Highly Stable Vapor-Deposited Glasses of Four Tris-naphthylbenzene

Oct 5, 2011 - those of ordinary glasses formed by cooling the supercooled liquid. ... glass formation to date, no systematic study of the influence of...
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LETTER pubs.acs.org/JPCL

Highly Stable Vapor-Deposited Glasses of Four Tris-naphthylbenzene Isomers Kevin Dawson,† Lei Zhu,† Laura A. Kopff,† Robert J. McMahon,† Lian Yu,†,‡ and M. D. Ediger*,† †

Department of Chemistry and ‡School of Pharmacy, University of Wisconsin-Madison, Madison, Wisconsin, United States

bS Supporting Information ABSTRACT: Recent reports have shown that physical vapor deposition can prepare organic glasses with remarkably high kinetic stability and low enthalpy in comparison with ordinary glasses formed by cooling the supercooled liquid. We have prepared vapor-deposited glasses of four isomers of tris-naphthylbenzene (TNB) and characterized them using differential scanning calorimetry. We find that highly stable glasses can be prepared from all four isomers of TNB by vapor deposition at 0.85 Tg. Glasses with low enthalpy were prepared for at least three of the four isomers. Because the TNB family of isomers contains both good and poor glass-formers, these results indicate a new dimension to the generality of stable organic glass formation via physical vapor deposition. SECTION: Macromolecules, Soft Matter

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s a liquid is cooled, the time required for molecular rearrangements lengthens. If a liquid can be cooled below the melting point without crystallizing, eventually the time scale for molecular motion will exceed the time available for equilibration. The temperature at which this occurs is denoted as the glasstransition temperature, Tg. Because glasses are out of equilibrium with respect to the supercooled liquid, their properties (density, enthalpy, structure, etc.) depend on the details of their preparation and can evolve over time because of physical aging. Essentially an infinite number of distinct glasses can, in principle, be formed from a given substance. In practice, cooling a liquid at experimentally accessible rates does not produce a set of glasses with substantially different properties. The reason for this is easily understood. For a typical organic glass-former, the rate of molecular relaxation decreases by a factor of 10 when the temperature is lowered by 3 K.1,2 Therefore, lowering the rate of cooling by a factor of 10 decreases Tg by only 3 K. The glass produced with the lower cooling rate will have properties that differ only slightly from the glass formed at the higher cooling rate. Recently, it has been reported that physical vapor deposition can prepare glasses whose properties differ dramatically from those of ordinary glasses formed by cooling the supercooled liquid. In particular, higher density, higher thermal stability, and lower enthalpy glasses have been prepared.3 13 It has been proposed that during physical vapor deposition at temperatures near 0.85 Tg, partial equilibration can occur within a liquid-like layer of a few monolayers at the surface of the glass. Because this equilibration can apparently occur up to 40 K below the conventional Tg, it has been estimated that thousands of years (at a minimum) would be required to prepare similar glasses by slow cooling of the supercooled liquid. Whereas nine organic molecules have been reported to show some aspects of stable r 2011 American Chemical Society

glass formation to date, no systematic study of the influence of chemical structure and glass-forming ability has been performed. The tris-naphthylbenzene (TNB) family of isomers,14 shown in Figure 1, is useful for understanding the influence of molecular structure on the properties of vapor-deposited glasses. The isomers have a roughly 20 K range of Tg values. The systematic decrease in Tg as the number of β substituents increases likely results from the enhanced flexibility of the β-substituted molecules. NMR experiments performed on the TNB isomers showed a high barrier for rotation (12 kcal/mol) for the α substituents in comparison with that of the β substituents (2 kcal/mol).14 The isomers also have a wide range of glass-forming abilities. It has been argued that glassforming ability of a series of molecules can be anticipated from the reduced Tg (Tg/Tm) of the systems, with molecules having higher reduced Tg being better glass-formers.15 The TNB isomers generally follow this rule, with the reduced Tg values indicated: α,α,α (0.76), α,α,β (0.74), α,β,β (0.80), and β,β,β (0.65).14 α,α,β-TNB is an excellent glass-former,16,17 and highly stable glasses have already been prepared by physical vapor deposition.3 In contrast, β,β,β-TNB is such a poor glass-former that a value for Tg had not been reported14 prior to this work. When β,β,βTNB is cooled at any rate accessible in a conventional differential scanning calorimeter (DSC), a solid with substantial crystallinity is obtained. Because it is a poor glass-former, β,β,β-TNB represents an opportunity to test the limits of stable glass formation. Conventionally, a poor glass-former would be vapor-deposited onto a very cold substrate to avoid crystallization.18 Under these Received: August 26, 2011 Accepted: October 5, 2011 Published: October 05, 2011 2683

