Water-Assisted Microphase Separation of Cationic Random Copolymers into Sub-5 nm Lamellar Materials and Thin Films

Abstract

Microphase separation of copolymers is a key technique to produce polymer bulk materials or thin films with ordered nanostructures for applications in various research fields including nanotechnologies, electronic devices, among many others. Herein, we report water-assisted microphase separation of amphiphilic random copolymers bearing quaternary ammonium cations and hydrophobic alkyl or oleyl groups in the solid state and the thin films. We investigated the effects of sample preparation protocols and the hydrophobic pendants (a butyl group: C4 − octadecyl or oleyl group: C18), composition, and molecular weight of the copolymers on the microphase separation behavior. By annealing under humid conditions, the copolymers bearing alkyl groups longer than an octyl group (C8) formed sub-5 nm lamellar structures comprising cationic layers and hydrophobic layers. Water hardly remained in the resulting lamellar materials under ambient conditions. The domain spacing was controlled between 3.7 and 5.3 nm by tuning the length of the hydrophobic pendants and composition and was independent of the molecular weight and molecular weight distribution. The cationic random copolymers carrying amorphous hydrophobic pendants provided transparent or translucent polymer materials containing small lamellar structures. The random copolymers further formed multilayered lamellar thin films on silicon substrates by spin-coating the copolymer solutions, followed by a humid annealing process. The layered lamellae were directly observed as terrace structures with about 4–5 nm steps by atomic force microscopy.

Microphase Separation of Cationic Homopolymers Bearing Alkyl Ammonium Salts into Sub-4 nm Lamellar Materials with Water Intercalation Channels

“…32 As shown in Figure 3a, T m of P3−P7 increased with increasing the carbon number of the pendant R units (Scheme 1) and was close to that the corresponding random copolymers. 32 In contrast, the T g of P1−P3, and P5 was approximately 30−40 °C higher than that of cationic random copolymers bearing octyl or dodecyl groups. This is probably because the homopolymers have cationic side chains denser than the random copolymers, and this factor strengthens the ionic interaction of their polymer chains and suppresses the mobility of polymer chains.…”

“…One is microphase separation of high χ–low N linear block copolymers, − where χ is the Flory–Huggins interaction parameter and N is degree of polymerization. Another is microphase separation of the side or graft chains of polymers including (1) random or alternating copolymers, − (2) double brush (Janus graft) (co)­polymers, − (3) brush polymers bearing block side chains, , (4) periodic graft copolymers, − and (5) homopolymers containing polar functional groups. − In the latter approaches, the domain spacing of the phase-separated structures is often controlled by tuning the copolymer composition or side chain structures. Therefore, common (co)­polymers with broad molecular weight distribution, typically prepared by free radical polymerization, can also be used for the construction of well-defined nanostructures, although the former approach requires block copolymers with narrow molecular weight distribution for controlling the domain structures and spacing.…”

“…For example, amphiphilic random copolymers bearing hydrophilic poly­(ethylene glycol) (PEG) − or quaternary ammonium cations and hydrophobic alkyl or oleyl groups as pendants form sub-10 nm lamellar structures via the crystallization or association of their side chains in the solid state and thin films. The domain spacing of the lamellar structures is controllable at the 0.1 nm level in the range between 3 and 10 nm by tuning the side chains and composition of the copolymers; the domain spacing is independent of the molecular weight and molecular weight distribution.…”

“…Among them, cationic random copolymers are found to show unique and attractive features in microphase separation (Figure b): (1) owing to the efficient segregation of the cationic units to hydrophobic segments, the cationic copolymers bearing amorphous hydrophobic pendants such as dodecyl or oleyl groups induce microphase separation to provide relatively flexible and self-standing bulk/film materials containing sub-10 nm lamellar structures. (2) Annealing the copolymers under humid conditions is effective for the formation of the lamellar structures. ,,− In this process, water molecules would be absorbed into the cationic segments to promote the microphase separation, whereas the final products just contained trace water, which is inevitably included under ambient air conditions. These results imply that the cationic (co)­polymers are not only effective for the formation of sub-10 nm microphase separation structures but also capable of the intercalation of water into the lamellar structures .…”

“…The domain spacing of the lamellar structures is controllable at the 0.1 nm level in the range between 3 and 10 nm by tuning the side chains and composition of the copolymers; the domain spacing is independent of the molecular weight and molecular weight distribution. Among them, cationic random copolymers are found to show unique and attractive features in microphase separation (Figure b): (1) owing to the efficient segregation of the cationic units to hydrophobic segments, the cationic copolymers bearing amorphous hydrophobic pendants such as dodecyl or oleyl groups induce microphase separation to provide relatively flexible and self-standing bulk/film materials containing sub-10 nm lamellar structures. (2) Annealing the copolymers under humid conditions is effective for the formation of the lamellar structures. ,,− In this process, water molecules would be absorbed into the cationic segments to promote the microphase separation, whereas the final products just contained trace water, which is inevitably included under ambient air conditions.…”