underwent an annealing process at Tg + 10 °C for 24 h invacuum in order to extract the residual solvent and residual stress asmuch as possible. AFM analysis showed no apparent dewetting afterthermal annealing. Films for dynamic mechanical analysis (DMA) testswere prepared using the solution casting method, with thickness ofabout 50 μm. Before measurement, the residual solvent was removedby thermal annealing at Tg + 10 °C for 72 h in a vacuum.

Advancing Contact Angle Measurements on the FilmSurface at Various Temperatures. The PS films were mountedonto a heating stage, which can adjust the temperature from 293 to473 K, with accuracy of ±1 K. The viscoelasticity of the PS films wascontrolled by the heating temperature. Advancing contact angles ofliquid droplets on PS film surfaces were measured in situ using theautomated axisymmetric drop shape analysis-profile (ADSA-P)method.58,59 An initial drop with a diameter larger than 3 mm wasdeposited onto the PS surface and ensured to be axisymmetric. Amotor-driven syringe was used to pump liquid steadily into the sessiledrop, and a sequence of images of the growing drop was thencaptured, using the drop shape analysis system (Kruss DSA 10-MK2,Germany). The dynamic contact angle behavior in advancing modewas obtained by tracing the evolution of contact angle (θ) and thediameter of the drop three-phase line (d) with liquid added into thedrop. The volume addition velocity used in this study was 0.26 μL ofliquid/s. Each experiment was repeated at least 15 times, using a freshpolymer film each time, to ensure the reproducibility of results.

Shape of Wetting Ridge Profile. To visualize the shape profile ofthe wetting ridge hidden beneath the droplets, the PS films werequenched to room temperature after dynamic wetting measurementsand then rinsed by ethanol to remove the drop. Afterward, films for

Table 1. Physical Parameters of Test Liquid and PS Used inthe Experiments

sample

surfacetension

(mN/m) at298 K

viscosityc

(Pa·s) at298 K

boiling point(K)

solubilityparameterδ (MPa1/2)

densityg

(g/mL)

glycerol 62.1a 0. 94 564 33.8d 1.261PEO 47.9a 0.085 479−482d 23.1e 1.128PS 40.7b 17.8f 1.050

aMeasured by surface tensiometer DCA-322 (Cahn Instruments Co.).bFrom J. Phys. Chem. 1970, 74, 632. cMeasured by cone−plateviscometer (Brookfield Co.). dIn ref 63. eHansen parameters estimatedby Hansen’s three-component method.63 fFrom Nature 1959, 183,818. gProvided by suppliers.

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measurements were dried with nitrogen gas to evaporate the residualethanol. The shape of the wetting ring ridge was obtained by reflectiongeometry using an optical microscope. The three-dimensional profilesof the “wetting ridge” were accessed by XEI-100 scanning probemicroscopy (psia Co., South Korea).Other Characterizations. Dynamic mechanical analysis (DMA)

was conducted with a Pyris Diamond dynamic mechanical analyzer(Perkin-Elmer Co.) at a frequency of 1 Hz over a temperature range of293−473 K with a heating rate of 2 K/min at 0.05% strain under anitrogen atmosphere. The test specimens had a gauge length of 20mm, a width of 10 mm, and a thickness of about 50 μm.The DSC curve was obtained with a Pyris Diamond differential

scanning calorimeter (Perkin-Elmer Co.). All specimens were firstheated to 403 K at a rate of 10 K/min under a nitrogen atmosphere toremove the thermal history. Subsequently, the samples were heated asecond time at a rate of 20 K/min, and Tg was determined from thesecond heating scan. The weights of all samples, determined byelectronic balance with 0.0001 g of sensitivity, were in the range 3−4mg.

