Polymers solvation polymer solutions

Finally, we want to describe two examples of those isolated polymer chains in a sea of solvent molecules. Polymer chains relax considerably faster in a low-molecular-weight solvent than in melts or glasses. Yet it is still almost impossible to study the conformational relaxation of a polymer chain in solvent using atomistic simulations. However, in many cases it is not the polymer dynamics that is of interest but the structure and dynamics of the solvent around the chain. Often, the first and maybe second solvation shells dominate the solvation. Two recent examples of aqueous and non-aqueous polymer solutions should illustrate this poly(ethylene oxide) (PEO) [31]... 

HOPC uses a column packed with porous materials that have a pore diameter close to a dimension of the solvated polymer to separate. A concentrated solution of the polymer is injected into the solvent-imbibed column by a high-pressure liquid pump until the polymer is detected at the column outlet. The injection is then switched to the pure solvent, and the eluent is fractionated. A schematic of an HOPC system is illustrated in Fig. 23.1. A large volume injection of a concentrated solution makes HOPC different from conventional SEC. 

The partitioning principle is different at high concentrations c > c . Strong repulsions between solvated polymer chains increase the osmotic pressure of the solution to a level much higher when compared to an ideal solution of the same concentration (5). The high osmotic pressure of the solution exterior to the pore drives polymer chains into the pore channels at a higher proportion (4,9). Thus K increases as c increases. For a solution of monodisperse polymer, K approaches unity at sufficiently high concentrations, but never exceeds unity. 

Nous s. solid polymer, aq, aqueous solution, gel. expanded polymergel m, meul electrode, dif. diffusion layer, solv, solvated molecules or ions... 

The relaxation of a mole of segments on the oxidized/neutral polymer borders involves the loss of qr electrons and the subsequent storage of qr positivecharges. At the same time, qr solvated monovalent anions penetrate into the polymer from the solution. 

On the other hand, Doblhofer218 has pointed out that since conducting polymer films are solvated and contain mobile ions, the potential drop occurs primarily at the metal/polymer interface. As with a redox polymer, electrons move across the film because of concentration gradients of oxidized and reduced sites, and redox processes involving solution species occur as bimolecular reactions with polymer redox sites at the polymer/solution interface. This model was found to be consistent with data for the reduction and oxidation of a variety of species at poly(7V-methylpyrrole). This polymer has a relatively low maximum conductivity (10-6 - 10 5 S cm"1) and was only partially oxidized in the mediation experiments, which may explain why it behaved more like a redox polymer than a typical conducting polymer. 

There are relatively few phase equilibrium data relating to concentrated polymer solutions containing several solvents. Nevertheless, In polymer devolatilization, such cases are often of prime Interest. One of the complicating features of such cases Is that. In many Instances, one of the solvents preferentially solvates the polymer molecules, partially excluding the other solvents from Interaction directly with the polymer molecules. This phenomenon Is known as "gathering". 

The randomly occupied lattice model of a polymer solution used in the Rory-Huggins theory is not a good model of a real polymer solution, particularly at low concentration. In reality, such a solution must consist of regions of pure solvent interspersed with locally concentrated domains of solvated polymer. 

It is known that polymer dynamics is strongly influenced by hydrodynamic interactions. When viewed on a microscopic level, a polymer is made from molecular groups with dimensions in the angstrom range. Many of these monomer units are in close proximity both because of the connectivity of the chain and the fact that the polymer may adopt complicated conformations in solution. Polymers are solvated by a large number of solvent molecules whose molecular dimensions are comparable to those of the monomer units. These features make the full treatment of hydrodynamic interactions for polymer solutions very difficult. 

The ion solvating polymers have found application mainly in power sources (all-solid lithium batteries, see Fig. 2.19), where polymer electrolytes offer various advantages over liquid electrolyte solutions. 

Figure 14 gives limiting viscosity numbers for hydrolyzed copolymer 11 as a function of shear rate. Since limiting viscosity number is a function of molecular size, these data show that solution pseudoplasticity occurs because of compaction of the solvated polymer with increasing shear. 

The polymer molecular weight may be greatly diminished if polymerization takes place under conditions where polymer precipitates from solution and/or is not well solvated. The reacting functional groups become inaccessible to each other and polymerization stops before the desired DP is reached. 

