Polymer/solvent interaction

In polymer solutions and blends, it becomes of interest to understand how the surface tension depends on the molecular weight (or number of repeat units, IV) of the macromolecule and on the polymer-solvent interactions through the interaction parameter, x- In terms of a Hory lattice model, x is given by the polymer and solvent interactions through... 

A good solvent is the technical as well as descriptive term used to identify a solvent which tends to increase coil dimensions. Since this is a consequence of thermodynamically favorable polymer-solvent interactions, good solvents also dissolve polymers more readily in the first place. 

Many ceUulosic derivatives form anisotropic, ie, Hquid crystalline, solutions, and cellulose acetate and triacetate are no exception. Various cellulose acetate anisotropic solutions have been made using a variety of solvents (56,57). The nature of the polymer—solvent interaction determines the concentration at which hquid crystalline behavior is initiated. The better the interaction, the lower the concentration needed to form the anisotropic, birefringent polymer solution. Strong organic acids, eg, trifluoroacetic acid are most effective and can produce an anisotropic phase with concentrations as low as 28% (58). Trifluoroacetic acid has been studied with cellulose triacetate alone or in combination with other solvents (59—64) concentrations of 30—42% (wt vol) triacetate were common. 

Gelatin stmctures have been studied with the aid of an electron microscope (23). The stmcture of the gel is a combination of fine and coarse interchain networks the ratio depends on the temperature during the polymer-polymer and polymer-solvent interaction lea ding to bond formation. The rigidity of the gel is approximately proportional to the square of the gelatin concentration. Crystallites, indicated by x-ray diffraction pattern, are beUeved to be at the junctions of the polypeptide chains (24). 

Dilute Polymer Solutions. The measurement of dilute solution viscosities of polymers is widely used for polymer characterization. Very low concentrations reduce intermolecular interactions and allow measurement of polymer—solvent interactions. These measurements ate usually made in capillary viscometers, some of which have provisions for direct dilution of the polymer solution. The key viscosity parameter for polymer characterization is the limiting viscosity number or intrinsic viscosity, [Tj]. It is calculated by extrapolation of the viscosity number (reduced viscosity) or the logarithmic viscosity number (inherent viscosity) to zero concentration. 

Polyisobutylene is readily soluble in nonpolar Hquids. The polymer—solvent interaction parameter Xis a. good indication of solubiHty. Values of 0.5 or less for a polymer—solvent system indicate good solubiHty values above 0.5 indicate poor solubiHty. Values of X foi several solvents are shown in Table 2 (78). The solution properties of polyisobutylene, butyl mbber, and halogenated butyl mbber are very similar. Cyclohexane is an exceUent solvent, benzene a moderate solvent, and dioxane a nonsolvent for polyisobutylene polymers. 

Table 2. Polymer—Solvent Interaction Parameters for Polyisobutylene and Butyl Rubber...

Tseng et al. [164] suecessfully used UNIFAC to optimize polymer-solvent interactions in three-solvent systems, determining polymer activity as a function of the solvent eomposition. The composition yielding the minimum in polymer aetivity was taken as the eriterion for optimum interaetion, and it eompared well with experimental measurements of dissolution rate and solution clarity. Better agreement was obtained using UNIFAC than using solubility parameter methods. 

Gch-s characterizing free energy of polymer-solvent interaction for sufficiently long chains, can be expressed as follows ... 

Part (a) is the driving force for the adsorption. If only (a) were present, adsorbed chains would lie flat on the surface. Parts (b) and (c) are the opposing forces (b) accounts for the entropy loss of a bond on the surface as compared to the solution, (c) represents the separation into a concentrated surface phase and a dilute solution. Part (d) arises from polymer-polymer, solvent-solvent and polymer-solvent interactions, which usually favour accumulation of segments. At equili-... 

Equation (23) predicts a dependence of xR on M2. Experimentally, it was found that the relaxation time for flexible polymer chains in dilute solutions obeys a different scaling law, i.e. t M3/2. The Rouse model does not consider excluded volume effects or polymer-solvent interactions, it assumes a Gaussian behavior for the chain conformation even when distorted by the flow. Its domain of validity is therefore limited to modest deformations under 0-conditions. The weakest point, however, was neglecting hydrodynamic interaction which will now be discussed. 

