Josef Janča

Retention characteristics of various polymers in thermal field-flow fractionation

Abstract The synthetic polymers, polystyrene, polyisoprene, polytetrahydrofuran, and polymethylmethacrylate, in the molecular-weight range from 7600 to 270,000, were successfully retained in a thermal field-flow fractionation (FFF) system. Two solvents, tetrahydrofuran and ethyl acetate were employed. Retention and selectivity levels were found to be comparable between polystyrene and the other polymers. The present results are the first to show decisively that thermal FFF is applicable to polymers other than polystyrene. Consequently, we consider the scope of thermal FFF in poymer analysis to be considerably enlarged by the present work.

Micro-thermal field-flow fractionation: new high-performance method for particle size distribution analysis.

Micro-thermal field-flow fractionation (mu-TFFF) was applied to the separation of polystyrene latices. This new high-resolution technique allows determination of the particle size distribution (PSD) if carried out under optimized experimental conditions. The optimum temperature of the accumulation wall, which influences the relaxation processes and, consequently, the zone broadening, was chosen on the basis of our prior work. The flow rate was chosen as a compromise between the theoretical optimum value, which is very low because the diffusion coefficients of the colloidal particles are very small, and a value allowing performance of the PSD analysis in a reasonable time. These experimental conditions can be manipulated easily due to the high versatility of mu-TFFF, which follows from a large decrease of the heat energy flux across the channel with its reduced dimensions in comparison with standard TFFF. The PSDs obtained from mu-TFFF data are compared with results from quasi-elastic laser light scattering (QELS) and transmission electron microscopy (TEM). It has been found that a baseline resolution of a model mixture of two samples of close average particle diameters can be achieved by an appropriate choice of the temperature drop in mu-TFFF, whereas only a broad, unresolved PSD of the mixed sample was obtained from the QELS measurement. The TEM of the mixed sample revealed the presence of two particle size populations. However, the number of particles which are practically counted on a TEM picture is several orders of magnitude lower than the number of particles taken into account in mu-TFFF or QELS. Consequently, the PSD obtained from the TEM did not represent the whole sample. Comparison of mu-TFFF with modern hydrodynamic chromatography (HC) has shown that the methods exhibit roughly the same resolution and time of analysis. Nevertheless, mu-TFFF is a more universal technique because the separation of the colloidal particles or of the macromolecules within a broad range of molar masses is carried out on the same channel, as demonstrated previously.

Micro-Thermal Field-Flow Fractionation in the Analysis of Polymers and Particles: A Review

Abstract The benefits of the miniaturization of thermal field-flow fractionation (TFFF) channel are reviewed in order to demonstrate that high-performance separations can be achieved with a micro-channel of optimized construction working under carefully chosen experimental conditions and operational variables. Micro-TFFF is highly competitive in comparison with size exclusion chromatography (SEC) of macromolecules for molar masses up to approximately one million g/mol. However, the versatility of micro-TFFF is superior to SEC for macromolecules of ultra high molar masses not only because there is not an inherent highest molar mass limit of the macromolecules that can be separated by micro-TFFF in contrast to SEC but also, as proved experimentally, due to the fact that shear degradation of the macromolecular coils, a well-known phenomenon in SEC, does not occur in micro-TFFF. Such mild conditions permit the analysis of macromolecular aggregates, micro-gels, and similar associative structures without any destruction. Moreover, colloidal submicron and micron-sized particles of synthetic, natural, or biological origin can also be separated and characterized by micro-TFFF without any modification of the separation system. The free choice of the carrier liquids affords an another advantage to micro-TFFF. It has been predicted theoretically and proven experimentally that high resolution is achieved more efficiently by increasing the temperature drop across the separation channel than with a decrease in channel thickness. This is due to the fact that the total heat flow between the hot and cold walls is substantially reduced in micro-TFFF. This makes the operation of the micro-TFFF system not only 30 times more economical from the viewpoint of electrical energy consumption but also safer. The experimental implementation and application to polymer and particle analysis confirmed the potential of micro-TFFF.

Micro‐channel Thermal Field‐Flow Fractionation: High‐Speed Analysis of Colloidal Particles

Abstract Micro‐thermal field‐flow fractionation (µ‐TFFF) was developed recently and applied for the characterization of the synthetic polymers and colloidal particles. In comparison with standard size TFFF channels, which were already used for the separation of the colloidal particles, the miniaturized channel allows one to shorten the time of the analysis and to achieve high resolution if the separation is performed under optimized experimental conditions. The relaxation processes leading to the establishment of the initial steady state after the injection of the sample into the channel can influence the retention and contribute to the zone broadening. These processes are considerably influenced by the temperature, which has to be carefully chosen. The choice of a convenient flow rate represents a compromise between an optimum flow rate (which is too slow due to the low diffusion coefficients of the colloidal particles) and a reasonable flow rate which takes into account the injection period and the stop‐flow procedure applied immediately after the injection of the sample in order to minimize the effect of the relaxation processes. All these parameters can easily be optimized in the µ‐TFFF due to its high versatility and to an important decrease of the heat energy flux across the channel, allowing an independent control of the temperatures of the cold and hot walls. The µ‐TFFF thus becomes high‐performance method for the separation of the colloidal particles and for the determination of their particle size distribution (PSD).

