Electrospun polyacrylonitrile–lauric acid composite nanofiber webs as a thermal energy storage material

Abstract

Phase-change materials have remarkable characteristics due to their simple phase-changeable nature. Within a certain temperature range, these materials can easily change from solid phase to liquid phase or vice versa. It is possible to build thermal energy storage mechanisms, thanks to their latent heat. In this study, composite nanofiber structures were prepared with lauric acid and polyacrylonitrile blends. Nanofiber webs were fabricated via electrospinning technique and combined with phase-change material due to their light weight and high surface area. Thermal energy storage properties were investigated via differential scanning calorimeter, and structural analysis was studied by Fourier transform infrared–attenuated total reflection spectroscopy. Scanning electron microscope was used to investigate the surface morphology of the fibers. Blended polyacrylonitrile–lauric acid nanofibers were successfully converted to nanofiber formation without losing their properties. Results showed that fabricated polyacrylonitrile–lauric acid composite nanofiber webs can be used as a thermal energy storage patch.

Introduction

One of the most efficient methods for storing thermal energy is latent heat storage due to its high storage density and small temperature variation while storing and releasing the heat. ¹ Temperature range, which is a unique property of all phase-change materials (PCMs), provides a large quantity of thermal energy during phase transitions and absorbs, retains, and releases the energy. ² Organic, inorganic, polymeric, and eutectic PCMs have been studied in detail due to their attraction to latent heat to store energy. ¹ Organic compounds used for PCMs are paraffin waxes, esters, acids, and alcohols; inorganic materials are salt hydrates, eutectics of inorganic salts, and metals and their eutectics are subgroups of PCM classification.^3–5 PCMs from organic compounds generally have low melting points and more suitable to use for room-heating thermal storage systems.^3,5,6 Generally, paraffin-based materials are used as a PCM and most of the studies are about the encapsulation of paraffin. However, fatty acids can be a good alternative for PCM, thanks to their appropriate temperature range of phase changes and higher capacitance values for latent heat. ⁷ It is not possible to use PCMs as they received, ⁸ thus PCMs should be incorporated with composite structures.

It is possible to combine PCMs in textiles by coating or encapsulation for thermoregulation of textiles and fibrous materials.^9,10 Studies showed that the encapsulation of PCMs with the textile material gave heat storage capacities between 0.9 and 4.4 J/g according to their changing concentration. ¹¹ Due to polyacrylonitrile’s (PAN) unique, tailor-made properties, PAN nanofibers (NFs) gain much attention to large-scale industrial applications. ¹² Chain of PAN polymer is essentially linear, which makes spinning possible for compatibility in the composite structure with the adjustment of fabrication parameters. ¹³ PAN solution can be easily used for electrospinning to produce submicron fibers. Electrospinning is a technique which allows producing fibers via applied electrostatic force between polymer solution feeding (needle) through the grounded or charged collector. Potential applications of PAN nanofibers are as follows: carbon nanofiber production, filtration, drug delivery systems, energy storage systems, solar cells, battery applications, and so on.^14–17 Moreover, PAN is an appropriate supporting skeleton for the creation of composite structures with PCMs while preserving the fiber form and preventing the leakages. ¹⁸ In this study, unique properties of PAN (as a supporting matrix polymer) were combined with lauric acid (LA) as a PCM to gain the energy storage properties in the level of nanofiber web. LA is one of the fatty acids with long molecular chain structure which is suitable for the heat storage systems. ¹⁹ Due to its mid-range phase-changing temperature value, LA can be a good candidate for the temperature-changing products in daily usage. Nanofiber web was produced via electrospinning technique which is a well-known method to produce submicron nanofibers ²⁰ together with PAN and LA as a single step, without the need for further processes to create the composite PAN-LA structure or encapsulation of PCM. Figure 1 shows the schematic explanation of the study. Electrospun PAN-LA composite structure was examined for their latent heat storage capacities while preserving the formation of the nanofiber.

Figure 1.

Schematic description of the study: graphical abstract.

Experiment

Materials

PAN was purchased from Sigma–Aldrich and a standard M_w 150,000 g/mol was used as received. N,N-dimethylformamide (DMF) was also purchased from Sigma–Aldrich and used without any further purification. LA M_w 200.32 g/mol was purchased from SAFC, Sigma–Aldrich and used as received.

