Length Scale of Dynamic Heterogeneity in Supercooled d-Sorbitol: Comparison to Model Predictions

A direct measurement of the length scale of dynamic heterogeneity in supercooled d-sorbitol has been performed using a multidimensional 13C solid-state NMR experiment. Several models predict that the growth of this length scale is linked to the slowing of dynamics as the glass transition is approached from above. At 275 K (Tg + 7 K), 2.5 ± 1.2 nm heterogeneities are detected in sorbitol. This result and recent results of similar measurements on glycerol, o-terphenyl, and poly(vinyl acetate) are compared to various models. The only model in quantitative agreement with these experimental data is based upon local fluctuations in the configurational entropy

Viscosity Dependence of Polystyrene Local Dynamics in Dilute Solution

Variable-temperature 2H NMR T1 measurements have been performed on backbone-deuterated atactic polystyrene at two Larmor frequencies in four solvents:  toluene, cis-decalin, dibutyl phthalate, and dioctyl phthalate. The time integral 〈σ〉 of the C−D vector orientation correlation function is extracted from the T1 data without assuming a specific model for C−D vector reorientation. The hydrodynamic Kramers' theory in the high-friction limit cannot describe the viscosity and temperature dependence of 〈σ〉. In contrast, 〈σ〉 is found to have a power law dependence on solvent viscosity with an exponent of 0.76 ± 0.05, whether the viscosity is varied by changing solvent or temperature. The internal energy barrier for polystyrene C−D vector orientation is determined to be 14 ± 3 kJ/mol using the power law viscosity dependence. The inapplicability of Kramers' theory is attributed to the lack of a clear separation between the time scales of polymer and solvent motions. As expected on the basis of this explanation, the viscosity exponents for polystyrene and five other polymers are found to correlate with the molecular weight of the side groups

Influence of Substrate Temperature on the Transformation Front Velocities That Determine Thermal Stability of Vapor-Deposited Glasses

Stable organic glasses prepared by physical vapor deposition transform into the supercooled liquid via propagating fronts of molecular mobility, a mechanism different from that exhibited by glasses prepared by cooling the liquid. Here we show that spectroscopic ellipsometry can directly observe this front-based mechanism in real time and explore how the velocity of the front depends upon the substrate temperature during deposition. For the model glass former indomethacin, we detect surface-initiated mobility fronts in glasses formed at substrate temperatures between 0.68<i>T</i>_g and 0.94<i>T</i>_g. At each of two annealing temperatures, the substrate temperature during deposition can change the transformation front velocity by a factor of 6, and these changes are imperfectly correlated with the density of the glass. We also observe substrate-initiated fronts at some substrate temperatures. By connecting with theoretical work, we are able to infer the relative mobilities of stable glasses prepared at different substrate temperatures. An understanding of the transformation behavior of vapor-deposited glasses may be relevant for extending the lifetime of organic semiconducting devices

Molecular Orientation in Stable Glasses of Indomethacin

Spectroscopic ellipsometry has been used to measure the properties of indomethacin prepared by physical vapor deposition at <i>T</i>_substrate/<i>T</i>_g = 0.78, 0.84, and 0.90. The as-deposited glasses exhibited high kinetic stability and had densities 0.8–1.2% higher than the ordinary glass prepared by cooling the liquid at 1 K/min. Deposition at the higher temperatures yielded glasses with positive birefringence (up to Δ<i>n</i> = 0.028), while the lowest-temperature sample was negatively birefringent (Δ<i>n</i> = −0.015). These results indicate that substrate temperature can be used to manipulate molecular orientation in high-density and high-stability glasses. The data for the supercooled liquid and the ordinary glass of indomethacin are reasonably consistent with the Lorentz–Lorenz equation, but significant deviations are noted with the as-deposited materials

Poly(ethylene oxide) Dynamics in Blends with Poly(vinyl acetate): Comparison of Segmental and Terminal Dynamics

Deuterium NMR at Larmor frequencies of 15.6 and 76.7 MHz was used to study the segmental dynamics of perdeuteriopoly(ethylene oxide) (d4PEO) in miscible blends with poly(vinyl acetate) (PVAc). Blends with PEO compositions of 2% and 50% were studied. The segmental dynamics of PEO are 9 orders of magnitude faster than the PVAc segmental dynamics for a 2% PEO blend near the blend Tg and could be described by the Lodge−McLeish model with a self-concentration of 0.3. The segmental dynamics of PEO in blends with PVAc show a weaker temperature dependence than the terminal dynamics of PEO in the same blends. We also compare the segmental and terminal dynamics of components in several other miscible polymer blends. For the fast component in a blend, it is commonly observed that terminal relaxation has a stronger temperature dependence than segmental relaxation. This effect correlates with the difference between the Tg values for the pure components and also with the ratio of the activation energies of the segmental dynamics for the two components in the blend

¹³C NMR Study of Segmental Dynamics of Atactic Polypropylene Melts

13C NMR T1 and NOE were measured for atactic polypropylene over the temperature range 325−425 K at 13C Larmor frequencies of 25, 75, and 125 MHz. Because of the wide range of Larmor frequencies employed, the data discriminate among commonly used models for segmental dynamics; the modified KWW distribution function provides a much better fit than the DLM and the modified log χ2 models. The temperature dependence of the segmental dynamics is very similar to that of the viscosity. An extrapolation of the calculated correlation times is consistent with previous NMR measurements near Tg. Interestingly, the full width half-maximum (fwhm) of the distribution of relaxation times remains unchanged at 1.3 decades, while previous NMR measurements nearer Tg reported a fwhm of 3.0 decades. Recent MD simulations are in qualitative agreement with the results reported here

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 Tg,DSC – 25 K (Tg,DSC is the glass transition temperature of bulk polystyrene). At Tg,DSC – 5 K, the mobility of these two subsets differs by 4 orders of magnitude. These data are interpreted as indicating the presence of a high-mobility layer at the film surface whose thickness is independent of polymer molecular weight and total film thickness. The thickness of the mobile surface layer increases with temperature and equals 7 nm at Tg,DSC