For many practical
applications, the most important factor is to have an improved interface
between the matrix and dispersed phase in a compressible composite
aerogel having a high degree of porosity and a large surface area.
Although some measure of compressibility is obtained in polymer-based
aerogels with a continuous backbone through the hybridization of the
stiff backbone [polyvinyltrimethoxysilane (P-VTMS), −C–C−]
and flexible backbone [poly(3-glycidyloxypropyl)trimethoxysilane
(P-GPTMS), −C–O–C−], it seems that the
extent of improvement is insignificant in terms of interface improvement,
surface area increase, and ordered mesoporous network. In this study,
the effects of the incorporation of graphene nanoplatelets (GnPs)
on aerogels made of a backbone consisting of −C–O–C–
(flexible backbone) were examined in terms of structural improvement
and were compared with aerogels made of a backbone consisting of −C–C–
(stiff backbone). Moreover, the inorganic siloxane cross-link density
between the underlying polymer chains was controlled by inducing hydrogen
bonding between polymer chains and GnPs. This approach reduces the
structural shrinkage during gelation and drying. The integration of
only 1 wt % GnP integrated into the backbone by using spinodal decomposition
phase separation processing allowed control of the pore size and the
surface area. Integration of GnPs through in situ exfoliation during
sol–gel transition is shown to be the best approach using the
lowest possible amount of GnPs to improve aerogels’ mesoporous
network made from polymerized GPTMS. A flexible backbone such as P-GPTMS
chains is supposed to result in a compliant aerogel, but the chains
tend to shrink extensively during gelation and drying, reducing the
porosity. P-GPTMS-derived aerogel suffers from a wrong combination
of flexible backbone conjugated with an extensive number of permanent
chemical cross-links and abundant remaining unreacted hydroxyl groups
that undergo permanent chemical shrinkage. To counteract this, the
GnP-reinforced prepolymer precursor (P-GPTMS) with fewer siloxane
cross-links was synthesized and studied. By use of this strategy,
the same elastic properties as those seen with the hybrid P-VTMS-
and hybrid P-GPTMS-derived aerogels were imparted, while also improving
the mechanical strength by up to 138% and the surface area by up to
205% by controlling the extent of GnP exfoliation during the sol–gel
transition. This exceptional effect of GnP on the surface area improvement
was shown to be of up to 2.05-fold for P-GPTMS and 2.63-fold for P-VTMS
material.
Phase change materials (PCM) have gained extensive attention in thermal energy storage applications. In this work, microencapsulation of vegetable-derived palmitic acid (PA) in bio-based polylactic acid (PLA) shell by solvent evaporation and oil-in-water emulsification was investigated. Fourier transform infrared spectroscopy and scanning electron microscopy were conducted to confirm the successful encapsulation of PA in PLA shells. Differential scanning calorimetry was performed to evaluate the thermal properties, thermal reliability, and core content of the fabricated PCM microcapsules (microPCM). Through a series of parametric studies, the effects of PCM and solvent content, oil phase-to-aqueous phase ratio, as well as surfactant type and content on the morphology, particle size, and thermal properties of the PCM microcapsules were investigated. Experimental results showed that PVA was a superior emulsifier to SDS in the emulsion systems being studied. There also existed an optimal PVA concentration to reduce the average size of microPCM. When the PVA concentration was above this optimal level, the emulsifier molecules tend to form micelles among themselves. This led to the adhesion of tiny microspheres on the surface of microPCM as well as larger microPCM. In short, this work has demonstrated the possibility of using the solvent evaporation method to fabricate 100% bio-based PCMpolymer microcapsules for thermal energy storage applications.
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