Optimization of solution fractionation of poly(vinyl chloride)

The optimization of solution fractionation of poly(vinyl chloride) (PVC) is described with regard to the variation of several parameters. The best separation efficiency is obtained by a single-step discontinuous solution fractionation at the following conditions: solvent: cyclohexanone (CHN); nonsolvent: ethylene glycol (EG); temperature: 130°C, isothermal; contact-time: 2 h per fraction; amount of polymer: 2 mg/g support (glass beads). An essential improvement with respect to the separation of high molecular weight species can be achieved, applying a re-deposition method of the polymer remaining in the column after half of the fractionation steps. This diminishes the unfavorable formation of a molecular weight- and concentration gradient along the column, occurring during fractionation. The optimally controlled single-stage discontinuous solution fractionation of PVC turns out to be superior to multistage processes. This is attributed to the very critical problem of formation of insoluble microcrystalline aggregates with increasing duration of stay of the polymer in the column. The efficiency of the fractionation technique can be established by the characterization of fractions (center fractions: M?_w/M?_n < 1.1) and by confirmation of small differences in molecular weight distributions, shown by other methods.     

Anomalies in the fractional precipitation of polyvinylchloride

An accurate fractional precipitation, using tetrahydrofuran as solvent and water as nonsolvent, of polyvinylchloride prepared by a strictly homogeneous polymerization process, shows that the anomalies in the fractionation are associated, as main factor, to the more crystalline part of PVC. In fact, the first fractions exhibit a markedly higher degree of crystallinity than the successive fractions and the unfractionated polymer, as determined by X-ray diffraction. Nuclear magnetic resonance indicates a slightly more syndiotactic configuration of the first fractions with respect to the others.     

Solution fractionation of polystyrene by different methods

Systematic investigations of the solution fractionation of polystyrene were made in the system butyl acetate (solvent) and 1-propanol (nonsolvent). For the method of “complete elution” it was possible to show, that (a) the amount of a dissolved fraction increases proportional to the loading of the support material with polymer, (b) the average molecular weight of a fraction is independent of the elution velocity. Subfractions from complete elution show an approach to a limiting value of the average molecular weight and a common high molecular range in gel permeation chromatograms. The existence of a critical limiting molecular weight M⁺ of polymer solubility, which is a necessary theoretical assumption of the complete elution method, could thus be verified experimentally. Comparison of fractions from complete elution with corresponding fractions from discontinuous solution fractionation demonstrates a better separation efficiency—more uniform fractions and less mutual overlapping of consecutive fractions—for the method of complete elution.     

Hypochromism of poly(N-vinylcarbazole)

The hypochromy of poly(N-vinylcarbazole) in relation to N-ethylcarbazole was estimated for samples of different origin (cationic and radical polymerization). The effect of solvent polarizability and the influence of temperature as well as of the addition of nonsolvent were studied. An increase in hypochromy of the ¹L_b transition and a decrease in that of the ¹L_a transition with increasing polarizability of the solvent were observed. No change in hypochromy connected with increasing temperature and addition of nonsolvent was found.     

Submicron-size particles of ultrahigh molecular weight polyethylene produced via nonsolvent and temperature-induced crystallization

Submicrometer size particles of ultrahigh molecular weight polyethylene (UHMWPE) were produced by crystallization from dilute (0.1-1.0 wt % of UHMWPE) solvent/nonsolvent emulsions. The procedure consisted of mixing a hot solution of UHMWPE in decalin or decane with a nonsolvent (tetraglyme) at approximately 160 degrees C, followed by rapid cooling of the mixture to zero or subzero temperatures. The rapid cooling causes microphase separation between the two liquids, resulting in the formation of an emulsion, which consists of microdroplets of the supercooled UHMWPE solution dispersed in tetraglyme. The consequent crystallization of the polymer in the microdroplets produces a suspension of fine crystals of UHMWPE, which can easily be isolated. The particles were characterized using scanning electron microscopy, differential scanning calorimetry, and Raman spectroscopy. Their degree of crystallinity is between that of GUR 1050 original (powder) and processed (molded) polymer. By changing the polymer concentration, solvent to nonsolvent ratio, and temperature, the size (from 0.1-1.0 microm) and shape (spheroids or rods) of the particles can be controlled. These particles may be used for immunochemical investigations and the study of the influence of UHMWPE wear debris on cell response.     