■ RESULTS AND DISCUSSIONDynamic Wetting Behavior on Polystyrene (PS) Thick

Films at Various Temperatures. The advancing contactangle measurements were first performed using glycerol as atest liquid on a PS (Mw = 56 530 g/mol) thick film withthickness of about 600 nm, which was sufficient to avoidsubstrate and ultrathinning effects.64,65 It was found that thethree-phase line smoothly advances with increasing liquiddroplet volume when the temperature of the PS film was below360 K, resulting in a constant advancing contact angle. Thisresult was the same as that obtained on a rigid H−Si surface.When the PS film temperature exceeds 370 K, the dynamicwetting of glycerol on PS film surface shows remarkabledifferences; i.e., stick−slip behavior is clearly observed. Thewetting behavior of the glycerol droplet on PS film surface at386 K shows a stick−slip pattern, as illustrated in Figure 1: asliquid is pumped into the drop and the drop volume increases,the contact angle increases linearly at a constant contact radius(i.e., the drop front remains hinged). The three-phase line thenabruptly slips to a new position on the solid surface as moreliquid is supplied. The slip of the three-phase line isaccompanied by a sudden decrease in the contact angle andby a sudden increase in the contact radius. By supplying moreliquid, the three-phase line again sticks to the solid surface at anew location, and the radius remains constant. This behavior isrepeated as the measurement continues, and a sawtooth-shapedcontact angle vs time curve emerges (Figure 1).When continuing to increase the temperature of PS film, the

transition of the three-phase line from stick−slip and then tosmooth motion was observed. In order to compare the stick−slip behavior on PS film surfaces at various temperatures, thejumping angle (Δθ) was employed as a parameter which couldbe determined by averaging the differences (Δθ = θ1 − θ2)between the contact angle before (θ1) and after (θ2) slipping(shown in Figure 1) during each measurement. The resultingrelationship between Δθ and the temperature of the PS films isshown in Figure 2. It was found that the jumping angle Δθ wasalmost zero when the PS film temperature was above 423 K andbelow 363 K. However, apparent stick−slip behavior wasobserved on PS film surfaces in the intermediate temperaturerange from 363 to 423 K and a maximum Δθ value was locatedat around 386 K. A characteristic temperature (Tjm) at whichmaximum Δθ occurs was defined to describe the temperaturedependent dynamic wetting behavior on PS film surfaces(shown in Figure 2).

Although stick−slip phenomena have been reported as beingdue variously to an external noise,66 swelling of a noninertcomponent on the polymer surface,59,67 vapor adsorption,58

and surface roughness and imhomogeneity,58,68,69 it can bededuced as previously reported61 that the stick−slip patternsoccurring on the PS film surfaces within a range oftemperatures in the present study are not attributable to anyof the factors described above. Glycerol is a viscous liquid with

Figure 1. Evolution of contact angle (a), contact diameter (b), volumeand (c) of glycerol droplet on the surface of PS (Mw = 56 530 g/mol)film at temperature 386 K, as liquid is continuously added with time.Pictures labeled “1” to “5” in panel d display the profile evolution of aglycerol droplet with liquid added into the drop during one stick−slipcycle. The stick−slip of the three-phase line and abrupt changes in thedrop radius by increases in the volume are evident. The definition ofjumping angle is shown in panel a: when a higher limit of θ1 is obtained,the triple line “jumps” from θ1 to θ2 (θ1 − θ2 = Δθ, jumping angle)with increase in drop volume. Film thickness: around 600 nm.

Figure 2. Jumping angle Δθ and tan δ plotted as a function oftemperature of PS (Mw = 56 530 g/mol) films. Glycerol used as testliquid. Film thickness: around 600 nm. DMA curves were obtained at2 °C/min and frequency 1 Hz.

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low vapor pressure, and its viscosity is temperature dependentand decreases with increasing temperature. The dynamicwetting behavior of glycerol on a rigid H−Si substrate wasinvestigated at various temperatures. The results indicate thatno “stick−slip” behavior was found within the range ofexperimental temperatures. This suggests that the temperaturedependent stick−slip behavior observed herein on PS films wasnot caused by the change of glycerol properties (e.g., viscosity,surface tension, evaporation rate). In addition, we alsoconducted the same experiment using PEO oligomer withlower viscosity and surface tension as a test liquid. The resultsare shown in Figure S1 (Supporting Information), whichpresents a similar relationship between jumping angle (Δθ) andthe temperature, and shows the same Tjm value, at 385 K. Theagreement of the temperature at peak jumping angle vstemperature curve (Tjm) obtained by dynamic wetting experi-ments using either PEO or glycerol as test liquid thus verifiesthat the Tjm has no correlation with the nature of the test liquid.Figure 3 shows the trace of the “wetting ridge” on PS film

