Probing Glass Transitions in Thin and Ultrathin Polystyrene Films by

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Probing Glass Transitions in Thin and Ultrathin Polystyrene Films by Stick−Slip Behavior during Dynamic Wetting of Liquid Droplets on Their Surfaces Biao 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 glass transition of thin and ultrathin polystyrene (PS) films by correlating the relationships between the temperature-dependent viscoelasticity of the PS films and stick−slip behavior on their surfaces during dynamic wetting of glycerol or oligo-poly(ethylene glycol) droplets. The peak temperature (Tjm) obtained from the jumping angle−film temperature curve, in which the jumping angle Δθ was employed to scale the stick−slip behavior, was nearly identical to the corresponding Tg (or Tα) 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 substrate chemistry (SiO2−Si and H−Si) further confirmed that the developed method is very sensitive for detecting the dynamics of ultrathin polymer films.

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INTRODUCTION With the recent advances in nanotechnology, increasing interest in thin film coatings and nanodevices of polymer materials exacerbates our concerns on the precise understanding of the physical properties and performances of nanodevices related to the segmental dynamics of polymers under confinement.1−5 The glass transition temperature (Tg) is defined as the temperature at which the dynamics of molecules dramatically slow 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 viscosity by many orders of magnitude.6−8 It is a key parameter for characterizing the mobility of polymer chains and determines the temperature range for application of many materials. The glass transition dynamics in thin or ultrathin polymer films (thickness comparable to the radius of gyration of chains) is a fundamental as well as a pressing problem in polymer condensed matter physics.9−12 Because of the finite size effects,13−16 surface and interface effects,17−21 and other impacts,22−25 the glass transition dynamics of nanometersthick polymer films significantly deviates from that in bulk. The glass transition phenomenon in nanostructured systems is a relatively 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 thin films,26 how the glass transition dynamics of ultrathin polymer films differ from those in bulk remains unclear and has become an intellectual challenge, partly due to the inherent measurement difficulties of the dynamics at nanoscales. Four types of © 2013 American Chemical Society

accepted methods have been developed to measure the Tg of polymer thin films, which are based on (1) the discontiguous changes of volume, thermal expansion, or density,20,22,23,26−32 (2) nanorheological properties,33−36 (3) the specific heat of the polymer,37,38 and (4) the segmental dynamics39−41 at the glass transition region. Some additional techniques, such as thermal probe,42 Raman43 and infrared spectroscopy,44 sum frequency generational spectroscopy,45 and birefringence46 are also sensitive to the glass transition of polymer films. Although considerable approaches have been developed for the Tg measurements of thin films, much controversy remains due to apparently contrasting and conflicting results which exhibit variously an increase,21,29,47,48 decrease,22,23,26,31,33−35 or no change of Tg36−39,45 with the thickness of thin films, as obtained by various techniques. The lack of consensus has provided a strong motivation for development of alternative techniques.9 The development of accepted models to describe the glass transition dynamics in confined geometries will also require continued efforts at developing new experimental approaches and insightful mechanisms. Such new experimental approaches and mechanisms are of significance, both in terms of unveiling the nature of glass transition dynamics and for providing effective ways for resolution of the controversies in understanding the glass transition dynamics of confined films. Received: November 17, 2012 Revised: January 21, 2013 Published: February 22, 2013 1875

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precleaned by piranha solution as above, in a 4% aqueous solution of HF for 5 min and then rinsed and dried them as described above. This procedure removes the native oxide layer and leaves the silicon surface terminated with Si−H groups with water contact angle of 72 ± 4°. Since the advancing contact angles on PS films were measured at temperatures ranging from 303 to 453 K, the oligo-poly(ethylene glycol) (PEO) with Mn of 400 and glycerol with high boiling point and lower vapor pressure were chosen as test liquids for advancing contact angle measurements. The physical parameters of these two liquids are given in Table 1. The boiling points of PEO and glycerol are well

