Characterizing the Structure of pH Dependent Polyelectrolyte Block Copolymer Micelles Albert S. Lee and Alice P. Gast* Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025 Vural Bu 1 tu 1 n and Steven P. Armes School of Chemistry, Physics and Environmental Science University of Sussex, Brighton BN1 9QJ, E. Sussex, UK Received December 1, 1998; Revised Manuscript Received April 20, 1999 ABSTRACT: We use fluorescence spectroscopy, dynamic light scattering (DLS), and small-angle neutron scattering (SANS) to characterize the structure of 2-(dimethylamino)ethyl methacrylate/2-(diethylamino)- ethyl methacrylate (DMAEMA/DEAEMA) block copolymer micelles. The copolymers exhibit a strong pH dependence, where protonation of the tertiary amines along the side chains cause the blocks to be soluble in water. Fluorescence results show a critical degree of protonation below which single chains aggregate to form micelles. This critical degree of protonation depends on the copolymer concentration and solution ionic strength. Dynamic light scattering experiments provide unimer and micelle size distributions, and the measured critical degrees of protonation are consistent with the fluorescence data. The micelle hydrodynamic radius measured from DLS depends on the solution ionic strength, because of the polyelectrolyte nature of the protonated copolymers. Small-angle neutron scattering experiments in conjunction with a starlike micelle model provide additional insights into the micellar structures. Introduction The study of micellar systems constitutes a broad area of research and applications ranging from household soaps to drug delivery systems. One specific class of micelles, polymeric micelles, comprise a copolymer having soluble and insoluble blocks, where the insoluble blocks aggregate to form a dense core and the soluble blocks extend out into the solvent to form a corona. 1,2 These micelles can assume various shapes, ranging from wormlike to spherical micelles. 3,4 In the past decade, the area of research in block copolymer micelles has been growing, and recently more attention has been focused in the area of aqueous block copolymer micelle systems. In general, aqueous copolymer systems have been difficult to prepare and to work with, often requiring a cosolvent such as methanol or THF to adequately dissolve the polymers to form stable, well-behaved micelles. 5 One way to avoid the use of cosolvents is to use block copolymers which are sensitive to the solution pH, where below a certain critical pH the copolymers dis- solve as unimers and aggregate to form micelles as the pH is increased beyond a critical value. 6-8 Recently Armes et al. have synthesized 2-(dimethylamino)- ethyl methacrylate/2-(diethylamino)ethyl methacrylate (DMAEMA/DEAEMA) copolymers which form micelles in aqueous solution above a critical pH. 6 The structures are shown in Figure 1. Under acidic conditions, the amine groups on the side chains are protonated, causing the copolymers to become hydrophilic and to remain as unimers in solution. Protonation of the side chains also causes the copolymers to behave as polyelectrolytes. The subsequent addition of base deprotonates the side chains, causing the DEAEMA block to become hydro- phobic. Above a critical pH, the copolymers aggregate to form micelles. The DEAEMA block forms the micelle core while the hydrophilic DMAEMA block extends out into the solvent to form the micelle corona. When excess base is added, the whole copolymer becomes deproto- nated, causing the micelles to aggregate and come out of solution. These copolymers could serve as a model for a delivery system, where the solute encapsulated in the micelle cores is released as the micelles break apart when they reach a target pH. 8 In this study, we use light scattering, fluorescence spectroscopy, and neutron scattering techniques to characterize the structure of DMAEMA/DEAEMA block copolymer micelles. We seek to find critical parameters such as the critical degree of protonation along the chain when micellization occurs. Because of the pH depen- dence and the weak polyelectrolyte nature of the copolymers, we expect a rich structural dependence of the unimers and micelles on parameters such as the solution ionic strength and pH. Experimental Section Materials. The DMAEMA/DEAEMA block copoly- mers were synthesized at Sussex using group transfer polymerization. The details of the synthesis are reported elsewhere. 6 Gel permeation chromatography analysis Figure 1. DMAEMA/DEAEMA block copolymer structure. 4302 Macromolecules 1999, 32, 4302-4310 10.1021/ma981865o CCC: $18.00 © 1999 American Chemical Society Published on Web 06/04/1999