- Email: [email protected]
cellulose acetate blends
Applied Energy 88 (2011) 3133–3139
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Electrospun phase change fibers based on polyethylene glycol/cellulose acetate blends Changzhong Chen a, Linge Wang b,⇑, Yong Huang c,d,⇑ a
School of Chemistry and Pharmacy Engineering, Nanyang Normal University, Nanyang 473061, China Department of Biomedical Science, University of Sheffield, Sheffield S10 2TN, UK c Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, Beijing 100190, China d State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China b
a r t i c l e
i n f o
Article history: Received 29 November 2010 Received in revised form 19 February 2011 Accepted 21 February 2011 Available online 14 April 2011 Keywords: Electrospinning Morphology PEG Phase change fiber Thermal properties
a b s t r a c t Ultrafine phase change fibers based on polyethylene glycol (PEG)/cellulose acetate (CA) blends in which PEG acts as a model phase change material (PCM) and CA acts as a supporting material, were successfully prepared via electrospinning. The effect of PEG content on the morphology, crystalline properties, phase change behaviors and tensile properties of the composite fibers was studied systematically by field-emission scanning electron microscopy (FE-SEM), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and a tensile tester, respectively. The SEM observation indicates that maximum PEG content in the fibers could reach up to 70 wt%, and the morphology and average diameter of the composite fibers vary with PEG content. Thermal analysis results show that the latent heats of the phase change fibers increase with the increasing of PEG content in the fibers, and the PEG/CA fibers with high enthalpies have a good capability to regulate their interior temperature as the ambient temperature alters. Therefore, the developed phase change fibers have enormous applicable potentials in thermal energy storage and temperature regulation. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Phase change fibers, also called thermo-regulating fibers or temperature-adaptable fabrics, have been extensively studied [1– 7] for thermal energy storage and temperature regulation as high performance nonwoven fabrics and coatings. Generally, phase change fibers work through energy storage and release of PCMs in the fibers during the phase change process as the ambient temperature alters. As renewable and clean energy storage materials, phase change fibers have been studied for several decades. As early as in 1980s, Vigo and Frost [1,2] prepared temperature-adaptable fabrics by immersing the hollow fibers into PCM solution such as hydrated inorganic salts solution and PEG aqueous solution. Though these fabrics based on PCMs could impart desirable thermal storage and release properties, they exhibited unreliable and poor thermal behaviors on repeated thermal cycles. Thus, other approaches have been developed to improve the performance of the thermo-regu-
⇑ Corresponding authors. Addresses: Department of Biomedical Science, University of Sheffield, Firth Court, Western Bank, Sheffield, S10 2TN, UK. Tel.: +44(0)1142224662; fax: +44(0)1142222787 (L. Wang), Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, Beijing 100190, China. Tel.: +86(0)1082543478; fax: +86(0)1062554670 (Y. Huang). E-mail addresses: [email protected] (L. Wang), [email protected] (Y. Huang). 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.02.026
lating fibers such as microencapsulation technique of PCM [3,4]. However, the surface coated fabrics became stiffer and less smooth as the add-on microPCM increased, and then the total hand value which determines the tactile comfort perceived by humans decreased. Zhang et al. prepared different thermo-regulating fibers by embed microPCM into the fibers via melt-spinning [5] and wet-spinning [6], respectively. In these methods, microPCMs were incorporated into fibers or fabrics, thus decreasing the weight percent of encapsulated PCMs and the corresponding enthalpies of the fibers. Hu and co-workers [7] copolymerized PEG with polyethylene terephthalate (PET), and melt-spun the phase change fibers of the copolymer, which have solid–solid phase change characteristics at a certain temperature range without obvious liquid substance appearing. The phase change temperature range and the enthalpy of these spun thermo-regulating fibers could be adjusted by controlling the molecular weight and the corresponding proportion of PCMs added. It is a pity that only a few types of thermo-regulating fibers could be made by these spinning methods because of their poor processability for many polymers and PCMs. In order to overcome the disadvantages mentioned above, electrospinning is introduced to prepared phase change fibers. Electrospinning [8,9] is a simple, convenient, and versatile technique for generating ultrafine fibers from a wide variety of polymers, polymer blends and nanoparticle-impregnated polymers. With their ultrafine size and huge surface-to-volume ratio, electrospun fibers can be applied in numerous areas [10] such as
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Fig. 1. Images of electrospun PEG/CA fibers collected by: (a) aluminum foil and (b) rotating drum.
Fig. 2. SEM images of: (a) electrospun CA fibers and (b) PEG particles.
healthcare, biotechnology, environmental engineering, and energy storage. McCann et al. [11] fabricated phase change nanofibers consisting of long chain hydrocarbon and TiO2–PVP composite by melt coaxial electrospinning for the first time. And then, the preparations of ultrafine phase change fibers [12–14] by electrospinning in our group and other research groups were also reported. Here, in order to further develop the new types and exploit the potential applications of this novel phase change fibers which were often called form stable fibrous PCMs [15–18] or shape-stabilized PCMs [19,20], in this work, ultrafine PEG/CA phase change fibers with various PEG content were prepared via electrospinning. CA acts as a supporting material and fiber template and PEG acts as a model phase change material in the composite fibers. The effect of PEG content in the composite fibers on morphology, crystalline properties, phase change behaviors and tensile properties of the electrospun PEG/CA fibers were systematically investigated by field-emission scanning electron microscopy (FE-SEM), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and a tensile tester, respectively.
