A totally new type of polymer is designed to lift weights, contracting and expanding the way muscles do. It has the potential to do lots of other stuff, too— like deliver drugs or even repair itself. The new capabilities are the result of both rigid and soft compartments with extremely different properties that are organized in specific ways.
The hybrid polymer cleverly combines the two types of known polymers: those formed with strong covalent bonds and those formed with weak non-covalent bonds, well known as supramolecular polymers. The integrated polymer offers two distinct “compartments” with which chemists and materials scientists can work to provide useful features.
“This would follow the first chapter of broadly useful covalent polymers, then the more recent emerging class of supramolecular polymers. Our discovery could transform the world of polymers and start a third chapter in their history: that of the ‘hybrid polymer.’
We can create active or responsive materials not known previously by taking advantage of the compartments with weak non-covalent bonds, which should be highly dynamic like living things. Some forms of these polymers now under development in my laboratory behave like artificial muscles.”
Softer Life Force
Polymers get their power and features from their structure at the nanoscale. The covalent rigid skeleton of Stupp’s first hybrid polymer has a cross-section shaped like a ninja star—a hard core with arms spiraling out. In between the arms is the softer “life force” material.
This is the area that can be animated, refreshed and recharged, features that could be useful in a range of valuable applications.
“The fascinating chemistry of the hybrid polymers is that growing the two types of polymers simultaneously generates a structure that is completely different from the two grown alone,” Stupp says. “I can envision this new material being a super-smart patch for drug delivery, where you load the patch with different medications, and then reload it in the exact same compartments when the medicine is gone.”
Stupp and his research team also discovered that the covalent polymerization that forms the rigid compartment is “catalyzed” by the supramolecular polymerization, thus yielding much higher molecular weight polymers.
Perfectly Cylindrical Filaments
The strongly bonded covalent compartment provides the skeleton, and the weakly bonded supramolecular compartment can wear away or be used up, depending on its function, and then be regenerated by adding small molecules. After the simultaneous polymerizations of covalent and non-covalent bonds, the two compartments end up bonded to each other, yielding a very long, perfectly shaped cylindrical filament.
To better understand the hybrid’s underlying chemistry, Stupp and his team worked with George C. Schatz, a chemistry professor at Northwestern. Schatz’s computer simulations showed the two types of compartments are nicely integrated with hydrogen bonds, which are bonds that can be broken. Schatz is a coauthor of the study.
“This is a remarkable achievement in making polymers in a totally new way, simultaneously controlling both their chemistry and how their molecules come together,”
says Andy Lovinger, a materials science program director at the National Science Foundation, which funded this research.
“We’re just at the very start of this process, but further down the road it could potentially lead to materials with unique properties— such as disassembling and reassembling themselves— which could have a broad range of applications,” Lovinger says.
Zhilin Yu, Faifan Tantakitti, Tao Yu, Liam C. Palmer, George C. Schatz, Samuel I. Stupp
Simultaneous covalent and noncovalent hybrid polymerizations
Science 29 Jan 2016 : 497-502
Illustration: Mark E. Seniw/Northwestern University