The effects of nanoparticles on morphology and thermal properties of erythritol/polyvinyl alcohol phase change composite fibers

2.1 Materials

The PVA (MW=130,000) was obtained from the Shanghai Kaiyuan Chemical Technology Co., Ltd. China. The erythritol (E) powder was obtained from Zibo Zhongshi Biotechnology Co., Ltd. (China). Nano SiO₂ (<15 nm, purity of 99.5%, 2.3~4.5 W/m·K), TiO₂ (<25 nm, purity of 99.8%, ~1.6 W/m·K), Al₂O₃ (<30 nm, purity of 99.99%, ~2.5 W/m·K) and carbon (<25 nm, purity of 99.5%, <2000 W/m·K) were obtained from Aladdin. All of the chemicals were used without further purification.

2.2 Sample preparation and electrospinning process

Firstly, the different contents of nanoparticles (0.5 wt%, 2 wt% and 4 wt%) were dispersed into deionized water under ultrasonic dispersion for 30 min to obtain uniform nanofluids. Heating the aforementioned solutions to above 95°C dissolved the PVA to prepare 10 wt% PVA solutions. Then, a series of mixture solutions were prepared by adding the appropriate mass of erythritol into the above uniform solutions by magnetic stirring for 30 min. After standing for 2 h, the mixture solutions become homogeneous and clear liquids which can be used for electrospinning. For comparison, pure PVA solution without nanoparticles was also prepared.

Electrospinning was performed at room temperature in air. The spinning device (DFS-100, Beijing Technova Technology Co., Ltd.) was used to fabricate composite fibers using a distance of 15 cm, flow rate of 2.4 ml/h, and a voltage power of 8 kV. All the fibers were dried in a vacuum at 80°C for 24 h to remove the residual solvent. For convenience, the composites which contain 60 wt%, 70 wt% and 80 wt% of erythritol are identified as E/PVA-1, E/PVA-2 and E/PVA-3, respectively.

2.3 Thermal cycles test

Thermal cycle test was used to study the structure, shape and thermal stabilities of composites by using a drying oven (WGL-45B, Tianjin, Instrument Co., Ltd.), in the temperature interval 20–160°C in air.

2.4 Characterization of the samples

The solution properties were composed of viscosity, surface tension and electric conductivity, which were measured using a viscometer (LVDV-I Prime, Brookfield, NY, USA), a surface tension apparatus (BZY-A, Shanghai Fangrui Instrument Co., Ltd., China) and a conductivity meter (DDSJ-308F, Shanghai Leici Instrument Co., Ltd., China), respectively. All measurements were carried out at 25°C and an average value was obtained from at least three repeated measurements.

Fibers were gold-coated and the morphology was observed by field emission-scanning electron microscopy (FE-SEM) (FEI Quanta 250F) at an acceleration voltage of 20 kV under low vacuum. The average fiber diameter (AFDs) of electrospun fibers was obtained by using an UTHSCSA Image Tool Program to measure from at least five SEM images for each sample. Thermal analysis was performed (each used about 10.0 mg) by differential scanning calorimetry (DSC) (DSC8500, PerkinElmer, USA). Both the heating rate and the cooling rate were 2°C/min ranging from 20 to 160°C in a nitrogen atmosphere. The thermal conductivities of composites were measured by thermal conductivity apparent (DRL-III, Xiangtan Xiangyi Co., Ltd.). An average value was obtained from at least three repeated measurements for every sample.

3.1 Fiber morphology

Figure 1 shows the SEM images of nanoparticles, the particles are granulated in shape. The nanoparticles are clustered into groups because of the high surface energy with such a small size (34).

Figure 1:

SEM images of nanoparticles: (a) SiO₂, (b) TiO₂, (c) Al₂O₃ and (d) Carbon.

Figure 2 shows the typical morphology of E/PVA-1, E/PVA-2 and E/PVA-3 composite fibers. Both the AFDs and the solution properties including viscosity, surface tension and electric conductivity are shown in Table 1. As shown in Figure 2, composite fibers possess smooth surfaces and a cylindrical shape same as PVA fibers (Figure S1). As displayed in Table 1, the AFDs of both PVA and composite fibers are 0.51 μm, 0.72 μm, 1.01 μm and 1.25 μm, respectively. Obviously, the addition of erythritol into PVA solution markedly increase the diameter of the resulting fibers. Such a result might be attributed to the property variations of mixture solutions compared with those of PVA solution, in which viscosity and surface tension increased, whilst the corresponding conductivity decreased as shown in Table 1. Based on the data in Table 1, electric conductivity of solution had the biggest influence on AFDs, followed by viscosity and surface tension. A similar phenomenon was reported in the literature (19).

Figure 2:

SEM images of composite fibers: (a–a′) E/PVA-1, (b–b′) E/PVA-2 and (c–c′) E/PVA-3 before and after 100 heating-cooling cycles.

In order to investigate the shape stability of composite fibers, the representative SEM images of E/PVA-1, E/PVA-2 and E/PVA-3 composite fibers before and after 100 heating-cooling thermal cycles are also shown in Figure 2. It can be found from Figure 2 that due to the high tackiness of sugar alcohol materials, the fibers look like an interconnected network with some conglutinations and crosslinks occurring between fibers after thermal cycles. Even so, the cycled composite fibers can still maintain a cylindrical shape, it can be attributed to the restriction of PCMs by the physical adsorption of fiber networks. The E/PVA-3 composites can retain good morphology after thermal cycles, it means the content of erythritol in composites can reach as high as 80 wt%, and possess high shape stability.

