Dynamic aspects of adsorbed polymer layers: Evidence of glass-like transitions

Measurements of the mass adsorbed of polymer layers adsorbed from dilute carbon tetrachloride solutions $(\approx$1.00 mg/mL) were made as a function of time using Fourier transform infrared spectroscopy in the mode of attenuated total reflection. The general experimental approach involved surrounding an initially adsorbed polymer layer with a solution of macromolecules having a greater affinity for the surface (either silicon oxide or germanium oxide). The polymers were amorphous and flexible. The weakly adsorbing molecules in these experiments were protio and deuterio polystyrene (hPS & dPS), and polydimethylsiloxane (PDMS) in control experiments. The more strongly adsorbing molecules in these experiments were protio and deuterio poly(methyl methacrylate) (hPMMA & dPMMA), and polyisoprene (PI) in control experiments.The work described focusses on two areas: (i) diffusion of chains through an adsorbed layer of different molecules "blanketting" the initial layer, and (ii) glass-like transitions which may occur within an adsorbed polymer layer. The desorption kinetics of the initial weakly adsorbed molecules through the strongly adsorbed molecular layer were empirically described with a stretched exponential relation in time, exp$(\rm-t/\tau)\sp\beta.$ The parameters $\tau$ & $\beta$ were important for the investigation of both areas mentioned above.Studies employing a range of PS molecular weights, M$\sb{\rm PS},$ measured time dependence of desorption through two different dPMMA layers. First, with a molecular weight of dPMMA, M$\sb{\rm dPMMA},$ of 146,000 g/mol adsorbed on top of initial PS layers, a $\rm\tau\sim M\sb{PS}\sp{2.6}$ dependence was measured. The calculation of an average characteristic relaxation time, $\langle\tau\rangle,$ resulted in $\rm\langle\tau\rangle\sim M\sb{PS}\sp{1.6}$ relation. The parameter $\beta$ was $\approx$0.2 for low M$\sb{\rm PS}$ and rose to an apparent limit of 0.5 for high M$\sb{\rm PS}.$ Similar results were obtained with M$\sb{\rm dPMMA}$ = 12,000 g/mol as the blanketting layer. Molecular weight dependence of $\rm\tau\sim M\sb{PS}\sp{1.9}$ and $\rm\langle\tau\rangle\sim M\sb{PS}\sp{1.5}$ were measured with an apparent limit of $\beta$ = 1/2 at high M$\sb{\rm PS}$ suggested once again.The second area of emphasis used desorption kinetics of initial weakly adsorbed molecules as a probe of the dynamics occurring within the strongly adsorbed blanketting layer. Studies of temperature dependent desorption kinetics were consistent with the empirical Vogel-Tamman-Fulcher relation which is often used successfully to describe relaxation time temperature dependence of conventional glasses. As the temperature was lowered, eventually differences occurred in the strongly adsorbed layers effectively quenching desorption of the "probe" molecules. Possible M$\sb{\rm dPMMA}$ dependence on the value of this "transition temperature" was studied. Control experiments with different molecular systems suggested that glass-like transitions within adsorbed polymer layers may be a general phenomenon.U of I OnlyETDs are only available to UIUC Users without author permissio