Water-Assisted Microphase Separation of Sodium Acrylate Random Copolymers Bearing Crystalline Alkyl Groups

Herein, we report water-assisted microphase separation of sodium acrylate (ANa) random copolymers bearing crystalline octadecyl or docosyl groups. The random copolymers are obtained from free radical copolymerization of t-butyl acrylate and octadecyl or docosyl acrylate (C18A or C22A), followed by the post-modification of the copolymers. The bulk random copolymers efficiently absorb water (13-37 wt.%) therein under humid conditions in relative humidity 90% above the melting temperature and induce the microphase separation of the hydrophobic alkyl groups and the hydrophilic ANa/main chains containing absorbed water. The phase separation bahavior, morphology, and domain spacing depend on the composition and weight fraction of the hydrophobic groups and hydrophilic segments. Typically, ANa/C18A (5/5 - 2/8) random copolymers form water-absorbed body-centered cubic or water-intercalated lamellar structures with 5-7 nm domain spacing. ANa/C18A (6/4) random copolymer not only forms a dry and crystalline lamellar structure below the melting temperature of the octadecyl groups but also stably maintains an amorphous lamellar structure above the melting temperature at least up to 150°C via the segregation of the polar main chains and octadecyl pendants.