Melting and Crystallization of Poly(ethylene oxide) Nanofilms Studied

ARTICLE pubs.acs.org/JPCC

Melting and Crystallization of Poly(ethylene oxide) Nanofilms Studied by Micromechanical Cantilevers Jun Zhao,*,†,‡ Xiaoqing Yin,† Jingdan Shi,† Xiaodong Zhao,† and Jochen S. Gutmann*,‡,§,|| †

)

Department of Polymer Science and Engineering, School of Chemical and Biological Engineering, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China ‡ Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany § Institute for Physical Chemistry, Johannes Gutenberg University, Jakob-Welder-Weg 10, D-55099 Mainz, Germany Physical Chemistry, University Duisburg-Essen, Campus Essen, D-45117, Essen, Germany ABSTRACT: Melting and crystallization of thin poly(ethylene oxide) (PEO) films are studied by micromechanical cantilevers (MC) consisting of a micromachined silicon substrate with native oxide layer and a thin poly(ethylene oxide) coating. Phase transitions of the PEO nanofilms, such as melting and crystallization, appear as signals in the temperature-dependent deflection traces. The melting temperature (Tm) of thin films with thickness of 20
700 nm measured by MC is about 7
10 K lower than that of bulk samples measured by differential scanning calorimetry (DSC) due to the size effect and different crystallization conditions. However, the crystallization temperature (Tc) obtained by MC and DSC is almost the same. Both Tm and Tc measured by MC increase with increasing film thickness. Besides, both Tm and Tc decrease with increasing number of heating
cooling cycles due to partial dewetting of the polymer films from the silicon oxide substrate surface, which results in a decreased contact area and adhesion strength at the interface.

1. INTRODUCTION Over the past decades, thermal analysis based on “bimaterial” micromechanical cantilevers (MC) has been performed to study the heat evolution of catalytic or chemical reactions and the heat associated with phase transitions.1
6 Furthermore, experimental results on thermogravimetric analysis using an oscillating, heated piezoresistive MC in helium gas have also been reported.7 When using MC array, one of the most outstanding advantages is its extremely high sensitivity, low noise, and immunity to external turbulences.8,9 For example, the quantitative measurement of the transition enthalpy of 10 ng of n-alkanes by MC has demonstrated the determination of a transition heat less than 1 mJ with an accuracy of 100 nJ and a time resolution of 200 ms.2 Further investigations probing the experimental limits of MC under ambient conditions obtained a heat sensitivity of 500 pJ for a sample mass of about 7 pg with a time resolution of 0.5 ms.3,4 The aim of this work is to apply MC to the thermal analysis of polymer films such as the observation of melting and crystallization. We focus our interest on the investigations of the thickness dependence of phase transitions in ultrathin films of nanometer thickness. Because of the low amount of polymer under investigation, phase transition in such films cannot be addressed by normal thermal analysis, typically differential scanning calorimetry (DSC),10
16 although nanocalorimetry is a newly emerging good effort to study the nanofilms.17,18 In previous MC experiments, the samples were typically attached to the MC apex.2
4 Such an experimental design requires the r 2011 American Chemical Society

measurements of mass change via the dynamic mode of MC. However, in this mode, the resonance frequency shift is sensitive to the exact position of the sample at the free end. It also mutually excludes the possibility for detection via deflection measurements because the MC deflection is more sensitive to the clamped end. Therefore, the use of thin film samples on the single side of MC in this work favors the observation of phase transitions via deflection traces.

2. EXPERIMENTAL SECTION 2.1. Materials. A poly(ethylene oxide) (PEO) sample with a number-average molecular weight (Mn) of 5.11  104 g mol
1 and a polydispersity index (Mw/Mn) of 1.12 was synthesized using a standard anionic polymerization method at the Max Planck Institute for Polymer Research, Mainz, Germany. The polymer was dissolved in deionized water to form dilute solutions with concentrations of 0.01
0.28 wt %. Arrays of eight rectangular silicon MCs at a pitch of 250 μm (Micromotive, Mainz, Germany) were used after a treatment with argon plasma to remove any organic contaminants. Each MC was 750 μm long, 90 μm wide, and about 0.8
0.9 μm thick. Received: June 8, 2011 Revised: October 5, 2011 Published: October 12, 2011 22347

dx.doi.org/10.1021/jp205381b | J. Phys. Chem. C 2011, 115, 22347–22353

The Journal of Physical Chemistry C

Figure 1. Schematic representation of the MC setup (a) and the inkjet printing method (b).

