Embedding Lauric Acid into Polystyrene ... - ACS Publications

Jun 30, 2017 - Key laboratory of Bio-based Material Science and Technology, Ministry of Education, Northeast Forestry University, 26 Hexing Rd,. Xiang...
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Research Article pubs.acs.org/journal/ascecg

Embedding Lauric Acid into Polystyrene Nanofibers To Make HighCapacity Membranes for Efficient Thermal Energy Storage Ping Lu,*,† Wenshuai Chen,‡ Min Zhu,§ and Simone Murray† †

Department of Chemistry and Biochemistry, Long Island University, One University Plaza, Brooklyn, New York 11201, United States ‡ Key laboratory of Bio-based Material Science and Technology, Ministry of Education, Northeast Forestry University, 26 Hexing Rd, Xiangfang District, Harbin, Heilongjiang 150040, P. R. China § Textile Development and Marketing Department, Fashion Institute of Technology, 227 W 27th St, Manhattan, New York 10001, United States S Supporting Information *

ABSTRACT: This article describes a simple and reliable approach to the fabrication of lauric acid (LA)-containing polystyrene (PS) nanofibers with LA/PS weight ratios up to 4 to 1 in the final dry mass of nanofibers. The obtained LAPS composite nanofibers achieved an unprecedented thermal energy storage capacity up to 78.4% of pristine LA because of the high LA loading as well as the lightweight and porous nature of the PS matrix. To the best of our knowledge, our result was higher than the previously reported values, which were generally less than 50%. The direct scanning electron microscopy (SEM) observation, IR spectra, Raman spectra, and differential scanning calorimeter (DSC) thermograms of LAPS nanofibers indicated that majority of LA was encapsulated inside the composite nanofibers. The X-ray diffraction (XRD) patterns showed that the crystal size of LA domains enlarged with the increase of LA loading in the composite nanofibers. In addition, the DSC run in T4P mode unveiled that the released latent heat during crystallization could raise the temperature of LAPS composite nanofibers above the onset temperature of crystallization, which was different from the reported results obtained from DSC run in traditional T1 mode. Furthermore, the undesirable supercooling effect was suppressed by increasing the percentage of LA in the composite nanofibers. Also, the LAPS composite nanofibers showed robust cycling stability and reusability during 100 continuous heating−cooling cycles in the temperature range of 0−80 °C. The LAPS composite nanofibers demonstrated excellent structural stability after solvent extraction and prolonged heat treatment. The entrapped LA was locked in the PS matrix without leaking out of the nanofibers even after continuous and repeated heating above the melting point of LA. KEYWORDS: Polystyrene, Lauric acid, Electrospinning, Phase change material, Thermal energy storage

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INTRODUCTION

during thermal energy charging and discharging. A large number of organic and inorganic materials have been utilized as PCMs, which include paraffins, fatty acids, fatty alcohols, esters, oils, salt hydrates, and metals.5 They have been widely employed in solar energy storage, energy-efficient buildings, thermal regulating textiles, and thermal protection wound dressings.6−9 Since PCMs experience repeated melting and solidifying, it is necessary to encapsulate them in a solid matrix to form shape-stabilized or form-stable structures. To date, PCMs have been embedded into various organic and inorganic materials such as poly(methyl methacrylate), polyethylene, poly(vinyl pyrrolidone), polyurethane, cellulose, graphene,

Storing heat from renewable thermal energy sources such as the sun and the geothermal field for consumption at a later time and a different location has attracted enormous attention in recent years.1 The heat harvested and saved by thermal energy storage systems includes sensible heat (e.g., temperature change of ocean and air), latent heat (e.g., phase change of materials), and chemical heat (e.g., heat released from chemical reactions).2 Among these three types of heat, latent heat has a higher energy density than sensible heat and is also less expensive than chemical heat.3 Therefore, the storing and releasing of latent heat through the melting and solidifying of phase change materials (PCMs) has become the most efficient and cost-effective method for a variety of thermal energy storage applications.4 PCMs are capable of storing and releasing a large amount of latent heat at nearly constant temperatures © 2017 American Chemical Society

Received: May 10, 2017 Revised: June 16, 2017 Published: June 30, 2017 7249

DOI: 10.1021/acssuschemeng.7b01476 ACS Sustainable Chem. Eng. 2017, 5, 7249−7259

Research Article

ACS Sustainable Chemistry & Engineering

ical strength. We have fabricated PS yarns with a highly porous interior structure through vapor-induced phase separation during electrospinning process.30,32 The empty space inside PS yarns could be used to accommodate a large amount of LA. As such, LAPS composite nanofibers with different LA/PS weight ratios were produced by simply co-electrospinning mixtures containing the desired amounts of LA and PS. Thanks to the lightweight and porous nature of PS, the loading of LA reached four times the mass of PS matrix, leading to an unprecedented 78.4% thermal energy storage capacity of pristine LA. Furthermore, the majority of LA was found to be encapsulated in a continuous PS matrix throughout the nanofibers. The small portion of LA on or close to the nanofiber surface was sipped into the nanofibers via the capillary effect during melting. Although the LA domains inside nanofibers expanded around 120% from solid to liquid, the LA was retained in LAPS nanofibers without any measurable leakage because of the excellent thermal mechanical flexibility of PS as well as the chemical compatibility of LA and PS. In addition, differential scanning calorimetry (DSC) run in T4P mode instead of traditional T1 mode was employed to investigate the actual temperature of LAPS composite nanofibers when latent heat was released. It was discovered that the released latent heat could raise the sample temperature above the onset crystallization temperature (Toc) toward the melting temperature (Tm), which has rarely been reported. Also, the supercooling of LA was minimized by incorporating LA into LAPS nanofibers to form larger domains.

