Polymer-solid contact

Length and Energy Scales of Minimal, Coarse-Grained Models for Polymer--Solid Contacts... 

In the following, we assume that the confining walls of the container or the supporting substrate (e.g., a wafer) are hard and non-deformable, that is, the properties of the solid do not change as it is brought into contact with the polymer material. If one measures the density profile of the polymer material at the polymer-solid contact, one will typically observe a steep rise of the density from zero (in the solid, confining wall) to the bulk density of the polymer material. The spatial extension of this polymer-solid interface is only a few ingstrom, that is, its width is dictated by the size of the atomistic constituents. On the other hand, in multicomponent polymer materials (i.e., polymer blends or polymer-solvent mixtures) one component vhll enrich at the solid surface, and the width of these enrichment layers may extend far away from the solid surface. 

If the multicomponent material is completely miscible in the bulk, then a component will typically adsorb at the preferential, solid surface. This phenomenon alters the composition profile near the polymer-solid contact on a length scale that is set by the molecular extension. The enrichment layers are much larger than the width of the polymer-solid interface but they carmot grow macroscopically large. 

If the multicomponent material exhibits two coexisting phases in the bulk, then one phase will be enriched at the polymer-solid contact and, provided that the solid is sufficiently preferential, this wetting layer of the preferred phase will grow macroscopically thick. One says that the preferred phase of the polymer material wets the solid [114]. 

The idea of a minimal coarse-grained model for polymer-solid contacts consists in not describing the structural details of the polymer-solid contact on the length scale of Angstroms because this length scale is not resolved. Instead, the aim is to tailor the interactions at the surface in order to match the difference in surface tension, Ay, of the coarse-grained model to the experimental data. In this way, the macroscopic interfacial thermodynamics is parameterized. 

To illustrate these techniques, we consider the coarse-grained model for hexadecane and carbon dioxide, which we have discussed in Section 1.2. The correction of the Lorentz-Berthelot rule is set to = 0.9 and the temperature is fixed atllcBr/E = 0.92. (In this case carbon dioxide was modeled as a simple Lennard-Jones bead without quadrupolar moment.) The short- and long-ranged interactions at the polymer-solid contact are described by a 9-3-potential of the form ... 

The use of the harmonic mean often leads to better predictions of interfacial tensions between polymers and better contact angles between liquids and polymer solids, but the criterion for maximization of the work of adhesion is the same as... 

In polymer-based ISEs, electrical contact between the membrane and inner reference electrode is made via an inner filling electrolyte. This type of ISE is the most common and they are usually referred to as liquid contact ISEs or very often simply ISEs. On the other hand, the contact can be obtained by the substitution of the aqueous inner solution with another polymeric material, to produce so-called solid-contact ISEs Table 2.1 provides current achievements in trace level... 

Solid-contact ISEs with conducting polymers as ion-to-electron transducers and plasticized PVC-based sensing membranes may be applied... 

Solid-state ISEs with conducting polymers are also promising for low-concentration measurements [60,63,74], even below nanomolar concentrations [60,74], which gives rise to optimism concerning future applications of such electrodes. In principle, the detection limit can be improved by reducing the flux of primary ions from the ion-selective membrane (or conducting polymer) to the sample solution, e.g., via com-plexation of primary ions in the solid-contact material. For example, a solid-state Pb2+-ISEs with poly(3-octylthiophene) as ion-to-electron transducer coated with an ion-selective membrane based on poly(methyl methacrylate)/poly(decyl methacrylate) was found to show detection limits in the subnanomolar range and a faster response at low concentrations than the liquid-contact ISE [74]. 

Conducting polymers have been studied as potentiometric ion sensors for almost two decades and new sensors are continuously developed. The analytical performance of solid-state ion sensors with conducting polymers as ion-to-electron transducer (solid-contact ISEs) has been significantly improved over the last few years. Of particular interest is the large improvement of the detection limit of such solid-contact ISEs down to the nanomolar level. Further optimization of the solid contacts as well as the ion-selective membranes will most certainly extend the range of practical applications. 

Briefly explain why a semi-crystalline polymer in contact with a good solvent has a lower melting point than in the dry, solid... 

Interfacial properties cannot be described without identifying the contacting medium. Interfacial properties of a polymer solid are dependent on the conditions under which the surface is equilibrated. The surface configuration of a polymer is a function of the contacting phase of polymer/contacting phase interface. In this context, the conventional sense of surface property (interface with air) is dependent on the history of the surface and the humidity of air. The surface dynamic change occurs when the interfacial equilibrium is broken and is driven by the interfacial tension in the new environment. 

The membrane in a broad sense is a thin layer that separates two distinctively different phases, i.e., gas/gas, gas/liquid, or liquid/liquid. No characteristic requirement, such as polymer, solid, etc., applies to the nature of materials that function as a membrane. A liquid or a dynamically formed interface could also function as a membrane. Although the selective transport through a membrane is an important feature of membranes, it is not necessarily included in the broad definition of the membrane. The overall transport characteristics of a membrane depends on both the transport characteristics of the bulk phase of membrane and the interfacial characteristics between the bulk phase and the contacting phase or phases, including the concentration polarization at the interface. The term membrane is preferentially used for high-throughput membranes, and membranes with very low throughput are often expressed by the term barrier. ... 

Ionophore-based solvent polymeric membranes are widely used as sensing membranes in ion-selective electrodes (ISEs) [24,25]. This type of potentiometric sensor has attracted great interest in the last decade due to the extraordinary improvement in the detection limit down to picomolar (10-12 M) levels [26,27], Furthermore, solid-contact ISEs have been developed by using various conducting polymers, including PEDOT, as the ion-to-electron transducer [28-31],... 

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