Peter Frantz

The origin and characterization of conformational heterogeneity in adsorbed polymer layers

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 (/segment) and for polystyrene (PS) adsorbed on oxidized silicon from a carbon tetrachloride solution where the segmental attraction is relatively weak (/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.

Regimes of polymer adsorption-desorption kinetics

Abstract We show, and support with experiments, two limiting regimes of polymer adsorption-desorption. When these processes are rate limited by diffusion they are expected to be non-exponential in time and the time constant is inversely proportional to the diffusion coefficient. When these processes are rate limited by the energetics of surface detachment, they are expected to be exponential in time and the time constant is exponential in the molecular weight. This suggests a unifying perspective from which to analyze polymer mobility at surfaces.

Orientation of adsorbed polymer in response to shear flow

Abstract We report preliminary evidence of shear-induced alignment of polyisoprene adsorbed from CCl4 solution on to a single solid surface. The experimental technique was Fourier transform infrared spectroscopy, in the mode of attenuated total reflection. The selective augmentation of vibrational modes oriented preferentially along the chain backbone implied chain orientation in the direction of flow. This occurred at low shear rates (about 1 s−1). The extent of orientation increased with increasing molecular weight, and did not depend on shear rate within the narrow range (0.4–1.6 s−1) available to us. After cessation of flow, relaxation toward the conformation in the quiescent state was slow.

End group effect in polymer adsorption: Competitive adsorption of carboxylic acid‐terminated and unfunctionalized polystyrene

The adsorption onto silica from dilute cyclohexane solution is studied using IR spectroscopy. The differential adsorption enthalpy from the carboxylic acid group is found to be about 5 KT