PNNL

Abstract

2D materials have attracted intense interest due to their potential for applications in the fields ranging from chemical sensing to catalysis, energy storage, and biomedicine. Recently, peptoids, a class of biomimetic sequence-defined polymers, have shown the ability to self-assemble into 2D crystalline sheets that exhibit unusual properties, such as high chemical stability and the ability to self-repair. In this study, we investigated the effect of peptoid sequence on the mechanism and kinetics of 2D assembly on mica surfaces using in situ AFM and time-resolved X-ray scattering. We explored three distinct sequences chosen to represent primary classes of chemical modification that can be implemented and still obtain 2D sheets. The results show that peptoid assembly on mica starts with deposition of aggregates, likely in the form of vesicles, that spread to establish 2D islands, which then grow by attachment of peptoids — either monomers or unresolvable small oligomers — following well known laws of crystal step advancement. Extraction of the solubility and kinetic coefficient from the dependence of growth rate on peptoid concentration reveals striking differences between the sequences. The sequence with the slowest growth rate in bulk and with the highest solubility shows almost no detachment i.e., once a growth unit attaches to the island edge there is almost no probability of detaching. Furthermore, a peptoid sequence with a hydrophobic alkyl chain in the hydrophilic block has enhanced hydrophobic interactions and exhibits rapid assembly both in bulk and on mica. These assembly outcomes suggest that hydrophobic interactions play a major role in assembly of peptoid sequences and contradict the results obtained in bulk solution, as well as the result predicted by theoretical simulations.