Solvent-non-solvent rapid-injection for preparing nanostructured materials from micelles to hydrogels

Aug 26, 2019
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An easy, robust, and quantitative method for preparing self-assembled materials using block polymers is of significant interest due to their wide-spread applications ranging from uses in everyday products such as adhesives, coatings, and packaging, to materials for highly-engineered products such as organic electronics, separation membranes, and therapeutic administration.

Traditional methods for colloidal block polymer self-assembly are often costly and time-consuming due to the lengthy preparation procedures or requirements in specialized set-ups. These complexities have been a major hurdle for large-scale applications and commercialization of block polymer self-assembly-based materials. For example, in slow water addition (a classical method for preparing aqueous block polymer colloids), multiple hours are needed to perform the self-assembly process. Others have reported that 10 days are needed for making block polymer-based hydrogels.

While we were exploring new methods for creating nanostructured colloids, we found that, rapid injection, a method widely used for preparing supramolecular colloidal structures, has shown great promise for block polymer self-assembly. The method is very easy to perform and takes very little time. All we need to do is to dissolve the block polymer in a good solvent, and then inject the polymer solution into a selective solvent like water (see Supplementary Video 1. The self-assembly begins when the polymer solution meets the water, and is completed within minutes. With just one polymer, we were able to prepare different materials including micelles, microgels, and hydrogels using rapid injection by changing the initial polymer concentration.

Rapid injection is a nonequilibrium processing method, which arguably represents the most easily adaptable continuous manufacturing scenario for block polymers. The convenience and effectiveness of the method has encouraged us to further investigate how to quantitatively control the self-assembly of the materials. We show that the final state of the nanostructured materials is related to the initial polymer solution. The method developed here will allow rational design of desired structures from a broad selection of ABA triblock copolymers for a variety of applications.

We found the method can be easily adjusted, and hydrogel printings, fibers, and coatings have been successfully demonstrated. Interestingly, the resulting materials exhibit hierarchical structures and amazing properties, such as high stretchability, toughness and resilience, and structural color. Taking advantage of the well-defined micellar structures, nanocomposite hydrogels were also prepared using gold nanoparticles. We are currently exploring different parameters to tune the hierarchical structures of the materials, and studying their relationship with mechanical and optical properties.

Robert J. Hickey

Assistant Professor, The Pennsylvania State University

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