surface after a dynamic wetting experiment at 380 K. It isapparent that three concentric “wetting ridges” with height ofabout 165 nm reside on the surface of the PS film. Each ridgecorresponds to one expansion of the three-phase line duringone stick−slip cycle. However, no concentric wetting ridge wasobserved when the film temperature was out of the regionwhere stick−slip behavior was exhibited. The dynamicmechanical analysis spectrum in Figure S2 (SupportingInformation) shows that the storage modulus of the PS filmbegins to decrease when the temperature is above 363 K, whichsuggests that the PS film changes from glassy to viscoelasticstate at this point. Therefore, this ridge near the three-phaseline shown in Figure 3 was formed due to the vertical forcecomponent of the liquid surface tension and the disjoiningpressure. Since the effect of the disjoining pressure on thesurface deformation cannot be resolved by our experimentaltechnique and it was same as that of the vertical forcecomponent of the liquid surface tension, it is thereforereasonable to neglect the effect of disjoining pressure in thefollowing section. Therefore, the height of the ridge (h) couldbe determined by the shear modulus (G) of the film: h = γ sinθ/G (γ is surface tension of the liquid) (Figure 4).50−52 WhenPS film changes from glassy to viscoelastic state (reducedmodulus) with increasing temperature, the component of liquidsurface tension acting perpendicularly to the solid causes adeformation or “wetting ridge” on the film surface. The filmwith lower modulus will cause large local deformations,resulting in a higher “wetting ridge” to prevent the spreadingof the drop. This may be the phenomenological reason why Δθincreases with increasing PS film temperature below 386 K, as

shown in Figure 2. However, with the film temperatureincreased to above 403 K, PS experiences a transition from therubbery to the viscous state, and the chain relaxation of PS isvery fast at high temperatures. With liquid added into the drop,the formed wetting ridge is able to propagate with the advancesof the three-phase line and thus cannot hold the meniscus ofthe drop, resulting in close to smooth motion of the three-phase line. The spreading behavior of liquid on a polymersurface with various mechanical states is shown in Figure 4.Therefore, the jumping angle may be an important parameterfor studying mechanical properties of polymer thin films, andthis will be reported on in further detail in the future.The damping factor tan δ of PS film as a function of

temperature obtained by dynamic mechanical analysis (DMA)is shown in Figure 2. It is clear that the curves of both tan δ andΔθ plotted as a function of temperature share the samefeatures. When Tg of PS was determined from the peak of thetan δ curve,70,71 the Tg of PS was almost the same as thetemperature (Tjm) at the observed peak in the Δθ vstemperature curve. Tan δ is an indicator of the capacity forviscoelastic dissipation of polymers.51,71 This perfect correlationbetween Δθ and tan δ demonstrates that the “stick−slip”behavior intrinsically originates from the viscoelastic dissipationin PS films. At the glass−rubbery transition region, theviscoelastic dissipation of PS film reaches a maximum; thus,the “stick−slip” behavior occurs during advancing contact anglemeasurement (Figure 4).The correlation between Δθ and tan δ of PS can be readily

interpreted based on the “viscoelastic break” mechanism, whichproposes that the viscoelastic dissipation as a main forceprevents the spreading of liquid.50−52 For a viscoelasticsubstrate, the spreading kinetics of a liquid are mainlycontrolled by the transformation of the excess capillarypotential energy of a drop released during spreading to theviscoelastic dissipation of substrate due to “wetting ridge”formation and motion.50−52 The balance between the excesscapillary free energy of a drop and the viscoelastic dissipation ofthe PS substrate determines the movement of the three-phaseline. The fraction of energy lost during deformation of

Figure 3. Optical image (a), AFM topographical image of the arrowhead indicating area in panel a (b), and line profile (c) of the concentric “wettingridge” on PS surface after dynamic wetting experiment at 380 K. Glycerol used as test liquid. The thickness of the film was around 600 nm.

Figure 4. Schematic representation of liquid spreading on polymersurfaces in various mechanical states.