Micro-thermal field-flow fractionation: New challenge in experimental studies of thermal diffusion of polymers and colloidal particles

The theoretical principles and methodological aspects of the thermal field-flow fractionation applied to study the thermal diffusion of the macromolecules in solution and colloidal particles in suspension were developed. The theoretical analysis indicated that the miniaturization of the separation channel for thermal field-flow fractionation should improve the performance of this technique. A new microchannel was conceived and built. The experimental results obtained for polymer samples with an extended range of molar masses from relatively low up to ultrahigh and for the colloidal particles confirmed that the achieved resolution is the same but the versatility of the microchannel is superior to that of the standard size channels owing to the substantial decrease in the heat energy flux. This important improvement allows us to achieve very high resolution when applying constant-field-force operation, it makes it much easier to program the temperature drop which is an advantageous operational mode from the viewpoint of the time of analysis, and it extends considerably the range of perfectly controlled temperatures of the cold and hot walls. The sample amount needed for one analysis can be as small as a few nanograms.

MICRO-CHANNEL THERMAL FIELD-FLOW FRACTIONATION: ANALYSIS OF ULTRA-HIGH MOLAR MASS POLYMERS AND COLLOIDAL PARTICLES WITH CONSTANT AND PROGRAMMED FIELD FORCE OPERATION

ABSTRACT Separation and analysis of ultra-high molar mass (UHMM) polystyrenes by micro-Thermal Field-Flow Fractionation (μ-TFFF) was found to be very efficient in the range of molar masses over one million grams per mol. Although, constant field force operation allows achieving very high resolution, the programming of the temperature gradient is an advantageous operational mode from the point of view of the time of analysis. The programming, as well as the substantial extension of the perfectly controlled temperature of the cold wall, is much easier with the μ-TFFF channel due to an important decrease of the heat energy flux compared with standard size channels. An example of the high performance analysis of colloidal particles by the μ-TFFF is presented.

Ultra-Micro-Thermal Field-Flow Fractionation

Abstract Micro-thermal field-flow fractionation, proposed conceptually and implemented experimentally several years ago, has developed rapidly in terms of theory, instrumentation, and numerous applications for the analysis and characterization of polymers and particles of synthetic, natural, and biological origin. Although the advances have been important, achieving the ultimate limits of miniaturization imposed by the physics as well as by recent technologies represents a challenge that was explored. The result of the reported experimental study is a new separation channel for ultra-micro-thermal field-flow fractionation, which was compared, in terms of performance, with the existing compact micro-thermal field-flow fractionation unit. The limits of the miniaturization are experimentally demonstrated with the use of suspensions of colloidal particles, which represent a more difficult case of separation than polymers in solution.

Quasi-Elastic Light Scattering Study of the Synthesis of Tailor-Made Suspensions of Uniform Polyaniline-Based Nanoparticles

Abstract The understanding of the mechanism of aniline oxidative polymerization allows one to regulate the synthesis of the colloidal panicle suspensions in such a way that a product of controlled average particle size and panicle size distribution is obtained. Quasi-elastic light scattering was used to determine these parameters for the panicles taken from a reaction mixture at various stages of the polymerization. It has been found that the panicle size and the panicle size distribution of the product depend on the reactiveness of the monomer (m-, o-toluidine. or aniline), on the concentrations of the monomer, oxidant and stabilizer-poly(vinyl alcohol-co-acetate). and on the temperature. The panicles of uniform size are formed within a limited period of time during the polymerization: in some cases until termination. Their sizes lie within the range of 200–600 nm for the polyaniline and within 400–3000 nm for poly(m-toluidine). Comparison of the prepared samples with commercially available uniform polys...

Experimental study of isopycnic focusing generated by coupled electric and gravitational field forces: Use in thin layer focusing and focusing field-flow fractionation

Various operational parameters affecting the formation of the density gradient generated by the electric field action on a binary pseudo-continuous carrier liquid composed of charged colloidal silica particles suspended in water and the isopycnic focusing of sample particles were investigated under conditions of static thin layer focusing and dynamic focusing field-flow fractionation. The properties and the behavior of the density gradient forming carrier liquid were studied. The experimental results are compared with theoretical predictions and discussed with respect to potential applications of the proposed concept not only for separation purposes but also for studies of interparticle interactions.

On the Precision of Particle Size Analysis by Micro‐Thermal Field‐Flow Fractionation

Abstract Micro‐Thermal Field‐Flow Fractionation of polymer colloidal particles was performed in two different laboratories. Short term repeatability of the experimental retentions obtained in a single laboratory has been found to be very high. The short term precision (expressed as percent standard deviation) of the determination of average particle diameter can reach the values greater than 1%, relative. Average repeatability of the retentions in both laboratories was better than 3%, relative when using identical experimental protocol. No other method of particle size analysis can provide the results of a comparable precision. Average repeatability of the width of the raw fractograms, which contains the information on particle size distribution, is of the order of 5%, relative. However, this value cannot be considered as the ultimate limit because the experiments were not carried at the low flow rate of the carrier liquid permitted to reach much higher resolution. The effect of the stability of the most important operational variables, such as the temperature drop between the cold and hot walls, the temperature of the cold wall, and the flow rate of the carrier liquid, on the precision of the analytical results is discussed. This article is dedicated to the memory of J. C. Giddings (1930–1996)