Phase Change Material Fibers

Aug 20, 2015 - Such PCM fibers may be used for the development of thermoregulating fabrics or in passive heat storage devices. ... Novel metal coated ...
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Electrospun Poly(vinyl alcohol)/Phase Change Material Fibers: Morphology, Heat Properties, and Stability Emilija Zdraveva,†,‡ Jian Fang,† Budimir Mijovic,‡ and Tong Lin*,† †

Institute for Frontier Materials, Deakin University, Waurn Ponds VIC3216, Australia Faculty of Textile Technology, University of Zagreb, 10000 Zagreb, Croatia

‡

ABSTRACT: A phase change material (PCM) from a mixture of plant oils was incorporated into electrospun poly(vinyl alcohol) (PVA) nanofibers using an emulsion electrospinning technique. Effects of PCM and PVA content in the emulsions on nanofiber morphology, heat properties, and phase change stability were examined. Higher PCM loadings in the nanofibers led to increased fiber diameter, gouged fiber surfaces, and higher heat enthalpies. The fibers maintained their morphological integrity even if the PCM melted. They showed reliable heat-regulating performance which can undergo at least 100 cycles of phase change. Such PCM fibers may be used for the development of thermoregulating fabrics or in passive heat storage devices.

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INTRODUCTION Phase changing materials (PCMs) function to store latent heat and regulate temperature. They have many practical applications such as for solar energy storing systems,1 heat sinks in electronics,2 air conditioning systems,3 thermal regulating fabrics,4 wound dressing for thermal protection of healthy tissues during cryosurgery of tumor cells,5 or heat biosensors for detecting biological species.6 To perform in a complete shape during phase transition, PCMs have to be encapsulated. PCMs are usually encapsulated on a micro or nano scale, depending on the application. In comparison to the microcapsules, the PCM nanocapsules have smaller size, higher surface area, higher heat transfer rate,7 and lower deformations when a load is applied.8 Recently, electrospinning has been reported as an alternative encapsulation method providing stable PCM fibrous membranes. Electrospinning is the process in which a polymer fluid is drawn under electrostatic forces to form solid fibers with diameters on nano or micro scale.9 It is a cost-effective fibermaking technique with controllability in fiber diameter and versatility in making fibers with multicomponent cross-sectional configurations. Two main approaches have been developed to encapsulate PCMs through electrospinning: (1) coaxial electrospinning to form core−sheath fibers with PCMs as core component and a polymer as protective sheath and (2) single-phase electrospinning of a mixture containing PCMs and polymer to form fibers with PCM dispersed in the polymer matrix. A range of PCM core−polymer sheath fibers have been prepared, such as fibers from short-chain hydrocarbons with titanium dioxidepoly(vinylpyrrolidone) (PVP) (sheath),10 mixture of crystal violet lactone (CVL), bisphenol A and 1-tetradecanol with poly(methyl methacrylate) (PMMA),11 soy wax with polyurethane (PU),12 polyethylene glycol (PEG) with polyvinylidene fluoride (PVDF),13 and dodecane with zein.14 Besides a core− sheath structure with a single core, fibers with a multicore (i.e., islands-in-a-sea fibers) (e.g., hydrocarbons-in-PVP) were also reported.15 Single-phase electrospun fibers were prepared by blending either polymeric or small molecule PCM with a matrix © 2015 American Chemical Society

material. Examples include PEG with poly(dl-lactide) (PLA), cellulose acetate (CA), or polyamide 6,6 (PA 6,6),16 fatty acids or ester derivatives with polyethylene terephthalate (PET),17 dodecane with zein,14 commercial PCM (Rubitherm-RT5) with polycaprolactone (PCL) or polystyrene.18 When PEG was used, it was chemically cross-linked into the matrix.19 Some studies were extended to improving the thermal stability and thermal conductivity by incorporating carbon nanofibers20 or SiO2 nanoparticles21 into PCM-containing electrospun fibers. SiO2 nanoparticles in the fibers were reported to assist in retaining the fibrous shape, thus preventing leakage of PCM when heated.22 In a fibrous material, the fiber morphology and structural stability are important parameters deciding its use performance. Because the phase change transition for most of the PCMs in electrospun fibers is of solid-melt type, melting of the PCMs should have an effect on fiber morphology, which adversely influences the fibrous properties. However, the effect of phase change transition on the morphology of PCM-containing fibers has been less reported.17g,18,22 Understanding the morphological stability during heating would assist in designing durable phase change fibers. In this study, we prepare a PCM-containing nanofiber and systematically examine the effect of heating and PCM content in the nanofibers on fiber morphology, thermal storing capability, thermal mechanical performance, crystalline features, and shape stability. Here, a water-borne polymer, poly(vinyl alcohol) (PVA), was chosen as the matrix material and an oilbased nonparaffin PCM was directly introduced into PVA fiber matrix through an emulsion electrospinning technique. The use of water-soluble polymer avoids using toxic organic solvents, and the PCM selected is from a renewable, nontoxic resource with high heat enthalpy and low volume change. Received: Revised: Accepted: Published: 8706

