Removal Of Residual Solvent Research Articles

Process variable implications for residual solvent removal and polymer morphology in the formation of gentamycin-loaded poly (L-lactide) microparticles.

The purpose was to determine the influence of process parameters in the precipitation with a compressed antisolvent (PCA) process on the morphology and residual dichloromethane (DCM) levels in gentamycin-loaded PLA microparticles. The three variables studied were the rate of CO2 co-flowed during the polymer and drug post-precipitation, the post-precipitation pure CO2 flush rate, and the post-precipitation CO2 flush volume. Residual DCM levels were determined from headspace gas chromatography-mass spectroscopy (GC-MS) with single ion monitoring. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were used to estimate the crystallinity within microparticles. DCM was extracted from drug-loaded microparticles by both supercritical CO2 extraction and vacuum drying for up to two days to determine a lower limit for solvent removal. Increasing either the post-precipitations CO2 flow rate or flush volume resulted in lower residual DCM levels in the microparticle. The CO2 co-flow rate showed an opposite trend. Increasing in value resulted in a higher DCM value after precipitation. XRD and DSC analysis on these samples suggest that those produced at lower CO2 co-flow rates have a higher degree of crystallinity, which increases the diffusivity of DCM through the polymer matrix. Finally, samples subjected to extended (48 hr) CO2 extraction resulted in DCM levels on the order of one to three ppm. Specific PCA process conditions during microparticle formation have a strong influence on the residual solvent levels within the microparticles. Polymer morphology affects the diffusivity of solvent through the polymer matrix, which in turn determines the solvent removal rates.

Relaxation phenomena in dense, glassy membranes prepared from a polymer solution, monitored by changes in dielectric properties

The drying of a solvent-cast polymer film is monitored in a non-invasive way, by measuring the changes in time of dielectric properties, using interdigitated or comb electrodes. Experimentally, the vitrification of the polymer solution is observed at a distinct time, followed by the slow evaporation of solvent from the glassy state. As the solvent diffusion coefficient is a strong function of the concentration in the polymer film, removal of residual solvent proceeds at a self-decreasing rate. With a simple model, solvent mass transfer coefficients in the glassy state are determined from the experiments. It is shown that volume relaxation may be slow compared to the drying process, when preparing thin solvent-cast membranes and using solvents that diffuse relatively fast through the glassy polymer film.

Spectroscopic Investigations of Neptunium's and Plutonium's Oxidation States in Sol-Gel Glasses as a Function of Initial Valence and Thermal History

ABSTRACTSeveral oxidation states of neptunium and plutonium, Pu(III), Pu(IV), Pu(VI), Np(IV), Np(V), and Np(VI), were studied in glasses prepared by a sol-gel technology. The oxidation state of these actinides in the sol-gel product was examined by absorption spectroscopy after solidification, aging, and thermal treatment. The oxidation state of the actinides in the starting solutions was essentially maintained through the solidification process of the silica matrix. However, during densification and removal of residual solvents at elevated temperatures, both actinides converted eventually to their tetra valent states while in the different sol-gel products. This finding is in accord with reports that tetravalent states of plutonium and neptunium are acquired in glass products prepared by dissolution of the actinide in molten glasses. Comparisons between room temperature spectra obtained from neptunium and plutonium in heated sol-gel products and from molten glass products showed subtle differences that can be related to the metal ion's environments.