SiO2, TiO2, and Al2O3, mainly in the forms of macro-, micro-, and nanocapsules.10−15 For most applications, however, these core−shell capsules with PCM loadings usually below 50% have to be incorporated into other macroscale structures such as films, foams, and fibers, thus further decreasing the weight ratio of encapsulated PCMs in the final materials.16−18 Recently, electrospinning has been developed as a direct method to encapsulate PCMs into fibers.19−22 Electrospinning represents a simple, convenient, and versatile method for the fabrication of nanofibers.23 It is the only available technique capable of producing continuous nanofibers of polymers, metals, and oxides from a variety of polymers and precursors in large quantities.24 Thanks to their large surface area, high porosity, high aspect ratio and superior mechanical flexibility, electrospun nanofibers provide a large heat transfer area and a strong driving force to speed up thermodynamic processes during thermal energy charging and discharging. PCMs can be encapsulated into polymer nanofibers through coaxial electrospinning to form core−sheath fibers or by co-electrospinning a mixture solution containing PCMs and polymers to produce blend nanofibers.19−21,25−29 For example, octadecane, a hydrocarbon PCM with an 18-carbon chain, was encapsulated into titanium dioxide−poly(vinyl pyrrolidone) (TiO2−PVP) nanofibers by a heating-assisted coaxial electrospinning.19 The nanofibers consisted of a TiO2−PVP sheath and an octadecane core. During the coaxial electrospinning, the core octadecane was melted by heating to 68 °C prior to injection with the sheath solution. By controlling the feeding rates of core melt and sheath solution, nanofibers with a maximum loading of 45% octadecane by weight were fabricated, which postponed the temperature decline against cooling. As compared with coaxial electrospinning, co-electrospinning of a PCM and polymer mixture is much easier to control because only one feeding rate is tuned. A plant oil-based PCM was incorporated into poly(vinyl alcohol) (PVA) through an emulsion coelectrospinning method.20 With a 47% maximum oil loading, the PCM domains were randomly distributed in the nanofibers. Upon heating above its melting point, however, the hydrophobic oil leaked from the hydrophilic PVP matrix to form large particles on the surface of nanofibers or in the interfiber spaces. In this study, we demonstrate a facile approach to the fabrication of lauric acid-containing polystyrene (LAPS) nanofibers with LA loadings up to four times the amount of PS matrix. These LAPS composite nanofibers showed superior structural and thermal stability during elongated and repeated heating−cooling cycles. We chose PS because it is an ideal thermal-insulating polymer that has a high mechanical durability, superb chemical resistance, and no toxicity.30 Fatty acids are a type of promising phase change materials because they have high energy storage density, controlled volume changes during the phase transition, little supercooling effect, appropriate phase change temperature range, and high thermal and chemical stability.5 Lauric acid, a 12-carbon-chain fatty acid, was selected as the active PCM component because it is a “food grade” PCM, which has been found in coconut oil (50%) and human breast milk (6.2%).31 The incorporation of LA in PS matrix can markedly enhance the energy efficiency while lowering the environmental disruption in practical household, commercial, and industrial applications. In order to achieve a high thermal energy storage capacity, the loading of the active PCM component (i.e., LA) in the polymer matrix should be maximized with well-balanced structural stability and mechan-

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EXPERIMENTAL SECTION

Chemicals and Materials. Polystyrene (PS, Mw ≈ 350 000, Mn ≈ 170 000) and lauric acid (LA, ≥98%) were obtained from SigmaAldrich. Anhydrous N,N-dimethylformamide (DMF, 99.9%) was purchased from Burdick & Jackson Honeywell, and ethanol (200 proof ACS grade) was received from Pharmco-AAPER. All chemicals were used as received without further purification. Deionized water with a resistivity of 18.2 MΩ·cm at 25 °C, purified using a Millipore Direct-Q 8 UV water purification system, was used for all the experiments. Fabrication of PS Yarns Comprised of Nanofibers. The yarns of PS nanofibers were prepared by electrospinning a 20 wt % solution of PS in anhydrous DMF at 20 °C and a relative humidity of 45%.30,32 In a typical procedure, the PS solution was loaded into a 3 mL syringe fitted with a 23 gauge diameter (ID 0.34 mm; OD 0.64 mm) and ca. 2.5 cm long flat metal needle (BD Medical) and delivered at 1 mL/h by using a syringe pump (Legato 110, KD Scientific). A voltage of 15 kV from a DC power source (ES30P-5W/DAM, Gamma High Voltage Research) was applied to the vertically positioned needle. The charged jet was spun into yarns of nanofibers and collected on a piece of aluminum foil plate (30 cm × 30 cm) placed ca. 25 cm below the tip of the needle. The as-prepared nonwoven mat of PS yarns was dried at ambient temperature under vacuum for 24 h prior to the subsequent characterization. Fabrication of LAPS Composite Nanofibers with Different LA/PS Ratios. The LA was embedded into PS through coelectrospinning mixtures of LA and PS with different ratios using the same electrospinning setup and parameters as described above. Typically, a 20 wt % PS solution was first prepared by dissolving 2 g of PS beads in 8 g of anhydrous DMF at ambient temperature with vigorous stirring for 24 h. Then 0.5, 1.0, 2.0, 4.0, 6.0, and 8.0 g of LA flakes were added into the 20 wt % PS solutions. Mixture solutions containing different LA/PS weight ratios were obtained after 24 h vigorous stirring and were used for electrospinning immediately. The received composite nanofibers were referred to as LA0.25PS, LA0.5PS, LA1.0PS, LA2PS, LA3PS, and LA4PS corresponding to the weight ratios between LA and PS, respectively. For simplification, the subscript of 7250