From the weak dependence of ef on the surrounding medium viscosity, it was proposed that the activation energy for bond scission proceeds from the intramolecular friction between polymer segments rather than from the polymer-solvent interactions. Instead of the bulk viscosity, the rate of chain scission is now related to the internal viscosity of the molecular coil which is strain rate dependent and could reach a much higher value than r s during a fast transient deformation (Eqs. 17 and 18). This representation is similar to the large loops internal viscosity model proposed by de Gennes [38]. It fails, however, to predict the independence of the scission yield on solvent quality (if this proves to be correct). 

According to our initial hypothesis, these anomalous effects are the experimental results occurring under kinetic control of conformational relaxation. Here conformational relaxation is exposed over its entire length to the influence of the electrochemical variables, the temperature, the polymer-polymer interactions, the polymer-solvent interactions, etc. These are the monitors that can be used to validate each new step of theoretical development during our attempt to integrate electrochemistry and polymer science. 

The conformational changes are stimulated by cathodic compaction of the structure, by anodic oxidation and swelling, or, in absence of any external electric field, by thermal energy.177,178,188 In any case, they are a combination of the polymer-solvent interactions. So A can be expressed as a sum of three terms... 

IX. POLYMER-SOLVENT INTERACTIONS FROM THE ELECTROCHEMICALLY STIMULATED CONFORMATIONAL RELAXATION MODEL... 

Figure47. Chronoamperometric responses to potential steps carried out using a polypyrrole electrode from -2000 to 300 mV vs. SCE for 50 s, in 0.1 M UCI04 solutions of different solvents. (Reprinted from H.-J. Grande, T. F. Otero, and I. Cantero, Conformational relaxation in conducting polymers Effect of the polymer-solvent interactions. 7. Non-Cryst. Sol. 235-237,619, 1998, Figs. 1-3, Copyright 1998. Reproduced with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.)...

Here x is the polymer-solvent interaction parameter defined by Flory for polymeric swelling its value becomes more negative as the interaction between the solvent and the polymer increases. By comparing Eqs. (45) and (46), we arrive at... 

The polymer-solvent interaction parameter, which is a key constant defining the physical chemistry of every polymer in a solvent, can be obtained from electrochemical experiments. Definition and inclusion of this interaction was a milestone in the development of polymer science at the beginning of the 1950s. We hope that Eq. 47 will have similar influence in the development of all the cross-interactions of electrochemistry and polymer science by the use of the ESCR model. A second point is that Eq. 47 provides us with an efficient tool to obtain this constant in electroactive... 

In good solvents, a polymer becomes well solvated by solvent molecules and the conformation of its molecules expands. By contrast, in poor solvents a polymer is not well solvated, and hence adopts a relatively contracted conformation. Eventually of course, if the polymer is sufficiently poor the conformation becomes completely contracted, there are no polymer-solvent interactions, and the polymer precipitates out of solution. In other words, the ultimate poor solvent is a non-solvent. 

One factor which affects the extent of polymer-solvent interactions is relative molar mass of the solute. Therefore the point at which a molecule just ceases to be soluble varies with relative molar mass, which means that careful variation of the quality of the solvent can be used to fractionate a polymer into... 

In equation (6.6), F is a constant and g is a function that varies according to the extent of polymer-solvent interaction, and has values close to zero for poor solvents and values close to 0.25 for good solvents. 

Kuwahara N. On the polymer-solvent interaction in polymer solutions. Journal of Polymer Science Part A.l, 7, (1963) 2395-2406. 

But for any arbitrary polymer concentration C2 there will be another osmotic pressure due to the previously considered polymer-solvent interaction and to the associated elastic reaction of the network. Recalling the general relationship tz= — (mi —mi)/vi, we may calculate oTo from Eq. (38). At equilibrium the total osmotic pressure arising from the effects of all solutes must be zero, i.e., — JCq. Hence from Eqs. 

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