Downstream Monomer Capture and Polymerization during Vapor Phase Fabrication of Porous Membranes

Abstract

Porous membranes can be formed by the polymerization of solid monomer by a vapor-phase initiator followed by sublimation of the unreacted monomer. This versatile bottom-up process can be used to deposit porous polymer membranes on a variety of substrates including planar, curved, and structured surfaces. In this paper, we incorporate additional thermoelectric coolers (TECs) into the reactor in order to study downstream monomer capture and polymerization. Better uniformity across the TECs is achieved by capturing the unreacted monomer from the preceding surface instead of capturing the monomer onto multiple surfaces at once. We show that the surface temperature of the downstream TEC affects the membrane morphology and the kinetics of polymerization. Lower capture temperatures lead to better surface coverage while higher temperatures promote the polymerization rate. The morphological differences are further confirmed by coating the membranes with a fluoropolymer to study variations in hydrophobicity. Our ability to capture and polymerize monomers across multiple TECs provides a sustainable route for enhancing the throughput of the process which is desirable for practical applications.

Influence of Oblique Angle Deposition on Porous Polymer Film Formation

“…The thickest point of the film occurs between points A and X as a result of the low monomer flow rate, which is consistent with our previous study of the growth of dense poly­(2-hydroxyethyl methacrylate) films using the monomer extension at low monomer flow rates . Region 1 is characterized by pillars as shown in Figure a, which are similar to those formed in our earlier studies in the absence of the monomer extension at higher flower rates. ,, The polymer pillars possess dual-scale porosity, with smaller nanometer-sized pores found within the pillars and larger micrometer-sized pores formed from the void spaces between the pillars. ,,, The pillars do not appear to have any preferential orientation with respect to the angle of delivery, unlike OAD of oxides, metals, and parylene-c, which result in highly structured films. − Instead of orienting relative to the angle of monomer delivery, the polymer pillars in our films orient parallel to the direction of freezing, which is perpendicular to the surface. This is consistent with freeze-casting studies that use unidirectional freezing to synthesize aligned porous polymer structures. , …”

Scratch-Resistant Porous Polymer Coatings with Enhanced Adhesion to Planar and Curved Substrates

“…The flow of the monomer was halted after achieving an approximate thickness of 100 μm, while the initiator continued to flow to enhance monomer conversion, as shown in our previous studies. 24,25 The excess solid MAA was sublimated after polymerization. Figure 2a shows porous PMAA deposited onto a silicon wafer.…”

“…In the conventional iCVD process, monomer and initiator molecules are introduced into the reactor in the gas phase and the monomer partial pressure is kept below the saturation pressure of the monomer at a given substrate temperature. − The monomer molecules adsorb to the surface of the cooled substrate, and initiator radicals that are formed by a resistively heated filament array diffuse to the substrate to begin polymerization to form dense pinhole-free polymer coatings with thicknesses of a few nanometers to several microns. In our modified iCVD process, we use operating conditions such that the monomer partial pressure and substrate temperature are below the triple point pressure and temperature of the monomer. − The monomer vapor therefore undergoes a phase change and deposits as solid pillar-like microstructures at the surface of the cold substrate. The initiator molecules are cleaved by a hot filament array, and the resulting radicals react with the solid monomer to begin polymerization.…”

Vapor Deposition of Functional Porous Polymer Membranes

“…One way to recycle the unreacted sublimating monomer is to add additional TECs downstream. Our recent work shows that these TECs serve as cold traps that can capture unreacted monomer, which leads to decreasing the monomer waste and allows for scaling up the process . Our future work will focus on further improving monomer-to-polymer conversion by studying the effect of directional monomer flow on maximizing the monomer capture rate and studying the use of additional cold traps to capture the unreacted monomer.…”