Large scale fractionation of polyethylene by means of the continuous polymer fractionation (CPF) method

The countercurrent extraction method recently developed for the continuous polymer fractionation (CPF) was applied to linear polyethylene (M?_w = 55 kg/mol; M?_n = 16,7 kg/mol). At temperatures higher than 130°C, moderately concentrated solution of polyethylene were extracted to remove the low-molecular-weight components. Discontinuous fractionation experiments served to detect the best suited solvents. Diphenyl ether was chosen to demonstrate that the present extraction can be performed even with the same single solvent used to prepare the feed. For very high-molecular-weight polymers, mixed solvents are, however, normally better than single ones, since they allow an easier tailoring of thermodynamic conditions, and yield much less viscous solutions. Mixtures of tetralin and triethylene glycol turned out to be best suited for polyethylene. By means of four successive CPF runs with the single solvent, polyethylene fractions with non-uniformities U = (M?_w/M?_n) ? 1 of approx. 0,3 to 0,4 were obtained on a 100 g scale. The rule of thumb that U can be halved in each CPF step without extensive optimization of the method was corroborated.     

Operating conditions for preparative successive solutional fractionation, 2. Miscellaneous factors including experimental performance,a modified procedure,and repetition of fractionation

Preparative successive solutional fractionation (SSF) of macromolecules has been rigorously computerized on the basis of a modified Flory-Huggins polymer dilute solution theory. The polymers having Schulz-Zimm and Wesslau distributions with the weight-to number-average degree of polymerization X?⁰_w/X?⁰_n = 2–5 and X?⁰_w = 300 have been employed for this purpose. Accurate control of amount of fractions, Q, and separation of two solution phases (i.e., a polymer-lean phase and a polymer-rich phase) were much more easy in SSF as compared to successive precipitational fractionation (SPF). Effect of the concentration dependence parameter p of the polymer/solvent thermodynamic interaction parameter χ on the molecular weight distribution (MWD) of the fractions have been examined in detail. A new modified SSF procedure was proposed in order to reduce the volume of solvent necessary for a single run. The relationships between initial concentration, amount of fraction Q and X?_w/X?_n for the first fractions isolated by SSF were established for the case where the original polymer had Schulz-Zimm and Wesslau distributions. In SSF repetition of fractionation was absolutely ineffective in contrast to SPF.     

Purification and properties of a ribonuclease from the excretory gland cells of Stephanurus dentatus

Ribonuclease (RNase) was isolated and purified from the excretory gland cells of Stephanurus dentatus by (NH4)2SO4 fractionation, column chromatography on phosphocellulose and affinity chromatography using 5' -UTP-agarose. The purified enzyme hydrolyzed yeast RNA, cyclic nucleotides, and polynucleotides. Hydrolysis products of RNA and synthetic substrates were analyzed by column chromatography and/or thin-layer chromatography and the results indicated that S. dentatus RNase should be classified as an endoribonuclease (EC 3.1.27.1). Experimentally infected pigs produced antibodies against the RNase as shown by immunodiffusion and immunoelectrophoresis. The RNase was also secreted by adult worm into an in vitro maintenance medium. The RNase may function in the worm to degrade TNA as a source of nucleotides for metabolic purposes.     

Fabrication of porous biodegradable polymer scaffolds using a solvent merging/particulate leaching method.