Comments on paper ‘residual solvent removal and n-hexane sorption in blends of atactic and isotactic polystyrene’: D. L. Faulkner, H. B. Hopfenberg and V. T. Stannett Polymer 1977, 18, 1130–1136

Comments on paper ‘residual solvent removal and n-hexane sorption in blends of atactic and isotactic polystyrene’: D. L. Faulkner, H. B. Hopfenberg and V. T. Stannett Polymer 1977, 18, 1130–1136

Morphological and physical property effects for solvent cast films of poly‐2, 5(6) benzimidazole

AbstractA polybenzimidazole having the structural repeat unit magnified image and designated AB‐PBI was formed into films by casting or precipitating from dilute solutions. Selected films were swollen in a plasticizing medium arid drawn about 2X. All films were dried in a vacuum oven to aid in the removal of residual solvent and moisture. Wide angle X‐ray scattering (WAXS) was used to show the type of ordering found in this material and to indicate the extent of molecular orientation. Neither a glass transition nor a crystalline melting point has been observed for this polymer, A Perkin‐Elmer DSC was used to scan the temperature range from −150 to H‐400°C in search of secondary transitions. The rheovibron proved more sensitive and a transition was located in the vicinity of −60°C. An activation energy‐was determined. The morphology of this polymer is subject to speculation. The ability of the chain to adopt a linear conformation leads to the possibility of a liquid‐crystalline nature in solution. A model based on the data in hand is suggested. Stress‐strain data was used to compare the mechanical properties of films prepared by several methods as well as to demonstrate the effect of orientation on strength.

Residual solvent removal and n-hexane sorption in blends of atactic and isotactic polystyrene

Atactic polystyrene, isotactic polystyrene, and intermediate polyblend films of the homopolymers were cast from solutions of 1,3,5-trimethylbenzene (mesitylene) or o-chlorotoluene. Residual solvent content, glass transition temperatures, and densities of the polyblends were determined. Consequent to these preliminary characterizations, n-hexane absorption kinetics and equilibria in unannealed blends were determined; maxima in the plots of weight change versus time were apparent for all compositions. The maximum is a consequence of the ‘unlocking’ of previously trapped residual casting solvent consequent to plasticizing invasion by the n-hexane in the unannealed film specimens. Neutron activation analysis for chlorine confirmed that the o-chlorotoluene content in the films was reduced dramatically consequent to n-hexane absorption. Residual solvent content was also reduced by thermally annealing the films at 110°C for 48 h under a vacuum and, therefore, no maximum in the sorption/time behaviour was observed in thermally annealed samples nor in films previously contacted with n-hexane vapour and subsequently vacuum stripped. Both the rate of sorption and the apparent sorption equilibrium are greatly reduced as the isotactic polystyrene content is increased. Predominantly Case II or relaxation controlled absorption kinetics were observed in the polyblends consequent to thermal annealing and/or ‘solvent’ annealing. Although the form of the sorption kinetics was quite similar for absorption of n-hexane in thermally annealed or ‘solvent-annealed’ films, the sorption rate and amount of penetrant sorbed at apparent equilibrium were both larger for the thermally annealed films. These results suggest that the history dependence, which so frequently affects polymeric glasses, appears to confound a simple interpretation of the sorption experiments reported here. Consistent with observations in related systems, Super Case II kinetics were observed for the more slowly sorbing films which, in this case, contained isotactic polystyrene. Moreover, film whitening related to micro-voiding accompanied n-hexane sorption in blends containing atactic polymer; however, the whitening in the isotactic polymer was less pronounced and densification, presumably related to solvent induced crystallization, occurred consequent to long term exposure of the isotactic polymer to n-hexane.

The barrier properties of polyacrylonitrile

The permeabilities and time lags characterizing transport in machine-cast polyacrylonitrile (PAN) films have been determined for helium, neon, argon, krypton, oxygen, nitrogen, and carbon dioxide. The measured permeabilities were decreased slightly and the observed time lags were increased consequent to scrupulous removal of residual solvent. The activation energies for permeation were not changed consequent to film purification, however. In all cases, the measured permeabilities were remarkably low. The permeability of PAN to helium was inordinately higher than the observed permeability of PAN to the other gases studied. The relatively high permeability to helium and the low activation energies for permeation suggest that the excellent barrier properties of PAN are related to the very small size of the fluctuating free volume elements involved in the rate-determining transport step for fixed gases in and through this particular polymer.