DOI: 10.1021/acssuschemeng.7b01476 ACS Sustainable Chem. Eng. 2017, 5, 7249−7259

Research Article

ACS Sustainable Chemistry & Engineering PS = 1 is omitted in the abbreviated expression. As a result, the weight ratios between LA and PS were 0.25/1, 0.5/1, 1/1, 2/1, 3/1, and 4/1 in the final products. Correspondingly, the percentages of LA in the composite fibers were 20%, 33%, 50%, 67%, 75%, and 80%, respectively. Solvent and Thermal Treatments of LAPS Composite Nanofibers. The LAPS composite nanofibers were treated with solvent and heat to study the extraction and release profiles of LA from the composite nanofibers and the dimensional stability of the membranes. Ethanol, which was chosen as the solvent to extract LA from the LAPS composite nanofibers, is a good solvent for LA (∼120 g of LA/100 g of alcohol at 20 °C) but a nonsolvent for PS.33 In a typical procedure for solvent extraction, a piece of LAPS membrane (5 cm × 5 cm) was immersed in 15 mL of ethanol for 24 h and then was washed with 5 mL of ethanol each 6 times under filtration. The ethanol-extracted membrane was dried in a desiccator at room temperature for the subsequent characterization. In a typical procedure for thermal treatment, a piece of LAPS membrane (5 cm × 5 cm) was laid flat on a clean aluminum foil and then was put into an oven at 60 °C for 24 h. After cooling to room temperature, the LAPS membrane and the underlying aluminum foil was examined after the removal of the membrane from the foil. Instrumentation and Characterization. The surface morphology and the structure porosity of the samples were examined using a field-emission scanning electron microscope (FE-SEM, Sirion 200, FEI) after gold coating for 60 s (Bio-Rad). The images were taken at an accelerating voltage of 10 kV and a working distance of 10 mm. The diameters of fibers were measured using the ImageJ software and then statistically analyzed using OriginPro 8.5 (OriginLab). Fourier transform infrared (FTIR) spectra were collected by a Nicolet iS50 spectrometer (Thermo Scientific, USA) under ambient conditions. Samples were analyzed by the attenuated total reflection (ATR) technique on a diamond crystal under 267 N (60 lbs) pressure. The spectra were recorded in the absorbance mode from an accumulation and an average of 128 scans at a 4 cm−1 resolution over a wavelength range of 4000−400 cm−1. Raman spectra were recorded from 3400 to 112 cm−1 with a 120 s exposure time using a Thermo Scientific DXR Raman microscope equipped with 10×/50× objective lens and a 780 nm argon laser source. Sample membranes were directly attached onto a clean glass slide for microscope focus and observation and Raman measurement. The overall crystalline phases of samples were determined on a Scintag X1 powder X-ray diffractometer (XRD) using a Ni-filtered Cu Kα radiation (λ = 1.5406 Å). Diffractograms were recorded from 2θ = 5° to 90° with a step size of 0.02° and a scanning rate of 0.04 s/step at an operating voltage of 45 kV and a filament current of 40 mA. The thermal stability and degradation profiles of samples were investigated by thermogravimetric analysis (TGA) on a simultaneous TGA/DSC Q600 thermoanalyzer (TA Instruments, USA). In a typical experiment, samples with a weight of around 10 mg were placed in a clean alumina (Al2O3) pan, and the TGA/DSC curves were recorded by heating the samples at 10 °C/min from ambient temperature (around 25 °C) to 600 °C in a dry nitrogen (N2) atmosphere with a purging rate of 100 mL/min. The thermal energy storage performance of samples was measured with a TA Q2000 differential scanning calorimeter (DSC) coupled with a refrigerated cooling system (RCS90, temperature range −90 to 550 °C). The DSC thermograms were collected by heating and cooling approximately 5 mg of each sample in a crimpled aluminum pan (Tzero series, TA Instruments) at a ramp rate of 10 °C/min in a temperature range of 0−80 °C in a dry N2 atmosphere with a flow rate of 100 mL/min. The enthalpies of melting (ΔHm) and crystallization (ΔHc) were calculated from the areas under the corresponding peaks using the TA Universal Analysis software.

