Transmission electron microscopy study of solvent-induced phase morphologies of environmentally responsive mixed homopolymer brushes on silica particles

摘要

We report in this work a transmission electron microscopy study of phase morphologies of environmentally responsive mixed poly(t-butyl acrylate) (PtBA)/polystyrene (PS) brushes and mixed poly(acrylic acid) (PAA)/PS brushes on 180 nm silica particles after treatments with nonselective good solvents and selective solvents, respectively. Mixed PtBA/PS brushes were grown from Y-initiator-functionalized silica particles by sequential atom transfer radical polymerization and nitroxide-mediated radical polymerization. Mixed PAA/PS brushes were prepared from mixed PtBA/PS brushes by removal of the t-butyl groups. For mixed PtBA/PS brushes with PtBA M, of 24.2 kDa and PS Mn of 23.0 kDa and the corresponding mixed PAA/PS brushes, random worm-like, nearly bicontinuous nanostructures were formed from lateral microphase separation when the particles were cast from nonselective good solvents (chloroform for mixed PtBA/PS brushes and N,N-dimethylformamide for mixed PAA/PS brushes). The feature sizes were on the order of polymer chain root-mean-square end-to-end distances (similar to 10 nm). In contrast, mixed PtBA/PS brushes with lower molecular weights (PtBA M-n = 10.4 kDa and PS M-n = 11.9 kDa) did not strongly phase separate after being cast from chloroform. After the solvents in the particle dispersions were gradually changed to selective solvents (n-octane for mixed PtBA/PS brushes and H2O for mixed PAA/PS brushes), isolated microdomains with an average size of 14-19 nm were formed as one grafted polymer collapsed and associated to form isolated microdomains, which were shielded by another grafted polymer yielding surface-tethered micellar structures. These results confirmed the theoretical predictions of the formation of "rippled" nanostructures and surface micellar structures of mixed homopolymer brushes induced by nonselective and selective solvents, respectively.