A sulfur-based polymer that can transmit light at long infrared wavelengths could enable a new class of lightweight plastic optics—as long as scientists can figure out how to reliably make it (ACS Macro Lett. 2019, DOI: 10.1021/acsmacrolett.8b00923).
The polymer allows light at wavelengths from 7.4 to 14 µm to pass. Thermal imaging devices, similar to night-vision goggles, use those wavelengths to detect heat signatures, allowing the military to identify targets and vehicles in darkness. Lenses and waveguides for devices that operate at those wavelengths are normally made from chalcogenide glasses containing sulfur, selenium, or tellurium, or from other IR-transparent materials such as germanium or crystalline silicon. Those kinds of optics are all heavier and more expensive than plastic optics would be and require more energy to make.
“I can’t find evidence in the literature that there’s a polymer that transmits this far into the long-wave infrared at these thicknesses,” says Darryl A. Boyd, a chemist in the optical sciences division of the US Naval Research Laboratory who led the work. Whereas most polymers thicker than about 1 mm are opaque at such wavelengths, his team’s material was transparent at thicknesses greater than 1.5 mm. It also showed a high index of refraction; lenses made from high-index materials have high focusing power, which means that a thinner lens can achieve the same focus as a thicker lens with a lower index. In organic polymers, the refractive index rarely exceeds 1.7, but Boyd and his team measured an index of 1.98 with 636.4 nm (red) wavelength light and 1.94 with 1548.4 nm (near-infrared) light.
The team created their material using a fairly new process called inverse vulcanization. Standard vulcanization involves strengthening a carbon polymer backbone with sulfur cross-linking; in inverse vulcanization, a sulfur polymer is the backbone and carbon the cross-linker. Boyd’s team stirred tetravinyltin into molten sulfur and then cured it for three to four hours at 125 to 130 °C. Boyd says this is the first time an organometallic compound has been used in inverse vulcanization and is therefore an early attempt to discern how a metal may affect the optical, thermal, or mechanical properties of the polymer.
The material started out rubbery but became brittle within a day, for reasons Boyd hasn’t figured out yet. He says he plans to pursue that question, as well as try the same synthesis method with compounds that have a metal other than tin to compare the resulting properties. The change in consistency is an important problem to solve for the materials to be useful for optics, Boyd says. Another processing issue is that the polymer contained some trapped bubbles, which outgassed over the course of a few days, leaving behind voids that would distort an image.
Jeffrey Pyun, a chemist at the University of Arizona who pioneered inverse vulcanization, calls the paper “the beginnings of a really good finding” but wants to see more work to characterize the structure of the polymer to validate that the material turned out to be what was expected. Using polymers for long-wave infrared optics is a new and challenging application, though, and this paper points in a promising direction, he says.
Tom Hasell of the University of Liverpool says more work needs to be done in understanding and processing these materials, but finds the work promising. “There is no denying that this is a great result,” he says, “but I think it shows more what is possible than what has been fully realized yet.” He adds that there’s still a lot of work to be done to improve the material to the point where it can make defect-free, damage-resistant lenses.
This story and headline were updated on Feb. 20, 2019, to correct the wavelength range transmitted by the polymer.
The caption was updated on Feb. 25, 2019, to clarify the wavelength range blocked by the polymer on the left.