into the CA solution and stirred for 2–4 h to prepare an uniform electrospinning solution. During the electrospinning, each of asprepared solutions was placed in a 5 ml syringe and was fed by a syringe pump (TS2-60, Baoding Changjing Pump Ltd., China) at a rate of 5 ml h 1. The stainless-steel needle (0.7 mm inner diameter) was connected a high voltage supply (BPS-20, Beijing Electrostatic Facility Ltd., China) with a voltage at 14 kV. An aluminum flat sheet and the rotating drum with the rotating speed of 2000 rpm wee earthed and used as the collector, respectively. The distance between the needle and the collector was fixed at 15 cm. The fibers were dried in vacuum at room temperature for 24 h to remove residual solvent. The electrospun fibers collected by aluminum foil and rotating drum are shown in Fig. 1. 2.3. Thermoregulating capability test
CA (Mn = 29,000 g/mol, degree of substitution (DS) = 2.45) was purchased from Aldrich. PEG (Mn = 10,000 g/mol) was purchased from Guangzhou Chemical Agent Company, China. All chemicals were used as received without further purification.
A test of three heating–cooling thermal cycles was performed to simulate a dramatic temperature change condition. A typical thermal cycling consisted of a consecutive heating and cooling process (in the temperature interval 35–90 °C). Both the heating and the cooling processes times were maintained at approximately 20 min. The PEG/CA composite fibrous mats and the CA fibrous mats (reference sample) were rolled to make a tube shape (thickness is about 1 mm) and covered with an aluminum foil, then placed onto a hot stage (STC200, Instec, USA). The temperatures of the inner core the sample and the hot stage were measured using thermal couple thermometers (CENTER 300, Taiwan, China) every 30 s.
2.2. Electrospinning
2.4. Measurements
A 15 wt% CA solution (acetone/DMAc = 2/1, w/w) was used as the original solution, and PEG with different content were mixed
The morphology of electrospun fibers was observed with a field-emission scanning electron microscope (FE-SEM, JSM-6700F,
2. Experimental part 2.1. Materials
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Fig. 3. SEM images of electrospun PEG/CA fibers with different PEG content: (a) 10 wt%; (b) 20 wt%; (c) 30 wt%; (d) 40 wt%; (e) 50 wt%; (f) 60 wt%; (g) 70 wt% and (h) the average fiber diameter of those fibers.
JEOL, Japan) at acceleration voltage of 20 kV under low vacuum. The average fiber diameter (AFD) of the electrospun fibers was obtained by using an UTHSCSA Image Tool Program to measure from at least five FE-SEM images for each sample. Thermal analysis was
performed on each sample (about 5.0 mg) by using a differential scanning calorimeter (DSC, PE DSC-100, USA). Both the heating rate and the cooling rate were 10 °C/min from 0 °C to 120 °C in a nitrogen atmosphere. The crystalline behavior of the samples was
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investigated by WAXD (D/Max-1200, Rigaku, Japan), Cu Ka radiation (k = 1.54 Å), Ni filtration, scanning from 2h = 5° to 40°. The tensile properties of the electrospun composite fibers were determined by means of a fiber tensile tester (TM-2, Institute of Chemistry, Chinese Academy of Sciences, China) with an extension rate of 10 mm/min at room temperature. A bundle of aligned fibers was collected and kept in a fibril form during the measurement. The size of the samples was about 20 mm length, 1 mm diameter, and 30 mm distance between two clamps. 3. Results and discussion 3.1. Fiber morphology Both PEG and CA are soluble separately in N,N-dimethylacetamide (DMAc)/acetone mixture with a mass ratio of 1:2. From Fig. 2, smooth and uniform CA fibers with average diameter of 720 nm were obtained electrospun from 15 wt% CA solution, on the contrary, it is not electrospinnable for PEG solution even with various concentrations, and PEG particles with average diameter of about 5.2 lm were formed by electrospray process from 20 wt% PEG solution. Therefore, CA is a fiber template for different weight percent of PEG in this study. The PEG content in the paper is equal to the mass ratio of PEG and the total solute (the weight of CA and PEG) in the solution. With the increase of PEG content, a series of mixed solutions were prepared, and the maximum concentration of PEG in the mixed solution is 70 wt% due to the solubility limit of PEG. Fig. 3a–g shows the typical morphology of electrospun PEG/CA composite fibers with different PEG content. Normally, very homogeneous and smooth PEG/CA fibers without beads are electrospun from the mixed solutions with lower PEG content (Fig. 3a–e). With the increase of PEG content in the composite fibers, PEG/CA fibers become non-uniform along the fiber axial and markedly inferior (Fig. 3f). Then, when PEG arrives to the maximum content, a corsslinked membrane-like structure have formed in the fibrous mats and there are many tubercles in fibers (Fig. 3g), which should be caused by the aggregation of the excessive PEG during electrospinning process. Therefore, mixed solution containing higher PEG concentration is an unfavorable factor to the formation of ultrafine fibers. Moreover, the average fiber diameter increased with the increase of PEG content from Fig. 3h. This result is attributed to the variations of the solutions properties such as a significant increase of viscosity and a decrease of conductivity of the mixed solution (seen from Fig. 4) by the addition of PEG into the CA solution, complimenting similar results reported in the literature [21–23].
