THE ORIGIN AND CHARACTERIZATION OF CONFORMATIONAL HETEROGENEITY IN ADSORBED POLYMER LAYERS

The equilibration of polymer conformations tends to be sluggish in pol ymer layers adsorbed onto highly attractive substrates, so the structu re of these layers must be understood in terms of the layer growth pro cess rather than equilibrium theory. Initially adsorbed chains adopt a highly Battened configuration while the chains which arrive later mus t adapt their configurations to the increasingly limited space availab le for adsorption. Thus, the chains adsorbed in the late stage of depo sition are more tenuously attached to the surface. This type of non-eq uilibrium 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 silic on from a carbon tetrachloride solution where the segmental attraction is relatively weak (1 k(B)T/segment). Measurements were based on Four ier transform infrared spectroscopy in attenuated total reflection (FT IR-ATR). In both cases, the chains arriving first adsorbed more tightl y, became flattened (as measured by the dichroic ratio), and occupied a disproportionately large fraction of the surface. This non-uniform s tructure persisted indefinitely for the strongly adsorbed PMMA chains, while the PS chains exhibited a gradual evolution, presumably reflect ing 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 t he surface space available at the time of adsorption. The inhomogeneit y in the size of adsorbing species (hemispheres) in this model is simi lar to the distribution of chain contacts in our measurements on adsor bed polymer layers. Owing to extensive variance around the mean, confo rmations having the mean number of chain contacts are least probable, which contrasts starkly with expectations based on equilibrium adsorpt ion theory.