Son Hoai Nguyen

Polystyrene-graft-poly(ethylene oxide) copolymers prepared by macromonomer technique in dispersion. 2. Mechanism of dispersion copolymerization

The reaction mechanism of dispersion copolymerization of methacryloyl-terminated poly(ethylene oxide) macromonomer and styrene in the polar media is discussed. The copolymerization products were analyzed by means of liquid chromatography. This analysis revealed presence of rather large amount of polystyrene in the final polymer product. It is assumed that the system undergoes phase separation in the course of copolymerization. The graft copolymer created in the first stage of polymerization acts as a detergent (amphiphile). The hydrophobic phase is hardly accessible for hydrophilic macromonomer but well dissolves styrene and benzoylperoxide initiator. Therefore, polystyrene homopolymer is intensively formed in this reaction locus. Its molar mass is rather low because of high initiator and low monomer concentration at the reaction loci. We suppose existence of at least two different polymerization loci; the continuous phase and the particle core. The existence of third polymerization locus, viz. particle surface layer cannot be excluded, either.

Reconcentration of diluted polymer solutions by full adsorption/desorption procedure — 1. Eluent switching approach studied by size exclusion chromatography

Abstract A Full Adsorption-Desorption (FAD) procedure is proposed for the controlled reconcentration of diluted polymer solutions. In the first step, macromolecules are quantitatively trapped from their diluted solution onto an appropriate adsorbent. In the second step, polymer is released by a small amount of desorption promoting liquid. The efficiency of this reconcentration process was tested with poly(methyl methacrylate) (PMMA), polytetrahydrofuran (PTHF), poly(ethylene oxide) (PEO) and polystyrene (PS). Bare nonporous silica packed into FAD minicolumns was used for reconcentration of dilute solutions of PMMA, PTHF and PEO in toluene and chloroform. Nonporous silica bonded with aliphatic C18 groups was applied to reconcentrate PS solutions from dimethylformamide (DMF). The desorbing liquid was tetrahydrofuran for PMMA and PTHF, DMF for PEO and toluene for PS. The efficiency of reconcentration was tested by size exclusion chromatography that was coupled on-line with the FAD minicolumn. The efficiency of reconcentration was found to be very high in the case of medium polar and polar polymers PTHF, PMMA and PEO: full recovery of macromolecules was demonstrated by the SEC peak sizes and shapes. Consequently, agreement was found between the values of mean molar mass and polydispersity obtained for a series of polymer samples before and after the reconcentration procedure if the size of the FAD column was optimized. On the other hand, it was more difficult to effectively reconcentrate polystyrene which was not fully adsorbed on bare silica from the most common solvents. Agreement was obtained for molecular characteristics of PS before and after reconcentration from dimethylformamide on C18 bonded silica for higher molar masses above ca. 9 × 104 g mol−1. Very diluted solutions containing 5 mg of polymer per litre of solution could be easily treated in this way and no problems are anticipated even with more diluted systems. The FAD procedure also allows ready exchange of the sample solvent and it can be used in various analytical methods, especially in multidimensional liquid chromatography of macromolecules where effluent from a column separating in the ‘first dimension’ is further separated in the ‘second- and higher dimension’ columns. The reconcentration of polymer mixtures including the selective reconcentration of one blend component and thus the preseparation of blends is also feasible. Preparative reconcentration based on the controlled full adsorption-desorption processes is also expected to be possible. In any case, the appropriate adsorbent and both adsorbing and desorbing liquid must be carefully chosen.

