Novel wearable polyacrylonitrile/phase-change material sheath/core nano-fibers fabricated by coaxial electro-spinning

Abstract

This study focused on the preparation of wearable polyacrylonitrile (PAN)/phase-change material (PCM) sheath/core nano-fibers by coaxial electro-spinning technology. PAN was used as the sheath material to make fibers comfortable to human skin, and isopropyl palmitate and paraffin oil were used as sample PCMs. The electro-spinning conditions are critical for encapsulating a PCM by a PAN shell and forming a uniform sheath–core structure. Our experiments show that these PCMs could be well encapsulated in PAN hollow fibers and function to stabilize environmental temperature in a narrow range. This study provides a versatile and feasible way to fabricate PAN/PCM sheath/core homo-thermal nano-fibers with various temperature stabilization windows. Using these homo-thermal nano-fibers as clothing materials will be able to abate people’s discomfort during a sudden environmental temperature change.

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1. Introduction

Phase change materials (PCMs) release or absorb tremendous heat during phase transitions.^1–4 PCMs have high heat storage capacity and a narrow temperature variation window, and are practically used in solar energy storage, smart temperature controlled buildings, cooling of equipment and homo-thermal textiles.^5,6 The performance of a PCM depends on several factors: (1) a PCM should possess a high latent heat; (2) a PCM needs to be well encapsulated to avoid possible leakage during its life cycle; (3) a PCM needs good heat transfer between the PCM and the surrounding environment. The heat transfer from a PCM to the environment can be improved by electro-spinning technology:^7,8 (1) electro-spinning prepares fibers with a diameter less than 1 μm, thus the PCM has a large amount of contact area to exchange heat with the environment; (2) the heat transfer path from the PCM phase to the environment is only a few hundred nanometers or less, the extremely short path will greatly promote its heat transfer. A PCM is normally encapsulated in a polymer material in the following ways: (1) mixture or blend, where the PCM is physically mixed with a material. For example, researchers spun fibers using the following mixtures, methyl stearate/polyacrylonitrile (PAN),⁹ fatty acid eutectics/polyethylene terephthalate,^10,11 and dodecane/poly(lactic acid).¹² (2) Micro-/nano-capsule: PCMs are first encapsulated in capsules, and then the capsules can be used as functional additives in any material. For instance, researchers have encapsulated octadecane in polyurea nanocapsules,¹³ octadecane in polystyrene nanocapsules,¹⁴ n-nonadecane in calcium alginate micro-capsules,¹⁵ and n-octadecane in resorcinol-modified melamine–formaldehyde micro-capsules.¹⁶ (3) Sheath/core polymer/PCM fibers: polymer and PCM are coaxial electro-spun to form a polymer sheath and PCM core structure. Researchers have made polyurethane/soy-bean wax sheath–core fibers,¹⁷ TiO₂–polyvinyl pyrrolidone/octadecane sheath–core fibers,¹⁸ and octadecane/polyvinyl butyral sheath–core fibers.¹⁹ In our studies, we focus on the potential homo-thermal fiber application of PCMs. The human body is sensitive to environmental temperature; either a higher or lower environmental temperature will cause body discomfort, especially during a sudden temperature change. The usage of homo-thermal fibers instead of common cotton as clothes will retard temperature change around the human body and make people feel comfortable. In this paper, we fabricated PAN/PCM sheath/core nano-fibers by coaxial electro-spinning. PAN is a common man-made clothing material which is soft and comfortable to our bodies; PAN has been found to be able to encapsulate some polymer materials inside by co-axial electro-spinning.^20,21 In this work, PCM is encapsulated in a PAN sheath to stabilize human skin temperature in a narrow range. Here, two types of PCMs were used, isopropyl palmitate (IPP) and paraffin oil. Both PCMs are non-toxic, and are liquid forms at room temperature. IPP has a bulk melting temperature of ∼13 °C, and paraffin oil is a mixture of low molecular weight wax with a melting transition at around −30 °C. The usage of IPP as PCM is to prove the functionality of PAN/PCM nano-fiber mats, and the usage of paraffin oil is to prove the versatility of this method for making PAN/paraffin wax nano-fibers. Wax is non-toxic, and has a high latent heat, normally 200–260 J g⁻¹, low melt vapor pressure, and negligible super-cooling.²² Additionally, paraffin wax has a melting temperature from 16 to 60 °C depending on its molecular weight.²³ Human body surface temperatures are different at diverse positions, which demand homo-thermal fibers with selective temperature regulation windows. Various types of waxes as PCMs are able to satisfy this requirement. Due to a limited temperature control capacity of our electro-spinning equipment, PCMs with a melting point higher than room temperature could not be spun without heating and were not used in our experiments.

