Acetic acid diffusion in polyisobutylene: Probing small molecule structures

In this study, time-resolved Fourier transform infrared, attenuated total r eflectance (FTIR-ATR) spectroscopy was used to characterize the diffusion o f acetic acid in polyisobutylene (PIB) at different vapor activities in ord er to understand complex diffusion mechanisms and probe molecular structure s. The relationship between the carbonyl (C=O) stretch at 1715 cm(-1), whic h represents a cyclic dimer, and an unknown structure at 1726 cm(-1) was in vestigated during the diffusion process to determine the structure of the u nknown species. A local equilibrium association model suggested that the un known species was a linear dimer in equilibrium with the cyclic dimer, A ma thematical model for diffusion with isomerization, assuming local equilibri um, predicted that each species would have the same effective diffusion coe fficient, but the individual diffusion coefficients could not be separated from the expression for the effective diffusion coefficient. Numerical simu lations of the two coupled continuity equations were performed to show that the individual diffusion coefficients can be separated and that they are e qual to each other for the acetic acid/PIB system. The values of the indivi dual diffusion coefficients are consistent with the structural. hypothesis from the local equilibrium assumption.