2.2. MC Array Setup. As shown in Figure 1a, an array of eight MCs was housed inside a sample cell sealed with a sapphire window. The sample cell was fabricated having a resistor foil heater and a thermoresistor (Pt100) to allow for measurements at elevated temperatures. An optical beam deflection technique was used for MC readout (SCENTRIS, Veeco Instr., Santa Barbara, CA). The bending of the MCs was measured via a beam deflection technique using eight super luminescence light sources and one position sensitive photodetector (PSD). The applied power of 10 W of the resistor foil heater produced a maximum temperature of about 62 K above the ambient temperature. In heating runs, the system reached its maximum temperature after a typical time scale of 20 min. In cooling runs, the heater was switched off and the system cooled to ambient temperature by heat convection and radiation loss. This cooling process was typically finished within 40 min. In a typical experiment, the thermal properties of the PEO films were investigated by several consecutive heating/cooling cycles. 2.3. Film Preparation via Inkjet Printing. As shown in Figure 1b, an NP-2 Nano-plotter (GeSiM, Dresden, Germany) was used to get a selective single coating of thin PEO films through a single droplet dispense onto the silicon MC array. During the coating process, a positioning system allows accurate placement with a resolution of about 2 μm and a repeatability of about 30 μm of single droplets of dilute PEO/water solution onto selected MC. The single droplet of the PEO solution that is dispensed from the piezoelectrically driven micropipet had a diameter of 60
100 μm corresponding to a volume of 0.1
0.4 nL. A stroboscope provided visual control to adjust piezo voltage and pulse width for reliable droplet ejection and to avoid satellite droplets. The vertical distance between the pipet tip and the top side of the MC was typically 0.4 mm. When deposited with a small pitch, the droplets merge into a continuous layer covering the entire MC length. After

ARTICLE

evaporation of the solvent at room temperature, a thin PEO film was formed on the top side of the MCs. These PEO films were further annealed in a vacuum oven at 50 °C for 2 h to remove any trapped solvent. The use of an inkjet printing is not restricted to aqueous solutions. The quality of the coating, however, depends sensitively on the evaporation of the solvent.19 2.4. Optical Microscopy. An optical microscope (Axiotech and Axiotech Vario, Carl Zeiss, Germany) equipped with an Olympus DP11 digital camera system was used in dark field mode to observe the morphology of PEO films on the MCs before and after the heating
cooling cycles. 2.5. Ellipsometry Measurements of Silicon Oxide Layer Thickness. An auto-nulling imaging ellipsometer (EP3, Nanofilm, Germany) in PCSA (polarizer
compensator
sample
analyzer) configuration was used for the measurements of silicon oxide layer thickness. A wavelength of 403.5 nm emitted from a xenon lamp was used, and the angle of incidence was set to 60°. The imaging lateral resolution was 1 μm. For the calculation of the layer thickness from the ellipsometric angles, ψ and Δ, a multilayer model for homogeneous silicon oxide layers covering the silicon substrate was applied. The following refractive indexes were used in the calculations: 1.4691 for silicon oxide, and 5.4343 for silicon. 2.6. Differential Scanning Calorimetry (DSC) Measurements. Bulk sample of PEO with a mass of 7.9 mg was sealed in aluminum pans, and its thermal properties were measured by using a Mettler
Toledo DSC822e. Indium and tin were employed for the temperature calibration at various ramp rates in both heating and cooling runs, the heat capacity was evaluated with respect to sapphire as a standard, and a nitrogen gas purge with a flux of about 30 mL min
1 was used to prevent oxidative degradation of samples during the heating and cooling runs. The PEO samples were first heated to 90 °C and kept isothermal for 1 min to eliminate the complex thermal history. Next, the samples were cooled at 10 K min
1 to 0 °C before the following measurements at different ramp rates of 5 and 20 K min
1, respectively. The temperature range was between 0 and 90 °C. After the heating run, the cooling run was started immediately. Peak point in the traces is taken as the melting and crystallization temperature. 2.7. Atomic Force Microscopy (AFM). A MultiMode Nanoscope IIIa (Veeco Instr., Santa Barbara, CA) was used in the tapping mode under ambient conditions. Silicon tips (model OMCL AC 160 TS) with a resonant frequency of about 300 kHz were used at a scan rate of 0.8 Hz.

