Effect of Acid-Base Interactions on Conformation of Adsorbed Polymer Chains

Sum frequency generation spectroscopy (SFG) and attenuated-total-reflection IR (ATR-IR) were used to investigate polymer adsorption on solid surfaces in CCl₄ (neutral), CHCl₃ (acidic), and acetone (basic) solvents. Fowkes showed that the adsorbed amount of the polymer from acidic and basic solvents is less than that from a neutral solvent (Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 3-7). Here, we show that besides the differences in adsorbed amount, chains adsorbed from an acidic solvent adopted a flat conformation with a much smaller ratio of segments of loops and tails to trains compared to those adsorbed from a neutral solvent. Sapphire (Al₂O₃) surfaces were saturated by train segments at 1.3 × 10^-5 volume fraction for both CCl₄ and CHCl₃ solutions, with a large fraction of the surface sites occupied by the PMMA segments, which was different from what was expected based on Fowkes' experiment. In contrast, PMMA segments were not able to replace acetone molecules from the surface in a time period of 2 h. Surface interaction parameters alone were unable to predict the differences in conformation of chains adsorbed from acidic or neutral solvents.