A.M. van Herk

Critical retention behaviour of polymers a study on the influence of some practical parameters

Liquid chromatography under critical conditions is an important tool for the microstructural characterization of telechelic polymers and block copolymers. Until now, only little information on the practical aspects of this technique is available. The influence of some important practical parameters was investigated, using polystyrene. Critical conditions depend strongly on the type of column packing. The solubility of polymers under critical conditions for different solvent-non-solvent combinations differs to a great extent. For different solvent-non-solvent pairs on reversed-phase systems, a roughly constant eluent strength in terms of the Hildebrand solubility parameter, under critical conditions is found. Temperature can be a useful tool for fine-tuning critical conditions. On normal-phase systems, however, the retention of polystyrene changes non-monotonously with temperature, which limits the use of temperature variations. It is not possible to obtain exact molecular mass independence on any of the investigated systems, which can not be ascribed to chemical differences. This makes the validity of the current theories on critical conditions questionable. Especially for the higher-molecular-mass polystyrenes, peak broadening increases significantly when going from size exclusion conditions to critical conditions. This phenomenon can limit the application of liquid chromatography under critical conditions to a certain molecular mass range. The composition of the solvent in which polystyrenes are dissolved prior to injection, has to be exactly the critical solvent composition, in order to suppress zone splitting as much as possible. For higher-molecular-mass polystyrenes this effect cannot be completely prevented.

Critical Retention Behavior of Homopolymers

Abstract The isocratic retention behaviour of a homopolymer at a specific temperature and varying solvent compositions can be divided into three modes: exclusion mode, transition mode (critical conditions) and adsorption mode. Under critical conditions (a specific non-solvent/solvent composition at a specific temperature) the retention of a homopolymer is independent of the molar mass. The critical conditions depend upon the temperature, the type of polymer and the type of non-solvent/solvent mixture. A difference in the critical conditions between polystyrene standards and polybutadiene standards was found. No significant influence of different types of column packing on the critical solvent composition was noticed. The exact mechanism of the retention of a polymer under critical condition is not clear yet. In order to determine the critical conditions, a new method has been developed. The method can be easily performed on existing equipment. The method contains a different setup of the data acquired by ...

BIMOLECULAR FREE-RADICAL TERMINATION AT LOW CONVERSION

A review on free-radical polymn. termination kinetics covering aspects of diffusion control of bimol. termination such as viscosity dependence and the effects of excluded vol. and coil size, chain-length dependence and theor. models. [on SciFinder (R)]

Determination of the chemical composition distribution of copolymers of styrene and butadiene by gradient polymer elution chromatography

Abstract In order to determine the chemical composition distribution (CCD) of styrene-butadiene copolymers, gradient polymer elution chromatography has been performed. The separation is mainly based on differences in solubility among the copolymer molecules with different chemical composition. The solubility of a copolymer is dependent on the following parameters: temperature, type of solvent/non-solvent mixture, molecular mass of the polymer and the chemical composition of the polymer. The resolution of the gradient polymer elution chromatographic separation and the molecular mass dependency are influenced by the solvent/non-solvent combination. In order to obtian a reliable separation according to chemical composition, the differences in solubility must be sufficientyl high and the molecular mass dependence must be negligible. In order to separate styrene-butadiene copolymers, synthesized by emulsion polymerization, a tetrahydrofuran-acetonitrile gradient was used. After calibration of the chromatographic system with styren-butadiene copolymer standards, the CCD of styrene-butadiene copolymers could be calculated.

Seeded emulsion polymerization of styrene: influence of acrylic acid on the particle growth process

The seeded batch emulsion copolymerization of styrene and acrylic acid was studied. The polymerization rate was investigated with pH as the main parameter. Some attempts were made to evaluate the average number of growing chains per particle during Stage II of the emulsion polymerization process. The final latex products were characterized by means of conductometric aqueous titration and potentiometric titration in an organic solvent mixture. The distribution of the acid groups over the aqueous phase, the particle surface, and the interior of the particles together with the kinetic results provided insight into important features governing the incorporation of acrylic acid. The results indicate that pH is the dominating parameter for the incorporation process. An optimal incorporation on the surface of the particles is observed for a low value of pH. In that case, all the acid groups are protonated.

Seeded emulsion polymerization of styrene : incorporation of acrylic acid in latex products

Seeded batch emulsion polymerization of styrene and acrylic acid was studied. The influence of pH and acrylic acid content on the polymerization rate and the amount of carboxylic acid groups incorporated in the final latex products was investigated. The distribution of the functional groups over the aqueous phase, the latex particle surface, and the interior of the latex particle was determined using aqueous conductometric titration and nonaqueous potentiometric titration at intermediate and complete overall conversions. Combined with kinetic results, the carboxylic acid group distribution history provided valuable information about the process of incorporation of acrylic acid in latex products. Two-step processes in which a shot of acrylic acid was performed in the last stage of the emulsion polymerization reaction were investigated as a strategy to increase the surface incorporation efficiency.

A topology map for novel vesicle-polymer hybrid architectures

The use of vesicles as templates for the polymerization of inserted monomers gives rise to novel vesicle-polymer hybrid morphologies where a polymer bead is phase-separated from the vesicle bilayer matrix. Complex vesicle-polymer architectures, such as the wrapped parachute morphologies shown in the Figure, are synthesized here and then examined using cryo-transmission electron microscopy (see also cover).

On-line monitoring and composition control of the emulsion copolymerization of VeoVA 9 and butyl acrylate by Raman spectroscopy

The batch and semi-continuous emulsion copolymerization of n-butyl acrylate and vinyl neononanoate (VeoVA 9) were monitored by remote on-line Raman spectroscopy. Monomer concentrations were calculated in real-time by a classical least squares (CLS) approach using the vinyl regions of the Raman spectra. During the batch copolymerization, conversion and copolymer composition calculated from on-line data showed good agreement with off-line gravimetry and NMR results, indicating that the CLS procedure indeed enables the calculation of individual monomer concentrations and on-line characterization of the copolymer formed. Real-time analysis of the data enabled control of the copolymer composition by suggesting how one of the comonomers should be added in a semi-continuous polymerization. NMR and gradient polymer elution chromatography corroborated the Raman results for the copolymer composition. Further, it was shown that only at high conversions (>80%) monomer partitioning influences the copolymerization behavior, since both monomers have a low solubility in water. The results demonstrate that Raman spectroscopy is very well suited as on-line sensor in emulsion copolymerization processes.

Evaluation of mixed-mode stationary phases in liquid chromatography for the separation of charged and uncharged oligomer-like model compounds

The possibility to separate charged and uncharged oligomer-like test compounds by high-performance liquid chromatography was investigated, using mixed-mode (reversed-phase/anion-exchange) stationary phases. In order to examine this possibility, several columns were evaluated for their reversed-phase, anion-exchange and mixed-mode properties using neutral and charged model compounds. The OmniPac PAX-500, the PRP-X100 and the mixed-phase PLRP-S/PL-SAX columns showed promising results for the separation of real samples.

Monitoring of originated polymer in pure monomer with gradient polymer elution chromatography (GPEC)

The anal. of the amt. of polymer in pure monomer with cloudpoint measurements gives only a qual. answer. Now by use of a liq. chromatograph and gradient elution a fully automated detn. the amt. of polymer can be measured and an impression of the molar mass can be achieved