Predicting Solute Diffusivity in Polymers Using Time-Temperature Superposition

We demonstrate a novel application of the time-temperature superposition (TTS) principle to predict solutediffusivityDin glassy polymers using atomistic molecular dynamicssimulations. Our TTS approach incorporates the Debye-Wallerfactor < u2 >, a measure of solute caging, along with concepts fromthermodynamic scaling methods, allowing us to balance con-tributions to the dynamics from temperature and < u2 > usingadjustable parameters. Our approach rescales the solute mean-squared displacement curves at several temperatures into a mastercurve that approximates the diffusive dynamics at a referencetemperature, effectively extending the simulation time scale fromnanoseconds to seconds and beyond. With a set of"universal"parameters, this TTS approach predictsDwith reasonable accuracy in a broad range of polymer/solute systems. Using TTS greatlyreduces the computational cost compared to standard MD simulations. Thus, our method offers a means to rapidly and routinelyprovide order-of-magnitude estimates ofDusing simulations.