3.2 Thermal properties

DSC analysis was conducted to investigate the thermal properties of the samples. DSC curves of erythritol powder and E/PVA composites before and after 100 thermal cycles are illustrated in Figure 3. The DSC characteristics of the samples such as phase change temperatures and enthalpy values are summarized in Table 2. The theoretical enthalpy values of composites are calculated by multiplying the latent heat of pure erythritol powder and the mass percentage of erythritol in composites. For the composites, the melting and crystallization temperatures are 118.10°C and 38.83°C, however, the melting points of composites gradually get bigger and the crystallization temperatures become gradually with the decreasing amount of erythritol in the composites, this is led to the low thermal conductivity of PVA as the supporting material in composites.

Figure 3:

DSC curves of samples before: (A) and after (B) 100 thermal cycles.

Table 2 confirmed that the latent heat of composites depends on the mass ratio of erythritol as PVA shows no phase transition in this temperature range (<160°C), and the enthalpy values increase with increasing the erythritol percent in composites. As shown in Table 2, the ∆H_m (∆H_c) of pure erythritol and three composites after thermal cycles are 332.3 (250.2) J/g, 194.5 (149.8) J/g, 224.7 (174.7) J/g and 258.9 (199.6) J/g, respectively. From these data, it can be indicated that the content of erythritol in composites can reach a high of 80 wt%, and the E/PVA-3 composites exhibit high thermal storage performance and excellent reusability.

It can be concluded from combining the results of the analyses in Section 3.1 with 3.2, that the erythritol content in the composites can reach as high as 80 wt%, the E/PVA-3 composites possess good shape stability and excellent thermal stability after thermal cycles. Therefore, the effects of nanoparticles on morphology and thermal properties of E/PVA composites are studied based on E/PVA-3 composites.

3.3 The effect of nanoparticles on fiber morphology

The morphology of E/PVA-3 composite fibers with nanoparticles (SiO₂, TiO₂, Al₂O₃ and carbon) before and after 100 thermal cycles are shown in Figure 4. These images demonstrate that composites are still cylindrical fibers. The AFDs decrease with increasing the percentage of nanoparticles, such a result is caused by the increase of solution electric conductivity as displayed in Table 1, the higher electric conductivity, the more slender AFDs. After thermal cycles, the morphology of composite fibers appeared the have some variations for the same reasons explained for Figure 2, however, fortunately, the addition of nanoparticles alleviate the conglutination phenomena between fibers.

Figure 4:

SEM images of E/PVA-3 composites with different content: (a) 0.5 wt%, (b) 2 wt%, (c) 4 wt% of nanoparticles and (d) 4 wt% composites after cycles.

It can be concluded that the smallest AFD is obtained from E/PVA-3 composite fibers with 4% nano carbon because of the highest solution for electric conductivity, and the addition of nanoparticles can markedly improve the morphology of composites after cycles; composites with 4% nano SiO₂ possess the best shape stability.

3.4 The effect of nanoparticles on composites thermal properties

In order to investigate the effects of nanoparticles on the thermal properties of composites, DSC curves of E/PVA-3 composites with nano SiO₂, TiO₂, Al₂O₃ and carbon are shown in Figure 5. The corresponding DSC data are illustrated in Table 3. In addition, the heat loss rates are the percentage difference of ∆H before and after thermal cycles.

Figure 5:

DSC curves of E/PVA-3 composites with different content of nanoparticles: (A) SiO₂, (B) TiO₂, (C) Al₂O₃ and (D) carbon.

Comparing the data of Table 2 with Table 3, the composites with nanoparticles have forward phase transition temperatures ranging from 114.82 to 116.77°C, and short melting processes. This is caused by the increase of heat transfer rates on fibers after the addition of nanoparticles. The enthalpy values decrease in different degrees with the increase of nanoparticle contents, such a phenomenon has also appeared in the literature (24). Moreover, the addition of nanoparticles, especially nano carbon, have a positive effect on the supercooling phenomenon of erythritol, it decreases 7.55°C of supercooling when the additive amount is 4%. This phenomenon might be caused by the nucleation of nanoparticles during the crystallization process of molten erythritol. After thermal cycles, there are no obvious variations in phase change temperatures, but there is a slight decrease in the enthalpies of the composites. The composites with 4% nano Al₂O₃ possess the best thermal stability with the lowest heat loss rate of 1.0%.

3.5 Thermal conductivity

Thermal conductivity is also one of the important symbols of phase change composites. Figure 6 displayed the effects of nanoparticles on thermal conductivity of E/PVA-3 composites. The thermal conductivity of E/PVA-3 composites is calculated to be 0.58 W/m·K. As shown in Figure 6, the addition of nanoparticles can effectively improve the thermal conductivity of composites, among these nano carbons have the strongest effect. The markedly enhanced thermal conductivity for 0~2% nano carbon in composites can be contributed to the high thermal conductivity of carbon and the increase of particle content per unit area in composites. The slight increase for 2~4% of nano carbon is due to the content (2%) of nano carbon being enough to build a heat conduction network on the surface and within the composites. Nevertheless, the thermal conductivity of E/PVA-3 composites is improved 167% after adding 4% nano carbon to it, it means the heat transfer rate of composites can be increased efficiently.

Figure 6:

Thermal conductivity of composites.