Crystallization in Sequence-Defined Peptoid Diblock Copolymers Induced by Microphase Separation

Abstract

Atomic level synthetic control over a polymer's chemical structure can reveal new insights into the crystallization kinetics of block copolymers. Here, we explore the impact of side chain structure on crystallization behavior, by designing a series of sequence-defined, highly monodisperse peptoid diblock copolymers poly-N-decylglycine-block-poly-N-2-(2-(2-methoxyethoxy)ethoxy)ethylglycine (pNdc-b-pNte) with volume fraction of pNte (ϕNte) values ranging from 0.29 to 0.71 and polydispersity indices ≤1.00017. Both monomers have nearly identical molecular volumes, but the pNte block is amorphous while the pNdc block is crystalline. We demonstrate by X-ray scattering and calorimetry that all the block copolypeptoids self-assemble into lamellar microphases and that the self-assembly is driven by crystallization of the pNdc block. Interestingly, the microphase separated pNdc-b-pNte diblock copolymers form two distinct crystalline phases. Crystallization of the normally amorphous pNte chains is induced by the preorganization of the crystalline pNdc chains. We hypothesize that this is due to the similarity of chemical structure of the monomers (both monomers have linear side chains of similar lengths emanating from a polyglycine backbone). The pNte block remains amorphous when the pNdc block is replaced by another crystalline block, poly-N-isoamylglycine, suggesting that a close matching of the lattice spacings is required for induced crystallization. To our knowledge, there are no previous reports of crystallization of a polymer chain induced by microphase separation. These investigations show that polypeptoids provide a unique platform for examining the effect of intertwined roles of side chain organization on the thermodynamic properties of diblock copolymers.