Fig. 4. Conductivity and viscosity of the PEG/CA mixed solutions vs. PEG content.
Fig. 5. WAXD patterns of: (a) pure PEG, (b) electrospun CA fibers and PEG/CA composite fibers with different PEG content: (c) 10 wt%; (d) 20 wt%; (e) 30 wt%; (f) 40 wt%; (g) 50 wt%; (h) 60 wt%; (i) 70 wt%.
3.2. Crystalline properties of PEG/CA composite fibers Fig. 5 is the WAXD patterns of pristine PEG, electrospun CA fibers and PEG/CA composite fibers with different PEG content. It is obvious that pristine PEG has two strong diffraction peaks at about 2h = 19.5° and 2h = 24° (Fig. 5a), and CA fibers shows a broad diffraction peak at about 2h = 21.3° (Fig. 5b), which indicates that PEG has good crystalline structure and CA fibers are amorphous form. Meanwhile, PEG/CA fibers with 10 wt% and 20 wt% PEG also are amorphous form for their broad diffraction peak like that of CA (Fig. 5c and d). And with the increase of PEG percent, PEG/CA composite fibers have two diffraction peaks with similar diffraction patterns and diffraction angles (Fig. 5e–i), which are the same as pristine PEG powders. It is easily inferred that the diffraction peaks of the PEG/CA composite fibers are aroused by PEG, and CA acts merely as a diluent in the composite fibers. The main difference between the WAXD curves of these PEG/CA fibers is the diffraction peaks’ shape, and the diffraction peaks become sharper and stron-
Fig. 6. DSC curves of electrospun CA fibers and PEG/CA fibers after washing.
ger with the increase of the PEG mass percent in the composite fibers, which means that the degree of crystallization of the composite fibers increases with increasing PEG content. 3.3. Phase change behaviors of PEG/CA composite fibers Fig. 6 shows the DSC curves of CA and PEG/CA composite fibers after water treatment. Obviously, the DSC curve of the washed composite fibers very coincides with that of CA fibers, and both of them have no endothermic peaks and exothermic peaks in the range of 30–130 °C. It indicates that PEG in the composite fibers is almost removed by aqueous rinsing treatment, and there is no
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Fig. 7. DSC: (A) heating and (B) cooling curves of PEG/CA fibers with different PEG content: (a) 10 wt%; (b) 20 wt%; (c) 30 wt%; (d) 40 wt%; (e) 50 wt%; (f) 60 wt%; (g) 70 wt% and (h) pristine PEG.
Fig. 9. Thermoregulating curves of: (a) the hot stage, (b) electrospun CA fibers and (c) PEG/CA fibers with 50 wt% PEG.
Fig. 8. FT-IR spectra of pristine PEG, electrospun CA fibers and PEG/CA fibers with 50 wt% PEG.
phase change for CA in the temperature range of the DSC test. In other word, the latent heats of the composite fibers from DSC are all contributed by the phase change of PEG. Fig. 7 shows the DSC curves of pristine PEG and electrospun PEG/CA fibers with different PEG content. From the figure, the phase transition temperatures of all the electrospun PEG/CA composite fibers are lower than those of PEG powder because the introduction of CA. The enthalpy of melting (DHm) and enthalpy of crystallization (DHc) of PEG are about 177.38 J/g and 167.75 J/g, respectively. Endothermic peaks and exothermic peaks of the composite fibers increased with the increase of PEG content, which means DHm and DHc of the composite
fibers have increased obviously with increasing PEG content from the intensity changes of DSC curves. DHm/DHc of the composite fibers from 10 wt% to 70 wt% PEG are 3.83 J/g/1.61 J/g, 16.35 J/g/ 10.87 J/g, 30.18 J/g/21.74 J/g, 57.48 J/g/43.86 J/g, 86.03 J/g/65.75 J/ g, 101.79 J/g/79.83 J/g, 120.18 J/g/104.40 J/g, respectively. Thus, the PEG mass percentage in the composite fibers becomes the dominating factor for the variations of enthalpies of the fibers. Compared with the phase change fibers prepared by other methods of the previous reports [3–6], the enthalpies of the electrospun phase change fibers are much higher because of the higher PEG content in the fibers. Theoretically, the latent heats of the PEG/CA composite fibers are obtained by multiplying the latent heats of pure PEG and the mass percent of PEG in the fibers. However, the real values of the enthalpies for all the composite fibers are obviously lower than the corresponding theoretical values. It is believed that the reduction of the enthalpies is mainly caused by a retardation of the crystallization process of PEG in the composite fibers, which not only hindered by the quench process of electrospinning [24,25] but also influenced by the hydrogen bonding interactions between PEG and CA chains [26,27]. The interactions of PEG and CA in the composite fibers are confirmed by FT-IR spectra shown in Fig. 8. Carbonyl peaks shifted from 1757 cm 1 to 1740 cm 1 in PEG/CA fibers are
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Fig. 10. The tensile properties of PEG/CA fibers with different PEG content: (A) the typical strain–stress curves of electrospun fibers; (B) ultimate strength and ultimate strain of the fibers vs. PEG content in the fibers. And the inset SEM image of (A) is the samples of electrospun fibers collected by rotating drum for tensile test.