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proper amounts of LA and PS in DMF under a set of optimal conditions: voltage (15 kV), feeding rate (1 mL/h), needle-tocollector distance (25 cm), temperature (20 °C), and relative humidity (45%). The spinning proceeded in a highly stable and reliable manner, producing a nonwoven mat of PS yarns and LAPS composite nanofibers 20 cm × 20 cm × 2 mm in dimensions within a few hours. The resultant PS yarns and LAPS composite nanofibers were then examined by SEM, and the representative results are shown in Figure 1. Since the collector used in our experiments had a flat and continuous surface, the obtained nanofibers were deposited in a randomly stacked fashion. The PS yarns and LAPS composite nanofibers with different LA/PS ratios were cylindrical in shape and uniform in size without the formation of beads and irregular segments along the fiber axes. The PS yarns (Figure 1A) had a quite smooth surface and the surface roughness of LAPS nanofibers generally increased with the increase of LA/PS ratio (Figure 1B−G), probably resulting from the phase separation between LA and PS during electrospinning. Furthermore, no notable porosity on the surface of PS yarns and LAPS composite nanofibers were observed. The size distributions of PS yarns and LAPS composite nanofibers are shown in Figure S1. The PS yarns had the largest average diameter with a broad size distribution range (1.93 ± 0.24 μm) among all the samples. The LA0.25PS composite nanofibers showed a dramatic decrease (∼50%) in average size to 0.95 μm after incorporating 20% LA. Furthermore, the size distribution range was also considerably narrowed. By doubling the amount of LA in the composite nanofibers, the average diameter of LA0.5PS nanofibers was only slightly reduced to 0.89 μm. The thinnest composite nanofibers with an average diameter of 0.78 μm were produced when LA accounted for 50% of the dry mass of nanofibers. It is obvious that the introduction of LA instead of the increased amount played a major role in the significant reduction of fiber size. In our previous study, we found that vapor-induced phase separation rather than thermally induced phased separation determined the structure of electrospun PS nanofibers.30 The addition of LA, even in a small amount, completely changed the vaporinduced phase separation behavior of PS in air due to the moderately hydrophilic nature of LA. Figure 1 (right column) shows the cross sections of PS yarns and LAPS composite nanofibers with different LA/PS ratios. A highly porous interior was clearly seen at the cross-section of a representative PS yarn cut through its vertical axis (Figure 1A, right column). Each PS yarn consisted of a bundle of distinguishable but possibly entangled nanofibers, resulting from the water vapor-induced phase separation of hydrophobic PS in air.30,32 With the addition of only 20% slightly hydrophilic LA, the porous interior disappeared and a solid cross-section was observed for the LA0.25PS composite nanofibers. Additionally, the crosssection became grainier with the increase of LA content (Figure 1B−D, right column), which was probably associated with the formation of LA-rich domain in the composite nanofibers. When the percentage of LA reached 50%, the average fiber size of LAPS gradually increased with the addition of more LA in the composite nanofibers (Figure 1E−G and Figure S1E− G). Specifically, the average fiber diameters were 0.87, 0.98, and 1.30 μm for LA2PS, LA3PS, and LA4PS, respectively. The increased fiber size was related to the increased LA quantity in the electrospinning solution and thus in the final composite nanofibers. However, the size increase was less dramatic as compared to the size change with the initial introduction of LA

RESULTS AND DISCUSSION

Structure of PS Yarns and LAPS Composite Nanofibers. The PS yarns and LAPS composite nanofibers with LA/ PS weight ratios of 0.25/1, 0.5/1, 1/1, 2/1, 3/1, and 4/1 were facilely fabricated by electrospinning solutions containing 7251

DOI: 10.1021/acssuschemeng.7b01476 ACS Sustainable Chem. Eng. 2017, 5, 7249−7259

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ACS Sustainable Chemistry & Engineering

The entrapment of LA in the composite nanofibers was further confirmed by infrared and Raman spectra. Figure S2 compares the infrared spectra of pristine LA and PS, in which LA had distinct and strong absorptions at 2915 cm−1 (CH3), 2848 cm−1 (CH2), 1695 cm−1 (CO), 1470−1410 cm−1 (CH3 and CH2), and 1085 cm−1 (C−O) and many other characteristic absorptions in the fingerprint region while PS had characteristic peaks at 3029 cm−1 (aromatic CH), 2923 cm−1 (aliphatic CH2 and CH), and 1493 and 1452 cm−1 (aromatic CC).34−36 Among these absorptions, the strong and unique carbonyl peak at 1695 cm−1 could be used as an indicator for the content of LA in the composite nanofibers. Figure 2A

Figure 2. Infrared spectra (A), Raman spectra (B), and X-ray powder diffraction patterns (C) of electrospun fibers with different LA/PS ratios: (a) pure PS, (b) LA0.25PS, (c) LA0.5PS, (d) LA1.0PS, (e) LA2PS, (f) LA3PS, and (g) LA4PS.

Figure 1. SEM images showing the overview (left column), surface (middle), and cross-section (right) of electrospun fibers with different LA/PS ratios: (A) pure PS, (B) LA0.25PS, (C) LA0.5PS, (D) LA1.0PS, (E) LA2PS, (F) LA3PS, and (G) LA4PS. The scale bars in panel G apply to the images in the same column.

shows the strength of the carbonyl peak increased with the incorporation of more LA into the PS matrix. The carbonyl peak of LA was not observed in the Raman spectra of LAPS composite nanofibers and other characteristic peaks of LA such as CH3 (1369 cm−1) and CC (970 cm−1) were also quite weak even for the LA4PS with 80% LA (Figure 2B and Figure S3).37 Since Raman is a scattering technique, the intensity of Raman

into PS. The cross sections of LA2PS, LA3PS, and LA4PS with 67%, 75%, and 80% LA, respectively, clearly showed the grainy domains of LA inside the composite nanofibers (Figure 1E−G, right column), confirming the encapsulation of LA in the PS matrix. 7252