The wetting dynamics of a solid surface by a liquid greatly depends on the corresponding mechanical properties of the solid film.49−55 Carre and Shanahan demonstrated that the spreading of a liquid on a soft solid was much slower than that on a rigid solid.50−52 Long et al.56,57 simply modeled the correlation between the dynamic wetting properties of a liquid with the viscosity and relaxation time of rubber films and found that the spreading speed of a liquid can be controlled by the viscoelasticity of the polymer films. Therefore, it can be inferred that observation of the dynamic wetting of liquid droplets on polymer films may be one potential way for detecting the viscoelasticity and glass transition of polymers. Automated axisymmetric drop shape analysis-profiling (ADSA-P) has been shown to be a very powerful method for studying the dynamic wetting on a surface by advancing and receding contact angle measurements.58,59 A phenomenon termed “stick−slip” may occasionally be observed during the evolution of a drop in advancing contact angle measurements, in which the wetting front remains static for most of the time but from time to time moves quite abruptly.58,60,61 Our group reported previously that the stick−slip patterns occurring during water drop spreading appear to be strongly dependent on the surface viscoelasticity of poly(styrene-b-isoprene-bstyrene) triblock copolymer films, in which there is a linear relationship between jumping angle (Δθ) from stick−slip patterns and the surface modulus of the film.61 The viscoelasticity of polymers is temperature dependent, the elastic modulus shows a sharp reduction, and the viscoelastic dissipation of polymers reaches its highest in the glass-torubbery transition region.33 This change in polymer viscoelasticity by temperature variance would bring about large perturbations 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 of temperature-dependent dynamic wetting of a liquid. To this end, we first measured the temperature-dependent dynamic wetting behavior of liquid on a PS thick film (around 600 nm thick) and verified that dynamic wetting is a feasible way to detect the glass transition of polymers. In the second part of this study, this approach was utilized to measure the glass transition of PS thin and ultrathin films with thickness down to 4.5 nm, and the effect of substrate/PS interfacial interactions on the glass transition of PS ultrathin films was also investigated.

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

sample

surface tension (mN/m) at 298 K

glycerol PEO PS

62.1a 47.9a 40.7b

viscosityc (Pa·s) at 298 K

boiling point (K)

0. 94 0.085

564 479−482d

solubility parameter δ (MPa1/2)

densityg (g/mL)

33.8d 23.1e 17.8f

1.261 1.128 1.050

a

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

above the glass transition temperature (Tg ≤ 378 K) of PS film. PS is insoluble in PEO and glycerol at room temperature and at temperatures as high as 403 K over extended periods of time, as confirmed by both the appropriate solubility parameters and dissolution experiments. In addition, PS film surfaces were not swollen by PEO and glycerol in the time scale of advancing contact angle measurements (∼3 min) even if the PS film was heated to 400 K. Film Formation. The PS films were prepared by the spin-coating method, from toluene solution onto the clean substrates. The thickness of the PS films was controlled only by varying the concentration of the PS solution, while keeping the spin speed constant at 2000 rpm/min. The thickness of the PS films was measured by spectroscopic ellipsometry (M-50, JASCO Co., Ltd.). All the samples underwent an annealing process at Tg + 10 °C for 24 h in vacuum in order to extract the residual solvent and residual stress as much as possible. AFM analysis showed no apparent dewetting after thermal annealing. Films for dynamic mechanical analysis (DMA) tests were prepared using the solution casting method, with thickness of about 50 μm. Before measurement, the residual solvent was removed by thermal annealing at Tg + 10 °C for 72 h in a vacuum. Advancing Contact Angle Measurements on the Film Surface at Various Temperatures. The PS films were mounted onto a heating stage, which can adjust the temperature from 293 to 473 K, with accuracy of ±1 K. The viscoelasticity of the PS films was controlled by the heating temperature. Advancing contact angles of liquid droplets on PS film surfaces were measured in situ using the automated axisymmetric drop shape analysis-profile (ADSA-P) method.58,59 An initial drop with a diameter larger than 3 mm was deposited onto the PS surface and ensured to be axisymmetric. A motor-driven syringe was used to pump liquid steadily into the sessile drop, and a sequence of images of the growing drop was then captured, using the drop shape analysis system (Kruss DSA 10-MK2, Germany). The dynamic contact angle behavior in advancing mode was obtained by tracing the evolution of contact angle (θ) and the diameter of the drop three-phase line (d) with liquid added into the drop. The volume addition velocity used in this study was 0.26 μL of liquid/s. Each experiment was repeated at least 15 times, using a fresh polymer film each time, to ensure the reproducibility of results. Shape of Wetting Ridge Profile. To visualize the shape profile of the wetting ridge hidden beneath the droplets, the PS films were quenched to room temperature after dynamic wetting measurements and then rinsed by ethanol to remove the drop. Afterward, films for

