Shape Shifters: Demonstrating Tunable Phase Shifting
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 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 2 distinct pathways: (1) at low temp, 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.
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.
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. 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 2D and 3D 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. http://science.energy.gov/bes/highlights/2015/bes-2015-10-c/
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