Role of Fragility in the Formation of Highly Stable Organic Glasses

In situ dielectric spectroscopy has been used to characterize vapor-deposited glasses of methyl-$m$-toluate (MMT), an organic glass former with low fragility ($m=60$). Deposition near $0.84T_g$ produces glasses of very high kinetic stability; these materials are comparable in stability to the most stable glasses produced from more fragile glass formers. Highly stable glasses of MMT, when annealed above $T_g$, transform into the supercooled liquid by a heterogeneous mechanism. A constant velocity propagating front is initiated at the free surface and controls the transformation of thin films. The transition to a bulk-dominated transformation process occurs at $5\mu \mathrm{m}$, the largest length scale reported for any glass. Contrary to recent conclusions, we find that physical vapor deposition can form highly stable organic glasses across the entire range of liquid fragilities.

Figure 1

Dielectric loss spectra of highly stable methyl-$m$-toluate (MMT) glasses during annealing $T_{\text{ann}}=175.5$, 176.8, and 179.0 K (films $\sim 15\mu \mathrm{m}$ thick). The loss amplitude rises from nearly zero with increasing annealing time as the stable glass transforms into the supercooled liquid. The lines are best fits using a fixed relaxation time at each $T_{\text{ann}}$. Dielectric spectra (left-hand panels) are obtained by subtracting dc conductivity from the original data (shown in right-hand panel for $T_{\text{ann}}=179.0\mathrm{K}$). Inset: Structure of MMT.

Figure 2

Time evolution of the dielectric response of $1.0\mu \mathrm{m}$ thick MMT glasses vapor deposited at various temperatures, during annealing at 177 K. The $y$ axis indicates the fraction of sample with the dielectric response of the supercooled liquid, evaluated at 1.77 Hz. Colors and symbols indicate deposition temperatures: 132 K (red circles), 137 K (dark yellow right-side triangles), 142 K (orange stars), 147 K (blue up triangles), 152 K (dark cyan down triangles), 157 K (magenta left-side triangles), and 167 K (black squares). Inset: Time required for complete transformation into the liquid ($t_{\text{trans}}$) as a function of deposition temperature ($T_{\text{dep}}$), divided by the liquid structural relaxation time (${\tau }_{\alpha }$).

Figure 3

Time required to transform stable glasses of MMT ($t_{\text{trans}}$) into the supercooled liquid as a function of film thickness. All films were deposited at 142 K and annealed at 179 K. The transformation time increases linearly with thickness and then becomes constant. Symbols indicate deposition rates: $0.2\mathrm{nm}/\mathrm{s}$ (orange filled stars), $1\mathrm{nm}/\mathrm{s}$ (red open stars). Inset: Subset of the transformation curves that provide $t_{\text{trans}}:480\mathrm{nm}$ (black squares), $1.5\mu \mathrm{m}$ (red circles), $1.9\mu \mathrm{m}$ (blue up triangles), $3\mu \mathrm{m}$ (cyan down triangles), $6\mu \mathrm{m}$ (magenta left-side triangles), and $35\mu \mathrm{m}$ (dark yellow right-side triangles).

Figure 4

Transformation growth front velocities for stable glasses of MMT, ethylbenzene, toluene, IMC, and TNB as a function of the liquid structural relaxation time (${\tau }_{\alpha }$) at the annealing temperature. For MMT, annealing temperatures range from 176 to 183 K. For all systems, the temperature dependence of the growth front velocity is slightly weaker than the temperature dependence of ${\tau }_{\alpha }$. Inset: Schematic illustration of stable glass (SG) to supercooled liquid (SCL) transformation for thin films.