caused by interactions of the OH group in PEG and the carbonyl group in CA. PEG/CA phase change fibers not only have excellent performances in thermal energy storage, but also have capability of temperature regulation. The thermoregulating capability test of the fibers was performed with a hot stage by self-designed direct method in this work, in which the hot stage simulated an exterior environment temperature swing. Fig. 9 shows the temperature– time relationship curves of the hot stage, electrospun CA fibers and PEG/CA composite fibers with 50 wt% PEG, respectively. As to PEG/CA composite fibers, in the beginning of the heating process, the fibers’ temperature increased as the temperature of the hot stage increased. And then, the fibers’ temperature responded less to the hot stage (exterior environment) temperature changes, and it only raised to about 58 °C (close to the Tm of PEG/CA fibers) while the hot stage temperature closed to 90 °C. This phenomenon is because of PEG absorbing heats isothermally during the melting process. In the cooling process, the temperature slowed down and stabilized around 47 °C due to the releasing of latent heats in the crystallization process of PEG. In contrast, the temperature–time curve of CA fibers followed closely with the variation of the hot stage. This result demonstrates that PEG/CA fibers have a good capability to regulate their interior temperature as the ambient temperature alters. Therefore, the ultrafine phase change fibers have potential applications in the thermal storage and temperature regulation as protecting cloths or textiles for valuable apparatus. 3.4. Tensile properties The tensile properties of the electrospun fibers could be affected by many different factors [25,28–30] such as the fibers’ alignment, the testing direction, the electrospinning solution concentration, the fibers’ component and so on. In this work, the tensile properties of the electrospun PEG/CA composite fibers were measured by examining a fibril that consisted of hundreds of aligned fibers similar to the inset image of Fig. 10A. Since there were many interspaces among the fibers in the testing fibril, the cross-section area of the fibril measured by a micrometer was not equal exactly to the real cross-section area of the fibers. Therefore, the collected data is semiquantitative to a certain extent. The typical stress–strain curves of CA fibers and PEG/CA fibers are shown in Fig. 10A, and Fig. 10B presents the tensile properties of the electrospun CA fibers and PEG/CA composite fibers with different PEG content in the fibers. From Fig. 10, the ultimate strength and the ultimate strain of CA fibers were about 8.5 MPa and 10%, respectively. It is clear that the ultimate strength and ultimate strain of all the composite fibers are lower than those of CA fibers,
and they decrease with the increase of PEG wt%. When PEG content is low in the fibers, the tensile properties have minor decrease, and as PEG content increases to 30 wt%, the ultimate strength and the ultimate strain of the fibers decrease sharply compared with CA fibers. Obviously, the reduction of the ultimate strength and ultimate strain of the PEG/CA composite fibers was attributing to the introduction of PEG, which broken the continuous phase structure of CA. Therefore, the addition of PEG was an unfavorable effect to the tensile properties of the composite fibers. 4. Conclusion Ultrafine PEG/CA composite fibers with various PEG percent were successfully prepared as phase change fibers by electrospinning. The morphology and the thermal properties of the composite fibers change dramatically by adjusting PEG content. Moreover, thermoregulating capability test demonstrates that PEG/CA electrospun fibers have a good capability to regulate their interior temperature as the ambient temperature alters. Therefore, the electrospun PEG/CA composite fibers with reliable thermal properties are suitable and promising in serving as phase change fibers for thermal energy storage and temperature regulation. Acknowledgements The authors thank for the financial supports from the National Natural Science Foundation of China (No. 50821062), the Foundation of Henan Educational Committee (No. 2011A430019) and the Science Research Projects Fund of Nanyang Normal University (No. ZX2010013). References [1] Vigo TL, Frost CM. Temperature-sensitive hollow fibers containing phase change salts. Textile Res J 1982;55:633–7. [2] Vigo TL, Frost CM. Temperature-adaptable hollow fibers containing polyethylene glycols. J Coated Fabrics 1983;12:243–54. [3] Shin Y, Yoo DI, Son K. Development of thermoregulating textile materials with microencapsulated phase change materials (PCM). II. Preparation and application of PCM microcapsules. J Appl Polym Sci 2005;96:2005–10. [4] Ying B, Kwok Y, Li Y, Zhu Q, Yeung C. Assessing the performance of textiles incorporating phase change materials. Polym Test 2004;23:541–9. [5] Zhang XX, Wang XC, Tao XM, Yick KL. Energy storage polymer/microPCMs blended chips and thermo-regulated fibers. J Mater Sci 2005;40:3729–34. [6] Zhang XX, Wang XC, Tao XM, Yick KL. Structure and properties of wet spunthermo-regulated polyarylonitrile-vinylidene chloride fibers. Textile Res J 2006;76:351–9. [7] Hu J, Yu H, Chen YM, Zhu MF. Study on phase-change characteristics of PET– PEG copolymers. J Macromol Sci B 2006;45:615–21. [8] Li D, Xia YN. Electrospinning of nanofibers: reinventing the wheel? Adv Mater 2004;16:1151–70.
