THE SOLUBILITY PARAMETER OF POLY(DL-LACTIC ACID)

The solubility parameter delta as defined by Hildebrand was determined for amorphous poly(DL-lactic acid) by means of high precision density measurements. These measurements were used to obtain the partial specific volumes VBAR of the polymer dissolved in a series of solvents characterized by different solubility parameters delta(s). The solubility parameter of the polymer delta(p) was calculated from these values applying an approximation of the type VBAR = VBAR(max) - A x (delta(s) - delta(p))2. This curve is adduced from the polymer-solvent interaction and shows the highest partial volume for the polymer dissolved in the solvent with maximum solvation power. The fit on the base of seven solvents resulted in a solubility parameter of about 10.3 cal0.5 cm-1.5 for poly(DL-lactic acid) at 25-degrees-C. An evaluation starting from the measurement of only two solutions, as described in the literature for low-molecular weight substances, gave no reliable results in this case. Determinations of the solubility parameter by means of viscosity numbers was performed for the most part using the same solvents and leads to a comparable value of about 10.0 cal0.5 cm-1.5. These experimental results are compared with solubility parameters calculated by means of incremental methods.