Article pubs.acs.org/Macromolecules
Calorimetric Glass Transition of Single Polystyrene Ultrathin Films Siyang Gao, Yung P. Koh, and Sindee L. Simon* Department of Chemical Engineering, Texas Tech University, Lubbock, Texas 79409-3121, United States ABSTRACT: The calorimetric glass transition (Tg) is measured for single polystyrene ultrathin films using a commercial rapid-scanning chip calorimeter as a function of cooling rate and film thickness. Films have been prepared in two ways: spin-cast films placed on a layer of inert oil or grease and films directly spin-cast on the back of the calorimetric chip. For the films on oil or on grease, the 160 nm thick films show results consistent with those of a bulk sample measured by conventional DSC. On the other hand, the 47 nm thick film on oil and 71 nm thick films both on oil and on grease show a Tg depression which decreases with increasing cooling rate; the magnitude of the Tg depression is similar to results reported in the literature for the most mobile substrate-supported films. For films directly spin-cast onto the sensor, a Tg depression is not observed for 47 and 71 nm thick films but is observed for a 16 nm thick film. These results are also within the range of the data on supported films in the literature but show a smaller depression than films on oil or grease. The effect of annealing is also investigated. For thick films and those directly spin-cast onto the sensor, annealing at 160 °C has no influence on heat flow curves; hence, Tg values remain unchanged. For the 47 and 71 nm thick films on either oil or grease, the depressed Tgs revert to the bulk values over the course of a day at 160 °C. Atomic force microscope (AFM) images show that annealing results in dewetting of the films with hole growth and thickening of the film to 200 nm, the latter of which is presumed to be the reason that Tgs revert to bulk values.
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studied stacked polystyrene ultrathin films and found Tg reductions similar to the supported polystyrene thin film work performed using other techniques. These results have been corroborated by Fukao and co-workers using dielectric relaxation spectroscopy,22 whereas Boucher and co-workers obtained much larger calorimetric depressions for their stacked films.24 On the other hand, recent work by Baer and coworkers26 indicates that the calorimetric Tg of stacked polystyrene ultrathin layers interleaved between polycarbonate is independent of layer thickness and equal to the bulk value, and their results are attributed to the importance of thin film interactions with the substrate or with neighboring polymeric domains. The role of interactions was first observed by Keddie, Jones, and Cory,8 who showed that poly(methyl methacrylate) thin films have a Tg depression on gold and a Tg elevation on native silicon surfaces. For polystyrene, Torkelson and coworkers18 find that the Tg depression of an ultrathin layer depends on the thickness and composition of the polymer underlayer, and Tsui and Russell, as well as Genesan and coworkers,9,23 report that the Tg depression decreases with increasing interfacial energy for polystyrene films on surfaces coated with polystyrene brushes or with styrene/methyl methacrylate copolymer. On the other hand, Fryer et al.7 report that the Tg of polystyrene films changes linearly with interface energy on rigid substrates, increasing at high energies and decreasing at low energies. However, in the recent study by
INTRODUCTION Changes in the glass temperature for nanoconfined materials have generated considerable interest since the early 1990s when a depression was first observed by Jackson and McKenna1 and verified by Jonas and co-workers.2 Several reviews have been written.3−5 The Tg of nanoconfined glass-formers is found to increase,6−8 decrease,7−25 or remain unchanged.12,18,26−30 Polystyrene ultrathin films have been especially widely studied, and in their case, Tg is generally observed to decrease or remain unchanged.7−30 For polystyrene ultrathin films supported on silica and other solid substrates, the Tg depression has been most commonly measured using X-ray reflectivity,6,8,9 ellipsometry,7,8,10−15,26 local thermal analysis,7 dielectric relaxation,16,22,25 fluorescence,18 and Brillouin light scattering,12,19 with typical cooling or heating rates from 0.5 to 2 K/min. Among these works, the glass transition temperature is empirically found to have a nonlinear dependence on the film thickness, independent of molecular weight:8,10 ⎡ ⎛ α ⎞δ ⎤ Tg(h) = Tgbulk ⎢1 − ⎜ ⎟ ⎥ ⎝h⎠ ⎦ ⎣
(1)
where Tg(h) is the Tg for a film of thickness h, Tbulk is the Tg of g the bulk material, and α and δ are fitting parameters which vary slightly among researchers. Values for α, δ, and Tbulk of 1.3 nm, g 1.28, and 373.8 ± 0.7 K, respectively, are reported by Keddie and co-workers.10 Although limited work on the Tg depression of thin films has been performed using conventional differential scanning calorimetry (DSC), Koh, McKenna, and Simon20,21 © 2013 American Chemical Society
