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Risultati finali
A test for easily predicting the dispersant capability of a stabilizer in scCO2 has been developed. The test is based on the fact that perfluoropolyether fluids can accurately mimic the behaviour of CO2. Accordingly, the test is carried out at room conditions using a fluoropolyether fluid instead of CO2. In case of VDF-based fluoropolymer, the test is based on the assumption that the capability of dispersing a polar VDF-based fluoropolymer in CO2 can be simulated by the capability of solubilising water in a PFPE fluid. In case of TFE-based fluoropolymers, the test consists in analyzing the capability of a surfactant to disperse a PTFE powder in a PFPE fluid.
A mathematical model able to simulate dispersion polymerization in supercritical medium has been developed. The model constitutive equations include material balances of all reactants, thermodynamic equations to evaluate phase behaviour and population balances for the polymer species as well. The reaction in both continuous and dispersed phase is accounted for. Diffusion controlled expressions for all kinetic rate coefficients are considered. The interphase transport of all polymer species is also included in a general form, accounting for the different behaviour of species at different chain length. The numerical solution of the population balances is carried out using a discretization method.
The model has been validated by comparison with experimental data for the model system methyl methacrylate in scCO2 using Krytox as stabilizer. These data have been produced in our laboratory and supplied by other partners of this same project. Further validation is going on with reference to the polymerization of fluorinated monomers in scCO2.
A Molecular Dynamics code has been developed which allows including two immiscible fluids and surfactant molecules. The behaviour of this system can, thus, be observed at the microscopic level. The evolution towards separation and the formation of an interface have been monitored. It was found that the surfactant chains distribute themselves across this interface and this distribution results in a decrease of the interfacial tension. In many cases, the self-assembly of micelles can also be seen. The effect of the surfactant architecture was studied by employing Dissipative Particle Dynamics, a mesoscopic tool offered in the Materials Studio software. The adsorption of ''palm tree'' and random copolymers on a specific wall (polymer particle) in the presence of a solvent (CO2) was modelled. Values of interaction parameters were screened and the ones that resulted in adsorption were selected. Depending on the surfactant architecture, the volume fraction of the surfactant in the mixture was varied. For the case of palm tree surfactants, it was found that that bilayer structures were formed onto the polymer particle surface. This could result in particle coagulation and, eventually, in the production of a poor quality product.
The phase equilibrium properties of CO2, VDF and PVDF were studied by the Gibbs Ensemble Monte-Carlo method, a simulation technique at the molecular level. The code developed is generic and can also be used for mixtures and/or polymeric chains. The phase behaviour of CO2 was modelled by using force fields from the literature, under both normal and supercritical conditions, and yielded very good agreement with the available experimental data. Intermolecular potentials for VDF have also been developed, which describe very well the corresponding phase diagrams. Using the same method, the properties of the mixture (CO2+VDF) have been predicted under selected conditions and it was found that CO2 was completely miscible with VDF for the range of pressures and temperatures studied. The latest literature force fields were tested for the chemical species under study. The COMPASS all-purpose force field produced excellent results for scCO2. For VDF, the simulation results under PCFF were in perfect agreement with the experimental data. Both force fields yielded the correct PVT behaviour for the PVDF polymer. Summation methods for the difficult-to-handle columbic forces were also tested on the grounds of accuracy and computational cost. It was found that the group-based cut off method provided accurate results and was much faster than the conventional Ewald method.
Possible end-users of the software codes developed can be found in the polymer as well as in the chemical industry in general.
Nice particles of PVDF polymer with high molecular weight were achieved by VDF polymerization in supercritical carbon dioxide by using PDMS-mMA and Novel fluorinated graft alternative maleic anhydride copolymer as stabilizers. Good, uniform and size of 200~500 micro porous particles were obtained in presence of PDMS-mMA. Nice, spherical and size of 200~500 nm particles were obtained in presence of the novel graft stabilizer.
Design and assembly of a continuous reaction system based on a CSTR. The system is constituted by a pumping section for the delivery of the solvent and the reactants, a reaction section with the jacketed CSTR whose T is controlled by a PID cascade control loop and a separation section for the recovery of the polymer. Monomer conversion can be computed on-line by a GC whose injection system has been adapted to the sampling of the fluid stream. Even if the apparatus has been aimed to the investigation of heterogeneous polymerization in scCO2 it can be easily fitted to investigate different classes of reactions in scfluids.
A completely new approach-power compensation calorimetry (PCC) was developed. In-situ monitoring of the progress of polymerisations inside the high-pressure autoclave could be achieved by this novel PCC system. This methodology uses a simple, but highly informative power compensating calorimetric system to follow the polymerisations under supercritical conditions.