2. Experimental

2.1. Materials

Dimethyl sulphoxide (DMSO, AR), IPP (98%, Lot# LCC0M30) and paraffin liquid (AR, Lot# F1526013) were purchased from Tianjing Fengchuan chemical technologies co. Ltd. (Tianjing, China), J&K Scientific (Shanghai, China), and Aladdin (Shanghai, China), respectively. DMSO was vacuum distilled before use, and other reagents were used without further purification. PAN-co-IA (M_w = 150 000 g mol⁻¹, IA% = 2 wt%, synthesized by radical polymerization in our lab) was dissolved in DMSO at a concentration of 10 wt%.

2.2. Electro-spinning

Electro-spinning equipment was self-assembled with the following parts. Two syringe pumps (Model TJ-3A/W0109-1B) were purchased from Longer Pump Corp. (Baoding, China). The high voltage power supply (Model TE-4020P50-50, 0–50 kV, 50 W) was bought from Dalian Teslaman Tech. Co. Ltd. (Dalian, China). The rotary drum (Model TL-RC5000, D = 10 cm, RPM = 1–5000) was obtained from Tongliweina Company (Shenzhen, China). The coaxial tip was bought from Nayi equipment technologies co. Ltd. (Changsha, China). A scheme of the spinning equipment is shown in Fig. 1. The PCM was pumped through the inner channel, and the PAN solution was pumped through the outer channel. During electro-spinning, the environmental temperature was around 20 °C and the humidity level was 45–50%, the applied voltage and the rotary drum speed were set to be 20 kV and 500 rpm, respectively. The flow rate of each component and the distance between needle tip and rotation drum were adjusted to obtain uniform sheath–core geometry nano-fibers.

Fig. 1 Scheme of self-assembled electro-spinning equipment (left) and an optical image of the coaxial tip (right). The inner tip has an inner diameter of 0.35 mm and outer diameter of 0.61 mm, the outer tip has an inner diameter of 1.12 mm.

2.3. Characterization

The surface morphologies of electro-spun fibers were recorded by a field emission scanning electron microscope (FE-SEM, Model JSM-7001F, made by JEOL Ltd, Japan) at an operation voltage of 3–5 kV after gold sputter coating. The diameter distributions of the electro-spun fibers were analyzed by ImageJ software; for each sample, 80 individual fibers were measured. The structure of the fibers was observed by field emission transmission electron microscopy (FE-TEM, Model Tecnai G2 F20 S-Twin, made by FEI, USA) at an operation voltage of 100 kV. The thermal properties were measured by a differential scanning calorimeter (DSC, Model 200F3, made by Netzsch, German) under a argon environment, the samples were initially quickly cooled down to −10 °C, then heated up to 50 °C, and again cooled down to −10 °C at a constant rate of 5 °C min⁻¹. The cooling and heating cycle was repeated for a total of 10 times.

3. Results and discussion

3.1. Effect of electro-spinning conditions on PAN/IPP fiber morphology

For coaxial electro-spinning, the flow rates of core and sheath components are important adjustable parameters to obtain a uniform encapsulation structure. In our initial trials, the distance between needle tip and rotary drum and drum rotation speed were set to be 16 cm and 500 rpm, respectively. The flow rate of IPP (core component) was pre-set to be 0.1 mL h⁻¹, and the flow rates of PAN solution (sheath component) was varied from 0.2 to 4.0 mL h⁻¹. The morphologies of the electro-spun fibers were observed by SEM, and images are shown in Fig. 2. The most frequently observed defects are micron size beads. Beads with a few to 10 micron size are observed in the fibers prepared at a PAN flow rate of 0.2 mL h⁻¹ (beads are noted inside red dash-dot circles in Fig. 2A). The other type of defect is a cleaved fiber surface (Fig. 2F) which is observed in some places. When the PAN solution flow rate was increased from 0.2 to 2 mL h⁻¹, the probability of forming beads and cleaved fiber surfaces decreased. When the PAN solution flow rate was increased to 2 mL h⁻¹, the electro-spun fibers formed a defect free structure (Fig. 2D). A further increase of PAN solution flow rate to 4 mL min h⁻¹ reduced the jetting stability, and beads were reformed as shown in Fig. 2E. The flow rates of each component need to be balanced to obtain a uniformly encapsulated sheath–core structure. For PAN/IPP coaxial electro-spinning, the best flow rates that we found in our experiments are 2 mL h⁻¹ and 0.1 mL h⁻¹ for PAN solution and IPP component, respectively.

