Plastic process produces puny pores

By Kimberly Patch, Technology Research News

The size of the microscopic pores in a material determines how the material scatters the sun's rays and how much light will shine through. Making microscopic pores precisely the right size, however, is tricky.

Researchers from the Massachusetts Institute of Technology have found a way to coat materials with layers of polymer that allows them to control the size of the polymer's microscopic pores.

The method has several key attributes. The water-based process is inexpensive and can be used to coat delicate materials like the plastic used for sunglasses.

Exposing the coating to a water solution of a certain acidity also changes the size of the pores even after they are in place. "When we change the pH of a water solution we can fill [the polymer layers] with air [forcing] them to become porous materials," said Michael Rubner, a professor of materials science and engineering at the Massachusetts Institute of Technology.

Pore size that changes depending on pH could eventually prove useful for biological applications like delivering drugs to specific areas within the body, according to Rubner.

The researchers have also found ways to pattern layers of the polymers on surfaces. "These are all water-soluble polymers [so] we can put water into an inkjet printer at the right pH and wherever the ink goes the polymers dissolve," said Rubner. This could lead to patterned surfaces that direct cell growth.

Controlling the size of pores means controlling the amount of air the coating contains, and thus the amount of glare that bounces off a material. "Air has the lowest refractive index you can have. So the more air you put into a material, the lower its refractive index is," said Rubner.

The refractive index of a material determines how much light it reflects. "If you pass light through a sheet of glass, at each surface you'll lose four percent of that light -- it's reflected," said Rubner.

Getting rid of glare means giving a material a top layer that has a refractive index that's lower than the refractive index of a material. The thickness of the coating also matters.

The right combination of refractive index and thickness will cause the waves to cancel each other out and the light to pass through the glass, said Rubner. "For example, if I took a beam of 100 percent intensity and measured it after it passed through [plain] glass it would be down to 92 percent," he said.

Glass with reflective coatings, however would cause disruptive interference so that "light that normally would be reflecting can't reflect anymore so it has to go through the glass. Now you've got 99.9 percent of the light you started off with coming out the other end rather than 92 percent," he said.

There's a caveat, however. This will tune out light waves of a certain wavelength over only a 10-nanometer range. "If you go to different wavelengths, you don't have the same kind of effectiveness," said Rubner. Visible light waves vary from 400 to 700 nanometers, and the sun's rays extend even further.

The way to broaden the antireflection capabilities is to grade the refractive index. This is commonly done in the natural world: a moth eye achieves this with a group of cones on the surface of the cornea, said Rubner. "Think of it as a bunch of mountains sitting on the surface of the cornea," he said. At the very top of the mountains there is a small amount of mountain and a lot of air, and the ratio changes all the way down, and near the bottom the ratio is reversed. "If you create a graded refractive index... you can broaden [the range of light waves] to hundreds of nanometers."

The researchers were able to make anti-reflective coating from their polymers by making very small pores. This was a challenge, said Rubner. "Just recently we figured out... that by applying some subtle games with the solutions that... convert [the polymers] from the nonporous to the porous state [we could] make the pores very very small," he said.

The researchers can now control the size of the pores from one micron, which is large enough that the material scatters light, down to a tenth of a micron, where it has anti-reflective properties, said Rubner.

To make the coatings, the researchers dip a glass or plastic substrate into a solution of water mixed with the polymer, a long, chain-like, molecule that can contain a negative or positive charge. "These polymer molecules will spontaneously... assemble on the surface into a very, very thin layer," said Rubner. Once a layer builds up, its charge repels any more polymer from attaching.

The researchers add another layer by putting the substrate in a negatively-charged vat of polymer solution. "The negatively-charge polymer is attracted to the positively-charge polymer. It neutralizes that charge and builds up a little excess charge," said Ruben. The excess charge repels further negatively-charged polymer.

The researchers can adjust the thickness of each layer from about half a nanometer to about 50 nanometers per layer. "The thinnest film that we put down is basically the thickness of the molecule itself," said Rubner.

The thickness of a layer depends the number of charges contained in the polymers that make up the layer. "If you have a lot of charge on the chain [it will] spread out on the surface," said Rubner. "If you put a few charges on the chain, a few charges will anchor, but the rest of the chain will loop away from the surface, so you end up getting a much thicker layer."

To make a 100-nanometer-thick film, which would have anti-reflection properties for visible light, the researchers could build up, for example, 10 layers that were 10 nanometers thick. "We do a fair amount of dipping... until we get precisely the thickness we want," said Rubner.

To change the size of the pores, the researchers change the number of charges on the negative polymer layers by dipping the substrate in liquid somewhere between a pH of seven and two. When it comes in contact with a high pH, a negatively-charged polymer is fully charged, and with a low pH it is only partially charged. As the pH rises, the molecules release hydrogen protons, which are positively charged. This increases the number of negative charges on the chain, which causes the molecule to flatten.

The film can change its thickness by as much as a factor of three this way, Rubner said. "You're filling it with air," he said.

For anti-reflective applications, the pore size can be permanently fixed. Once the layers reach a desired thickness, they can be set by baking them at a temperature of 60-90 degrees Celsius, said Rubner. "Once you have the conditions you want -- the right velocity, the right thickness -- we [heat it] up in the oven at a relatively low temperature and we chemically fix the structure so it can never change again."

The researchers are currently working on improving the adhesion of layers on different substrates. At the same time they're looking to use the controllable pore sizes in biological applications. "You could change what is passing through a filter, for example, or you could have a drug inside a [controllable] multilayer and then open the pore and release it," said Rubner.

Something like a change in pH could open the pores, and it's well known that tumors in the human body create a different pH locally around them than the body normally has, said Rubner. "It's speculative, and we have no evidence to prove that we can do that yet, but it's potentially possible [to] direct some of these materials to tumors and have them release a drug," he said.

The other potential biological applications include culturing cells. "You want to control where those cells are anchored, and what nutrients are feeding them," said Rubner. To do this "you need to have patterned surfaces to control where the cells attach and how they interact with the substrate [that] they're sticking to."

The method is ready to use in making anti-reflection coatings now, said Rubner. All that's needed is to "identify the application, look at the substrate that you want to put [the coating] on, and... make sure that [it] adheres well and it doesn't rub off easily," he said.

Using the coatings for uses like drug delivery is probably five years away, he said.

Rubner's research colleagues were Jeri'Ann Hiller and Jonas D. Mendelsohn. They published the research in the September, 2002 issue of the journal Nature Materials. The research was funded by the National Science Foundation (NSF).

Timeline:   Now, 5 years
Funding:   Government
TRN Categories:  Chemistry; Materials Science and Engineering
Story Type:   News
Related Elements:  Technical paper, "Reversibly Erasable Nanoporous Anti-Reflection Coatings from Polyelectrolyte Multilayers," Nature Materials, September, 2002.


January 15/22, 2003

Page One

Heat's on silicon

Remote monitoring aids data access

Metal stores more hydrogen

Device demos terabit storage

Plastic process produces puny pores


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