May 22, 2015 August 7, 2015 August 20, 2015 August 20, 2015 DOI: 10.1021/acs.iecr.5b01822 Ind. Eng. Chem. Res. 2015, 54, 8706−8712

Article

Industrial & Engineering Chemistry Research

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EXPERIMENTAL SECTION Materials. Poly(vinyl alcohol) (98−99% hydrolyzed, Mw = 146 000−186 000), sodium dodecyl sulfate (SDS), Triton X100 and Nile red were purchased from Sigma-Aldrich. Phase change material (a mixture of plant oils with a peak melting point at ∼40 °C) was kindly supplied by Pure Temp - Entropy Solutions, Inc. All chemicals were used as received. Preparation of Oil/Water Emulsion. Emulsion solutions were prepared by adding 15 wt % of the PCM and SDS (8.4 mmol/L) in 70−80 °C water and then mixing by a high-speed shear mixer (Ultra turrax T50 basic, IKA Labrotechnik, maximum speed 10 000 rpm) for 6 min (pulse on 2 min, pulse off 1 min). The oil/water (O/W) emulsions were then sonicated for 5 min with an ultrasonic homogenizer (Omni Sonic Ruptor 400) before mixing with preprepared PVA aqueous solutions. The solutions were then cooled in an ice− water bath and sonicated for 15 min (Sonication bath, DeconF5300b). A small amount of Triton X-100 (0.07−0.2%) was added into the solution for improving spinning ability. Two PVA concentrations, 9 w/w% and 7 w/w%, were used in this study, and the PVA/PCM mass ratios in the solution were controlled at 100/10, 100/30, 100/50, 100/70, and 100/90. The solution conditions and also the resulting fibers are also referred respectively to as PCM-10, PCM-30, PCM-50, PCM70, and PCM-90. Electrospinning. Electrospinning was conducted using a purpose-made electrospinning setup consisting of a highvoltage power supply (ES30P, Gamma High Voltage Research), a syringe pump (KD scientific), and a grounded rotating drum collector (diameter: 12 cm; length: 10 cm). During electrospinning, the applied voltage, flow rate of the emulsion solution, and spinning distance were maintained as 18−20 kV, 0.6 mL/h, and 20 cm, respectively. Randomly oriented fiber mats were prepared. Characterizations. Surface morphology was observed on Zeiss Supra 55VP or Zeiss Leo 1530 scanning electron microscopy (SEM) instruments. The samples were gold coated using a Bal-Tec sputter coater (SCD 050). The fiber diameter was averaged from the measurement of 100 randomly selected fibers using ImageJ 1.48 software. ANOVA one-way and Tukey tests were performed for the mean fiber diameter. The mean populations of p < 0.05 were considered significantly different. For examining the cross-sectional morphology, the fibers were embedded in an epoxy resin and ultrathin slides were prepared on an Ultramicrotome (Leica, UC-6). Confocal imaging was taken on a Leics SP5 confocal microscope with DPSS 561 laser, dry objective, and the settings of excitation at 553 nm and emission at 530 nm. The samples were prepared by adding a trace amount of fluorescent dye into the oil phase of the emulsion before electrospinning. Transmission electron microscopy (TEM) images were taken with a JEOL 2100 transmission electron microscope at an accelerating voltage of 200 kV. Phase change behavior was investigated using differential scanning calorimetry (DSC) results on a TA Instruments DSC (Q200). The samples were tested in a cyclic mode (10 or 100 cycles of heating and cooling) from 0 to 80 °C with a heating rate of 10 °C/min in nitrogen atmosphere at a flow rate of 50 mL/min. Prior to each testing the samples were heated in a ramp mode from −20 to 80 °C with isothermal state for 5 min at the final temperature to erase any thermal history. Final curves were given from the last cycle. The heats of fusion and crystallization were calculated from the

areas below the peaks using the TA Universal Analysis software. Thermal stability of the electrospun mats was examined on a TA Instruments Q50 in a temperature range of 30−700 °C. The heating rate was controlled at 10 °C/min in nitrogen atmosphere at a flow rate of 50 mL/min for balance gas and 60 mL/min for sample gas. Crystal structure of the fibers was examined by X-ray diffraction analysis (XRD) using a diffractometer (Panalytical X’Pert Powder XRD) with Cu radiation of 1.54 Å. The samples were scanned in the 2θ range of 5−45° with the step size of 0.05°. Thermomechanical analysis of the electrospun nanofiber mats was conducted on a dynamic mechanical analysis in a single-frequency tension mode. Nanofiber mats were cut into a rectangular shape of 5 × 20 mm2, and their thickness was measured by a Mitutoyo micrometer. The samples were tested in the temperature range of −15 to 150 °C, with a heating rate of 3 °C/min under liquid nitrogen at 1 Hz frequency. The tensile properties were measured on a Universal Testing Machine (Zwick/Röel UTM 1445) using the 200 N load cell at a crosshead rate of 10 mm/ min. The samples were cut into 5 × 30 mm2 pieces and tested in triplicate.

