Letter pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2019, 10, 278−282
Boundary Geometry Effects on the Coalescence of Liquid Crystalline Tactoids and Formation of Topological Defects Orla O’Keeffe,†,§ Pei-Xi Wang,†,§ Wadood Y. Hamad,‡ and Mark J. MacLachlan*,† †
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1 FPInnovations, 2665 East Mall, Vancouver, British Columbia, Canada V6T 1Z4
‡
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S Supporting Information *
ABSTRACT: In many lyotropic liquid crystals, the evolution of macroscopic anisotropic phases is mediated by tactoids, which are discrete ordered microdroplets existing in continuous disordered phases. Here we report the effects of boundary conditions on the movement and transformation of liquid crystalline tactoids of cellulose nanocrystals (CNCs) in nonspherical droplets. Using an in situ photopolymerization method, we obtained three-dimensional views of the initial emergence and expansion of macroscopic ordered phases. These processes, as well as the evolution of topological defects, were significantly influenced by the boundary geometry (or Marangoni flows and pinning effects) of the droplets. This phenomenon helps explain the influence of the substrate on the photonic properties of chiral nematic films of CNCs and may also provide new insights into the self-assembly process in lyotropic liquid crystals.
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efforts have been made to control the behavior of tactoids in CNC dispersions.18,19,33 However, tactoids are difficult to study because of the unstable fluid states of these ordered microdroplets. To address this problem, we developed an in situ photopolymerization method to capture and solidify liquid crystalline tactoids in cross-linked polymer matrices,34−36 providing high structural stability for cross-sectional optical and electron microscopy observations. This method is less restricted by the transparency or thickness of soft matter systems, which may severely limit the performance of confocal microscopy generally used for three-dimensional imaging. In this paper, we describe the influence of boundary conditions on the evolution of liquid crystalline tactoids as well as the emergence of macroscopic ordered phases and topological defects. We tracked the phase transitions in nonspherical liquid crystal droplets from cross-sectional views. In oblate spheroidal droplets having obtuse contact angles, the continuous ordered phase first appears beneath the upper liquid−air interface at the horizontal edge of the droplet and then vertically spreads along the side face to the bottom, forming topological defects near the edge (Figure 1a−c). However, with an acute contact angle, the long-range ordered phase initially appears at the bottom center of the droplet and then horizontally spreads from the center to the edge without forming topological defects, which is opposite to that in oblate spheroidal droplets (Figure 1d−f). This phenomenon might be caused by Marangoni flows, pinning effects, or other factors.
ellulose nanocrystals (CNCs) are rod-shaped nanoparticles (lengths of 100−400 nm and widths of 10−30 nm) obtained by hydrolysis of cellulose in sulfuric acid.1 In aqueous dispersions, CNCs can self-assemble into left-handed chiral nematic (cholesteric) liquid crystals, and the helical organization of CNCs could be preserved in solid films after drying,2−6 which have unique optical properties arising from the chiral nematic order. The structural and liquid crystalline properties of CNCs as well as their transfer to solid-state materials have been studied extensively.7−21 The formation of liquid crystalline phases in CNC dispersions is mediated by tactoids, which appear as spherical or ellipsoidal birefringent microdroplets with periodic layers that are 10−15 μm apart (Figure S1). Tactoids are discrete anisotropic microdroplets that exist in continuous isotropic phases, and they have been observed since the 1920s in many lyotropic liquid crystals, such as vanadium pentoxide,22−24 tobacco mosaic virus,25 polypeptides,26 chromonics,27,28 carbon nanotubes,29 and chitin.30 In these systems, phase transitions are mediated by the tactoids, which emerge from isotropic phases, grow larger by coalescence, and eventually fuse together to form macroscopic anisotropic phases.31 The transformation of tactoids kinetically determines the structure of long-range ordered phases in these lyotropic liquid crystals, and this process is usually accompanied by the generation of topological defects (i.e., microdomains where the continuity of the liquid crystalline ordering breaks down) due to the random orientation of different tactoids during coalescence. As topological defects directly impair the optical properties (e.g., their iridescent colors and selective reflection of circularly polarized light) of chiral nematic films formed by CNCs,32 © XXXX American Chemical Society
Received: December 13, 2018 Accepted: January 2, 2019
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DOI: 10.1021/acs.jpclett.8b03733 J. Phys. Chem. Lett. 2019, 10, 278−282
Letter
The Journal of Physical Chemistry Letters
Figure 1. Sketches showing the evolution of discrete liquid crystalline tactoids and continuous chiral nematic phases in the droplets of aqueous CNC dispersions on (a−c) hydrophobic PTFE and (d−f) hydrophilic glass substrates. As shown in panel c, when a hydrophobic PTFE substrate is used, the growth of chiral nematic layers along the curved droplet boundary will lead to the formation of disclinations (i.e., folded chiral nematic layers).
