Immobilization of polymer-decorated liquid crystal droplets on chemically tailored surfaces

Abstract

We demonstrate that the assembly of an amphiphilic polyamine on the interfaces of micrometer-sized droplets of a thermotropic liquid crystal (LC) dispersed in aqueous solutions can be used to facilitate the immobilization of LC droplets on chemically functionalized surfaces. Polymer 1 was designed to contain both hydrophobic (alkyl-functionalized) and hydrophilic (primary and tertiary amine-functionalized) side chain functionality. The assembly of this polymer at the interfaces of aqueous dispersions of LC droplets was achieved by the spontaneous adsorption of polymer from aqueous solution. Polymer adsorption triggered transitions in the orientational ordering of the LCs, as observed by polarized light and bright-field microscopy. We demonstrate that the presence of polymer 1 on the interfaces of these droplets can be exploited to immobilize LC droplets on planar solid surfaces through covalent bond formation (e.g., for surfaces coated with polymer multilayers containing reactive azlactone functionality) or through electrostatic interactions (e.g., for surfaces coated with multilayers containing hydrolyzed azlactone functionality). The characterization of immobilized LC droplets by polarized, fluorescence, and laser scanning confocal microscopy revealed the general spherical shape of the polymer-coated LC droplets to be maintained after immobilization, and that immobilization led to additional ordering transitions within the droplets that were dependent on the nature of the surfaces with which they were in contact. Polymer 1-functionalized LC droplets were not immobilized on polymer multilayers treated with poly(ethylene imine) (PEI). We demonstrate that the ability to design surfaces that promote or prevent the immobilization of polymer-functionalized LC droplets can be exploited to pattern the immobilization of LC droplets on surfaces. The results of this investigation provide the basis of an approach that could be used to tailor the properties of dispersed LC emulsions and to immobilize these droplets on functional surfaces of interest in a broad range of fundamental and applied contexts.

Polymer 1

Figure 1

A-C) Bright-field and D-F) polarized light micrographs of dispersed 5CB droplets incubated in A, D) a buffer solution (10 mM HEPES, pH 7); B, E) a solution of polymer 1 at 0.1 mg/mL; and C, F) a solution of polymer 1 at 1.0 mg/mL. The 5CB emulsions were under continuous agitation for 2 h. G-I) Schematic illustrations of the director profiles for G) bipolar, H) preradial, and I) radial configurations. Point defects in the 5CB droplets shown in bright-field images are indicated by the white arrows. Scale bar = 3 μm.

Figure 2

A) Polarized light and B) fluorescence micrographs of a 5CB droplet dispersed in a mixture of polymer 1 and polymer 1_TMR (4:1, 0.1 mg/mL) for 2 h. Excess polymer was removed from the bulk aqueous solution prior to imaging. Scale bar = 3 μm.

Figure 3

Schematic illustrations representing multilayered films A) presenting azlactone functionality (surface 1), G) terminated with a layer of BPEI (surface 2) and, J) treated to hydrolyze the azlactone functionality (surface 3). Polymer 1-coated LC droplets were dispersed onto the surface of these multilayered films: (B-D) surface 1, (H, I) surface 2, and (K-M) surface 3. Images in E and F were collected using uncoated LC droplets placed on surface 1. The droplets were given time to sediment to the surface of these films (B, E, H, and K) and then rinsed with fresh buffer (C, F, I, L, see text). In an additional step, droplets were rinsed with buffer containing 1.5 M NaCl (D, M). Scale bar = 15 μm.

Figure 4

A) Bright-field micrograph of a polymer 1-laden 5CB droplet freely moving above a multilayer film presenting azlactone functionality (surface 1). B) Bright-field micrograph of the same 5CB droplet after contacting the surface of the film (immobile). Scale bar = 5 μm.

Figure 5

(A, B, E, F) Bright-field and (C, D, G, H) polarized light micrographs of polymer 1-laden 5CB droplets immobilized on (A-D) surface 1 and (E-H) surface 3. The images were captured with the focal plane of the microscope positioned at the middle of the droplets (A, C, E, G) or positioned at the apex of the droplets (B, D, F, H; see text). Scale bar = 3 μm.

Figure 6

Confocal fluorescence micrographs of a polymer-laden LC droplet immobilized on surface 1 captured in the (A, B) x-y plane (bottom-up view) or the (C, D) x-z plane (side-on view). Images were captured by collecting the red channel (A, C; polymer 1_TMR adsorbed to the surface of the LC droplets) or the green channel (B, D; FITC-dextran dispersed in the aqueous solution). Scale bar = 5 μm.

Figure 7

Polarized light micrograph of polymer 1-coated 5CB droplets on a multilayer film terminated with a layer of BPEI (i.e., surface 2) and patterned with circular region of PVDMA. The dashed boxes in A) and B) correspond to magnified regions shown in B) and C), respectively. See text for additional details. Scale bars = A) 400 μm, B) 100 μm, and C) 20 μm.