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RESULTS AND DISCUSSION Fiber Morphology and PCM Distribution. Figure 1 shows representative SEM images of nanofibers electrospun

Figure 1. SEM images of electrospun nanofibers prepared from 9 w/w % PVA solutions: (a) pure PVA, (b) PCM-10, (c) PCM-30, (d) PCM50, and (e) PCM-70.

from a PVA solution and PCM/PVA emulsions. Pure PVA nanofibers electrospun from 7 w/w% and 9 w/w% PVA solutions were uniform with a smooth surface and a circle cross-sectional morphology. The fibers electrospun from the PCM-containing emulsion had a rough surface, though they were uniform in morphology as well. With increasing PCM content, the fiber surface became rougher. When the PCM content was low, in the range of 10% ∼ 30% (based on mass ratio with PVA), the fibers showed a circular cross section and the surface was almost smooth, except for voids scattered on the surface. The random pores were attributed to the detachment of PCM particles from the area due to the low PCM−PVA adhesion. When the PCM content was higher than 50% (based on mass ratio with PVA), the fiber surface became rougher and the cross section turned irregular. At the maximum PCM content (PCM-90 in the case of 7w/w% PVA), thinner fibers with occasional cluster forms and branches resulted. This was presumably due to high viscosity of the emulsion. The high PCM content also led to interconnection of the PCM particles in the PVA matrix. 8707

DOI: 10.1021/acs.iecr.5b01822 Ind. Eng. Chem. Res. 2015, 54, 8706−8712

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Industrial & Engineering Chemistry Research Based on the SEM images, the fiber average diameter was calculated. Figure 2 shows the mean fiber diameter of the

Figure 2. Average fiber diameter of the electrospun PVA and PVA/ PCM fibers. (Whiskers indicate SD, and boxes represent SE).

Figure 3. Confocal image of fibers electrospun from PCM-50 solution (from 9 w/w% PVA) containing a small amount of Nile red (scale bar of 10 μm).

electrospun fibers. The ANOVA one-way analysis showed mean diameter significantly different (at the level of p < 0.05) between all samples, and the Tukey test further confirmed significant mean difference, with an exception between the PCM-50 and PCM-90 samples in the case of the 7 w/w% PVA. The presence of PCM in PVA fiber increased the fiber diameter. With increasing PCM content, the fiber diameter increased until the PCM content was larger than 70% (based on mass ratio with PVA). For the fibers prepared from 7 w/w% PVA, increasing the PCM content to 70% (based on PVA) resulted in the fiber diameter increasing from 228.18 ± 42.41 to 634.66 ± 112.36 nm. When 9 w/w% PVA emulsions were electrospun, increasing the PCM content from 0 to 70% (based on PVA) resulted in increasing the fiber diameter from 360.63 ± 51.14 to 760.18 ± 118.86 nm. When the PCM content was increased to 90% (based on PVA), the fiber diameter decreased to 539.54 ± 224.75 nm, similar to the diameter of those electrospun from the emulsion containing the same content of PVA but 50% of PCM (based on PVA). This can be explained by some PCM particles forming clusters, which separated from the suspension. Based on the fiber average diameter, 44% PCM was separated from the fibers during electrospinning. Figure 3 shows the confocal image of PCM-containing nanofibers. Here a small amount of Nile red was added into the oil phase of the electrospinning solution. Discontinuous red segments were observed along the fibers, confirming the presence of PCM. Figure 4 shows the TEM image of PCM-50. Most of the PCMs contained in the fibers were spherical particles (Figure 4a). They swelled when the fibers were immersed in acetone. Only a few fibers showed a core−sheath structure (Figure 4b). Figure 4c shows the cross-sectional SEM image of the PCM/ PVA fibers (PCM-70). As expected, the fiber showed pertrusions and voids. After heat treatment in a 60 °C oven (5 min in, 5 min out equivalent to one round) for 10 rounds, continuous channels formed in the sample which came from the merging of the PCM during heat treatment (Figure 4d). Thermoregulating Behavior. Figure 5a shows the DSC results of 9 w/w% PVA/PCM fibers. The plotted peaks were acquired from the tenth heating−cooling cycle. During heating, a unimodal endothermic peak (curve down) was observed for all PCM loadings, assigned to the melting of PCM. During cooling, multiple temperature peaks occurred, suggesting a multiple crystallization process (inset graph in Figure 5a), which is not the case for the bulk PCM material. Table 1 gives

Figure 4. TEM images of PCM-50 fibers electrospun from 9 w/w% emulsion with (a) discontinuous PCM phase and (b) continuous PCM phase; cross-sectional SEM images of PCM-70 (from 9 w/w% PVA) (c) before and (d) after heat treatment.

