Ordered Teflon Thin-Films for Columnar Liquid Crystal AlignmentMadeline Van Winkle, Joseph J. Reczek

Denison University, Department of Chemistry and Biochemistry, Granville, OH 43023

Experimental Approach (cont.)

I then used contact angle measurements and atomic force microscopy to analyze the deposited films, including film thickness and structure. Finally, I used polarized optical microscopy to investigate the effects of the highly ordered Teflon thin-films on alignment of the Reczek Group’s DACLC mixture AGB 2-106A (Figure 3) upon recrystallization in the liquid crystal phase.

Figure 3. Drawing of AGB 2-106A mixture. Donor pictured on the left. Acceptor pictured on the right.

Discussion

Analysis of the deposited Teflon films showed a correlation between contact angle measurements and surface characteristics under the atomic force microscope (Figure 4). Highly ordered films exhibited anisotropic effects, causing water droplets to elongate in the direction of deposition and decreasing the contact angle (typically 50-70°).

Highly ordered Teflon thin-films did have an effect on DACLC alignment (Figure 5), probably due to their increased ability to nucleate in a single direction within the physical ridges of the polymer film. However, uniform alignment was not exhibited throughout the entire sample. This could be the result of defects in the Teflon film that formed during melting of the DACLC.

Acknowledgements

I would like to thank Dr. Joseph Reczek for his mentorship and guidance. I would also like to thank Dave Burdick in the Machine Shop for helping me build my apparatus as well asDr. Annabel Edwards, the Reczek Group, Riley Sechirist, Dr. Jordan Fantini and the Fantini Group, Mr. Philip Waite, and the Department of Chemistry and Biochemistry for their support. Funding for this research was provided by the National Science Foundation.

Literature Cited

1. Leight, K.; Esarey, B.; Murray, A.; Reczek, J. Predictable Tuning of Absorption Properties in Modular AromaticDonor−Acceptor Liquid Crystals. Chem. Mater. 2012, 24, 3318-3328. 2. Sechrist, R. Electronic Characterization of Organic Materials. Senior Thesis, Denison University, Granville, OH, 2015. 3. Wittmann, J.C.; Smith, P. Highly oriented thin films of poly(tetrafluoroethylene) as a substrate for oriented growth of materials. Nature. 1991, 352, 414-417.

Introduction

Donor-acceptor columnar liquid crystals (DACLCs) are a promising new material for use in solar cells due to their absorption of a wide range of wavelengths in the UV/Vis spectrum1 as well as their behavior as semiconductors2. Upon melting and recrystallization, DACLCs naturally crystallize in a random, disorganized formation. However, DACLCs must be oriented in a single direction to increase the efficiency of their electrical conductivity.

It has been shown that Teflon, a very unreactive fluoropolymer, can direct alignment of many liquid crystals when mechanically deposited onto a glass substrate in a single direction3 . My goal was to uniformly align DACLC mixtures (Figure 1) by recrystallizing them on highly oriented Teflon thin-films so that accurate measurements of their conductivity can be taken later on.

(a) (b) (c)

Figure 1. (a) Electron movement through a single DACLC stack. (b) Randomly crystallized DACLC layer. (c) Uniformly aligned DACLC layer.

Results

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(b)

Figure 4. (a) Contact angle image with corresponding droplet, AFM image, and graph of cross-section. Contact angle measured ~104°, and water droplet formed a round, beaded shape. AFM shows a dense, but disordered Teflon film ~5 nm thick. (b) Water droplet formed an elongated, rectangular shape with a contact angle ~50°. AFM shows a highly ordered Teflon film ~10 nm thick deposited in linear streaks.

(a) (b)

Figure 5. (a) AGB 2-106A under polarized optical microscope in the liquid crystalline phase (~145° C). Recrystallized between two highly ordered Teflon films, each ~20 nm thick. (b) AGB 2-106A in liquid crystalline phase (~130° C) between clean glass slides.

Future Directions1)Prevent defects in Teflon thin-films during melting and recrystallization of DACLC sample. Possible methods include melting the DACLC prior to placing it on the film and/or spin-coating the DACLC onto the film.

2)Obtain complete DACLC alignment on a highly ordered Teflon thin-film.

3)Investigate conductive properties of the aligned DACLC.

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