Depolymerization and Etching of Poly(lactic acid) via TiCl₄ Vapor Phase Infiltration

This study investigates the use of TiCl₄ vapor phase infiltration (VPI) to cleave ester groups in the main chain of a polymer and drive depolymerization and film etching. Prior investigations have demonstrated that the infiltration of TiCl₄ into PMMA results in dealkylation of its ester bond, cleaving its side groups. This study investigates the VPI of TiCl₄ into poly(lactic acid), which is a prototypical polymer with an ester group in its main chain. Utilizing <i>in situ</i> quartz crystal microbalance (QCM) measurements and spectroscopic ellipsometry, PLA is observed to depolymerize readily at 135 °C with extended TiCl₄ precursor exposure, resulting in significant thickness and mass reduction, whereas at 90 °C, depolymerization is significantly slower and etching is negligible. Utilizing Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and a residual gas analyzer (RGA), dealkylation is shown to be the primary depolymerization mechanism. FTIR and XPS analyses reveal the consumption of carbonyl and methoxy groups and the emergence of hydroxyl, chlorine, and titanium moieties. <i>In situ</i> RGA measurements provide further insights into the byproducts formed during the TiCl₄ and water exposure steps, indicating that the depolymerized components undergo further breakdown into other substances. Residuals left after 135 °C TiCl₄ VPI are easily removed with a 0.1 M HCl aqueous solution. These findings highlight the expanding functionality of VPI, revealing its capability as both an additive and subtractive process and suggesting its broader applications.