Cocrystallization of monomer units in lactic acid-based biodegradable copolymers, poly(l-lactic acid-co-l-2-hydroxybutanoic acid)s

•Cocrystallization occurs in the copolymers regardless of their compositions.•The effects of crystallization conditions on cocrystallization are studied.•Melt-crystallization and precipitation are favorable to cocrystallization.•The d and Tm values of copolymers depend on l-lactic acid unit content.•The spherulites of copolymers are formed when melt-crystallized.

The copolymers of l-lactic acid (LLA) and l-2hydroxybutanoic acid (L-2HB) [P(LLA-co-L-2HB)s] having relatively random monomer sequences were synthesized by polycondensation. It was first revealed that the cocrystallization of LLA and L-2HB units occurs in P(LLA-co-L-2HB) copolymers for a wide comonomer unit (L-2HB unit) content range of 27-74 mol% during precipitation and melt-crystallization. The interplane distance values of the precipitated and melt-crystallized P(L-2HB), PLLA, and P(LLA-co-L-2HB) samples increased monotonically with an increase in L-2HB unit content, whereas the Tm values of the P(LLA-co-L-2HB) copolymer samples decreased with the deviation of L-2HB unit content from 0 or 100% and became minimum at the L-2HB unit content of 74 mol%. In melt-crystallized P(LLA-co-L-2HB) copolymer samples, crystallites were formed by random incorporation of monomer units without selecting the types of monomer units, whereas in precipitated P(LLA-co-L-2HB) samples, the crystallites were formed by the rather predominant incorporation of L-2HB units having a larger side chain. The cocrystallization of P(LLA-co-L-2HB) copolymers is expected to provide the versatile biodegradable materials having a wide variety of degradation rates and behavior by altering the monomer unit contents and also by crystallinity and crystalline size which cannot be attained by non-crystallizable biodegradable copolymers and will thereby widen the applications of LLA-based polymers.

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