Manipulating the properties of stable organic glasses using kinetic facilitation.

In contrast to ordinary glasses, when highly stable organic glasses are annealed at temperatures above T(g), they transform heterogeneously into the liquid state by a constant velocity propagating front that initiates at the free surface. The evolution of this growth front has been interpreted as kinetic facilitation, i.e.

Stable glasses of indomethacin and α,α,β-tris-naphthylbenzene transform into ordinary supercooled liquids.

Secondary ion mass spectrometry has been used to characterize translational motion in vapor-deposited glasses of indomethacin (IMC) and α,α,β-tris-naphthylbenzene (TNB). Vapor deposition onto substrates at ∼0.99 T(g) produced ordinary glasses that evolve according to fickian diffusion during annealing.

Direct measurement of molecular motion in freestanding polystyrene thin films.

An optical photobleaching technique has been used to measure the reorientation of dilute probes in freestanding polystyrene films as thin as 14 nm. Temperature-ramping and isothermal anisotropy measurements reveal the existence of two subsets of probe molecules with different dynamics. While the slow subset shows bulk-like dynamics, the more mobile subset reorients within a few hundred seconds even at T(g,DSC) - 25 K (T(g,DSC) is the glass transition temperature of bulk polystyrene).

Temperature-ramping measurement of dye reorientation to probe molecular motion in polymer glasses.

A temperature-ramping anisotropy measurement is introduced as an efficient way to study molecular motion in polymer glasses. For these experiments, fluorescent molecules were dispersed in the polymer glass and the reorientation of these dyes was used as a probe of segmental dynamics. For thick samples of polystyrene, poly (4-tert-butyl styrene), and poly(2-vinyl pyridine), temperature-ramping anisotropy measurements have a shape similar to differential scanning calorimetry measurements and nearly the same transition temperature.

Transformation of stable glasses into supercooled liquids: growth fronts and anomalously fast liquid diffusion.

Physical vapor deposition onto substrates near 0.85T(g) can prepare organic glasses with low enthalpy, high density, and high thermal stability. Isotopically labeled multilayer films of tris(naphthyl)benzene and indomethacin stable glasses were prepared and secondary ion mass spectrometry was used to study the evolution of these materials upon heating above T(g).

Self-diffusion of supercooled tris-naphthylbenzene.

We have measured self-diffusion coefficients for deeply supercooled tris-naphthylbenzene using secondary ion mass spectrometry. Isotopically labeled multilayer films were prepared by vapor deposition. For samples deposited within a few K of T(g), the evolution of the concentration profile was observed to be Fickian on all accessible length and time scales.

Stable glass transformation to supercooled liquid via surface-initiated growth front.

Highly stable glasses of tris-naphthylbenzene transform into a liquid when annealed above the glass transition temperature T_{g}. In contrast to the predictions of standard models, the observed transformation is spatially inhomogeneous. Secondary ion mass spectrometry experiments on isotopically labeled multilayer films show that the liquid grows into the stable glass with sharp growth fronts initiated at the free surface and at the interface with the substrate.

Calorimetric evidence for two distinct molecular packing arrangements in stable glasses of indomethacin.

Indomethacin glasses of varying stabilities were prepared by physical vapor deposition onto substrates at 265 K. Enthalpy relaxation and the mobility onset temperature were assessed with differential scanning calorimetry (DSC). Quasi-isothermal temperature-modulated DSC was used to measure the reversing heat capacity during annealing above the glass transition temperature Tg.

Direct measurement of molecular mobility in actively deformed polymer glasses.

When sufficient force is applied to a glassy polymer, it begins to deform through movement of the polymer chains. We used an optical photobleaching technique to quantitatively measure changes in molecular mobility during the active deformation of a polymer glass [poly(methyl methacrylate)]. Segmental mobility increases by up to a factor of 1000 during uniaxial tensile creep.

Molecular view of the isothermal transformation of a stable glass to a liquid.

