Polymer-Water Interfaces dynamics

In this section, we describe the role of fhe specific membrane environment on proton transport. As we have already seen in previous sections, it is insufficient to consider the membrane as an inert container for water pathways. The membrane conductivity depends on the distribution of water and the coupled dynamics of wafer molecules and protons af multiple scales. In order to rationalize structural effects on proton conductivity, one needs to take into account explicit polymer-water interactions at molecular scale and phenomena at polymer-water interfaces and in wafer-filled pores at mesoscopic scale, as well as the statistical geometry and percolation effects of the phase-segregated random domains of polymer and wafer at the macroscopic scale. 

Lavielle L (1988) Orientation phenomena at polymer-water interfaces. In Andrade JD (ed) Polymer surface dynamics. Plenum, New York, p 45... 

A major incentive of this article will be to stress the complicating traits of the membrane environment on effective proton transport and fuel cell performance. The polymer affects distribution and structme of water and dynamics of protons and water molecules at multiple scales. In order to describe the conductivity of the membrane, one needs to take into account explicit polymer-water interactions at molecular level, interfacial phenomena at polymer-water interfaces at mesoscopic scale and the statistical geometry and topology of randomly distributed aqueous and polymeric domains at macroscopic scale. 

Assuming that the size and shapes of water-filled domains are known, as well as the structure of polymer/water interfaces, proton distributions at the microscopic scale can be studied with molecular dynamics simulations (Feng and Voth, 2011 Kreuer et al., 2004 Petersen et al., 2005 Seeliger et al., 2005 Spohr, 2004 Spohr et al., 2002) or using the classical electrostatic theory of ions in electrolyte-filled pores with charged walls (Commer et al., 2002 Eikerling and Komyshev, 2001). An advanced understanding of spatial variations of proton mobility in pores warrants quantum mechanical simulations. 

It is very well known that the nature of the monolayer partially depends on the strength of interfacial interactions with substrate molecules and that of polymer in-tersegmental interactions. And it is normal to expect that the viscoelastic properties of polymer monolayer are also dependent on these factors. The static and dynamic properties of several different polymer monolayers at the air - water interface have been examined with the surface quasi-elastic Light Scattering technique combined with the static Wilhelmy plate method [101]. 

Lin et al. [17] studied the dynamics of copolymers adsorbed on an air-water interface. These measurements complemented the static measurements described above and in Fig. 4. The extent of the polymer films perpendicular to the surface is small compared to penetration distance and wavelength so that EWDLS is most sensitive to variation of composition in the plane of the interface. Figure 7 shows the measured normalized autocorrelation I (/) for different surface pressures. Frames a-d were taken during the first compression of the monolayer, and frames e-h were taken during the second compression. The difference between the two sets of measurements is an indication of structural changes induced by compression cycling. The frames e-g can be compared to the data in Fig. 4. The solid lines in the three frames are fits to a sum of two exponential functions, each with a characteristic decay time. The fast decay constant has a characteristic associated with diffusive motion of the disks. The slow decay constant ( several seconds) was ascribed to the dynamics of the associations of disks. 

Dynamic properties of interfaces have attracted attention for many years because they help in understanding the behaviour of polymer, surfactant or mixed adsorption layers.6 In particular, interfacial rheology (dilational properties) is crucial for many technological processes (emulsions, flotation, foaming, etc).1 The present work deals with the adsorption of MeC at the air-water interface. Because of its amphiphilic character MeC is able to adsorb at the liquid interface thus lowering the surface tension. Our aim is to quantify how surface active this polymer is, and to determine the rheological properties of the layer. A qualitative and quantitative evaluation of the adsorption process and the dilata-tional surface properties have been realised by dynamic interface tension measurements using a drop tensiometer and an axisymmetric drop shape analysis. 

The result of the interactions of some copolymer mimics of AMP with model bacterial membranes has been studied via atomistic molecular dynamics simulation (Figure 3.2). The model bacterial membrane expands homogeneously in a lateral manner in the membrane thickness profile compared with the polymer-free system. The individual polymers taken together are released into the bacterial membrane in a phased manner and the simulations propose that the most possible location of the partitioned polymers is near the l-palmitoyl-2-oleoyl-phosphatidylglycerol clusters. The partitioned polymers preferentially adopt facially amphiphilic conformations at the lipid-water interface, although lack intrinsic secondary structures, such as an a-helix or P-sheet, found in naturally occurring AMP [23]. 

It needs to be mentioned here that many other experimental techniques are available for studying monolayers at the air-water interface. Most frequently, surface potential is measured to evaluate the molecular orientation of amphiphiles at the interface. This method is, however, better suited to the study of small molecules. Polymeric amphiphiles, due to their conformational dynamics, are difficult to analyze and simple dielectric layer models do not apply, or produce large errors. Grazing incidence X-ray diffraction provides information on molecular packing, and spectroscopic methods are used to study molecular interactions and the structural changes of molecules upon compression. Fluorescence microscopy is useful for studying two-dimensional organization of small molecular mass amphiphiles however, it is not applied to polymer monolayers. For a more comprehensive overview of experimental methods used to study monolayers at the air-water interface, the reader is referred to more specialized articles, e.g. [18]. 

---