Block Copolymers: Synthesis, Self-Assembly, and Applications

Abstract

Research on block copolymers (BCPs) has played a critical role in the development of polymer chemistry, with numerous pivotal contributions that have advanced our ability to prepare, characterize, theoretically model, and technologically exploit this class of materials in a myriad of ways in the fields of chemistry, physics, material sciences, and biological and medical sciences. The breathtaking progress has been driven by the advancement in experimental techniques enabling the synthesis and characterization of a wide range of block copolymers with tailored composition, architectures, and properties. In this review, we briefly discussed the recent progress in BCP synthesis, followed by a discussion of the fundamentals of self-assembly of BCPs along with their applications.

Keywords: applications; block copolymers; self-assembly; synthesis.

Figure 1

Representative architectures of linear block terpolymers, “comb” graft polymers, miktoarm star terpolymers, and cyclic block terpolymers.

Figure 2

The number of publications with block copolymer as topic against year. The data were obtained from Web of Science (2017 Clarivate Analytics).

Scheme 1

Synthesis of the poly(N-isopropyl acrylamide-b-2-methacryloyloxyethylphosphorylcholine-b-N-isopropyl acrylamide) (PNIPAM-b-PMPC-b-PNIPAM) triblock copolymers via atom transfer racial polymerization (ATRP) using a bifunctional ATRP initiator.

Scheme 2

Synthesis of polyallene-based triblock copolymer using conventional free radical and ATRP. Reprinted from Reference [30]. (Copyright (2017) Nature Publishing Group).

Scheme 3

Synthetic scheme of PI-b-PtBuA and PS-b-PI block copolymers using a modified nitroxide-mediated polymerization (NMP) initiator.

Scheme 4

(A) Synthesis of PS-b-PI-b-P2VP-b-PtBA-b-PEO pentablock copolymers via sequential living anionic polymerization (LAP). (B) Synthesis of PDMS-b-PtBA diblock copolymers.

Figure 3

Illustration of complex architectures using living anionic polymerization and coupling chemistry. Adapted from Reference [55]. (Copyright (2017) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

Scheme 5

One pot polymerization of PMMA-b-PCL using combination of metal-free ATRP and ring opening polymerization (ROP) simultaneously under sunlight. Reprinted from Reference [56]. (Copyright (2017) Royal Society of Chemisty).

Figure 4

(a) Preparation of PMMA-b-PEG block copolymer via ATRP and azide-alkyne click reaction. (b) Size exclusion chromatography (SEC) curves of PMMA, polyethylene glycol (PEG), and PMMA-b-PEG diblock copolymer. Reprinted from Reference [57]. (Copyright (2005) Royal Society of Chemistry).

Figure 5

Synthesis of cyclic PS-b-PEO copolymer. Reprinted from Reference [11]. (Copyright (2012) American Chemical Society).

Figure 6

Synthetic strategies to block copolymers (BCPs) using thiol-based coupling reactions. Reprinted from Reference [59]. (Copyright (2014) Royal Society of Chemistry).

Scheme 6

Formation of block copolymers via a reversible addition-fragmentation chain transfer (RAFT) polymerization and a hetero Diels-Alder reaction. Adapted from Reference [60]. (Copyright (2008) American Chemical Society).

Figure 7

(a) Equilibrium morphologies of AB diblock copolymers in bulk: S and S’ = body-centered-cubic spheres, C and C’ = hexagonally packed cylinders, G and G’ = bicontinuous gyroids, and L = lamellae. (b) Theoretical phase diagram of AB diblocks predicted by the self-consistent mean-field theory, depending on volume fraction (f) of the blocks and the segregation parameter, χN; CPS and CPS’ = closely packed spheres. (c) Experimental phase diagram of polystyrene-b-polyisoprene copolymers, in which f_A represents the volume fraction of polyisoprene, PL = perforated lamellae. Reproduced from Reference [69]. (Copyright (2012) Royal Society of Chemistry).

Figure 8

TEM and small-angle X-ray scattering (SAXS) images of sPS-b-fPI. (A) ordered hexagonal structures without annealing; (B) less ordered structures of sample annealed at 120 °C; (C) SAXS of the corresponding samples in (A,B). Reprinted from Reference [72]. (Copyright (2010) Royal Society of Chemistry).

Figure 9

Gyroid-forming PI-b-PS-b-PEO block copolymers and the preparation process of NbN superconductors. Reprinted with permission from Reference [75]. (Copyright (2016) the authors).

Figure 10

The types of formed nanostructures of amphiphilic diblock copolymers due to the inherent curvature of the polymer, as estimated by chain packing parameter, p. Reprinted from Reference [80]. (Copyright (2009) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

Figure 11

TEM images and corresponding schematic diagrams of various morphologies formed by amphiphilic PS_m-b-PAA_n copolymers( m and n denote the degrees of polymerization of PS and PAA, respectively): (a) spherical micelles; (b) rods; (c) bicontinuous rods; (d) small lamellae; (e) large lamellae; (f) vesicles; (g) hexagonally packed hollow hoops (HHHs); (h) large compound micelles (LCMs). Reprinted from Reference [69]. (Copyright (2012) Royal Society of Chemistry).