Received: September 25, 2012 Revised: December 5, 2012 Published: January 10, 2013 562
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Table 1. WLF Parameters for Ultrathin Films from Flash DSC interface N/A Krytox oil
Apiezon grease directly spin-cast
film thickness (nm) bulk 160 71 47 160 71 160 71 47 16
sample mass (ng)b N/A 8.3 7.0 5.4 15.6 9.4 51.9 31.1 21.2 2.7
Tg,ref (K) at 0.1 K/s 374.2 375.6 369.0 365.7 376.0 368.9 376.4 376.3 375.9 361.0
± ± ± ± ± ± ± ± ± ±
0.2 0.5 0.4 1.2 0.7 0.8 0.6 0.4 0.7 0.4
C1 N/A 19.3 10.3 8.9 13.2 13.2 11.7 12.1 16.6 19.8
± ± ± ± ± ± ± ± ±
C2 (K) 3.1 1.5 1.8 2.7 1.7 2.8 3.7 4.6 7.9
N/A 67.6 10.3 48.3 44.1 63.1 37.6 39.2 56.9 123.3
± ± ± ± ± ± ± ± ±
11.1 1.5 10.7 9.8 9.1 9.7 12.8 16.3 50.6
ΔH/R (kK)a 124 93 65 57 97 65 101 100 95 44
± ± ± ± ± ± ± ± ± ±
0.4 0.2 1 1 2 1 2 2 1 2
c
ma 144 107 76 67 113 76 117 117 109 58
± ± ± ± ± ± ± ± ± ±
0.4c 0.2 1 1 2 1 2 2 1 3
a Determined using Tg values at 0.1 K/s. bDetermined from ΔCp assuming a value of ΔCp = ΔCp,bulk[1 − (α/h)δ], where ΔCp, bulk is 0.29 J/(g K), α is 2.35 nm, and δ is 0.73 based on data from ref 21. cDetermined from a linear fit of the data in ref 21 due to the limited frequency range over which the data were obtained.
Nealey and co-workers,25 the Tg of polystyrene thin films was independent of substrate pretreatment and equal to the bulk value for films thicker than 20 nm. There is also limited work that shows no change in Tg for supported polystyrene ultrathin films. Both Schick and coworkers and Allen and co-workers measured single ultrathin films down to 3 nm using nanocalorimetry27−30 and reported no Tg depressions in their studies. The discrepancy between the nanocalorimetry results and those showing Tg depressions was initially attributed to the high frequency and rates used in nanocalorimetry. In fact, a cooling rate dependence of the Tg depression was found by Fakhraai and Forrest,13 Schönhals and co-workers,17 and Koh and Simon.21 However, subsequent work by Allen and co-workers28 showed no Tg reductions even after annealing which effectively increased the time scales of the measurements to those comparable with conventional DSC. In addition, in Schick’s work,29,30 no Tg depression was found even at the relatively low frequency of 40 Hz. The reason for a lack of a Tg depression in nanocalorimetry is, thus, unclear, and as part of the present work, we followed the same procedure as Schick’s group and measure the calorimetric Tg for films directly spin-cast on the sensor. As will be discussed, we find differences between results for films directly spin-cast on the native silica substrate and those for films spin-cast on mica and then transferred to the sensor on a thin layer of oil or grease. A further complication which must be considered is the role of annealing and degradation. Kremer and co-workers show no Tg reductions for polystyrene films down to 20 nm thick.31,32 They annealed their samples under vacuum and high temperature (up to 200 °C) and attributed the Tg depressions observed by other researchers to oxidation or to plasticization by water from film preparation and transfer techniques.31 Their interpretation has been questioned by McKenna.33,34 Several works by Dalnoki-Veress and co-workers35,36 have also directly disputed the claims from Kremer and co-workers. Raegen et al.35 found that the depressed Tg values in supported polystyrene films were independent of the measurement atmosphere. More recent work by Bäumchen et al.36 shows that a Tg depression is observed for a freely standing polystyrene ultrathin film, and no depression is observed for the same film after placing it on a substrate without further annealing, indicating the importance of free surface effects relative to annealing and sample preparation. Here, we examine the influence of high-temperature annealing on the T g depression of supported films in order to further test Kremer’s
suggestion that measured Tg depressions are artifacts associated with film preparation. A commercial rapid-scanning chip calorimeter is used to study single polystyrene ultrathin films due to its high sensitivity. In addition, its broad range of cooling/heating rates allows us to obtain data over 4 decades in cooling rate, from 0.1 to 1000 K/s, and to compare the results with those from conventional DSC, which allows controlled cooling at 0.5 K/s (30 K/min) and lower. The paper is organized as follows: the material and sample preparation are described in the Experimental Section, the results for Tg as a function of film thickness for various rates and annealing times are then shown, and we end with the discussion and conclusions.
