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.

Poly(n-butyl methacrylate) in ionic liquids with tunable lower critical solution temperatures (LCST).

We describe the lower critical solution temperature (LCST)-type phase behavior of poly(n-butyl methacrylate) (PnBMA) (M =13,000 and 48,000) dissolved in 1-alkyl-3-methylimidazolium bis{(trifluoromethyl) sulfonyl}amide ionic liquids (ILs). The temperature-composition phase diagrams of these PnBMA/IL systems are strongly asymmetric with the critical composition shifted to low concentrations of PnBMA. As the molecular weight increases from 13,000 to 48,000, the critical temperature decreases by 20 °C, and the critical composition shifts to a lower concentration. On the basis of the LCST of PnBMA, we designed a thermosensitive poly(n-butyl methacrylate)-poly(ethylene oxide) (PnBMA-PEO) diblock copolymer that exhibits a free chain/micelle transition in an IL as the temperature increases above the lower critical micellization temperature (LCMT). Furthermore, using IL blends as solvents, both the LCST of PnBMA and the LCMT of PnBMA-PEO can be tuned over a wide range by mixing two different alkyl methylimidazolium ILs without modifying the chemical structure of the polymers.

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.

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.

Free volume and finite-size effects in a polymer glass under stress.

Molecular dynamics simulations of the nonlinear creep response of a polymer glass under tension and compression have been performed at the glass transition temperature. The dynamics were measured as the deformation proceeds using the bond autocorrelation function, and the relaxation times measured as the system is compressed or elongated exhibit a universal response. In tension, the volume increases with strain rate and the relaxation times decrease. In compression, however, the volume decreases by approximately the same amount for all of the applied stresses. Thus, decreases in free volume take place alongside a decrease of the relaxation times by over a factor of 100. We find direct evidence that a characteristic length scale exists below which the deformation of the system exhibits distinct anomalies.