Precise Synthesis of ABA Triblock Copolymers Comprised of Poly(ethylene oxide) and Poly(β-benzyl-<scp>l</scp>-aspartate): A Hierarchical Structure Inducing Excellent Elasticity

Abstract

ABA triblock copolymers comprised of poly(β-benzyl-l-aspartate) (PBLA) as an A segment and poly(ethylene oxide) (PEO) as the B segment were precisely synthesized by a ring-opening polymerization of the N-carboxyanhydride of BLA initiated from amines at both terminals of the PEO chain. The copolymers have a number-average molecular weight (Mn) of 1.5 × 104−3.6 × 104 and a monodisperse molecular weight distribution in the range 1.04−1.07. The copolymer films casted from a dichloromethane solution were flexible and elastic despite their low Mn. The film formed a hierarchical structure, i.e., the chains adopted an α-helix−72 helix−α-helix conformation and was highly crystallized, forming a long-range ordering structure which resulted in a rodlike construct arranged like ripples, as confirmed by Fourier transformed infrared spectroscopy, X-ray diffraction, and atomic force microscopy. Differential scanning calorimetry confirmed that the film showed a melting endotherm of the PEO segment at 313−326 K depending on the PBLA content. The film became very soft and behaved like rubber above the melting temperature. When the film was successively cooled, the crystalline diffraction became less intense, especially in terms of the PBLA segments whose conformation partially transformed from α-helices to β-sheets, and the film showed enhanced strength and drastically increased deformation, with a strain of more than 500% accompanied by a necking phenomenon. The relationship between the mechanical properties and the structure is also discussed.