Implications of Diversity: A Study of the Evolution of the Adsorption States of Macromolecules at Surfaces

180 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1995.We study the adsorption of macromolecules from solution onto single solid surfaces, focusing on the evolution of compositional heterogeneity of the layer. The origin and development of adsorption states within the physisorbed layer is noted in both chemically homogeneous and heterogeneous layers and is inferred from measurements of surface excess, bound fraction, segmental orientation, and adsorption-desorption kinetics made using Fourier transform infrared spectroscopy in the mode of attenuated total reflectance. The macromolecular systems studied were intentionally varied with regard to chemical composition, surface affinity, flexibility, and solvent quality and concentration.The work focuses on two primary issues: (i) the interplay of adsorption and desorption at high surface coverage, and (ii) the nature of the distribution and equilibration of adsorption sub-states within a physisorbed layer. These issues reflect realistic systems more often encountered than the theoretically treated situations of competitive adsorption onto a bare surface or equilibration of adsorbed layers approximated as homogeneous populations.We find that when considering the competition for surface sites by polymers in a nearly saturated layer, classical predictions based on free energy calculations of segmental interaction with a bare surface are no longer sufficient. Such studies were conducted using various molecular weights of isotopically labeled and normal polystyrene, (h-PS, d-PS) as well as polystyrene being displaced by polyisoprene (PI). In these systems the desorption of the original layer was rate limiting, and these kinetics were critical. In systems in which complete displacement was expected by purely energetic arguments (PS displaced by PI), the desorption of the primary layer, which controlled the accumulation of the displacer polymer, could even be kinetically prevented from going to completion.In addition to its importance in regard to competitive adsorption, the ease of displacement of an originally bound layer was found to reflect heterogeneity in adsorption states. In the case of weakly adsorbing PS, the distribution of substates in the layer was found to be bimodal, with populations varying in sticking energy on the scale of thermal energy.The bimodal distribution of adsorption states in a spontaneously adsorbed layer was also found in a tightly binding system, polymethylmethacrylate, PMMA, in better than theta solvent. Through studies of the conformational distribution of chains in both starved and saturated layers, it was determined that the collective strength of surface interactions for all the bound segments of the molecule prevent relaxation.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD