Composite nanofibrous sheets of fatty acids and polymers as thermo-regulating enclosures

Abstract

In this work we report fabrication and study of Polyvinyl Alcohol (PVA)-Fatty acids composite nanofibrous mats with an aim to develop thermo-regulating enclosures. Fatty acids, an important class of phase change materials (PCM), can serve as energy storage/recovery materials due to their high latent heats. PCM-PVA composite nanomats are fabricated by incorporating the PCMs into electrospun polymeric mats by two methods: (i) in ‘drop-cast’ composite mats, the molten liquid binary mixtures of the PCMs are gently dropped on to the electrospun PVA nanomats and fill the voids by capillary-suction, and (ii) in ‘mixed-and-spun’ composite mats, the fatty acid mixture solution is mixed with the PVA spinning solution and electrospun. The nanofibrous-mats of PVA and a series of mixtures of lauric acid (LA) and stearic acid (SA) prepared using the above two methods are characterized using differential scanning calorimetry (DSC), X-Ray diffractometry, and scanning electron microscopy. Phase diagrams constructed using the transition temperatures from DSC are interpreted as phase diagrams of the LASA binary mixture inside a nanofibrous polymeric medium, as opposed to its bulk phase diagram. The LASA mixture in PVA nanomats exhibits a eutectic phase diagram similar to their phase behavior in bulk. However, the eutectic temperature (Te) is significantly altered in composite nanomats as compared to its bulk value of 39.5°C: Te=34°C in “drop cast” composite mats, and further drops to 30°C in the “mixed-and-spun” composite mats. Tensile tests show considerable softening in the composite mats. Application of regular solution model suggests that an enhanced attraction between LA and SA components, attributed here to nanoconfinement, may be responsible for this change. SEM images show partial wetting of the PVA nanofibers by the fatty acids in “drop cast” composite mats and uniform fibers in “mixed-and-spun” composite mats samples. Thermoregulation ability of these mats is confirmed by monitoring the temperature vs. time profile inside a vial filled with hot water and wrapped around by the composite nanomats.