Polystyrene-graft-poly(ethylene oxide) copolymers prepared by macromonomer technique in dispersion. I. Liquid chromatographic separation of product mixtures

Products of the radical dispersion copolymerization of methacryloyl-terminated poly(ethylene oxide) (PEO) macromonomer and styrene were separated and characterized by size exclusion chromatography (SEC), full adsorption-desorption (FAD)/SEC coupling and eluent gradient liquid adsorption chromatography (LAC). In dimethylformamide, which is a good solvent for PEO side chains but a poor solvent for polystyrene (PS), amphiphilic PS-graft-PEO copolymers formed aggregates, which were very stable at room temperature even upon substantial dilution. The aggregates disappeared at high temperature or in tetrahydrofuran (THF), which is a good solvent for both homopolymers and for PS-graft-PEO. FAD/SEC procedure allowed separation of homo-PS from graft-copolymer and determination of both its amount and molar mass. Effective molar mass of graft-copolymer was estimated directly from the SEC calibration curve determined with PS standards. Presence of larger amount of the homo-PS in the final graft-copolymer products was also confirmed with LAC measurements. The results indicate that there are at least two or maybe three polymerization loci; namely the continuous phase, the particle surface layer and the particle core. The graft copolymers are produced mainly in the continuous phase while PS or copolymer rich in styrene units is formed mostly in the core of monomer-swollen particles.

Dynamic desorption of macromolecules from solid surfaces by low-molecular-weight displacers

Abstract A novel dynamic approach to polymer desorption study has been tested. The adsorbent under study was packed into an HPLC-like adsorption–desorption column (ADC). Constant amount of polymer was dynamically preadsorbed within the ADC from an adsorption promoting liquid (adsorli) and desorbed subsequently by changing composition of a displacer consisting of an adsorli and a desorption promoting liquid (desorli). Polymer in the ADC effluent was detected with help of an evaporative light scattering detector using an appropriate calibration for each system. Dynamic integral desorption isotherms were obtained by plotting desorbed amount of polymer versus displacer composition. Both the attachment and detachment of polymer onto/from nonporous silica adsorbent surface were found to be very fast as far as the adsorbent surface was undersaturated. The courses of desorption isotherms were measured and compared for various systems, viz. polymer nature, polymer molar mass and molar mass distribution as well as adsorli and desorli nature. The effects of experimental conditions such as flow rate, temperature, amount of preadsorbed polymer and ADC size on the shapes of desorption isotherms were evaluated, as well. All above parameters influence the courses of desorption isotherms and it is shown that the described measurements can give valuable information on the desorption processes in the dynamic systems. The described dynamic desorption measurements are fast and the results obtained are repeatable. Moreover, both the sample and the sorbent consumption are low.

Reconcentration of Diluted polymer solutions by full adsorption/desorption procedure : II. Desorption of macromolecules by a narrow pulse of displacer

Diluted polymer solutions can be effectively reconcentrated applying full adsorption/desorption processes. Macromolecules from diluted solutions are quantitatively retained within a bed of appropriate adsorbent. Next, the polymer is released by a high-strength desorbing liquid that is introduced into the sorbent bed as a narrow pulse. To evaluate the above reconcentration procedure, medium-polarity polymers, mainly poly(methyl methacrylate)s of various molar mass distributions were chosen as model species. Nonporous silica was used as an adsorbent, toluene and chloroform as adsorbing liquids, and tetrahydrofuran as a desorbing liquid in an HPLC-like apparatus. The concentration profiles of both the desorbing liquid pulse and desorbed polymer were monitored with the usual LC detectors. On-line size exclusion chromatography was employed in selected cases to determine molar mass and molar mass distribution of desorbed macromolecules. The effect of some experimental parameters on the reconcentration efficiency was elucidated, viz. the nature of the sample solvent-adsorbing liquid, flow rate of desorbing liquid, molar mass, molar mass distribution, and nature of reconcentrated polymer, as well as relations among the amount of the polymer to be reconcentrated and the volume of the desorbing liquid pulse. It is shown that very high reconcentration factors can be readily obtained by the full adsorption–desorption procedure if the experimental conditions are carefully optimized.