The interactions between solvent and polymer depend not only on the nature of the polymer and type of solvent but also on the temperature. Increasing temperature usually favors solvation of the macromolecule by the solvent (the coil expands further and a becomes larger), while with decreasing temperature the association of like species, i.e., between segments of the polymer chains and between solvent molecules, is preferred. In principle, for a given polymer there is a temperature for every solvent at which the two sets of forces (solvation and association) are equally strong this is designated the theta temperature. At this temperature the dissolved polymer exists in solution in the form of a nonexpanded coil, i.e., the exponent a has the value 0.5. This situation is found for numerous polymers e.g., the theta temperature is 34 °C for polystyrene in cyclohexane, and 14 °C for polyisobutylene in benzene. 

In plastisol propellants, however, all polymerization reactions are complete before propellant manufacture begins. Solidification is accomplished through solvation (or solution) of the solid resin (or polymer) particles in the nonvolatile liquid, which has been selected to be a plasticizer for the resin. Solvation or curing is accomplished by heating to a temperature at which the resin particles dissolve rapidly (within a matter of a few minutes) in the plasticizer to form a gel which on returning to room temperature has the characteristics of a rubbery solid. 

The effectiveness of a polymeric flow enhancer is influenced decisively by the state of solution and the solvation characteristics. In the case of polyelectrolytes, in particular, the chemical nature plays a significant role, e.g., it was found for poly(acrylamide)-coacrylate that a significant increase in effectiveness arises with the increasing number of ionic groups. It is therefore necessary to consider, for example, such factors as the question of critical concentration, polymer-polymer and polymer-solvent interactions, the thermodynamic quality of the solvent, the proportion of ionic molecular groups and their behavior in the presence of lower-molecular-weight charge carriers. 

Cellulose ucelate is converted into libers by solvating the polymer in an acetone solution which is ihen spun by a process called tin spinning. In ihe spinning process, the polymer solution is metered through a spinneret into a column cimuiining warm air which evaporates the acetone. The solidified filaments are oiled and package collected al the base uf llte spinning lube. 

These emulsions are liquid-liquid systems comprising immiscible polymer solutions in nonpolar solvents and BG copolymer emulsifiers. The emulsifying power of BG copolymers has been attributed (12) to coalescence barriers formed by accumulation of the BG copolymers in the emulsion interface. This interface apparently has the structure of a double layer consisting of the different subchains of the BG copolymers which are solvated by the organic solvent. The chemically different sequences in BG copolymers are separated in different layers in the interface because polymer chains of different chemical structures are usually incompatible (3, 4), particularly in nonpolar solvents. 

Silica precursors, tetraethoxysilane and tetramethoxysilane (TEOS and TMOS) are able to solvate some organic polymers. This enables the silica precursor to polymerize in the environment of an organic polymer solution. The number of polymers that form solutions with sol-gel formulations is, however, limited. Some initially soluble polymers tend to precipitate during gelation when a change in solvent composition leads to phase separation. 

In ISFETS utilizing polymeric ion-selective membranes, it has been always assumed that these membranes are hydrophobic. Although they reject ions other than those for which they are designed to be selective, polymeric membranes allow permeation of electrically neutral species. Thus, it has been found that water penetrates into and through these membranes and forms a nonuniform concentration gradient just inside the polymer/solution interface (Li et al., 1996). This finding has set the practical limits on the minimum optimal thickness of ion-selective membranes on ISFETS. For most ISE membranes, that thickness is between 50-100 jttm. It also raises the issue of optimization of selectivity coefficients, because a partially hydrated selective layer is expected to have very different interactions with ions of different solvation energies. 

If a polymer molecule in solution behaves as a random coil, its average end-to-end distance is proportional to the square root of its extended chain length (see page 25) - i.e. proportional to Ai 5, where Mr is the relative molecular mass. The average solvated volume of the polymer molecule is, therefore, proportional to M 5 and, since the unsolvated volume is proportional to A/r, the average solvation factor is proportional to (i.e. Af 5). The intrinsic viscosity of... 

In this chapter we will mostly focus on the application of molecular dynamics simulation technique to understand solvation process in polymers. The organization of this chapter is as follow. In the first few sections the thermodynamics and statistical mechanics of solvation are introduced. In this regards, Flory s theory of polymer solutions has been compared with the classical solution methods for interpretation of experimental data. Very dilute solution of gases in polymers and the methods of calculation of chemical potentials, and hence calculation of Henry s law constants and sorption isotherms of gases in polymers are discussed in Section 11.6.1. The solution of polymers in solvents, solvent effect on equilibrium and dynamics of polymer-size change in solutions, and the solvation structures are described, with the main emphasis on molecular dynamics simulation method to obtain understanding of solvation of nonpolar polymers in nonpolar solvents and that of polar polymers in polar solvents, in Section 11.6.2. Finally, the dynamics of solvation with a short review of the experimental, theoretical, and simulation methods are explained in Section 11.7. 

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