Polymer Capsules Exhibiting Emergent Properties Due to Internal Chemical Reactions
Raghavan, Srinivasa R.
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Life is an “emergent” property – i.e., one exhibited by living systems due to interactions between their parts, but not shown by any of the parts on their own. Such emergent behavior is seen at various length scales, down to that of a single cell. In a single eukaryotic cell, there exist internal compartments called organelles, such as the nucleus and mitochondria. While the organelles each have distinct functions, it is the overall cell that has the property of life – i.e., the whole is greater than the sum of the parts. Inspired by these attributes of living cells, this dissertation attempts to create millimeter-scale polymer capsules that exhibit emergent properties. To achieve these properties, I use the lumen of the capsule as the site for a chemical reaction, which in turn bestows a specific behavior to the overall capsule. Three studies are described in this vein in this dissertation. First, in chapter 3, I design capsules that rapidly inflate and then burst, ejecting their core contents. These capsules have a core of catalytic silver (Ag) particles and a crosslinked polymer shell. When a fuel (hydrogen peroxide, H2O2) is added to water, a catalytic reaction generates gas (O2) in the capsule. The gas causes the capsule to inflate over time until the shell finally ruptures. The inflation extent and duration, as well as the rupture intensity can be tuned by altering the core and shell composition. Also, instead of a catalytic reaction, capsule inflation can be achieved by combining reactants, one in the capsule and the other in the solution, that together generate a different gas (e.g., CO2). In chapter 4, I describe a second emergent property of capsules having a design similar to those in chapter 3. The capsules here contain both the catalytic Ag particles as well as a solute (dye). I monitor the release of dye in the presence of the O2-generating reaction (“active release”) or by diffusion alone (“passive release”). Active dye release is shown to be much faster than passive release, i.e., the dye is “pumped” out due to the reaction. More interestingly, when the dye is Rhodamine 6G (R6G), it gets released in a series of pulses, i.e., dye releases for a few seconds, then it stops, and then resumes. The number and duration of the pulses can be tuned. The reason for pulsed release is attributed to the binding interaction of R6G to Ag particles. Finally, in chapter 5, I show the use of an external trigger (temperature) to induce emergent behavior in capsules. For this, I first coat individual capsules with an impermeable shell made of wax. This is done by briefly immersing a cold capsule in molten wax so that a wax layer solidifies around the capsule. This layer is impermeable when the capsule is placed in water, but when heated, the wax melts and the contents can diffuse out. I construct multicompartment capsules (MCCs) that have one or two wax-coated inner compartments. The MCCs are used to conduct cascade reactions such as the iodine clock reaction. Also, an MCC with H2O2 in a compartment and Ag particles in the lumen is used to demonstrate thermally triggered capsule inflation.