This study developed a solvent merging/particulate leaching method for preparing three-dimensional porous scaffolds. Poly(L-lactic-co-glycolic acid) (PLGA) and sodium chloride particles were dry-mixed and cast into a special mold, through which a liquid could pass due to a pressure difference. An organic solvent was then poured into the mold to dissolve and merge the PLGA particles under negative pressure. A nonsolvent was conducted into the PLGA/salt composite to solidify and precipitate the merged PLGA matrix. Finally, a large amount of water was passed through the mold to leach out the salt particles so as to create a porous structure. The results revealed that a highly porous three-dimensional scaffold (>85 vol %) with a well interconnected porous structure could be achieved by this process. Porosity and the pore size of the scaffold were controlled using the ratio and the particle size of the added salt particles. A larger-volume scaffold was produced using a larger mold. This work provides a continuous and simple procedure for fabricating a bulk three-dimensional porous scaffold for tissue engineering.     

Studies on the hydrolysis reaction of model substances of cellulose in the presence of a polymer catalyst, 3 Catalytic activity of poly(vinyl alcohol-co-vinylsulfonic acid) in mixtures of organic solvents and water

The reaction rates of dextrin hydrolysis catalyzed by poly(vinyl alcohol-co-vinylsulfonic acid) in the mixtures of organic solvents and water were measured and compared with those catalyzed by sulfuric acid. In the case of sulfuric acid the reaction rate decreased continuously with increasing content of organic solvent, whereas in the case of the copolymer it passes a minimum and increases again at higher contents of organic solvent with the exception of dimethylsulfoxide. The dependence of the logarithm of the rate constant on the reciprocal of the dielectric constant of the mixed solvent showed the degree of dissociation of sulfuric acid to be a predominant factor for the reaction rate in the system catalyzed by sulfuric acid. For the copolymer-catalyzed reaction, however, it was shown that other factors besides the dielectric constant of the mixed solvent were also significant. In the system ethanol/water the thermodynamic parameters were determined. The viscosity and the pH of the reaction mixtures were also measured. The aggregation of substrate and copolymer as well as the enhancement of interactions between protons and polyanions with increasing organic solvent content were also proposed to be factors determining the reaction rate.     

Über die elementarprozesse bei der anionischen polymerisation des styrols in tetrahydropyran (THP)

The anionic polymerization of styrene was studied in tetrahydropyran as solvent by kinetical and conductivity measurements in order to determine all thermodynamic and kinetical parameters of the formerly proposed three-way-mechanism. These parameters are compared with those in other ethereal solvents. It turns out that the largely differing individual propagation rate constants of the contact-ion pairs, of the solvent-separated ion-pairs, and of the free anions are hardly influenced by the solvent. The unusually great influence of the solvent on the overall polymerization rate is caused by shifting the equilibria between the above mentioned different species.     

Biodegradable polymeric microcellular foams by modified thermally induced phase separation method.

Thermally induced phase separation (TIPS) for the fabrication of porous foams based on various biodegradable polymers of poly(L-lactic acid) and its copolymers with D-lactic acid and/or glycolic acid is presented. Diverse foam morphologies were obtained by systematically changing several parameters involved in the TIPS process, such as polymer type and concentration, coarsening conditions, solvent/nonsolvent composition, and the presence of an additive. The produced foams had microcellular structures with average pore diameters ranging from 1 to 30 microns depending on the process parameters, which were characterized by scanning electron microscopy (SEM) and mercury intrusion porosimetry. Additionally, Pluronic F127 was used as an additive porogen to control the pore geometry and size.     

Effect of thermodynamic strength of solvent on structure formation of crosslinked copolymers

The effect of the thermodynamic strength of the solvent used in the copolymerization of mono- and bifunctional monomers on the molecular-topologic structure of soluble copolymers with bridge bonds was investigated. The investigation was carried out taking as an example radical copolymerization of methyl methacrylate (MMA) and 9,10-anthrylenedimethyl dimethacrylate. The presence of anthracene groups in the molecules of the crosslinking agent made it possible to follow the number of intra- and interchain bridge bonds formed in the polymer system. Toluene/octane mixtures with an octane content from 0 to 35 vol.-% were used as solvents of different thermodynamic strengths with respect to PMMA. It was found that over a narrow range of changes in the composition of the solvent (from 30 to 33 vol.-% of octane) the values of M?_n and [η] of the copolymers increase drastically. It is shown that this increase is due to an increase in the number of interchain crosslinks occurring when the strength of the solvent under the copolymerization conditions used decreases.     

