Modeling Ternary Liquid-Liquid Equilibria of Polylactides-Water-1,4-Dioxane with PC-SAFT Eos for TIPS Scaffold Fabrication

Poly(L-Lactic Acid) or PLLA and Poly(D-Lactic Acid) or PDLA and their copolymers are biocompatible and biodegradable polymers that could be used for fabrication of porous scaffolds suitable for tissue engineering applications. Among the many fabrication procedure listed in literature, phase separation methods seems to be adequate for producing porous structures with pore size distribution compatible with tissue growth. It is believed that the pore size distribution and pore topology can be tailored by adjusting the thermodynamic and kinetic parameters of the process. Thermally Induced Phase Separation processing of polylactides can be exploited by controlled cooling of solution of polylactides in 1,4-dioxane, that is a good solvent of the polymer, containing a given amount of water, that acts as a non-solvent. Upon cooling the system will demix and two phases will appear, namely a polymer lean solution and a polymer rich solution, that is going to give rise to the solid phase of interest. Although in the case of optically pure PLLA it is know that crystallization plays a major role in the formation of the solid phase, knowledge of the liquid-liquid equilibria of the ternary solution is fundamental in any modelling or optimization effort of the whole TIPS fabrication process. Liquid-liquid equilibria of ternary systems comprising a macromolecular component is a challenging task for every kind of thermodynamic model. Moreover the presence of water and its peculiar role of non solvent, that upon cooling promotes phase separation, calls for the use of a model that can endow the underlying physics of hydrogen bonding. Actually, as shown by Mannella, modelling the phase behaviour of the binary mixture made by 1,4-dioxane and water is a task that can’t be accomplished by several commonly used activity coefficient models. Moreover it seems that even the modelling capabilities of Equation of State based on statistical mechanics theory, like the Sanchez Lacombe EOS, are deeply challenged by this mixture. The Perturbated Chain – Statistical Associating Fluid Theory (PC-SAFT) Equation of State is a model based on a well defined picture of the intermolecular interactions that has been shown to be able to predict pure substances properties and phase equilibria of water, polar substances, macromolecular species and components of biological interests. In the present work we discuss the modelling the pure component properties of 1,4-Dioxane as well as those of the polylactides, the binary Vapor-Liquid Equilibria of 1,4-Dioxane –Water and the ternary systems PDLA-1,4-Dioxane-Water and PLLA-1,4-Dioxane-Water with PC-SAFT EOS. Comparisons with several sets of experimental data from the literature show that PC-SAFT EOS performs very well respect to this very challenging systems, correctly depicting the underlying physics.