Studying and controlling chromophore orientation in polyelectrolyte layer-by-layer thin films

Abstract

Many properties such as electrical conductivity, liquid crystallinity, and optical properties depend on specific molecular orientation. Currently, this orientation is achieved using either inorganic crystals or rigorously processed organic materials. This thesis studied a film assembly technique that provides a mechanism for aligning polymeric materials as films are assembled. This eliminates the need for a post-processing alignment step and allows easier film construction. The fabrication technique for the polymeric thin films is layer-by-layer (LBL) electrostatic assembly. This technique takes advantage of the electrostatic attraction between oppositely charged polyelectrolytes to assembly films. For example, a polyanion and polycation are dissolved in separate aqueous solutions at a given pH and salt concentration. A charged substrate is immersed into one solution, rinsed with water, and immersed in the other solution. This cycle is repeated to assemble films. Typically, the polyelectrolytes in these films are randomly arranged; however, a few researchers have demonstrated that ordered films could be assembled with the LBL technique. We investigated the driving force of this order and how to control it by studying the processing parameters, polyelectrolytes, and surfaces used to assemble ordered LBL films. For the processing study, two side-chain chromophoric polyanions were used. One, PolyS 119, contained sulfonic acid groups while the other, PAZO, contained carboxylic acid groups. These were assembled with four polyamines. The pH of the polyanion solutions was varied to observe its effect on chromophore orientation. Chromophore orientation was measured by FTIR and UV/isible spectroscopy.

(cont.) From these studies it was determined that stronger electrostatic attraction leads to more orientation; thus, orientation can be controlled by adjusting the strength of the attraction using different pH values since the ionization of the polyelectrolytes changes with the pH. The salt concentration of the solutions has a more subtle effect. The salt can change the ionization of polyelectrolytes in solution, which changes their ionization and influences the electrostatic attraction. Again, it was demonstrated that chromophore orientation could be adjusted by changing the solution conditions. LBL film structure is influenced by the processing conditions as well as the chemical structure of the polyelectrolytes. Since we studied how processing parameters affect orientation, we also wanted to determine the role of chemical structure. Other research on liquid crystals has shown that the spacer length between the side- and main polymer chain affects rigid side-chain orientation. For this study, a series of polymers of varying spacer length were synthesized. These were then assembled with the same four polyamines used in the previous studies. More orientation with increased spacer length was observed indicating that chromophores with longer spacers have more freedom from the main polymer chain and this allows more orientation. This effect is independent of the polyamine used to assemble the films. Another chemical struture study was changing the location of the charged group from the side-chain chromophores to the polymer main-chain ...