Thermo-regulated sheath/core submicron fiber with poly(diethylene glycol hexadecyl ether acrylate) as a core

Abstract

Side-chain crystallizable comb-like polymers can be used as polymeric thermal energy-storage materials. Diethylene glycol hexadecyl ether acrylate (C16E2AA) was synthesized with sodium alkoxide using diethylene glycol hexadecyl ether (C16E2) and acryloyl chloride as reactants. C16E2AA was prepared by free radical polymerization to form a comb-like polymeric phase change material, poly(diethylene glycol hexadecyl ether acrylate) (PC16E2AA). A series of sheath/core composite submicron fibers were coaxially electrospun using PC16E2AA as the core and poly(acrylonitrile-co-vinylidene chloride) as the sheath. Results indicate that C16E2AA and PC16E2AA were synthesized in high yield. The average molecular weight and polydispersity index of PC16E2AA were 29,800 g/mol and 3.17, respectively. PC16E2AA melted at 33.8℃ and crystallized at 25.8℃. The melting and crystallization enthalpy for PC16E2AA were 90 and 85 J/g, respectively. PC16E2AA was thermally stable below 326℃. Coaxial submicron fibers with smooth surface and average diameter ranging from 341 to 371 nm were electrospun. The optimum feed rate of the sheath and core components for fabricating thermo-regulated submicron fibers with high enthalpies were 0.125 and 0.375 mL/h, respectively. The fibers can absorb 50 J/g at approximately 37℃ and release 48 J/g of heat at approximately 32℃.

Keywords

energy-storage material comb-like polymer side-chain crystallization electrospinning submicron fiber

Phase change materials (PCMs) are reusable energy-storage materials that can absorb significant amounts of energy as latent heat and release it into the surroundings during a solid–liquid or solid–solid phase change over a defined temperature range.¹ PCMs have been used for energy-saving buildings,^2,3 solar energy storage,⁴ waste heat recycling,⁵ both smart fibers and fabrics^6,7 and garment cooling.⁸ The research on traditional solid–liquid organic PCMs (such as paraffin waxes, fatty alcohols and aliphatic polyesters) has attracted increasing attention.¹ However, severe supercooling and leakage into the surrounding environment during solid–liquid transitions limit their practical application.^6,9 PCMs can be encapsulated to obtain microencapsulated phase change materials (microPCMs) or nanoencapsulated phase change materials (nanoPCMs) that remain in the solid state forever. Leakage of PCMs during the phase change process can be decreased when polymers such as polyethylene glycol, aliphatic polyesters and block copolymers^10,11 are used as PCMs. However, few materials that satisfy the demands are available. The fabrication, structure and properties of a series of comb-like polymers with crystallizable side chains have been investigated for decades.^12–16 However, these polymers are seldom used as polymeric phase change materials (PPCMs) because the temperature range between the melting and crystallization temperature for some PPCMs is too high or the enthalpy too low to be applied.¹⁷ In other words, the polymers are typically not desirable energy-storage materials. A new type of PPCM-poly(polyethylene glycol octadecyl ether methacrylate) (PC18E2MMA) was recently synthesized and characterized, and its potential for application as a PPCM was investigated.¹⁸ However, the polymer’s melting and crystallizing temperatures were higher than room temperature, limiting its potential applications.

Conventional PCMs have been used to manufacture thermo-regulated fibers, fabrics, foams and coatings.^6,19–23 Thermo-regulated textiles containing PCMs can regulate their inner temperature as the ambient temperature changes by absorbing heat from the environment or releasing heat to the environment via the corresponding PCM phase change from solid to liquid or liquid to solid.²⁴ Thermo-regulated fibers have been fabricated using microPCMs or n-alkanes as additives.^6,21–23 However, there is little available information on the use of PPCMs as fabricate additives for producing thermo-regulated fibers.

In the present study, we fabricated a novelty PPCM, poly(diethylene glycol hexadecyl ether acrylate), in which the alkyl group (—C₁₆H₃₃) was employed as a crystallizable side chain. Moreover, two flexible ether bonds (O—CH₂CH₂—O) were inserted between the crystallizable side chain and backbone chain to enhance the crystallization of the alkyl side chain. In the end, the electrospun sheath/core bi-component submicron fibers using PC16E2AA as the core were fabricated, and poly(acrylonitrile-co-vinylidene chloride) (PAN-co-VDC) was used as a sheath to enhance the strength and surface performance of the fiber. Moreover, the phase change properties and microstructure were investigated in detail.

