Effect of carbon nanotubes on phase transitions of nematic liquid crystals

Abstract

Phase diagrams of multi‐wall carbon nanotube (MWNT)/nematic liquid crystal (E7) and buckminsterfullerene (C₆₀‐I _h)/nematic liquid crystal (E7) binary systems have been investigated by means of polarizing optical microscopy and differential scanning calorimetry. It was found that the isotropic–nematic phase transition temperature (T _NI) of the liquid crystal component was enhanced by the incorporation of MWNT within a small composition gap. A chimney‐type phase diagram can be identified in the MWNT/E7 mixture over a narrow range of ∼0.1–0.2% MWNT concentration. Upon substituting the nanotubes with isotropic fillers such as fullerene, the (C₆₀‐I _h)/E7 blend showed no discernible change of T _NI in the same concentration range of the chimney of the MWNT/E7 mixture, suggesting a significant contribution of anisotropy (or the aspect ratio) of the nanotubes to the entropy of the system containing liquid crystal molecules. This enhanced T _NI phenomenon may be attributed to anisotropic alignment of liquid crystal molecules along the carbon nanotube bundles.

Acknowledgments

Support for this work by the NSF‐DMR 02‐09727, Ohio Board of Regents through Research Challenge Grant, Toyo Tire and Rubber Company, and Collaborative Center for Polymer Photonics sponsored by WPAFB/Air Force Office of Scientific Research/University of Akron, is gratefully acknowledged. The authors are indebted to Dr Liming Dai for supplying the MWNT samples and Dr Scott Meng for critically reviewing the manuscript.