DOI: 10.1021/acssuschemeng.7b01476 ACS Sustainable Chem. Eng. 2017, 5, 7249−7259

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ACS Sustainable Chemistry & Engineering scattering is directly related to the surface composition of the LAPS composite nanofibers. The pristine LA had strong Raman signals in a wide range from 1500 to 900 cm−1 (Figure S3), whereas these peaks were not observed in the spectra of the composite nanofibers.38 The lack of LA peaks in Raman spectra indicates that the majority of LA was encapsulated inside the polymer matrix, which agreed well with the SEM observations (Figure 1). With the increase of LA/PS ratio, the characteristic peak of PS at 1078 cm−1, assigned to the aromatic C−H, gradually weakened along with another characteristic peak at 1673 cm−1 for aromatic CC, resulting from the reduced percentage of PS in the composite nanofibers. Figure 2C shows the XRD patterns of PS yarns and LAPS nanofibers with different LA/PS ratios. The PS yarns showed a broad peak centered at around 2θ = 21°, which is typical for amorphous materials. The LAPS composite nanofibers had sharp peaks at 2θ = 6.3°, 9.5°, 16.0°, 19.2°, 20.2°, 21.4°, and 23.7°, which are characteristic peaks of crystalline LA (PDF No. 00-008-0528).39 The intensity of these peaks increased with the increase of LA/ PS ratios. The relative crystalline size of LA in the composite nanofibers could be estimated by using the Scherrer equation and the strongest peak at 2θ = 21.5°. Since the full width at half-maximum (fwhm) of the peak at 2θ = 21.5° gradually decreased with incorporation of more LA, the crystal size of LA in the composite nanofibers increased accordingly, which was consistent with the direct SEM observation of nanofiber cross sections (Figure 1). It also indicates that phase separation between LA and PS took place during electrospinning, which led to the formation of LA-rich and PS-rich regions in the composite nanofibers. Thermal Performance of LAPS Composite Nanofibers. The thermal stability of LAPS nanofibers with different LA/PS ratios was evaluated by using simultaneous thermogravimetric analysis (TGA)−DSC technique in nitrogen atmosphere. TGA measures the mass of a sample as a function of temperature or time, which alone is not able to identify certain physical and chemical changes such as glass transition, melting, crystallization, and cross-linking. The simultaneous TGA−DSC technique provides parallel measurement of weight change and true differential heat flow on the same sample. Furthermore, the heat flow is dynamically normalized using the instantaneous sample weight at any given temperature instead of the initial weight, leading to superior accuracy. Figure 3 shows the simultaneous TGA−DSC thermograms of PS yarns and LAPS nanofibers with different LA/PS weight ratios. The pristine PS degraded in one step with nearly 100% weight loss in the temperature range of 350−450 °C (Figure S4A). The pristine LA also underwent a one-step weight loss of 97.8% in the temperature range of 100−300 °C (Figure S4B). In addition to the peaks at 413.6 and 237.8 °C, which were associated with the endothermic decomposition of PS and LA, respectively, the peak at 43.3 °C was assigned to the melting of LA (Figure S4). The PS yarns showed a similar degradation profile as that of pristine PS (Figure 3A). For the LAPS composite nanofibers, two weight-loss stages were observed, which belonged to the decomposition of LA (100−300 °C) and PS (350−450 °C), respectively. With the increase of LA/ PS ratio, the weight loss in the first stage increased, while the weight loss in the second stage decreased accordingly. The percentage of LA in the composite nanofibers could be estimated from the weight loss in the first stage. Specifically, the weight losses in the first stage were 16.6%, 30.6%, 47.2%, 65.7%, 73.6%, and 78.4% for LA0.25PS, LA0.5PS, LA1.0PS, LA2PS,

Figure 3. Simultaneous TGA−DSC thermograms of (A) PS yarns and (B) LA0.25PS, (C) LA0.5PS, (D) LA1.0PS, (E) LA2PS, (F) LA3PS, and (G) LA4PS composite nanofibers.

LA3PS and LA4PS nanofibers, respectively. These measured weigh losses closely matched the calculated weigh ratios of LA in the composite nanofibers, suggesting a uniform distribution of LA throughout the entire nanofibers. With the increase of LA/PS ratio, the peak at around 416 °C, which was assigned to the endothermic decomposition of PS, gradually weakened because of the decreased PS ratio in the composite nanofibers. Meanwhile, the melting peak of LA in the composite nanofibers strengthened and shifted to slightly higher temperatures, which was likely related to the peak broadening effect. The decomposition peak of LA in DSC curves was not strong enough to be distinguishable for LA0.25PS and LA0.5PS nanofibers, indicating a low energy consumption in its degradation. A weak peak at 213.7 °C associated with the LA degradation appeared in the DSC thermogram of LA1.0PS (Figure 3D). This peak increased and slightly shifted to higher temperatures with the increase of LA content in the composite nanofibers. Additionally, the degradation of LA in the composite nanofibers did not affect the thermal properties of the PS matrix, and the onset temperature for PS degradation remained the same (∼395 °C) for all the composite nanofibers. In order to get the best accuracy and precision during DSC experiments, the heat flow was calibrated according to the following equation, which is based on the Tzero theory.40,41 7253

DOI: 10.1021/acssuschemeng.7b01476 ACS Sustainable Chem. Eng. 2017, 5, 7249−7259

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ACS Sustainable Chemistry & Engineering q=−

⎛1 dT ΔT 1⎞ dΔT + ΔT0⎜ − ⎟ + (Cr − Cs) s − Cr Rr Rr ⎠ dt dt ⎝ Rs (1)

ΔT = Ts − Tr

(2)

ΔT0 = T0 − Ts

(3)

where Ts is the sample temperature, Tr is the reference temperature, T0 is the temperature of the center sensor, which is called the Tzero thermocouple, Rs is the thermal resistance of the sample sensor, Rr is the thermal resistance of the reference sensor, Cs is the heat capacity of the sample sensor, and Cr is ΔT the heat capacity of the reference sensor. In eq 1, − R is the r

principal heat flow rate, which represents the traditional heat

(

flow; ΔT0

1 Rs

−

1 Rr

)

