Structure of pH-Dependent Block Copolymer Micelles: Charge and Ionic Strength Dependence

Abstract

We characterize the structures of various polyelectrolyte block copolymer micelles in dilute aqueous solution as a function of pH and ionic strength. The block copolymers carry a common core block, 2-(diethylamino)ethyl methacrylate (DEAEMA), and one of three coronal blocks, 2-(dimethylamino)ethyl methacrylate (DMAEMA), poly(ethylene oxide) (PEO), and DMAEMA, whose side chain amine groups are selectively quaternized with benzyl chloride (Q-DMAEMA). The PEO−DEAEMA, DMAEMA−DEAEMA, and Q-DMAEMA−DEAEMA copolymers form micelles with electrostatically neutral, weakly charged, and highly charged coronae, respectively. We adjust the fractional charge α on the DEAEMA and DMAEMA blocks by adjusting the solution pH. For DMAEMA−DEAEMA micelles increasing the fractional charge α swells the micelle corona while decreasing the aggregation number due to electrostatic repulsions. The decrease in aggregation number is also observed with increasing α for the PEO−DEAEMA and Q-DMAEMA−DEAEMA micelles, due to electrostatic repulsions between the hydrophobic DEAEMA blocks. Increasing the ionic strength causes the DMAEMA−DEAEMA micelle corona to shrink as the salt screens electrostatic repulsions within the corona. In all three copolymers increases in the ionic strength cause the micelle aggregation number to increase by screening the electrostatic repulsions between chains. Trends in the corona thickness with varying fractional charge and ionic strength are compared with a number of theoretical models providing additional insight into the micelle structure.