pH-controlled adsorption of polyelectrolyte diblock copolymers at the solid/liquid interface

We investigated the pH dependence of the adsorption of polyelectrolyte diblock copolymers from aqueous solution onto the native oxide surface of silicon using spectroscopic ellipsometry. The observed adsorption behavior is closely related to the chemical structure and the hydrophilic-hydrophobic balance of the copolymers. These amphiphilic copolymers contain hydrophobic residues comprising either 2-(diethylamino)ethyl methacrylate (DEA) or 2-(diisopropylamino)ethyl methacrylate (DPA). The copolymers also contain hydrophilic residues that are either (1) methyl-quaternized or benzyl-quaternized 2-(dimethylamino)ethyl methacrylate (designated Me-DMA or Bz-DMA, respectively) or (2) sulfopropyl betainized 2-(dimethylamino)ethyl methacrylate (Bet-DMA). The DEA and DPA residues can be tuned to become hydrophilic by adjusting the solution pH. Thus, these diblock copolymers can be molecularly dissolved in acidic media without using cosolvents as a result of the protonation of the tertiary amine groups. At low solution pH, adsorption of the copolymers is only about 0.5-1.5 mg/m(2), which is expected for polyelectrolyte adsorption. Above a pH of 7, there is a pronounced increase in the adsorbed amount. This change in adsorption coincides with the formation of copolymer micelles in the bulk solution. Hence, it is likely that the interfacial layer consists of adsorbed micelles. In the Bz-DMA-b-DEA copolymer, only a small fraction of DEA blocks (22%) is needed to achieve a relatively large increase in the adsorption at higher pH. On the other hand, control experiments confirm that the corresponding homopolymers show no sharp change in the extent of adsorption with pH. Changing the hydrophobic residues from DEA to DPA does not significantly affect the extent of adsorption. However, substitution of the hydrophilic Bz-DMA residues with Bet-DMA significantly increases the extent of adsorption at higher pH. This is probably because the electrically neutral betainized block is less hydrophilic than the cationic Bz-DMA block. This work provides insight into the major influences on the block copolymer adsorption and thus creates a framework for tuning adsorption behavior.