Quantitative Evaluation of the Swelling and Crosslinking Degrees in Two Organic Gel Packings for SEC

The elution behavior of different solvent/polymer systems has been studied by means of size‐exclusion chromatography (SEC) in two organic column packings based on polystyrene/divinylbenzene (PS/DVB) copolymer, named μ‐styragel and TSK‐Gel H_HR. Both packings offer similar characteristics (pore size, particle size, efficiency) but some differences arise when eluting the same systems. The different elution behavior observed in both polymeric gels has been analyzed in terms of their swelling and crosslinking degrees. To evaluate the magnitude of these parameters, a new equation is proposed which has been derived from the Flory‐Huggins theory of swelling equilibrium, and takes into account binary and ternary thermodynamic interaction functions. The equation appropriately predicts the experimental results, showing that the gel that exhibits more solute‐packing attractive interactions (or adsorption as secondary mechanism) is also the one with the highest crosslinking degree or lowest swelling ratio. In this sense, the μ‐styragel column was the most crosslinked polymer network. In addition, the influence of solvent nature, polymer nature, and solute composition on the observed swelling degree has also been compared in both packings.     

Synthèse par voie radicalaire de polymères à extrémités hydroxylées, 10. Synthèse de poly(méthacrylate de méthyle)s par le rayonnement UV: étude de l'influence de la nature du solvant mutuel sur la polymolécularité

Methyl methacrylate was polymerized under UV irradiation (254 nm) at room temperature by hydrogen peroxide in order to give hydroxytelechelic polymers. The polymerization was carried out in the presence of ethanol. An alcohol may be used as a diluent, as a mutual solvent of hydrogen peroxide and the monomer, and as a solvent or nonsolvent of the polymer. The influence of the alcohol is pointed out on the various characteristics of the polymerization; the results are compared with those obtained in methanol.     

Characterization of ethylene-propene copolymers prepared with heterogeneous titanium-based catalysts

Ethylene-propene (EP) copolymers prepared with various heterogeneous titanium-based Ziegler-Natta catalysts, i. e., δ-TiCl₃, β-TiCl₃, and MgCl₂-supported titanium catalysts were fractionated by successive solvent extraction. Wide composition distributions were observed for all samples. Composition distributions of some samples were investigated precisely by temperature-programmed elution column fractionation. The fractionation data showed that these copolymers are a mixture of polyethylene and of copolymers with different structures, i. e., random and block copolymers. In every sample random copolymers were found most abundantly, the propylene sequences being present only as mmmm pentads. These data suggest that random copolymer is formed on an isospecific site.     

Continuous polymer fractionation of poly(methyl vinyl ether) and a new Kuhn-Mark-Houwink relation

Continuous polymer fractionation (CPF) — a new large-scale method — was applied to the technical poly(methyl vinyl ether) (PVME) using toluence as solvent and petroleum ether as non-solvent. Two different devices served as fractionation apparatus, namely a column packed with glass beads and a set of mixer-settler extractors. Advantages and draw-backs of these devices are discussed. The experiments yielded numerous PVME fractions, varying in molar mass from 22 to 87 kg/mol and in molecular non-uniformity U = (M¯_w/M?_n) – 1 from 0,25 to 0,41 according to gel-permeation chromatography as compared with ca. U = 1,30 for the starting material. The following Kuhn-Mark-Houwink relation was established for PVME in ethyl methyl ketone at 30°C: [η]/(mL/g) = 2,6 . 10^?3M?^0,86.     

