Procedure for the Transfer of Polymer Films Onto Porous Substrates with Minimized Defects

To study polymer thin films in membrane applications, materials must be transferred from a polished, smooth substrate to porous substrates in a fashion that does not fold, wrinkle, tear, or plastically deform the film. This video describes a 3D printable drain chamber designed to achieve this aim and concerns steps four through six in the process flow shown here. Samples used in this work consist of silicon wafer substrates into which are spin coated a stack of water soluble polyacrylic acid followed by a random copolymer mat and a polystyrene block polymethyl methacrylate block copolymer.

The aim of steps four through six is to transfer this pore-forming block copolymer from the silicon wafer onto a porous anodized aluminum oxide membrane to enable subsequent transformation into a functional filtration device. The drain chamber consists of a top ramp that enables the introduction of the polymer film stack and a bottom portion which holds the porous substrate and deionized water. The top part is printed from PLA from a filament printer, and the bottom portion was printed from an inkjet 3D printer.

A sample transfer tool is also printed from PLA using the filament printer. The first part attaches to a laboratory jack so the sample can be raised and lowered smoothly and without vibrations from hand movements. The second part holds the BCP wafer piece in place.

With that, the transfer process can begin. First, the BCP tool is assembled. The clamp portion is threaded so a screw can secure it to the lab jack.

The anodized aluminum oxide substrate is then placed inside of the bottom portion of the drain chamber using forceps. Next, a rubber O-ring is inserted to ensure a waterproof seal. The ramp portion of the drain chamber is screwed on tightly.

Last, clear vinyl tubing is attached to the spout at the bottom portion of the drain chamber and the other end to the syringe which is inside of a syringe pump to be able to control the drainage rate. Next, the silicon wafer with the polymer stack is placed on the transfer tool, and the drain chamber is also filled with deionized water. The silicon wafer is then lowered slowly into the water, which causes the polyacrylic acid layer to begin to dissolve.

This dissolution delaminates the polymer film from the silicon wafer leaving it to float on the water surface. The transfer tool is next removed, and the draining is activated by starting the syringe pump. The water is suctioned through the aluminum oxide substrate and out of the chamber by the pump.

Shown here is the draining taking place at a rate of one milliliter per minute. The angle of the ramp and design of the chamber directs the polymer film towards the center of the cylinder. Here, the membrane and the water level have reached the cylindrical body of the device, creating a more prominent meniscus, which holds the membrane taut and centered as it is lower to the underlying, anodized aluminum oxide membrane.

Once the water is drained and the polymer is in contact with the membrane, the chamber can be unscrewed to allow the remaining water to evaporate. The total process, including setup and water draining, takes about 15 minutes, but this time can be shortened by increasing the draining rate and by constructing the chamber to require a smaller water volume. Shown here are films prepared by directly scooping them off of a water bath surface using the porous membrane.

The tearing of the membrane and the placement off center are evident. The distance of the membrane center to the substrate center is shown on the right for various placed membranes. Here we see the improvement in reproducibility utilizing the drain chamber.

The intact films are evident, and the distance of membrane center to substrate center is shown to be minimized. The drain chamber has reduced the chance of film tearing and folding over the manual method. A clean laboratory environment free from dust will limit incidental contamination.

This method for thin film membrane transfer definitively reduces the damage done to the membrane and improves the accuracy and reproducibility of its placement on the substrate. By removing the difficult task of manipulating thin films on the water surface by hand, modifications to the basic drain chamber design can allow for the implementation of different polymer sample sizes and porous substrate types.