Naslov
Size-exclusion chromatography of syntetic polyelectolytes
(Size-exclusion chromatography of syntetic polyelectrolytes)

Autori
Šegudović, Nikola

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
5th International symposium "Chromatography and hypheneted techniques",Book of abstracts / Marsel, J. - Bled : Slovenian Chemical Society, 1998, 30-33

Skup
5th International symposium "Chromatography and hypheneted techniques"

Mjesto i datum
Bled, Slovenija, 05.10.1998. - 09.10.1998

Vrsta sudjelovanja
Pozvano predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
size-exclusion chromatography; syntetic polyelectrolytes; molecular mass distribution

Sažetak
Molecular mass is one of the most important parameters for end-use properties of polymers. According to the nature of polymerization reactions it is not a unique feature and it is characterized by one or more molecular mass averages or by molecular mass distribution. High performance size-exclusion chromatography (HPSEC ) is practically the most powerful method for molecular mass distribution determination ( MMDD). HPSEC is a modern separation, liquid chromatography technique which provides a separation of macromolecular species from macromolecular mixtures according to their size (hydrodynamic volume ) in solution and it is used very often in both research and industry. The sample solution is introduced onto the column, which is filled with rigid, semi-rigid or soft structure porous particle packing, and is carried by a mobile phase ( solvent ) through the column. The size separation takes places by a repeated exchange of the solute macromolecules between the bulk solvent of the mobile phase and the stationary liquid phase within the pores of the packing. The pore size of the packing particles determines the molecular size range within which separation occurs. In classical size-exclusion chromatography ( SEC ) a distribution of the solute macromolecules between the interparticle volume ( V0 ) and the accessible pore volume ( Vp ) takes place and the retention volume Vr is determined by : Vr = V0 + Vp Kd ( 1 ) Kd ( distribution coefficient ) is related to the change in Gibbs free energy ?G at the point where the solute macromolecules pass from the mobile into stationary phase. ?G = ?H - T?S = -RT ln Kd ¸ ( 2 ) The change in ?G may be due to different effects : - inside the pore, which is limited in dimension, the macromolecules cannot occupy all possible conformations and therefore, the conformational entropy ?S decreases - when penetrating the pores, the macromolecules may interact with the pore walls resulting in a change in enthalpy ?H. In an ideal SEC separation is exclusively directed by conformational changes of the macromolecules and ?H by definition is zero and Ksec = exp ( ?S / R ) ( 3 ) As the conformational entropy decreases ( ?S ? 0 ) the distribution coefficient of the ideal SEC is Ksec ? 1. The maximum value Ksec = 1, is related to zero change in entropy i.e. to a situation where all pores are accessible to the solute macromolecules. At Ksec = 0 the solute macromolecules are too large to penetrate the pores. Separation range is limited by 0 ? Ksec ? 1. By involving enthalpy change in the separation process SEC mode move to liquid adsorption chromatography ( LAC ).The ideal SEC and LAC are extreme of the same principle. In the real SEC and LAC modes, retention responds to both enthalpic and entropic interaction and only the predominance of one of these interactions decides which mode is operating. HPSEC alone, is just a liquid chromatographic separation tecnhique which cannot say anything about molecular mass averages or molecular mass distribution. Elution curve (chromatogram ) is a direct result of SEC process which, at the best, shows which amount of the sample leaves the column in a certain elution volume. The relationship between molecular mass and the elution volume must be determined through the calibration curve, as the first step in extracting molecular mass information from SEC. Calibration curve can be established by using a narrow molecular mass standard, broad molecular mass standard, universal calibration or absolute detectors. A numbers of homopolymers and copolymers are characterized by HPSEC, but some problems have arisen in molecular mass distribution of polyelectrolytes. A polyelectrolyte is defined as any polymeric substance in which the monomeric units of its constituent macromolecules possess ionizable groups. Electrochemically, a polyelectrolyte can be classified as either a polyacid, a polybase or polyampholyte depending upon the nature of its ionization in water solution. In contrast to a simple electrolyte like sodium chloride, in which the size of the oppositely charged ions are similar in magnitude. a