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Glasses of Organic Molecules Prepared by Physical Vapor Deposition: Will it Form a Stable Glass? Madeleine Beasley 1 , Mike Tylinski 1 , Yeong Zen Chua 2 , Christoph Schick 2 , and M.D. Ediger 1 1 University of Wisconsin Madison 2 University of Rostock Stable Glasses Prepared by Physical Vapor Deposition Enhanced Surface Mobility Kinetic Stability of Vapor-Deposited Glasses Predicting What Molecules Will Make a Stable Glass Acknowledgmen ts This work is funded by the National Science Foundation. O P O O O O P O O O O P O O O HO OH HO OH HO OH O O Tributyl phosphate (TBP) T g = 140 K Benzyl alcohol (BA) T g = 169 K o-Terphenyl (OTP) T g = 242 K Triethyl phosphate (TEP) T g = 137 K Trimethyl phosphate (TMP) T g = 137 K Propylene glycol (PG) T g = 168 K Ethylene glycol (EG) T g = 152 K 2-Ethyl-1-hexanol (2E1H) T g = 143 K Methyl m-toluate (MMT) T g = 169 K Ethylbenzene (EB) T g = 116 K Toluene (TOL) T g = 117 K Ethylcyclohexane (ECH) T g = 100 K Zhang, W.; Brian, C. W.; Yu, L. J. Phys. Chem. B., 2015, 119, 5071–5078. L. Berthier and M.D. Ediger, Facets of Glass Physics, Phys. Tod. 69 41-46. (2016).; DOI: 10.1063/PT.3.3052 During physical vapor deposition, molecules on the surface have diffusion coefficients at least 8 times faster than bulk diffusion. This enhanced mobility allows the molecules to sample many configurations before being buried by subsequent deposition. Surface diffusion measurements have been performed on some organic molecules to test this hypothesis. The surface mobility allows molecules to find positions deeper in the potential energy landscape than those of an ordinary liquid cooled glass. Isothermal annealing experiments are performed in order to quantify the kinetic stability of vapor deposited glasses. When stable glasses are heated above T g , they can take thousands of times longer to transform into the supercooled liquid than an ordinary liquid cooled glass. Isothermal Annealing Experiments Indomethacin (IMC) T g = 315 K Nifedipine (NIF) T g = 315 K Itraconazole (ITR) T g = 330 K M. Tylinski, unpublished M. Tylinski, unpublished M. Tylinski, unpublished Glasses are non-equilibrium amorphous solids. Glasses are traditionally prepared by cooling a liquid past its melting point to form a supercooled liquid. As the temperature of the system is decreased, the relaxations of the molecules become so slow that the material falls out of equilibrium, forming a glass. The properties of a glass change over time and do depend on how the glass has been prepared. Stable glasses have lower enthalpy, entropy, and volume compared to an ordinary liquid cooled glass. Many stable glasses have also been shown to have anisotropic packing. Stable glasses are prepared by physical vapor deposition (PVD). In PVD, gas molecules are condensed onto a temperature controlled substrate held below T g . Each molecule gets a chance to be a part of the mobile surface layer. This results in an extremely well- packed glass. A game of Tetris gives us a good graphical depiction of the difference in the packing between an ordinary liquid cooled glass and a stable glass prepared by PVD. When annealed at temperature above T g , the heat capacity of a stable glass slowly increases until the sample has completely transformed into a supercooled liquid. An ordinary liquid cooled glass would transform thousands of times faster. We use log(t transformation /t a ) to quantify the kinetic stability for the vapor deposited glasses. Larger values indicate greater kinetic stability. Previously reported stable glasses have values above 3. A typical liquid-cooled glass has a log(t transformation /t a ) value of zero. The value of log(t transformation /t a ) is calculated by taking the amount of time for a 590 nm vapor-deposited film to transform when annealed at T ann near T g and normalizing by the structural relaxation time (t a ) of the supercooled liquid at T ann . Our goal is to understand and to be able to predict why certain molecules form stable glasses and why others do not. Currently we have two hypothesis to explain why systems do not form stable glasses. The low surface mobility hypothesis states that the molecules simply do not have high enough surface mobility at our deposition temperatures to effectively sample all possible configurations. To test this we will decrease the deposition rate to compensate for slower surface mobility. If this is the issue, experiments with slower deposition rates should result in stable glass formation. Preliminary results indicate that this hypothesis is correct. One hypothesis to predict stable glass formation has to do with the fragility parameter m. It was thought that low fragility systems would be unable to form stable glasses. However, this was shown to not be true when stable glasses of methyl-m-toluate and ethylcyclohexane were prepared. We have found a correlation with a new fragility parameter, log(t a ) at 1.25 T g . We see a good correlation between the transformation time normalized by t a and our new fragility parameter. The above plot shows many molecules that have been vapor deposited. In order to test the predictive ability of this theory, we will calculate where a new molecule will fall on this plot and then experimentally test its stable glass Abstract: Organic stable glasses are materials with properties expected of “million-year-old” glasses such as high density, low enthalpy, enhanced kinetic stability, and low vapor uptake. Their spectacular properties make stable glasses of interest for many fields ranging from pharmaceutics to organic electronics. These glasses are prepared via physical vapor deposition (PVD). It is thought that the mechanism that allows for stable glass formation is fast surface mobility at deposition temperatures. This enhanced surface mobility has been observed by surface diffusion measurements at temperatures near T g . In the past 9 years, stable glasses of at least 27 molecules have been prepared. However, recent research has shown that stable glass formation is not universal. We seek to understand why some molecules do not form stable glasses. Additionally, we hope to elucidate structural trends and to find a method for predicting a molecule’s stable glass forming ability. N O O OH O Cl N H O O O O NO 2 Cl Cl O O N N N O N N N N N O

