Microfluidic Production of Monodisperse Biopolymer Microcapsules for Latent Heat Storage

Microencapsulation of phase change materials in a polymer shell is a promising technology to prevent them from leakage and to use them as a handleable powder state. However, the microencapsulation process is a time-consuming process because the typical shell-forming step requires polymerization or evaporation of the solvent. In this study, we report a simple and rapid flow process to prepare monodisperse biocompatible cellulose acetate (CA) microcapsules encapsulating n-hexadecane (HD) for latent heat storage applications. The microcapsules were prepared by combining microfluidic droplet formation and subsequent rapid solvent removal from the droplets by solvent diffusion. The diameter and shell thickness of the microcapsules could be controlled by adjusting the flow rate and the HD-to-CA weight ratio in the dispersed phase. We found that 1-hexadecanol added to the microcapsules played the role of a nucleation agent and mitigated the supercooling phenomenon during crystallization. Furthermore, cross-linking of the CA shell with poly(propylene glycol), tolylene 2,4-diisocyanate terminated, resulted in the formation of a thin and dense shell. The microcapsules exhibited a 66 wt % encapsulation efficiency and a 176 J g^-1 latent heat storage capacity, with negligible supercooling. We believe that this microflow process can contribute to the preparation of environmentally friendly microcapsules for heat storage applications.

Interfacial photocrosslinking of polymer particles possessing nucleobase photoreactive groups for hollow/capsule polymer fabrication

Herein, polystyrene-based particles possessing nucleobases in polymer side chains were prepared and nucleobase groups were applied to the interfacial photocrosslinking as photoreactive groups for the first time for fabricating hollow/capsule particles.

Carboxy-Functionalized pH Responsive Capsule Polymer Particles Fabricated by Particulate Interfacial Photocrosslinking

pH-responsive capsule particles show promise for various applications, such as self-healing materials, micro/nanoreactors, and drug delivery systems. Herein, carboxy-functionalized capsule polymer particles possessing neutral-alkali pH responsive controlled release capability were...

Synthesis of uniform submicron poly(lactic acid)-based particles/capsules by radical precipitation polymerization

Poly(l-lactic acid) (PLLA) is a well-known biopolymer, usually synthesized via step-growth or ring-opening polymerization from lactic acid or a lactide monomer, respectively. PLLA microspherical particles are produced by dispersion polymerization with a ring-opening lactide monomer using a particular copolymer chain as a stabilizer. This is not easy to achieve when dehydration is needed. Here, a robust and simple synthesis of a nearly monodisperse, submicron PLLA-based particle/capsule was proposed via radical precipitation polymerization without the use of surfactant. A commercial PLLA was first glycolyzed with ethylene glycol to obtain a low molecular weight glycolyzed PLLA (GPLLA). Then, the GPLLA was copolymerized with methacrylic acid and ethylene glycol dimethacrylate monomers using a benzoyl peroxide initiator. Active sites on the GPLLA backbone were generated by hydrogen abstraction of benzoyloxy radicals that further copolymerized before self-assembly to form the polymer particles. Uniform particle size of about 580 nm with a low polydispersity index (PDI) of 0.012 was obtained. This method was also implemented to produce nearly monodisperse capsules containing linalool. The particle size of PLLA-based capsules was about 280 nm with narrow particle size distribution (PDI of 0.120). The PLLA-based capsules effectively inhibited microbial growth of Staphylococcus aureus, Escherichia coli and Candida albicans and were not toxic to human cells.

pH-Responsive Capsule Polymer Particles Prepared by Interfacial Photo-Cross-Linking: Effect of the Alkyl Chain Length of the pH-Responsive Monomer

pH-responsive capsule particles have immense potential for use in various advanced fields, such as microreactors and drug delivery. Moreover, the interfacial photo-cross-linking of spherical polymer particles is a promising strategy to create various functional capsule particles. In this study, pH-responsive capsule polymer particles were prepared by interfacial photo-cross-linking with photo-reactive polymers possessing different pH-responsive monomer units of different alkyl chain lengths, namely, 2-dimethylaminoethyl methacrylate, 2-diethylaminoethyl methacrylate, and 2-diisopropylaminoethyl methacrylate. Using these different pH-responsive monomers, regulation of the controlled release properties of pH-responsive capsule particles was achieved. All capsule particles prepared from these three different polymers released encapsulated molecules under acidic conditions; however, more acidic conditions were necessary for releasing encapsulated molecules with the increasing alkyl chain length. The afforded results indicated that pH-responsive monomers of different alkyl chain lengths could be successfully employed to regulate the pH-responsive controlled release property of the capsule particles prepared by interfacial photo-cross-linking.

