Unveiling the Heat-Conducting Superpowers of Stretchy Polymers: A Revolutionary Discovery from MIT
Unleashing the Heat-Conducting Potential of Stretchy Polymers
Imagine a material that can switch its heat-conducting properties like a light switch, transforming from a poor conductor to a high-performance one with a simple stretch. This is not science fiction but a groundbreaking discovery by engineers at the Massachusetts Institute of Technology (MIT). The team has found that a common olefin block copolymer can dramatically enhance its heat conduction when quickly stretched, offering a new avenue for developing adaptive materials.
The Heat-Conducting Polymers: A Natural Switch
Most materials have inherent heat-conducting capabilities. For instance, plastic is typically a poor thermal conductor, while materials like marble are efficient heat conductors. When you place one hand on a marble countertop and the other on a plastic cutting board, the marble will conduct more heat away from your hand, creating a colder sensation. Typically, a material's thermal conductivity cannot be changed without re-manufacturing it. However, MIT engineers have discovered that a relatively common material can switch its thermal conductivity by simply stretching it quickly.
The Stretchy Material: A Thermally Reversible Olefin Block Copolymer
The thermally reversible material is an olefin block copolymer, a soft and flexible polymer used in various commercial products. When the material is quickly stretched, its ability to conduct heat more than doubles, and this transition occurs within just 0.22 seconds, the fastest thermal switching observed in any material. When the material returns to its unstretched form, it reverts to its plastic-like properties.
The Microscopic Structure: The Key to Heat-Conducting Superpowers
The key to this new phenomenon is that when the material is stretched, its microscopic structures align in ways that suddenly allow heat to travel through easily, increasing the material's thermal conductivity. In its unstretched state, the same microstructures are tangled and bunched, effectively blocking heat's path.
From Spandex to Heat-Conducting Polymers: An Unexpected Discovery
The team initially sought more sustainable alternatives to spandex, a synthetic fabric made from petroleum-based plastics that is traditionally difficult to recycle. They were investigating fibers made from a different polymer known as polyethylene. However, they realized that polyethylene had other properties more interesting than its elasticity. The unique feature of polyethylene is its backbone of carbon atoms arranged along a simple chain, making it a good conductor of heat.
The Microscopic Structure of Polymers: A Messy Tangled Chain
The microstructure of most polymer materials, including polyethylene, contains many carbon chains. However, these chains exist in a messy, spaghetti-like tangle known as an amorphous phase. Despite carbon's good heat conductivity, the disordered arrangement of chains typically impedes heat flow, making polyethylene and most other polymers generally low thermal conductors.
From Amorphous to Crystalline: Unlocking Heat-Conducting Potential
In previous work, MIT Professor Gang Chen and his collaborators found ways to untangle the mess of carbon chains and push polyethylene to shift from a disordered amorphous state to a more aligned, crystalline phase. This transition effectively straightened the carbon chains, providing clear highways for heat to flow through and increasing the material's thermal conductivity. However, this switch was permanent.
The Stretchy Olefin Block Copolymer: A Reversible Heat-Conducting Switch
When the team explored polyethylene, they also considered other closely related materials, including olefin block copolymer (OBC). OBC is predominantly an amorphous material, made from highly tangled chains of carbon and hydrogen atoms. Scientists had assumed that OBC would exhibit low thermal conductivity. However, when the team carried out experiments to test the elasticity of OBC, they found something quite different.
The Stretching Experiment: Unlocking the Heat-Conducting Potential
As they stretched and released the material, they realized that its thermal conductivity was high when stretched and lower when relaxed, over thousands of cycles. This switch was reversible, while the material stayed mostly amorphous. This was an unexpected discovery, as scientists had assumed that increasing conductance in OBC would be permanent, similar to polyethylene.
The Microscopic Structure: Unraveling the Stretching Mystery
The team then took a closer look at OBC and how it might be changing as it was stretched. They observed that, in its unstretched state, the material consists mainly of amorphous tangles of carbon chains, with just a few islands of ordered, crystalline domains scattered here and there. When stretched, the crystalline domains seemed to align and the amorphous tangles straightened out, similar to what Gang Chen observed in polyethylene.
The Stretchy Tangles: A Reversible Heat-Conducting Switch
However, rather than transitioning entirely into a crystalline phase, the straightened tangles stayed in their amorphous state. In this way, the team found that the tangles were able to switch back and forth, from straightened to bunched and back again, as the material was stretched and relaxed repeatedly.
The Fast Thermal Switching: A Revolutionary Discovery
The team also found that this thermal switching happens extremely fast: The material's thermal conductivity more than doubled within just 0.22 seconds of being stretched. This fast thermal switching is a revolutionary discovery, offering a new avenue for developing adaptive materials that can quickly react to dissipate heat.
The Future of Heat-Conducting Polymers: A World of Possibilities
The researchers are working on further optimizing the polymer and on engineering new materials with similar properties. The material could be used to engineer systems that adapt to changing temperatures in real time. For instance, switchable fibers could be woven into apparel that normally retains heat. When stretched, the fabric would instantly conduct heat away from a person's body to cool them down. Similar fibers can be built into laptops and infrastructure to keep devices and buildings from overheating.
The Impact of Heat-Conducting Polymers: A Game-Changer for Industry and Society
'We need cheap and abundant materials that can quickly adapt to environmental temperature changes,' says Svetlana Boriskina, principal research scientist in MIT's Department of Mechanical Engineering. 'Now that we've seen this thermal switching, this changes the direction where we can look for and build new adaptive materials.' The team's results are being worked into models to see how they can tweak a material's amorphous structure to trigger an even bigger change when stretched. If they can make further improvements to switch their thermal conductivity from that of plastic to that closer to diamond, it would have a huge industrial and societal impact.