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Glasses of Organic Molecules Prepared by Physical Vapor Deposition: Will it Form a Stable Glass?

Madeleine Beasley1, Mike Tylinski1, Yeong Zen Chua2, Christoph Schick2, and M.D. Ediger1

1University of Wisconsin Madison2University of Rostock

Stable Glasses Prepared by Physical Vapor Deposition Enhanced Surface Mobility

Kinetic Stability of Vapor-Deposited Glasses

Predicting What Molecules Will Make a Stable Glass

AcknowledgmentsThis work is funded by the National Science Foundation.

OP

O

O

OO

PO

O

OO

PO

O

O

HO

OH

HO

OH HO

OH

OO

Tributyl phosphate (TBP) Tg = 140 K

Benzyl alcohol (BA) Tg = 169 K

o-Terphenyl (OTP) Tg = 242 K

Triethyl phosphate (TEP)Tg = 137 K

Trimethyl phosphate (TMP)Tg = 137 K

Propylene glycol (PG)Tg = 168 K

Ethylene glycol (EG)Tg = 152 K

2-Ethyl-1-hexanol (2E1H)Tg = 143 K

Methyl m-toluate (MMT)Tg = 169 K

Ethylbenzene (EB)Tg = 116 K

Toluene (TOL)Tg = 117 K

Ethylcyclohexane (ECH)Tg = 100 K

Zhang, W.; Brian, C. W.; Yu, L. J. Phys. Chem. B., 2015, 119, 5071–5078.

L. Berthier and M.D. Ediger, Facets of Glass Physics, Phys. Tod. 69 41-46. (2016).; DOI: 10.1063/PT.3.3052

During physical vapor deposition, molecules on the surface have diffusion coefficients at least 8 times faster than bulk diffusion. This enhanced mobility allows the molecules to sample many configurations before being buried by subsequent deposition. Surface diffusion measurements have been performed on some organic molecules to test this hypothesis. The surface mobility allows molecules to find positions deeper in the potential energy landscape than those of an ordinary liquid cooled glass.

