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.
■ INTRODUCTIONBlock polymers are an intriguing and useful class of materials due to the ability to precisely tune morphology and properties by changing parameters including composition, molecular weight, and block sequencing. 1 While diblock and triblock copolymers have been studied extensively in the literature, fewer reports have focused on multiblock copolymers. Interestingly, multiblock copolymers have been shown to have superior mechanical properties relative to diblock and triblock copolymers due to the ability of these molecules to bridge multiple nanoscale domains. 2−11 For this reason, incorporating brittle poly(lactide) (PLA) into a multiblock copolymer architecture is an attractive option for improving its mechanical properties. Additionally lactide, as well as other renewable cyclic esters such as β-methyl-δ-valerolactone, 12 menthide, 13 ε-decalatone, 9 and δ-decalactone, 14 can be polymerized with α,ω-dihydroxyl functionality by using difunctional alcohols as initiators. Hence, sustainable-based multiblock copolymers can be prepared from homopolymer and block polymer macromonomers by step-growth methods using difunctional isocyanates and acid chlorides, among other approaches. 4,15−20 Because of the potential to improve mechanical properties and the simple synthetic routes, multiblock copolymers are poised to play an important ro...