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viscoelastic polymers is a function of the damping factor (tanδ).51,71 In the glass transition region, the tan δ of PS filmachieves its maximum value. Accordingly, the greater the valuetan δ of PS film, the more excess energy that can be stored bythe drop, thus resulting in a larger jumping angle (Δθ). This isthe main reason why the temperatures at which both tan δ andjumping angle Δθ reached their maximum values are almost thesame. This mechanism suggests that the glass transition of PSfilm could be detected by investigating the dynamic wettingbehavior of liquid droplets on film surfaces at varioustemperatures.This suggestion was further confirmed by the comparison

between the Tg and Tjm values of PS of various molecularweights, as shown in Figure 5. Since Tg of PS with Mw below

the entangled molecular weight is hard to detect by DMA dueto its poor film forming ability and high fragility, DSC, which isa very popular method for obtaining Tg of polymers, wasemployed to measure the Tg of PS samples with variousmolecular weights. The results show that Tjm is also molecularweight dependent and increases with increasing molecularweight before reaching a threshold value Mw. Both molecularweight dependent Tg and Tjm could be readily fitted with theFox−Flory equation (Tg = Tg

∞ − A/Mn),72 in which the

resulting fitting parameters are Tg∞ = 378 K, A = 108 (mol K)/

kg and Tjm∞ = 386 K, A = 99 (mol K)/kg. The Mw-dependent

parameter A obtained by two different methods is very close to100 (mol K)/kg, which was the value reported by Fox andFlory.72 Alternatively, it was found that the Tjm

∞ value is 8 Khigher than the Tg

∞ measured by DSC. This small discrepancybetween Tjm

∞ and Tg∞ can be satisfactorily rationalized by the

different properties utilized for observation of Tg and Tjm,namely, heat capacity and damping factor (tan δ), respectively.The DSC technique measures the steplike change of heatcapacity due to the equilibrium to nonequilibrium transition atTg. However, Tjm, corresponding to the peak temperature of tanδ, is a measure of segment dynamics, whereby the segmentalrelaxation time becomes comparable to the experimental timescale. Usually, the glass transition temperature obtained by thepeak of the tan δ curve is higher than that obtained by DSC,sometimes as much as several tens of kelvin higher, due to thevarious mechanisms used for the definition of Tg.

70,73,74

Accordingly, if we make an 8 K upward offset adjustment ofthe Tg vs Mw curve, it matches well with the Tjm vs Mw curve

(Figure 5, inset). This consistency in Tjm and Tg for PS withvarious Mw values supports the assertion that Tjm could be asuitable parameter for measuring the glass transition ofpolymers. More specifically, the Tjm corresponds to the α-relaxation temperature, at which the chain relaxation time iscomparable to the laboratory time scale (i.e., minutes) andexhibits significant dissipation capacity.

Glass Transition of Supported Thin and Ultrathin PSFilms. In this section, the influence of thickness on the glasstransition of PS thin films is explored. For these studies, PEOoligomer with lower viscosity was selected as a test liquid fordynamic wetting measurements. Figure 6 displays the Δθ vs

temperature curve of PS films with thickness ranging from 20to 4.5 nm. It can be observed that the jumping angle (Δθ)decreases with decreasing thickness of the PS films. Thisobservation is related to the magnitude of deformation in a thinfilm, since it decreases with decreasing thickness of the thinfilm. The reason for this is mainly attributed to the fact that thepolymer film stiffens as the film thickness is reduced for d < 20nm.65 For the ultrathin films, the jumping angle is also sensitiveenough to detect the viscoelasticity change by variance oftemperature. In the case of the 4.5 nm PS film, a maximumvalue of Δθ was clearly observed at around 345 K. According tothe discussion in the above section, this characteristic peaktemperature (Tjm) can be considered as the apparent Tg of PSthin films of various thicknesses.Figure 7 shows the plot of the apparent Tg (or Tjm) vs

thickness of PS thin films on SiO2−Si substrate measured bythe dynamic wetting method (solid circles in Figure 7). Thedata exhibit a substantial depression in Tjm with decreasing filmthickness below a threshold film thickness value (≈100 nm),which is qualitatively similar to the Tg behavior of thesupported PS films documented in the literature.23,26,46 Thelowest Tjm is about 42 K lower than the Tg of 386 K for bulksamples measured by DMA. The Tjm of supported PS ultrathinfilms over a full range of thicknesses obtained in this study wasfitted with the empirical function (eq 1), originally proposed byKeddie et al.26 to describe the thickness dependence of theapparent Tg of thin films:

