LETTER pubs.acs.org/JPCL
Structural Variations of an Organic Glassformer Vapor-Deposited onto a Temperature Gradient Stage Zahra Fakhraai,† Tim Still,‡ George Fytas,‡ and M. D. Ediger*,† † ‡
Department of Chemistry, University of Wisconsin—Madison, Madison, Wisconsin 53706, United States Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
bS Supporting Information ABSTRACT: The structural properties of vapor-deposited organic glasses have been shown to depend significantly upon the temperature of the substrate during the deposition. In order to systematically study this effect, a stage was designed to maintain a linear temperature gradient of 80 K across a substrate during vapor deposition. Brillouin light scattering was used to measure the longitudinal sound velocity of glasses of indomethacin at each deposition temperature by performing measurements at many positions along the direction of the temperature gradient. Glasses with exceptionally high longitudinal moduli are formed at deposition temperatures between 0.7Tg and 0.9Tg, where Tg is the glass transition temperature. In this regime, the observed dependence of the deposition rate on the longitudinal modulus is consistent with glass formation via enhanced surface mobility. Glasses prepared at low temperatures were observed to have longitudinal moduli up to 20% less than the modulus of ordinary glass of indomethacin. SECTION: Macromolecules, Soft Matter
structure of the material.19,20 In contrast to the exceptionally stable glasses that are produced at 0.85Tg, at temperatures far below Tg, the molecules condense on the surface without a chance to find efficient packing arrangements. Such glasses have been reported to have lower densities than glasses produced by cooling the supercooled liquid.21-23 Low-temperature depositions are used to prepare glasses of materials that readily crystallize upon cooling from the liquid phase.24-26 It has been predicted that at deposition temperatures below 0.25Tg, crystallization can be completely avoided.27 Glasses made under such conditions are often unstable upon heating and can transform irreversibly even at temperatures below Tg.23 These glasses can also exhibit different packing structures. For example, it has been reported that the formation of hydrogen bonds in methanol depends on the deposition temperature and that at low deposition temperatures, there is not enough surface mobility to allow the hydrogen bonding to occur.26 Given the interesting differences between glasses vapordeposited at high and low temperatures, a systematic study of
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s a glass ages toward equilibrium, the density increases as the molecules find more efficient packing arrangements.1,2 However, aging is an extremely slow process limited by molecular mobility in the glass and the distance from equilibrium.3 Aging a glass on any reasonable time scale only changes the density by a few tenths of one percent;1 mechanical properties such as the longitudinal or shear modulus only change by a few percent.4-6 Clearly, aging is not an efficient method to control such properties. Recently, it has been shown that physical vapor deposition can prepare exceptionally stable glasses with low enthalpy,7,8 high density,9,10 and high mechanical moduli.11 It has been estimated that a glass prepared by cooling the liquid would require thousands or millions of years of aging to achieve the properties of a glass vapor-deposited onto a substrate at 0.85Tg.12 It is hypothesized that at this temperature, enhanced surface mobility13-18 allows the molecules to sample configurations and find efficient packing arrangements before they are buried by further deposition. As a result, lower deposition rates yield higher-density and higher-stability glasses.12 During physical vapor deposition of organic materials, the temperature of the substrate plays a key role in defining the final r 2011 American Chemical Society
Received: December 22, 2010 Accepted: January 31, 2011 Published: February 10, 2011 423
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Figure 1. (a) Schematic of the temperature gradient stage. Blocks of copper (Cu) on each side are controlled at different temperatures, producing a linear gradient across a stainless steel bridge (SS). The black dots indicate the position of the temperature sensors that are used to measure or control the temperature at each point. (b) Molecular structure of indomethacin (IMC). (c) Schematic drawing of the BLS experimental setup. Light scattered inelastically by the thermal density fluctuations in the temperature-gradient sample is directed into a Fabry-Perot interferometer that measures the Stokes and anti-Stokes modes shown in (d).