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LETTER

Figure 1. Structures of the four tris-naphthylbenzene isomers. The α,α,β-TNB molecule is marked to show the α and β positions on the naphthyl ring.

conditions, low density and high enthalpy glasses are prepared.20 For a poor glass-former, one might anticipate that the high surface mobility required for stable glass formation would result in extensive crystallization during the process of vapor deposition. Here we report the preparation and characterization of vapordeposited glasses of the four isomers of the TNB family. All isomers were deposited onto substrates at 0.85 Tg with a deposition rate of 0.2 nm/s. Subsequent characterization by DSC indicated that all isomers (including the poor glass-former) formed glasses with much higher kinetic stability than the ordinary glass. Vapor-deposited glasses of the four isomers exhibited nearly identical kinetic stability, as quantified by the increase in the onset temperature for transformation into the supercooled liquid. We verified that three of the four isomers formed low enthalpy glasses; the DSC measurements on β,β,β-TNB did not allow a determination of the enthalpy because of rapid crystallization as the vapordeposited glasses transformed into the supercooled liquid. Vapor-deposited glasses of all four TNB isomers have high kinetic stability in comparison with the corresponding ordinary glasses (prepared by cooling the supercooled liquid at ∼40 K/min). In Figure 2a, we show heat capacity data for α,β,β-TNB in a format that emphasizes the difference between the vapor-deposited and ordinary liquid-cooled glasses. The higher onset temperature Tonset (defined in the inset to panel a) for the asdeposited sample indicates that this glass has higher kinetic stability than the ordinary glass; that is, higher temperature (more energy) is required to release the molecules from the solid state. Much smaller increases in Tonset have been observed when ordinary glasses are aged. For example, in previous experiments, samples of α,α,β-TNB cooled from the supercooled liquid were aged for 15 days at Tg -19 K, and only an 8 K increase in the Tonset was achieved.3 In contrast, Figure 2a shows that vapor-deposition increases Tonset for α,β,β-TNB by 35 K. Heat capacity measurements shown in Figure 2b indicate that vapor-deposited glasses of all four TNB isomers show a very large peak, consistent with low enthalpy content. Whereas such a peak superficially resembles the behavior of a melting crystal, this is not the correct explanation. In Figure 2a, for example, the asdeposited glass is at least 99% amorphous, according to wideangle X-ray scattering; the as-deposited scattering intensities were compared with scattering peaks in the same sample after

Figure 2. Differential scanning calorimetry measurements of the heat capacity for tris-naphthylbenzene glasses. Panel a compares Cp for the asdeposited and ordinary glasses of α,β,β-TNB. The high Tonset and large enthalpy overshoot for the as-deposited sample indicate increased kinetic stability and low enthalpy relative to the ordinary glass. Panel b shows Cp data (offset for clarity) for as-deposited (main figure) and ordinary (inset) glasses of all four TNB isomers.

full crystallization. (unpublished data) In addition, the large Cp peak in Figure 2a occurs far below the known melting point. The Cp peaks in Figure 2 are highly exaggerated versions of the “enthalpy overshoot” seen in experiments on aged ordinary glasses.2,19 When a low enthalpy glass suddenly transforms into the higher enthalpy supercooled liquid, a large amount of heat flows into the sample quickly; the enthalpy content of the as-deposited glasses is quantified below. Figure 3 shows that vapor-deposited glasses of the four isomers of TNB have remarkably similar kinetic stability. In all cases, the difference between Tonset for the vapor-deposited and ordinary glasses is between 32 and 35 K. We emphasize that the deposition conditions used for this study were based on previous optimization of stable glass formation of only one of the isomers (α,α,β-TNB).4,5 The observation that these deposition conditions produce highly stable glasses (of nearly uniform stability) for the entire TNB family further argues for the generality10 of stable glass formation for organic molecules. Vapor-deposited glasses of the TNB isomers have quite low enthalpies in comparison with the ordinary glasses, as illustrated 2684

