The enantiomeric crystallization of polylactides has removed the limitations of innate poor thermal and mechanical properties of the homopolymers. The supercritical fluid technology is an emerging panoramic version of biomedical polymer synthesis and has proven to be a domineering substitute to toxic organic solvents. Herein, we report an intriguing, efficient and a novel polymerization process using supercritical dimethyl ether (sc-DME) for preparation of polylactides leading to the stereocomplex polylactide (s-PLA) nanoparticles. The process has generated high molecular weight homopolymers (Mn ≥ 200 000 g mol–1) starting from monomers which ultimately crystallized to a dry powder of s-PLA nanoparticles. The optimum processing parameters are d/l-lactide polymerization using sc-DME at 130 °C, 400 bar for 5 h with a 30% monomer concentration, keeping the ratio [monomer]:[tin­(II)­2-ethylhexanoate]:[1-dodecanol] as 3000:1:1 while the stereocomplexation as sc-DME at 70 °C, 350 bar for 2 h. We have investigated the effects of monomer concentration, molecular weights of homopolymers, times, temperatures, and pressures on the degree of stereocomplexation. The degree of s-PLA was analyzed by DSC and XRD. The s-PLA has improved melting point and thermal degradation than homopolymers. The Young’s modulus of s-PLA increased to 1.4 GPa with tensile strength (∼43 MPa) higher than homopolymers (∼13 MPa) with 3.2% elongation at break. The dry s-PLA powder shows a diversity of particle size ranging from 30 to 600 nm analyzed by SEM. The s-PLA finds potential applications in polymer nanofabrication, biomedical stents and encapsulation, melt-blending, solution casting, and molding.

Polylactide (PLA) is a eco‐friendly and biodegradable material that can be synthesized from renewable resources. PLA features poly(d‐lactic acid) (PDLA) and poly(l‐lactic acid) (PLLA) enantiomers. Supercritical fluid (SCF) technology is a very promising method for the stereocomplexation between PDLA and PLLA enantiomers. This study acquires stereocomplex (sc‐)PLA particles with diverse sizes and behaviors by controlling the experimental conditions. Various parameters including polymer concentration, reaction temperature, stirring speed, pressure reducing speed, and final temperature were controlled to adjust size and behavior of sc‐PLA particles. Additionally, we analyzed the effect of subsequent processing following SCF (such as homogenization, mechanical stirring, and sonication) on the size and morphological behavior of sc‐PLA particles. Finally, the mechanical strengths of different PLA composites featuring different sc‐PLA filler sizes were determined. The mechanical strength of PLA composites was significantly improved when using smaller filler sizes. POLYM. ENG. SCI., 58:1193–1200, 2018. © 2017 Society of Plastics Engineers

Engineering the polylactide via stereocomplexation with supercritical fluid (SCF) technology paved way to fabricate polymers with enhanced thermal and mechanical properties. We aimed to establish a SCF medium with excellent solubility for PLA without any additional solvent/co-solvent. We, therefore, employed supercritical dimethyl ether to synthesize 100% stereocomplex polylactide from high molecular weight homopolymers with an excellent yield. The remarkable solubility of the homopolymers in dimethyl ether is the key for quick conversion to s-PLA. This study proves a rapid synthesis route of dry s-PLA powder with sc-DME at 250 bar, 70 o C and 1.5 h, which are reasonably achievable processing parameters compared to the conventional methods. The degree of stereocomplexation was evaluated under the effect of pressures, temperatures, times, homopolymer-concentrations and molecular weights. An increment in the degree of stereocomplexation was observed with increased temperature and pressure. Complete conversion to s-PLA was obtained for PLLA and PDLA with M n~2 00 kg·mol -1 with a total homopolymer to total DME ratio of 6:100% w/w at prescribed reaction conditions. The degree of stereocomplexation was determined by DSC and confirmed by XRD. Considerable improvement in thermo-mechanical properties of s-PLA was observed. DSC and TGA analyses proved a 50 o C enhancement in melting transition and a high onset temperature for thermal degradation of s-PLA respectively.