Molecular characterization of block copolymers by means of liquid chromatography I. Potential and limitations of full adsorption-desorption procedure in separation of block copolymers

Abstract The full adsorption–desorption (FAD) procedure was applied to the selected model di- and tri-block copolymers. The dynamic integral desorption isotherms were measured for various homo- and block-copolymers of poly(methyl methacrylate) and poly(glycidyl methacrylate) in a system of non-porous silica–dichloroethane adsorli–tetrahydrofuran desorli. The aim was to evaluate separation selectivity of the FAD approach toward molar mass and chemical composition of macromolecules. It was demonstrated that under optimum conditions the FAD procedure can discriminate parent homopolymers from di-block copolymers, as well as di-block from tri-block copolymers when the adsorptivities of blocks differ sufficiently. The molar mass of both kinds of polymer chains affected the course of their desorption in present system of adsorbent–adsorli/desorli. Consequently the block copolymers studied could not be effectively fractionated according to their composition by a single FAD procedure. A combined method, full adsorption–desorption plus size exclusion chromatography was proposed for the species with selectively adsorbing blocks to provide a two dimensional fractionation of block copolymers.

Coupling of Full Adsorption/Desorption with Size Exclusion Chromatography: Application to Separation and Characterization of Minor Macromolecular Admixtures in Polymer Blends

Abstract Potential of a full adsorption - desorption (FAD)/size exclusion chromatography (SEC) procedure in discrimination, reconcentration and characterization of minor macromolecular admixtures in polymer blends was evaluated. Separation of chemically different macromolecules by means of the FAD method is based on differences in their selective adsorption/desorption onto/from appropriate sorbent. The on-line SEC column allows independent characterization of blend components. Various contingencies were discussed, namely those when either a major or minor blend component is more strongly adsorbed on the FAD column packing. The FAD/SEC approach was tested with several model polymer blends containing up to 99% of a major component: It was shown that if the affinities of blend components toward FAD column packing differ sufficiently, this proposed procedure can provide precise and reliable data on molar masses and molar mass distributions of minor homopolymer admixtures. It is anticipated that the FAD method will allow full analysis and characterization of minor admixtures present in many polymer blends, even at very low concentrations below 1%.

Molecular characterization of statistical copolymers: 1. Potential and limitations of full adsorption–desorption procedure in separation of statistical copolymers

Dynamic integral desorption isotherms for a series of poly(methyl methacrylate) homopolymers and poly(methyl methacrylate)–polystyrene statistical copolymers were measured. Nonporous silica was the full adsorption–desorption (FAD) column packing and various adsorption-promoting and desorption-promoting liquids were used. The aim of this study was to evaluate the applicability of the FAD approach for separation of statistical copolymers. The effects of the adsorbing liquid and desorbing liquid nature were demonstrated on the positions and shapes of desorption isotherms. The desorption isotherms also strongly depended on both (co)polymer molar mass and copolymer chemical composition. This indicates large fractionation potential of the FAD procedure. Simultaneously, the interference of both above parameters prevents the direct use of FAD for fractionation of the copolymers. It is anticipated that the fractionation and/or reconcentration potential of the FAD procedure can be very effectively utilized in combination of FAD with size-exclusion chromatography and/or with gradient elution liquid adsorption chromatography.

Separation and Characterization of Polymers Applying Full Adsorption/Desorption Approaches

The present contribution is a review of the recent progress in combining a full adsorption/desorption procedure with size exclusion chromatography (FAD/SEC) for the separation and characterization of (co)-polymers. FAD includes complete and selective adsorption of the polymer sample to be separated from an adsorption promoting-liquid (adsorli) onto an appropriate adsorbent, which is packed in an especially designed LC-like (micro)column. In the following steps, macromolecules are successively displaced from the adsorbent by different eluents with increasing desorbing strength. Adsorption and desorption of macromolecules are generally governed by their molecular characteristics, primarily by their molar mass (MM) and their chemical nature, that is, by their chemical composition (CC). Therefore, fractionation of polymers according to these parameters can be reached in the course of the FAD process. In this respect, a coupling of an SEC instrument with the FAD column has turned out to be very advantageous. The on-line SEC enables monitoring the amount, molar mass and molar mass distribution of macromolecules leaving the FAD column. In this way, constituents of polymer blends can be discriminated and independently characterized. Advantages and problems associated with the applications of FAD approaches are outlined. The assessment of dynamic adsorption/desorption of macromolecules onto/from solid surfaces as part of the optimization procedure is discussed as well.