Microencapsulation of CHO cells in a hydroxyethyl methacrylate-methyl methacrylate copolymer

Chinese hamster ovary fibroblasts, as model cells, have been microencapsulated in a hydroxyethyl methacrylate-methyl methacrylate copolymer (HEMA-MMA) by interfacial precipitation. The polymer containing approximately equal to 75 mol% HEMA, dissolved in polyethylene glycol 200 (PEG 200) was coextruded with the cell suspension (4-6 X 10(5) cells/ml in the alpha-MEM with 10% foetal calf serum +/- Ficoll 400/PBS) through a concentric needle assembly. Polymer solution droplets, containing cells, were blown off the end of the needle assembly by a coaxial filtered air stream into a nonsolvent bath containing phosphate buffered saline (PBS) with 5 ppm Pluronic L101, overlaid with hexadecane. The nascent capsules hang at the hexadecane/PBS interface while the solvent is extracted into the aqueous nonsolvent, to precipitate the polymer around the cells. The resultant capsules were 500 microns-1 mm in diam. with a microporous sponge-like interior, and also very tough and flexible. The cells survived encapsulation based on subculture ability, retention of some fluorescein diacetate (FDA) activity over 5 d and direct light microscopic evidence of cell growth over 10 d after histological sectioning and staining. However, cell growth was not uniformly observed (especially in the FDA assay) and this was attributed to space limitations for growth within the microporous interior. Continued development of this process and adaptation to cells such as pancreatic islets is expected to lead to hybrid artificial organs which are capable of ameliorating metabolic disorders such as diabetes.     

Preparation and characterization of chitin beads as a wound dressing precursor

Chitin was dissolved in N, N-dimethylacetamide/5% lithium chloride (DMAc/5%LiCl) to form a 0.5% chitin solution. Chitin beads were formed by dropping the 0.5% chitin solution into a nonsolvent coagulant, ethanol. The beads were left in ethanol for 24 h to permit hardening, consolidation, and removal of residual DMAc/5%LiCl solvent in order to give spherical chitin beads uniform size distribution. The ethanol-gelled chitin beads had an average diameter of 535 microm. The chitin beads were subsequently activated in 50% (w/v) NaOH solution and reacted with 1.9 M monochloroacetic acid/2-propanol solution to introduce a carboxymethylated surface layer to the chitin beads. The bilayer character of the surface-carboxymethylated chitin (SCM-chitin) beads was verified by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and confocal microscopy. The bilayered SCM-chitin beads were found to absorb up to 95 times their dry weight of water. These SCM-chitin beads have potential as a component of wound dressings.     

Study on the possibility of reverse-order fractionation in successive precipitational fractionation

An attempt has been made to account for the occurrence of reverse-order fractionation (ROF) phenomena in successive precipitational fractionation (SPF) in terms of the newly established rigorous fractionation theory, assuming complete thermodynamic equilibrium conditions between a polymer-rich phase and a polymer-lean phase and to throw light on operating conditions under which ROF occurs. For this purpose, the simulation technique was employed. ROF occurs not only between 1st and 2nd fractionation steps but also between two successive higher order steps even under thermodynamic equilibrium state. The molecular weight distribution of the original polymer contributes very sensitively to ROF: For the Schulz-Zimm type polymer ROF is highly observed at a large amount of fraction Q in a narrow range of initial concentration v whereas for the Wesslau type polymer large ROF occurs at small Q over a relatively wide v range. In general, there exist appropriate ranges of the weight-average degree of polymerization of the initial polymer X? the solvent nature expressed by the concentration dependence of the polymer/solvent interaction parameter p, and the initial polymer concentration v for ROF. A broad original polymer gives rise to large ROF. The theoretical predictions obtained here can explain very clearly the experimental results by Fujisaki and Kobayashi concerning the effect of X?, p, and v on ROF in SPF of polyacrylonitrile.