The transient electric birefringence of nanomaterials: Alignment mechanism, characterization, and its application towards aligned polymer nanocomposites.

摘要

The experimental and theoretical bases of transient electric birefringence (TEB) and its application in a variety of nanomaterials, including Na-montmorillonite, cellulose nanocrystals (CNXL), and CdSe quantum dots (QDs) will be presented. TEB is a phenomenon attributed to the alignment of particles in response to an applied electric field and has been used for the determination of particle sizes, optical anisotropies, polarizability anisotropies Λalpha, and permanent dipole moments mu. In this thesis, I will present the relations between the physical parameters just mentioned and the experimental measurements of the rise, saturation, and decay of particle alignment. Detailed experimental setup and data analysis procedures will also be discussed. In the case of Na-montmorillonite, our findings imply a wide size and shape distribution and that the shape of the material is best modeled somewhere between a rod and a disk. The alignment mechanism was dominated by an induced-dipole, with some of the data suggesting the presence of a small permanent dipole; however, the later findings were inconclusive. CNXL, on the other hand, is shown to be polydisperse only in size and is modeled well as a rigid rod. At intermediate field strength, the alignment response was governed by both an induced (∼95%) and permanent moment (∼5%). CNXL was also shown to produce very large birefringence, owing to its equally large optical anisotropy. CdSe QDs are nearly monodisperse in both size and shape, and are an ideal representation of a rigid rod. Their alignment mechanism contained a large permanent dipole contribution (∼88%), which is attributed to their wurtzite crystalline structure. In the last chapter, preliminary results on nanocomposites containing carbon nanotubes (CNTs) and QDs are presented. Results for the nanoparticles in polylaurylmethacrylate are promising. TEM images of both composites showed a net alignment of particles in the field direction. However, aggregation of the CNTs in the polymer was a problem and further work is needed to keep them adequately exfoliated during the curing process. Aggregation of the QDs was not observed.