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Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Electrospun phase change fibers based on polyethylene glycol/cellulose acetate blends Changzhong Chen a, Linge Wang b,⇑, Yong Huang c,d,⇑ a
School of Chemistry and Pharmacy Engineering, Nanyang Normal University, Nanyang 473061, China Department of Biomedical Science, University of Sheffield, Sheffield S10 2TN, UK c Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, Beijing 100190, China d State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China b
a r t i c l e
i n f o
Article history: Received 29 November 2010 Received in revised form 19 February 2011 Accepted 21 February 2011 Available online 14 April 2011 Keywords: Electrospinning Morphology PEG Phase change fiber Thermal properties
a b s t r a c t Ultrafine phase change fibers based on polyethylene glycol (PEG)/cellulose acetate (CA) blends in which PEG acts as a model phase change material (PCM) and CA acts as a supporting material, were successfully prepared via electrospinning. The effect of PEG content on the morphology, crystalline properties, phase change behaviors and tensile properties of the composite fibers was studied systematically by field-emission scanning electron microscopy (FE-SEM), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and a tensile tester, respectively. The SEM observation indicates that maximum PEG content in the fibers could reach up to 70 wt%, and the morphology and average diameter of the composite fibers vary with PEG content. Thermal analysis results show that the latent heats of the phase change fibers increase with the increasing of PEG content in the fibers, and the PEG/CA fibers with high enthalpies have a good capability to regulate their interior temperature as the ambient temperature alters. Therefore, the developed phase change fibers have enormous applicable potentials in thermal energy storage and temperature regulation. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Phase change fibers, also called thermo-regulating fibers or temperature-adaptable fabrics, have been extensively studied [1– 7] for thermal energy storage and temperature regulation as high performance nonwoven fabrics and coatings. Generally, phase change fibers work through energy storage and release of PCMs in the fibers during the phase change process as the ambient temperature alters. As renewable and clean energy storage materials, phase change fibers have been studied for several decades. As early as in 1980s, Vigo and Frost [1,2] prepared temperature-adaptable fabrics by immersing the hollow fibers into PCM solution such as hydrated inorganic salts solution and PEG aqueous solution. Though these fabrics based on PCMs could impart desirable thermal storage and release properties, they exhibited unreliable and poor thermal behaviors on repeated thermal cycles. Thus, other approaches have been developed to improve the performance of the thermo-regu-
⇑ Corresponding authors. Addresses: Department of Biomedical Science, University of Sheffield, Firth Court, Western Bank, Sheffield, S10 2TN, UK. Tel.: +44(0)1142224662; fax: +44(0)1142222787 (L. Wang), Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, Beijing 100190, China. Tel.: +86(0)1082543478; fax: +86(0)1062554670 (Y. Huang). E-mail addresses: [email protected] (L. Wang), [email protected] (Y. Huang). 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.02.026
lating fibers such as microencapsulation technique of PCM [3,4]. However, the surface coated fabrics became stiffer and less smooth as the add-on microPCM increased, and then the total hand value which determines the tactile comfort perceived by humans decreased. Zhang et al. prepared different thermo-regulating fibers by embed microPCM into the fibers via melt-spinning [5] and wet-spinning [6], respectively. In these methods, microPCMs were incorporated into fibers or fabrics, thus decreasing the weight percent of encapsulated PCMs and the corresponding enthalpies of the fibers. Hu and co-workers [7] copolymerized PEG with polyethylene terephthalate (PET), and melt-spun the phase change fibers of the copolymer, which have solid–solid phase change characteristics at a certain temperature range without obvious liquid substance appearing. The phase change temperature range and the enthalpy of these spun thermo-regulating fibers could be adjusted by controlling the molecular weight and the corresponding proportion of PCMs added. It is a pity that only a few types of thermo-regulating fibers could be made by these spinning methods because of their poor processability for many polymers and PCMs. In order to overcome the disadvantages mentioned above, electrospinning is introduced to prepared phase change fibers. Electrospinning [8,9] is a simple, convenient, and versatile technique for generating ultrafine fibers from a wide variety of polymers, polymer blends and nanoparticle-impregnated polymers. With their ultrafine size and huge surface-to-volume ratio, electrospun fibers can be applied in numerous areas [10] such as
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C. Chen et al. / Applied Energy 88 (2011) 3133–3139
Fig. 1. Images of electrospun PEG/CA fibers collected by: (a) aluminum foil and (b) rotating drum.
Fig. 2. SEM images of: (a) electrospun CA fibers and (b) PEG particles.
healthcare, biotechnology, environmental engineering, and energy storage. McCann et al. [11] fabricated phase change nanofibers consisting of long chain hydrocarbon and TiO2–PVP composite by melt coaxial electrospinning for the first time. And then, the preparations of ultrafine phase change fibers [12–14] by electrospinning in our group and other research groups were also reported. Here, in order to further develop the new types and exploit the potential applications of this novel phase change fibers which were often called form stable fibrous PCMs [15–18] or shape-stabilized PCMs [19,20], in this work, ultrafine PEG/CA phase change fibers with various PEG content were prepared via electrospinning. CA acts as a supporting material and fiber template and PEG acts as a model phase change material in the composite fibers. The effect of PEG content in the composite fibers on morphology, crystalline properties, phase change behaviors and tensile properties of the electrospun PEG/CA fibers were systematically investigated by field-emission scanning electron microscopy (FE-SEM), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and a tensile tester, respectively.
