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 E L). Specifically, the dihydroxyl-terminated LE/E E L served as a macromonomer in a step-growth polymerization in which stoichiometric quantities of sebacoyl chloride were added, resulting in (LE/E E L) ⟨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.