Elsevier

Thermochimica Acta

Volume 690, August 2020, 178673
Thermochimica Acta

Effect of halloysite nanotubes on multifunctional properties of coaxially electrospun poly(ethylene glycol)/polyamide-6 nanofibrous thermal energy storage materials

https://doi.org/10.1016/j.tca.2020.178673Get rights and content

Highlights

  • Nanoencapsulation of phase change material (PCM) in core/shell structured fibers.

  • Mechanical testing of nanofibrous thermal energy storage (TES) material.

  • Determination of multifunctional properties of nanofibrous TES material.

  • Halloysite nanotubes incorporation enhanced the durability and efficiency of TES material.

Abstract

Coaxial electrospinning of poly(ethylene glycol) (PEG)/polyamide 6 (PA6) was successfully used in development of nanofibrous thermal energy storage (TES) material. Halloysite nanotubes (HNTs) were introduced into the core/shell structured TES materials at various concentrations (0.5, 1, 3 and 5 wt. %). Surface activation of HNT was also conducted by piranha etching in order to increase the affinity between piranha-etched nanotubes (HNT-P) and PEG. The core/shell structured materials were characterized using SEM, FTIR, TGA, DSC, tensile and thermal cyclic tests. With incorporation of 3 wt. % HNT-P into the core/shell nanofibers, tensile modulus and latent heat of melting values were increased by 25 % and 21 %, respectively. Additionally, PEG encapsulation efficiency of the neat core/shell nanofibers was increased from 78 % to 96 % with 3 wt. % HNT-P addition. The neat core/shell samples preserved 88 %, whereas 3 wt. % HNT and HNT-P added nanofibrous samples preserved 94 % of their initial melting enthalpies.

Introduction

Energy storage and retrieval has been one of the biggest concerns in the last century due to increasing demands and depleting finite resources. Considering the projections suggest that the energy consumption will rise by 48 % by the year 2040, the necessity in orienting the industry towards the renewable energy sources is undeniable [1]. Although technological advancements in energy harvesting from renewable resources such as wind, solar radiation and ocean waves may provide solution to this problem in the future, low cost and efficient storage of the harvested energy is currently the main obstacle in the widespread implementation. At this stage, TES systems which stock energy by heating or cooling a medium have been the most convenient solution to energy storage problems.

Phase change materials (PCMs) have the ability of storing thermal energy in the form of latent heat during melting owing to their high melting enthalpy values. At the same time, they possess the ability of releasing significant amount of the absorbed energy during liquid to solid phase transition. According to the chemical structure, PCMs can be classified as organic, inorganic and eutectic. Utilization of organic phase change materials which absorb thermal energy in the form of latent heat has been considered to be one of the most promising TES systems due to their several properties such as high energy storage density, very low temperature difference in the process of heat absorption and release, low corrosiveness and reusability with no significant degradation in energy storage efficiencies. Due to these properties of the organic PCMs, wide applications of such materials in industrial areas as solar energy storage, building coating materials, smart textiles and air conditioning systems are common [2].

Although PCMs possess advantageous properties such as TES in the form of latent heat, disadvantages such as low thermal conductivity and leakage in the liquid state are the obstacles needed be overcome for development of convenient TES materials. In order to cope with this problem, several PCM encapsulation methods have been developed for production of thermally intelligent TES materials named as micro encapsulated phase change materials (MEPCM) and nano encapsulated phase change materials (NEPCM) [3]. Form stable PCMs can be encapsulated by in-situ polymerization, layer by layer assembly, electrospinning, etc. There have been many studies about development of TES materials from various polymeric substances by using these encapsulation methods. Salaün et al. [4] utilized in-situ polymerization as the encapsulation method. They used melamine formaldehyde as the shell material to encapsulate n-hexadecane and n-eicosane mixture with different concentrations. Phase change temperature of the resulting microencapsulated PCMs ranged between 268 K (-5 °C) and 303 K (30 °C) with latent heat of 163−170 J/g even after 13 heating-cooling cycles. Chaiyasat et al. [5] prepared MEPCM by using suspension polymerization as the encapsulation method. They have used poly(divinyl benzene) as the shell material in order to encapsulate octadecane. They have achieved microencapsulation with average particle size of 1.5 μm. The latent heat of melting value of the MEPCMs was 192 J/g at a temperature of 295.6 K (22.6 °C). Although encapsulation methods such as in-situ polymerization and layer by layer assembly provide highly adjustable and functional coatings to the PCMs, they generally result in MEPCMs rather than NEPCMs due to some limiting factors arising from the nature of particle formation under centrifugal forces or layer formation [[6], [7], [8]].

