Surface Viscoelastic Parameters of Poly((dimethylamino)ethyl methacrylate-methyl methacrylate) Diblock Copolymer Solutions: pH Dependence of the Evolution of the Equilibrium Values Andrew J. Milling and Randal W. Richards* IRC in Polymer Science and Technology, University of Durham, Durham DH1 3LE, UK Fiona L. Baines, Steven P. Armes, and Norman C. Billingham School of Chemistry, Physics and Environmental Sciences, University of Sussex, Brighton BN1 9QJ, UK Received September 26, 2000; Revised Manuscript Received February 22, 2001 ABSTRACT: The dynamic interfacial properties of a diblock copolymer poly((dimethylamino)ethyl methacrylate-b-methyl methacrylate) have been obtained by surface quasi-elastic light scattering. The temporal evolution of the capillary wave frequency and damping as well as the surface viscoelastic properties (surface tension, dilational modulus, and dilational viscosity) of a freshly formed air-water interface for a range of pH values (at fixed scattering vector) has been investigated. The equilibration dynamics were dependent on the solution pH, but the equilibrium state was obtained by the time the interface was 12 h old. Tensiometry showed that a surface phase transition occurs between pH 7.0 and pH 6.0. The surface light scattering data clearly showed the differences in temporal response in this same range of pH. For solutions with pH 5.0, the air-water interface is populated by unimers. For higher pH solutions, micelles are the initial species at the surface, but these disaggregate for solutions with pH 6.0 and 6.5. The disaggregation process is faster for the solution with pH 6.5, and this is associated with resonance between the capillary and dilational modes enabling energy to be transferred to the dilational mode, accelerating the disaggregation of the micelles. Introduction The adsorption of block copolymers at interfaces is of interest for both academic study and technological applications. In solution, block copolymers can display a wide range of structural behavior, depending upon the solvency conditions and the molecular architecture. In many cases preferential solvation of one block bestows surface activity and self-aggregation to micelles and eventually organization with considerable long-range order. At low concentrations, solution structures of partially soluble copolymers are generally confined to unimers and micelles; the micellar states themselves may be of various geometries depending on copolymer composition and solution concentration. The majority of block copolymers studied have electrically neutral blocks; a relatively novel class of block copolymers composed of an electrically neutral block and a poly- electrolyte block have been explored of late. The aqueous solution behavior of these copolymers is dependent upon the ionic strength and, for weak polyelectrolytes, the solution pH. The linear diblock copolymer poly((di- methylamino)ethyl methacracrylate-b-methyl methacry- late) (poly(DMAEMA-b-MMA)) is one such a copolymer, and its organization in both aqueous solution at the interface of the aqueous solution with air and a solid 1 has been explored by others using dynamic light scat- tering, neutron reflectometry, and ellispometry. How- ever, the role of these polymers in modifying surface fluctuations (capillary waves) and associated interfacial surface viscoelastic moduli has not been reported. Surface quasi-elastic light scattering (SQELS) is a nonperturbative method for the investigation of capil- lary wave phenomena that has been applied to pure liquid interfaces, surfactant and polymer excess layers (Gibbs films), and spread polymer films (Langmuir films) at fluid interfaces. For adsorption from solution, the dynamics of adsorption and relaxation may be slow, and hence the rapid data acquisition times of SQELS experiments allow “snapshots” of the interface to be made over time during surface reorganization and surface-bulk equilibration. Consequently, insight into the mechanism of interfacial structure formation may be gained. We report here the findings of SQELS experiments performed on aqueous solutions of poly(DMAEMA-b- MMA) at fixed concentration but over a range of pH. First, the theoretical background to the SQELS experi- ment and the extraction of pertinent information is outlined. The details of the light scattering and tensio- metric experiments are given before concluding with a discussion of the results and a comparison with the surface organization data from the earlier neutron reflectometry experiments on similar copolymers. Theoretical Background The surfaces of liquids are continually roughened by thermal fluctuations that can be decomposed into a discrete set of Fourier modes, the capillary waves. 2 The relationship between the frequency and wavenumber (q) of these Fourier modes is described by a dispersion equation. 3,4 For a pure liquid surface the dispersion equation is couched in terms of the surface tension (γ 0 ), the bulk density (F), and viscosity (η). 5 The complex frequency of the capillary wave, ω, * To whom correspondence should be addressed. ω ) ω 0 + iΓ (1) 4173 Macromolecules 2001, 34, 4173-4179 10.1021/ma001675p CCC: $20.00 © 2001 American Chemical Society Published on Web 05/05/2001