into the CA solution and stirred for 2–4 h to prepare an uniform electrospinning solution. During the electrospinning, each of asprepared solutions was placed in a 5 ml syringe and was fed by a syringe pump (TS2-60, Baoding Changjing Pump Ltd., China) at a rate of 5 ml h 1. The stainless-steel needle (0.7 mm inner diameter) was connected a high voltage supply (BPS-20, Beijing Electrostatic Facility Ltd., China) with a voltage at 14 kV. An aluminum flat sheet and the rotating drum with the rotating speed of 2000 rpm wee earthed and used as the collector, respectively. The distance between the needle and the collector was fixed at 15 cm. The fibers were dried in vacuum at room temperature for 24 h to remove residual solvent. The electrospun fibers collected by aluminum foil and rotating drum are shown in Fig. 1. 2.3. Thermoregulating capability test
CA (Mn = 29,000 g/mol, degree of substitution (DS) = 2.45) was purchased from Aldrich. PEG (Mn = 10,000 g/mol) was purchased from Guangzhou Chemical Agent Company, China. All chemicals were used as received without further purification.
A test of three heating–cooling thermal cycles was performed to simulate a dramatic temperature change condition. A typical thermal cycling consisted of a consecutive heating and cooling process (in the temperature interval 35–90 °C). Both the heating and the cooling processes times were maintained at approximately 20 min. The PEG/CA composite fibrous mats and the CA fibrous mats (reference sample) were rolled to make a tube shape (thickness is about 1 mm) and covered with an aluminum foil, then placed onto a hot stage (STC200, Instec, USA). The temperatures of the inner core the sample and the hot stage were measured using thermal couple thermometers (CENTER 300, Taiwan, China) every 30 s.
2.2. Electrospinning
2.4. Measurements
A 15 wt% CA solution (acetone/DMAc = 2/1, w/w) was used as the original solution, and PEG with different content were mixed
The morphology of electrospun fibers was observed with a field-emission scanning electron microscope (FE-SEM, JSM-6700F,
2. Experimental part 2.1. Materials
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Fig. 3. SEM images of electrospun PEG/CA fibers with different PEG content: (a) 10 wt%; (b) 20 wt%; (c) 30 wt%; (d) 40 wt%; (e) 50 wt%; (f) 60 wt%; (g) 70 wt% and (h) the average fiber diameter of those fibers.
JEOL, Japan) at acceleration voltage of 20 kV under low vacuum. The average fiber diameter (AFD) of the electrospun fibers was obtained by using an UTHSCSA Image Tool Program to measure from at least five FE-SEM images for each sample. Thermal analysis was
performed on each sample (about 5.0 mg) by using a differential scanning calorimeter (DSC, PE DSC-100, USA). Both the heating rate and the cooling rate were 10 °C/min from 0 °C to 120 °C in a nitrogen atmosphere. The crystalline behavior of the samples was
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investigated by WAXD (D/Max-1200, Rigaku, Japan), Cu Ka radiation (k = 1.54 Å), Ni filtration, scanning from 2h = 5° to 40°. The tensile properties of the electrospun composite fibers were determined by means of a fiber tensile tester (TM-2, Institute of Chemistry, Chinese Academy of Sciences, China) with an extension rate of 10 mm/min at room temperature. A bundle of aligned fibers was collected and kept in a fibril form during the measurement. The size of the samples was about 20 mm length, 1 mm diameter, and 30 mm distance between two clamps. 3. Results and discussion 3.1. Fiber morphology Both PEG and CA are soluble separately in N,N-dimethylacetamide (DMAc)/acetone mixture with a mass ratio of 1:2. From Fig. 2, smooth and uniform CA fibers with average diameter of 720 nm were obtained electrospun from 15 wt% CA solution, on the contrary, it is not electrospinnable for PEG solution even with various concentrations, and PEG particles with average diameter of about 5.2 lm were formed by electrospray process from 20 wt% PEG solution. Therefore, CA is a fiber template for different weight percent of PEG in this study. The PEG content in the paper is equal to the mass ratio of PEG and the total solute (the weight of CA and PEG) in the solution. With the increase of PEG content, a series of mixed solutions were prepared, and the maximum concentration of PEG in the mixed solution is 70 wt% due to the solubility limit of PEG. Fig. 3a–g shows the typical morphology of electrospun PEG/CA composite fibers with different PEG content. Normally, very homogeneous and smooth PEG/CA fibers without beads are electrospun from the mixed solutions with lower PEG content (Fig. 3a–e). With the increase of PEG content in the composite fibers, PEG/CA fibers become non-uniform along the fiber axial and markedly inferior (Fig. 3f). Then, when PEG arrives to the maximum content, a corsslinked membrane-like structure have formed in the fibrous mats and there are many tubercles in fibers (Fig. 3g), which should be caused by the aggregation of the excessive PEG during electrospinning process. Therefore, mixed solution containing higher PEG concentration is an unfavorable factor to the formation of ultrafine fibers. Moreover, the average fiber diameter increased with the increase of PEG content from Fig. 3h. This result is attributed to the variations of the solutions properties such as a significant increase of viscosity and a decrease of conductivity of the mixed solution (seen from Fig. 4) by the addition of PEG into the CA solution, complimenting similar results reported in the literature [21–23].
