Polymer solutions enthalpy change

Polymerization of 4-bromo-6,8-dioxabicyclo[3.2.1 ]octane 2 7 in dichloromethane solution at —78 °C with phosphorus pentafluoride as initiator gave a 60% yield of polymer having an inherent viscosity of 0.10 dl/g1. Although it is not described explicitly, the monomer used seems to be a mixture of the stereoisomers, 7 7a and 17b, in which the bromine atom is oriented trans and cis, respectively, to the five-membered ring of the bicyclic structure. Recently, the present authors found that pure 17b was very reluctant to polymerize under similar conditions. This is understandable in terms of a smaller enthalpy change from 17b to its polymer compared with that for 17a. In the monomeric states, 17b is less strained than 17a on account of the equatorial orientation of the bromine atom in the former, whereas in the polymeric states, the polymer from 17b is energetically less stable than that from 17a, because the former takes a conformation in which the bromine atom occupies the axial positioa Its flipped conformation would be even more unstable, because the stabilization by the anomeric effect is lost, in addition to the axial orientation of the methylene group. 

Another factor that has inhibited the precise determination of H bond enthalpies of polymer forming compounds has been the uncertainty of the species present. Thus the determinations of the H bond enthalpy per mole in pure alcohol, as given by Mecke (1375), is of restricted value because of the variety of species present. On the other hand, solution determinations of AH have the advantage that van der Waals attractions play only a minor role. In an inert solvent, for example, little change is expected in the van der Waals attractions... 

In the above considerations, the hydrophobic portions of both the membrane polymer and the small molecules that enter the membrane are expected to interact in the hydrophobic microphases in the membrane. It therefore becomes useful to find a numerical measure of the miscibility of these hydrophobic portions of molecules. In the case of complete molecules, both small and polymeric, the solubility parameter concept has been useful in the past. This concept is related to the enthalpy change which occurs on mixing in regular solution theory as developed by Hildebrand and coworkers (10) and as used for polymer solution theory by Flory (11). The Hildebrand solubility parameter is a measure of the attraction between molecules of the same kind, including dispersion forces, polar forces, and hydrogen bonding... 

The first extrapolation by a square root of concentration in Zr seems to be incorrect, because the ionic strength in these solutions is not affected by the concentration of Zr but by that of the background electrolyte. At constant ionic strength, the dissolution enthalpy should be independent of the Zr concentration. The observed concentration dependeney may indicate a change in solution speciation, hence polymer formation may have occurred. The inclusion of the data of the authors in the overall assessment of Zr hydrolysis requires a re-interpretation of their experimental data in terms of mono-mer/polymer transition. Polymer formation in similar solutions was analysed by [75TRI/SCH]. A re-interpretation was found to be important in similar solutions [75TRI/SCH] (see Appendix A entry) aceording to the reaction ... 

Enthalpies of solution or mixing, expressed as the enthalpy change per unit mass of polymer, are given in the table at infinite dilution, i.e., a very small amount of polymer and a large excess of solvent were mixed isothermaUy to form a homogeneous solution. By thermodynamics, or A H "" are obtained from the fol-... 

In an attempt to clarify the mechanism whereby electrolytes influence the solvency of water for a polymer, Garvey and Robb (1979) measured the heats of dilution of poly(vinyl pyrrolidone) in aqueous electrolyte solutions. The heat of dilution was found to be negative (i.e. mixing is favoured by the enthalpy changes) so that phase separation must inevitably be controlled by entropy effects. Thermodynamics demands that in the salting-out of a polymer by a second component, this component must be negatively adsorbed from the polymer. This can be expressed mathematically by... 

The Handbook is again divided into seven chapters (1) Introdnction, (2) Vapor-Liqnid Eqitilibritmi (VLB) Data and Gas Solirbilities at Elevated Pressirres, (3) Liqnid-Liqnid EqniUbrinm (LLE) Data of Polymer Solntions at Elevated Pressirres, (4) High-Pressnre Flnid Phase EqniUbrinm (HPPE) Data of Polymer Solutions, (5) Enthalpy Changes in Polymer Solntions at Elevated Pressirres, (6) PVT Data of Polymers and Solntions, and (7) Pressnre Dependence of the Second Virial Coefficients 042) of Polymer Solutions. Finally, appendices quickly route the user to the desired data sets. Thus, the book covers aU the necessary areas for researchers and engineers who work in this field. 

ENTHALPY CHANGES IN POLYMER SOLUTIONS AT ELEVATED PRESSURES... 

Enthalpy changes for polymer solutions at elevated pressrrres... 

Some implicit databases are provided within the Polymer Handbook by Schuld and Wolf or by Orwoll and in two papers prepared earlier by Orwoll. These four sources list tables of Flory s %-function and tables where enthalpy, entropy or volume changes, respectively, are given in the literature for a large number of polymer solutions. The tables of second virial coefficients of polymers in solution, which were prepared by Lechner and coworkers (also provided in die Polymer Handbook), are a valuable source for estimating the solvent activity in the dilute polymer solution. Bonner reviewed vapor-liquid equilibria in concentrated polymer solutions and listed tables containing temperature and concentration ranges of a certain number of polymer solutions." Two CRC-handbooks prepared by Barton list a larger number of fliermodynamic data of polymer solutions in form of polymer-solvent interaction or solubility parameters." ... 

Measurement of the enthalpy change caused by solving a given amount of the solute polymer in an (increasing) amount of solvent, i.e., the solution experiment... 

---