Monomer Structure and Discharge Power Influence the Multiscale Architecture of Plasma-Polymerized Hydrocarbon...

Plasma polymer nanoparticles (pp-NPs) have emerged as linker-free nanocarriers for bioactive cargo and as nanofuels for laser-driven nuclear fusion, yet the structural factors governing their functionality remain poorly understood. Here, pp-NPs were synthesized via plasma polymerization of hexane, cyclohexane, and benzene to determine how monomer structure and specific energy input control radical concentration, hydrogen content, and hierarchical topology. Electron spin resonance and variable-energy positron annihilation lifetime spectroscopy establish that unquenched radical concentrations (10¹⁴-10¹⁶ spins mg^-1) and free-volume size (0.5-0.6 nm) increase from aliphatic to aromatic monomers, whereas free-volume concentration decreases inversely. These trends arise from biradical formation in benzene plasma, steric constraints of aromatic rings, and denser packing caused by reduced hydrogen content, as confirmed by elastic recoil detection analysis. Higher discharge power unexpectedly reduces cross-link density in pp-NPs; statistical network reconstruction reveals that unsaturated carbon bonds reduce the concentration of elastically active network chains more strongly than dehydrogenation increases it. Atomic force microscopy demonstrates that aliphatic monomers yield smoother overall NP surfaces but with a more locally roughened profile (roughness exponent α = 0.68), whereas pp-benzene NPs exhibit higher global roughness yet smoother local morphology (α = 0.75). Upon deposition, pp-NPs assemble into porous coatings with interparticle voids tunable via NP size (65-560 nm). Together, these findings elucidate how plasma chemistry governs the hierarchical architecture of pp-NPs from subnanometer free volumes to nanoscale surface morphology and mesoscopic interparticle voids, establishing design principles for tailoring radical concentration, hydrogen content, and multiscale porosity for advanced applications.