Effect of Block Size and Sequence on the Micellization of ABC Triblock Methacrylic Polyampholytes

Abstract

Fluorescence spectroscopy and static and dynamic light scattering were employed to probe the effect of block size and sequence on the micellization properties of a series of water-soluble, low-polydispersity, low-molecular-weight, block and random methacrylic polyampholytes : B 8 M 12 A 16 , B 12 M 12 A 12 , B 16 M 12 A 8 , B 12 A 12 M 12 , and (B-co-M-co-A) 12 , where B is 2-(dimethylamino)ethyl methacrylate (DMAEMA), M is methyl methacrylate (MMA), and A is methacrylic acid (MAA). The values of the pyrene fluorescence emission intensity ratio I i /I 3 , indicative of micelle formation, are reported for both acidic and basic solution pH values, over a wide range of copolymer concentrations (0.0001-0.5% w/w) and at three different temperatures (10, 25, and 50 °C). The critical micellization concentration (CMC) values ordered as CMC-(B 16 M 12 A 8 ) > CMC(B 12 M 12 A 12 ) > CMC(B 8 M 12 A 16 ) at low solution pH. Under basic conditions, the CMC for B 16 M 12 A 8 was slightly lower than that for B 12 M 12 A 12 . Micelles were not formed by the random copolymer (B-co-M-co-A) 12 at either pH value, nor by the triblock B 12 A 12 M 12 at basic pH. A small increase in the CMC with increasing temperature was observed for the BMA copolymers at basic pH. Dynamic light scattering data indicated that, for the micelles formed by the BMA polyampholytes, both DMAEMA and MAA blocks were in the corona regardless of which block was charged, while the middle hydrophobic MMA block constituted the micellar core. The constraints imposed by such a conformation of the polyampholyte blocks resulted in relatively low micelle aggregation numbers of 10-20 and a micelle size about half the contour length of the polymer. The B 12 A 12 M 12 copolymers formed larger micelles than the BMA polyampholytes ; with the MMA block in the center, the micelle radius is determined by the length of the fully-stretched polymer.