Getting into Shape: Reflections on a New Generation of Cylindrical Nanostructures' Self-Assembly Using Polymer Building Blocks

Abstract

Cylinders are fascinating structures with uniquely high surface area, internal volume, and rigidity. On the nanoscale, a broad range of applications have demonstrated advantageous behavior of cylindrical micelles or bottlebrush polymers over traditional spherical nano-objects. In the past, obtaining pure samples of cylindrical nanostructures using polymer building blocks via conventional self-assembly strategies was challenging. However, in recent years, the development of advanced methods including polymerization-induced self-assembly, crystallization-driven self-assembly, and bottlebrush polymer synthesis has facilitated the easy synthesis of cylindrical nano-objects at industrially relevant scales. In this Perspective, we discuss these techniques in detail, highlighting the advantages and disadvantages of each strategy and considering how the cylindrical nanostructures that are obtained differ in their chemical structure, physical properties, colloidal stability, and reactivity. In addition, we propose future challenges to address in this rapidly expanding field.

Figure 1

TEM or AFM images of typical cylindrical nanostructures obtained using PISA or CDSA, or from bottlebrush polymer synthesis. Reproduced with permission from refs (18) (PISA worms; Copyright 2016 Wiley), (10) (CDSA cylinders; Copyright 2010 Springer Nature), and (66) (bottlebrush polymers; Copyright 2015 Springer Nature).

Figure 2

(A) Synthetic route for the RAFT aqueous dispersion polymerization of HPMA using a water-soluble PGMA₅₆ macroCTA to form PGMA₅₆-b-PHPMA₁₅₅ diblock copolymer worms. (B) Representative TEM image of the PGMA₅₆-b-PHPMA₁₅₅ diblock copolymer worms after drying a dilute aqueous dispersion at 20 °C. (C) The worms, in combination with PVA, exhibit enhanced cryoprotective behavior relative to controls, which was attributed to the inhibition of ice crystal formation in the presence of the worm micelles. Reproduced with permission from ref (18). Copyright 2016 Wiley.

Figure 3

Schematic illustrating the seeded growth process of PDHF₁₄-b-PEG₂₂₇. These nanofibers exhibit exciton transfer from the core to the lower-energy polythiophene coronas in the end blocks, which occurs in the direction of the interchain π–π stacking with very long diffusion lengths (>200 nm) and a large diffusion coefficient (0.5 cm²/s). Reproduced with permission from ref (24). Copyright 2018 AAAS.

Figure 4

PISA is conducted via chain extension of a soluble macroCTA with a monomer that produces an insoluble polymer. Pure cylindrical micelle morphologies are obtained at a given weight fraction of the hydrophobic and hydrophilic blocks at a certain concentration. In this example, photo-PISA was conducted in the presence of GOx to remove O₂ to prepare cylindrical micelles. Reproduced with permission from ref (26). Copyright 2017 American Chemical Society.

Figure 5

(A) CDSA facilitates controlled epitaxial growth of 1D cylinders using a “seeded-growth” protocol. Reproduced with permission from ref (40). Copyright 2016 Springer Nature. (B) Crystallization-driven epitaxial growth of PCL cylinders. Scale bars = 1000 nm. Reproduced with permission from ref (19). Copyright 2017 American Chemical Society. (C) Controlled growth of cylindrical micelles with nP3HT cores. Scale bars = 200 nm. Reproduced with permission from ref (34). Copyright 2011 American Chemical Society. The plots in B and C show the dependence of the average length of the cylinders on the ratio of block copolymer unimers which had been added to seed micelles during the preparation procedure.

Figure 6

Four routes to prepare bottlebrush polymers: (A) bottlebrush synthesis by grafting functionalized polymers to a functional backbone (grafting-to); (B) grafting-from, involving the polymerization of the side chains from a polymeric initiator/CTA; (C) transfer-to, similar to grafting-from but differing in the attachment and behavior of the CTA moieties; and (D) preparation of bottlebrush polymers via polymerization of macromonomers in the grafting-through method.

Figure 7

Features of bottlebrush polymers. (A) AFM images of bottlebrush polymers with poly(n-butyl acrylate) side chains. The rigidity of the bottlebrush polymer increases with increasing side-chain length. Reproduced with permission from ref (66). Copyright 2015 Springer Nature. (B) Dependence of bottlebrush polymer structure on side-chain grafting density (Z) and the degree of polymerization (N_sc). Reproduced with permission from ref (59). Copyright 2016 AAAS. (C) Dependence of bottlebrush polymer structure on the length of the polymer backbone. The inflection in the plot of maximum dimension as a function of backbone length indicates a transition from globular to cylindrical structures. Reproduced with permission from ref (58). Copyright 2013 American Chemical Society.

Figure 8

Key differences between the cylindrical nanostructures produced by PISA, CDSA, or bottlebrush polymer synthesis. The variable P signifies the persistence length of the cylindrical constructs. Images reprinted with permission from refs (18) (PISA worms; Copyright 2016 Wiley) and (10) (CDSA cylinders; Copyright 2010 Springer Nature).

Figure 9

Internal structure of cylindrical micelles can be semicrystalline, glassy, or amorphous. (A) Pyrene release from glassy micelles occurs around and above the T_g of the core block. Reproduced with permission from ref (62). Copyright 2011 Wiley. (B) TEM confirms the crystallinity of cylindrical micelles produced by CDSA. Reproduced with permission from ref (19). Copyright 2017 American Chemical Society. (C) Unlike cylindrical micelles, bottlebrush polymers most likely do not possess an internal hydrophobic compartment.

Figure 10

Taking advantage of the inherent chain-end reactivity of cylindrical nanostructures. (A) Multiblock cylindrical assemblies prepared via crystallization of each new block from reactive semicrystalline faces at the cylinder ends. Scale bars = 500 nm (TEM) and 2000 nm (LCSM). Reproduced with permission from ref (69). Copyright 2014 Springer Nature. (B) End-to-end self-assembly of bottlebrush polymers occurs through hydrophobic interactions at the bottlebrush chain ends. Scale bar = 100 nm. The inset is from a 500 nm scan. Reproduced with permission from ref (70). Copyright 2011 American Chemical Society.