Figure 2. (a−d) Cross-sectional POM micrographs showing the growth of continuous chiral nematic phases (bright birefringent regions) in CNC droplets (100 μL in volume) on PTFE substrates. These images were taken from vertical slices of the droplets after being captured by in situ photopolymerization, and the imaging areas were near the edges of the droplets. In each of the images, the curved upper boundary was the liquid− air interface of the droplet, while the horizontal bottom boundary was the liquid−PTFE interface. The abnormal shape shown in panel d may be caused by the inhomogeneous polymerization due to the refraction of ultraviolet light in the thick liquid crystalline phase. Standing times were 1, 3, 6, and 24 h for panels a−d, respectively. (e−h) Cross-sectional SEM micrographs showing the edge region of a CNC droplet deposited on a PTFE substrate (volume of 100 μL, standing time of 8 h). Microstructures of its lower (f) and upper (g and h) boundaries reveal parallel (planar) anchoring of CNCs at the liquid−air interface. The horizontal bottom boundary in panel e was the liquid−PTFE interface. (i−l) SEM images showing the edge region of a CNC droplet on a PTFE substrate (volume of 100 μL, standing time of 24 h). The chiral nematic layers are aligned parallel to the upper liquid−air interface (j) and the bottom liquid−PTFE interface (k and l). Scale bars are (a−d) 500 μm, (e) 50 μm, (f) 20 μm, (g) 2 μm, (h) 1 μm, (i and j) 50 μm, (k) 25 μm, and (l) 10 μm.
(PAAm) precursors, i.e., acrylamide (0.6 g, monomer), N,N′methylenebis(acrylamide) (60 mg, cross-linker), and 2hydroxy-4′-(2-hydroxyethoxy)-2-methyl-propiophenone (3 mg, photoinitiator), to give a homogeneous mixture. This mixture was stored in the dark at 4 °C to protect the precursors until further experiments were conducted.
These observations reveal an early stage of the evolution of liquid crystalline phases and topological defects in nonspherical geometrical confinement and might help explain the effects of the substrate on the structure of liquid crystal films.37 In a typical experiment, an aqueous dispersion of CNCs (4.0 mL, 4.0 wt %, pH 2.4) was combined with polyacrylamide 279
DOI: 10.1021/acs.jpclett.8b03733 J. Phys. Chem. Lett. 2019, 10, 278−282
Letter
The Journal of Physical Chemistry Letters
Figure 3. (a−d) Cross-sectional POM micrographs showing the growth of continuous chiral nematic phases in CNC droplets (volume of 100 μL) on hydrophilic glass substrates (in each of the images, the bottom boundary was the liquid−glass interface). The curvature of the samples may result from the mechanical inhomogeneity of the polymer network, which might be caused by the variation of ultraviolet light intensity at different regions during photopolymerization. Standing times were 1, 3, 6, and 24 h for panels a−d, respectively. (e−h) Cross-sectional SEM micrographs showing the near-edge (e and g) and central (f and h) regions of a CNC droplet deposited on a glass substrate (volume of 100 μL, standing time of 24 h), where the bottom boundary was the liquid−glass interface. The thickness of the continuous liquid crystalline phase and the number of chiral nematic layers increase from the edge (g) to the center (h). Scale bars are (a−d) 500 μm, (e and f) 100 μm, (g) 50 μm, and (h) 25 μm.
