Polypeptoid Monomer Sequence and Chemical Composition as Independent Controls of Interfacial Tension and Elasticity at Air/Fluid Interfaces.

ABSTRACT

We use sequence-specific polypeptoids to characterize the impact of the monomer sequence on the adsorption of surface-active polymers at fluid/fluid interfaces. Sets of 36 repeat unit polypeptoids with identical chemical composition, but different sequences of hydrophobic moieties along the oligomer chain (taper, inverse taper, blocky, and evenly distributed), are designed and characterized at air/water interfaces. Polypeptoids are driven to the interfaces by decreasing the solvent quality of the aqueous solution. In situ processing of the adsorbed layers causes a collapse of polypeptoids and the formation of irreversibly adsorbed, solvent-avoiding layers at interfaces. Differences in thermodynamic properties, driven by solubility, between the collapsed structures at interfaces are studied with measurements of interfacial tension. The dilatational modulus of polypeptoid-coated interfaces is used as a proxy to probe the extent of the coil-globule collapse at the interface. The role of hydrophobicity is investigated by comparing four sequences of polypeptoids with an increased size of the hydrophilic side chains. In each set of polypeptoids, the composition of molecules, not the sequence, controls the surface concentration. The molecules are described in terms of the distribution of the hydrophobic monomers on the backbone of the polymer. Inverse taper (IT) and blocky (B) sequences of hydrophobic moieties favor the formation of highly elastic interfaces after processing, while taper (T) and distributed (D) showed lower elasticity after processing, which is achieved by replacing good solvent with poor solvent and then nonsolvent. These structures allow for the study of the impact of the chemical composition and sequence of monomers on the properties of polymer-coated interfaces.