Fig. 4. Conductivity and viscosity of the PEG/CA mixed solutions vs. PEG content.
Fig. 5. WAXD patterns of: (a) pure PEG, (b) electrospun CA fibers and PEG/CA composite fibers with different PEG content: (c) 10 wt%; (d) 20 wt%; (e) 30 wt%; (f) 40 wt%; (g) 50 wt%; (h) 60 wt%; (i) 70 wt%.
3.2. Crystalline properties of PEG/CA composite fibers Fig. 5 is the WAXD patterns of pristine PEG, electrospun CA fibers and PEG/CA composite fibers with different PEG content. It is obvious that pristine PEG has two strong diffraction peaks at about 2h = 19.5° and 2h = 24° (Fig. 5a), and CA fibers shows a broad diffraction peak at about 2h = 21.3° (Fig. 5b), which indicates that PEG has good crystalline structure and CA fibers are amorphous form. Meanwhile, PEG/CA fibers with 10 wt% and 20 wt% PEG also are amorphous form for their broad diffraction peak like that of CA (Fig. 5c and d). And with the increase of PEG percent, PEG/CA composite fibers have two diffraction peaks with similar diffraction patterns and diffraction angles (Fig. 5e–i), which are the same as pristine PEG powders. It is easily inferred that the diffraction peaks of the PEG/CA composite fibers are aroused by PEG, and CA acts merely as a diluent in the composite fibers. The main difference between the WAXD curves of these PEG/CA fibers is the diffraction peaks’ shape, and the diffraction peaks become sharper and stron-
Fig. 6. DSC curves of electrospun CA fibers and PEG/CA fibers after washing.
ger with the increase of the PEG mass percent in the composite fibers, which means that the degree of crystallization of the composite fibers increases with increasing PEG content. 3.3. Phase change behaviors of PEG/CA composite fibers Fig. 6 shows the DSC curves of CA and PEG/CA composite fibers after water treatment. Obviously, the DSC curve of the washed composite fibers very coincides with that of CA fibers, and both of them have no endothermic peaks and exothermic peaks in the range of 30–130 °C. It indicates that PEG in the composite fibers is almost removed by aqueous rinsing treatment, and there is no
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Fig. 7. DSC: (A) heating and (B) cooling curves of PEG/CA fibers with different PEG content: (a) 10 wt%; (b) 20 wt%; (c) 30 wt%; (d) 40 wt%; (e) 50 wt%; (f) 60 wt%; (g) 70 wt% and (h) pristine PEG.
Fig. 9. Thermoregulating curves of: (a) the hot stage, (b) electrospun CA fibers and (c) PEG/CA fibers with 50 wt% PEG.
Fig. 8. FT-IR spectra of pristine PEG, electrospun CA fibers and PEG/CA fibers with 50 wt% PEG.
phase change for CA in the temperature range of the DSC test. In other word, the latent heats of the composite fibers from DSC are all contributed by the phase change of PEG. Fig. 7 shows the DSC curves of pristine PEG and electrospun PEG/CA fibers with different PEG content. From the figure, the phase transition temperatures of all the electrospun PEG/CA composite fibers are lower than those of PEG powder because the introduction of CA. The enthalpy of melting (DHm) and enthalpy of crystallization (DHc) of PEG are about 177.38 J/g and 167.75 J/g, respectively. Endothermic peaks and exothermic peaks of the composite fibers increased with the increase of PEG content, which means DHm and DHc of the composite
fibers have increased obviously with increasing PEG content from the intensity changes of DSC curves. DHm/DHc of the composite fibers from 10 wt% to 70 wt% PEG are 3.83 J/g/1.61 J/g, 16.35 J/g/ 10.87 J/g, 30.18 J/g/21.74 J/g, 57.48 J/g/43.86 J/g, 86.03 J/g/65.75 J/ g, 101.79 J/g/79.83 J/g, 120.18 J/g/104.40 J/g, respectively. Thus, the PEG mass percentage in the composite fibers becomes the dominating factor for the variations of enthalpies of the fibers. Compared with the phase change fibers prepared by other methods of the previous reports [3–6], the enthalpies of the electrospun phase change fibers are much higher because of the higher PEG content in the fibers. Theoretically, the latent heats of the PEG/CA composite fibers are obtained by multiplying the latent heats of pure PEG and the mass percent of PEG in the fibers. However, the real values of the enthalpies for all the composite fibers are obviously lower than the corresponding theoretical values. It is believed that the reduction of the enthalpies is mainly caused by a retardation of the crystallization process of PEG in the composite fibers, which not only hindered by the quench process of electrospinning [24,25] but also influenced by the hydrogen bonding interactions between PEG and CA chains [26,27]. The interactions of PEG and CA in the composite fibers are confirmed by FT-IR spectra shown in Fig. 8. Carbonyl peaks shifted from 1757 cm 1 to 1740 cm 1 in PEG/CA fibers are
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Fig. 10. The tensile properties of PEG/CA fibers with different PEG content: (A) the typical strain–stress curves of electrospun fibers; (B) ultimate strength and ultimate strain of the fibers vs. PEG content in the fibers. And the inset SEM image of (A) is the samples of electrospun fibers collected by rotating drum for tensile test.
