US re­searchers de­velop self heal­ing hy­brid rub­ber

Chemical Industry Digest - - News & Views -

Re­searchers

from Har­vard John A Paul­son School of En­gi­neer­ing and Ap­plied Sciences (SEAS) in the US have de­vel­oped tough, hy­brid rub­ber with­out stress points so it doesn’t crack. Punc­tured car tyres that au­to­mat­i­cally re­pair them­selves may soon be­come a re­al­ity, due to this de­vel­op­ment.

The re­searchers cre­ated self-heal­ing hy­dro­gels, which rely on water to in­cor­po­rate re­versible bonds that can pro­mote heal­ing.

En­gi­neer­ing self-heal­ing prop­er­ties in dry ma­te­ri­als -such as rub­ber -has proved chal­leng­ing as rub­ber is made of poly­mers of­ten con­nected by per­ma­nent, co­va­lent bonds. While these bonds are in­cred­i­bly strong, they will never re­con­nect once bro­ken. In order to make rub­ber self-heal­able, the team needed to make the bonds con­nect­ing the poly­mers re­versible, so that the bonds could break and re­form. “Pre­vi­ous re­search used re­versible hy­dro­gen bonds to con­nect poly­mers to form rub­ber but re­versible bonds are in­trin­si­cally weaker than co­va­lent bonds,” said Li-Heng Cai, a post­doc­toral fel­low at SEAS.

“This raised the ques­tion, can we make some­thing tough which can still self-heal?” added Cai who along with Jin­rong Wu, a pro­fes­sor from Sichuan Uni­ver­sity, China, and col­leagues de­vel­oped a hy­brid rub­ber with both co­va­lent and re­versible bonds. “These two types of bonds are in­trin­si­cally im­mis­ci­ble, like oil and water,” said Cai.

The re­searchers de­vel­oped a molec­u­lar rope, called ran­domly branched poly­mers to tie these two types of bonds to­gether. This rope al­lows two pre­vi­ously un­mix­able bonds to be mixed ho­mo­ge­neously on a molec­u­lar scale. In do­ing so, they were able to cre­ate a trans­par­ent, tough, self heal­ing rub­ber.

Typ­i­cal rub­ber tends to crack at cer­tain stress point when force is ap­plied. When stretched, hy­brid rub­ber de­vel­ops so-called crazes through­out the ma­te­rial, a fea­ture sim­i­lar to cracks but con­nected by fi­brous strands. These crazes re­dis­tribute the stress, so there is no lo­calised point of stress that can cause cat­a­strophic fail­ure. When the stress is re­leased, the ma­te­rial snaps back to its orig­i­nal form and the crazes heal. The self­heal­ing abil­ity is ap­peal­ing for a wide va­ri­ety of rub­ber prod­ucts, ex­perts said.

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