The Remarkable “Flip−Flop” Self-Assembly of a Diblock Copolymer in Aqueous Solution

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Abstract

In this paper, it is shown that tertiary amine methacrylate-based diblock copolymers can form both micelles and reverse micelles in aqueous solution at room temperature. This remarkable behavior is due to subtle variations in the hydrophilic−hydrophobic balance of the two block sequences, which are based on 2-(diethylamino)ethyl methacrylate (DEA) and 2-(N-morpholino)ethyl methacrylate (MEMA). 1H NMR spectroscopy studies indicate that these DEA−MEMA copolymers dissolve molecularly in water at pH 6−7 at 20 °C. DEA−core micelles are formed at higher pH, and MEMA−core micelles are formed at pH 6.5 in the presence of 1.0 M Na2SO4. Both types of micelles were characterized in terms of their intensity-average diameters and mean aggregation numbers using dynamic and static light scattering, respectively. In addition, one of the copolymers (60 mol % MEMA, Mw = 30 000, Mw/Mn = 1.05) was selected for a detailed small-angle neutron scattering (SANS) study. SANS data for the micelles were analyzed in terms of a weakly interacting polydisperse sphere model and were in reasonable agreement with the light scattering data. The DEA−core micelles are larger than the MEMA−core micelles, but both light scattering and SANS data indicated that the DEA−core micelles had a lower mean aggregation number than the MEMA−core micelles. This suggests that the MEMA−core micelles are relatively dense and compact compared to the more loosely packed DEA−core micelles.

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