Accelerated Hydrolysis of Amorphous Polylactide Containing Salicylate Additives

Abstract

Blends of amorphous-grade polylactide (PLA) with low levels of salicylic acid (SA), disalicylide (DS), or oligosalicylate (OS) were prepared using scalable melt processing techniques. The glass transition temperature (T_g), tensile properties, and shelf life stability of the salicylate-containing blends were nearly identical to neat PLA. The inclusion of salicylate additives accelerated sample mass loss in artificial seawater at 50 °C by up to a factor of 3 compared to neat PLA. Water uptake occurred after the PLA molar mass dropped below 15 kg/mol, corresponding to the point at which the T_g of the remaining material dropped below 50 °C. The onset of mass loss was observed after PLA hydrolyzed into 2 kg/mol fragments, coinciding with the leaching of SA and DS; OS remained in the blends for longer immersion periods. The rate of molar-mass reduction in 1 wt % salicylate-containing blends was up to twice as fast as the rate of neat PLA. Degradation-induced PLA crystallization occurred once T_g dropped below 50 °C, and mass loss slowed after the degree of crystallinity (X_c) exceeded 50%. We conclude that a low concentration of carboxylic acid groups from salicylate-containing compounds is sufficient to enhance PLA degradability without sacrificing material performance.