dT

and (Cr − Cs) dts are the thermal

resistance and capacitance imbalances between the sample dΔT and the reference, respectively; and −Cr dt is the heating rate imbalance between the sample and the reference. The Tzero technique requires two separate calibrations. The first calibration is to run empty cells and then two sapphire disks with known mass as the sample and the reference to determine the Rs, Rr, Cs, and Cr. The second is to use a known enthalpy standard (i.e., indium) to calibrate the temperature and to determine the cell constant. DSC running in this mode is called T4 mode, which uses all four terms in eq 1, while the traditional DSC running in T1 mode uses only the first term. By further correcting the difference between the sample and the reference pan masses and pan materials to eliminate the slight difference in pan heat capacities and thermal resistances, an enhanced mode T4P, based on T4 mode, was used in our experiments to investigate the heat flow during the heating−cooling cycles. Therefore, this method is more accurate than the traditional technique used in DSC experiments, which shows the real temperature and heat flow instead of the programmed ones. The DSC thermograms of PS yarns and LAPS nanofibers with different weight ratios during the first heating−cooling cycle in the range of 0−80 °C are shown in Figure 4. For easy comparison, the heat flow range (y-axis) was intentionally set to be the same for all the samples. The DSC thermogram of PS yarns was flat and featureless (Figure 4A) because PS is chemically stable and has no physical change below 80 °C, which agreed well with the simultaneous TGA−DSC thermogram. During heating, a unimodal endothermic peak at 44.9 °C was observed for the pristine LA (Figure S5), which was assigned to the melting of LA (Tm). During cooling, the crystallization of LA started at 40.0 °C (Toc), which was even lower than its onset melting temperature (Tom = 43.7 °C). A crystallization process typically consists of two steps, nuclei formation and crystal growth.20 When the melted LA is cooled to a certain temperature where the free enthalpy of the crystal becomes smaller than the free enthalpy of the melt, crystallization occurs if sufficient number of nuclei are present in the melt. However, the melt can be maintained all the way down below its freezing point (or melting point) without becoming a solid if there are not enough nuclei. This is called supercooling effect, in which the crystallization temperature is lower than the melting temperature (Tc < Tm) as evidenced in Figure S5. Supercooling is disadvantageous for efficient thermal energy storage applications because it makes the latent heat to be released below the desired temperature and sometimes heat

Figure 4. DSC thermograms of (A) PS yarns and (B) LA0.25PS, (C) LA0.5PS, (D) LA1.0PS, (E) LA2PS, (F) LA3PS, and (G) LA4PS composite nanofibers during the first heating−cooling cycle.

is released in a broad temperature range.3 As such, a large temperature difference between charging and discharging of thermal energy is required to fully utilize the latent heat. As a first-order transition, crystallization involves an exothermic latent heat. Interestingly, the sample temperature stopped decreasing further after it passed the onset temperature of crystallization (Toc) under continued cooling (10 °C/min), but instead increased back to 42.9 °C, forming an irregular crystallization peak above the Toc. This phase change behavior was different from the previously reported ones in which the heat flow was recorded in T1 mode.10,21,25,26 In traditional T1 mode, the heat flow is plotted against the programmed sample temperature instead of the actual sample temperature as measured in T4 mode.40,41 Therefore, the crystallization peak in T1 mode was always similar to the melting peak with a continuously decreased sample temperature while latent heat was released. Actually, the exothermic crystallization of some materials, especially phase change materials, can be so quick and vigorous that the latent heat released during the crystallization process is simply not able to be dissipated away from the sample cell. Consequently, the sample temperature was increased by the crystallization-generated latent heat back toward the melting point as shown in Figure S5. It should be noted that the crystallization-induced reheating 7254

DOI: 10.1021/acssuschemeng.7b01476 ACS Sustainable Chem. Eng. 2017, 5, 7249−7259

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ACS Sustainable Chemistry & Engineering Table 1. Summary of the Phase Change Properties of LA, PS, and LAPS with Different LA/PS Ratios sample

Toma (°C)

Tm1b (°C)

LA LA0.25PS LA0.5PS LA1.0PS LA2PS LA3PS LA4PS

43.7 42.9 41.8 32.9 32.4 36.5 37.8

44.9 43.9 43.8 44.3 44.6 44.5 45.2

Tm2b (°C)

ΔHmc (J/g)

Tocd (°C)

Tc1e (°C)

37.6 41.6 42.3 42.7

180.2 6.8 34.8 73.6 107.7 128.4 141.3

40.0 39.2 40.4 41.7 42.3 41.6 41.7

41.5 38.7 40.3 41.7 42.1 42.2 42.3

Tc2e (°C) 21.1

34.4

ΔHcf (J/g) 185.2 6.6 36.4 73.2 107.5 129.4 143.0

Tom, onset temperature of melting. Tm1 and Tm2, first and second melting temperature. ΔHm, enthalpy of melting (or fusion). dToc, onset temperature of crystallization. eTc1 and Tc2, first and second crystallization temperature. fΔHc, enthalpy of crystallization. a