We have used neutron reflectivity to measure translational motion on the nanometer length scale in exceptionally stable glasses of tris(naphthylbenzene). These glasses are prepared by vapor deposition onto a substrate held somewhat below the glass transition temperature (T(g) = 342 K). When the most stable samples are annealed at 345 K, no translational motion is observed on the 12 nm length scale for over 10,000 s and full mixing requires more than 60,000 s.

Dye reorientation as a probe of stress-induced mobility in polymer glasses.

The reorientation of dye molecules can be used to monitor the segmental dynamics of a polymer melt. We utilize this technique to measure stress-induced mobility in a lightly cross-linked poly(methyl methacrylate) (PMMA) glass during tensile creep deformation. At 377 K (18 K below the glass transition temperature Tg), the mobility increased by a factor of 100 during deformation with a stress of 20 MPa.

Hiking down the energy landscape: progress toward the Kauzmann temperature via vapor deposition.

Physical vapor deposition was employed to prepare amorphous samples of indomethacin and 1,3,5-(tris)naphthylbenzene. By depositing onto substrates held somewhat below the glass transition temperature and varying the deposition rate from 15 to 0.2 nm/s, glasses with low enthalpies and exceptional kinetic stability were prepared.

Influence of substrate temperature on the stability of glasses prepared by vapor deposition.

Physical vapor deposition of indomethacin (IMC) was used to prepare glasses with unusual thermodynamic and kinetic stability. By varying the substrate temperature during the deposition from 190 K to the glass transition temperature (Tg=315 K), it was determined that depositions near 0.85Tg (265 K) resulted in the most stable IMC glasses regardless of substrate.

Organic glasses with exceptional thermodynamic and kinetic stability.

Vapor deposition has been used to create glassy materials with extraordinary thermodynamic and kinetic stability and high density. For glasses prepared from indomethacin or 1,3-bis-(1-naphthyl)-5-(2-naphthyl)benzene, stability is optimized when deposition occurs on substrates at a temperature of 50 K below the conventional glass transition temperature. We attribute the substantial improvement in thermodynamic and kinetic properties to enhanced mobility within a few nanometers of the glass surface during deposition.

Neutron reflectivity measurements of the translational motion of tris(naphthylbenzene) at the glass transition temperature.

The translational dynamics of the low molecular weight glass-former tris(naphthylbenzene) have been studied on the length scale of a few nanometers at the glass transition temperature Tg. Neutron reflectivity was used to measure isotopic interdiffusion of multilayer samples created by physical vapor deposition. Deposition with the substrate held at Tg-6 K allows observation of dynamics characterizing the equilibrium supercooled liquid.

Self-diffusion of supercooled o-terphenyl near the glass transition temperature.

Self-diffusion coefficients for the low molecular weight glass former o-terphenyl have been measured near Tg by isothermally desorbing thin film bilayers of deuterio and protio o-terphenyl in a vacuum chamber. We observe translational diffusion that is about 100 times faster at Tg + 3 K than the Stokes-Einstein prediction. Predictions from random first order transition theory and a dynamic facilitation approach are in reasonable agreement with our results; in these approaches, enhanced translational diffusion is associated with spatially heterogeneous dynamics.

Isothermal desorption measurements of self-diffusion in supercooled o-terphenyl.

Isothermal desorption of o-terphenyl thin-film bilayers was used to measure self-diffusion coefficients of supercooled o-terphenyl near the glass transition temperature (Tg=243 K). Diffusion coefficients from 10(-15.5) to 10(-12) cm2 s(-1) were obtained between 246 and 265 K.

Self-diffusion of tris-naphthylbenzene near the glass transition temperature.

We present a direct measurement of self-diffusion of a single-component glass-forming liquid at the glass transition temperature. Forward recoil spectrometry is used to measure the concentration profiles of deuterio and protio 1,3-bis-(1-naphthyl)-5-(2-naphthyl)benzene (TNB) following annealing-induced diffusion in a vapor-deposited bilayer. These experiments extend the range of measured diffusion coefficients in TNB by 6 orders of magnitude.