Modification of PVDF nanoparticles by controlled free radical graft copolymerization in supercritical carbon dioxide

PVDF (polyvinylidenefluoride) membranes are widely used in microfiltration (MF) and ultrafiltration (UF) due to their excellent processability, chemical resistance, well-controlled porosity, and good thermal properties [1]. Moreover this polymer is also frequently adopted in bio-medical devices owing to its good biocompatibility [2]. On the other hand, the use of PVDF membranes in biomedical applications is limited to some extent by the hydrophobicity of their exposed area, which mainly arises from the nature of the pore surface. Prompted by these considerations in the present work we have studied the modification of PVDF nanoparticles with hydroxyethylmethacrylate (HEMA) thus obtaining a modified macromolecular material that could be used in the preparation of more hydrophilic membranes. The nanoparticles modification is carried out by atom transfer radical polymerization (ATRP) in supercritical carbon dioxide as reaction medium. The use of supercritical carbon dioxide (scCO2) allows to perform the modification process in the absence of any toxic solvent and without dissolution of the polymer matrix thus preserving its original morphology. scCO2 has been object of relevant interest as a medium for polymer processing. The origin of such a plethora of research efforts is the capability of CO2 to swell and plasticize amorphous domains of macromolecular materials. As a consequence of this attitude the free volume of the amorphous domains of the polymer is increased and this leads to enhancement of the diffusion rate of low molecular weight molecules in the matrix. Owing to the aforementioned mass transfer acceleration effect scCO2 has been extensively studied as a solubilizing and carrier agent to transport small molecules inside polymer materials [3]. If the solute is a reactive monomer, the scCO2 assisted infusion can be followed by polymerization and through this route it is possible to prepare a massive composite, a surface modified organic polymer compound or to induce grafting of suitable polar monomers onto the polymer backbone [4]. The ATRP procedure has been used to prepare graft copolymers with regularly-spaced polymer chains from polymeric macroinitiators that have pendant chemical groups containing radically transferable halogen atoms [5]. The halogen atom serves as initiation site for the polymerization of side chains and the reaction is often catalyzed by a suitable coordination complex of the redox couple CuII/CuI. Our research has been designed and carried out to find the operative conditions that allows the operator to have high degree of grafting. To this purpose we have studied the effect of the nature of the ligand, of the copper salt, of the temperature and time of reaction in the ATRP experiments. According to collected data the best results in scCO2 were obtained using Cu(I)Cl /4,4′-dimethyl-2,2′-dipyridyl as catalytic complex at 65°C, in the presence of benzoyl peroxide with reaction time of 16 hours. Additionally, we used electrochemical techniques in polar organic solvents to investigate the effectiveness of the fluorinated polymer as ATRP macroinitiator and to monitor the relative concentration of the components of the redox couple Cu(I)/Cu(II) during the graft copolymerization. [1] Wang, D.; Li, K.; Teo, W. K. J. Membr. Sci. 1999, 163, 211. [2] U. Klinge, B. Klosterhalfen, A.P. Ottinger, K. Junge, V. Schumpelick; Biomaterials,2002, 23, 3487–3493. [3] Berens A. R.; Huvard, G. S.; Korsmeyer, R. W.; Kunig, F. W. Journal of Applied Polymer Science 1992, 46, 231-242 [4] Li, D.; Han, B.; Liu, Z. Macromol. Chem. Phys. 2001, 202, 2187-2194 [5] J. F. Hester, P. Banerjee, Y. Y. Won, A. Aktakul, M. H. Acar, A. M. Mayes, Macromolecules, 2002, 35, 7652. Acknowledgements: the financial support of Università di Palermo, Project 2012-ATE-0377 is gratefully acknowledged.