the onset and peak melting (Tom, Tm) and crystallization (Toc, Tc) temperatures for the fibers electrospun from 9 w/w% PVA emulsions. Compared to pure PCM, the onset and peak melting and crystallization temperatures decreased for all PCM loaded fibers, except for the sample with the lowest PCM loading (in the case of Toc, Tc1) which showed the opposite trend. Generally, the change in temperatures resulted from the effect of the surrounding matrix. A crystallization process typically comprises two stages: nuclei formation and crystal growth. The size of PCM domains was reported to have an effect on the nucleation process, and a smaller PCM domain led to a decrease in the number of stable nuclei necessary for the crystallization to start.23 As a result, supercooling effect, which shows a temperature difference in the onset or peak during melting and crystallization, could take place for PCMs. In our case, the bulk PCM material at the heating−cooling rate of 10 °C/min performed with a supercooling effect of ∼4 °C and ∼10 °C for the onset and peak temperatures, respectively. For the PCM-loaded fibers, the supercooling decreased in both onset and first peak crystallization, whereas for the second crystallization peak, the 8708

DOI: 10.1021/acs.iecr.5b01822 Ind. Eng. Chem. Res. 2015, 54, 8706−8712

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Industrial & Engineering Chemistry Research

Figure 5. (a) DSC thermograms of the 9 w/w% PVA/PCM fibers after 10 cycles of heating−cooling and inset graph of the multiple crystallization peaks; (b) DSC thermograms of the PCM-70 fibers (from PVA 7w/w%) after 100 cycles of heating−cooling.

enthalpy showed a similar variation with the PCM content. The consecutive heat-and-cooling cycles showed negligible change of the enthalpy value. After 100 cycles of heat and cooling, the melting and crystallization enthalpies for the fibers from 7 w/w % PVA (PCM-70) were 79.48 J/g and 78.76 J/g, respectively, which decreased less than 1 J/g compared to the results of the tenth cycle. Crystalline Change. Figure 6 shows the XRD patterns of PVA/PCM fibers at the lowest and the highest PCM loadings.

Table 1. Onset and Peak Temperatures of Melting and Crystallization sample

Tom

Tm (°C)

Toc

Tc1 (°C)

Tc2 (°C)

Tc3 (°C)

Tc4 (°C)

raw PCM PCM-10 (9 w/w% PVA) PCM-30 (9 w/w% PVA) PCM-50 (9 w/w% PVA) PCM-70 (9 w/w% PVA)

36.65 34.81

40.34 37.29

33.08 33.30

30.05 32.50

− 29.68

− 26.89

− 22.81

35.02

37.95

32.01

31.26

26.48

−

−

35.09

38.25

32.39

32.00

25.72

−

−

35.29

38.46

32.83

32.21

26.51

−

−

supercooling firmly increased. The highest supercooling (∼15 °C) was observed on the fibers with the lowest PCM loading. This can be explained by fibers with lower PCM loading possibly having smaller PCM size and hence increase the supercooling effect. After 100 cycles of heating and cooling, the fibers still maintained the thermal regulating feature (Figure 5b), confirming the good cycling stability (representative samples PCM-70 from 7 w/w% PVA). The heat enthalpy of the nanofibers was calculated based on the area under the peak. As shown in Table 2, both melting and crystallization enthalpies increased with increasing PCM content in the fibers. With the variation of PCM content, the melting enthalpies changed from 17.81 to 84.41 J/g and 18.77 to 84.58 J/g for the fibers prepared from 9 w/w% PVA emulsions, and from 15.88 to 96.01 J/g and 15.68 to 96.88 J/g for those from 7w/w% PVA, respectively. The crystallization

Figure 6. XRD patterns of the PVA/PCM fibers (electrospun from 9 w/w% PVA emulsions) before and after 10 heat−cool cycles.

The samples were exposed at 60 °C in a convective oven. The exposure was for 10 rounds, where 5 min in the oven and 5 min at room temperature was equivalent to one round. The strong peak at 2θ = 19.4° was assigned to the (101) plane of PVA.24 The intensity of (101) diffraction reduced with the increase in the PCM content in the fibers. This change was highest for the highest PCM loading, indicating that the crystallinity of the polymer was affected by its presence. The crystalline structure of the natural plant PCM was observed with the sharp peaks at 7.5°, 11.1°, 15.0°, 21.0°, 22.0°, and 24.0°. A slight increase in the intensities was observed as the PCM loading progressed. This confirmed good crystallinity of the PCM in the fibers. The clear matrix and PCM peaks indicate their immiscibility. After heat−cool treatment, the PCM characteristic peaks increased in intensity, and the intensity increase became more obvious for higher PCM loading. This can be explained by the fact that small PCM loading is kept firmly in a discontinuous state. For the fibers with higher PCM loadings, PCM particles tended to merge into a continuous network. This resulted in higher matrix crystallinity, indicated by the increase in its main peak, thus indicating phase separation.

