New development of 'impossible' polymer capable of increasing both stiffness and flexibility

Scientists have developed something long thought impossible: a polymer that's both stiff and stretchable, without one quality compromising the other. Materials research has shown that the stiffer a polymer, the less flexible it will be — but no longer. 

The findings are published in the journal Science Advances and supported by the U.S. National Science Foundation. The University of Virginia research team behind these findings shows that strength and "stretchability" don't necessarily have to be opposites when it comes to polymers.  

Liheng Cai, assistant professor of materials research and principal investigator of an NSF Faculty Early Career Development award, says, "We are addressing a fundamental challenge that has been thought to be impossible to solve since the invention of vulcanized rubber in 1839." The research was led by doctoral student Baiqiang Huang.  

Credit: Matt Cosner, University of Virginia School of Engineering and Applied Science

Says Huang, "This limitation has held back the development of materials that need to be both stretchable and stiff, forcing engineers to choose one property at the expense of the other."   

He adds, "Imagine, for example, a heart implant that bends and flexes with each heartbeat but still lasts for years." 

Cai and team discovered that polymer qualities of stiffness and flexibility have the same molecular basis. What they have in common: They're both based on cross-linked polymer strands.  

The more cross-links are added the stiffer the material becomes. But it also grows more fragile, and less supple or "stretchable," the more cross-links are added in the process. That is only when they are added in a linear fashion.  

Instead of linear cross-links, Cai and colleagues tried for a "bottlebrush" structure. This nonlinear network of material is made up of "core" cross-link strands branching out into polymer "side chains."  

On a molecular level, these allow the material to have a foldable or accordion-like structure when it moves — imbuing the polymer with qualities of firmness and flexibility simultaneously and independently of each other.  

Andrew Lovinger, program director in the NSF Division of Materials Research, says, "This very clever polymer architecture by Professor Cai opens the door to materials with unprecedented combinations of properties and exemplifies the very rich design potential offered by polymers."  

Materials with this capability show promise for medical, health care and robotics innovations, such as implants, prosthetics, electromechanical devices and more.