New approach to the molecular characterization of hydrophobically modified polyacrylamide I.V. Blagodatskikh a, * , O.V. Vasil’eva a , E.M. Ivanova a , S.V. Bykov a , N.A. Churochkina a , T.A. Pryakhina a , V.A. Smirnov b , O.E. Philippova b , A.R. Khokhlov b a Nesmeyanov Institute of Organoelement Compounds of RAS, Vavilova 28, 119991 Moscow, Russian Federation b Physics Department, Moscow State University, 119992, Leninskie Gory, Moscow, Russian Federation Received 22 January 2004; received in revised form 17 May 2004; accepted 16 June 2004 Abstract A new approach to the molecular characterization of hydrophobically associating copolymers of acrylamide is developed. It is based on the study of associative properties: the formation of intermolecular aggregates was followed by dynamic and static light scattering (DLS and SLS), while the formation of hydrophobic domains was detected by fluorescence spectroscopy with pyrene as a probe. In aqueous media, hydrophobic aggregation begins at concentrations much lower than the overlap concentration. The addition of co-solvent, acetonitrile, shifts the aggregation to the semi-dilute region. The dissolution of hydrophobic aggregates is controlled both by fluorescence spectroscopy with pyrene as a probe and by DLS and SLS. Absolute M w values are measured by SLS in mixed solvent of optimal composition. Molecular weight distribution (MWD) is characterized by GPC using calibration with secondary standards characterized by SLS. This approach allowed us to follow MWD evolution during a micellar copolymerization of acrylamide, N-nonylacrylamide and acrylic acid. It is found that the molecular weight heterogeneity remarkably growths with the increase of conversion. q 2004 Elsevier Ltd. All rights reserved. Keywords: Hydrophobically associating polymers; Molecular weight characterization 1. Introduction Hydrophobically modified water-soluble polymers and polyelectrolytes have attracted increased interest over past decades due to their ability in controlling viscosity at various shear rates [1–3]. Among these polymers, hydro- phobically modified polyacrylamides (HMPAAm) are especially attractive, in particular, for enhanced oil recovery. There are two main methods of synthesis of HMPAAm: (1) micellar radical polymerization and (2) chemical modification of polyacrylamide. The vast majority of investigations in the field of HMPAAm are concerned the micellar polymerization technique, which offers a real versatility in tuning such characteristic parameter of copolymers as monomer sequence distribution. A peculiar feature of this method is as follows: water-soluble monomers (acrylamide or acrylamide and ionazable como- nomer, e.g. sodium acrylate) are dissolved in water, while insoluble hydrophobic comonomer is solubilized in micelles of surfactant. The most detailed revue of micellar polymerization one can find in the work [4]. HMPAAm prepared by chemical modification are discussed in the work [5]. The key parameters controlling rheological behaviour of binary and charged ternary polymers are the chemical nature, the content and the blockiness in the distribution of hydrophobic monomer units and the molecular weight of polymer. Inhomogeneity in these parameters is also an important factor. To provide the synthesis of copolymers with the controllable properties, it is necessary to estimate the correlation between synthesis conditions and molecular characteristics of the prepared polymer. One can note that MW and MWD characterization of these polymers is rather difficult and, to our knowledge, no 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.06.040 Polymer 45 (2004) 5897–5904 www.elsevier.com/locate/polymer * Corresponding author. Tel.: C7-95-1358119; fax: C7-95-1355085 E-mail address: blago@ineos.ac.ru (I.V. Blagodatskikh).