Phase Behavior and Electrophoretic Deposition of LPEI-PAA Polyelectrolyte Complexes
This project aims to discover a new means of overcoming the drawbacks of traditional layer-by-layer dip coating through the use of polyelectrolyte complexes (PECs) and electrophoretic deposition. The layer-by-layer process, by which oppositely charged polyelectrolytes or other charged particle are alternately adsorbed onto a substrate to produce a thin film of precisely controllable thickness, is versatile and simple to implement but suffers from requiring numerous, long deposition steps to produce uniform micron thick films. PECs are small associations of oppositely charged polyelectrolytes that, when mixed in non-stoichiometric ratios, can form charged water- soluble particles. There is much still to be determined about the phase behavior of PECs. However, it has been shown that they can exist over a range of conditions. Electrophoretic deposition is a technique used in many commercial applications for the deposition of charged particles onto a conducting substrate. It has even been shown to enhance the deposition of polyelectrolyte single layers and multilayers.
This study examines the phase behavior of PECs made of linear poly(ethyleneimine) (LPEI) and poly(acrylic acid) (PAA). PEC behavior is studied over a pH range of 4.0 to 6.0, with no salt added to the system. This study also reports the results of tests examining how soluble PECs responded to changes in pH and whether solid PECs could be made to dissolve through the addition of different salts. Light scattering is used to examine the particle size distribution and effective diameter of PECs in solution. This information is then used to electrophoretically deposit PECs with 10%, 30%, 70%, and 90% excess LPEI. Stylus profilometry is used to assess the thickness of deposited films. The results showing that PEC layers deposited under an applied voltage were 40% to 400% thicker than PECs deposited with no applied potential.