Electrospinning is a versatile, fast and cost effective method in production of nanofibers which utilizes electrostatic forces in developing fibrous morphologies out of polymer melts or solutions. This way, not only the specific surface area can be enhanced but also higher encapsulation efficiency of the PCMs can be achieved. Moreover, low thermal conductivity being another major problem behind utilization of organic PCMs in the TES systems, can be partly overcome by the high specific surface area of the nano sized electrospun fiber morphology. There have been many studies utilizing electrospinning method in the production of NEPCMs. Chen et al. [8] developed ultrafine TES nanofibers by using lauric-myristic acid as eutectic PCM and poly(metha-phenylene isophthalamide) as the shell material. Uniaxial electrospinning method in which both encapsulating agent and PCMs are mixed in the same solution was used. Nanofibers with average fiber diameters between 210–377 nm were acquired with TES capacities of around 25 J/g. In another study, Golestaneh et al. [9] used co-electrospinning in development of heat storing nanofibrous mats by utilizing capric–lauric acid and capric-palmitic acid eutectic PCMs and poly(ethylene terephthalate) (PET) as the supporting matrix. By co-electrospinning the capric–lauric acid/PET and capric-palmitic acid/PET solutions on to the same collector, fiber structured TES materials were produced. According to the DSC results, phase change temperature of the TES ranged from 286 K (13 °C) to 306 K (33 °C) with TES capacities of about 90 J/g.

Leakage being one of the major problems for utilization of PCMs leading to both shape stability problem and high operating costs, can be solved by increasing the encapsulation efficiency and ensuring the shape stability of material over several usages. Thus, the coaxial and multiaxial electrospinning methods which produce core/shell structured ultrafine nanofibers with higher encapsulation efficiency with respect to previously mentioned methods have been widely implemented for such purposes. Noyan et al. [10] developed core/shell structured ultrafine nanofibers having PEG as the PCM and poly(acrylonitrile) (PAN) as the shell material by using coaxial electrospinning. After 10th heating/cooling cycle, they have achieved up to 70 % encapsulation efficiency in the core/shell structured nanofibers with 105 J/g TES capacity and average fiber diameter of 654 nm.

Although the shape stability problem of the organic PCMs in the liquid state can be solved by encapsulation in the fiber morphology, the low thermal conductivity of the organic PCMs still imparts a limitation to the widespread usage of such materials in the industry. Thus nanoparticulate additives have been introduced into the structure. Babapoor et al. [11] investigated the effect of various inorganic additives (SiO2, Al2O3, Fe2O3, and ZnO) and their concentration on the coaxially electrospun PEG/PA6 core/shell nanofibers. They observed that with the introduction of inorganic additives into the fiber structure, the average fiber diameter decreased. This was due to the increase in the electrical conductivity of the electrospinning solution resulting in induction of higher electrostatic forces (higher surface charge density). When the latent heat values of the nanofibers were investigated, it was seen that the TES property decreased for the composite structure. This was attributed to the retardation of the crystallization of PCM during electrospinning in the presence of particles. Additionally, by the introduction of Al2O3 to the core/shell structured nanofibers, maximum increase in thermal conductivity of the composite material was observed. In an another study, Thanakkasaranee and Seo [12] developed PEG/HNT composites by melt extrusion method for investigating the shape stability of the composite material. It was observed that due to the porous structure of the HNTs particles, the PEG molecules were encapsulated in the nanotube particles displaying no leakage upon heating over the melting temperature of PEG. Moreover, HNT presence in the PEG matrix increased the thermal conductivity of the composite structure. On the other hand, there are several ways of increasing the HNTs dispersion in the polymer matrix such as covalent functionalization of the nanotube surfaces (silane coupling, surface graft polymerization, etc.) or non-covalent functionalization (intercalation of HNTs, enlargement of the lumen, etc.) [[13], [14], [15], [16], [17], [18]]. In this study, the HNTs surface was activated by piranha etching in order to increase the polymer/HNT interaction [17].