Synthesis of Micrometer-Sized Poly(methyl acrylate) by Temperature-Step Microsuspension Polymerization with Iodoform Based on the "Radical Exit Depression" Effect§

Previously, we have reported the successful preparation of micrometer-sized poly(methyl methacrylate) particles without submicrometer-sized byproduct particles by microsuspension iodine-transfer polymerization (ms ITP), in which the radical exit depression (RED) effect was expected, with the benzoyl peroxide initiator at 8 wt % relative to the monomer. However, it was difficult to apply it simply under a similar condition for methyl acrylate (MA), which is more hydrophilic than methyl methacrylate (MMA), because the polymerization rate in the water phase (R_p^w) arising from the oligomer radicals exiting from the monomer droplets is high, resulting in a lot of submicrometer-sized byproduct particles. In this study, the problem was overcome by utilizing a two-step temperature process in the microsuspension polymerization with iodoform (ms I) of MA, which supports the proposed mechanism in the ms ITP of MMA in the previous paper. Although the control of the molecular weight (M_n) and the molecular weight distribution (M_n/M_w) was restricted, the preparation of micrometer-sized particles without byproduct particles was realized and a high conversion was reached within a practical time that meets the demands of the industry by utilizing the ms I. The optimal conditions for MA were 70 °C for 2 h, followed by 80 °C for 4 h with a high content of initiator (8 wt % relative to a monomer).

Paraffin wax microsphere embedded epoxy composites for potential thermal management in electronic devices

Miniaturization of electronic devices with more computing power has created a challenging set of aspects in thermal management. Present work is based on phase change materials microsphere and its incorporation in the epoxy network to develop a new class of potting material facilitating thermal management for miniaturized electronic devices. A facile and scalable method was implemented to synthesize paraffin wax microspheres (PMPs). It was dispersed into a room temperature curing epoxy network to fabricate the epoxy composite with high latent heat of fusion and high thermal stability. PMPs obtained have spherical morphology with an average diameter of approximately 5 µm. The PMP/epoxy composite can store 34.34 and 49.3 J g−1 of latent heat energy at 30 and 40 wt% PMP loading, respectively. Leakage test reveals that leaching declined as the size of PMP is reduced. Incorporation of PMP into the epoxy network reduces the compressive strength, but still resilient enough to protect electronic devices. This is an added advantage over the potential to mitigate the issue of hot spot in electronic devices as demonstrated by infrared thermography. The application of such composite is not limited only as electronic potting materials but also has the potential for other thermal energy storage applications.

Relation between colour- and phase changes of a leuco dye-based thermochromic composite

Reversible colour change of leuco dye-based composites is in general closely related to their phase change, thus the two phenomena should occur at around the same temperature and should be influenced similarly. However, spatial confinement of the analysed sample affects the change in colour differently compared to its phase transition and the most pronounced effects can be observed during cooling. The bulk composite is coloured while still liquid and the colour hysteresis does not exhibit a loop. In an open-porous medium the colouration coincides well with the crystallization and the colour hysteresis widens to about 4 °C. Microencapsulated composite exhibits two crystallization processes, one of them taking place at the bulk crystallization temperature and the other one at about 20 °C lower. Under such conditions the composite is coloured just before the onset of the second crystallization, i.e. about 15 °C below crystallization in the bulk, and the corresponding colour hysteresis widens to 18 °C. The two crystallization forms are thermally independent and have the same crystalline structure. These effects should be taken into account when designing future applications where the phase-changing materials are implemented.

Synthesis of micrometer-sized poly(methyl methacrylate) particles by microsuspension iodine transfer polymerization (ms ITP)

Micrometer-sized poly(methyl methacrylate) (PMMA) particles were successfully prepared without submicrometer-sized by-products for the first time by applying microsuspension iodine transfer polymerization with iodoform as a chain transfer agent.