3. RESULTS AND DISCUSSION As compared to a single MC, the use of an MC array allows using a number of the MCs as uncoated references to compensate thermal drift of the setup. Furthermore, the signal-to-noise ratio can be improved remarkably by recording differential responses of a number of identical coated MCs with respect to the reference MCs.20 In our experiments, four of the eight MCs in each array were single-side coated with PEO films of various thicknesses (20
700 nm), whereas the others were left uncoated as references (see Figure 2 for an example). For the bare silicon MC, the resonance frequency, fRs, follows:21 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 K ð1Þ fRs ¼ 2π 0:24lwFs ts 22348

dx.doi.org/10.1021/jp205381b |J. Phys. Chem. C 2011, 115, 22347–22353

The Journal of Physical Chemistry C

ARTICLE

observed on the opposite side of any target MC by optical microscope. The apparent thicknesses of the PEO films were determined from the resonance frequency shifts after coating. After singleside coating with PEO films, the resonance frequency, fR, changes to:21 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 K ð4Þ fR ¼ 2π 0:24lwðFs ts þ Fc tc Þ where the subscript “c” represents the coating. In eqs 1 and 4, the K’s of MC before and after coating are not distinguished because the discrepancy between them is negligible (when the tc is equal to ts, there is only about 7.8% increase for the K).22 From a combination of eqs 1 and 4, the apparent film thickness tc can be calculated via "  # Fs ts fRs 2 tc ¼
1 ð5Þ Fc fR

Figure 2. Optical microscopy images of an array of eight MCs (denoted as nos. 1
8 sequentially from the left to the right), where the right four MCs are coated with different thickness of PEO (680 nm for no. 5, 310 nm for no. 6, 170 nm for no. 7, and 53 nm for no. 8), while the left four MCs are bare, before (a) and after (b) six heating
cooling cycles.

and the spring constant, K, follows:22 K ¼

wEs ts 3 4l3

ð2Þ

where the subscript “s” represents the substrate, l is the length, w is the width, t is the thickness, F is the mass density, and E is Young’s modulus. Combing eqs 1 and 2, we can get: rffiffiffiffiffiffiffiffiffiffiffiffi 0:96Fs ts ¼ 2πl2 fRs ð3Þ Es The average resonance frequency of the uncoated silicon MCs was measured to be fRs = 2028.9 ( 3.8 Hz. Using l = 7.5  10
4 m, Fs = 2.33  103 kg m
3, and Es = 1.66  1011 Pa,23 we obtained a ts = 832 ( 2 nm, which is consistent with the data provided by the producer. Although several efforts have been reported to single-side coat the MC with polymers, the inkjet printing is the most versatile way.19,24
27 Figure 2a shows an optical microscopy image of an array of eight MCs where the left four MCs (nos. 1
4) were kept uncoated, while the right four MCs (nos. 5
8) were coated with PEO films of different thickness. It can be seen that almost complete covering of polymer films over the whole length and width was achieved for all four MCs. The coating is quite homogeneous for most of the four coated MCs as can be judged from the colored interference fringes, which reflect the variation of film thickness caused by inhomogeneous droplet evaporation. No polymer samples were

