(488o) Liquid Water Transport in Polylactide Using Time-Resolved FTIR-ATR Spectroscopy
Bio-based polymers that biodegrade and can be synthesized from renewable sources are of great interest to replace commodity plastics for a variety of applications, including packaging (e.g., water bottles). Polylactic acid or polylactide (PLA) is one bio-based polymer that has recently attracted commercial interest because of its biodegradability. However, a deeper understanding of water transport in PLA is required for its application as a future barrier material. In this study, the diffusion and sorption of liquid water in PLA was measured using both gravimetry and time-resolved Fourier transformed infrared-attenuated total reflectance (FTIR-ATR) spectroscopy. Similar to the former technique, the latter technique measures mutual diffusion coefficients in the presence of a concentration gradient. However, unlike the former technique, the latter also provides molecular-level contrast between diffusant(s) (e.g., water) and the polymer in real time via the mid-infrared spectrum (chemical bond vibrations absorbing light at different wavelengths). Experiments were conducted on commercial PLA samples, as well as, synthesized PLA and PLA derivatives. Non-Fickian behavior was observed in all diffusion experiments, comparable to what has been observed in the literature for coupled diffusion and relaxation phenomena. Both the diffusion of water and polymer relaxation was measured independently with time-resolved FTIR-ATR spectroscopy. A diffusion-relaxation model was developed that combined Fickian diffusion and viscoelastic relaxation. Each set of data was regressed to the model to calculate a relaxation time constant and a diffusion coefficient, where the model regressed well to the data and only one fitting parameter was required for each data set. The synthesized PLA derivatives had similar relaxation time constants compared to PLA, but diffusivities almost an order of magnitude lower. These results provide new insights into the diffusion-relaxation phenomena in glassy polymers.