It enables us to follow changes in temperature, pressure, heater power and stirrer current during the reaction. PCC has been applied in monitoring homopolymerisation process of MMA, GMA, DMA, e-caprolacton, vinylidene fluoride and copolymerisation process of DMA and MMA in scCO2. The effects of stabilizer, initiator concentration, and stirring rate and monomer concentration have been studied in detail.
The results demonstrate that PCC is a very effective way to monitor polymerisation process. PCC also has been used to determine the viscosity of polymer plasticized by scCO2.
A novel technique to measure sorption and swelling in the same equipment (i.e. at the same operating conditions) has been developed and validated.
The equipment is a commercial magnetic suspension balance and the novelty is the two-step experimental procedure. Namely, two subsequent measurements of sample weight and fluid density are carried out. The first one in the presence of the component under examination (CO2 or monomers in this case) and the second one after the addition of a defined amount of an inert (typically Argon). Main advantages are that (1) the polymer sample does not need any pre-treatment before its use (can be used as powder, for example) and (2) the assumption of isotropic volume expansion (typically used in more conventional approaches) is not required.
The technique has been validated by comparing the results obtained for the model system poly-methyl methacrylate in scCO2 with those obtained using a conventional technique (balance combined with swelling measurement by visualization). The range of operating conditions ensuring reliable results has been identified. A minimum pressure of the component under examination is needed to avoid too large impact of the inert addition on the sorption behaviour.
The main achievement of the present technique is that sorption and swelling measurements can be carried out in the same experiment using commercial equipment and changing only the experimental procedure.
Polymerization conditions for the synthesis in sc-CO2 of (vinylidene fluoride/polydimethylsiloxane) graft copolymers. The optimized values of pressure, temperature, monomers concentration, polydimethylsiloxane molecular weight and functionality have been disclosed. Further, The impact of the polydimethylsiloxane macromonomers on polymerization kinetics, reactor fouling, powder properties and the molecular weight distribution has been assessed. It turned out that the incorporation of polydimethylsiloxane macromonomers is particularly efficient when the reaction is carried out in sc-CO2 environment.
Design and assembly of an experimental apparatus to perform dielectric spectroscopy in high-pressure fluid systems. A simple equivalent circuit has been defined in order to determine static dielectric constant and electrical conductivity from the measurement. Such apparatus can be considered a general purpose system suitable for the investigation of phase behaviour of scfluid systems, solubility of compounds, water in CO2 micro-emulsions, phase boundaries and envelopes.
A novel technique to measure sorption has been developed. The technique is based on "in situ" measurements performed by gas chromatography. The equipment is constituted by a pressure vessel containing the polymer and the gaseous mixture at desired temperature, pressure and composition and connected by an high pressure valve with a GC for analyses of gaseous concentration in situ. When the volume variation of the polymer cannot be neglected both mass uptake and volume swelling can be computed by the use of a not absorbable molecular probe.
The technique has been validated by comparing the results obtained for the model system scCO2-PVDF with those obtained by other researchers (Briscoe et al. and CPERI) using different techniques. A relevant achievement of the present technique is given by the possibility to achieve both sorption and swelling in the same experiment.
Method for the simultaneous measurement of sorption and swelling of supercritical fluids in polymers
The experimental method developed to measure diffusion in films is a combination of a gravimetric determination of mass transfer from a supercritical fluid phase to a solid phase within a high-pressure autoclave, and a simultaneous optical assessment of visual changes.
The apparatus consists of a magnetic suspension microbalance (MSB) connected to a view cell (VC). The view cell enables the determination of volume change of the solid sample by using a CCD camera. Solubility and swelling measurements of CO2 in polymer films can be carried out at temperatures up to 80oC and pressures up to 300bar. Sorption and swelling measurements were carried out simultaneously at the same temperature and pressure. Swelling isotherms for different pressures were obtained and, thus, the swelling or volume change was calculated. By knowing the exact volume V (t) of the sample at each time t, the total mass gain can be calculated by taking into account the buoyancy effect. Therefore, solubility can be corrected taking into account the swelling effect. Solubility and swelling measurements of CO2 in PVDF films were carried out at 40oC and pressures up to 210bar. Instead of one fluid, it is also possible to use a mixture of two (for example CO2 and VDF). The microbalance cell can also be connected to a gas chromatograph. In this respect, it is possible to measure the change of the mixture composition using chromatograph during the desorption period.
The process developed can be applied to solubility measurements of other supercritical fluids, as well that are of interest to the European Polymer Industry, such as ethylene, propylene, etc.
Graft copolymers of vinylidene fluoride and polydimethylsiloxane. Compatibilisation of the two polymer components is believed to take place, through an improved dispersion of the silicone elastomer. The crystallization temperature of VDF is not affected by polydimethylsiloxane while the heat degradation of the polymers as measured by the TGA method is proportionally increasing with the concentration of PDMS rubber present.
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