Hydrodynamic Properties of Polymer Mixtures in Solution † Cristina-Eliza Brunchi, Maria Bercea,* and Simona Morariu “Petru Poni” Institute of Macromolecular Chemistry, 41-A Grigore Ghica Voda Alley, 700487 Iasi, Romania The paper investigates the hydrodynamic properties of polyacrylonitrile (PAN) and poly(N-(4-carboxyphe- nyl)maleimide) (PMI) solutions in dimethylformamide (DMF) in comparison with PMI/PAN/DMF ternary mixtures. The experimental data obtained by viscometry have been discussed by means of two methods: first, the plots obtained with the classical Huggins equation are analyzed, and in parallel, an evaluation of the parameters given by the new Wolf model is presented. The experimental data obtained for binary polymer/ solvent and ternary polymer/polymer/solvent mixtures fit well with this last method and allow the calculation of intrinsic viscosities and other hydrodynamic parameters, which provide new information about the competition between different types of interactions for polymer mixtures in solution. The compatibility of the two polymers dissolved in a common solvent is also discussed on the basis of two parameters: Δb and R, reflecting the interactions between the polymer segments and polymer/polymer miscibility, respectively. The sign of these parameters shows that the PMI/PAN blends in DMF are miscible in the range of PAN mass fraction between 0.36 and 0.75. Introduction The viscometric behavior of polyacrylonitrile (PAN) solutions was investigated in different concentration regimes: extremely dilute, dilute, and semidilute. 1,2 In dilute polymer solution, the reduced viscosity is proportional to concentration, as indicated by the Huggins equation. The viscosity of a polymer solution in the extremely dilute region usually reveals some abnormalities such as the curves of reduced viscosity (η sp /c) plotted against concentration that show either an upward or a downward turn as the concentration is below a definite concentration. 1-4 The experimental works encounter some difficulties at such low concentrations of polymer because systematic errors appear in the evaluation of the intrinsic viscosity ([η]), being originated from the improper technique by which the viscosity of polymer solution is determined at very low concentrations. Upward changes in the slope of η sp /c as a function of concentration were reported for polyelectrolytes due to the fact that electrostatic repulsion between the charged groups of the macromolecules strongly increases upon dilution. 5 Despite considerable efforts, the understanding of the anoma- lous viscosity behavior of neutral polymer solutions at low concentrations is still far from complete. In the case of PAN solutions, such behaviors were attributed either to the polymer losses caused by the adsorption of the polymer molecules on the walls of the viscometer which decreases considerably the effective diameter of the viscometer capillary, either to an expansion of the individual coils, drainage, kinetic energy effects, or conformational changes of the macromolecules in solution. 1,2 Many efforts were carried out to obtain a reliable equation for determining the intrinsic viscosity by using the viscometric data obtained at different concentrations. A brief presentation of the most important methods and their applicability limits is recently published. 5 A new phenomenological approach was proposed as an alternative method for the determination of the intrinsic viscosity for the polyelectrolytes, in the presence and in the absence of salts. 6,7 The value of [η] is determined from the initial slope of the dependence of ln η rel (where η rel is the relative viscosity) as a function of concentration at sufficiently low shear rates and polymer concentrations. This model has been successfully applied for salt-free aqueous solutions of poly(N-butyl-4-vinylpyridinium bromide) samples with different quaternization degrees 6 and sodium polystyrene sulfonate, 7 as well as for a salted solution of sodium polystyrene sulfonate, 6 cationic polysaccharides based on dextran in salt-free aqueous solution, and water-methanol mixtures. 8 We obtained previ- ously accurate values of the intrinsic viscosity by applying the new model for neutral polymers in solution, that is, PAN in dimethylformamide (DMF). 9 The polymers containing N-substituted maleimides, such as poly(N-(4-carboxyphenyl)maleimide) (PMI), exhibit special properties due to the rigid imide rings in the backbones. Thus, N-phenylmaleimide polymers are used in some electro-optical applications which requires the improvement of the thermal stability, processability, electro-optical coefficients, and optical loss. 10-12 These properties are related to each other, and the enhancement of one property often adversely affects other properties. For example, rigid polymers with high glass transition temperatures usually have some difficulties in processing. The above-mentioned conflicting properties can be optimized by using the copolymerization technique 13,14 which enables us to control the concentrations of the hard and soft segments as well as the chromophores. Also, the polymer blends are extensively investigated because of their growing interest for practical applications. Blends are an inexpensive way to improve the polymer properties, producing desirable materials without the effort of devising a new synthesis. In some cases, by synergistic effects, such combinations may possess unique properties that are different from those of individual components. However, the superior properties of polymer blends are deter- mined by the miscibility of homopolymers on the molecular scale. † Part of the “Sir John S. Rowlinson Festschrift”. * To whom correspondence may be addressed. E-mail: bercea@icmpp.ro. Tel.: +40 232 217454. Fax: +40 232 211299. J. Chem. Eng. Data 2010, 55, 4399–4405 4399 10.1021/je1005714  2010 American Chemical Society Published on Web 07/08/2010