Structure, humidity adaptivity, and elasticity of hydrogen-bonded complexes formed by self-assembly of a triblock copolymer and a homopolymer†

Abstract

Specific aggregation structures largely influence the macro-physical performances of materials. Small changes in molecular components, conformation, crystallisation and microphase transitions can dramatically alter material properties. Here, we combine the triblock copolymer polystyrene-b-poly(ethylene oxide)-b-polystyrene (SES) with the homopolymer poly(acrylic acid) (PAA), fabricating elastomers with multi-hierarchical architectures. Unlike the single molecular chain of traditional elastomers, the flexible domain of these elastomers is formed via hydrogen bonding of the complex chains (A/E) of poly(ethylene oxide) (PEO) and PAA. The polystyrene (PS) blocks are microphase-separated and hydrophobically associated, providing physical crosslinks among the flexible complex chains and enhancing the strength of the elastomers. The influence of the molecular weight of the PS blocks, molecular chain length of PAA, and relative humidity on the elastomer's macro-physical properties was revealed using small-angle X-ray scattering, transmission electron microscopy, differential scanning calorimetry, Fourier transform infrared spectroscopy, and tensile stress–strain measurements. The PS block forms a hard spherical domain that affects the hydrogen-bonding complex behaviours of the PAA and PEO segments. Enlarging the PS sphere would hinder the formation of the PAA/PEO complex, but this trend is mitigated by shortening the PAA chain length. Hydrophobic steric hindrance by the PS sphere disturbs the association behaviours of the PAA and PEO segments. These structural differences result in different mechanical properties of the elastomers. Moreover, the elastomers adapt to changes in the relative humidity and show an obvious humidity-induced glass transition. The elastomers possess good elasticity properties under low and high humidity conditions. The methods and ideas presented in this study will enable the construction of new types of elastomers.