The effect of alcoholic solutions on the thermo-mechanical properties of poly (lactic acid)

The effects of aqueous and deuterium oxide solutions of straight-chain alcohols with one to four carbon atoms on the thermo-mechanical properties of poly(lactic acid) - PLA - films were assessed by dynamic mechanical analysis (DMA). The glass transition temperature (Tg) of PLA films immersed in 25%, 50%, 75%, and 100% (v/v) alcoholic solutions were compared with the Tg's of films immersed in 100% HPLC water and deuterium oxide. The Tg's of PLA films immersed in methanol and ethanol (one to two carbon atoms) aqueous solutions decreased as the concentration of the alcohol solutions increased. For methanol and ethanol, the Tg's decreased by 8.6 ℗ʻC and 7.6 ℗ʻC per every 25% (v/v) increase of alcohol, respectively. When the PLA films were immersed in alcohol solutions with three to four carbon atoms, the Tg's reduced until 75% (v/v). Tg's of PLA films immersed in 1-propanol and 2-propanol decreased by 8.6 ℗ʻC and 8.9 ℗ʻC for every 25% increase of propanol, respectively. Aqueous and deuterium oxide solutions had similar effects on Tg. The sorption amount of ethanol in PLA films at 40 ℗ʻC was measured by quantitative proton nuclear magnetic resonance (1H-qNMR). PLA films experienced the most significant sorption amount of ethanol (9.16%) at 4 h of immersion. The crystallinity (Xc) of PLA immersed in ethanol in the first 12 h, was less than 3%. When the time reached 24 h, the Xc exceeded 20%, and after 48 h stabilized at about 30% meanwhile the ethanol sorption stabilized at around 5%. The results of this work help in the development of PLA films needed to be in contact with aqueous-alcoholic solutions.