Synthesis and Self-assembly of Asymmetric Molecular Brushes with Azobenzene-containing Side Chains
Self-assembly of amphiphilic copolymers presents a promising platform for the design and synthesis of nanomaterials,depending heavily on the topological polymer chemistry and properties of their compositions.In particular,asymmetric molecular brushes are composed of two different side chains grafted on a linear backbone,possessing distinct assembly behaviors in comparison with conventional linear copolymers,owing to the asymmetric double-brushes and combined effects of backbone and brushes.Additionally,the introduction of unique functionalities and responsiveness into the self-assembly system of asymmetric molecular brushes endows extra opportunities to pursue morphologic diversity and intriguing functionalities.In this study,a novel functional acrylate monomer Br-acrylate-Cl with Br-containing initiating group and alkylchlorine group was designed and synthesized.The functional macro-agent,poly(Br-acrylate-Cl),was first prepared by reversible addition-fragmentation chain transfer(RAFT)homopolymerization of Br-acrylate-Cl monomer.Azobenzene-containing side chains were then directly grown from the backbone via atom transfer radical polymerization(ATRP)of azobenzene-containing monomer initiated by the pendant Br-containing initiating groups on the backbone,affording PA-g-PAzo molecular brushes.The obtained PA-g-PAzo molecular brushes were subsequently used in postpolymerization substitution to generate PA-g-PAzo/(TPE-HEG)asymmetric molecular brushes with azobenzene and hexaethylene glycol-containing side chains and tetraphenylethylene groups.Due to the special architecture and inter/intramolecular association of pendant azobenzene and tetraphenylethylene groups,PA-g-PAzo/(TPE-HEG)asymmetric molecular brushes exhibited unique self-assembly behavior.Therefore,π-π and solvophobic interaction-driven self-assembly of PA-g-PAzo/(TPE-HEG)in CH3OH-THF mixed solvents was studied,and the morphologies of the assemblies were characterized by transmission electron microscopy(TEM).Well-defined bundle-like aggregates,sheaf-like aggregates,and 2D platelets were constructed via self-assembly of PA-g-PAzo/(TPE-HEG)through a heating-cooling-aging process.The formation of sheaf-like aggregates was systematically investgated.Intriguingly,the azobenzene moieties undergo trans-cis isomerization upon UV irradiation and further promote a morphology evolution of the assemblies,leading to the formation of rectangular platelets.Our study presents an efficient method for preparing asymmetric molecular brushes based on the functional monomer stragtegy,and opens a new avenue to design molecular-brush-based nanomaterials with tunable morphologies.
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