b

c

(Figure 4E), resulting from the increased LA amount on or close to the LA2PS nanofiber surface. Furthermore, the crystallization peak at 42.5 °C fell into the range between Tm1 (44.6 °C) and Tm2 (41.6 °C), demonstrating the weakening of the supercooling effect. Moreover, the melted LA in LA2PS nanofibers crystallized at 42.3 °C, and the crystallization process released much latent heat, which raised the sample temperature slightly above the Toc to 42.5 °C. This trend continued for LA3PS and LA4PS nanofibers. Specifically, the sample temperature was raised 0.9 °C for LA3PS and 1.1 °C for LA4PS above their corresponding Toc, indicating that a large amount of latent heat was released during crystallization. In addition, the two melting peaks (Tm1 and Tm2) moved closer to each other to form an overlapped peak (Figure 4F,G), which was probably related to the increased amount of LA on or close to the nanofiber surface. Figure 4G shows a second crystallization peak at 34.4 °C, which was likely related to the second crystallization of melted surface LA. No significant supercooling was observed for LA3PS an LA4PS nanofibers. Table 1 summarizes the phase change properties of pristine PS and LA, as well as LAPS nanofibers with different weight ratios. The enthalpy of samples was calculated according to the area under the peak and the sample mass. The difference between the enthalpy of melting (ΔHm) and the enthalpy of crystallization (ΔHc) for each sample was small and negligible. Both ΔHm and ΔHc increased with increasing LA content in the nanofibers. Specifically, the enthalpy of melting increased more than 20 times from 6.8 J/g for LA0.25PS to 141.3 J/g for LA4PS by increasing LA percentage from 20% to 80%. Meanwhile, the enthalpy of crystallization also increased around 20 times from 6.6 J/g for LA0.25PS to 143.0 J/g for LA4PS. For efficient energy storage applications, it is of critical importance that the materials have a high capacity of thermal energy charging and discharging. Therefore, the latent heat absorbed or released per unit mass (or volume) should be as close to the incorporated phase change materials as possible. To the best of our knowledge, the reported nanomaterials containing PCMs have never achieved more than 50% heat storage capacity of the phase change materials alone. For example, Tian et al. achieved a 15.5% heat storage capacity by incorporating 17.4% octadecane into polyurethane foams.10 Cai et al. reached a 42.8% storage capacity by including 60% LA into polyamide fibers.21 Lin et al. achieved 43.7% capacity by adding 47.4% oilbased PCM into poly(vinyl alcohol) fibers.20 The enthalpy of melting for pristine LA was 180.2 J/g and its enthalpy of crystallization was 185.2 J/g. By incorporation of 50% LA, the LA1.0PS nanofibers achieved a 40.8% heat storage capacity of pristine LA. With an increase of LA content to 67%, the LA2PS nanofibers reached a 59.8% storage capacity of LA. The maximum storage capacity of 78.4% was accomplished by

of samples persisted even after increasing the cooling rate to 50 °C/min. Eventually, the temperature rise ceased when an equilibrium was reached between the released latent heat of the crystallization and the cooling of the system.42 Figure 4B shows a small peak at 43.9 °C during the heating of LA0.25PS nanofibers, which was due to the melting of incorporated LA in the composite nanofibers. The crystallization of LA in LA0.25PS started at 39.2 °C and continued until 21.1 °C. It is well-known that PS is an excellent thermal insulator that is capable of reducing heat transfer significantly. In LA0.25PS nanofibers, PS accounted for 80% of the dry mass and thus was the major component. It is highly likely that the majority of LA, if not all, was encapsulated inside the PS matrix. During cooling, the crystallization-induced exothermic latent heat probably was partially isolated from the environment by PS, leading to a gradual release of heat (Figure 4B, inset). More importantly, the domain size of phase change materials also had an effect on the crystallization process. It was reported that the formation of small domains of phase change materials could decrease the number of nuclei in the melt. This can lead to a delayed start of the crystallization process, namely supercooling, and a broadened temperature range of heat release.43 The broadening effect of the crystallization peak was not observed for the LA0.5PS nanofibers with 33% LA (Figure 4C), which clearly indicated that the domain size of LA played a major role in crystallization process. Also, the supercooling effect was weakened by increasing the LA domain size. Specifically, the difference between Tc and Tm was 3.5 °C for LA0.5PS, while it was 5.2 °C for LA0.25PS. With a further increase of LA to 50%, a second endothermic peak (Tm2) appeared before the major endothermic peak (Tm1) for LA1.0PS nanofibers during heating (Figure 4D). Since both LA and PS accounted for 50% in the LA1.0PS nanofibers, a small portion of LA was probably distributed on or close to the surface of nanofibers, which led to a bimodal melting behavior. It has been extensively studied that small particles on supporting surfaces have a lower melting point than the same bulk material.44 Small particles have a higher proportion of surface atoms or molecules than large particles. These surface atoms or molecules are more weakly bound to the particle than the ones inside the particle and thus less constrained in thermal motion. These small LA domains on or close to the LA1.0PS nanofiber surface melted before the entrapped LA in nanofibers. Because the Tm2 peak was much weaker than the Tm1 peak, the surface LA was expected to be much less than the entrapped LA in LA1.0PS nanofibers. Another important observation was that the supercooling effect was further weakened with the increase of LA domain size, and the melting and crystallization peaks begin to overlap with each other. After increasing LA to 67%, the Tm2 peak moved closer toward the Tm1 peak, and these two peaks partially overlapped 7255

DOI: 10.1021/acssuschemeng.7b01476 ACS Sustainable Chem. Eng. 2017, 5, 7249−7259

Research Article

ACS Sustainable Chemistry & Engineering

Structural Stability of LAPS Composite Nanofibers during Phase Changes. The structural stability of polymer matrix in composite nanofibers is of vital importance to the application of nanofiber-based PCMs. Since LA is converted from solid to liquid phase during thermal energy charging, its structural rigidity disappears after melting. Therefore, the PS matrix served as the frame to support the entire structure. This required the PS phase to be continuous throughout the composite nanofibers without notable weak points along the fibers. Ethanol, a good solvent for LA (∼120 g/100 g at 20 °C) but a nonsolvent for PS, was used to remove LA from LA1.0PS nanofibers through solvent extraction. Figure S6 (curve a) shows the infrared spectrum of LA1.0PS nanofibers after ethanol extraction. The representative carbonyl peak of LA at 1700 cm−1 as well as other LA peaks disappeared from the original LA1.0PS spectrum (Figure 2A, curve d), suggesting a complete removal of LA from the composite nanofibers. Furthermore, the simultaneous TGA−DSC curve of LA1.0PS after ethanol extraction (Figure S7A) only showed the thermal degradation profile of PS, confirming that only PS was left after extraction. Figure 6A shows the SEM image of ethanol-extracted LA1.0PS