EXPERIMENTAL SECTION

Materials. 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 with molecular 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 400 and glycerol were purchased from Aldrich Chemical Co. and used as test 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 of H2SO4:H2O2 (3:1) preheated to 363 K for 30 min, then rinsed thoroughly in excessive deionized water, and dried with nitrogen. This process removes any organic contaminants, leaving the silicon surface as a native oxide layer covered with the Si−OH groups with water contact angle less than 10°. Unless otherwise stated, the native oxidecovered silicon wafer (SiO2−Si) was used as the supporting substrate of the PS films. If hydrogen-passivated silicon (H−Si) is desired, as outlined in a reported method,29 we submerged the silicon wafer, 1876

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measurements were dried with nitrogen gas to evaporate the residual ethanol. The shape of the wetting ring ridge was obtained by reflection geometry using an optical microscope. The three-dimensional profiles of the “wetting ridge” were accessed by XEI-100 scanning probe microscopy (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 of 293−473 K with a heating rate of 2 K/min at 0.05% strain under a nitrogen atmosphere. The test specimens had a gauge length of 20 mm, 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 first heated to 403 K at a rate of 10 K/min under a nitrogen atmosphere to remove the thermal history. Subsequently, the samples were heated a second time at a rate of 20 K/min, and Tg was determined from the second heating scan. The weights of all samples, determined by electronic balance with 0.0001 g of sensitivity, were in the range 3−4 mg.

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RESULTS AND DISCUSSION Dynamic Wetting Behavior on Polystyrene (PS) Thick Films at Various Temperatures. The advancing contact angle measurements were first performed using glycerol as a test liquid on a PS (Mw = 56 530 g/mol) thick film with thickness of about 600 nm, which was sufficient to avoid substrate and ultrathinning effects.64,65 It was found that the three-phase line smoothly advances with increasing liquid droplet volume when the temperature of the PS film was below 360 K, resulting in a constant advancing contact angle. This result was the same as that obtained on a rigid H−Si surface. When the PS film temperature exceeds 370 K, the dynamic wetting of glycerol on PS film surface shows remarkable differences; i.e., stick−slip behavior is clearly observed. The wetting behavior of the glycerol droplet on PS film surface at 386 K shows a stick−slip pattern, as illustrated in Figure 1: as liquid 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 then abruptly slips to a new position on the solid surface as more liquid is supplied. The slip of the three-phase line is accompanied by a sudden decrease in the contact angle and by a sudden increase in the contact radius. By supplying more liquid, the three-phase line again sticks to the solid surface at a new location, and the radius remains constant. This behavior is repeated as the measurement continues, and a sawtooth-shaped contact 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 to smooth motion was observed. In order to compare the stick− slip behavior on PS film surfaces at various temperatures, the jumping angle (Δθ) was employed as a parameter which could be 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 resulting relationship between Δθ and the temperature of the PS films is shown in Figure 2. It was found that the jumping angle Δθ was almost zero when the PS film temperature was above 423 K and below 363 K. However, apparent stick−slip behavior was observed on PS film surfaces in the intermediate temperature range from 363 to 423 K and a maximum Δθ value was located at around 386 K. A characteristic temperature (Tjm) at which maximum Δθ occurs was defined to describe the temperature dependent dynamic wetting behavior on PS film surfaces (shown in Figure 2).

