Richard Osgood1 Yassine Ait-El-Aoud1 Richard Pang1 Michael Okamoto1 Svetlana Boriskina2 Hadi Zandavi2 Alkim Akyurtlu3 Steven Kooi4 Gang Chen2 Hongwei Sun5 Brian Koker6 Leila Deravi7 Amrita Kumar7

1, U.S. Army NSRDEC, Natick, Massachusetts, United States
2, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
3, University of Massachusetts Lowell, Lowell, Massachusetts, United States
4, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
5, University of Massachusetts Lowell, Cambridge, Massachusetts, United States
6, University of Massachusetts Lowell, Lowell, Massachusetts, United States
7, Northeastern University, Boston, Massachusetts, United States

Thermally managing the body’s heat flow is important for working efficiently indoors and outdoors. Power radiated by the human body is in the 7-12 μm Longwave Infrared (LWIR) regime. By using textiles that release heat in warm environments, and trap heat in cold environments, one can work at higher activity levels, and not sweat profusely or lose dexterity, respectively. Saharan ants reflect sunlight and emit infrared (IR) through special fiber-like hairs. We manipulate the IR to manage temperature, building on successful temperature-adaptive insulation research with temperature-dependent fiber shape changes.1

Fibers may absorb/reflect/backscatter LWIR too strongly, keeping a textile-wearer hot in warm climates. We investigate and control heat flow through these fibers and related films. Materials like ultra-high molecular-weight polyethylene absorb less IR radiation, and nano-structured- and micro-fibers alter IR scattering properties, permitting radiative cooling.2 New research has shown that nanomaterials can substantially modify blackbody emission, produce unusually large scattering and reduce unwanted IR absorption. We seek fibers that expand and contract in a cephalopod-like fashion to enable a textile with temperature-adaptiveness, perhaps due to fiber-compatible semiconductor-carbon thermoelectric materials with a temperature differential-controllable electrical output.3

We designed, fabricated, and analyzed polymer fibers and films for thermal absorption, emission, and scattering to enable radiative-cooling and heat-trapping textiles. The IR response of films with sub-monolayer nanoparticle arrays enabled design of temperature-responsive fibers. To control the LWIR, we mixed ultra-high-molecular-weight, medium-density, and low-linear-density polyethylene in different ratios in single-filament fibers with diameters in the range of 100 μm. These fibers were melt-extruded in a micro-compounder and incorporated uniformly 5 types of nanoparticles (which were scalable to high volume processes): silica, TiO2, Si, cephalopod granules, and Ag, with sizes 60 nm, 500 nm, 2 μm, 500 nm, and 120 – 300 nm, respectively; different concentrations and polymer-only fibers were also analyzed. Individual fibers were characterized (composition and surface morphology) and measured with visible light and IR micro-spectrophotometry;1-d stripes of these fibers revealed thermal properties of a textile. We quantified and controlled IR scattering (not just reflectivity/transmission, as has been found in a recent investigation) to understand absorption for models/predictions of thermal transport across fiber arrays into the body. Fibers of a construction-grade polyethylene fibrous fabric were found to have high LWIR transmission, and were a positive control. Si nanoparticles were found to strongly scatter IR.

[1] B. DeCristofano, S. Fossey, et. al., Proc. MRS (2011) 1312.
[2] J. Tong ACS Photonics 2 (2016) 769.
[3] G. Fernandes et. al., Nanotech. (2012) 135704.