To the best of our knowledge, there has not been any published study investigating the effects of HNT addition on the morphology, chemical structure, thermal, mechanical and TES properties of core/shell structured composite nanofibers. This study aimed to investigate the effects of HNT and surface activated HNT-P addition into the core structure of the PEG/PA6 core/shell nanofibrous TES material. PEG was utilized as the PCM in the core structure and the PA6 was used as the shell material. Having Al and Si based inorganic nanotube structure, HNTs possess high thermal stability. Additionally, since HNTs are naturally occurring materials with several unique structural properties, they possess high potential as nano additives in multifunctional engineering composite materials. In the scope of the study, HNT surfaces were activated by piranha etching process in order to increase the interaction between PEG and the nano additive leading to higher encapsulation and less agglomeration of the nano particles in the core structure of the nanofibers. Different HNT or HNT-P compositions (0.5, 1, 3 and 5 wt. %) were implemented in the composite nanofibers and the above mentioned properties were investigated. With addition of HNTs into the core structure, enhancement of multifunctional properties of nanofibers, such as mechanical and TES characteristics was aimed. Finally, the TES property and durability of the composite material were also determined by thermal cyclic test.

Section snippets

Materials

Polyamide-6 (Tecomid NB PA6) (Viscosity Average Molecular Weight (Mv): 26,000 g/mol) was obtained from Eurotec Engineering Plastics (Corlu, Turkey) and used as the shell material. It had a density of 1.13 g/cm3 and a melting temperature of 496 K.

As the organic PCM, poly(ethylene glycol) (PEG) (Mn: 6000 g/mol) was purchased from Merck and used as the core material. PEG had density of 1.2 g/cm3 and melting temperature between 333−338 K.

Halloysite Nanotubes (Al2Si2O5(OH)4.2H2O) were acquired from

Characterization of HNTs and HNT-P

The HNTs and piranha-etched HNT-P were both characterized by XRD analysis, TGA and FTIR given in Fig. 1, Fig. 2, Fig. 3, respectively. According to the XRD analysis results in Fig. 1, both HNTs and HNT-P showed (001) diffraction peak at 12° (2θ) indicating dehydrated halloysite structure having 0.74 nm of interlayer spacing, whereas the hydrated form of HNT has interlayer spacing of 1 nm [17,19]. The (020) reflection at 20° (2θ) also supports the presence of dehydrated state HNT [17]. There was

Conclusions

In the present study, PEG and PA6 were utilized in development of core/shell structured nanofibrous energy storage materials using coaxial electrospinning method. HNTs were added into the core solution in order to determine their effects on morphology, chemical structure and multifunctional properties such as, thermal, mechanical and TES characteristics of core/shell composite nanofibers. Upon SEM analysis, it was seen that the AFD and uniformity of the nanofibers decreased with HNT presence in

CRediT authorship contribution statement

Refik Baris Yilmaz: Investigation, Methodology, Writing - original draft, Formal analysis. Goknur Bayram: Conceptualization, Investigation, Writing - review & editing, Supervision. Ulku Yilmazer: Supervision, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was financially supported by the Middle East Technical University (Research Project Number: YLT-304-2018-3577) and the Scientific and Technological Research Council of Turkey (TUBITAK) (BIDEP 2210-C).

References (29)

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