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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Surprise! Weaker bonds can make polymers stronger    
    By adding weak linkers to a polymer network, chemists dramatically   
   enhanced the material's resistance to tearing    
      
     Date:   
         June 22, 2023   
     Source:   
         Massachusetts Institute of Technology   
     Summary:   
         Chemists discovered a new way to make polymers stronger: introduce   
         a few weaker bonds into the material. Working with polyacrylate   
         elastomers, they could increase the materials' resistance to   
         tearing up to tenfold by using a weaker type of crosslinker to   
         join some of the polymer building blocks.   
      
      
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   FULL STORY   
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   A team of chemists from MIT and Duke University has discovered a   
   counterintuitive way to make polymers stronger: introduce a few weaker   
   bonds into the material.   
      
   Working with a type of polymer known as polyacrylate elastomers, the   
   researchers found that they could increase the materials' resistance to   
   tearing up to tenfold, simply by using a weaker type of crosslinker to   
   join some of the polymer building blocks.   
      
   These rubber-like polymers are commonly used in car parts, and they are   
   also often used as the "ink" for 3D-printed objects. The researchers are   
   now exploring the possible expansion of this approach to other types of   
   materials, such as rubber tires.   
      
   "If you could make a rubber tire 10 times more resistant to tearing,   
   that could have a dramatic impact on the lifetime of the tire and on the   
   amount of microplastic waste that breaks off," says Jeremiah Johnson,   
   a professor of chemistry at MIT and one of the senior authors of the   
   study, which appears today in Science.   
      
   A significant advantage of this approach is that it doesn't appear to   
   alter any of the other physical properties of the polymers.   
      
   "Polymer engineers know how to make materials tougher, but it invariably   
   involves changing some other property of the material that you don't   
   want to change. Here, the toughness enhancement comes without any   
   other significant change in physical properties -- at least that we can   
   measure -- and it is brought about through the replacement of only a   
   small fraction of the overall material," says Stephen Craig, a professor   
   of chemistry at Duke University who is also a senior author of the paper.   
      
   This project grew out of a longstanding collaboration between Johnson,   
   Craig, and Duke University Professor Michael Rubinstein, who is also a   
   senior author of the paper. The paper's lead author is Shu Wang, an MIT   
   postdoc who earned his PhD at Duke.   
      
   The weakest link Polyacrylate elastomers are polymer networks made from   
   strands of acrylate held together by linking molecules. These building   
   blocks can be joined together in different ways to create materials with   
   different properties.   
      
   One architecture often used for these polymers is a star polymer   
   network. These polymers are made from two types of building blocks:   
   one, a star with four identical arms, and the other a chain that acts   
   as a linker. These linkers bind to the end of each arm of the stars,   
   creating a network that resembles a volleyball net.   
      
   In a 2021 study, Craig, Rubinstein, and MIT Professor Bradley Olsen   
   teamed up to measure the strength of these polymers. As they expected,   
   they found that when weaker end-linkers were used to hold the polymer   
   strands together, the material became weaker. Those weaker linkers,   
   which contain cyclic molecules known as cyclobutane, can be broken with   
   much less force than the linkers that are usually used to join these   
   building blocks.   
      
   As a follow-up to that study, the researchers decided to investigate   
   a different type of polymer network in which polymer strands are   
   cross-linked to other strands in random locations, instead of being   
   joined at the ends.   
      
   This time, when the researchers used weaker linkers to join the acrylate   
   building blocks together, they found that the material became much more   
   resistant to tearing.   
      
   This occurs, the researchers believe, because the weaker bonds are   
   randomly distributed as junctions between otherwise strong strands   
   throughout the material, instead of being part of the ultimate strands   
   themselves. When this material is stretched to the breaking point, any   
   cracks propagating through the material try to avoid the stronger bonds   
   and go through the weaker bonds instead. This means the crack has to   
   break more bonds than it would if all of the bonds were the same strength.   
      
   "Even though those bonds are weaker, more of them end up needing to   
   be broken, because the crack takes a path through the weakest bonds,   
   which ends up being a longer path," Johnson says.   
      
   Tough materials Using this approach, the researchers showed that   
   polyacrylates that incorporated some weaker linkers were nine to 10   
   times harder to tear than polyacrylates made with stronger crosslinking   
   molecules. This effect was achieved even when the weak crosslinkers made   
   up only about 2 percent of the overall composition of the material.   
      
   The researchers also showed that this altered composition did not alter   
   any of the other properties of the material, such as resistance to   
   breaking down when heated.   
      
   "For two materials to have the same structure and same properties at   
   the network level, but have an almost order of magnitude difference in   
   tearing, is quite rare," Johnson says.   
      
   The researchers are now investigating whether this approach could be   
   used to improve the toughness of other materials, including rubber.   
      
   "There's a lot to explore here about what level of enhancement can be   
   gained in other types of materials and how best to take advantage of it,"   
   Craig says.   
      
       * RELATED_TOPICS   
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                   # Materials_Science # Chemistry #   
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             o Polymer o Polyethylene o Triboelectric_effect o   
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             Chemical_bond   
      
   ==========================================================================   
   Story Source: Materials provided by   
   Massachusetts_Institute_of_Technology. Original written by Anne   
   Trafton. Note: Content may be edited for style and length.   
      
      
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   Journal Reference:   
      1. Shu Wang, Yixin Hu, Tatiana B. Kouznetsova, Liel Sapir, Danyang   
      Chen,   
         Abraham Herzog-Arbeitman, Jeremiah A. Johnson, Michael Rubinstein,   
         Stephen L. Craig. Facile mechanochemical cycloreversion of polymer   
         cross- linkers enhances tear resistance. Science, 2023; 380 (6651):   
         1248 DOI: 10.1126/science.adg3229   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/06/230622142355.htm   
      
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