caused by interactions of the OH group in PEG and the carbonyl group in CA. PEG/CA phase change fibers not only have excellent performances in thermal energy storage, but also have capability of temperature regulation. The thermoregulating capability test of the fibers was performed with a hot stage by self-designed direct method in this work, in which the hot stage simulated an exterior environment temperature swing. Fig. 9 shows the temperature– time relationship curves of the hot stage, electrospun CA fibers and PEG/CA composite fibers with 50 wt% PEG, respectively. As to PEG/CA composite fibers, in the beginning of the heating process, the fibers’ temperature increased as the temperature of the hot stage increased. And then, the fibers’ temperature responded less to the hot stage (exterior environment) temperature changes, and it only raised to about 58 °C (close to the Tm of PEG/CA fibers) while the hot stage temperature closed to 90 °C. This phenomenon is because of PEG absorbing heats isothermally during the melting process. In the cooling process, the temperature slowed down and stabilized around 47 °C due to the releasing of latent heats in the crystallization process of PEG. In contrast, the temperature–time curve of CA fibers followed closely with the variation of the hot stage. This result demonstrates that PEG/CA fibers have a good capability to regulate their interior temperature as the ambient temperature alters. Therefore, the ultrafine phase change fibers have potential applications in the thermal storage and temperature regulation as protecting cloths or textiles for valuable apparatus. 3.4. Tensile properties The tensile properties of the electrospun fibers could be affected by many different factors [25,28–30] such as the fibers’ alignment, the testing direction, the electrospinning solution concentration, the fibers’ component and so on. In this work, the tensile properties of the electrospun PEG/CA composite fibers were measured by examining a fibril that consisted of hundreds of aligned fibers similar to the inset image of Fig. 10A. Since there were many interspaces among the fibers in the testing fibril, the cross-section area of the fibril measured by a micrometer was not equal exactly to the real cross-section area of the fibers. Therefore, the collected data is semiquantitative to a certain extent. The typical stress–strain curves of CA fibers and PEG/CA fibers are shown in Fig. 10A, and Fig. 10B presents the tensile properties of the electrospun CA fibers and PEG/CA composite fibers with different PEG content in the fibers. From Fig. 10, the ultimate strength and the ultimate strain of CA fibers were about 8.5 MPa and 10%, respectively. It is clear that the ultimate strength and ultimate strain of all the composite fibers are lower than those of CA fibers,
and they decrease with the increase of PEG wt%. When PEG content is low in the fibers, the tensile properties have minor decrease, and as PEG content increases to 30 wt%, the ultimate strength and the ultimate strain of the fibers decrease sharply compared with CA fibers. Obviously, the reduction of the ultimate strength and ultimate strain of the PEG/CA composite fibers was attributing to the introduction of PEG, which broken the continuous phase structure of CA. Therefore, the addition of PEG was an unfavorable effect to the tensile properties of the composite fibers. 4. Conclusion Ultrafine PEG/CA composite fibers with various PEG percent were successfully prepared as phase change fibers by electrospinning. The morphology and the thermal properties of the composite fibers change dramatically by adjusting PEG content. Moreover, thermoregulating capability test demonstrates that PEG/CA electrospun fibers have a good capability to regulate their interior temperature as the ambient temperature alters. Therefore, the electrospun PEG/CA composite fibers with reliable thermal properties are suitable and promising in serving as phase change fibers for thermal energy storage and temperature regulation. Acknowledgements The authors thank for the financial supports from the National Natural Science Foundation of China (No. 50821062), the Foundation of Henan Educational Committee (No. 2011A430019) and the Science Research Projects Fund of Nanyang Normal University (No. ZX2010013). References [1] Vigo TL, Frost CM. Temperature-sensitive hollow fibers containing phase change salts. Textile Res J 1982;55:633–7. [2] Vigo TL, Frost CM. Temperature-adaptable hollow fibers containing polyethylene glycols. J Coated Fabrics 1983;12:243–54. [3] Shin Y, Yoo DI, Son K. Development of thermoregulating textile materials with microencapsulated phase change materials (PCM). II. Preparation and application of PCM microcapsules. J Appl Polym Sci 2005;96:2005–10. [4] Ying B, Kwok Y, Li Y, Zhu Q, Yeung C. Assessing the performance of textiles incorporating phase change materials. Polym Test 2004;23:541–9. [5] Zhang XX, Wang XC, Tao XM, Yick KL. Energy storage polymer/microPCMs blended chips and thermo-regulated fibers. J Mater Sci 2005;40:3729–34. [6] Zhang XX, Wang XC, Tao XM, Yick KL. Structure and properties of wet spunthermo-regulated polyarylonitrile-vinylidene chloride fibers. Textile Res J 2006;76:351–9. [7] Hu J, Yu H, Chen YM, Zhu MF. Study on phase-change characteristics of PET– PEG copolymers. J Macromol Sci B 2006;45:615–21. [8] Li D, Xia YN. Electrospinning of nanofibers: reinventing the wheel? Adv Mater 2004;16:1151–70.
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