Table 2. Melting and Crystallization (Cycles 1 and 10) Enthalpy of PVA/PCM Fibers sample 9 w/w% PVA

7 w/w%PVA

raw PCM PCM-10 PCM-30 PCM-50 PCM-70 PCM-10 PCM-30 PCM-50 PCM-70 PCM-90

Hm1 (J/g)

Hm10 (J/g)

Hc1 (J/g)

Hc10 (J/g)

221.2 17.96 48.70 73.41 84.72 15.88 44.05 65.08 80.43 96.58

221.5 17.81 48.06 72.37 84.41 15.88 43.35 63.75 79.92 96.01

221.8 18.18 48.78 71.89 83.83 15.58 43.89 64.80 79.29 96.19

221.5 18.77 49.27 72.34 84.58 15.68 43.47 63.62 78.77 96.88 8709

DOI: 10.1021/acs.iecr.5b01822 Ind. Eng. Chem. Res. 2015, 54, 8706−8712

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Industrial & Engineering Chemistry Research

Figure 7. (a) Storage modulus and (b) loss tangent (tan δ) of PVA/PCM fibers (from 9 w/w% PVA).

Tensile Properties. Figure 8 shows the stress−strain curves of representative PCM-10 and PCM-70 (both from 9 w/w%

Dynamic Mechanical Temperature Behavior. Figure 7a shows the storage modulus (E′) of the pure 9 w/w% PVA and PVA/PCM fibers before and after heat−cool treatment (conditions same as in previous section) in the temperature range of −15 to 150 °C. It presents the results of the lowest and highest PCM loading in fibers. When the temperature is increased up to 80 °C, the modulus (E′) decreased gradually, followed by a sharp decrease in E′ with further increase the temperature. The first stage was assigned to the glassy state, while the second presented further relaxation.25 The additional step was observed at around 40 °C, which was associated with the melting of the PCM. The addition of the PCM to PVA fibers apparently decreased the modulus, by approximately up to 60%. This was attributed to the poor PCM−PVA interaction26 and the uneven dispersion of PCM particles in the fibers. This is different from the core−sheath electrospun soy wax/PU fibers, in which an increased storage modulus with increasing PCM loading was reported, which was explained by the good dispersion of PCM in the fibers.12b After heat−cool treatment, the storage modulus was obviously decreased, with a deeper step around the melting point for the highest PCM loading. It was interesting to note that the fibers with the lowest PCM loading had no storage modulus step at the phase change temperature and the storage modulus after heat−cool treatment was slightly decreased. This suggests that the PCM content is too small to affect the storage modulus of the fibers. Figure 7b shows the damping or the system energy dissipation (tan δ). Two peaks were observed ,with the first one at around 5−10 °C, corresponding to the relaxation in the polymer amorphous states,27 and the second peak at 101.34 °C (with an onset Tg at approximately 72.80 °C), which represented the relaxation in the crystalline states of PVA.27b In comparison with some literature reports,27b the first peak shifted to lower temperature, presumably because of the absorption of moisture, as in accordance with the TGA results. The damping increased in the whole temperature region with increasing PCM loading for the control samples. This suggests that the PCM particles enhance the matrix chain movement, thus decreasing the storage modulus. For the heat−cool treated fibers, the damping decreased at its maximum peak, especially for fibers with the lowest PCM content. The peak around the solid−liquid transition was higher for the heat−cool treated fibers with the highest PCM loading. For the lowest PCM loading, the peak was broader, lower, and shifted toward 38 °C, as acquired by DSC, closer to its melting point. Finally, fibers with lower PCM loading showed better thermomechanical behavior.

Figure 8. Stress−strain curves of PVA/PCM fibers (9 w/w% PVA).

PVA) before and after cyclic heat−cool treatment (at 60 °C, conditions same as in previous sections). After cyclic heat−cool treatment, the system with the lower PCM loading showed increased strength and break strain, while the fibers with higher PCM content showed an obvious decrease in tensile strength but increase in break strain. This is presumably because the PCM at a low content has a uniform distribution within PVA fibers. Stability of PCM in PVA Nanofibers upon Heat Treatment. To examine the PCM stability in PVA fibers, the PVA/PCM nanofibers were exposed to different temperatures and durations in a standard convective oven. The temperature ranged from 50 to 80 °C, which is above the melting point of the PCM. The time durations were 5 min, 15 min, 30 min, 1 h, and 2 h. Further, repeated heating and cooling (total of 10 cycles) was performed at 60 °C at the same conditions as in previous sections. When treatment was performed at lower temperatures (50 and 60 °C) for up to 2 h, pores were still identified on the fiber surfaces (Figure 9a). At higher temperature (70 and 80 °C), the fibers showed a smoother surface even if exposed for shorter time, indicating that the PCM fully covered the fibers surface (Figure 9b). Representative samples are given from the PVA 7w/w%. For the fibers from 9 w/w% PVA emulsion, fiber morphology was still maintained after 10 cycles of heat and cooling treatment (Figure 9c,d). With increased PCM loading, the number of pores on the surface increased, indicating PCM leaking. The PCM typically has weak interaction with PVA because of the mismatch of the polarity between PCM and PVA. As a result, PCM tends to emigrate from the hydrophilic matrix 8710