From shifts of resonance frequency, the apparent thickness of PEO coating (Fc = 1.20  103 kg m
3) on the MCs shown in Figure 2a was calculated to be 680 nm for no. 5, 310 nm for no. 6, 170 nm for no. 7, and 53 nm for no. 8, respectively. Figure 3 shows the time-dependent deflection traces of an MC coated with 53 nm thick PEO (mass is 3.7 ng, the no. 8 of the MC array in Figure 2) and the average of four references (bare MCs, nos. 1
4 of the MC array in Figure 2). Figure 3a shows the absolute signal, while Figure 3b shows the differential signal of the PEO-coated MC with respect to the reference. The deflection traces of the reference in Figure 3a are shifted down by 300 nm for clarity. Upon heating, the deflection of the reference MCs changed rapidly at the beginning, eventually slowing until a stable maximum deflection was reached. The negative value of the deflection corresponds to an MC bending upward when heated. The same nonlinear time response behavior was observed upon cooling. Again, the change in deflection was stronger at the start of cooling and slowed gradually until the deflection almost returned to the initial value of the heating
cooling cycle. For the following five successive heating
cooling cycles, a very good repeatability was seen, and the deflection traces could be largely overlapped. In our heating
cooling system, constant power was used in the heating run, while the cooling run was up to the heat convection and radiation loss. Therefore, the temperature versus time curves for both runs followed an exponential function. For the applied power of 10 W, the instantaneous temperature, T, as a function of run time, t, was given as T = Tmax
(Tmax
Tamb) exp(
t/(Tmax
Tamb)/3.944 s K
1) for the heating run and T = Tamb
(Tamb
Tmax) exp(
t/(Tmax
Tamb)/4.867 s K
1) for the cooling run, respectively (Tmax denotes the maximum system temperature, and Tamb is the ambient temperature). Because of the finite time allowed for cooling, both Tmax and Tamb increased slightly in the course of multiple heating
cooling cycles. Tmax increased from 82 to 85 °C, and Tamb increased from 22 to 25 °C between the first and sixth heating
cooling cycles (see Figure 3c). Using the exponential relation between temperature and time, the time-dependent deflection measurements (Figure 3b) may be converted into temperature/deflection traces (Figure 4). For the reference, the deflection Δz changed linearly with ramping temperature (Figure 4). The deflection change per unit 22349

dx.doi.org/10.1021/jp205381b |J. Phys. Chem. C 2011, 115, 22347–22353

The Journal of Physical Chemistry C

ARTICLE

Figure 4. Temperature traces of differential deflection of the same MCs as shown in Figure 3: (a) heating run and (b) cooling run.

more reliable than the absolute values toxidetop and toxidebottom. For this reason, although this simplification would exaggerate the bending effect under the same temperature change, we simplified this trilayer model as a bilayer model: 832 ( 2 nm thick silicon substrate as calculated above covered only on the top side by 2.83 ( 0.09 nm (toxide = toxidetop
toxidebottom) thick silicon oxide. When the thermal expansion effect is considered, the deflection follows:28 ΔzT ¼ 2 Figure 3. Time traces of MC deflection (a,b) and temperature profile (c). The MC is coated with 53 nm thick PEO (no. 8 MC of the array in Figure 2) and the average of four references (bare MCs, nos. 1
4 MC of the array in Figure 2): absolute signal (a) and differential signal (b). The relative error of four bare MCs is about 4.8%. The traces of bare MCs in (a) have been shifted downward along the y (deflection) axis by 300 nm for clarity.

temperature ΔzT was calculated to be
6.71 ( 0.32 nm K
1 from the average over MC nos. 1
4 in the heating run of the heating
cooling cycle. Upon change of temperature, the difference in thermal expansion coefficient of the silicon substrate and silicon oxide layer would lead to a bending. According to the ellipsometry measurements, the top side oxide layer had a thickness, toxidetop = 7.02 ( 0.15 nm, while the bottom side oxide layer had a thickness, toxidebottom = 4.20 ( 0.04 nm. As far as ellipsometry measurements are concerned, the differential value toxide is much