Fig. 2 SEM images of PAN/IPP electro-spun fiber mats. IPP flow rate was 0.1 mL h⁻¹, and PAN solution flow rate was (A) 0.2 mL h⁻¹, (B) 0.4 mL h⁻¹, (C) 1.0 mL h⁻¹, (D) 2.0 mL h⁻¹, and (E) 4.0 mL h⁻¹. (F) Typical defects: (up) large bead and (down) cleaved fiber surface. Red dash-dot circles indicate beads.

Fig. 3 shows SEM and TEM images of PAN/IPP sheath–core fibers obtained at a PAN solution flow rate of 2 mL h⁻¹. The fibers have a smooth surface and relatively narrow diameter distribution. As shown in Fig. 3B, the electro-spun fibers have a diameter in the range from 0.4 to 1.2 μm, and the number average diameter is 0.72 ± 0.14 μm. The HR-TEM images are shown in Fig. 3C. A clear interface is observed between the IPP core and PAN sheath, which proves that no diffusion occurs between the IPP and PAN components during the electro-spinning process. It is noted that although the IPP core is encapsulated in a PAN sheath, the PAN sheath thickness varies along the fiber length. Fig. 3D shows a sketch of the PAN/IPP fiber structure, the thickest PAN shell is ∼400 nm while the thinnest portion is only ∼100 nm. The thin PAN sheath portions might break at un-stable spinning conditions. Additionally, the effect of distance between the needle tip and collecting drum on fiber diameter was investigated. Under the same electro-spinning conditions, we reduced the distance between needle tip and drum from 16 to 14 to 10 cm, and the fiber average diameter slightly decreased from 0.72 to 0.67 to 0.60 μm, respectively (Table 1). The decrease of the fiber diameter is ascribed to the formation of a stronger electrical field when the distance decreases.

Fig. 3 Morphologies of PAN/IPP fibers obtained at a PAN flow rate of 2 mL h⁻¹. (A) SEM images, (B) fiber diameter distribution, (C) HR-TEM images, and (D) sketch of the sheath–core structure. Arrows show uniform sheath thickness.

Table 1 Fiber outer diameters at different distance between needle tip and collecting drum

Distance (cm)	16	14	10
Diameter (μm)	0.72 ± 0.14	0.67 ± 0.17	0.60 ± 0.13

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3.2. Thermal properties of PAN/IPP nano-fibers

PAN/IPP nano-fibers prepared at a PAN flow rate of 2 mL h⁻¹ exhibit the most uniform fiber structures, and the thermal properties of this fiber were characterized by DSC. The heat flow curves of PAN/IPP nano-fibers and IPP are shown in Fig. 4, and the measured melting temperature, crystallization temperature, melting enthalpy, and crystallization enthalpy are listed in Table 2. The heat flow curves of cycles 2 to 9 totally overlap, while that of cycle 1 is slightly different. For the measurement of the first cycle, the sample was initially quickly cooled down from room temperature to a low temperature. By comparison, before cycles 2 to 9, the sample was cooled down from 50 °C at a constant rate of 5 °C min⁻¹. The different thermal behavior between cycle 1 and cycles 2 to 9 is caused by the different thermal history before sample measurements. PAN/IPP nano-fibers exhibit a single melting and crystallization peak, while IPP shows multiple peaks. The multiple melting or crystallization peaks suggest that IPP may have various types of crystalline structures. Here, we denote each pair of melting and crystallization peaks of bulk IPP with symbols α, β, and γ. Each pair of peaks corresponds to a particular crystalline structure of IPP. Park et al. electro-spun polyethylene glycol/poly(vinylidene fluoride) sheath/core fibers, and observed that the melting or crystallization of the core component inside the fibers was slower than bulk core material.²⁴ They ascribed the delay to the existence of the sheath component which would slow down the heat transfer from the environment to the core component during the DSC measurements. Here, both melting and crystallization temperatures of PAN/IPP nano-fibers are much lower than the α peak temperatures of IPP, but are close to that of the β peak. The different thermal behavior between IPP material and IPP inside the PAN/IPP fibers may be ascribed to the existence of PAN or a confined geometry which induces the formation of only the β-type crystalline structure of IPP during the heating or cooling process. The detailed reasons for the different crystallization behaviors between IPP bulk material and IPP inside the PAN nano-fiber sheath requires further investigation.

Fig. 4 DSC curves of (A) PAN/IPP nano-fibers, and (B) bulk IPP. Green dash-dot lines are the baselines for enthalpy calculation.

