Light Makes Pores in Polymer Gel Bigger
by Prachi Patel
Materials Research Society | Published: 15 February 2013
Light can increase the size of pores in a novel polymer gel network, chemists have found. When exposed to light, the gel swells and grows in the direction of brightest light. It could be used to make filters and light-harvesting materials, says Jeremiah Johnson of the Massachusetts Institute of Technology, who published details of the light-reactive gel in a paper published online in the journal Angewandte Chemie.
Researchers have used light to interact with and modify polymers for decades. Polymers that degrade when exposed to ultraviolet light, for instance, are used to pattern silicon integrated circuits. More recently, research groups have made polymers that repair themselves or change shape under light. In all these applications, the bonds inside polymers change in some way as a reaction to light.
Light irradiation is also used commonly to initiate the cross-linking of monomers to form polymers in the production of plastics. Trithiocarbonate groups, which consist of one carbon atom and three sulfur atoms, are typically used to initiate such cross-linking.
Johnson and his colleague Huaxing Zhou designed a new kind of trithiocarbonate. They attached organic side groups to two of the sulfur atoms in the group. In the presence of ultraviolet light, the bond between the sulfur atom and the carbon atom in the side group breaks. When the researchers add monomers—they use N-isopropylacrylamide—these monomers insert themselves in between the sulfur atom of the thiocarbonate group and the organic side group. As long as there are monomers in the mixture, the two side groups keep growing into longer polymeric chains. The polymerization stops when the light is shut off.
The researchers then treat the resulting polymer with tris-tetrazine. These molecules link together the polymer chains in a honeycomb-shaped network, making them cross-link to become a gel.
As long as the researchers add more monomers and there is light, the polymer chains keep growing. This increases the size of the pores in the polymer network, and makes the gel swell. The gel stops growing when it has consumed all the monomer.
By adding more or less monomer to the gel, the researchers can control how large or small they want the pores in the material. By placing masks over the gel, they could pattern regions with different-sized pores. “Pore size is related to the ability to filter things,” Johnson says. “Such gels could be used for the purification of chemicals or wastewater.”
The researchers show that the gel growth can occur in natural sunlight. They also found that if they shone light on one side of the vial containing the gel, the gel grew more in the direction of strongest light. “It’s like a flower growing towards the sun,” Johnson says. “No synthetic material does that.” He thinks the material could be used to create a system that efficiently harvests light for conversion into electricity.
“This work represents a unique approach to increasing polymer chain molecular weight and network pore size at any position or time within a material,” says April Kloxin, a professor of chemical and biomolecular engineering at the University of Delaware. “The ability to use not only ultraviolet light but broad spectrum sunlight is an exciting contribution, in addition to the tuning of pore size.”
Read the abstract in Angewandte Chemie here.
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