ξ= − υT T h[1 ( / ) ]g gbulk

(1)

where Tgbulk is the bulk Tg, h is the thickness of the PS film, ξ is a

characteristic length, and υ is an exponent. The black boldcurve in Figure 7 represents the best fit to the data from this

Figure 5. Comparison between Tg (■) of PS films of variousmolecular weights obtained by DSC and Tjm of PS film (●) obtainedby the dynamic wetting method. The inset shows the overlap of theTjm vs Mw curves with the Tg vs Mw curve subjected to an 8 K upwardoffset. Glycerol used as test liquid. Film thickness: around 600 nm.

Figure 6. Temperature dependence of jumping angle for PS thin filmswith thicknesses ranging from 20 to 4.5 nm. PEO as test liquid. Mw =56 530 g/mol.

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study, with Tjmbulk = 386 K, ξ = 2.6 nm, and υ = 1.3. The thin

dashed curve represents the fit using the parameters ξ = 3.2 nmand υ = 1.8 reported in a ref 26, but the Tg

bulk was selected asour measured bulk Tg of 386 K by DMA. The two curvesoverlap at low and high film thickness, and only at intermediatethickness do these two fitting results exhibit a slight deviation.The agreement between the fits and our data demonstrates thataccurate glass transition values of ultrathin films, with thicknessdown to several times the value of Rg of PS, can be obtained viathe dynamic wetting method.Effect of Substrate Chemistry on Glass Transition of

Ultrathin PS Films. Figure 8 presents the temperature

dependence of jumping angle of 11 nm thickness PS filmsdeposited on SiO2−Si and H−Si substrates. It is obvious thatthe apparent Tg of the 11 nm PS film on SiO2−Si substrate is356 K, much smaller than that on the H−Si substrate (380 K).As well, the jumping angle of PS films deposited on SiO2−Sisubstrate was much larger than that on H−Si substrate, whichmay be related to its viscoelastic properties, and will beinvestigated in detail in the future. Figure 7 displays the plot ofmeasured apparent Tg vs thickness of PS thin films depositedon the two different substrates. It is clear that the thresholdthickness for apparent Tg reduction for PS films deposited onH−Si substrate (∼30 nm) is lower than that on the SiO2−Si

surface (∼100 nm). At the same time, the Tg depression of PSthin film deposited on SiO2−Si substrate is obviously enhancedcompared with that on H−Si substrate.The difference in the apparent Tg of PS thin films on the

different substrates is due to the deviation of interactionbetween the PS chains and the H−Si and SiO2−Si surfaces.The importance of polymer−substrate interfacial interactionson the Tg was first reported for poly(methyl methacrylate)(PMMA) films on Au and SiO2 substrates.75 In the presentstudy, the interaction at the polymer/substrate interface isfeasible as a reason for the Tg discrepancy between PS ultrathinfilms deposited on SiO2−Si and Si−H substrates. Table S1(Supporting Information) shows that the thermodynamic workof adhesion required for PS/SiO2−Si separation is 87.0 mJ/m2,while that for PS/Si−H is 90.8 mJ/m2. The interaction betweenPS and SiO2−Si is relatively weak, as evidenced by thedewetting of PS on the SiO2−Si surface.

76,77 The relatively highthermodynamic work of adhesion at the Si−H/PS interfacesuggests that the PS chains have more tendency to adsorb onthe H-terminated silicon surface and form strong interactionswith the H−Si surface. The more favorable attractiveinteraction between PS and H−Si surface is also supportedby the fact that more residual PS remains on the H-terminatedsilicon surface than on the SiO2−Si surface after rinsing the 50nm supported PS film with toluene. In addition, it wasdemonstrated78,79 that PS thin film coated on a SiO2−Si surfacehas a negative interfacial potential, which represents a repulsiveinteraction between SiO2−Si and PS. However, PS thin filmsdeposited on H-terminated silicon surfaces have a positiveinterfacial potential, which means a more attractive interactionoccurs between H−Si and PS films. Napolitano et al.40,80

reported that the adsorption of PS chains at a PS/aluminuminterface effectively increases the glass transition temperature ofPS thin films. The strong adhesion of PS chains on the H−Sisurface would be expected to restrict the mobility of the chainsnear the interface, elevating the Tg of ultrathin films. Thevariation of apparent Tg of PS films with change of filmthickness and substrate chemistry indicates that the dynamicwetting technique is a very sensitive way to detect the glasstransition of polymer films.

