Molecular-Scale Asymmetry and Memory Behavior in Poly(vinyl acetate) Monitored with Mobility-Sensitive Fluorescent Molecules

A fluorescence technique employing a mobility-sensitive fluorescence molecule (julolidenemalononitrile, JMN) is shown to sense the molecular relaxations occurring after temperature jumps in glassy and rubbery poly(vinyl acetate) (PVAc). Results suggest that molecular environments relax in a manner similar to bulk relaxations. After temperature jumps near T_g, the technique senses when the local probe environments reach equilibrium, and bulklike “asymmetry” behavior is shown to occur on the local level. The molecular mobility sensor, JMN, reproduces all details found in bulk volume or enthalpy asymmetry, including the τ_eff paradox. After multiple temperature jumps, the fluorescence technique also detects “memory effects” on the molecular scale. In addition, this technique senses the increase in mobility caused by the plasticization of PVAc by water which decreases the times needed to reach fluorescence equilibrium at a particular temperature. However, these equilibrium times are constant for a given value of T_g - T_a (T_a = annealing temperature) regardless of water content. Since a short exposure to humid air can severely plasticize PVAc, it is concluded that T_g is a very important reference point for making comparisons of PVAc properties or attempting to extract meaningful parameters from theoretical fits of relaxation data. The sensitivity of the fluorescence technique to the relaxation time scales in PVAc is similar to that of dilatometry and bulk enthalpy relaxation.