Dynamic behavior adsorbed polymer layers

Most spraying processes work under dynamic conditions and improvement of their efficiency requires the use of surfactants that lower the liquid surface tension yLv under these dynamic conditions. The interfaces involved (e.g. droplets formed in a spray or impacting on a surface) are freshly formed and have only a small effective age of some seconds or even less than a millisecond. The most frequently used parameter to characterize the dynamic properties of liquid adsorption layers is the dynamic surface tension (that is a time dependent quantity). Techniques should be available to measure yLv as a function of time (ranging firom a fraction of a millisecond to minutes and hours or days). To optimize the use of surfactants, polymers and mixtures of them specific knowledge of their dynamic adsorption behavior rather than equilibrium properties is of great interest [28]. It is, therefore, necessary to describe the dynamics of surfeictant adsorption at a fundamental level. The first physically sound model for adsorption kinetics was derived by Ward and Tordai [29]. It is based on the assumption that the time dependence of surface or interfacial tension, which is directly proportional to the surface excess F (moles m ), is caused by diffusion and transport of surfeictant molecules to the interface. This is referred to as the diffusion controlled adsorption kinetics model . This diffusion controlled model assumes transport by diffusion of the surface active molecules to be the rate controlled step. The so called kinetic controlled model is based on the transfer mechanism of molecules from solution to the adsorbed state and vice versa [28]. 

Fig. 13 Example of the analysis of the adsorption kinetics for a layer of PDADMAC adsorbed from a dipping solution with an ionic strength of 100 mM. a Adsorption dynamics for the layer of polymer. The solid line shows the best fit to Eq. (3). The two exponential components are shown dashed line fast step and dotted line slow step, b Plot of logarithm of (Tco — F) versus time, where Too represents the surface concentration at the steady-state of the adsorption process, after long adsorption times where the fastest exponential becomes negligible, it can be fitted to a straight line (solid line), c Short time behavior of the adsorption kinetics, a plot of In (F — F — A2C2 versus time gives a straight line (solid line). Reprinted from Ref. [97], Copyright (2011), with permission from Elsevier...

To overcome this gap, together with the investigation of stmcture and dynamics of the adsorbed layers, respectively in Chaps. 6 and 7, here we investigated the kinetics of adsorption of polystyrene on silicon wafers covered by silicon oxide, the most commonly investigated system for studies of the deviation from bulk behavior under ID confinement. In the next sessions, after reviewing current theoretical models on adsorption of polymer chains (Sect. 5.2), we will present recent experimental results (Sect. 5.3) and the outcome of molecular dynamics simulations on a similar model system (Sect. 5.4). 

The presence of interfaces on the dynamics of PVDF coiffined in alumina nanopores has been also investigated by dielectric spectroscopy [34]. A strong deviation of the relaxation behavior of PVDF embedded within the nanopores is observed as compared to that of the bulk. When the restriction in size is larger, that is, when the pore diameter is comparable with the size of the adsorbed layer, the existence of a highly constrained relaxation associated with the polymer-alumina interfacial layer is observed [34]. 

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