Conformal Growth of Ultrathin Hydrophilic Coatings on Hydrophobic Surfaces Using Initiated Chemical Vapor Deposition

Prevention and characterization of thin film defects induced by contaminant aggregates in initiated chemical vapor deposition

As initiated Chemical Vapor Deposition (iCVD) finds increasing application in precision industries like electronics and optics, defect prevention will become critical. While studies of non-ideal morphology exist in the iCVD literature, no studies investigate the role of defects. To address this knowledge gap, we show that the buildup of short-chain polymers or oligomers during normal operation of an iCVD reactor can lead to defects that compromise film integrity. We used atomic force microscopy to show that oligomer aggregates selectively prevented film growth, causing these hole-like defects. X-ray diffraction and optical microscopy demonstrated the crystallinity of the aggregates, pointing to a flat-on lamellar or mono-lamellar structure. To understand the origin of the aggregates, spectroscopic ellipsometry showed that samples exposed to the reactor consistently accrued low-volatility contaminants. X-ray photoelectron spectroscopy revealed material derived from polymerization in the contamination, while scanning electron microscopy showed the presence of defect-causing aggregates. We directly linked oligomeric/polymeric contamination with defect formation by showing an increased defect rate when a contaminant polymer was heated alongside the sample. Most importantly, we showed that starting a deposition at a high sample temperature (e.g., 50 °C) before reducing it to the desired setpoint (e.g., 9 °C) unilaterally prevented defects, providing a simple method to prevent defects with minimal impact on operations.

Kinetically Limited Bulk Polymerization of Polymer Thin Films by Initiated Chemical Vapor Deposition

An experimental study and kinetic model analysis of the initiated chemical vapor deposition (iCVD) of polymer thin films have been performed at saturated monomer vapor conditions. Previous iCVD kinetic studies have focused on subsaturated monomer conditions where polymer deposition kinetics is known to be limited by monomer adsorption. However, iCVD kinetics at saturated conditions have so far not been systematically investigated, and it remains unclear whether the adsorption-limited phenomenon would still apply at saturation, given the abundance of monomer for reaction. To probe this question, a series of depositions of poly(vinylpyrrolidone) (PVP) thin films as a model system were performed by iCVD at substrate temperatures from 10 to 25 °C at both fully saturated (100%) and subsaturated (50%) conditions. While the deposition rates at subsaturated conditions exhibit the expected adsorption-limited behavior, the deposition rates at saturated conditions unexpectedly show two distinct deposition regimes with reaction time: an initial adsorption-limited regime followed by a kinetically limited steady-state regime. In the steady-state regime, the deposition kinetics is found to be thermally activated by raising substrate temperature with an overall activation energy of +86 kJ/mol, which agrees reasonably well with the experimentally determined value of +89 kJ/mol in the literature for bulk PVP polymerization and a mechanistically derived value of +91 kJ/mol based on the bulk free radical polymerization mechanism of PVP. These findings open new operating windows for iCVD polymerization and thin-film growth in which fast polymer deposition can be achieved without substrate cooling that can greatly simplify the iCVD scale-up to roll-to-roll processing and enable iCVD polymerization of highly volatile monomers relevant for diverse applications in biomedicine, smart wearables, and renewable energy.

Simulation Study on the Screening of Hydrophobic Surface Materials for Pipeline Drag Reduction Based on Adsorption Properties

Hydrophobic surface drag reduction techniques are effective in reducing the frictional resistance of fluids, the adsorption of liquid molecules on hydrophobic surfaces can reflect the resistance to fluid flow through such solid surfaces. Based on molecular simulation technology, we investigate the adsorption characteristics of water molecules on hydrophobic surfaces to achieve rapid screening of hydrophobic materials in fire-fighting water supply systems. The Monte Carlo method was used to simulate the adsorption process of polymers and to analyze the effects of temperature and fixed adsorption quantity. Contact angle tests were also done to verify polymer hydrophobicity. The isothermal adsorption heat, water molecule distribution, and energy distribution were studied by molecular mechanics and molecular dynamics methods. Then, adsorption localization simulations and electrostatic potential distributions were used to predict possible adsorption sites on hydrophobic surfaces and single-molecule chains. Finally, the interaction energy, diffusion coefficient, and free volume were investigated to explain the adsorption mechanism at the molecular level. Simulation results show that, overall, PTFE was more hydrophobic and PES was more hydrophilic and at 298 K, the number of adsorbed water molecules was ranked as follows: PTFE < PVDF < PVC < PMMA < PPS < CSM < BD-HDI < BD-MDI < BD-TDI < PES. Furthermore, PTFE, PVDF, PVC, PES, and PPS have more stable adsorption configurations on the (0 -1 0) surface. According to the findings, hydrogen bonding dominates the interaction between water molecules and hydrophilic polymers, whereas π-π interactions increase water molecules' diffusion resistance in polymers with benzene rings. In addition, PES contains many sulfone groups and ether bonds, which disorganize the chain arrangement to provide more free volume, whereas the water adsorption rate of PTFE is reduced because its molecular chains are less convoluted and more organized.