MULTIFUNCTIONAL POLYMERIC MICROCAPSULES ENCLOSING PHASE CHANGE MATERIALS: SYNTHESIS, CHARACTERIZATION, AND APPLICATIONS

Abstract

Storage of solar energy in the form of Latent heat is the most elegant and realistic route to store thermal energy to surmount environmental/energy crisis. Phase change materials (PCMs) with high-energy storage ability, close melting/crystallization temperatures, reproducible performance and being renewable in nature play crucial role in this application. PCMs undergo phase transformation while absorption/release of large amount of latent heat at constant temperature, enabling them to be useful in various thermal management applications of batteries, buildings, food/medicine transportation, packaging, textiles, electronic devices, and many more. Based on the nature of source PCMs are classified into organic PCMs (paraffin, fatty acids/alcohol, etc.), inorganic PCMs (salt hydrates, metal alloys, etc.), and eutectics (combination of any two different PCMs). As phase transformation is common with all PCMs, enclosing the PCM once melted becomes the common challenge with all PCM. A molten PCM may contaminate the surrounding, some being corrosive may corrode the surrounding/container, may result in irreversible phase separation, often associated with poor thermal conductivity and supercooling behaviour, hence direct application of PCMs get limited. These challenges can be addressed by enclosing PCMs inside suitable confinement depending on the application being targeted. This technique of enclosing PCMs is known as encapsulation. In case the support material is porous in nature, PCMs are loaded inside pores to obtain a composite material. Based on the size of container used, encapsulation is further categorized as: macro-encapsulation (size range), microencapsulation (size range) and nano-encapsulation (size range). Out of these, microencapsulation is the most prominent one due to its synthetic simplicity, and practical feasibility, where capsules of core-shell nature are formed containing the PCM in core and shell wall remains impermeable to PCM. Besides overcoming leakage/contamination/volume expansion issues, it increases heat-transfer area, which facilitates rapid heat transfer. By tuning the shell wall microencapsulated PCM can be imparted with dual/multiple functionalities. This feature will increase their application scope. This thesis reports synthesis, characterization, and selected applications of various multifunctional PCM microcapsules, where the mechanism of microcapsule formation was separately taken up in each case and explained based on fundamental principles of colloid and interface science. To begin with, functionalized copper nanoparticles (CuNPs) interlocked polydivinylbenzene (PDVB) microcapsules containing hexadecane (HD) as PCM were synthesized targeting thermal buffering application in food packaging. Fourier transform infrared spectroscopy (FT-IR) and transmission electron microscopy (TEM) characterizations confirmed successful functionalization of CuNPs and their presence in the PDVB shell. Scanning electron microscope (SEM) images confirmed formation of well-defined core-shell microstructure of spherical shape, while composition and chemical structures were confirmed through series of other spectroscopic methods. Differential scanning calorimeter (DSC) confirmed PCM entrapment and microcapsules with 1.0% CuNPs showed highest melting enthalpy of 132 J/g with encapsulation ratio of 60.5%, and the PCM content was retained even after 100 melting-freezing cycles. A significant increase in the thermal conductivity by 319.5% was observed compared to bare microcapsules. Most importantly, optimized microcapsules provided an excellent thermal buffering effect of more than 6.5 h for 240 g of chocolate to raise its temperature from 5oC to 35oC, confirming its potential in food packaging application. Secondly, titanium dioxide (TiO2) nanoparticle decorated [poly(4-methylstyrene-co-divinylbenzene)] microcapsules containing PCM were synthesized adopting one-pot non-Pickering emulsion templated suspension polymerization technique. TiO2 nanoparticles were hydrophobized using trace quantity of tertradecyltrimethylammonium bromide (TTAB, cationic stabilizer) due to electrostatic interaction between them and used as a particle stabilizer. SEM images confirmed formation of core-shell microstructure of spherical geometry. Presence of TiO2 over the shell surface was confirmed through energy-dispersive X-ray (EDX), and X-ray diffraction (XRD) analysis. DSC analysis demonstrated that PCM microcapsules with 2.6 wt.% of TiO2 achieved highest phase change enthalpy of 174 J/g with encapsulation efficiency of 76.6%, and enthalpy content could be maintained beyond 100 thermal cycles (melting-freezing cycles). TGA analysis showed addition of TiO2 improved thermal stability of PCM microcapsules. Presence of TiO2 imparted photocatalytic activity with PCM microcapsules through synergistic photothermal effect. Thus, bifunctional microcapsules reported in this work is expected to stimulate applications in the biomedical field, residential buildings in polluted urban sites, at industrial establishments as thermal energy storage, and pollution abatement. Thirdly, a Lego-inspired glass capillary microfluidic device capable of encapsulating both organic and aqueous PCMs with high reproducibility and 100% yield was designed. Oil-in-oil-in-water (O/O/W) and water-in-oil-in-water (W/O/W) core-shell double emulsion droplets were formed to encapsulate hexadecane (an organic PCM) and calcium chloride hexahydrate-based SP21EK (an aqueous PCM) in a UV-curable polymeric shell, Norland Optical Adhesive (NOA). The double emulsions were consolidated through on-the-fly polymerization which followed thiol-ene click chemistry for photoinitiation. The microcapsules were monodispersed and exhibited the highest encapsulation efficiency of 65.4% and 44.3% for HD and SP21EK-based materials, respectively, determined using DSC. TGA analysis confirmed much higher thermal stability of both encapsulated PCMs compared to pure (bulk) PCMs. Polarization microscopy revealed that microcapsules could sustain over 100 melting-crystallization cycles. Bifunctional microcapsules with remarkable photocatalytic activity along with thermal energy storage performance were obtained with 1 % (by weight) of TiO2 NPs into the polymeric shell. The presence of TiO2 NPs in the shell was confirmed by higher opacity and whiteness of these microcapsules and was quantified by EDX spectroscopy. Young’s modulus of HD-based microcapsules estimated using micromanipulation analysis increased from 58.5 MPa to 224 MPa after TiO2 incorporation in the shell. Lastly, a novel approach of Pickering emulsion templated in situ polymerization technique was used to tailor complex polymeric microstructures comprising TiO2 NPs embedded poly(melamine-urea-formaldehyde) (polyMUF) composite as shell and hexadecane (HD, soft template) as core PCM. Initially, TiO2 NPs were hydrophobized by chemisorbing long-chain bio-based myristic acid via bidentate chelating complex and wettability of NPs were precisely tuned by varying grafting density of myristic acid to achieve highly stable oil-in-water (O/W) Pickering emulsion. Subsequently, optimized TiO2 NPs were applied for targeted encapsulation strategy to achieve various microstructures and morphologies ranging in the particle diameter from 5 to 20 μm. Careful manipulation of reaction parameters and copolymer component resulted complex and novel microstructures: smooth, raspberry-like, partially budded, hollow, filled, single-holed, and closed-cell-like. The particle properties such as morphology, size, shell thickness, and core content were governed by the TiO2 NPs content, core: shell ratio, copolymer component, conversion, and pH value. Based on the results of a series of control experiments, novel mechanisms behind the formation of various novel microstructures were proposed.