Crystallization-Driven Thermoreversible Gelation of Coil-Crystalline Cyclic and Linear Diblock Copolypeptoids.

Methanol solutions of cyclic and linear coil-crystalline diblock copolypeptoids [i.e., poly(N-methyl-glycine)--poly(N-decyl-glycine)] (5-10 wt %) have been shown to form free-standing gels consisting of entangled fibrils at the room temperature. The gelation is thermally reversible and mechanically nonreversible. The gel-to-sol transition at the elevated temperature is induced by the melting of the PNDG crystalline domains which results in the morphological change of the fibrillar network into an isotropic solution. Variable-temperature NMR studies reveal that the cyclic polymer gels have higher gel-to-sol transition temperatures than the linear analogs. The hydrophobic segment is substantially less solvated in the cyclic polymers than the linear analogs both in gel and sol states. Rheological measurements reveal that the cyclic gels are stiffer than the linear counterparts, presumably due to the enhanced crystallinity in the fibrillar network in the formers relative to the latters. This study is the first example of thermoreversible gelation of coil-crystalline block copolymers, where the crystallization of the solvophobic segment has been shown to drive the gelation through the formation of crystalline fibrils.