Keewook Paeng

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). At T(g,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 T(g,DSC).

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. Although the Eyring model can describe the increase in mobility at low stress, it fails to describe mobility after flow onset. In this regime, mobility is strongly accelerated and the distribution of relaxation times narrows substantially, indicating a more homogeneous ensemble of local environments. At even larger stresses, in the strain-hardening regime, mobility decreases with increasing stress. Consistent with the view that stress-induced mobility allows plastic flow in polymer glasses, we observed a strong correlation between strain rate and segmental mobility during creep.

Molecular mobility in supported thin films of polystyrene, poly(methyl methacrylate), and poly(2-vinyl pyridine) probed by dye reorientation

Temperature-ramping anisotropy measurements were used to probe the molecular mobility of fluorescent probes in polystyrene, poly(methyl methacrylate), and poly(2-vinyl pyridine) films supported upon silicon wafers with native oxide coatings. All polymer films showed evidence of high mobility at the free surface. The fraction of a film with high mobility was characterized as a mobile surface layer thickness, which increased with temperature. The mobile surface layer thickness for supported films of polystyrene and poly(methyl methacrylate) reasonably matched that previously deduced from freestanding films of these polymers; for poly(methyl methacrylate), enhanced mobility extends about 4 nm into the film from the free surface at Tg. For supported polystyrene and poly(methyl methacrylate) films, the results are consistent with no decrease in mobility near the solid substrate but do not eliminate this possibility. On the other hand, the mobility of supported poly(2-vinyl pyridine) thin films provides some evidence for slower-than-bulk relaxation near the solid substrate.

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. Generally, the mobility increased as the stress, strain, and strain rate increased. After removing the stress, we observed that the enhanced mobility slowly disappeared during strain recovery. At 377 K, when the stress is lower than 11 MPa, almost no mobility enhancement was observed. Once the stress crossed this threshold value, the mobility dramatically increased.

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. We present results using different fluorescent molecules and different temperature-ramping rates; such experiments show potential for accessing slow molecular motions considerably below T(g). Temperature-ramping anisotropy measurements were performed on freestanding poly (4-tert-butyl styrene) films of varying thicknesses. The anisotropy decay of a 22 nm film was shifted about 12 K lower in temperature as compared to a bulk sample.

Fast Crystal Growth Induces Mobility and Tension in Supercooled o-Terphenyl

A photobleaching method was used to measure the reorientation of dilute probes in liquid o-terphenyl near a crystal growth front. Near the glass-transition temperature Tg, mobility in the supercooled liquid was enhanced within ∼10 μm of the crystal growth front, by as much as a factor of 4. This enhanced mobility appears to be caused by tension created in the sample as a result of the density difference between the supercooled liquid and crystal. The maximum observed mobility enhancement corresponds to a tension of about -8 MPa, close to the cavitation limit for liquid o-terphenyl. Whereas the observed mobility near the growing crystal is not large enough to explain the extraordinary fast crystal growth observed near Tg in o-terphenyl and some other low-molecular-weight glassformers, these observations suggest that cavitation or fracture plays a key role in releasing tension and allowing fast crystal growth to occur at a steady rate.

Fast crystal growth from organic glasses: comparison of o-terphenyl with its structural analogs.

Crystal growth kinetics and liquid dynamics of 1,2-diphenylcyclopentene (DPCP) and 1,2-diphenylcyclohexene (DPCH) were characterized by optical microscopy and dielectric spectroscopy. These two molecules are structurally homologous and dynamically similar to the well-studied glassformer ortho-terphenyl (OTP). In the supercooled liquid states of DPCP and DPCH, the kinetic component of crystal growth ukin has a power law relationship with the primary structural relaxation time τα, ukin [proportionality] τα(–ξ) (ξ ≈ 0.7), similar to OTP and other fragile liquids. Near the glass transition temperature (Tg), both DPCP and DPCH develop much faster crystal growth via the so-called GC (glass to crystal) mode, again similar to the behavior of OTP. We find that the α-relaxation process apparently controls the onset of GC growth, with GC growth possible only at sufficiently low fluidity. These results support the view that GC crystal growth can only occur in systems where the liquid and crystal exhibit similar local packing arrangements.