3l2 ðαoxide
αs Þ toxide þ ts 

t oxide 1 þ ts

2

3

7 6 7 6 7 6 ! 7 6  2   7 6 2 t E t t E t oxide oxide oxide oxide s s 5 4 3 1 þ þ 1 þ þ ts Es ts ts 2 Eoxide toxide

ð6Þ where the subscript “oxide” represents the covering silicon oxide layer, and the α terms are the coefficients of thermal expansion. Using αs = 3  10
6 K
1 for silicon, αoxide = 5.5  10
7 K
1 for silicon oxide, and Eoxide = 7.5  1010 Pa for silicon oxide,29 we obtained ΔzT =
7.63 nm K
1 for the estimated response of a bimaterial MC, which is in good agreement with the experimental results (
6.71 ( 0.32 nm K
1) shown in Figure 4, especially after considering the exaggeration effect of the simplification of trilayer model as bilayer model as mentioned above. Therefore, we can conclude that the deflection of the bare MCs mainly resulted from the thickness difference of the oxide layers between the opposite sides. 22350

dx.doi.org/10.1021/jp205381b |J. Phys. Chem. C 2011, 115, 22347–22353

The Journal of Physical Chemistry C

Figure 5. DSC traces of 7.9 mg of PEO in the bulk form at various ramp rates.

Figure 3 also shows the time traces of deflection of a PEOcoated MC (effective thickness 53 nm, no. 8 MC of the array in Figure 2). The positive value of the differential deflection with respect to the reference shown in Figure 3b means that the MC bent downward upon heating. In comparison with the uncoated references, the PEO-coated cantilevers show similar nonlinear response patterns, due to the exponential temperature profile during heating. As compared to the bare references, the deflection traces of coated MC in Figure 3 also show some characteristic features, not found for the references. First, the maximum value of the deflection in each cycle decreased with increasing number of cycles. Besides, for the first three heating
cooling cycles, a peak appeared in the initial stages of the heating and cooling. For the fourth and fifth cycles, the peak only appeared in the heating run and turned into a step in the cooling run. After the fifth cycle, no more peaks or steps were detected. Using the same time/temperature conversion as for the reference MCs, the time traces of the PEO-coated MC shown in Figure 3b may be converted into temperature traces. With increasing cycle number, the peak appearing in the heating run shifted to lower temperatures (Figure 4a) as did the peak in the cooling run (Figure 4b). Furthermore, the monotonic decrease of the maximum deflection with increasing cycle times can be seen for both the heating and the cooling runs. Figure 5 shows the DSC traces of PEO in the bulk form at various ramp rates. As expected, an endothermic melting peak appeared in the heating run, and an exothermic crystallization peak appeared in the cooling run. As is typical for polymeric materials, the melting peak shifted to higher temperatures, while the crystallization peak shifted to lower temperatures with increasing ramp rates. Comparing Figures 4 and 5, we can unequivocally attribute the characteristic peak and step appearing in the MC traces to the melting and crystallization of PEO films on the silicon MC. However, our experimental setup could not measure the instantaneous temperature change due to the phase transitions where a time resolution of milliseconds was required.3
5 Because the deflection value for the PEO-coated MC was the differential signal with respect to the reference cantilevers, the effect of the different thickness of silicon oxide layers on the opposite sides of silicon substrate on the deflection traces has already been compensated for. This allows us to interpret the response of the PEO-coated MC in terms of a bilayer with

ARTICLE

Figure 6. Comparison between MC and DSC measurements obtained from Figures 4 and 5. The DSC data are the extrapolated values with the same ramp rate as the MC. The solid lines are linear fits to the MC data.