Isothermal annealing experiments are performed in order to quantify the kinetic stability of vapor deposited glasses. When stable glasses are heated above Tg, they can take thousands of times longer to transform into the supercooled liquid than an ordinary liquid cooled glass.

Isothermal Annealing Experiments

N

O

O

OH

O Cl

NH

O

O

O

ONO2

Cl Cl

O

O

N

N

N

ON N

NN

N

O

Indomethacin (IMC)Tg = 315 K

Nifedipine (NIF)Tg = 315 K

Itraconazole (ITR)Tg = 330 K

M. Tylinski, unpublished

M. Tylinski, unpublished

M. Tylinski, unpublished

Glasses are non-equilibrium amorphous solids. Glasses are traditionally prepared by cooling a liquid past its melting point to form a supercooled liquid. As the temperature of the system is decreased, the relaxations of the molecules become so slow that the material falls out of equilibrium, forming a glass. The properties of a glass change over time and do depend on how the glass has been prepared. Stable glasses have lower enthalpy, entropy, and volume compared to an ordinary liquid cooled glass. Many stable glasses have also been shown to have anisotropic packing.

Stable glasses are prepared by physical vapor deposition (PVD). In PVD, gas molecules are condensed onto a temperature controlled substrate held below Tg. Each molecule gets a chance to be a part of the mobile surface layer. This results in an extremely well-packed glass. A game of Tetris gives us a good graphical depiction of the difference in the packing between an ordinary liquid cooled glass and a stable glass prepared by PVD.

When annealed at temperature above Tg, the heat capacity of a stable glass slowly increases until the sample has completely transformed into a supercooled liquid. An ordinary liquid cooled glass would transform thousands of times faster.

We use log(ttransformation/ta) to quantify the kinetic stability for the vapor deposited glasses. Larger values indicate greater kinetic stability. Previously reported stable glasses have values above 3. A typical liquid-cooled glass has a log(ttransformation/ta) value of zero. The value of log(ttransformation/ta) is calculated by taking the amount of time for a 590 nm

vapor-deposited film to transform when annealed at Tann near Tg and normalizing by the structural relaxation time (ta) of the supercooled liquid at Tann.

Our goal is to understand and to be able to predict why certain molecules form stable glasses and why others do not. Currently we have two hypothesis to explain why systems do not form stable glasses. The low surface mobility hypothesis states that the molecules simply do not have high enough surface mobility at our deposition temperatures to effectively sample all possible configurations. To test this we will decrease the deposition rate to compensate for slower surface mobility. If this is the issue, experiments with slower deposition rates should result in stable glass formation. Preliminary results indicate that this hypothesis is correct.

One hypothesis to predict stable glass formation has to do with the fragility parameter m. It was thought that low fragility systems would be unable to form stable glasses. However, this was shown to not be true when stable glasses of methyl-m-toluate and ethylcyclohexane were prepared. We have found a correlation with a new fragility parameter, log(ta) at 1.25 Tg. We see a good correlation between the transformation time normalized by ta and our new fragility parameter. The above plot shows many molecules that have been vapor deposited. In order to test the predictive ability of this theory, we will calculate where a new molecule will fall on this plot and then experimentally test its stable glass forming ability.

Abstract: Organic stable glasses are materials with properties expected of “million-year-old” glasses such as high density, low enthalpy, enhanced kinetic stability, and low vapor uptake. Their spectacular properties make stable glasses of interest for many fields ranging from pharmaceutics to organic electronics. These glasses are prepared via physical vapor deposition (PVD). It is thought that the mechanism that allows for stable glass formation is fast surface mobility at deposition temperatures. This enhanced surface mobility has been observed by surface diffusion measurements at temperatures near Tg. In the past 9 years, stable glasses of at least 27 molecules have been prepared. However, recent research has shown that stable glass formation is not universal. We seek to understand why some molecules do not form stable glasses. Additionally, we hope to elucidate structural trends and to find a method for predicting a molecule’s stable glass forming ability.