LA4PS after incorporating 80% LA. The greatly improved heat storage capacity of LAPS nanofibers could be attributed to the light weight and high porosity of the PS matrix. The DSC thermograms of LA1.0PS during 100 continuous heating−cooling cycles are shown in Figure 5. It is likely that

Figure 5. DSC thermograms of LA1.0PS composite nanofibers during 100 continuous cycles of heating−cooling.

the first heating−cooling cycle redistributed LA in the nanofibers due to the melting of LA. The weak peak at 37.6 °C disappeared after the first circle, probably due to the absorption of surface melted LA into the nanofibers through capillary force.21 Although the melting peak slightly shifted from 45.8 °C for the first cycle to 47.1 °C for the second cycle, their crystallization peaks were essentially the same. Starting from the third cycle, both the melting and crystallization peaks slowly shifted to the lower temperatures. Specifically, the melting and crystallization temperatures down-shifted by 1.0 and 1.8 °C, respectively, for the 100th cycle as compared with the second cycle. After the second heating−cooling cycle, the change of heat flow during the crystallization process became more dramatic than that of the first two cycles. Because of the thermal expansion and contraction of PS as well as the melting and crystallization of LA, open channels might have formed to connect the internal LA to the nanofiber surface. Thus, the crystallization process released the latent heat much more quickly. However, the changes in crystallization heat flow and peak shape did not alter the amount of heat released during crystallization (i.e., ΔHc). The enthalpy of crystallization slightly decreased from 66.2 J/g for the second cycle to 64.6 J/g for the 100th cycle. Meanwhile, the enthalpy of melting decreased from 68.6 to 67.0 J/g. The enthalpies of melting and crystallization at the 10th, 20th, 30th, 40th, 50th, 60th, 70th, 80th, 90th, and 100th heating−cooling cycles are listed in Table S1. Essentially, variations in the enthalpies of melting and crystallization were quite small and negligible. Additionally, as compared with the enthalpy of melting, the corresponding enthalpy of crystallization was slightly smaller, probably resulting from the heat loss during thermal charging and discharging. As such, the LA1.0PS nanofibers retained a high thermal energy storage capacity without notable deterioration during 100 continuous heating−cooling cycles, demonstrating a robust cycling stability and reusability.

Figure 6. SEM images showing electrospun LA1.0PS nanofibers after (A) ethanol extraction and (B) thermal treatment (60 °C, 24 h). Their corresponding fiber size distributions are shown in C and D, respectively. The scale bar in image A applies to B.

nanofibers. The nanofibers were continuous without any observable broken pieces, and there were no identifiable cavities on the fiber surface, demonstrating the continuity of PS phase throughout the entire composite nanofibers. Furthermore, the size of extracted LA1.0PS nanofibers increased to 0.87 from 0.78 μm for the composite nanofibers before ethanol extraction, revealing that the majority of LA was encapsulated inside the nanofibers instead of being deposited on the nanofiber surface. This agreed well with the SEM, Raman, and DSC results. The slight increase of size after extraction was mainly due to the solvent swelling effect to PS matrix. The interior of nanofibers became highly porous as shown in Figure 7A because of the complete removal of LA from the composite nanofibers after ethanol extraction. Also, the nanofibers tended 7256

DOI: 10.1021/acssuschemeng.7b01476 ACS Sustainable Chem. Eng. 2017, 5, 7249−7259

Research Article

ACS Sustainable Chemistry & Engineering

majority of LA was encapsulated inside PS and the PS matrix prevented LA from leaking out of the nanofibers during prolonged and repeated heating above the melting point of LA. Thanks to the lightweight, flexible, porous, and thermally insulating PS matrix and food-grade high-capacity LA, the structurally stabilized LAPS composite nanofibers can be used in broad thermal energy storage applications such as solar energy storage, thermal regulating textiles, and thermal therapy devices.

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CONCLUSIONS In summary, we have demonstrated a facile approach to the fabrication of high-capacity thermal storage membranes through coelectrospinning LA and PS with weight ratios up to 4 vs 1. The obtained LAPS composite nanofibers with LA/ PS ratios from 0.25/1 to 4/1 were uniform in size. The addition of LA into PS first dramatically reduced the fiber size down to 0.78 μm (LA1.0PS) from 1.93 μm (PS yarns) and then gradually increased the size up to 1.30 μm (LA4PS). Furthermore, SEM images and Raman spectra indicated that majority of LA was encapsulated inside the composite nanofibers. Moreover, the XRD patterns suggested that the crystal size of LA domains in the composite nanofibers grew larger by incorporating more LA. The thermal stability of LAPS nanofibers was evaluated by simultaneous TGA−DSC technique. The measured weight loss of LA closely matched the calculated percentage of LA in the composite nanofibers, while the degradation of LA did not affect the thermal properties of PS. It should be noted that the thermal energy storage performance of LAPS nanofibers was measured in T4P mode instead of the traditional T1 mode. Our DSC results showed that the temperature of samples was increased by the crystallization-released latent heat during cooling, which has been rarely reported. Furthermore, the supercooling effect, which was not desired in thermal storage applications, was greatly weakened by increasing LA content in the composite nanofibers. Moreover, our LAPS composite nanofibers achieved a 78.4% storage capacity, which was higher than the previously reported values (