PEO layer on the top side of a silicon MC. Because the coefficient of thermal expansion of PEO coating (1.2  10
4 K
1) is about 40 times larger than that of the silicon substrate, the MC bends toward the substrate (downward) upon increase of temperature (Figures 3b and 4). However, the simple bimaterial model does not account for the appearance of a peak at the phase transition temperature of the PEO layer. In fact, there are two more effects associated with the melting of PEO layer, which exercise opposite effects on the cantilever deflection: an endothermic effect related to the heat of fusion (about 165 J g
1) and a rapid jump in volume (about 9%). In our response signal, both effects are present in addition to the change of the thermal expansion coefficient.30 The endothermic effect hinders the thermal increase of the deflection, while the volume jump effect favors it. Therefore, the appearance of the peak was a combination of these two effects. As a matter of fact, on closer observation the so-called peak can also be looked at as a step at least for the first heating run shown in Figure 4a. In a similar way, the exothermic heat of crystallization combined with a drop in volume led to a peak upon crystallization of the PEO layer in the cooling runs (Figure 4b). Figure 6 shows the comparison of the phase transition temperature obtained from MC and DSC measurements where the peak point is taken as the characteristic temperatures for both techniques. It should be mentioned that the ramp rates in the MC measurements were not constant but decreased gradually with the proceeding of heating or cooling run. From the temperature versus time curves, the instantaneous ramp rates at the peak or step temperature were measured to be about 6.8 and 3.7 K min
1 for the heating and cooling runs, respectively. For comparison, both Tm and Tc for the DSC measurements were extrapolated to the same ramp rates as those for the MC measurements. From Figure 6, it can be seen that the Tm measured by MC for the first heating
cooling cycle was about 8 K lower than that by DSC measurements, while the Tc by MC measurements for the first cycle was almost the same as that by DSC measurements. Because the surface of the native oxide layer on silicon substrate was not hydrophilic (water contact angle ca. 50° in ambient atmosphere), partial dewetting of PEO melt is inevitable for thin films.31
34 As shown in Figure 2b, bright spots appeared on the top surface of the polymer films after six heating
cooling 22351

dx.doi.org/10.1021/jp205381b |J. Phys. Chem. C 2011, 115, 22347–22353

The Journal of Physical Chemistry C

ARTICLE

Figure 7. AFM height (left) and phase (right) images of PEO-coated MC (no. 8 MC of the array in Figure 2) before the first heating
cooling cycle (top) and after the sixth heating
cooling cycle (bottom).

Figure 8. Dependence of Tm and Tc on the thickness of PEO films. The solid lines are guides to the eyes.

cycles. Under the optical microscope, most of the PEO aggregates were observed to keep their shape even at the temperature well above the Tm, and the number of the aggregates was observed to increase gradually with increasing cycles. These results indicate an irreversible partial dewetting of the PEO films from the surface of the silicon MC. However, because most of the substrate surface was still covered by the polymer film according to both optical microscopy (see Figure 2b) and AFM (see Figure 7), the partially dewetted PEO films still show a response behavior different from the reference MCs. As a result of the progressing dewetting, the maximum deflection of the PEO-coated MC decreased with increasing cycle times as shown in Figures 3b and 4. For the PEO films with a thickness of 53 nm used in our experiments, the increase of the surface roughness due to the partial dewetting would reduce the gross crystallization rate and the final degree of crystallinity and perfection of crystals.34,35 Therefore, both phase transition temperatures decreased with increasing cycle times as shown in Figures 4 and 6.

Figure 8 shows the film thickness dependence of phase transition temperatures of thin PEO films measured by MC. It can be seen that for the thickness below 100 nm both phase transition temperatures decreased with decreasing thickness, while for the thickness above 200 nm the temperature dependence is much less pronounced. This is consistent with the literature results of PEO films and other semicrystalline polymers such as poly(di-n-hexyl silane), where both the crystallization rate and the final degree of crystallinity decreased rapidly with decreasing thickness for the thickness less than 100 nm and remained constant for the thickness greater than 200 nm.35
38 For ultrathin films, a confinement-induced anomalous chain transport at the crystal/melt interface was used to explain the thickness dependence.35 As a matter of fact, the trend of the phase transition temperatures with increasing film thickness shown in Figure 8 can be used to explain the remarkable difference between MC and DSC measurements shown in Figure 6. Lower phase transition temperatures obtained by MC than those by DSC might result from the combination of the effect of low thickness (bulk samples were used for DSC) and the different crystallization conditions.