DOI: 10.1021/acs.iecr.5b01822 Ind. Eng. Chem. Res. 2015, 54, 8706−8712

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Industrial & Engineering Chemistry Research

Figure 11. TGA curves of PVA/PCM fibers prepared from 9 w/w% PVA emulsions; inset graph of weight loss in the low-temperature region.

weight loss of ∼6% for the pure PVA was assigned to water evaporation. With increasing PCM content, this was diminished because of the decrease in the moisture absorptive capacity.

Figure 9. Heat treatment of PCM-70 (PVA 7w/w%) at (a) 60 °C for 2 h, (b) at 70 °C for 30 min; heat treatment of (c) PCM-30 (PVA 9w/ w%) and (d) PCM-70 (PVA 9w/w%) at 60 °C for 10 cycles (5 min in the oven, 5 min at room temperature).

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CONCLUSIONS PCM/PVA fibers have been prepared by an emulsion electrospinning technique. The PCM shows random distribution in the nanofibers. Higher PCM loading in the fibers leads to increased heat enthalpies, gouged fiber surface, and an irregular cross section. Upon heat treatment, the fiber integrity is maintained even if the PCM melts. The PCM/PVA fibers show reliable thermoregulating performance. Further work on the stabilization of the matrix and optimization of the PCM loadings, to prevent leakage, may lead to novel fibrous phase change materials for various applications.

when heat is applied. Under optical microscopy, big PCM particles were observed to either stack on the fibers surface or fall between the fibers (Figure 10). This became noticeable at higher PCM loadings.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Gil, A.; Oró, E.; Peiró, G.; Á lvarez, S.; Cabeza, L. F. Material selection and testing for thermal energy storage in solar cooling. Renewable Energy 2013, 57, 366. (2) Tan, F. L.; Tso, C. P. Cooling of mobile electronic devices using phase change materials. Appl. Therm. Eng. 2004, 24, 159. (3) Diaconu, B. M.; Varga, S.; Oliveira, A. C. Experimental assessment of heat storage properties and heat transfer characteristics of a phase change material slurry for air conditioning applications. Appl. Energy 2010, 87, 620. (4) Bendkowska, W.; Wrzosek, H. Experimental study of the thermoregulating properties of nonwovens treated with microencapsulated PCM. Fibres Text. East. Eur. 2009, 17, 87. (5) Lv, Y.; Zou, Y.; Yang, L. Feasibility study for thermal protection by microencapsulated phase change micro/nanoparticles during cryosurgery. Chem. Eng. Sci. 2011, 66, 3941. (6) Wang, C.; Hossain, M.; Ma, L.; Ma, Z.; Hickman, J. J.; Su, M. Highly sensitive thermal detection of thrombin using aptamerfunctionalized phase change nanoparticles. Biosens. Bioelectron. 2010, 26, 437. (7) Liu, C.; Rao, Z.; Zhao, J.; Huo, Y.; Li, Y. Review on nanoencapsulated phase change materials: Preparation, characterization and heat transfer enhancement. Nano Energy 2015, 13, 814. (8) Sukhorukov, G.; Fery, A.; Möhwald, H. Intelligent micro- and nanocapsules. Prog. Polym. Sci. 2005, 30, 885. (9) Bhardwaj, N.; Kundu, S. C. Electrospinning: a fascinating fiber fabrication technique. Biotechnol. Adv. 2010, 28, 325.

Figure 10. Optical image of the electrospun PCM-50 (PVA 9 w/w%) fibers.

Thermal Stability. Figure 11 shows TGA curves of PCM/ PVA fibers. The presence of PCM in the fibers affected the onset temperature of weight loss. The onset temperature for the PCM-containing fibers was very similar to each other regardless of the PCM content. However, it was different from that of pure PVA. The decomposition of the PCM/PVA fibers was divided into three zones. The first step is almost equal to the weight fraction of the PCMs in the matrices, with nearly 45% for the highest loading system. The second decomposition zone was above ∼250 °C, related to the degradation of the polymer. The final weight loss was at a temperature above 400 °C, resulting in the least residue left of ∼5%. For pure PCM, it degraded in one-step with nearly 100% of weight loss, assigned to evaporation.28 The inset graph presents the weight loss in the low-temperature region, up to 100 °C. This decay in the 8711