the effect of substrate temperature is needed for further development of this field. Here, we use a high-throughput method to systematically study the effect of the substrate temperature on the mechanical properties of vapor-deposited glasses. This is accomplished by making glasses at a large number of deposition temperatures covering a temperature range of 190 K. To avoid the time-consuming task of making these samples one at a time, a stage was designed to maintain an 80 K linear temperature gradient across the substrate during the deposition. We utilized this stage to prepare a wide range of glasses of indomethacin (IMC), a pharmaceutical that is also a good glass former. Brillouin light scattering (BLS) was employed to measure the mechanical moduli of these glasses. It is shown that the deposition temperature can change the longitudinal modulus of IMC glasses by more than 35%. The highest modulus glasses were prepared near 0.70Tg, while the lowest modulus materials were prepared at 0.40Tg. BLS probes thermal density fluctuations by inelastic coupling of the scattered light to the thermal phonons, and we use it here to determine the moduli of vapor-deposited glasses. In BLS, the scattering wave vector q couples to the phonons propagating in the same direction with an equal wave vector. As a result, the BLS spectrum at a given q consists of a doublet with a Doppler frequency shift of 2πf = ω = (cl,tq, where cl,t is the speed of sound with longitudinal (l) or transverse (t) polarization in the medium. For the experimental geometry in this study (Figure 1), the scattering vector lies in the plane of the sample, and its amplitude q = 4π/λ sin θ/2 only depends on the scattering angle θ and the wavelength of the incident laser beam λ.11,28 Figure 1 illustrates the essential elements of the experiments reported here. Figure 1a shows a schematic drawing of the temperature gradient stage used to prepare vapor-deposited glasses of IMC (molecular structure shown in Figure 1b). Figure 1c illustrates how BLS was used to probe the as-deposited glass at various spatial positions. Using a micrometer screw that is fixed on a goniometer, the sample could be moved accurately along the direction of the temperature gradient, allowing measurements at positions corresponding to many different deposition temperatures Tdep. A typical BLS spectrum is shown in Figure 1d, featuring a single Brillouin doublet at f ≈ 5.5 GHz on both sides of the strong elastic line. The longitudinal modulus
Figure 2. Longitudinal sound velocity as a function of the deposition temperature for IMC glasses deposited at 0.2 ( 0.02 nm/s. Each color represents data taken on a single temperature gradient sample. All measurements were performed at room temperature. The dashed line shows the sound velocity of an ordinary IMC glass at room temperature.11 One sample yielded smaller values of cl compared to other samples (black circles) but recovered larger values after 100 days of storage at ∼250 K (red circles).
M = Fc2l in the plane of the sample was determined for each Tdep utilizing the density F(Tdep) at each position along the sample as measured by ellipsometry. Figure 2 shows a plot of the longitudinal sound velocity as a function of the deposition temperature for several different IMC samples. Each color represents measurements on a single temperature gradient film. The deposition rate for all of the samples was controlled at 0.2 ( 0.02 nm/s. Samples with overlapping temperature ranges were prepared to demonstrate the reproducibility of the major trends. Within the scatter in the data, the sound velocities depend only on the deposition temperature and do not depend on whether that deposition temperature was at the edge or the middle of the sample. The data in Figure 2 show three distinct regimes. As Tdep is decreased below Tg (315 K), the longitudinal sound velocity 424
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increases, indicating more compact packing as compared to the ordinary glass. In this regime, enhanced mobility near the surface of the film allows molecules to nearly reach equilibrium packing arrangements before they are buried by further deposition; equilibrium at lower temperatures is associated with better packing and a larger sound velocity. Upon further decrease of Tdep below 235 K (the second regime), the mobility decreases to the point that the molecules cannot get close to equilibrium; the packing becomes less efficient, and the sound velocity decreases toward the ordinary glass value. Figure 2 shows a third regime for Tdep below 170 K. Here, the as-deposited glass apparently has the same sound velocity as the ordinary glass for most of the measured samples. For one sample (black circle), the sound velocities of the as-deposited glass were much lower than that of the ordinary IMC glass. We interpret these results as follows. Below 170 K, surface mobility is negligible on the time scale of the deposition. In this regime, we expect to form poorly packed, low-density glasses during the deposition because the molecules have essentially no opportunity to find good packing arrangements.22,23,27 This explains the low speed of sound observed in the one sample. We suspect that all of the glasses that were produced in this temperature range had this low-density structure at the end of the deposition but that all of the samples (except one) collapsed into a somewhat denser glass during reheating to ambient temperature. Consistent with this view, earlier studies of ethylbenzene glasses vapor-deposited at low temperature