Polymer - solvent interactions, magnitude

For the solubility of TPA in prepolymer, no data are available and the polymer-solvent interaction parameter X of the Flory-Huggins relationship is not accurately known. No experimental data are available for the vapour pressures of dimer or trimer. The published values for the diffusion coefficient of EG in solid and molten PET vary by orders of magnitude. For the diffusion of water, acetaldehyde and DEG in polymer, no reliable data are available. It is not even agreed upon if the mutual diffusion coefficients depend on the polymer molecular weight or on the melt viscosity, and if they are linear or exponential functions of temperature. Molecular modelling, accompanied by the rapid growth of computer performance, will hopefully help to solve this problem in the near future. The mass-transfer mechanisms for by-products in solid PET are not established, and the dependency of the solid-state polycondensation rate on crystallinity is still a matter of assumptions. 

A characteristic feature of a dilute polymer solution is that its viscosity is considerably higher than that of either the pure solvent or similarly dilute solutions of small molecules. The magnitude of the viscosity increase is related to the dimensions of the polymer molecules and to the polymer-solvent interactions. Viscosity measurements thus provide a simple means of determining polymer molecular dimensions and thermodynamic parameters of interactions between polymer and solvents. These aspects will also be considered in a later part of this chapter. 

The quantity A2 in Eq. (2) is an important measure of polymer-solvent interaction. Its thermodynamic significance is somewhat complicated, the magnitude of the effect being determined by both the heat and entropy of solution. For any given polymer, A 2 is highest in the best solvent and lowest in the worst. If A2-becomes appreciably negative, the solvent becomes too weak to keep the polymer in solution and precipitation takes place. 

It is unlikely that this branching analysis can be reliably applied to graft or to block copolymers because chain dimensions—from which branching indices are determined—are influenced by polymer-polymer and polymer-solvent interactions. The magnitude of the problem is indicated by published data that indicate that graft copolymers can exhibit an intrinsic viscosity, for the same molecular weight, which is not even intermediate between the two homopolymers [6, 7]. A fortuitous match of cohesive energy densities... 

The different crosslinking behaviours of photopolymer films formed from different solvents can be explained with the help of different polymer conformations in these solvents. Studies were performed with one selected photopolymer (27). The second virial coefficients (A2), determined by light scattering, revealed the most coiled photopolymer structure in THF solution. This is to be expected since the magnitude A2 (found to be lowest in THF solution) is related to the Flory-Huggins constant Xi widely used in fundamental polymer studies as measure for the polymer-solvent interactions. If A2>0, the interactions between polymer molecules and solvent molecules are more attractive than the solvent-solvent interactions. Such solvents are considered to be thermodynamically good for the polymer. Thus, the low value for THF means that this solvent can be considered to be poor , leading to a more... 

Because of these uncertainties, equations 1, 2, 3, 4, and 5 may not be relied upon as a means of quantitative evaluation of A until more data for other polymer-solvent systems become available. The equation-of-state thermodynamics is, however, useful in its ability to give us insight into the physical factors and their relative magnitudes which contribute to the polymer-polymer interaction parameter. The results in this work clearly show that the dependence of A on concentration and temperature is moderate. This gives a justification as a good approximation to the use of a constant polymer-polymer interaction parameter in the polymer interface theories where the polymer concentration encompasses the whole range Wi = 0 to 1 across the phase boundary. 

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