Self-Assembly of Flexible Linear–Semiflexible Bottlebrush–Flexible Linear Triblock Copolymers

Abstract

Block copolymer (BCP) self-assembly is a fundamental process in which incompatible blocks spontaneously form organized microstructures with broad practical applications. Classical understanding is that the domain spacing is limited by the contour length of the polymer backbone. Here, using a combination of molecular design, chemical synthesis, small-/wide-angle X-ray scattering, transmission electron microscopy, and electron tomography, we discover that this molecular picture does not hold for architecturally semiflexible BCPs. For strongly segregated linear− semiflexible bottlebrush−linear triblock copolymers, the size of the bottlebrush domain can be twice the bottlebrush backbone contour length. The mechanism of such anomalous self-assembly is likely that the interfacial repulsion between the incompatible blocks is large enough to pull a part of the linear end blocks into the bottlebrush domain. This effectively increases the bottlebrush domain size. Moreover, the semiflexible bottlebrush widens the regime for the cylinder morphology that is associated with the volume fraction of the end blocks f C SFB ∈ (0.10, >0.41). This window is much wider than that for flexible linear BCPs, f C F ∈ (0.14, 0.35), and that predicted by the recent self-consistent field theory for linear-bottlebrush BCPs of the same chemistry and molecular architecture. Our experimental findings reveal previously unrecognized mechanisms for the self-assembly of architecturally complex BCPs.

Dynamic Mechanical Properties of Self-Assembled Bottlebrush Polymer Networks

“…The self-assembled microstructure remains to be a hexagonal cylinder up to the fraction of 0.41. 22 measurements. We dissolve the polymer in toluene at a concentration of 100 mg/mL, pipette about 0.5 mL of solution onto the bottom plate of the rheometer, and cover the solution with a glass cap.…”

“…For an ABA triblock copolymer, the probability of forming a loop is proportional to the concentration of A blocks, which is inversely proportional to the volume pervaded by the middle B block. Because all side chains are grafted to a backbone, the size of the semiflexible bottlebrush, 22 R SFB ≈ 13 nm, is much smaller than that of the flexible linear counterpart of the same MW, R FL ≈ bN 1/2 ≈ 34 nm, in which b = 1.3 nm is the size of a PDMS Kuhn monomer and N = 670 is the number of Kuhn monomers for a linear PDMS of 255 kDa. For an LBBL triblock copolymer, therefore, the probability for the two end linear blocks to meet to form loops is much higher than that for conventional linear triblock copolymers.…”

“…Moreover, previous studies showed that, for LBBL polymers, the diameter of a spherical domain, ∼10 nm, is comparable to the size of the bottlebrush. 22 As a result, the semiflexible bottlebrush does not have to bend to have the two end linear blocks connected to the same spherical domain. This process can avoid the entropic penalty associated with constraining polymer configuration, which is often seen in the loop formation for conventional linear triblock copolymers, where the middle block must change from a linear to a ring-like configuration, as illustrated in Figure 4a(ii).…”

“…Recently, we studied the self-assembly of strongly segregated LBBL triblock polymers (Figure 1). 22 In an LBBL polymer, the two end linear blocks are poly(benzyl methacrylate) (PBnMA), whereas the middle block is a bottlebrush formed by polymerizing 51 methyl acrylate-terminated polydimethylsiloxane (MA-PDMS) with a molecular weight (MW) of 5000 g/mol. The polydispersity index of the bottlebrush is relatively narrow of 1.15.…”

“…Using the bottlebrush PDMS (bbPDMS) as a macroinitiator, we synthesize a series of PBnMA-bbPDMS-PBnMA triblock copolymers with the volume fraction f of the end blocks ranging from 0.045 to 0.41 (Table 1 and Supporting Information). 22 Importantly, all triblock copolymers have the same middle block, which is a semiflexible bottlebrush with the backbone contour length L max ≈ 13 nm about twice that of the persistence length. We found that the bottlebrush can rearrange its constituent linear side chains to form domains remarkably larger than the contour length of the bottlebrush backbone.…”