Crystallization and Mechanical Properties of Poly(l-lactide)-Based Rubbery/Semicrystalline Multiblock Copolymers

The crystallization and mechanical properties of triblock and multiblock copolymers containing 70 vol % semicrystalline poly(l-lactide) (L) and 30 vol % rubbery poly(ethylene-co-ethylene) (E/E_E) were investigated. The multiblock copolymer was synthesized directly from the triblock copolymer (denoted LE/E_EL). Specifically, the dihydroxyl-terminated LE/E_EL served as a macromonomer in a step-growth polymerization in which stoichiometric quantities of sebacoyl chloride were added, resulting in (LE/E_EL)_〈3.6〉, a multiblock copolymer with an average of 3.6 triblock copolymer units connected together. Additionally, triblock and multiblock copolymers were blended together in order to systematically tune 〈n〉 and uncover the role of block number on properties. Dynamic mechanical analysis (DMA) indicated that despite differences in 〈n〉, all samples had an order-to-disorder transition temperature T_ODT ≈ 190 °C, which is above the melting temperature (T_m) of poly(l-lactide). Small-angle X-ray scattering measurements (SAXS) of the block copolymers at T_m < T < T_ODT showed that the samples had identical morphology (hexagonally packed cylinders) and domain spacing. Isothermal crystallization experiments were performed using differential scanning calorimetry (DSC) and indicated that samples with higher 〈n〉 had a lower percentage crystallinity after 1 h of crystallization, which we associate with the differences in the average chain architecture. Uniaxial tensile measurements demonstrate a brittle-to-ductile transition at 〈n〉 = 1.8 for specimens with limited crystallinity. Finally, the effect of crystallinity on mechanical properties was investigated by annealing select samples.