nearly oblate spheroidal geometries with curved edges due to the obtuse contact angle (∼100°), the macroscopic liquid crystalline phase initially appears beneath the upper liquid−air interface near the edge of the droplet (Figure 2a) and then spreads along the boundary both horizontally to the center of the upper interface (Figure 2b,c) and vertically to the bottom of the edge (Figure S2). Eventually, the edge region becomes filled with curved and folded chiral nematic layers while leaving a continuous isotropic phase with discrete liquid crystalline tactoids in the center of the droplet (Figure 2d). We further examined the cross sections of the droplets by SEM, which provides more insights into the evolution of longrange liquid crystalline phases. In a droplet that was captured by in situ photopolymerization after ∼8 h, SEM micrographs reveal that the tactoids preferentially moved to the horizontal edge of the droplet and coalesced into a continuous ordered phase (Figure 2e). As indicated by POM observations, the chiral nematic layers of this phase were more or less curved along the boundary of the droplet. This phenomenon is possibly caused by the planar anchoring of CNCs near the boundary, as confirmed by high-resolution SEM images (Figure 2f−h). SEM observations also revealed the microstructures of the topological defects near the edge of the oblate spheroidal droplets. In a sample that was captured after standing for 24 h, the edge region was completely filled by continuous chiral nematic layers (Figure 2i). In addition, the layers are first oriented along the upper boundary (Figure 2j) because of the parallel anchoring at the interfaces. They are then nearly vertically aligned at the side interface (Figure 2k) and dramatically turn horizontal before reaching the bottom boundary (Figure 2l). The parallel anchoring and curved geometry of the droplet boundary cooperatively lead to the formation of topological defects (disclinations) and stacks of curved chiral nematic layers.
Liquid crystal droplets (∼0.10 mL) were prepared by dropping the dispersion described above on polytetrafluoroethylene (PTFE) or glass substrates, where the contact angle is ∼100° or ∼40°, respectively. After the droplet-covered substrates were kept in sealed containers (to inhibit evaporation) in the dark for a period of time (1, 3, 6, and 24 h), ultraviolet irradiation (300 nm, 8 W) was applied for 30 min to initiate polymerization in the droplets. Cross-linked polymer networks rapidly formed to capture and solidify tactoids and continuous ordered phases, enabling observations by cross-sectional polarized optical microscopy (POM) and scanning electron microscopy (SEM). Previously, it was demonstrated that the structure and photonic properties of CNC films are strongly dependent on the substrate upon which they form. Films developed on hydrophobic PTFE substrates have significantly shorter helical pitches and blue-shifted birefringence compared to the features of those formed on hydrophilic substrates.37 To investigate the origin of the surface effects, we examined the evolution of lyotropic liquid crystalline phases in aqueous droplets of CNCs on different substrates, e.g., PTFE and glass, by both optical and electron microscopy, and observed that the boundary geometry of the droplets has a crucial influence on the movement and transformation of tactoids. To study the evolution of the liquid crystalline phase of CNCs on the PTFE and glass substrates, droplets of aqueous CNCs were combined with monomer, a cross-linking agent, and a photoinitiator. Previous studies have demonstrated that the polymerization of these monomers combined with CNCs gives a cast of the liquid crystalline phase without much distortion. Rapid in situ photopolymerization of the droplets enabled capture of the liquid crystalline tactoids and other ordered microstructures within cross-linked polymer networks. These polymer casts of the droplets could then be vertically sliced to give cross-sectional views of the original CNC droplets. On PTFE substrates, where the CNC droplets have 280
DOI: 10.1021/acs.jpclett.8b03733 J. Phys. Chem. Lett. 2019, 10, 278−282
The Journal of Physical Chemistry Letters
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When using hydrophilic glass substrates (contact angle of