DOI: 10.1021/acs.iecr.5b01822 Ind. Eng. Chem. Res. 2015, 54, 8706−8712

Article

Industrial & Engineering Chemistry Research (10) McCann, J. T.; Marquez, M.; Xia, Y. Melt coaxial electrospinning: a versatile method for the encapsulation of solid materials and fabrication of phase change nanofibers. Nano Lett. 2006, 6, 2868. (11) Li, F.; Zhao, Y.; Wang, S.; Han, D.; Jiang, L.; Song, Y. Thermochromic core−shell nanofibers fabricated by melt coaxial electrospinning. J. Appl. Polym. Sci. 2009, 112, 269. (12) (a) Hu, W.; Yu, X. Encapsulation of bio-based PCM with coaxial electrospun ultrafine fibers. RSC Adv. 2012, 2, 5580. (b) Hu, W.; Yu, X. Thermal and mechanical properties of bio-based PCMs encapsulated with nanofibrous structure. Renewable Energy 2014, 62, 454. (13) (a) Nguyen, T. T. T.; Lee, J. G.; Park, J. S. Fabrication and characterization of coaxial electrospun polyethylene glycol/polyvinylidene fluoride (Core/Sheath) composite non-woven mats. Macromol. Res. 2011, 19, 370. (b) Van Do, C.; Nguyen, T. T. T.; Park, J. S. Fabrication of polyethylene glycol/polyvinylidene fluoride core/shell nanofibers via melt electrospinning and their characteristics. Sol. Energy Mater. Sol. Cells 2012, 104, 131. (14) Pérez-Masiá, R.; López-Rubio, A.; Lagarón, J. M. Development of zein-based heat-management structures for smart food packaging. Food Hydrocolloids 2013, 30, 182. (15) Wang, N.; Chen, H.; Lin, L.; Zhao, Y.; Cao, X.; Song, Y.; Jiang, L. Multicomponent phase change microfibers prepared by temperature control multifluidic electrospinning. Macromol. Rapid Commun. 2010, 31, 1622. (16) (a) Chen, C.; Liu, K.; Wang, H.; Liu, W.; Zhang, H. Morphology and performances of electrospun polyethylene glycol/ poly (dl-lactide) phase change ultrafine fibers for thermal energy storage. Sol. Energy Mater. Sol. Cells 2013, 117, 372. (b) Chen, C.; Wang, L.; Huang, Y. Electrospinning of thermo-regulating ultrafine fibers based on polyethylene glycol/cellulose acetate composite. Polymer 2007, 48, 5202. (c) Chen, C.; Wang, L.; Huang, Y. Electrospun phase change fibers based on polyethylene glycol/ cellulose acetate blends. Appl. Energy 2011, 88, 3133. (17) (a) Chen, C.; Wang, L.; Huang, Y. Morphology and thermal properties of electrospun fatty acids/polyethylene terephthalate composite fibers as novel form-stable phase change materials. Sol. Energy Mater. Sol. Cells 2008, 92, 1382. (b) Yibing, C.; Huizhen, K.; Tingting, Z.; Ju, D.; Hui, Q.; Honglei, W.; Zhangfei, X.; Qufu, W.; Yong, Z.; Hao, F. Preparation, Morphology and Properties of Electrospun Laurie Acid/ PET Form-Stable Phase Change Ultrafine Composite Fibres. Polym. Polym. Compos. 2011, 19, 773. (c) Chen, C.; Wang, L.; Huang, Y. A novel shape-stabilized PCM: Electrospun ultrafine fibers based on lauric acid/polyethylene terephthalate composite. Mater. Lett. 2008, 62, 3515. (d) Ke, H.; Cai, Y.; Wei, Q.; Xiao, Y.; Dong, J.; Hu, Y.; Song, L.; He, G.; Zhao, Y.; Fong, H. Electrospun ultrafine composite fibers of binary fatty acid eutectics and polyethylene terephthalate as innovative form-stable phase change materials for storage and retrieval of thermal energy. Int. J. Energy Res. 2013, 37, 657. (e) Cai, Y.; Ke, H.; Lin, L.; Fei, X.; Wei, Q.; Song, L.; Hu, Y.; Fong, H. Preparation, morphology and thermal properties of electrospun fatty acid eutectics/polyethylene terephthalate form-stable phase change ultrafine composite fibers for thermal energy storage. Energy Convers. Manage. 2012, 64, 245. (f) Ke, H.; Li, D.; Zhang, H.; Wang, X.; Cai, Y.; Huang, F.; Wei, Q. Electrospun form-stable phase change composite nanofibers consisting of capric acid-based binary fatty acid eutectics and polyethylene terephthalate. Fibers Polym. 2013, 14, 89. (g) Cai, Y.; Gao, C.; Xu, X.; Fu, Z.; Fei, X.; Zhao, Y.; Chen, Q.; Liu, X.; Wei, Q.; He, G.; Fong, H. Electrospun ultrafine composite fibers consisting of lauric acid and polyamide 6 as form-stable phase change materials for storage and retrieval of solar thermal energy. Sol. Energy Mater. Sol. Cells 2012, 103, 53. (h) Chen, C.; Wang, L.; Huang, Y. Ultrafine electrospun fibers based on stearyl stearate/polyethylene terephthalate composite as form stable phase change materials. Chem. Eng. J. 2009, 150, 269. (i) Chen, C.; Liu, S.; Liu, W.; Zhao, Y.; Lu, Y. Synthesis of novel solid−liquid phase change materials and electrospinning of ultrafine phase change fibers. Sol. Energy Mater. Sol. Cells 2012, 96, 202. (j) Ke, H.; Li, D.; Wang, X.; Wang, H.; Cai, Y.; Xu, Y.; Huang, F.; Wei, Q. Thermal and mechanical properties of nanofibers-