Figure 1. Evolution of contact angle (a), contact diameter (b), volume and (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 a glycerol droplet with liquid added into the drop during one stick−slip cycle. The stick−slip of the three-phase line and abrupt changes in the drop radius by increases in the volume are evident. The definition of jumping 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 of temperature of PS (Mw = 56 530 g/mol) films. Glycerol used as test liquid. Film thickness: around 600 nm. DMA curves were obtained at 2 °C/min and frequency 1 Hz.

Although stick−slip phenomena have been reported as being due variously to an external noise,66 swelling of a noninert component on the polymer surface,59,67 vapor adsorption,58 and surface roughness and imhomogeneity,58,68,69 it can be deduced as previously reported61 that the stick−slip patterns occurring on the PS film surfaces within a range of temperatures in the present study are not attributable to any of the factors described above. Glycerol is a viscous liquid with 1877

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

low vapor pressure, and its viscosity is temperature dependent and decreases with increasing temperature. The dynamic wetting behavior of glycerol on a rigid H−Si substrate was investigated at various temperatures. The results indicate that no “stick−slip” behavior was found within the range of experimental temperatures. This suggests that the temperature dependent stick−slip behavior observed herein on PS films was not caused by the change of glycerol properties (e.g., viscosity, surface tension, evaporation rate). In addition, we also conducted the same experiment using PEO oligomer with lower viscosity and surface tension as a test liquid. The results are shown in Figure S1 (Supporting Information), which presents a similar relationship between jumping angle (Δθ) and the temperature, and shows the same Tjm value, at 385 K. The agreement of the temperature at peak jumping angle vs temperature curve (Tjm) obtained by dynamic wetting experiments using either PEO or glycerol as test liquid thus verifies that 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 is apparent that three concentric “wetting ridges” with height of about 165 nm reside on the surface of the PS film. Each ridge corresponds to one expansion of the three-phase line during one stick−slip cycle. However, no concentric wetting ridge was observed when the film temperature was out of the region where stick−slip behavior was exhibited. The dynamic mechanical analysis spectrum in Figure S2 (Supporting Information) shows that the storage modulus of the PS film begins to decrease when the temperature is above 363 K, which suggests that the PS film changes from glassy to viscoelastic state at this point. Therefore, this ridge near the three-phase line shown in Figure 3 was formed due to the vertical force component of the liquid surface tension and the disjoining pressure. Since the effect of the disjoining pressure on the surface deformation cannot be resolved by our experimental technique and it was same as that of the vertical force component of the liquid surface tension, it is therefore reasonable to neglect the effect of disjoining pressure in the following section. Therefore, the height of the ridge (h) could be determined by the shear modulus (G) of the film: h = γ sin θ/G (γ is surface tension of the liquid) (Figure 4).50−52 When PS film changes from glassy to viscoelastic state (reduced modulus) with increasing temperature, the component of liquid surface tension acting perpendicularly to the solid causes a deformation or “wetting ridge” on the film surface. The film with lower modulus will cause large local deformations, resulting in a higher “wetting ridge” to prevent the spreading of the drop. This may be the phenomenological reason why Δθ increases with increasing PS film temperature below 386 K, as