Table 2 Melting point, crystallization point, and enthalpies measured from DSC curves in Fig. 4

Sample	Cycle#	Melting temp (°C)	Melting enthalpy (J g^−1)	Crystallization temp (°C)	Crystallization enthalpy (J g^−1)
IPP	1^st	14.3	127.0	7.2	131.0
	2^nd to 10^th	14.5	123.9	7.2	131.0
PAN/IPP	1^st	8.8	27.2	−0.2	34.6
	2^nd to 10^th	8.9	30.8	−0.2	34.6

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According to the feeding rates, the encapsulation ratio of IPP in the PAN/IPP nano-fibers is 28.1 wt%. From DSC results, the encapsulation ratio of IPP can be calculated from the following equation,

Encapsulation ratio (%) = ΔH_m,PAN/IPP/ΔH_m,IPP × 100%

where, ΔH_m,PAN/IPP and PAN/ΔH_m,IPP are the melting enthalpy of PAN/IPP fibers and bulk IPP, respectively. The sum of the melting enthalpies of the α, β, and γ peaks is used as the melting enthalpy of the bulk IPP (Table 2). Based on DSC, the calculated encapsulation ratio is 24.8%, which is lower than the value, 28.1%, calculated based on the feeding flow rates. DMSO has a high boiling temperature (189 °C) and is hard fully remove from fibers. My previous study observed that PAN fibers may contain ∼4 wt% solvent even after vacuum drying at 60 °C for 2 days.²⁵ Additionally, PAN easily absorbs moisture from the atmosphere. The existence of solvent and moisture inside the sample will lead to a smaller melting enthalpy value as measured by DSC, and results in a lower calculated encapsulation ratio. The DSC curves of PAN/IPP nano-fibers at various heating and cooling rates are included in Fig. S1 in ESI.† During heating and cooling cycles, the DSC curves of PAN/IPP fibers (Fig. 4A and S1†) overlapped for cycle 2 and latter cycles, which proves that the prepared fibers have good stability during repeated phase changes.

3.3. PAN/paraffin oil sheath–core nano-fibers

PAN/paraffin oil sheath/core fibers were electro-spun under the following conditions: the applied high voltage was 18 kV, the distance between needle and drum was 15 cm, the drum rotation speed was 500 rpm, the PAN solution (sheath) flow rate was 0.5 mL h⁻¹, and the paraffin oil (core) flow rate was 0.1 mL h⁻¹. SEM and HR-TEM images of the prepared PAN/paraffin oil fibers are shown in Fig. 5. The fibers have a smooth surface. The average diameter of the fibers is 406 ± 87 nm (measured from Fig. 5B from 20 fibers). The TEM image (Fig. 5C) shows that paraffin oil is well encapsulated in the PAN sheath. The DSC curves of paraffin oil, PAN nano-fibers, and PAN/paraffin oil nano-fibers are shown in Fig. S2 in ESI,† all samples show no thermal transition in the temperature range between −30 and 50 °C. Based on the feeding flow rates of the PAN solution and paraffin oil, the encapsulation ratio of paraffin oil inside the nano-fibers is 61%. In comparison to IPP, the encapsulation of paraffin oil in PAN nano-fibers could reach a much higher volume fraction. Here, we prove that paraffin oil (a mixture of low molecule weight waxes) can be well encapsulated in PAN nano-fibers at a relatively high ratio of 61%. Using paraffin wax with various melting temperatures,²³ nano-fiber mats with adjustable stabilization temperature windows can be fabricated under similar processing conditions.

Fig. 5 (A and B) SEM and (C) HR-TEM images of PAN/paraffin oil sheath/core nano-fibers.

4. Conclusions

This study represents the fabrication of PAN/PCM sheath/core nano-fibers using coaxial electro-spinning technology. PAN is used as the shell material to provide a comfortable feeling for the human body. IPP and paraffin oil have been used as sample PCMs to fabricate homo-thermal fibers. TEM proves that these PCMs can be well encapsulated, and DSC analysis shows that PCM can stabilize temperature in a narrow range. The encapsulation ratios are ∼28% and 61% when IPP and paraffin oil was used as PCM, respectively. The human body has different temperatures at different places; customized PAN/PCM materials will provide extreme comfort when the environmental temperature suddenly changes. Using the same spinning method, various types of PCMs, for example paraffin waxes, can be encapsulated inside PAN nano-fibers to stabilize body temperature at different temperature levels to provide homo-thermal functions. For wearable applications of these homo-thermal fibers, further work is needed to prove their structural and functional stabilities after multiple washes.

Acknowledgements

This work was supported by One Hundred Person Project of the Chinese Academy of Sciences.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00281a

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