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/EEL). Specifically, the dihydroxyl-terminated LE/EEL served as a macromonomer in a step-growth polymerization in which stoichiometric quantities of sebacoyl chloride were added, resulting in (LE/EEL)(< 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 approximate to 190 degrees 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.