Figure 12

The schematic representation of the micelles from polyferrocenylsilane (PFS)—containing BCPs via epitaxial growth. Reprinted from Reference [82]. (Copyright (2007) American Association for the Advancement of Science).

Figure 13

The schematic illustration of linear amphiphilic PBA-b-PEO-b-PBA and cyclic PBA-b-PEO-b-PBA self-assembly in aqueous media. The cyclic BCP shows an increased T_c. Reprinted from Reference [83]. (Copyright (2010) American Chemical Society).

Figure 14

Morphology diagram for PB-PEO in water (1 wt %) as a function of molecular size and composition, where N_PB and W_PEO are the degree of polymerization and weight fraction of the PB and PEO blocks, respectively. Reprinted from Reference [84]. (Copyright (2003) American Association for the Advancement of Science).

Figure 15

The structure of poly(benzofulvene (BF)-b-isoprene-b-benzofulvene) (FIF) block copolymer and mechanical analysis. Reprinted from Reference [92]. (Copyright (2016) American Chemical Society).

Figure 16

(A) Stress–strain behavior of tetrafunctional multigraft copolymers compared to commercial thermoplastic elastomers. Reprinted from Reference [96]. (Copyright (2006) American Chemical Society). (B) Hysteresis curve of a tetrafunctional multigraft copolymer with 14 vol % PS and 5.5 branching points. Reprinted from Reference [97]. (Copyright (2006) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

Figure 17

Surface force microscopy (SFM) phase image of (A) cylinder-forming PS-b-PMMA and (B) lamella-forming PS-b-PMMA on the substrate modified by R64 (PS mole fraction of 0.64) and R55 (PS mole fraction of 0.55), respectively, at various film thicknesses of block copolymer after thermally annealing thin films at 170 °C for 24 h. Reprinted from Reference [119]. (Copyright (2008) American Chemical Society).

Figure 18

Atomic force microscopy (AFM) height images of sawtooth patterns and phase images of solvent-annealed PS-b-PEO thin films. (A,D) When the M-plane sapphire was annealed at 1400 °C and 1500 °C, a pitch of ~48 and ~24 nm and a peak-to-valley depth of ~6 and ~3 nm were obtained, respectively. Highly ordered PEO cylindrical microdomains having areal densities of 0.74 to 10.5 terabit/inch2 from PS-b-PEO (M_n = 26.5 kg/mol) (B), PS-b-PEO (M_n = 25.4 kg/mol) (C), PS-b-PEO (M_n = 21.0 kg/mol) (E), and PS-b-PEO (M_n = 7.0 kg/mol) (F) BCP thin films annealed in o-xylene vapor were obtained. Scale bars, 100 nm. Reprinted from Reference [123]. (Copyright (2009) American Association for the Advancement of Science).

Figure 19

Atomic force microscopy height images for (a) cyclic PS13K-b-PEO5K and (b) PS13K-b-PEO5K diblock copolymers (inlet: 2D fast Fourier transform (FFT) of AFM images). Scale bars are 250 nm. Reprinted from Reference [11]. (Copyright (2012) American Chemical Society).

Figure 20

(A) Formation mechanism of ordered mesoporous silica through the solvent evaporation -induced aggregating assembly (EIAA) process by using diblock copolymer PEO-b-PMMA as a template, tetraethylorthosilicate (TEOS) as the silica source, and acidic tetrahydrofuran (THF)/H₂O mixture as the solvent; (B) Typical field-emission scanning electron microscopy (FESEM) image of mesoporous silica calcinated in air at 550 °C. Reprinted from Reference [150]. (Copyright (2011) American Chemical Society).

Figure 21

Synthesis of nitrogen-doped mesoporous carbon spheres (NMCS) with extra-large pores through the assembly of diblock copolymer micelles. Reprinted from Reference [152]. (Copyright (2015) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

Figure 22

Ternary diagram mapping out the morphologies found for various composites directed by PI-b-PEO using (3-glycidyloxypropyl)trimethoxysilane (GLYMO) and aluminum sec-butoxide as inorganic precursors. At the bottom of the diagram, schematics of the morphologies found for the pure PI-b-PEO are shown. Hatched areas along the PI-b-PEO axis indicate areas where no data was available from the diblock copolymer diagram. Each white region within the diagram is labeled with a schematic representing the morphology of the composites formed. The yellow (light) regions in these schematic morphologies on the right and left are a representation of the PEO/inorganic domains. Closed dark points on a gray background indicate areas where biphasic behavior is observed. Reprinted from Reference [161]. (Copyright (2009) American Chemical Society).