Evidences of Nonideal Mixing in Poly(ethylene Glycol)/Organic Solvent Mixtures by Brillouin Scattering M. Pochylski, ‡ F. Aliotta,* ,² Z. Blaszczak, ‡ and J. Gapin ´ ski ‡ Istituto per I Processi Chimico Fisici del CNR, sezione di Messina, Italy, and Department of Physics, Adam Mickiewicz UniVersity, Poland ReceiVed: July 12, 2005; In Final Form: NoVember 8, 2005 The concentration dependence of the hypersonic properties of solutions of poly(ethylene glycol) of mean molecular mass 600 g/mol (PEG600) in benzene and toluene has been investigated by Brillouin scattering. The two solvents are very similar in structure and chemical properties, but while benzene is nonpolar, toluene possess a modest dipole. In both solvents a high-frequency relaxation process has been observed at high concentrations which has been assigned to conformational rearrangements of the polymeric chains, triggered by reorientation of the side groups. In both cases, the concentration dependence of the adiabatic compressibility deviates significantly from linearity, indicating the existence of nonideal mixing phenomena driven by aggregation processes taking place in the systems. However, there is no temperature dependence for solutions of PEG600 in benzene; on the contrary, the results obtained for solutions of PEG600 in toluene are noticeably dependent on the temperature. The comparison of the experimental data with the results of previous experiments on similar systems allows a general picture for weakly interacting mixtures of hydrogen-bonded systems and organic solvents to be developed. In particular, in the presence of a nonpolar solvent molecule the local structure of the mixture is dominated by solute self-association processes and any resulting solute-solvent correlation is barely induced by excluded volume effects. At high enough dilution the self-aggregation of solute molecules produces a variety of new local topologies that cannot be observed in bulk solute, and as a consequence, the concentration evolution of the system is too rich to be described in terms of a linear combination of a few components over the whole concentration range. The situation seems to be simpler for the polar toluene solvent molecules, where a three-component model seems able to fit the experimental concentration dependence of the hypersonic velocity. This result is interpreted to imply that the interaction between the solvent dipoles and the active sites of the solute produces a relatively stable heterocoordination, while the relevance of self-association is partially reduced. 1. Introduction In the last few decades, the continuing quest for the engineering of new polymeric materials and devices has drawn a growing interest toward the investigation of the structural and dynamical properties of molten polymers and polymer solutions. In particular, poly(ethylene oxide) (PEO) and poly(ethylene glycol) (PEG) are of great industrial and biomedical relevance due to their high solubility in a wide number of organic solvents as well as in water. 1 Their high solubility is a consequence of the structure of the polymer chain, along which hydrophobic monomer units are arranged together with hydrophilic oxygen atoms. This type of structure means that there are a number of active sites available for both inter- and intramolecular hydrogen bonds. 2 While from the biomedical point of view the main interest is devoted to aqueous solutions, many industrially interesting properties come from the dynamical properties of the chain, responsible both for the transport mechanisms through the polymer network and for the self-aggregation processes among the polymer chains that drive the long-range order of the polymer network. The conformational dynamics of the polymer chain are known to be solvent sensitive. 2-11 In the presence of a noninteracting or moderately interacting solvent, any change in the local density of the polymeric component influences the localized fast motions of the polymeric chains. 12-14 In an attempt to separate the effects of the change in the local density of the hydrogen bonds from those induced by solvent- polymer interaction, we recently performed a Brillouin scattering investigation on solutions of poly(ethylene glycol) with mean molecular mass of 600 g/mol (PEG600) in CCl 4 . 15 Carbon tetrachloride is a good candidate for this kind of investigation due to its intrinsically nonpolar nature. A fast relaxation process was detected and assigned to conformational rearrangements of the polymeric chains, triggered by the reorientation of the side groups. The concentration dependence of the hypersonic velocity was interpreted assuming the existence of different concentration regimes, originated by the competition between the self-association process of the polymer chains (dominant at high polymer contents) and the tendency of their active sites to coordinate with the solvent molecules (this becoming relevant in the semidiluted region). Such heteroassociation should be promoted through electrostatic interaction in a fashion very similar to what has been recently observed for methanol/CCl 4 mixtures 16 and theoretically supported by some numerical results from ab initio molecular orbital simulations in acetone/CCl 4 clusters. 17,18 * To whom correspondence should be addressed. E-mail: aliotta@me.cnr.it (F.A.); pochyl@amu.edu.pl (M.P.). ² Istituto per I Processi Chimico Fisici del CNR. ‡ Adam Mickiewicz University. 485 J. Phys. Chem. B 2006, 110, 485-493 10.1021/jp053813o CCC: $33.50 © 2006 American Chemical Society Published on Web 12/10/2005