Homopolymer nanobowls with a controlled size and denting degree

Abstract

Homopolymer nanobowls hold promising potential applications in many fields because of their designability, large specific surface area and high packing density. However, it is still challenging to prepare nanobowls with a controlled size (diameter D) and denting degree (including relative opening width W/D and relative concave depth H/D). Herein, we propose a strategy for preparing such nanobowls with controlled D, W/D, and H/D values by self-assembly of a homopolymer, poly(N-([2,2′-bipyridyl]-4-yl)acrylamide) (PBPy), that was synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization. In the homopolymer PBPy, the bipyridine (BPy) pendants are functional moieties that exhibit intramolecular π–π stacking and hydrogen bonding, as well as coordination with Fe²⁺ ions. To control the size and denting degree of nanobowls, the initial concentration of PBPy (C_ini), self-assembly temperature (T_s), solution pH, and molar ratio of Fe²⁺ to BPy were tuned. It was confirmed that relatively uniform nanobowls can be prepared at a C_ini of 0.2 mg mL⁻¹ because of moderate π–π stacking and hydrogen bonding interactions. D and W/D increase with T_s as it affects the intermolecular interactions. H/D increases but W/D decreases with the hydrophilicity of PBPy and hydrogen bonding that can be promoted by decreasing the solution pH. Additionally, Fe²⁺ ions are added to manipulate the denting degree more effectively than pH, as they can coordinate with BPy moieties and water to form [Fe(BPy)_x(H₂O)_y]²⁺ (0 ≤ x + y ≤ 6). This not only causes additional coordination among polymer chains but also enhances the hydrophilicity of PBPy and hydrogen bonding interactions. Overall, this study provides fresh insights for preparing nanobowls with a controlled size and denting degree.