Hetero-stereocomplex formation between substituted poly(lactic acid)s with linear and branched side chains, poly(l-2-hydroxybutanoic acid) and poly(d-2-hydroxy-3-methylbutanoic acid)

Differential scanning calorimetry and wide-angle X-ray diffractometry first revealed the formation of hetero-stereocomplex (HTSC) between biodegradable, optically active, and isotactic poly(2-hydroxyalkanoic acid)s having different chemical structures and opposite configurations, i.e., l-configured substituted poly(lactic acid) (PLA) [poly(l-2-hydroxybutanoic acid), P(l-2HB)] with linear side chains (ethyl groups) and d-configured substituted PLA [poly(d-2-hydroxy-3-methylbutanoic acid), P(d-2H3MB)] with branched side chains (isopropyl groups) in solution and in bulk from the melt. The melting temperature of P(l-2HB)/P(d-2H3MB) HTSC crystallites was 197–204 °C, which is much higher those of P(l-2HB) and P(d-2H3MB) homo-crystallites (100–101 °C and 158–165 °C, respectively). The interplain distances and crystalline lattice sizes of P(l-2HB)/P(d-2H3MB) HTSC crystallites were respectively larger and smaller than those of P(l-2HB)/P(d-2HB) and P(l-2H3MB)/P(d-2H3MB) homo-stereocomplexes. The HTSC formation of substituted PLA with opposite configurations reported in the present study will provide a versatile way to prepare poly(2-hydroxyalkanoic acid)-based biodegradable materials having a wide variety of physical properties and biodegradability.