Layer-by-Layer\nDeposition of Kraft Lignin and PEDOT:PSS

Kraft lignin (KL) is a sustainable\ncarbon-based substance with\na potential use in photovoltaic materials. However, its conductivity\nis low, but it can be improved via incorporation with a conductive\npolymer, such as poly(3,4-ethylene dioxythiophene) (PEDOT): poly(styrenesulfonate)\n(PSS). This study examines the factors affecting the interaction of\nKL and PEDOT:PSS (PS) in a solution state using a quartz crystal microbalance\nwith dissipation (QCM-D) and a stagnation point refractometer (SPAR).\nThe results confirmed that aqueous environments, e.g., pH and ionic\nstrengths, considerably affected particle size and zeta potential\nof KL and PS due to protonation, deprotonation, particle aggregation,\nand charge screening. The polymers exhibited the largest adsorbed\nmass and thickness at pH 6 and 10 mM NaCl on a solid surface, which\nwas attributed to the relatively linear structure of PEDOT chains,\nexposing more adsorptive sites for interaction with KL. A 10 mM NaCl\nconcentration facilitated the screening of charges on PS and KL surfaces,\ndiminishing repulsive forces and enabling hydrophobic and cationic-π\ninteraction, which led to increased adsorption. Contact angle and\nSEM investigations of the adsorbed layer revealed the water contact\nangle increasing and the morphology changing from a smoother layer\nto a porous surface, providing further evidence of adsorption. Furthermore,\nthe conductivity was improved by the introduction of a PS adlayer\non ITO glass when it was sandwiched between KL adsorbed layers. These\nfindings provide insight into KL and PS interaction and suggest that\nKL can be used with PS for conductive materials, such as photovoltaics,\nimparting the waterproofness of the films.