based form-stable PCMs consisting of glycerol monostearate and polyethylene terephthalate. J. Therm. Anal. Calorim. 2013, 114, 101. (18) Chalco-Sandoval, W.; Fabra, M. J.; López-Rubio, A.; Lagaron, J. M. Electrospun heat management polymeric materials of interest in food refrigeration and packaging. J. Appl. Polym. Sci. 2014, 131. DOI: 10.1002/app.40661 (19) (a) Chen, C.; Wang, L.; Huang, Y. Crosslinking of the electrospun polyethylene glycol/cellulose acetate composite fibers as shape-stabilized phase change materials. Mater. Lett. 2009, 63, 569. (b) Seifpoor, M.; Nouri, M.; Mokhtari, J. Thermo-regulating nanofibers based on nylon 6, 6/polyethylene glycol blend. Fibers Polym. 2011, 12, 706. (20) Cai, Y.; Zong, X.; Zhang, J.; Du, J.; Dong, Z.; Wei, Q.; Zhao, Y.; Chen, Q.; Fong, H. The Improvement of Thermal Stability and Conductivity via Incorporation of Carbon Nanofibers into Electrospun Ultrafine Composite Fibers of Lauric Acid/Polyamide 6 Phase Change Materials for Thermal Energy Storage. Int. J. Green Energy 2014, 11, 861. (21) Cai, Y.; Ke, H.; Dong, J.; Wei, Q.; Lin, J.; Zhao, Y.; Song, L.; Hu, Y.; Huang, F.; Gao, W.; Fong, H. Effects of nano-SiO2 on morphology, thermal energy storage, thermal stability, and combustion properties of electrospun lauric acid/PET ultrafine composite fibers as form-stable phase change materials. Appl. Energy 2011, 88, 2106. (22) Nguyen, T. T. T.; Park, J. S. Fabrication of electrospun nonwoven mats of polyvinylidene fluoride/polyethylene glycol/fumed silica for use as energy storage materials. J. Appl. Polym. Sci. 2011, 121, 3596. (23) (a) Zhang, X.; Fan, Y.; Tao, X.; Yick, K. Fabrication and properties of microcapsules and nanocapsules containing n-octadecane. Mater. Chem. Phys. 2004, 88, 300. (b) Tang, X.; Li, W.; Shi, H.; Wang, X.; Wang, J.; Zhang, X. Fabrication, characterization, and supercooling suppression of nanoencapsulated n-octadecane with methyl methacrylate−octadecyl methacrylate copolymer shell. Colloid Polym. Sci. 2013, 291, 1705. (24) (a) Nakane, K.; Yamashita, T.; Iwakura, K.; Suzuki, F. Properties and structure of poly(vinyl alcohol)/silica composites. J. Appl. Polym. Sci. 1999, 74, 133−138. (b) Assender, H. E.; Windle, A. H. Crystallinity in poly(vinyl alcohol). 1. An X-ray diffraction study of atactic PVOH. Polymer 1998, 39, 4295. (25) Peresin, M. S.; Habibi, Y.; Zoppe, J. O.; Pawlak, J. J.; Rojas, O. J. Nanofiber composites of polyvinyl alcohol and cellulose nanocrystals: manufacture and characterization. Biomacromolecules 2010, 11, 674. (26) Jose, M. V.; Thomas, V.; Johnson, K. T.; Dean, D. R.; Nyairo, E. Aligned PLGA/HA nanofibrous nanocomposite scaffolds for bone tissue engineering. Acta Biomater. 2009, 5, 305. (27) (a) Nishio, Y.; Manley, R. S. J. Cellulose-poly(vinyl alcohol) blends prepared from solutions in N,N-dimethylacetamide-lithium chloride. Macromolecules 1988, 21, 1270. (b) Naebe, M.; Lin, T.; Staiger, M. P.; Dai, L.; Wang, X. Electrospun single-walled carbon nanotube/polyvinyl alcohol composite nanofibers: structure−property relationships. Nanotechnology 2008, 19, 305702. (28) Yu, S.; Wang, X.; Wu, D. Microencapsulation of n-octadecane phase change material with calcium carbonate shell for enhancement of thermal conductivity and serving durability: Synthesis, microstructure, and performance evaluation. Appl. Energy 2014, 114, 632.

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DOI: 10.1021/acs.iecr.5b01822 Ind. Eng. Chem. Res. 2015, 54, 8706−8712