Melting point depression solid polymers

The final colligative property, osmotic pressure,24-29 is different from the others and is illustrated in Figure 2.2. In the case of vapor-pressure lowering and boiling-point elevation, a natural boundary separates the liquid and gas phases that are in equilibrium. A similar boundary exists between the solid and liquid phases in equilibrium with each other in melting-point-depression measurements. However, to establish a similar equilibrium between a solution and the pure solvent requires their separation by a semi-permeable membrane, as illustrated in the figure. Such membranes, typically cellulosic, permit transport of solvent but not solute. Furthermore, the flow of solvent is from the solvent compartment into the solution compartment. The simplest explanation of this is the increased entropy or disorder that accompanies the mixing of the transported solvent molecules with the polymer on the solution side of the membrane. Flow of liquid up the capillary on the left causes the solution to be at a hydrostatic pressure... 

The melting temperature depends on water content, as depicted in Figure 6.24b. This phenomenon is comparable to the melting point depression commonly observed for impure solid materials (e.g., imperfect crystals). Flory has derived an equation for the melting temperature Tm as a function of the volume fraction of polymer q> in concentrated polymer-... 

The melting point depression of solid polymers (with no liquid diluent) may be derived from classical solution theory considerations. For ideal solutions of low molecular weight molecules, the equation analogous to equation 3.100 is... 

An attempt to quantitatively examine these observations in terms of theory is given in Fig. 7.5. The solid line in this plot is computed from Eq. (7.18) with x i = 0 and neglect of the elastic contribution. For the low values of v, where neither the thermodynamic interaction term nor the elastic term make an appreciable contribution to the melting point depression, the data follows the simplest theoretical expectation. As the polymer concentration in the mixed phase decreases, a contribution to the melting point depression of the omitted terms is expected. Small deviations from the simplified theory are observed. A small positive value of xi believed to be appropriate for this system, brings the observed and calculated values very close... 

In dry spinning, the dope is extruded through spinnerets located at the top of a tower. As the uncoagulated filaments flow down the tower, they are brought into contact with an inert gas heated above the boiling point of the dope solvent. The solvent evaporates from the filaments as they pass down the column and solidify. Conceptually, dry spinning can be considered to be a special case of melt spinning, in which the polymer solidification or crystallization temperature has been depressed by the solvent. The solidification temperature of the filaments will continuously increase as the solvent evaporates until solid filaments are formed. The filaments are continuously removed from the bottom of the tower, washed free of solvent, and then for the most part processed like the wet-spun fibers. 

---