Controlling Rheology of Fluid Interfaces through Microblock Length of Sequence-Controlled Amphiphilic Copolymers

Abstract

Previous studies have demonstrated that films of sequence-controlled amphiphilic copolymers display contact angles that depend on microblock size. This suggests that microblock length may provide a means of tuning surface and interfacial properties. In this work, the interfacial rheology of a series of sequence-controlled copolymers, prepared through the addition of bicyclo[4.2.0]oct-1(8)-ene-8-carboxamide (monomer A) and cyclohexene (monomer B) to generate sequences up to 24 monomeric units composed of (A _m B _n ) _i microblocks, where m, n, and i range from 1 to 6. Interfacial rheometry is used to measure the mechanical properties of an air-water interface with these copolymers. As the microblock size increases, the interfacial storage modulus, G', increases, which may be due to an increase in the size of interfacial hydrophobic domains. Small-angle X-ray scattering shows that the copolymers have a similar conformation in solution, suggesting that any variations in the mechanics of the interface are due to assembly at the interface, and not on solution association or bulk rheological properties. This is the first study demonstrating that microblock size can be used to control interfacial rheology of amphiphilic copolymers. Thus, the results provide a new strategy for controlling the dynamics of fluid interfaces through precision sequence-controlled polymers.

Keywords: amphiphilic copolymers; block copolymers; interfacial rheology; precision copolymers; sequence-controlled copolymers.

Figure 1.

a) Chemical structure of copolymers. b) Illustration of the copolymers. Modified from Li et al.^[3]

Figure 2.

a) Interfacial storage modulus G′ and b) loss modulus G″ at variable frequencies for polymers with varying sequence and microblock lengths. c) An example of G′ and G″ for P3, showing viscoelastic fluid behavior and the crossover frequency.

Figure 3.

Interfacial storage and loss modulus, G′ and G″, for ω = 100 rad s⁻¹.

Figure 4.

Interfacial complex viscosity at variable frequencies for polymers with varying sequence and microblock lengths.

Figure 5.

Brightfield images of a) H1, b) P1, c) P2, d) P3, e) P4, f) P5, g) P6, and h) Pr on the air–water interface.

Figure 6.

Confocal fluorescent images on the polymer films of a) H1, b) P1, c) P2, d) P3, e) P5, f) P6, and g) Pr. Shown are single representative images from z-stack acquisitions. All images were acquired with identical microscope exposure settings and displayed with identical brightness/contrast settings.

Figure 7.

Brightfield images of the polymer films of a) H1, b) P1, c) P2, d) P3, e) P5, (f) P6, and g) Pr corresponding to the confocal fluorescent images displayed in Figure 6.

Figure 8.

SAXS curve plotted for varied 1% w/v copolymers in THF. a) The SAXS pattern fits of the sphere model (except for P2). b) The SAXS pattern fits of the polymer excluded volume model (except for P2). Data are vertically shifted for clarity.