Kinetic Traps in Polymer Adsorption: Exchange of Polystyrene between the Adsorbed State and a Good Solvent

The conformations of polystyrene (PS) adsorbed onto silicon oxide from carbon tetrachloride (CCl₄) were investigated through measurements of exchange rates between the adsorbed state and free solution. Infrared spectroscopy in attenuated total reflection (FTIR-ATR) was used to measure PS adsorbed onto oxidized silicon from dilute carbon tetrachloride solution at 25.0°C. Protiopolystyrene (PS-h; M_w = 575 000 or 96 400) was allowed to adsorb for a variable time from a solution of concentration 1 mg mL^-1; then this solution was replaced by a solution of deuteriopolystyrene (PS-d; M_w = 550 000 or 85 000) at the same concentration. An initial rapid exchange was completed over periods of minutes. No subsequent exchange was detectable over periods up to 10 h, indicating that a major fraction of the adsorbed polymer was kinetically trapped. With increasing surface residence time before exchange, the rapid exchange rate slowed moderately and the trapped fraction (T_f) increased. For exchange of chains of matched chain length, after a residence time of 16 h, T_f ≈ 0.6 (M_w ≈ 550 000) and T_t ≈ 1.0 (M_w ≈ 90 000). However, T_f extrapolated to zero for residence times less than 1 h. In addition, PS-h could be entirely displaced by CH₂Cl₂, indicating for this reason also that the adsorption was reversible in spite of being kinetically metastable.