Abstract
The present thesis deals with the use of magnetic fields as a handle to manipulate matter at a molecular level and as a tool to probe molecular properties or inter molecular interactions. The work consists of in situ optical studies of (polymer) liquid crystals and molecular aggregates in high magnetic fields up to 20T, together with a description of the methods and set-ups developed and the theoretical interpretations of the obtained results. Chapter 2 gives a general treatment of the magnetic field induced orientation mechanism and shows how this effect can be described in the frame of the Boltzman Statistics. More specifically, the Liquid Crystals, Polymer Liquid Crystals and Molecular Aggregates systems are studied in the presence of magnetic fields. The chapter continues with a presentation of the set-ups developed and used for the purpose of this thesis and it ends with a section of data analysis methods. Chapter 3 describes the magnetic field induced alignment of Side Chain Polymer Liquid Crystals and the particular role of the mesogen/backbone coupling in this phenomenon. Chapter 4 focuses on the Isotropic-Nematic phase transition of Side Chain Polymer Liquid Crystals in magnetic fields, an example of nonlinear physical phenomena: the magnetic field produces an energetic effect much larger that the direct energy it introduces into the system. Chapter 5 presents a study of thin Liquid Crystals cells in magnetic fields, discussing the effect of magnetic field on the surface anchoring and presents the first direct observation of a surface Isotropic-Nematic transition. Finally, Chapter 6 deals with Molecular Aggregates solutions in the presence of magnetic fields; a combined study of magnetic field induced orientation and system characterization
