Unexpected behavior of polydimethylsiloxane/poly(2-(dimethylamino)ethyl acrylate) (charged) amphiphilic block copolymers in aqueous solution

Abstract

We report the synthesis and self assembly of amphiphilic block copolymers based on poly(dimethylsiloxane) and poly(2-(dimethylamino)ethyl acrylate) (PDMS-b-PDMAEA). We consider the self-catalyzed hydrolysis of PDMAEA in aqueous solution, and its effect on the self-assembly of the amphiphilic copolymer by investigating poly(dimethylsiloxane)-b-[poly(2-dimethylamino)ethyl acrylate)-co-poly(acrylic acid)] (PDMS-b-(PDMAEA-co-PAA)), obtained from hydrolysis of PDMAEA, and poly(dimethylsiloxane)-b-poly(2-(trimethylammonium iodide)ethyl acrylate) (PDMS-b-PTMAIEA), obtained from quaternization of PDMAEA. We observe that the thermal properties and self-assembly behavior in water of these amphiphilic diblock copolymers depend on the structure of the hydrophilic block, and are strongly influence by hydrolysis of the PDMAEA block. Self-assembly in aqueous solution leads to the formation of spherical micellar structures with diameters between 40 and 70 nm, which were investigated by dynamic light scattering (DLS) and transmission electron microscope (TEM). PDMS-b-(PDMAEA-co-PAA) is shown to be pH responsive, and behaves as a highly negatively charged polymer at high pH values (pH > 10) and as a highly positively charged polymer at low pH values (pH < 2). Critical micelle concentrations (cmc) of the amphiphilic block copolymers were determined by fluorescence measurement with N-phenyl-1-naphthylamine (PNA) as a fluorescence probe. Surprisingly, surface tension measurements suggest that PDMS-b-PTMAIEA is “non-surface active”, forming micelles in bulk solution instead of adsorbing at the air–water interface to form a Gibbs monolayer, even when the polymer concentration was over 100 times above its cmc value. This remarkable behavior is consistent with recent reports that suggest that “non-surface activity” can be observed for both cationic and anionic amphiphilic diblock copolymers due to the effect of image charge repulsion at the air–water interface.