Soft Report - Dipartimento di Fisica - Sapienza

Dielectric Properties of Polyelectrolyte Aqueous Solutions:the Scaling ApproachPolyelectrolytes are macromolecules with manyionizable groups. In polar solvents, these groups candissociate, leaving charges on the polymer chain andreleasing counterions in solution. Polyelectrolytesfind widespread applications as functional poly-mersin many industrial processes as thickeners,dispersants, ion-exchange resins, enhanced oilrecovery agents etc. Moreover, DNA and a variety ofbiologically important macromolecules arepolyelecrolytes. Their properties depend on thefraction of dissociated ionic groups, on the solventquality for the polymer backbone, the dielectricconstant of the solvent and the salt concentra-tion.Fig. 1. Top - Different chain conformations areobserved in the different concentration regimesand in different solvents. Bottom: Scalingbehaviour of the dielectric parameters as afunction of polymer concentration. a): theexpected exponent (α=1) for the “good solvent”model is retrieved using pure ethylene glycol (EG)as solvent, a good solvent for the polymeremployed; in contrast, the exponent α increasesabove 1 with the increase of the water content(poor solvent) in the mixture. b): the reversal isalso true: the expected exponent for a poorsolvent (1/3) is obtained for pure water solution.Long-range electrostatic interactions be-tweenpolyions and counterions and between dif-ferentcharged groups on the same polymer chainintroduces new scale lengths and the fine balancebetween long-range electrostatic and short-rangepolymer-solvent interactions gives rise to a com-plexphenomenology.We have recently investigated [1-3] the radio-wavedielectric behaviour of aqueous polyelectro-lytesolutions in the light of the scaling approachproposed by Dobrynin and Rubinstein [4], as afunction of the solvent quality and in differentconcentration regimes, showing how the differentpolyion conformations reflect on the low-frequen-cypermittivity and electrical conductivity.As can be seen in Fig. 1 a) and b), for the par-tiallycharged PMVP-Cl polymer dissolved in a mixedsolvent the predicted scaling behaviour is observedin a wide concentration range. For this polymer,water and ethylene glycol [EG] behave as poor andgood solvent, respectively, and in both cases apower law is clearly evidenced. In particular, whenthe data are analyzed according to the “good solventmodel” (Fig. 1 a), the expected value of theexponent in the polyion concentration dependenceα=1 is attained for X=1 (pure ethylene glycol), whilea progressive deviation is observed as the quality ofthe solvent changes, approaching the “poor solvent”,i.e., as the mole fraction of water is progressivelyincreased. Analogously, when the same data areanalyzed according to the “poor solvent model” (Fig.1 b), the expected value of the exponent α=1/3 isretrieved at X=0 (pure water), whereas deviationsare apparent when the EG con-centration in thesolution increases (good-solvent condition). Theobserved scaling behaviour of the dielectricproperties of polyelectrolytes in solvents of differentquality gives strong support to the existence of a“necklace” conformation of the chain, under poorsolvent condition.References[1] F.Bordi, C.Cametti, T.Gili, S.Sennato, S.Zuzzi,S.Dou, R.H.Colby, Phys. Rev. E 72, 031806 (2005).[2] F.Bordi, C.Cametti, T.Gili, S.Sennato, S.Zuzzi,S.Dou, R.H.Colby, Journal of Physical Chemistry122, 234906 (2005).[3] Bordi, C. Cametti, S. Sennato, S. Zuzzi, S. Dou,R. H. Colby, Phys Chem Chem Phys. 2006; 8:3653-3658[4] A. V. Dobrynin, M. Rubinstein, Prog. Polym. Sci.30, 1049 (2005).AuthorsF. Bordi § , C. Cametti § , S. Sennato § , S. Dou # , R. H.Colby #§Dip. di Fisica, Università di Roma ″La Sapienza″,Roma and INFM CRS-SOFT# Dept. of Material. Sci. and Engineering, ThePennsylvania State University, University Park, PA,USA.87SOFT Scientific Report 2004-06