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Tuesday, 10 December 2019

The newly discovered Plastic hardens 1800 times on Heating

The soft, transparent gel at 25 ° C does not support weight (top), but quickly becomes rigid and opaque when heated to 60 ° C, becoming strong enough to support weight (bottom).

Heat hardening plastic

Researchers at Hokkaido University in Japan have developed a hydrogel that does the opposite of what polymer-based materials - such as plastic bottles - usually do: the material hardens when heated and softens when cooled.

The new material, which hardens 1,800 times when exposed to heat, could protect motorcyclists and race car drivers during accidents.



Takayuki Nonoyama and his colleagues were inspired by how proteins remain stable within living things that survive in extreme heat environments, such as hot springs. Normally, heat "denatures" proteins, altering their structure and breaking their bonds. But proteins in thermophiles remain stable with heat, thanks to specially reinforced electrostatic interactions such as ionic bonds.

They mimicked this behavior by using a low-cost, non-toxic polyacrylic gel.


Phases of Polymers

The poly [acrylic acid] polyelectrolyte gel (PAAc) was immersed in an aqueous calcium acetate solution. PAAc itself behaves like any other polymer-based material, softening when heated. But when calcium acetate is added, the molecules of materials interact in a similar way to thermophilic proteins, causing PAAc to behave very differently.

As the temperature rises, the originally uniform gel separates into a dense polymer "phase" and a sparse polymer "phase." When it reaches a critical temperature of around 60 ° C, the dense phase undergoes severe dehydration, which strengthens ionic bonds and hydrophobic interactions between polymer molecules.

This causes the material to change rapidly from a soft, transparent hydrogel to a rigid, opaque plastic - 1,800 times stiffer, 80 times stronger and 20 times stronger than the original hydrogel.

Simply lowering the temperature causes the behavior to reverse, which opens up numerous possibilities for application.

Molecular structures and the mechanisms behind instant hydrogel thermal hardening.

Temperature sensitive intelligent materials

The team demonstrated one of the possible applications by combining the material with a fiberglass. The resulting composite fabric is soft at room temperature, but when it was rotated on an asphalt surface for five seconds at a speed of 80 km / hour, the heat generated by the friction was sufficient to harden the material with only minor abrasions. forming on the contact surface.

"Clothing made of similar fabric can be used to protect people during traffic accidents or sports, for example. Our material can also be used as a heat-absorbing window covering to keep indoors cooler," said Nonoyama.

"This polymeric gel can easily be made from versatile, inexpensive, non-toxic raw materials commonly found in everyday life. Specifically, polyacrylic acids are used in disposable diapers and calcium acetates are used in food additives. Our study contributes to basic research on new temperature sensitive polymers and applied research on intelligent temperature sensitive materials, "added Professor Jian Ping Gong.




Bibliography:

Instant Thermal Switching from Soft Hydrogel to Rigid Plastics Inspired by Thermophile Proteins

Takayuki Nonoyama  Yong Woo Lee  Kumi Ota  Keigo Fujioka  Wei Hong  Jian Ping Gong

Advanced Materials 2019

https://doi.org/10.1002/adma.201905878

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