Methods

Polymeric PAN-LA composite nanofiber webs have been fabricated via electrospinning method. DMF was used as a solvent while preparing an electrospinning solution. PAN-LA blends were prepared according to their weight ratios and dissolved in the DMF solution. For obtaining a homogeneous solution, magnetic stirrer was used. The weight ratio of the solid material is 50:50, and PAN-LA and 8wt% of PAN-DMF were kept constant for all fabrications. Later on, electrospinning solution was fed into a syringe and the process initiated. The electrospinning parameters were set as follows: 15 cm distance between collector and tip, feeding ratio of the solution was 0.8 mL/h, applied voltage was 16 kV, and nanofibers were collected on the aluminum foil.

Fourier transform infrared–attenuated total reflection (FTIR-ATR; Perkin Elmer, Spectrum One, with a Universal ATR attachment with a diamond and ZnSe crystal) spectroscopy was used for structural analysis, and differential scanning calorimeter (DSC) was used to investigate the thermal behavior of samples. Morphologic properties of nanofiber webs were observed using scanning electron microscopy (SEM), QUANTA 400 F with 10 kV accelerating voltage was used for proving the fiber formation, and also the diameters of the nanofibers were measured according to the obtained SEM images. Additional software (ImageJ) was used to measure the diameter of the nanofiber.

Results and discussions

FTIR-ATR was used to record the characteristic peaks of the samples. Results showed that when powder form of LA and PAN was physically blended at 50:50 ratio or their NF was produced from 50:50 ratio of PAN:LA, same FTIR-ATR spectra were observed. It means that LA and PAN combined and mixed well during the nanofiber fabrication process without losing their chemical structure compared to their bulk form. Produced PAN-LA composite nanofiber web which represented the same structural properties of its blend demonstrated that during the electrospinning process, no chemical reaction or bonding occurred according to FTIR-ATR results. Produced composite electrospun web exhibits both PAN and LA structural peaks.

Figure 2(a) and (b) represents the characteristic peaks of LA and PAN powders, respectively, before obtaining their blends. According to FTIR-ATR results, PAN displays all the characteristic peaks compatible with the literature. The bands located at around 2940 cm⁻¹ were assigned to the stretching vibrations of C–H groups. ¹⁶ Absorption band of around 2243 cm⁻¹ was assigned as C≡N triple bond^21–23 and there is also a strong band at 1454 related to the bending vibration of –CH in –CH₂. The band at 1076 cm⁻¹ is ascribed to the –CH bending mode in CH.^24–26 FTIR-ATR spectra of LA show that absorption bands for C=O, OH, and CH₃ bonds are also coherent with its long chain acidic structure.

Figure 2.

FTIR-ATR spectra of (a) LA and (b) PAN powders.

It is seen from Figure 3 that all the characteristic peaks of PAN and LA are also preserved for their composite nanofiber formation when compared to the PAN-LA powder blend. During the composite nanofiber fabrication process, no significant chemical reaction occurred, due to which same blend peaks were seen in the PAN-LA nanofiber.

Figure 3.

FTIR-ATR spectrum of PAN-LA powder blend and PAN-LA nanofiber.

Figure 4 presents the DSC curves of PAN-LA composite nanofiber web and PAN nanofiber web. For the PAN nanofiber web, no enthalpy change occurred through increased temperature. For the composite nanofiber webs, the addition of PCM material (LA) changes the thermal behavior of the nanofiber web; thus, at the phase-change temperature of LA, the curve was obtained. Furthermore, when heat-treated sample is compared with the non-heat-treated one, total enthalpy value is enhanced. The DSC curves are integrated between the temperature interval of 41.5°C and 44.5°C. The result of this integration is as follows: for PAN-LA sample, ∆H (total enthalpy) was 4.483 J/g; for PAN-LA 1-h heat-treated sample, ∆H (total enthalpy) was 7.000 J/g. This result approved that heat treatment on the sample had a positive effect to improve the heat storage capacity. Thermal treatment which was applied to the PAN-LA samples after their fabrication causes swelling, which is observed by increasing the diameter of the nanofiber. The applied heat treatment and the phase-changing behavior of LA during the treatment may increase the diameter. However, it was promoted to capsulation of PCM in the nanofiber network and increased enthalpy values.

