Natural Sporopollenin Microcapsules Facilitated Encapsulation of Phase Change Material into Cellulose Composites for Smart and Biocompatible Materials.

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Sporopollenin exine capsules (SECs) are empty microcapsules that are 25 μm in diameter and have extensive networks of ∼200 nm diameter holes obtained by chemically removing all external and internal cytoplastic materials from the natural pollen grains. We have demonstrated that a phase change material (PCM) such as n-eicosane (EIS), a natural paraffin wax, can be successfully encapsulated in the SECs to produce [EIS@SEC]. The high stability and robust nature of SECs retain EIS in the microcavity even during phase transitions, enabling EIS to fully maintain its phase change property while also protecting the EIS from elevated temperatures and corrosive environments. [EIS@SEC] can, therefore, be incorporated into cellulose (CEL) composites with a synthetic process that uses the simple ionic liquid butylmethylimmidazolium chloride to produce [CEL+EIS@SEC] composites. Similar to EIS alone, EIS in the [CEL+EIS@SEC] composites melts when heated and crystallizes when cooled. The energies associated with the crystallization and melting processes enable the [CEL+EIS@SEC] composites to fully exhibit the properties expected of PCMs, i.e., heating the surroundings when they cool and absorbing energy from the surroundings when they warm. The efficiency of latent heat storage and release of [CEL+EIS@SEC] composites was estimated to be around 57% relative to pure EIS. The fact that the DSC curves of the [CEL+EIS@SEC] composites remain the same after going through the heating-melting cycle 220 times clearly indicates that SEC effectively retains EIS in its cavity and protects it from leaking and that the [CEL+EIS@SEC] composites are highly stable and reliable as a phase change material. The [CEL+EIS@SEC] composites are superior to any other available materials based on encapsulated PCM because they are not only robust, reliable, and stable and have strong mechanical properties. They are also are sustainable and biocompatible because as they are synthesized from all naturally abundant materials using a green and recyclable synthesis. These features enable the [CEL+EIS@SEC] composites to be uniquely suited as high performance materials for such uses as dressings to treat burnt wounds, smart textiles for clothing, smart building materials, and energy storage.