(495g) Nano within Nano: Kinetics of Vapor-Phase Free Radical Polymerization of Nanolayers Under Nano-Confinement

Nanostructures with ultra-high aspect ratios (typically >100) offer unparalleled large surface areas for a fixed device volume and are thus of great interest in many fields, including membranes used in filtration, (bio)sensors, drug delivery, and energy storage. Through coating surfaces with functional polymer nanolayers (<100-nm thick), one can precisely engineer the surface properties (e.g. surface energy) of such devices without sacrificing the innate bulk substrate properties (e.g. robustness), thus achieving optimal device performance. Initiated chemical vapor deposition (iCVD) has introduced the unprecedented capability to coat micro- to nanoscale structures with conformal and multi-functional polymer coatings. Previously, it has been established that the coating conformality can be controlled by the fractional saturation of monomers in the gas phase (i.e. P_M/P_M^sat); nonetheless, the effect of initiators on iCVD deposition kinetics – especially under nano-confinement – is far less understood compared to that of monomers. Here, we present the first systematic investigation of the respective effect of initiators (e.g. tert-butyl peroxide, TBPO) and monomers (e.g. 2-Hydroxyehtyl Methacrylate, HEMA) on the deposition kinetics of pHEMA nanolayers inside ultra-high-aspect-ratio nanopores.

Our results demonstrated successful deposition of uniform pHEMA nanolayers along 200 nm-diameter pores (aspect ratio > 275) (Fig. A), albeit 16- to 100-fold slower deposition rates inside pores (DR_pore) relative to that on flat surfaces (DR_flat). DR_norm, defined as DR_pore/ DR_flat,was found invariant with regard to P_m/P_m^sat (Fig. B); whereas DR_norm first increased with P_I/P_I^sat (i.e. fractional saturation of TBPO in iCVD chamber) until reaching maxima at the critical P_I/P_I^sat~0.005, and then decreased upon further increase in P_I/P_I^sat (Fig. C). Such distinct behaviors in DR_norm with regard to initiator and monomer concentrations were attributed to the nano-confinement-induced amplification of radical-wall collisions, due to the highly frequent bouncing of the radicals between the pore walls. This phenomenon was absent in the case of monomers as their interaction with the walls was governed by the BET isotherm. Using a collision-based kinetic model, we were able to numerically simulate this characteristic shape of DR_norm, thus lending support to this hypothesis. This amplification of radical-wall collisions can potentially be extended to various nanoconfined geometries important to the next-generation miniaturized devices.