Shape Shifters: Demonstrating Tunable Phase Shifting

Reversible self-assembled structures balance two competing attractions to enable stimuli-responsive materials.

The Science

Scientists devised a new approach that balances attractions between particles and promises to become a useful tool to create designer materials that can repair damage. The discovery of a cross point between temperature-dependent interactions between particles – not previously thought possible – opens the doors for more nimble synthesis. The crosspoint exploits the temperature-dependent behavior (solubility and adsorption) of a polymer. Researchers discovered that a liquid polymer-colloid mixture on cooling and heating forms different solid phases reversibly. These solids are formed by two distinct pathways: (1) at low temperature, pressure from collisions with the surrounding non-adsorbing polymer forms a colloidal crystal and (2) at high temperature, the polymer sticks (adsorbs) to particles, forming a random aggregate.

The Impact

This research opens a new pathway to stimuli-responsive self-assembled structures. Using the crosspoint pathway, it may now be possible to (1) thermally control viscoelastic properties, (2) heal defects that occur during assembly, (3) more controllably sequester and release objects, and (4) exert fine control over inter-particle interactions for sequential assembly of two- and three-dimensional materials with precisely organized optical and mechanical functions.

Summary

A new approach that balances attractions between particles promises to become a useful tool to fine-tune self-assembly and add functionality, such as error correction during assembly and damage repair. Previous polymer-directed routes for particle self-assembly were in stark contrast to biological systems that can form, reconfigure, and repair complex assemblies within cells by balancing assembly and disassembly processes.

In this research, understanding of the pathways for both assembly and disassembly was developed. The transformative aspect is the identification by researchers at New York University of a crossing point between the two pathways for a polymer-colloid mixture previously thought impossible; the crossing point mimics the biological assembly-disassembly capability. Adsorption properties of polymers change with temperature. At low temperature, colloidal crystals are formed due to pressure from collisions with surrounding non-adsorbing polymer. Actually, the colloids are squeezed together to increase the volume available to the non-adsorbing polymer. This mechanism for forming colloidal crystals was well known, but what was observed next was quite surprising. On heating, the colloidal crystal melted to form a liquid polymer-colloid mixture. Beyond this point, the solubility of the polymer decreased as the temperature increased; eventually, the polymer was able to weakly stick (adsorb) to the particles, creating bridges that solidify the liquid to a random aggregate gel.

At the crossover point between colloidal crystal deformation and gel formation, these new attractions (so-called enthalpic attraction, in thermodynamic terminology) completely balance the forces exerted by the volume available to the polymer from the particles being squeezed together (so-called entropic attraction). The crossing point depends on the change in solubility of the non-adsorbing polymer, resulting in a liquid-to-solid transition on cooling and heating. Most importantly, this process is thermally reversible at each stage of assembly and disassembly, which could allow entry into and out of the particles. As a result, it may be possible to heal defects in assembled structures, and to fabricate two- and three-dimensional materials with desired optical and mechanical properties. The general nature of these interactions suggests that they can be applied over a broad range of self-assembly approaches, such as the DNA-directed assembly of particle networks, to stimuli-responsive functional materials.

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