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EXPERIMENTAL SECTION
A high-molecular-weight polystyrene with 1 998 000 g/mol numberaverage molecular weight and PDI of 1.02 (Sigma-Aldrich) was used in this study. Films were prepared by spin-coating polystyrene from toluene (99.999% purity, Sigma-Aldrich). Concentrations of 0.22, 0.45, 0.62, and 0.91 wt % were used to prepare 16 ± 3.2, 47 ± 4.2, 71 ± 3.6, and 160 ± 4.9 nm films, with thickness determined by making a scratch on the film and using an atomic force microscope (AFM, Advanced Scanning Probe Microscope XE-100) in tapping mode to measure the film thickness. Films spin-cast on mica were floated on water, picked up with a ringlike support, and then annealed for 24 h at ambient conditions and for an additional 24 h at 50 °C under vacuum. The films were then cut and transferred using a hair to the sensor area of the flash DSC chip which has been reported37 to be ∼0.2−0.5 mm2. The sample masses ranged from 5 to 16 ng as reported in Table 1 and calculated from the step change in heat flow at Tg. A layer of Krytox oil (GPL107 from DuPontTM, Tg = −63.2 °C) of ∼10 ng based on the step change in heat flow at the Tg of the oil was used to enhance the thermal contact between the chip and the sample. Apiezon-N grease (SPI Supplies) was also used between the sensor and thin films for some samples since it was previously used by Schick and co-workers38 for measuring semicrystalline samples. The oil or grease layer also ensures that the thin film stays on the sensor during measurements. As previously mentioned, substrate surface energy can be important for the Tg depression. The surface energy of the Krytox oil is 16−20 mJ/m2 according to manufacture data,39 and that of Apiezon-N grease is determined by the two-liquid method to be 27 mJ/m2 using a goniometer and water and diiodomethane liquids. The polystyrene sample is stable in the Krytox oil, and no plasticization or change in Tg occurred during bulk modulus measurements made over the course of months at elevated temperature and pressures in our laboratory.40−42 The Tg of bulk polystyrene, on the other hand, is depressed a maximum of 2 K by absorption of an equilibrium amount (3 wt %) of Apiezon grease as measured by conventional DSC; the 2 K depression 563
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was observed on the first DSC run after sealing equal amounts of grease and polystyrene in the pan and did not evolve over the course of weeks at 85 °C or on multiple scans from 25 to 135 °C. It is noted that plasticization may suppress Tg changes in nanoconfined films,43−45 and this is further addressed later. Films were also spin-cast directly onto the backside of the calorimetric sensor, which is native silicon oxide and is used as received, and in this case, no oil or grease was used between the film and the sensor. The spin-cast material initially covered the entire back side of the sensor; the excess material was removed using a hair dipped in toluene, such that the film remaining covered only the sample side of the sensor. Sample sizes for films directly spin-cast onto the sensor ranged from 3 to 52 ng, as shown in Table 1 and calculated from the step change in heat flow at Tg; these sample sizes are consistent with the assumed sample thickness given the reported sensor area (∼0.2− 0.5 mm2 37). Annealing was then accomplished for 24 h under in-situ conditions and for another 24 h at 50 °C under vacuum. The morphology of the back of sensor and the films before and after DSC scans was examined by AFM: the films are uniform with no signs of dewetting. The root-mean-squared roughness (Rq) is 11 nm, identical to that of the untreated silicon substrate, and the roughness does not change after DSC scans or with annealing. The glass transition temperatures of the polystyrene thin films were measured using a Mettler Toledo Flash DSC 1 with freon intercooler and nitrogen purge. A piece of indium of ∼1 μg was placed on the top of the sample to ensure that temperature calibration was correct. Temperature corrections were made based on the onset of indium melting, and (Tm,IN − Tm, meas) ranged from −0.5 to −3.5 K, with an average of −1.3 K, and was stable for a given sample and heating rate. For annealing experiments and films spin-cast directly onto the chip, indium was placed on the sample only at the end of the experimental series. The calorimetric chip was conditioned and corrected according to Mettler Toledo procedures prior to use. Heating scans were performed from 50 to 160 °C after cooling at rates ranging from 0.1 to 1000 K/s. Sensitivity increases with increasing heating rate; hence, for the smallest sample/thinnest film (3 ng/16 nm thick), a heating rate of 5000 K/s was needed to obtain a good signal-to-noise ratio. For thicker samples heating rates of 600− 1000 K/s were used. Data were not collected during the first heating scan, and two to six heating scans were performed for each sample to check reproducibility. Measurements with the flash DSC can be made on cooling, but even at a cooling rate of 1000 K/s, the signal from our samples is too weak to accurately determine Tg on cooling because the transition is broader on cooling than on heating. Hence, to cover a large range of cooling rates, measurements are made on heating after cooling at prespecified rates. The limiting fictive temperature (Tf′) is determined from the heating scans. The value of Tf′ measured on heating after cooling at a given rate is equivalent (within 1 K) to the Tg value measured on cooling at the same rate.46−48 Consequently, we often refer to Tf′ as Tg in the Results and Discussion sections of this work. Three methods were used to obtain Tf′. Data were analyzed using the Mettler Toledo software as well as using two different calculation procedures developed for Excel. The data used for the Mettler Toledo software calculations were smoothed over 3−4 K (using the Mettler Toledo software), whereas the unsmoothed raw data were used for the Excel calculations. All calculations are based on Moynihan’s definition of Tf′:49 T ≫ Tg
∫T′ f
(C pl − C pg) dT =
T ≫ Tg
∫T ≪T
g
(C p − C pg) dT
can be incurred if Cpl and Cpg are drawn incorrectly. Consequently, in our work, we first superpose the liquid and glass lines for all of the runs for a given sample and then calculate Tf′ using the same glass and liquid lines. Using this procedure, both the Mettler Toledo software and Excel calculations using eq 2 give the same Tf′ values within ±1.1 K. A simplified version of eq 2 was also applied to the data for Excel calculations of Tf′. When overshoots are present such that Tf′ is below the onset of devitrification (i.e., Tf′ is below the transition region observed on heating), eq 2 can be simplified: T ≫ Tg
∫T′ f
(C pl − C p) dT = 0
(3)
The advantage of applying this equation is that only the extrapolated liquid line is needed. We found that Tf′ values calculated from eqs 2 and 3 agree with each other in all cases for flash DSC data having moderate to large overshoots. The only discrepancy between the two methods is for the small overshoots obtained on heating at 600 K/s after cooling at 1000 K/s; in this case, eq 2 gives a value 0.3 K higher than eq 3, but still well within the error of the measurements. The effect of annealing at 160 °C on the Tg values measured was investigated using a procedure that involved annealing for a specific time, cooling the sample at a given rate q, either 0.1 or 60 K/s, heating to obtain Tg(q), and then further annealing at 160 °C; the cycle was repeated until the cumulative time of annealing was 2 days. For the 160 nm thick film and for films directly spin-cast on the sensor, annealing had no effect on the heat flow curves or the Tg values. This was not the case for the thinner films on oil or grease, and thus, in order to determine the mechanism operable during annealing, experiments were also performed on 71 nm thick films supported on a mica surface with a thin layer of oil or grease between the film and substrate. The films were annealed under vacuum at 160 °C, and the morphology was investigated using AFM in tapping mode as a function of annealing time. Conventional DSC (Mettler Toledo DSC 1) was also used to measure the limiting fictive temperature (Tf′ ≈ Tg) as a function of cooling rate for a bulk sample of ∼10 mg. The DSC experiments were performed from 25 to 135 °C under nitrogen gas, and the cooling system was maintained at 5 °C. The heating rate was fixed at 0.17 K/s (10 K/min), and the cooling rates varied from 0.0017 to 0.5 K/s (0.1 to 30 K/min). Tf′ values were obtained by the Richardson method50 using the Mettler Toledo software, which is equivalent to Moynihan’s method for determining Tf′.49 The temperature calibration was checked periodically using indium on heating at 10 K/min.
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RESULTS Films on Apiezon Grease or Krytox Oil. The heat flow versus temperature behavior for the 160 nm thick polystyrene film on Krytox oil is shown in Figure 1 as a function of cooling rate q for heating at 600 K/s. The enthalpy overshoot at the glass transition increases in magnitude and shifts to higher temperatures as the cooling rate decreases from 1000 to 0.1 K/ s. This behavior is expected and well understood. It arises from the kinetics associated with the glass transition: slower cooling results in lower volume, enthalpy, and mobility in the glassy state at a given temperature, and consequently, slowly cooled samples must be heated to higher temperatures before their mobility increases enough to allow the material to recover its equilibrium liquid density; the result is a shift in the enthalpy overshoot to higher temperatures and an increase in the area under the overshoot with decreasing cooling rate. We also draw attention to the good reproducibility and superposition of the liquid and glass lines in Figure 1. As mentioned in the Methodology section, this reproducibility is important to accurately determine Tf′ from the data. In order to illustrate the method for determining the limiting fictive temperature (Tf′ ≈ Tg) from the data as well as
(2)
where Cpl is the liquid heat capacity, Cpg is the glass heat capacity, and Cp is the apparent heat capacity of the sample. The Tf′ values calculated from eq 2 depend strongly on how the liquid and glass lines are drawn when large overshoots are present. Typically, for small to moderate overshoots in conventional DSC, changes in the liquid and glass lines result in only small changes (