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

shown in Figure 2. However, with the film temperature increased to above 403 K, PS experiences a transition from the rubbery to the viscous state, and the chain relaxation of PS is very fast at high temperatures. With liquid added into the drop, the formed wetting ridge is able to propagate with the advances of the three-phase line and thus cannot hold the meniscus of the drop, resulting in close to smooth motion of the threephase line. The spreading behavior of liquid on a polymer surface with various mechanical states is shown in Figure 4. Therefore, the jumping angle may be an important parameter for studying mechanical properties of polymer thin films, and this 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 and Δθ plotted as a function of temperature share the same features. When Tg of PS was determined from the peak of the tan δ curve,70,71 the Tg of PS was almost the same as the temperature (Tjm) at the observed peak in the Δθ vs temperature curve. Tan δ is an indicator of the capacity for viscoelastic dissipation of polymers.51,71 This perfect correlation between Δθ and tan δ demonstrates that the “stick−slip” behavior intrinsically originates from the viscoelastic dissipation in PS films. At the glass−rubbery transition region, the viscoelastic dissipation of PS film reaches a maximum; thus, the “stick−slip” behavior occurs during advancing contact angle measurement (Figure 4). The correlation between Δθ and tan δ of PS can be readily interpreted based on the “viscoelastic break” mechanism, which proposes that the viscoelastic dissipation as a main force prevents the spreading of liquid.50−52 For a viscoelastic substrate, the spreading kinetics of a liquid are mainly controlled by the transformation of the excess capillary potential energy of a drop released during spreading to the viscoelastic dissipation of substrate due to “wetting ridge” formation and motion.50−52 The balance between the excess capillary free energy of a drop and the viscoelastic dissipation of the PS substrate determines the movement of the three-phase line. The fraction of energy lost during deformation of 1878

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(Figure 5, inset). This consistency in Tjm and Tg for PS with various Mw values supports the assertion that Tjm could be a suitable parameter for measuring the glass transition of polymers. More specifically, the Tjm corresponds to the αrelaxation temperature, at which the chain relaxation time is comparable to the laboratory time scale (i.e., minutes) and exhibits significant dissipation capacity. Glass Transition of Supported Thin and Ultrathin PS Films. In this section, the influence of thickness on the glass transition of PS thin films is explored. For these studies, PEO oligomer with lower viscosity was selected as a test liquid for dynamic wetting measurements. Figure 6 displays the Δθ vs

viscoelastic polymers is a function of the damping factor (tan δ).51,71 In the glass transition region, the tan δ of PS film achieves its maximum value. Accordingly, the greater the value tan δ of PS film, the more excess energy that can be stored by the drop, thus resulting in a larger jumping angle (Δθ). This is the main reason why the temperatures at which both tan δ and jumping angle Δθ reached their maximum values are almost the same. This mechanism suggests that the glass transition of PS film could be detected by investigating the dynamic wetting behavior of liquid droplets on film surfaces at various temperatures. This suggestion was further confirmed by the comparison between the Tg and Tjm values of PS of various molecular weights, as shown in Figure 5. Since Tg of PS with Mw below

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

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

temperature curve of PS films with thickness ranging from 20 to 4.5 nm. It can be observed that the jumping angle (Δθ) decreases with decreasing thickness of the PS films. This observation is related to the magnitude of deformation in a thin film, since it decreases with decreasing thickness of the thin film. The reason for this is mainly attributed to the fact that the polymer film stiffens as the film thickness is reduced for d < 20 nm.65 For the ultrathin films, the jumping angle is also sensitive enough to detect the viscoelasticity change by variance of temperature. In the case of the 4.5 nm PS film, a maximum value of Δθ was clearly observed at around 345 K. According to the discussion in the above section, this characteristic peak temperature (Tjm) can be considered as the apparent Tg of PS thin 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 by the dynamic wetting method (solid circles in Figure 7). The data exhibit a substantial depression in Tjm with decreasing film thickness below a threshold film thickness value (≈100 nm), which is qualitatively similar to the Tg behavior of the supported PS films documented in the literature.23,26,46 The lowest Tjm is about 42 K lower than the Tg of 386 K for bulk samples measured by DMA. The Tjm of supported PS ultrathin films over a full range of thicknesses obtained in this study was fitted with the empirical function (eq 1), originally proposed by Keddie et al.26 to describe the thickness dependence of the apparent Tg of thin films:

