Solvents system

Unlike with droplets of low-molecular-weight liquids or monomers that subdivide into smaller droplets under a strong electric field, polymer solutions undergo a degree of elongational flow and orientation in an electric field. It is the entanglement of the partially oriented, expanded conformations of polymer chains that makes their electrospinning possible in the first place. Solvents that yield open conformations of polymer chains and 

TABLE 4.1 Data for solvents commonly used in electrospinning of polymers 

The dramatic influence of solvents on electrospinnabflity is well illustrated by a study that investigated 18 different solvents for PS (M = 119,000 g/mol) (see Fig. 4.3). All but one of these dissolved the PS, but only four (DMF, MEK, THF, DCE) yielded electrospinnable solutions (at 10-30% w/v). Both the dipole moment and conductivity of the solutions were identified as key properties that determined electrospinnability (Jarusuwannapoom et al. 2005). Son et al. (2004d) studied PEO dissolved in five solvents at different concentrations selected to obtain [17] 10 (i.e., entangled semi- 

The electrospinning process fundamentally requires the transfer of electric charge from the electrode to the spinning droplet at the terminus of the tip. A minimal electrical conductivity in the solution is therefore essential for electrospinning solutions of zero conductivity cannot be electrospun. Solvents commonly used in electrospinning have conductivities that are 

The effect due to conductivity is not always easy to isolate or quantify. Changing solvent composition will often change the surface tension and dielectric constant as well, and consequent changes in electrospinning behavior cannot he uniquely attributed to changes in conductivity. Adding 

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G.H. Peters, D.M.F van Aalten, O. Edholm, S. Toxvaerd, and R. Bywater. Dynamics of proteins in different solvent systems Analysis of essential motion in lipases. Biophys. J., 71 2245-2255, 1996. 

Prediction of pKaS of Titratable Residues in Proteins Using a Poisson-Boltzmann Model of the Solute-Solvent System... 

Furthermore, most physicochemical properties are related to interactions between a molecule and its environment. For instance, the partitioning between two phases is a temperature-dependent constant of a substance with respect to the solvent system. Equation (1) therefore has to be rewritten as a function of the molecular structure, C, the solvent, S, the temperature, X etc. (Eq. (2)). 

The hydrophobic constant r is a measure of the contribution of a substituent X to the lipophilidty of compound R-X compared with R-H. The constant representing the solvent/solvent system, analogously to Hammett s p constant for the reaction type, was arbitrarily set to 1 for octanol/water and thus does not appear in Eq. (7). The lipophilidty constant ti allows the estimation of log P values for congeneric series of compounds with various substituents (see Eq. (8)). 

Protonated and diprotonated carbonic acid and carbon dioxide may also have implications in biological carboxylation processes. Although behavior in highly acidic solvent systems cannot be extrapolated to in vivo conditions, related multidentate interactions at enzymatic sites are possible. 

TABLE 8.31 Half-Wave Potentials (vs. Saturated Calomel Electrode) of Organic Compounds at 25°C The solvent systems in this table are listed below ... 

An interesting situation is obtained when the catalyst-solvent system is such that the initiator is essentially 100% dissociated before monomer is added and no termination or transfer reactions occur. In this case all chain initiation occurs rapidly when monomer is added, since no time-dependent initiator breakdown is required. If the initial concentration of catalyst is [AB]o,then chain growth starts simultaneously at [B"]q centers per unit volume. The rate of polymerization is given by the analog of Eq. (6.24) ... 

We concluded the last section with the observation that a polymer solution is expected to be nonideal on the grounds of entropy considerations alone. A nonzero value for AH would exacerbate the situation even further. We therefore begin our discussion of this problem by assuming a polymer-solvent system which shows athermal mixing. In the next section we shall extend the theory to include systems for which AH 9 0. The theory we shall examine in the next few sections was developed independently by Flory and Huggins and is known as the Flory-Huggins theory. 

Figure 8.2 Schematic illustrations of AGm versus X2 showing how jUj -may be determined by the tangent drawn at any point, (a) The polymer-solvent system forms a single solution at all compositions, (b) Compositions between the two minima separate into equilibrium phases P and Q.

Table 8.3 Theta Temperatures for a Few Polymer-Solvent Systems...

Table 9.2 Values for the Mark-Houwink Coefficients for a Selection of Polymer-Solvent Systems at the Temperatures Noted...

Quick-breaking foams consist of a miscible solvent system such ethanol (qv) [64-17-5] and water, and a surfactant that is soluble in one of the solvents but not in both. These foams are advantageous for topical appHcation of pharmaceuticals because, once the foam hits the affected area, the foam coUapses, deUvering the product to the wound without further injury from mechanical dispersion. This method is especially usehil for treatment of bums. Some personal products such as nail poHsh remover and after-shave lotion have also been formulated as quick-breaking foams. 

Hbls Process. Chemische Werke Huls AG has developed a process to produce soda ash and hydrochloric acid from salt via an amine—solvent system (12). A potential advantage of the Huls process is that, under some market conditions, hydrochloric acid may be more easily sold than either ammonium or calcium chloride. 

Akzo Process. Akzo Zout Chemie has developed a route to vinyl chloride and soda ash from salt usiag an amine—solvent system catalyzed by a copper—iodide mixture (13). This procedure theoretically requires half the energy of the conventional Solvay processes. 

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