Time domain responses of glassy polymers
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The properties of glassy polymers are generally studied in the vicinity of the glass transition temperature using frequency and time domain responses. In this work, we primarily focus on studying two time domain responses of glassy polymers namely the dielectric, the structural recovery, and aging responses. A proper understanding of these properties is essential for a better prediction of the performances of polymers. Time domain dielectric measurement provides a powerful means to study the glassy responses in short times (as low as micro seconds) to long times (>200 seconds) in a single measurement device. We built a Time Domain Dielectric Spectrometer (TDS) in our laboratory at Texas Tech to take advantage of this ability. Successful working of TDS was demonstrated by studying the dielectric response of poly (vinyl acetate) in a pulse-probe experiment. We see memory effect and this was in quantitative agreement with linear Boltzmann superposition for small applied fields. However, evidence of breakdown of linearity was observed at relatively larger applied fields. This is an interesting result, as we were able to demonstrate the ability to delineate between linear and nonlinear behaviors in the time domain. We also introduce a filtration tool called Empirical Mode Decomposition (EMD) to improve the data analysis in dielectric and structural recovery experiments whose data are time varying in nature corrupted with nonlinear noise. Structural recovery and aging experiments of glassy polymers are very well understood for temperature formed glasses compared to concentration formed glasses. Previous work from our group has shown that concentration formed glasses qualitatively mimic temperature formed glasses but were quantitatively different. Further, our preliminary work on the structural recovery of epoxy film subjected to CO2 plasticizer jumps showed that the effective retardation time for concentration formed glass and temperature formed glass (subjected to same final condition) do not converge to the same point as equilibrium is approached. Hence, we further investigated this behavior by studying the aging and structural recovery of epoxy film subjected to CO2 plasticizer jumps. The results were surprising and we observe evidence for the existence of a new glassy state.