Evaporation Induced Self-Assembly of Megamolecular Polysaccharide through Planar and Linear Air-LC Interfaces
Authors: Joshi Gargi , Okeyoshi Kosuke , Okajima Maiko K , Kaneko Tatsuo 
Published: February 2020

[Introduction]

Liquid crystalline polymers like cellulose, chitin, DNA and collagen self-assemble in living organisms to provide a variety of functions. Deriving inspiration and biomimicking their structural hierarchy has led researchers to explore a number of fields especially evaporation based self-assembly of particles. One of these is the use of air-liquid crystalline (LC) interface for organization of polymer LC domains into highly ordered films. However, drying induced deposition in a restricted geometry has never been considered before. Understanding this regulation is crucial to further applications in advanced biomedical materials, such as selective semipermeable membranes, drug delivery systems, and bioactuators. We have researched the cyanobacterial exopolysaccharide, sacran.1 The LC unit of sacran has been observed to be huge rods, 1-2 μm in diameter with length greater than 20 μm. With a Mw ∼ 107 gmol-1 and critical LC concentration ∼ 0.2 wt%, it has the ability to form anisotropically swelling hydrogels.2
Fig. A shows the method to control the evaporation front for deposition of the polymer, by varying the width of the container. It was shown that drying 0.5 wt% solution through a planar evaporation front led to hierarchical assembly from nano and submicron range ultimately resulting in a dried layered structure (Fig. B) demonstrating uniaxial swelling.3 Additionally, it has been observed that drying the solution in a container with thickness less than capillary length leads to nucleation and deposition of micro-rods to form a vertical membrane which swells along its length. Next, the meniscus curve was analyzed theoretically so as to clarify the rationale behind splitting of the interface in a linear front followed by vertical deposition.4

[Result & Discussion]

Drying the solution in a confined space with narrow gap (Δy0
> Lc
) is followed by formation of a dense layer at the air-LC interface. This interface restricts further evaporation creating instability in the system leading to splitting of the meniscus. The standard equation of meniscus was extended to validate two menisci and an equation was derived to calculate the approximate value of lengths. Equation for the meniscus curvature, z = z0.e(±κx)
; where κ-1
= capillary length.
Expanding, z(x) = A[e(x-Δx0/2)] + e(-(x+Δx0/2))]

Now, total length of the curve l,

dl = [(dz)2 + (dx)2]1/2

Thus, putting the value of dz and then integrating, the equation to estimate dl was derived. Calculating the values of meniscus length at various nucleation positions, a plot was prepared and meniscus lengths for a single meniscus and a splitted (double) meniscus were compared. A splitted interface with two menisci, imposes twice as much area for evaporation as an interface of single meniscus. Furthermore, it was found that it is not dependent up on the point of nucleation. This splitted interface allows water to evaporate efficiently.

[Conclusion]

We have studied the evaporation induced self-assembly behavior of a polysaccharide solution through planar as well as linear evaporative fronts. The planar front leads to deposition of layered, in-plane oriented film. Whereas, a linear front leads to nucleation followed by a vertically deposited membrane formed by the splitting of the interface. This splitted interface actually is capable of providing approximately twice the surface area as compared to the normal one. We foresee that optimizing drying conditions and concentration, other polymeric solutions can also split meniscus and form highly oriented vertical deposition.5

[References]

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G. Joshi; K. Okeyoshi; M. K. Okajima; T. Kaneko Soft Matter
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K. Okeyoshi; G. Joshi; T. Kaneko et. al.
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K. Okeyoshi; G. Joshi; M. K. Okajima; T. Kaneko Adv. Mater. Inter.
2018, 3, 1870013.

G. Joshi et al., in preparation.

Figure 1