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Figure 3. Overview of the kinetic stability (Tonset) and enthalpy content (Tf) of vapor-deposited TNB isomers. Tonset for the vapor-deposited glass is ∼35 K higher than Tonset for the ordinary glass for each isomer, indicating significant increase in kinetic stability, even for the poor glassformer β,β,β-TNB. The as-deposited samples also show significantly lower enthalpy than the ordinary glass, as quantified by the low fictive temperatures Tf. Dielectric measurements of Tg are in good agreement with the DSC measurements on the ordinary glasses. For all quantities, the error is smaller than the symbol size, except where indicated.

for α,α,β-TNB in Figure 4. The Cp data for three different vapordeposited samples and one ordinary glass, all of the same isomer, have been integrated to obtain the curves shown; the enthalpy curves have been overlapped at high temperature, where all samples are the same supercooled liquid. For α,α,β-TNB, the vapor-deposited samples are ∼12 J/g lower in enthalpy than the ordinary glass, which represents ∼25% of the enthalpy difference between the ordinary glass and the crystal.5 To interpret this enthalpy difference further, we extrapolate the enthalpy of the supercooled liquid17 to low temperature, as shown by the dashed line in Figure 4. The fictive temperature Tf is the temperature at which a glass has the same enthalpy as that expected for the supercooled liquid. For an ordinary glass prepared by cooling into the glass and immediately reheating at a comparable rate, Tf is approximately equal to the Tonset identified by DSC. In our DSC experiments on ordinary glasses of the TNB isomers, Tf and Tonset always agreed to within 3 K. Figure 3 shows that the fictive temperatures for three vapordeposited TNB isomers are lower than Tf for the ordinary glasses by 30 to 40 K. For context, an ordinary glass of α,α,β-TNB that was aged for 15 days obtained a Tf of 330 K, whereas our vapordeposited samples of α,α,β-TNB have a Tf of 306 K.3 From the Tf values and extrapolated dielectric relaxation data,(unpublished data), we can estimate how long one would have to age an ordinary glass to obtain glasses with fictive temperatures as low as those obtained by vapor deposition. These estimates vary from 102 years for α,α,α-TNB to 1013 years for α,α,β-TNB. The fictive temperature for β,β,β-TNB could not be calculated from the DSC data because the full glass-to-liquid transformation was not observed owing to rapid crystallization. For comparison, Figure 3 also includes the Tg values for the isomers as determined by dielectric spectroscopy (identifying Tg as the temperature where the relaxation time is 100 s); as expected, the dielectric Tg value agrees well with the DSC Tonset values for the ordinary glass of each isomer.

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Figure 4. Enthalpy as a function of temperature for three vapor-deposited samples of α,α,β-TNB (red, black, and blue curves). The enthalpy of the ordinary glass (teal) is shown for comparison. The fictive temperature Tf is defined as the temperature at which the glass enthalpy line intersects the extrapolated supercooled liquid curve. Good reproducibility of the fictive temperature is observed among the three vapor-deposited samples. The liquid line is calculated using the Cp data of Magill in ref 17. (Cp data for these vapor-deposited samples can be found in the Supporting Information.)

We now utilize the DSC data on the TNB isomers to discuss the relationship between glass-forming ability (avoiding crystallization upon cooling a liquid) and the ability to form stable glasses via vapor deposition (without crystallization). In previous reports of stable glass formation, if the sample was slowly heated above the glass-to-liquid transformation temperature, crystallization was always observed, even in those cases when the corresponding ordinary glass did not show crystallization upon heating.3 5,10,11 This observation might be interpreted to mean that the same surface mobility that gives rise to stable glass formation also results in the formation of crystal nuclei. Whereas the DSC data presented here for stable glasses of the α,α,α, α,α,β, and β,β,β isomers do show crystallization upon heating, no crystallization was observed for as-deposited samples of α,β,β-TNB either in DSC experiments (Figure 2a) or in wideangle X-ray scattering.(unpublished data) To our knowledge, this is the first report of the formation of a stable glass that resists crystallization upon subsequent heating at rates as low as 10 K/ min. Therefore, the surface mobility that allows stable glass formation need not result in a greater tendency toward thermally induced crystallization than that observed for the ordinary glass. Crystallization has been avoided in some previous experiments on stable glasses where very fast heating rates (g36 000 K/min) were utilized.7,8,12,13 Further information about crystallization of as-deposited samples can be found in the Supporting Information. The successful formation of a stable glass of β,β,β-TNB is surprising given that this molecule is a poor glass-former. Different vapor depositions of β,β,β-TNB produced solids with levels of crystallinity as low as 2% (based on wide-angle X-ray scattering, unpublished data). This result stands in contrast with the traditional practice of physical vapor deposition. Typically, very low substrate temperatures (0.2 to 0.4 Tg) have been used to avoid crystallization when vapor-depositing poor glass-formers.18 This practice results in a high enthalpy glass20 rather than the low enthalpy glass reported here. It is significant that β,β,β-TNB, which apparently has facile pathways between amorphous and 2685