■ CONCLUSIONSThe study of dynamics in thin polymer films remains a vibrantand productive research area. Although there have been anumber of key developments in recent years, it is obvious thatsome apparent controversies still remain. Detailed answers tothe remaining questions will require continued effort atdeveloping new experimental approaches and novel mecha-nisms.9 In this paper, a new method was developed to detectthe glass transition of polystyrene (PS) thin and ultrathin filmsin situ. By investigating the relationships between thetemperature of PS thick films (∼600 nm) and stick−slipbehavior on their surface during dynamic wetting of glycerol oroligo-poly(ethylene glycol) droplets, it was found that thestick−slip motion of the droplets can be observed only in theglass transition region, where the PS exhibits high viscoelasticproperties. The jumping angle Δθ was employed to scale thestick−slip behavior on the film surfaces with various temper-atures. It was found that the temperature (Tjm) for PS films, atwhich the jumping angle Δθ reached a maximum, corre-sponded well to the Tg (or Tα) of PS obtained from themaximum tan δ value determined from DMA analysis. Themeasured Tjm of PS films with various molecular weights was

Figure 7. Tjm as a function of PS films thickness deposited on SiO2−Si(circle) and H−Si (diamond) substrates as measured by dynamicwetting method. Also shown are a least-squares fit of the measuredapparent Tg to eq 1 (solid curve) as well a curve drawn from the sameequation with the parameter values of Keddie (dashed curve).26 Mw =56 530 g/mol.

Figure 8. Temperature dependence of jumping angle of PS filmsdeposited on H−Si and SiO2−Si substrates with a thickness of 11 nm.Mw = 56 530 g/mol.

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found to be greatly coincident with the Tg data obtained byDSC. These results indicate that the Tjm, where the jumpingangle (Δθ) reaches a maximum value, is very close to the Tg ofthe PS film and that dynamic wetting is therefore a suitablemethod for studying the apparent Tg of polymer thin films. Thegood consistency between the Tjm and Tg values is satisfactorilyinterpreted by the viscoelastic dissipation-controlled spreadingon a soft substrate. The value of Δθ is dictated by the balancebetween the excess capillary free energy of the drop and theviscoelastic dissipation of PS substrate in the course of thedeformation of PS films by the three-phase line during liquidspreading.The apparent Tg of PS thin and ultrathin films deposited on

SiO2−Si and H−Si substrates was measured using the dynamicwetting approach. A depression in apparent Tg was observed forPS thin films with thickness below 100 nm when deposited on aSiO2−Si substrate. The decrease in Tjm demonstrates theenhanced relaxation rate of PS confined in thin films. Inaddition, the measured apparent Tg of PS thin films on SiO2−Sisubstrate was much lower than that on H−Si substrate at thesame thickness due to the more favorable adsorption of PS onH−Si surfaces. The change of the measured apparent Tg withvariance of film thickness and substrate chemistry also confirmsthat the method developed in this paper is very sensitive fordetecting the dynamics of polymer thin films. A detailed studyof the effect of substrate/polymer interactions on thin filmdynamics and dynamic wetting behavior is proceeding and willbe discussed in detail in a subsequent paper.

■ ASSOCIATED CONTENT*S Supporting InformationJumping angle of PEO oligomer on PS surface at varioustemperatures; DMA spectra of PS film and the thermodynamicwork of adhesion required for PS/SiO2−Si and PS/H−Siseparation. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Fax 86-571-8684-3600; e-mail wxinping@yahoo.com orwxinping@zstu.edu.cn.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe are thankful for support from the National Natural ScienceFoundation of China (NSFC, No. 21174134) and the NaturalScience Foundation of Zhejiang Province (Grant No.Z4100463).

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