Figure 1. Fabrication of Janus membranes using physicochemical inhibition. (a) A schematic of a three-step process for the fabrication of a Janus membrane. Step (i): the hydrophilic support is filled with glycerol/inhibitor (HQ) mixture. Step (ii): the filled membrane is placed on the back-cooled stage (15 oC) of the iCVD reactor, and the PTFE deposition is performed for a specific time (5-15 min). Step (iii): the glycerol/inhibitor mixture is removed from the sample by placing it inside an isopropanol bath. (b) A top-down representative scanning electron microscopy (SEM) image of the hydrophilic PVDF support. (c) A top-down SEM image of the Janus membrane surface showing the PTFE structure deposited on the top surface of the support without blocking the pores. (d) A cross-section SEM image of the Janus membrane surface showing the PTFE structure deposited on the hydrophilic PVDF support. The dashed line shows the PVDF-PTFE interface. Here, the pressure and time of deposition were 900 mTorr and five minutes, respectively.
Although it has been shown that, 28–30 by changing the iCVD condition and moving from a diffusion-limited process toward a reaction-limited process, we are able to apply polymeric coating within the entire porous substrates, selective area deposition of polymers has not yet been achieved. Here, using the developed approach, we show that the PTFE deposition can be limited to the solid domains on the top surface of the substrate. As a result, the surface porosity of the substrate does not decrease after the coating. In the developed method, we note two different inhibition mechanisms: physical and chemical. Once the primary radicals are absorbed into the glycerol mixture, the relatively faster rate of diffusion, compared to that of the surface diffusion, prevents the formation of a polymer film on the liquid surface. As a result, the deposition of PTFE on the liquid domain will be delayed. The proposed mechanism can be found in Section S2, Supporting Information. In addition to the physical inhibition, the chemical inhibition also exists because of the quenching of the absorbed radicals in glycerol through the HQ present in the liquid mixture.
To unravel the difference between physical and chemical inhibition, we performed similar experiments on liquid films. For this purpose, the liquid films were prepared via spin-coating of the glycerol/inhibitor mixture on pieces of the silicon wafer with comparable areas. The detailed information for sample preparation can be found in Section S3, Supporting Information. Figure 2 (a) shows the Fourier transformed infrared spectroscopy (FTIR) of liquid samples before and after five minutes of PTFE deposition. As shown, the FTIR spectra of glycerol/inhibitor mixture remained unchanged after five minutes of iCVD deposition, indicating that no detectable PTFE was deposited on the surface of the glycerol/inhibitor. On the other hand, the PTFE characteristic peaks, located between 1150-1225 cm-1, were salient in the spectra of the pure glycerol sample processed in the iCVD chamber. This observation indicates that, during the first five minutes of reaction, HQ molecules present in glycerol quenched the adsorbed primary radicals. Increasing the deposition time to seven minutes, we found that the inhibition process becomes ineffective, and the characteristic peaks of PTFE were again observed in the spectra. The data showing the FTIR of the samples processed for a longer time are shown in Section S3, Supporting Information.