A study on the degradation of polylactic acid in the presence of phosphonium ionic liquids

In this overview study, two ionic liquids (IL) with different anions (decanoate, tetrafluoroborate) but with the same phosphonium-based cation that showed promising plasticizing/lubricating behavior in polylactic acid (PLA) were screened for their effects on the polymer degradation under thermomechanical, thermo-oxidative (at 160 °C), hydrolytic (100% humidity, 60 °C), conditions, and during soil immersion. Depending on the particular medium and conditions used, degradation was followed by changes in molecular weight, melt viscosity, sample weight and appearance, morphology, crystallinity, acid number, and pH. The effects of the IL containing a decanoate anion were more pronounced on lubrication and also on degradation as evidenced by reduced melt viscosities and accelerated thermomechanical, isothermal, hydrolytic, and soil degradation. The IL containing the tetrafluoroborate anion showed higher thermal stability compared with the IL containing decanoate anion as also confirmed from thermal degradation rate constants which were calculated from random chain scission statistics. Accelerated hydrolytic degradation was observed in PLA containing the tetrafluoroborate based IL but to a lesser extent than the decanoate based IL. The catalytic role of the decanoate anion in hydrolytic degradation was confirmed through experiments with model compounds. X-ray diffraction (XRD) data on the materials exposed to soil degradation provided evidence that the initially amorphous polymer attained a certain degree of crystallinity as a result of the significant MW reduction.