Structure and Dynamics of Adsorbed Polymer
We study the time dependent composition and structure of macromolecules physisorbed from solution to a single solid surface. Measurements of the surface excess, segmental orientation, bound fraction, segment-surface interaction enthalpy, and adsorption-desorption kinetics were made using Fourier transform infrared spectroscopy in the mode of attenuated total reflection. The polymers were amorphous, linear and flexible. The work described focusses on three areas: (i) time scales of equilibration as an adsorbed molecule relaxes towards its lowest energy configuration, (ii) the specific conformational structure of the chains composing the layer, and (iii) the dependence of equilibrium conditions on the presence of solvent flow past the layer. In order to approach these questions quantitatively, it was first necessary to characterize our experimental system. This was done through studies of competitive adsorption of the polymers employed in the study, and through careful analysis of the oxidized substrate. We find that the classical description of polymer adsorption, based on free energy considerations, is quite limited in its range of applicability. In a weakly adsorbing system, polystyrene adsorbing from dilute cyclohexane onto silicon oxide, rearrangements in the direction of conformational equilibrium were remarkably slow. The time scales of equilibration increased exponentially with length of adsorbed chains. In a more tenaciously bound system, polymethyl -methacrylate in dilute carbon tetrachloride, relaxations were prohibited by steric impedance and segment-surface interactions. The layer was frozen in a nonequilibrium state. For this system, a distribution of conformations was constructed, showing that the earliest arriving molecules were bound in flattened conformations, while the later chains retained their solution-like isotropy. Finally, preliminary evidence of shear induced alignment is presented. The significance is to demonstrate the crippling restriction of present descriptions of adsorption properties to idealized quiescent interfacial environments.
- Pub Date:
- POLY(METHYL METHACRYLATE);
- Physics: Molecular; Chemistry: Physical; Engineering: Materials Science