showed transformation to a higher-density glass at temperatures modestly below Tg.10 Our hypothesis is supported by the observation that the lone sample to show sound velocities lower than the ordinary glass (black circles) was not stable over the course of 100 days of storage well below room temperature (T = 250 K). The red circles in Figure 2 show the measured sound velocities after storage; all of the glasses with low sound velocities had evolved into glasses with the same sound velocity as the ordinary glass. Figure 3 shows measurements of the sound velocity for IMC glasses prepared in two different deposition rate regimes (0.2 versus 2-4 nm/s). The blue line in this figure is the speed of sound expected for the supercooled liquid equilibrated at Tdep and then measured at room temperature. The blue line is obtained by extrapolating previous BLS measurements11 on IMC and is explained in the Supporting Information. Down to a temperature of Tdep = 270 K, within the scatter of the data, the measured sound velocity follows the predicted equilibrium line at both high and low deposition rates. In this regime, both deposition rates are apparently low enough that incoming molecules have enough time to equilibrate in the near-surface layer before they are buried by further deposition. Because we expect equilibration to produce the maximum possible speed of sound, we refer to the blue line in Figure 3 as the limiting cl. As the deposition temperature is decreased below 270 K, the sound velocity is less than the expected equilibrium value; in this regime, the samples prepared at the lower deposition rate are closer to the equilibrium line, consistent with the idea that slower deposition allows greater equilibration. It is remarkable that at temperatures down to 0.57Tg, the system can still partially move toward equilibrium during the deposition, which indicates that enhanced surface mobility likely persists down to such low temperatures. The peak value of the sound velocity for glasses deposited at 0.2 nm/s is 2550 ( 25 m/s, 5-7% higher than the value of the sound velocity for the ordinary glass, indicated by the dashed line
Figure 3. Longitudinal sound velocity as a function of the deposition temperature for two deposition rate regimes. Green and orange stripes show the position of the peak values at each deposition rate, and green and orange lines are trend lines to guide the eye. The blue line indicates the expected values of the sound velocity for the supercooled liquid equilibrated at Tdep and measured at room temperature (see Supporting Information for details).
in Figure 3. On the equilibrium line (blue line in Figure 3), this value represents the sound velocity of an equilibrium supercooled liquid at 260 ( 8 K. This 55 K shift in the fictive temperature relative to Tg is unprecedented. This value must be considered with care as the equilibrium line has been linearly extrapolated far below Tg, where no experimental data exists. Figure 4 shows the longitudinal moduli calculated from data of Figure 3 using the relationship M = Fc2l . The inset shows the density F of IMC glasses at each deposition temperature obtained from ellipsometry measurements on temperature gradient samples. The details of the ellipsometry measurements and density calculations from those measurements will be discussed elsewhere. The actual density data are shown as blue circles, and the black line is the trend line used to calculate the modulus. Both cl and F are measured at room temperature; thus, the values displayed in Figure 4 represent room-temperature moduli. Figure 4 illustrates that the substrate temperature can modulate the longitudinal modulus of vapor-deposited IMC glasses by more than 35%. The peak modulus at 235 K is ∼15% higher than the ordinary glass value. This is much higher than the increase in the modulus that can be achieved by aging an ordinary glass.4,6 An even larger change in the value of the modulus is observed for the low-temperature glass produced below 170 K (black cross symbols in Figure 4). Because there was no density data available for this particular sample, the density of ordinary glass was used to calculate the value of the longitudinal modulus in this regime. The low sound velocity values of this sample (and it subsequent low-temperature transformation) indicate inefficient packing, and it is expected that the actual density will be lower than that of the ordinary glass in this range. Thus, the lowest moduli in this figure (black crosses) represent an upper bound on the actual moduli; because most of the variation in the modulus comes from cl and not F, this is likely a small correction. There is evidence that vapor-deposited films can show molecular orientation29 and structural anisotropy.30 Because the speed of sound is measured in the plane of the films, the calculated moduli represent the in-plane values.33 For samples deposited at 265 K, we verified that the longitudinal speed of sound was isotropic.11 425