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The Journal of Physical Chemistry Letters crystalline portions of the potential energy landscape, can be prepared low on the amorphous part of the landscape (where a relatively small number of states exist) without the even lower energy crystalline states being accessed. The ability to form β,β,β-TNB stable glasses that are almost completely amorphous despite rapid crystallization in the supercooled liquid might be due to the critical size of a viable crystal nucleus exceeding the thickness of the mobile surface layer under these deposition conditions. In summary, recent reports have shown that a number of organic molecules can form stable glasses if vapor-deposited onto substrates held somewhat below Tg.3 13,21 23 Simulations of vapor-deposited systems have also shown some aspects of stable glass formation.24,25 The formation of these materials is thought to result from enhanced surface mobility at the glass surface,26 which allows partial equilibration in a thin interfacial layer27 at temperatures where equilibration would otherwise be essentially impossible.3 Because these materials have potential applications in organic electronics and next-generation photoresists,28 31 it is important to understand the conditions that might make stable glass formation impossible and the extent to which the properties of one molecular system might be transferable to others. For the TNB family of isomers, deposition conditions optimized for one isomer produce highly stable glasses of all isomers, with remarkable consistency of the resulting kinetic stability. In addition, our results indicate that highly stable glasses with low crystallinity can be formed even for molecular systems that crystallize rapidly upon cooling from the liquid.

’ EXPERIMENTAL METHODS Previously reported synthetic procedures14 were utilized to prepared the four isomers of TNB: α,α,α-TNB [1,3,5-tris(1-naphthyl)benzene], α,α,β-TNB [1,3-bis(1-naphthyl)-5-(2-naphthyl)benzene], α,β,β-TNB [1-(1-naphthyl)-3,5-bis(2-naphthyl)benzene], and β,β,β-TNB [1,3,5-tris(2-naphthyl)benzene]. DSC experiments on the synthesized material gave melting points for each isomer within 1 K of literature values.14 Physical vapor deposition was used to prepare amorphous samples of the TNB isomers as previously described.4,32 A clean quartz crucible was loaded with crystalline material of one isomer of TNB. The crucible was then heated in a vacuum chamber to achieve a deposition rate of 0.2 nm/s, as measured by a quartz crystal microbalance (QCM). Roughly 2 to 3 mg of material were deposited directly into DSC pans that were attached to a temperature-controlled copper block. After completion of the deposition, the pans were removed from the chamber, hermetically sealed, cleaned, and weighed using a Cahn C-35 balance. The details of the DSC analysis have been previously reported.3 5 DSC was carried out using a TA Instruments Q1000 with each sample undergoing three heating ramps using a heating rate of 10 K/min and cooling rates of roughly 40 K/min.4 These three ramps allow us to measure the as-deposited, crystalline, and ordinary glass properties of the samples. (See the Supporting Information for additional details.) For α,α,α-TNB and α,α, β-TNB, our calculated ΔHfusion values were compared with those in the literature as a check of sample mass.17,33 Our enthalpy of fusion values were always within 5% of the literature values. Tm values for the vapor-deposited samples agreed with those of the starting material to within 1 K. When calculating Tf for α,α,α and α,α,β-TNB, literature values for Cp were used for the supercooled liquid.17,33 For α,β,β-TNB, the Cp data of α,α,β-TNB was utilized. The Tonset and Tf values for the vapor-deposited

