Depolymerization\nand Etching of Poly(lactic acid)\nvia TiCl<sub>4</sub> Vapor Phase Infiltration

This study investigates the use of TiCl<sub>4</sub> vapor\nphase\ninfiltration (VPI) to cleave ester groups in the main chain of a polymer\nand drive depolymerization and film etching. Prior investigations\nhave demonstrated that the infiltration of TiCl<sub>4</sub> into PMMA\nresults in dealkylation of its ester bond, cleaving its side groups.\nThis study investigates the VPI of TiCl<sub>4</sub> into poly(lactic\nacid), which is a prototypical polymer with an ester group in its\nmain chain. Utilizing <i>in situ</i> quartz crystal microbalance\n(QCM) measurements and spectroscopic ellipsometry, PLA is observed\nto depolymerize readily at 135 °C with extended TiCl<sub>4</sub> precursor exposure, resulting in significant thickness and mass\nreduction, whereas at 90 °C, depolymerization is significantly\nslower and etching is negligible. Utilizing Fourier transform infrared\nspectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and a\nresidual gas analyzer (RGA), dealkylation is shown to be the primary\ndepolymerization mechanism. FTIR and XPS analyses reveal the consumption\nof carbonyl and methoxy groups and the emergence of hydroxyl, chlorine,\nand titanium moieties. <i>In situ</i> RGA measurements provide\nfurther insights into the byproducts formed during the TiCl<sub>4</sub> and water exposure steps, indicating that the depolymerized components\nundergo further breakdown into other substances. Residuals left after\n135 °C TiCl<sub>4</sub> VPI are easily removed with a 0.1 M HCl\naqueous solution. These findings highlight the expanding functionality\nof VPI, revealing its capability as both an additive and subtractive\nprocess and suggesting its broader applications.