We describe the histogram of chain conformations that results when flexible polymers adsorb more quickly than the surface structure equilibrates. By infrared analysis of conformational substates in a model system (poly(methyl methacrylate) adsorbed primarily by hydrogen bonding onto oxidized silicon), this histogram was measured and found to be bimodal. One large population of chains spread into flattened conformations. A second large population of chains, finding less free surface available at the time of adsorption, became attached more tenuously. The history dependence of adsorption could be understood as the irreversible adsorption of objects with flexible shape. This interpretation differs decisively from the traditional considerations of minimizing the free energy.

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Abstract. The equilibration of polymer conformations tends to be sluggish in polymer layers adsorbed onto highly attractive substrates, so the structure of these layers must be understood in terms of the layer growth process rather than equilibrium theory. Initially adsorbed chains adopt a highly flattened configuration while the chains which arrive later must adapt their configurations to the increasingly limited space available for adsorption. Thus, the chains adsorbed in the late stage of deposition are more tenuously attached to the surface. This type of non-equilibrium growth process is studied for polymethylmethacrylate (PMMA) adsorbed on oxidized silicon where the segmental attraction is strong (4 k B T /segment) and for polystyrene (PS) adsorbed on oxidized silicon from a carbon tetrachloride solution where the segmental attraction is relatively weak (1 k B T /segment). Measurements were based on Fourier transform infrared spectroscopy in attenuated total reflection (FTIR-ATR). In both cases, the chains arriving first adsorbed more tightly, became flattened (as measured by the dichroic ratio), and occupied a disproportionately large fraction of the surface. This non-uniform structure persisted indefinitely for the strongly adsorbed PMMA chains, while the PS chains exhibited a gradual evolution, presumably reflecting an equilibration of the adsorbed layer occurring after long times. On the theoretical side, the initial heterogeneity of these adsorbed polymer layers is modelled using a random sequential adsorption (RSA) model where the size of the adsorbing species is allowed to adapt to the surface space available at the time of adsorption. The inhomogeneity in the size of adsorbing species (hemispheres) in this model is similar to the distribution of chain contacts in our measurements on adsorbed polymer layers. Owing to extensive variance around the mean, conformations having the mean number of chain contacts are least probable, which contrasts starkly with expectations based on equilibrium adsorption theory.

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The conformations of polystyrene (PS) adsorbed onto silicon oxide from carbon tetrachloride (CCI4) 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; Afw = 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; Afw = 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 (TV) increased. For exchange of chains of matched chain length, after a residence time of 16 h, TV = 0.6 (Af" ~550 000) and TV = 1.0 (M" = 90 000). However, TV extrapolated to zero for residence times less than 1 h. In addition, PS-h could be entirely displaced by CH2CI2, indicating for this reason also that the adsorption was reversible in spite of being kinetically metastable. The higher trapped fractions after increased equilibration are tentatively interpreted to reflect logjammed states induced by steric hindrance between intertwined adsorbed chains.