Studies on Comonomer Compositional Distribution of Poly(L-lactic acid) Copolymers and Its Effect on the Thermal, Mechanical, and Oxygen Barrier Properties of Fractions

To investigate the effects of the different types and content of monomer units and molecular weights Mns on the thermal, mechanical and barrier properties of the copolymers. Here, three unsaturated poly(L-lactic acid-co-butyrate itaconate) (P(LA-BI)), poly(L-lactic acid-co-butyrate fumaric) (P(LA-BF)), and poly(L-lactic acid-co-butyrate maleic) (P(LA-BM)) copolymers were synthesized by melt polycondensation and subjected to repeated grading. Based on a solvent/non-solvent (chloroform/n-heptane) mixture, an original sample was fractionated into 9–10 fractions. As the fractionation proceeded, the alterations in the monomer units content and Mns of the three copolymers showed disperate trends. The P(LA-BI) with 10.4% PBI content reaches a maximum value of Mn 8.82 × 104 and has the best physical properties. The elongation at break and oxygen transmission rate (OTR) were 491.3% and 106 cm3/m2 d–1 respectively, and the degree of crosslinking was 7.7%. P(LA-BF) exhibited the second-best Mn and physical properties, while P(LA-BM) displayed the worst physical properties. The elongation at break was 3.4% and the oxygen transmission rate (OTR) was 389 cm3/(m2 d) at 7.3% BM.