4. CONCLUSIONS Melting and crystallization of thin PEO films on the silicon substrate with native oxide layer could be observed within the deflection traces of MC. With increasing number of heating
cooling cycles, both phase transitions shifted to lower temperature due to the partial dewetting of the polymer films from the substrate. As compared to the phase transition temperatures of bulk materials measured by DSC, the melting temperature Tm measured by MC was about 7
10 K lower, while the crystallization temperature Tc was almost the same. It was also found that with increasing thickness of PEO films, both phase transition temperatures increased at thicknesses below 100 nm and turned over to a constant value for the thickness above 200 nm. 22352

dx.doi.org/10.1021/jp205381b |J. Phys. Chem. C 2011, 115, 22347–22353

The Journal of Physical Chemistry C

’ AUTHOR INFORMATION Corresponding Author

*Telephone: +86-10-62334516 (J.Z.), +49-201183-2566 (J.S.G.). Fax: +86-10-62332599 (J.Z.), +49-201183-4934 (J.S.G.). E-mail: [email protected] (J.Z.), jochen.gutmann@uni-due. de (J.S.G.).

’ ACKNOWLEDGMENT We would like to thank Dr. R€udiger Berger, Dr. Masaya Toda, Mrs. Christine Herrmann, and Dr. Jijun Wang of Max Planck Institute for Polymer Research, Mainz, Germany, for the MC, ellipsometry, DSC, and AFM measurements, respectively. The PEO was kindly provided by Mr. Thomas Wagner. The financial support from the joint program between the Max Planck Society (MPG), Germany, and the Chinese Academy of Sciences (CAS), People’s Republic of China, is also greatly appreciated. ’ REFERENCES (1) Gimzewski, J. K.; Gerber, Ch.; Meyer, E.; Schlittler, R. R. Chem. Phys. Lett. 1994, 217, 589–594. (2) Berger, R.; Gerber, Ch.; Gimzewski, J. K.; Meyer, E.; G€untherodt, H. J. Appl. Phys. Lett. 1996, 69, 40–42. (3) Nakagawa, Y.; Sch€afer, R.; G€untherodt, H.-J. Appl. Phys. Lett. 1998, 73, 2296–2298. (4) Nakagawa, Y.; Sch€afer, R. Angew. Chem., Int. Ed. 1999, 38, 1083–1085. (5) Goericke, F.; Lee, J.; King, W. P. Sens. Actuators, A 2008, 143, 181–190. (6) Kasper, M. Master Degree Thesis, University of Illinois at Urbana
Champaign, 2010. (7) Berger, R.; Lang, H. P.; Gerber, Ch.; Gimzewski, J. K.; Fabian, J. H.; Scandella, L.; Meyer, E.; G€untherodt, H.-J. Chem. Phys. Lett. 1998, 294, 363–369. (8) Lavrik, N. V.; Sepaniak, M. J.; Datskos, P. G. Rev. Sci. Instrum. 2004, 75, 2229–2253. (9) Ji, H.-F.; Armon, B. D. Anal. Chem. 2010, 82, 1634–1642. (10) Chen, H.; Xu, H.; Cebe, P. Polymer 2007, 48, 6404–6414. (11) Chen, H.; Cebe, P. Macromolecules 2009, 42, 288–292. (12) Chen, H.; Liu, Z.; Cebe, P. Polymer 2009, 50, 872–880. (13) Zhao, J.; Swinnen, A.; Van Assche, G.; Manca, J.; Vanderzande, D.; Van Mele, B. J. Phys. Chem. B 2009, 113, 1587–1591. (14) Zhao, J.; Shan, J.; Van Assche, G.; Tenhu, H.; Van Mele, B. Macromolecules 2009, 42, 5317–5327. (15) Zhao, J.; Hoogenboom, R.; Van Assche, G.; Van Mele, B. Macromolecules 2010, 43, 6853–6860. (16) Zhao, J.; Bertho, S.; Vandenbergh, J.; Van Assche, G.; Manca, J.; Vanderzande, D.; Yin, X.; Shi, J.; Cleij, T.; Lutsen, L.; Van Mele, B. Phys. Chem. Chem. Phys. 2011, 13, 12285–12292. (17) Minakov, A. A.; van Herwaarden, A. W.; Wien, W.; Wurm, A.; Schick, C. Thermochim. Acta 2007, 461, 96–106. (18) Gotzen, N.-A.; Huth, H.; Schick, C.; Van Assche, G.; Neus, C.; Van Mele, B. Polymer 2010, 51, 647–654. (19) Bietsch, A.; Zhang, J.; Hegner, M.; Lang, H. P.; Gerber, Ch. Nanotechnology 2004, 15, 873–880. (20) Lang, H. P.; Berger, R.; Andreoli, C.; Brugger, J.; Despont, M.; Vettiger, P.; Gerber, Ch.; Gimzewski, J. K.; Ramseyer, J. P.; Meyer, E.; G€untherodt, H.-J. Appl. Phys. Lett. 1998, 72, 383–385. (21) Chen, G. Y.; Warmack, R. J.; Thundat, T.; Allison, D. P.; Huang, A. Rev. Sci. Instrum. 1994, 65, 2532–2537. (22) Young, W. C.; Budynas, R. G. Roark’s Formulas for Stress and Strain, 7th ed.; McGraw
Hill: New York, 2002. (23) Wortman, J. J.; Evans, R. A. J. Appl. Phys. 1965, 36, 153–156.