the entangled molecular weight is hard to detect by DMA due to its poor film forming ability and high fragility, DSC, which is a very popular method for obtaining Tg of polymers, was employed to measure the Tg of PS samples with various molecular weights. The results show that Tjm is also molecular weight dependent and increases with increasing molecular weight before reaching a threshold value Mw. Both molecular weight dependent Tg and Tjm could be readily fitted with the 72 Fox−Flory equation (Tg = T∞ in which the g − A/Mn), resulting fitting parameters are T∞ = 378 K, A = 108 (mol K)/ g kg and T∞ jm = 386 K, A = 99 (mol K)/kg. The Mw-dependent parameter A obtained by two different methods is very close to 100 (mol K)/kg, which was the value reported by Fox and Flory.72 Alternatively, it was found that the T∞ jm value is 8 K higher than the T∞ measured by DSC. This small discrepancy g ∞ between T∞ jm 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 heat capacity due to the equilibrium to nonequilibrium transition at Tg. However, Tjm, corresponding to the peak temperature of tan δ, is a measure of segment dynamics, whereby the segmental relaxation time becomes comparable to the experimental time scale. Usually, the glass transition temperature obtained by the peak of the tan δ curve is higher than that obtained by DSC, sometimes as much as several tens of kelvin higher, due to the various mechanisms used for the definition of Tg.70,73,74 Accordingly, if we make an 8 K upward offset adjustment of the Tg vs Mw curve, it matches well with the Tjm vs Mw curve

Tg = Tgbulk[1 − (ξ /h)υ ]

(1)

where is the bulk Tg, h is the thickness of the PS film, ξ is a characteristic length, and υ is an exponent. The black bold curve in Figure 7 represents the best fit to the data from this Tbulk g

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surface (∼100 nm). At the same time, the Tg depression of PS thin film deposited on SiO2−Si substrate is obviously enhanced compared 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 interaction between the PS chains and the H−Si and SiO2−Si surfaces. The importance of polymer−substrate interfacial interactions on the Tg was first reported for poly(methyl methacrylate) (PMMA) films on Au and SiO2 substrates.75 In the present study, the interaction at the polymer/substrate interface is feasible as a reason for the Tg discrepancy between PS ultrathin films deposited on SiO2−Si and Si−H substrates. Table S1 (Supporting Information) shows that the thermodynamic work of 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 between PS and SiO2−Si is relatively weak, as evidenced by the dewetting of PS on the SiO2−Si surface.76,77 The relatively high thermodynamic work of adhesion at the Si−H/PS interface suggests that the PS chains have more tendency to adsorb on the H-terminated silicon surface and form strong interactions with the H−Si surface. The more favorable attractive interaction between PS and H−Si surface is also supported by the fact that more residual PS remains on the H-terminated silicon surface than on the SiO2−Si surface after rinsing the 50 nm supported PS film with toluene. In addition, it was demonstrated78,79 that PS thin film coated on a SiO2−Si surface has a negative interfacial potential, which represents a repulsive interaction between SiO2−Si and PS. However, PS thin films deposited on H-terminated silicon surfaces have a positive interfacial potential, which means a more attractive interaction occurs between H−Si and PS films. Napolitano et al.40,80 reported that the adsorption of PS chains at a PS/aluminum interface effectively increases the glass transition temperature of PS thin films. The strong adhesion of PS chains on the H−Si surface would be expected to restrict the mobility of the chains near the interface, elevating the Tg of ultrathin films. The variation of apparent Tg of PS films with change of film thickness and substrate chemistry indicates that the dynamic wetting technique is a very sensitive way to detect the glass transition of polymer films.