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α,α,β-TNB samples reported here agree with values previously reported for the same deposition conditions within error (3 K).5 Because of the fast crystallization of β,β,β-TNB, we could not consistently obtain ordinary glass samples when cooling the liquid in the DSC (maximum cooling rate of 40 K/min). For this isomer, a special method was used to create the ordinary glass for the third heating ramp. After the first two heating ramps were complete, the DSC pan was removed from the DSC and placed on a hot plate to melt the crystals. After melting, the pan was immediately submersed in liquid nitrogen. To create a well-defined thermal history, the sample was returned to the DSC and held at 343 K (Tg + 12 K) for 5 min. The sample then was cooled at 10 K/min to 253 K, and the third heating ramp was initiated.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional figures showing DSC data for α,α,β-TNB glasses are available. Further information about crystallization of vapor-deposited samples can also be found. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Research supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award DE-SC0002161 (vapor deposition and data analysis) and by the National Science Foundation under award DMR-0907031 (DSC instrument) and under award CHE-1011959 (TNB synthesis). ’ REFERENCES (1) Ediger, M. D.; Angell, C. A.; Nagel, S. R. Supercooled Liquids and Glasses. J. Phys. Chem. 1996, 100, 13200–13212. (2) Angell, C. A.; Ngai, K. L.; McKenna, G. B.; McMillan, P. F.; Martin, S. W. Relaxation in Glassforming Liquids and Amorphous Solids. J. Appl. Phys. 2000, 88, 3113–3157. (3) Swallen, S. F.; Kearns, K. L.; Mapes, M. K.; Kim, Y. S.; McMahon, R. J.; Ediger, M. D.; Wu, T.; Yu, L.; Satija, S. Organic Glasses with Exceptional Thermodynamic and Kinetic Stability. Science 2007, 315, 353–356. (4) Kearns, K. L.; Swallen, S. F.; Ediger, M. D.; Wu, T.; Yu, L. Influence of Substrate Temperature on the Stability of Glasses Prepared by Vapor Deposition. J. Chem. Phys. 2007, 127, 154702. (5) Kearns, K. L.; Swallen, S. F.; Ediger, M. D.; Wu, T.; Sun, Y.; Yu, L. Hiking Down the Energy Landscape: Progress toward the Kauzmann Temperature via Vapor Deposition. J. Phys. Chem. B 2008, 112, 4934–4942. (6) Swallen, S. F.; Kearns, K. L.; Satija, S.; Traynor, K.; McMahon, R. J.; Ediger, M. D. Molecular View of the Isothermal Transformation of a Stable Glass to a Liquid. J. Chem. Phys. 2008, 128, 214514. (7) Leon-Gutierrez, E.; Garcia, G.; Lopeandia, A. F.; Fraxedas, J.; Clavaguera-Mora, M. T.; Rodriguez-Viejo, J. In Situ Nanocalorimetry of Thin Glassy Organic Films. J. Chem. Phys. 2008, 129, 181101. (8) Leon-Gutierrez, E.; Garcia, G.; Clavaguera-Mora, M. T.; Rodriguez-Viejo, J. Glass Transition in Vapor Deposited Thin Films of Toluene. Thermochim. Acta 2009, 492, 51–54. (9) Ishii, K.; Nakayama, H.; Hirabayashi, S.; Moriyama, R. Anomalously High-Density Glass of Ethylbenzene Prepared by Vapor Deposition at Temperatures Close to the Glass-Transition Temperature. Chem. Phys. Lett. 2008, 459, 109–112. 2686

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(31) Lee, S. S.; Loo, Y. L. Structural Complexities in the Active Layers of Organic Electronics. Annu. Rev. Chem. Biomol. Eng. 2010, 1, 59–78. (32) Dawson, K. J.; Kearns, K. L.; Yu, L.; Steffen, W.; Ediger, M. D. Physical Vapor Deposition as a Route to Hidden Amorphous States. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 15165–15170. (33) Tsukushi, I.; Yamamuro, O.; Ohta, T.; Matsuo, T.; Nakano, H.; Shirota, Y. A Calorimetric Study on the Configurational Enthalpy and Low-Energy Excitation of Ground Amorphous Solid and LiquidQuenched Glass of 1,3,5-Tri-α-naphthylbenzene. J. Phys.: Condens. Matter 1996, 8, 245–255.

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