Figure 4.

DSC curves for PAN nanofiber web: 50:50 PAN-LA nanofiber web and 1-h heat-treated 50:50 PAN-LA nanofiber web.

Previous studies^6,27 showed that nanofiber formation cannot be well preserved with the composite structure of polymer and PCM, but this study showed that PAN and LA are in good compliance to sustain the nanofiber structure for the applied fabrication conditions.

SEM images in Figure 5(a)–(c) show that fiber diameter was about 293.4 nm for 1-h heat-treated PAN-LA sample and 204.6 nm for PAN-LA nanofiber web. Both nanofiber diameter variation and beads are available and are seen as an inhomogeneity compared to PAN nanofiber web under the same fabrication parameters, and the average nanofiber diameter of PAN nanofiber was 395 nm. While PAN nanofiber web exhibited a smooth and bead-free formation, under same fabrication conditions, LA-PAN blend caused slight inhomogeneity on the nanofiber structure. Figure 5(d) represents the nanofiber diameter distribution histograms for PAN-LA and PAN-LA 1-h heat-treated samples with diameter distribution curves.

Figure 5.

(a) 50:50 PAN-LA nanofiber web, (b) 1-h heat-treated 50:50 PAN-LA nanofiber web, (c) SEM image of 8wt% PAN nanofiber web and an inset diameter histogram is placed (395 nm), and (d) nanofiber diameter distribution histograms for PAN-LA and PAN-LA 1-h heat-treated nanofibers.

Decreased nanofiber diameter is expressed to increase the surface area. In the thermodynamic point of view, when the total enthalpy of the composite structures with same weights is compared, higher surface area promotes to raise the total enthalpy of the system. The LA addition was assisted to reduce the nanofiber diameter of PAN nanofibers and this is another added advantage of its phase-changing nature.

According to SEM results, the addition of LA to the PAN nanofiber system helps to decrease the nanofiber diameter while creating an inhomogeneity for diameter distribution, which can be the result of the swelling of the fibers after heat treatment is applied. However, as a result of the applied heat treatment on the PAN-LA composite nanofiber web, fiber diameter increased due to the encapsulation of LA domains inside the nanofiber matrix. Coefficient of variation (CV%) values of nanofiber diameters were calculated and represented in Figure 6. CV% values increased in the order of PAN, PAN-LA, and PAN-LA 1-h heat-treated sample through 15%–29%.

Figure 6.

(a) Average nanofiber diameter values with error bars for PAN, PAN-LA, and PAN-LA 1-h heat-treated samples; (b) coefficient of variation (CV) values of nanofiber diameters for PAN, PAN-LA, and PAN-LA 1-h heat-treated samples.

In the literature, PAN-LA composite structure was obtained via electrospinning of PAN after it was magnetron sputtered coated with functional silver as a nanolayer and LA was adsorbed to the surface. ²⁸ In this study, single-step fabrication of PAN-LA has succeeded through electrospinning, thanks to the well-combined solutions of PAN and LA. Thus, further steps for creating the composite structure or capsulation of the materials are not needed. PCM and polymer matrix gathered as a nanofiber, and it was prevented for the composite structure of PAN-LA.

Conclusion

In this study, PAN-LA composite nanofiber web was successfully fabricated via electrospinning method. Also, nanofiber formation was done through fabrication and after that, thermal treatment was maintained. Based on the results, the produced composite nanofiber webs can be used as a thermal energy storage material in the patch form, as in the state of being fabricated. This study proposes the facile method to combine polymeric structure with the chosen PCM to build a phase-changing nanofiber web without the need for complex methods, encapsulation processes, or chemical treatments. A blend of LA with PAN assisted to decrease the nanofiber diameter compared to PAN nanofibers. In terms of increased surface area, decreased nanofiber diameter was appreciated. Moreover, it was examined that the applied simple thermal treatment process had a positive effect on the improvement of the heat capacity of the web and it could be explained with the improved capsulation of PCMs during the heat treatment process. Produced nanofiber web could be used in different application areas; applying heat treatment on the composite structure is recommended to enhance the total enthalpy of the patch.