Orientation-on-demand thin films: Curing of liquid crystalline networks in ac electric fields

Science

Volumn 272, Issue 5259, 1996, Pages 252-255

  Körner, Hilmar ^a   Shiota, Atsushi ^a   Bunning, Timothy J ^b   Ober, Christopher K ^a  

^a Cornell University   (United States)

^b SCIENCE APPLICATIONS INTERNATIONAL CORPORATION   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ANISOTROPY; CROSSLINKING; CURING; DIELECTRIC PROPERTIES; ELECTRIC FIELDS; FREQUENCIES; LIQUID CRYSTALS; MOLECULAR ORIENTATION; SYNCHROTRON RADIATION; THERMOSETS; X RAY DIFFRACTION;

DIELECTRIC ANISOTROPY; MOLECULAR ASSEMBLY; NETWORK STRUCTURE;

THIN FILMS;

EID: 0030128251     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.272.5259.252     Document Type: Article

References (21)

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12. ⊥) is almost constant, which leads to a crossover frequency at which the sign of the dielectric anisotropy Δε changes and molecular reorientation occurs It is known from LC display technology that one can use this effect to introduce dual frequency addressability to a display and was used to control the molecular orientation before final cure in our system.
13. L. M. Blinov and V. G. Chigrinov, in Electrooptic Effects in Liquid Crystal Materials, L. Lam and D. Langevin, Eds. (Springer-Verlag, New York, 1994); W. Haase, in Side-Chain Liquid Crystal Polymers, C. B. McArdle, Ed. (Blackie, Edinburgh, UK, 1989).
14. L. M. Blinov and V. G. Chigrinov, in Electrooptic Effects in Liquid Crystal Materials, L. Lam and D. Langevin, Eds. (Springer-Verlag, New York, 1994); W. Haase, in Side-Chain Liquid Crystal Polymers, C. B. McArdle, Ed. (Blackie, Edinburgh, UK, 1989).
15. The introduction of methyl groups leads to three isomers. Isomeric mixtures are known from LC displays to show better dual frequency switching behavior.
16. Compound DCN without these bulky lateral groups does not melt and decomposes without showing LC behavior. Cross-linking is therefore impossible Both substituted compounds (DCN and CHDCN) can be synthesized in a two-step reaction involving inexpensive, commercially available materials.
17. We reoriented both DCN and CHDCN in an applied E field by changing the frequency between low (10 Hz) and high (10,000 Hz) values while the material was curing. The direction of the nematic director could be rotated from homeotropic (perpendicular to film plane) to planar (parallel to film plane) alignment with respect to the electrode surface; that is, at low frequencies, we observed mesogens oriented parallel to the E field, but at high frequencies, the compound changed its director by 90° Compared to known low-molecular weight compounds with similar structure, we expect the crossover frequency at 190°C to be on the order of 5000 Hz, but detailed dielectric measurements during curing have yet to be carried out to clarify this question.
18. At 190°C, DCN cures within 3 hours, and at 250°C, in 30 min, forming a densely cross-linked structure
19. The calculations were carried out with the appropriate corrections for uniaxially oriented fibers after background correction. The d spacings from wide-angle XRD data were 5.2 Å for DCN and 4.8 Å for CHDCN.
20. In contrast, the radial width of the reflections, which can approximately describe the order of the nematic phase, shows a higher order for CHDCN than for DCN, where the wide-angle reflections are unusually broad along the radial direction The radial width of a reflection offers information about the domain size of the nematic phase of a polymer. Although the orientation for CHDCN is weaker, its domain size is larger than that for DCN.
21. We thank the Cornell Materials Science Center and the National Science Foundation (DMR-9321573) for partial support of this work Support for H K from the Industry-Cornell Alliance for Electronic Packaging, the Fulbright Foundation, and the Laser Hardened Materials Branch of Wright-Patterson Air Force Base is appreciated Support for T J B. under Air Force contract F33615-95-C-5423 is acknowledged. Helpful discussions with W. Adams and E. J Kramer are gratefully acknowledged. Access to the Cornell High Energy Synchrotron was essential for this work.