Polymer processing using supercritical fluids : doctoral dissertation

english

doktorska dela

2009

Univerza v Mariboru, Fakulteta za kemijo in kemijsko tehnologijo

The traditional methods for polymer processing use environmentally hazardous volatile organic solvents and chlorofluorocarbons. Due to the increase of hazardous solvent emission and generation of aqueous waste streams, there is an obvious need of finding new and cleaner methods for the processing of polymers. Supercritical carbon dioxide (scCO2) has attracted particular attention for these applications due to its tremendous potential as a plasticizer in polymer processing. A particular interest is shown to the use of supercritical fluids for processing polymers destined for biomedical applications (as microspheres, microcapsules, foams, membranes, polymer/drug composites). The method offers important advantages related to the absence of harmful organic solvents or, when necessary, the efficient extraction of solvents and impurities, the mild processing conditions and the control of particle and foams morphology by simple variation of pressure and temperature. Despite the huge potential of scCO2 as a "green" solvent for processing biocompatible and biodegradable polymers, the phase equilibrium data, essential for process design, are quite scarce. Optimum processing techniques and parameters (pressure, temperature) still need consideration andstudy. The data are especially scarce regarding the scCO2 processing of polymer/ceramic composites for biomedical applications. On this basis, this thesis is aimed to open new perspectives over the use of scCO2 as a "green" solvent for the processing of biodegradable polymers and composites used as biomaterials. Two biodegradable polymers were chosen for this study, poly(L-lactide) (PLLA) and poly(D,L-lactide-co-glycolide) (PLGA). Their composite with a bioactive ceramic powder, hydroxyapatite (HA), was also studied. The main idea followed by this thesis was the obtaining of porous polymeric or composite material scaffolds suitable for tissue engineering under mild temperature conditions and without the use of additional organic solvents. The behavior of the two polymers under dense CO2 had been studied and explained. More data about the polymer-gas phase equilibrium, necessary for understanding and optimizing the effect of processing parameters, were obtained by determining the solubility and diffusion coefficients of CO2 in the polymers for certain values of temperature and pressure. The solubility ofCO2 was measured for each polymer for three different temperatures (308, 313and 323 K) in the pressure range 10 - 30 MPa. The temperatures were chosen higher than the critical temperature for CO2, but still low enough so as not to affect the bioactivity of any drugs or proteins that could be introduced in the system during processing. The same range of temperature and pressure was employed for all tests involving the studied polymers or their composite materials. The efficiency of mixing in the presence of scCO2 for obtaining composite materials from PLLA and HA and respectively PLGA and HA was studied by comparison with coprecipitation. The solubility and diffusion coefficient of CO2 in the composite materials were afterward determined, and the results were compared with the ones obtained for the polymer alone in order to determine the effect of the ceramic filler on the gas uptake. The possibility of obtaining porous scaffolds was assessed by using a pressure quench technique using dense CO2 as blowing agent, with and without the presence of aporogen. The effect of pressure, temperature, depressurization rate and porogen on the final porous structure was investigated. The experimental results were compared with literature data and with data obtained by mathematical modeling, employing equations of state commonly used for polymersor polymer/solvent systems. The results indicate that gas foaming of biodegradable polymers represents a promising technique for obtaining tissue engineering scaffolds with the desired structure. Still the processing parameters need to be studied and optimized, according to the nature of the substrate and of the aimed final product.

07.08.2023