Phosphonium-based ionic liquids as new Greener plasticizers for poly(vinyl chloride) biomedical applications

Poly(vinyl chloride) (PVC) is one of the most widely used thermoplastic materials mostly due to its known advantages such as availability, low cost, processability and broad range of properties, that enable its large variety of applications ranging from building construction (tubes, pipes, flooring) to biomedical applications (blood bags, catheters). Depending on the envisaged processing method and on the intended application, different levels of viscosity and of material softness and flexibility may be needed and this can be controlled and achieved by the incorporation of external plasticizers into rigid PVC. Plasticizers are usually physically mixed with PVC and their migration and release when in contact with a certain surrounding medium may occur in different extents. The most commonly employed plasticizers for PVC are phthalate esters. However, these are now being banned or highly controlled by the European Union for several applications such as plastic children toys and medical devices due to their potential toxicity and low biodegradability. Therefore, both alternative plasticizers and substitute plastic materials have been assessed for the substitution of phthalate-plasticized PVC but, for many applications, no material has yet been found that can satisfactorily substitute soft PVC and no low molar mass plasticizer has been able of replacing phthalate esters without inducing migration. Ionic Liquids (ILs) are readily modifiable materials with physical properties that make these compounds amenable to a wide range of applications. Due to some of these properties (negligible vapor pressures, high thermal stability and a large liquid-phase temperature range) together with the large number of different ILs (with different properties) that can be prepared by the conjugation of different cations and anions, recently ILs have been proposed as new solvent media for polymerization reactions and/or as polymer plasticizers. In this work, the effects of PVC molecular weight and of IL plasticizer type and composition on the properties of plasticized PVC were studied. Two different molecular weight PVC materials (1200 and 2000 Da) were incorporated (by film casting) with increasing amounts of a phosphonium-based IL (0, 5, 10 and 20% w/w), comprised by similar cations but with different anions having different hydro-phylicities (namely Cl-, Br-, N(SO2CF3)(2)(-) and N(CN)(2)(-)). A conventional organic plasticizer (diisononyl phthalate, DINP) was also used for comparison purposes. Films were prepared by a solvent film casting method and using THF as the organic solvent. The migration/leaching (in water and in pH 7.0 HBSS buffer solutions) of employed plasticizers from prepared films was quantified at different temperature and stirring conditions. Because the migration of plasticizers can lead to changes in the thermal stability of PVC, obtained films (before and after leaching) were characterized by DMTA, TGA and DSC. Samples were also characterized by water and water-vapor sorption, SEM-EDX, FTIR-ATR and by contact angle determination. Considering one of the desired final biomedical applications (plasticized PVC materials for blood bags) blood biocompatibility tests were also carried out for pure ILs and for plasticized films, and in order to verify their toxicity against rabbit blood cells. Obtained results showed that all the studied ILs presented similar or even superior performances as biocompatible plasticizers to the employed conventional PVC plasticizer (DINP). Amongst all employed ILs, (trihexyl) tetradecylphosphonium bis(trifluoromethylsulfonyl) imide presented the most interesting and promising results regarding plasticized PVC thermal stability, blood biocompatibility and plasticizer leaching results.