Covalent layer-by-layer assemblies of polyelectrolytes and homobifunctional spacers

Area of Science:

• Materials Science
• Polymer Chemistry
• Surface Chemistry

Background:

• Step-by-step organic film construction typically relies on physical interactions or covalent bonding between alternating polymeric chains.
• Overcompensation mechanisms, like charge overcompensation in polyelectrolyte multilayers, are crucial for continued film buildup.
• Existing methods are often limited by the specific polymeric nature of the interacting species.

Purpose of the Study:

• To introduce a novel film architecture based on step-by-step construction utilizing covalent bonding.
• To explore the use of the Sharpless click reaction for building films from polyelectrolytes and neutral bifunctional molecules.
• To investigate the influence of various parameters on the film buildup process.

Main Methods:

• Utilized the Cu(I)-catalyzed click reaction for film assembly.
• Employed poly(acrylic acid) (PAA) functionalized with ethylene glycol (EG) arms, each terminating in either an alkyne or azide group.
• Used bifunctionalized EG spacers also terminated with alkyne or azide groups for cross-linking.

Main Results:

• Demonstrated the regular buildup of films that uniformly cover the substrate surface.
• Observed that film roughness correlates with the thickness of the film core.
• Found that the grafting density of reactive moieties on PAA did not affect the thickness increment per bilayer.
• Identified that the combination of alkyne-functionalized EG spacers with azide-functionalized PAA chains resulted in faster film growth compared to the reverse.
• Determined that longer EG arms on PAA and longer EG spacers generally led to larger thickness increments per bilayer, with an exception for very long spacers (50 EG units) likely due to steric hindrance.

Conclusions:

• The click reaction provides a versatile method for constructing well-defined organic films with tunable thickness.
• The choice of functional groups on the PAA and spacers influences the film growth rate.
• Spacer length significantly impacts film thickness, with optimal lengths observed before potential size exclusion effects impede deposition.