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cooling, the material crystallized and appeared white to the eye. The glass sandwich was then mounted on the temperature gradient stage using type H Apiezon grease. The gradient stage was calibrated under ultrahigh vacuum (UHV) conditions (P ≈ 5 10-8 Torr) in two configurations. In the first configuration, both sides of the stage were controlled at the same temperature. The control temperature was raised by 2 K/min at both ends until visual observation indicated that the crystal had melted. The melting point was accurate within (1 K over the entire stage. In the second configuration, a temperature gradient of 40 K was maintained across the stage. The temperature at both ends was raised by 1 K/min to maintain a 40 K gradient at all times. As the melting front moved across each RTD, the temperature was compared with the melting point. The measured temperature was accurate within (1 K for all three RTDs on the SS bridge. During the actual deposition, a gradient of 80 K was typically maintained across the stage, which we expect to be accurate to (2 K. For physical vapor deposition of IMC, glass substrates were mounted on the stage using type N or H Apiezon grease, depending on the desired temperature range. IMC crystalline powder (Sigma-Aldrich) was placed in an aluminum oxide crucible and electrically heated inside of the UHV chamber. The deposition rate was controlled by monitoring a quartz crystal microbalance (Sycon instruments) and adjusting the current in the heating wires accordingly. The crucible was positioned at a distance of about 20 cm from the center of the temperature gradient stage. The deposition was continued until a thickness of about 4 μm was achieved at the center of the substrate. Due to the geometry of the deposition, the thickness and therefore the deposition rate were about 10% lower at the two ends of the substrate. Depositions with overlapping temperature ranges (Figure 2) show that thickness and rate variations did not affect, within the experimental error, values of the longitudinal sound velocity calculated from the BLS experiments. The BLS experiments described in this letter were performed using a six-pass tandem Fabry-Perot interferometer (JRS Scientific Instruments, see Figure 1c) with a 150 mW Nd:YAG laser (λ = 532 nm) and vertically polarized light (VV) in a transmission geometry. All spectra were recorded at room temperature using θ = 70°, that is, q = 0.01355 nm-1, yielding the longitudinal sound velocity cl. Except for samples deposited below 170 K, no evidence of aging at the measurement temperature (295 K) was observed, and repeated measurements of the sound velocity on the same sample yielded the same value within experimental error. The samples prepared for this study were too thin to reliably acquire transverse sound velocities. The transverse sound velocities for samples deposited at 0.85Tg were measured in an earlier study on samples with a thickness of about 20 μm.11
Figure 4. Calculated longitudinal modulus for IMC glasses as a function of the deposition temperature (black squares deposited at 0.2 nm/s; red circles deposited at 2-4 nm/s). This calculation utilized density data obtained by ellipsometry (blue squares), which is shown in the inset. The solid black line in the inset shows the trend line used to calculate the modulus. The data shown by cross symbols represent an upper bound for the moduli of these low-temperature, unstable glasses.
In summary, we designed a temperature gradient stage to allow systematic study of the structural properties of vapordeposited glasses as a function of the substrate temperature during the deposition. Measurement of the sound velocity of IMC glasses as a function of deposition temperature indicate that materials with fictive temperatures as low as Tg-55 K can be produced by slow physical vapor deposition. Using the deposition temperature as a control parameter, glasses of the same material can be produced with highly variable packing efficiencies, resulting in variation in the modulus of more than 35%. This is a much larger range than is accessible for glasses obtained by cooling the supercooled liquid and subsequent aging. It would be interesting to understand how the packing configurations adopted by such glasses differ from that of ordinary glasses. The temperature gradient stage allows for systematic studies of other structural properties, and hopefully, this question can be addressed in the future. The temperature gradient stage will also allow the systematic study of the electronic31,32 and optical properties29,34 of vapor-deposited glasses.
’ EXPERIMENTAL METHODS The temperature graduate stage was constructed from a rectangular bridge of SS (40 mm 25 mm 2 mm) attached to two blocks of copper for which the temperature could be independently controlled. Both copper blocks were thermally connected to a 100 K reservoir at all times. The temperature of each block was controlled (Omega CNi3222 temperature controller) within (0.5 K by 100 W cartridge heaters. Temperature measurements (using Omega four wire platinum resistance thermometers, RTDs) on each block and at three points along the SS bridge confirmed the stability and linearity of the temperature gradient along the stage. Calibration of the temperature gradient stage was performed using the melting point of hexacontane (373.9 K). Hexacontane was sandwiched between two glass microscope slides and melted on a hot stage outside of the deposition chamber such that the material was uniformly spread between the two glass slides. Upon
’ ASSOCIATED CONTENT
bS
Supporting Information. Description of the procedure by which the equilibrium line in Figure 3 is obtained. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 426
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’ ACKNOWLEDGMENT Z.F. and T.S. contributed equally to this work. The authors would like to thank NSERC for the postdoctoral fellowship that supported Z.F. and the DOE Office of Basic Energy Sciences (DE-SC0002161) for financial support for this study.
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