Liquid mixing enables scalable fabrication of soft polymer structures

Researchers have developed and demonstrated an efficient and scalable technique that allows them to fabricate soft polymer materials in a dozen different structures or “morphologies,” from nanoscale ribbons and sheets to rods and branched particles. The technique allows users to precisely tune the morphology of materials at the micro- and nano-scale. The paper, “Fluid Flow Templating of Polymeric Soft Matter with Diverse Morphologies,” is published open access in the journal Advanced Materials.

“This advance is important because the technique can be used with a wide variety of polymers and biopolymers. Because the morphology of these polymer micro- and nanostructures is critical to their applications, it allows us to obtain new polymer functions by simply controlling the structure instead of polymer chemistry,” says Orlin Velev, corresponding author of the paper and the S. Frank and Doris Culberson Distinguished Professor of Chemical and Biomolecular Engineering at North Carolina State University.

“For example, nanosheets can be used to design better batteries, while dendricolloids – branching networks of polymer fibers that have an extremely high surface area – can be used in environmental remediation technologies or in the creation of new lightweight metamaterials.”

Basically, all the different morphologies are produced using a well-known process called polymer precipitation. In this process, a polymer is dissolved in a solvent, producing a polymer solution. This polymer solution is then introduced into a second liquid, which causes the polymer to come back together as a soft material.

What’s new here is that the researchers discovered how to precisely control the structure of the resulting soft polymer material by manipulating three sets of parameters during the manufacturing process.

The first set of parameters is the shear rate, which refers to how fast the fluids are stirred when the two fluids mix together. The second set of parameters is the concentration of the polymer in the polymer solution. The last set of parameters is the composition of the solvent in which the polymer was initially dissolved, as well as the composition of the liquid to which the polymer solution is added.

“We identified the critical parameters that affect the final morphology of polymer materials, which in turn gives us great control and flexibility,” says Rachel Bang, first author of the paper and a recent Ph.D. graduate of NC State. “Because we now understand the role of each of these factors and how they all affect each other, we can reproducibly optimize the morphology of polymeric particles.”

“Although we have demonstrated how to produce a dozen different morphologies, we are still in the early stages of exploring all possible outcomes and applications,” says Velev.

Researchers have already demonstrated that dendricolloids can be used to make membranes for the growth of living cells or to create hydrophobic or hydrophilic coatings. The researchers also worked with collaborators to demonstrate that nanosheets have the potential to be used as more effective separators in lithium-ion batteries.

“The technique can also be used with a variety of natural biopolymers, such as plant proteins, and could be used to support a variety of applications, such as the development of plant-based meat analogs, which require precise control of protein particle morphologies in multiple length scales,” adds co-author Professor Simeon Stoyanov of the Singapore Institute of Technology and Wageningen University in the Netherlands. “Furthermore, because our technique is based on mixing liquids using conventional mixers, it can easily be scaled up for practical manufacturing.”

“We are currently working with food science researchers to determine how protein microribbons could be used to control the texture of certain food products,” says Velev. “And we are also working with collaborators to explore how our technique can be used to produce biopolymer-based materials for use in biodegradable soft electronics.

“We are open to working with additional partners to explore potential applications for polymers and biopolymers in all of these morphologies.”

NC State has issued or pending patents on the fabrication of microrods, nanofibers, dendricolloids and their application in electrochemical energy sources.