Effects of biodegradable imidazolium-based ionic liquid with ester group on the structure and properties of PLLA

Blends of poly(L-lactide) (PLLA) and a functional ionic liquid (IL), 3-methyl-1-(ethoxycarbonylethyl) imidazolium tetrafluoroborate ([AOEMIm][BF4]) with various weight ratios were prepared by melt compounding and their interactions, compatibilities, processing characteristics, crystallization behaviors, mechanical properties and thermal degradation were investigated in this study. The interaction between PLLA and [AOEMIm][BF4] was first studied by infrared spectroscopy (IR) investigation, and then the Flory-Huggins interaction parameter between PLLA and [AOEMIm][BF4] was calculated. The compatibility was also supported by both sample appearance and scanning electron microscopy (SEM) results. Crystallization and melting conditions were obtained from differential scanning calorimetry (DSC), and the glass transition temperature (Tg) of PLLA showed a gradual decrease with the increasing addition of [AOEMIm][BF4]. The balancing torque data were obtained by Haaker torque rheometry, and the molecular weight data of processed PLLA obtained by gel permeation chromatography (GPC) tests reflected a positive effect of [AOEMIm][BF4] on the melt-processing of PLLA. A strong increase of the mechanical parameters of PLLA, such as the elongation at break and the impact strength, was observed with the incorporation of [AOEMIm][BF4] up to the content of 7 phr (phr is the abbreviation for parts per hundreds of resin, n phr denoted n g IL/100 g PLLA), while the tensile strength decreased slightly. The thermal stability of PLLA took a sustained downward trend, with the increasing contents of [AOEMIm][BF4] in its matrices, as presented from thermogravimetric analysis (TGA). PLLA blended with 5 phr [AOEMIm][BF4] achieved excellent overall performance; as well, the mechanical properties improved considerably, and damage to the thermal stability was acceptable.