Stereocomplex polylactide (scPL) shows better mechanical properties than both poly(L-lactide) (PLL) and poly(D-lactide) (PDL). However, scPL is still brittle. In this work, di- and triblock copolymers of poly(propylene glycol) (PPG) and polylactide (PLL or PDL) were synthesized for the preparation of scPL films. The influences of the block structure, PPG block length and blend ratio on stereocomplexation, film morphology and mechanical properties of the scPL films were investigated. From DSC and WAXD results, the 50/50 wt% blend films exhibited the highest stereocomplexation. Phase separation between the scPL and PPG phases was not found from their SEM images. The tensile stress and elongation at break of the blend films increased with the sterecomplex crystallinities. The longer PPG blocks improved both the stereocomplexation and elongation at break of the blend films. In conclusion, the block structures and PPG block lengths influenced on both the stereocomplexation and mechanical properties of the scPL films.

Keywords: stereocomplex, polylactide, poly(propylene glycol), block copolymers, mechanical properties

Poly(L-lactic acid) (PLLA) or poly(L-lactide) (PLL) is an important bio-renewable and biodegradable polyester that has been widely studied for use in bioplastic applications. However, PLL products are limited for practical applications due to their high brittleness and low thermal resistance. Efficient plasticizers for PLL, such as poly(ethylene glycol) (PEG)^1-3 and poly(propylene glycol) (PPG),^4,5 etc. have been reported to reduce the brittleness of the PLL. However, phase separation and migration behavior in these plasticizers have been reported, which reduced the plasticization effect.^6,7 The attachment of these plasticizer molecules by block copolymerization and reactive blending can improve the phase compatibility to reduce the migration effect of the plasticizers.^8,9
Polylactides (PL) has two enaniomers: poly(L-lactide) (PLL) and poly(D-lactide) (PDL). The PLL/PDL blending produced a stereocomplex PL (scPL), which exhibited better mechanical properties and heat resistance than both the PLL and PDL.^10-14 The stronger interactions on the stereocomplex crystallites of the scPL induced a higher melting temperature (T_m≈230 ℃) and faster crystallization rate than in either the original PLL or PDL (T_m≈180 ℃).10 The faster crystallization of the scPL enhanced the good heat resistance. However, the scPL is still brittle due to its Tg being similar to those of PLL and PDL. Although stereocomplex formation of scPL has been extensively reported, there are only a limited number of works on the improvement of scPL flexibility by block copolymerization.
The PDL-PEG-PDL triblock copolymers have been synthesized and blended with PLL to form more flexible scPL films.^8,15-18 The stereocomplexation of PLL-PEG-PLL/PDLPEG-PDL blends has also been reported.¹⁹ Good phase compatibility between the PL and PEG phases was observed for these triblock copolymer blend films. The triblock copolymer blending showed synergistic effects of stereocomplexation between the two enantiomeric PLL and PDL chains, and plasticization of the PEG blocks. To the best of our knowledge, the influence of the PPG-PLL/PPG-PDL block copolymer blending on the stereocomplexation and mechanical properties of the scPL has not been reported so far. The PPG has also been shown to give efficient plasticization to improve the flexibility of the PLL^4,5 and scPL.⁶ However, phase separation was observed for scPL/PPG blends with higher PPG molecular weight.⁶
In this paper, the stereocomplex blend films of PPG-PLL/PPG-PDL block copolymer blends prepared with different block structures (di- and triblock), PPG block lengths (2000 and 4000 g/mol) and blend ratios (75/25, 50/50 and 25/75 wt%) were prepared as scPL films by solution blending before film casting. The obtained results were also compared with the neat di- and triblock copolymer films (100/0 and 0/100 wt%).