This molecular model shows the structure of the new hybrid polymer with removable supramolecular compartments. Image: Mark E. Seniw, Northwestern University.
This molecular model shows the structure of the new hybrid polymer with removable supramolecular compartments. Image: Mark E. Seniw, Northwestern University.

Imagine a polymer that can deliver a substance to the environment and then be chemically regenerated to do so again. Or a polymer that can lift weights, contracting and expanding the way muscles do.

These abilities require polymers possessing both rigid and soft nano-sized compartments with extremely different properties that are organized in specific ways, and a completely new hybrid polymer of this type has now been developed by Northwestern University researchers. This hybrid polymer might one day be used in artificial muscles or other life-like materials, for delivering drugs, biomolecules or other chemicals, in materials with self-repair capability, and in novel energy sources.

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, known as ‘supramolecular polymers’. The strongly bonded covalent compartment provides the skeleton, while 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. This work is detailed in a paper in Science.

"We have created a surprising new polymer with nano-sized compartments that can be removed and chemically regenerated multiple times," said materials scientist Samuel Stupp, director of Northwestern's Simpson Querrey Institute for BioNanotechnology and senior author of the paper.

"Some of the nanoscale compartments contain rigid conventional polymers, but others contain the so-called supramolecular polymers, which can respond rapidly to stimuli, be delivered to the environment and then be easily regenerated again in the same locations," he explained. "The supramolecular soft compartments could be animated to generate polymers with the functions we see in living things.

"Our discovery could transform the world of polymers and start a third chapter in their history: that of the hybrid polymer. This would follow the first chapter of broadly useful covalent polymers, then the more recent emerging class of supramolecular polymers. 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.".

Polymers get their features and abilities from their structure at the nanoscale. The covalent rigid skeleton of Stupp's 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 supramolecular polymer: this is the area that can be animated, refreshed and recharged, features that could be useful in a range of valuable applications.

To better understand the hybrid's underlying chemistry, Stupp and his team worked with George Schatz, a world-renowned theoretician and professor of chemistry at Northwestern and co-author of the paper. Schatz's computer simulations showed that the two types of compartment are nicely integrated with hydrogen bonds.

"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 said. "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.

"This is a remarkable achievement in making polymers in a totally new way – simultaneously controlling both their chemistry and how their molecules come together," said 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. "

This story is adapted from material from Northwestern University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.