ARTICLE

(24) Baller, M. K.; Lang, H. P.; Fritz, J.; Gerber, Ch.; Gimzewski, J. K.; Drechsler, U.; Rothuizen, H.; Despont, M.; Vettiger, P.; Battiston, F. M.; Ramseyer, J. P.; Fornaro, P.; Meyer, E.; G€untherodt, H.-J. Ultramicroscopy 2000, 82, 1–9. (25) Betts, T. A.; Tipple, C. A.; Sepaniak, M. J.; Datskos, P. G. Anal. Chim. Acta 2000, 422, 89–99. (26) Bumbu, G.-G.; Kircher, G.; Wolkenhauer, M.; Berger, R.; Gutmann, J. S. Macromol. Chem. Phys. 2004, 205, 1713–1720. (27) Zhao, J.; Berger, R.; Gutmann, J. S. Appl. Phys. Lett. 2006, 89, 033110. (28) Timoshenko, S. P. J. Opt. Soc. Am. 1925, 11, 233–255. (29) Cao, Z.; Zhang, T.-Y.; Zhang, X. J. Appl. Phys. 2005, 97, 104909. (30) Brandrup, J.; Immergut, E. H.; Grulke, E. A.; Abe, A.; Bloch, D. R. Polymer Handbook, 4th ed.; John Wiley & Sons: New York, 1999. (31) Reiter, G.; Sommer, J.-U. Phys. Rev. Lett. 1998, 80, 3771–3774. (32) Reiter, G.; Sommer, J.-U. J. Chem. Phys. 2000, 112, 4376–4383. (33) Sommer, J.-U.; Reiter, G. J. Chem. Phys. 2000, 112, 4384–4393. (34) Ok, S.; Demirel, A. L. J. Macromol. Sci., Part B: Phys. 2003, 42, 611–620. (35) Dalnoki-Veress, K.; Forrest, J. A.; Massa, M. V.; Pratt, A.; Williams, A. J. Polym. Sci., Part B: Polym. Phys. 2001, 39, 2615–2621. (36) Frank, C. W.; Rao, V.; Despotopoulou, M. M.; Pease, R. F. W.; Hinsberg, W. D.; Miller, R. D.; Rabolt, J. F. Science 1996, 273, 912–915. (37) Despotopoulou, M. M.; Miller, R. D.; Rabolt, J. F.; Frank, C. W. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 2335–2349. (38) Despotopoulou, M. M.; Frank, C. W.; Miller, R. D.; Rabolt, J. F. Macromolecules 1996, 29, 5797–5804.

22353

dx.doi.org/10.1021/jp205381b |J. Phys. Chem. C 2011, 115, 22347–22353