Hau-Nan Lee

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.

Heterogeneous dynamics during deformation of a polymer glass

The origins of molecular mobility in polymer glasses, particularly under deformation, are not well understood. A concerted experimental and computational approach is adopted to examine the segmental motion of a polymeric glass undergoing creep and constant strain rate deformations. Through a combination of molecular dynamics simulations and optical photobleaching experiments we are able to directly probe how dynamic heterogeneity evolves during deformation. Two distinct regimes emerge from our analysis; early in the deformation, the dynamics of the glass are strongly heterogeneous, as evidenced by the spectrum of relaxation times measured experimentally and the participation ratio of the atomic non-affine displacements measured computationally. After the onset of flow, the dynamics become significantly more homogeneous, and the participation ratio increases considerably.

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.

Interaction between physical aging, deformation, and segmental mobility in poly(methyl methacrylate) glasses

Optical photobleaching experiments were used to investigate the interaction between physical aging, segmental mobility, and mechanical properties in polymer glasses. Mechanical creep experiments were performed on lightly cross-linked poly(methyl methacrylate) glasses with systematically varying aging histories. By directly measuring the molecular mobility of polymer glasses under deformation, we observe that stresses in the preflow regime and flow regime have qualitatively different influences on the aging process. In the preflow regime, the effects of aging and stress on mobility act as two independent processes; stress causes an increase in segmental mobility but does not erase the influence of previous aging. In contrast, as a sample enters the flow regime, plastic deformation takes the glass into a high mobility state that is independent of any predeformation aging history.