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

study, with Tbulk jm = 386 K, ξ = 2.6 nm, and υ = 1.3. The thin dashed curve represents the fit using the parameters ξ = 3.2 nm and υ = 1.8 reported in a ref 26, but the Tbulk was selected as g our measured bulk Tg of 386 K by DMA. The two curves overlap at low and high film thickness, and only at intermediate thickness do these two fitting results exhibit a slight deviation. The agreement between the fits and our data demonstrates that accurate glass transition values of ultrathin films, with thickness down to several times the value of Rg of PS, can be obtained via the dynamic wetting method. Effect of Substrate Chemistry on Glass Transition of Ultrathin PS Films. Figure 8 presents the temperature

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CONCLUSIONS The study of dynamics in thin polymer films remains a vibrant and productive research area. Although there have been a number of key developments in recent years, it is obvious that some apparent controversies still remain. Detailed answers to the remaining questions will require continued effort at developing new experimental approaches and novel mechanisms.9 In this paper, a new method was developed to detect the glass transition of polystyrene (PS) thin and ultrathin films in situ. By investigating the relationships between the temperature of PS thick films (∼600 nm) and stick−slip behavior on their surface during dynamic wetting of glycerol or oligo-poly(ethylene glycol) droplets, it was found that the stick−slip motion of the droplets can be observed only in the glass transition region, where the PS exhibits high viscoelastic properties. The jumping angle Δθ was employed to scale the stick−slip behavior on the film surfaces with various temperatures. It was found that the temperature (Tjm) for PS films, at which the jumping angle Δθ reached a maximum, corresponded well to the Tg (or Tα) of PS obtained from the maximum tan δ value determined from DMA analysis. The measured Tjm of PS films with various molecular weights was

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

dependence of jumping angle of 11 nm thickness PS films deposited on SiO2−Si and H−Si substrates. It is obvious that the apparent Tg of the 11 nm PS film on SiO2−Si substrate is 356 K, much smaller than that on the H−Si substrate (380 K). As well, the jumping angle of PS films deposited on SiO2−Si substrate was much larger than that on H−Si substrate, which may be related to its viscoelastic properties, and will be investigated in detail in the future. Figure 7 displays the plot of measured apparent Tg vs thickness of PS thin films deposited on the two different substrates. It is clear that the threshold thickness for apparent Tg reduction for PS films deposited on H−Si substrate (∼30 nm) is lower than that on the SiO2−Si 1880

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found to be greatly coincident with the Tg data obtained by DSC. These results indicate that the Tjm, where the jumping angle (Δθ) reaches a maximum value, is very close to the Tg of the PS film and that dynamic wetting is therefore a suitable method for studying the apparent Tg of polymer thin films. The good consistency between the Tjm and Tg values is satisfactorily interpreted by the viscoelastic dissipation-controlled spreading on a soft substrate. The value of Δθ is dictated by the balance between the excess capillary free energy of the drop and the viscoelastic dissipation of PS substrate in the course of the deformation of PS films by the three-phase line during liquid spreading. The apparent Tg of PS thin and ultrathin films deposited on SiO2−Si and H−Si substrates was measured using the dynamic wetting approach. A depression in apparent Tg was observed for PS thin films with thickness below 100 nm when deposited on a SiO2−Si substrate. The decrease in Tjm demonstrates the enhanced relaxation rate of PS confined in thin films. In addition, the measured apparent Tg of PS thin films on SiO2−Si substrate was much lower than that on H−Si substrate at the same thickness due to the more favorable adsorption of PS on H−Si surfaces. The change of the measured apparent Tg with variance of film thickness and substrate chemistry also confirms that the method developed in this paper is very sensitive for detecting the dynamics of polymer thin films. A detailed study of the effect of substrate/polymer interactions on thin film dynamics and dynamic wetting behavior is proceeding and will be discussed in detail in a subsequent paper.

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ASSOCIATED CONTENT

* Supporting Information S

Jumping angle of PEO oligomer on PS surface at various temperatures; DMA spectra of PS film and the thermodynamic work of adhesion required for PS/SiO2−Si and PS/H−Si separation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax 86-571-8684-3600; e-mail [email protected] or [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for support from the National Natural Science Foundation of China (NSFC, No. 21174134) and the Natural Science Foundation of Zhejiang Province (Grant No. Z4100463).

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