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

TL;DR

This paper shows how the sequence of polypeptoid monomers affects the elasticity and tension of fluid interfaces, independent of their chemical composition.

Contribution

The study demonstrates that monomer sequence, not just chemical composition, controls interfacial properties of polypeptoids.

Findings

Inverse taper and blocky sequences produce highly elastic interfaces after processing.

Taper and distributed sequences result in lower interface elasticity.

Chemical composition controls surface concentration, while sequence affects interfacial elasticity.

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.

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