polyelectrolyte is always composed of a macroion in which the charged group are interconnected by chemical bond, together with an equivalent number of small oppositely charged counterions. Virtually, all of the unique properties of polyelectrolytes result from the interaction of interconnected ionic group of the macroion, and in turn, from the interaction of the charged macroion with its compensating counterion. In aqueous solution all of the synthetic polyelectrolytes mentioned above tend to adopt a randomly coiled configuration, suitably modified by the mutual interaction ( generally repulsive ) of charged groups on the polyion chain. Solution containing charged macroions display a large deviation from thermodynamic ideality. Even in dilute solutions of strong polyelectrolyte, the activity coefficient of small counterion is only about 0.25. These nonidealities are a consequence of the large electrostatic potentials which exist in the vicinity of polyions with multiple charge; counterions become "trapped " in these regions of high potential and essentially lose their identity as independent mobile species. Qualitatively, solution of polyelectrolyte does indeed display some behavioral similarities to both nonionic polymer solutions on one hand, and to simple electrolyte solutions on the other. It is instructive to compare the concentration dependence of the reduced viscosity ( ?sp / c ) of polyelectrolyte with that of its parent, nonionic polymer with the same chain length. Dilution of the nonionic polymer results in a linear decrease of reduced viscosity according to the well-known Huggins equation. The effect of dilution on the reduced viscosity for the related polyelectrolyte is remarkably different. In this case, the reduced viscosity increases sharply with dilution and sometimes ( ?sp /c ) approaches the ordinate almost asymptotically. The main problem in separating a polyelectrolyte by SEC is that the polymer size in solution is governed not only by molecular mass, but also by the number of the attached ionogenic groups, the type of the counter-ion ( its charge and mobility ), and the polarity and electrical screening properties of the solvent. Abnormal chain expansion occurs when an ionic polymer is dissolved, because the number of electrical charge within the polymer coil generally exceeds that in the bulk solvent. Osmotic forces drive solvent into the coil to expand it and cause the counterion to diffuse out away from the backbone chain into the bulk portions of the solvent.This process leaves a net residue of charged groups ( cationic or anionic ) on the polymer chain. The charge groups remaining on the polymer chain are resposible for large intramolecular repulsive forces and lead to a further chain expansion. The addition of strong electrolyte to the solvent suprresses the loss of counterion from the charged sites on the polymer and permits a return of the polymer to normal physical and thermodynamic solution properties, in which state the polymer can be separated reproducibly. Maleic acid polymers and copolymers as auxiliary agents ( dispersing agents, emulsifier, coatings, adhesives, sizing agent, etc. ) are a typical example of synthetic polyelectrolyte. Due to the presence of two weak carboxylic groups on the maleic acid unit and a strong sulfo group on the alternating sryrene unit, a partially sulfonating copolymers of maleic acid and styrene as an anionic polyelectrolyte are specially interesting to study HPSEC of polyelectrolyte. MMD of partially sulfonated alternating copolymers of maleic acid and styrene with different degree of sulfonation were determined on ?-Bondagel column set ( E- High and two E-Linear ) with dimethylformamide ( DMF ) and DMF enriched with a different amount of LiBr as mobile phases. The position of the chromatograms (separation range ) depends on the degree of sulfonation and quality of mobile phase (Table I ). In pure DMF calculated MM using polystyrene calibration curve have been almost two order in magnitude higher than the expected ones.The increasing concentration of LiBr shifts chromatograms toward the higher elution volume or lower MM. The concentration of LiBr of 0.1 M / L has not been enough to shift chromatograms on the expected MM range for the sample with the highest degree of sulfonation ( 83 mol % S ). References : R.W. Armstrong., V.P. Strauss., Polyelectrolytes in Enciklop. Polym. Sci and Tech. Eds. N. Gaylord and H. Mark, N.Y.,1969 p 781 H. Pasch., Adv. in Polym. Sci., 128 (1997 ) 1 N. Šegudović., S. Sertić., M. Kovač-Filipović., V. Jarm.,J. Chromatogr., A 704 (1995) 149 V. Jarm., M. Kovač-Filipović., N. Šegudović., J. Appl. Polym. Sci., 58 (1995) 1967

Izvorni jezik
Engleski

Znanstvena područja
Kemija

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