Experimental details

Experimental materials

Diethylene glycol hexadecyl ether (C16E2) (99%) was purchased from Sigma-Aldrich. The solvent was mixed with toluene and distilled to remove residual water before using. Acryloyl chloride (AC) (analytical reagent (AR)) was purchased from Beijing Ouhe Science and Technology, Inc. Sodium was purchased from the Tianjin Institute of Fuchen Fine Chemical. Absolute alcohol (AR), toluene (AR), sodium hydroxide (AR) and azodiisobutyronitrile (AIBN) were purchased from Tianjin Fengchuan Chemical Reagent Science and Technology Co., Ltd. The AIBN was recrystallized in alcohol before use. PAN-co-VDC with a mass ratio of 70%:30% was a procured from Fushun Huifu Retardant Fiber Company. N,N-dimethylformamide (DMF) (AR) was obtained from the Tianjin Institute of Guangfu Fine Chemical Ltd.

Synthesis of diethylene glycol hexadecyl acrylate (C16E2AA)

The synthesis scheme of C16E2AA is presented in Scheme 1. Predetermined amounts of C16E2 and toluene were added to a three-necked flask with a thermometer, fractionating column and constant pressure drop funnel. The temperature was set to 105℃ with magnetic stirring. An excess amount of sodium was added under nitrogen and reacted for 3 h until no bubbles were observed on the liquid surface. The resultant solution was cooled to room temperature and filtered to remove residual sodium. The flask containing sodium diethylene glycol hexadecyl ether was immersed in an ice-water bath. A predetermined amount of AC mixed with toluene was added dropwise into the flask with magnetic stirring. The reaction continued for 12 h at room temperature. The resultant mixture was filtered to remove the generated NaCl. Residual toluene was removed using a rotatory evaporator. The raw C16E2AA product was obtained.

The raw product was dissolved in chloroform and washed with a 5 wt.% aqueous NaOH solution in a separatory funnel several times until the color did not change. The bottom organic phase was collected. The residual chloroform was removed with a rotatory evaporator. The product was dried at room temperature for 24 h. The yields were 86%, 87%, 91%, 93% and 85% for molar ratios of 1:1, 1:1.1, 1:1.2, 1:1.3 and 1:1.4, respectively. An optimized value was reached at 1:1.2. The reaction was influenced by the molar ratio of C16E2 to AC.

Synthesis of poly (diethylene glycol hexadecyl acrylate) (PC16E2AA)

Scheme 2 shows the polymerization scheme of comb-like polymer PC16E2AA. A predetermined amount of C16E2AA was mixed with toluene in a three-necked flask with a nitrogen inlet and thermometer. AIBN was added as an initiator using 0.5 wt.% of C16E2AA under magnetic stirring under the protection of nitrogen. The reaction was maintained at 65℃ for 30 h. Two-thirds of the solvent volume was distilled using a rotatory evaporator. Absolute ethanol was added to the solution as a precipitant to form the raw product. The raw product was redissolved and precipitated twice. C16E2AA dissolved well in ethanol. By contrast, PC16E2AA was insoluble in ethanol. Any residual C16E2AA was removed. The product was vacuum-dried at 40℃ for 24 h. The yield was approximately 93%.

Coaxial electrospinning of sheath/core composite submicron fibers

PC16E2AA was dissolved in toluene at 50℃ with a concentration of 60 wt.% and was used as the core component. P(AN-co-VDC) was dissolved in DMF at 50℃ with a concentration of 20 wt% and was used as the sheath component. Submicron fibers were fabricated using an electrospinning machine purchased from Beijing Future Material Sci-tech, Ltd. The fibers were coaxially electrospun using a sheath/core composite spinneret. The electrospinning voltage was 30 kV, and the sheath feed rate was fixed to 1.25 mL/h; the feed rate of the core varied from 0.125 to 0.625 mL/h, as shown in Table 1. The distance between the spinneret and the aluminum foil was 20 cm.

Table 1.

Coaxial electrospinning parameters

Sample no.	Concentration of core component (wt.%)	Feed rate of sheath component (mL/h)	Feed rate of core component (mL/h)
F₁	0	1.250	0.000
F₂	60	1.250	0.125
F₃	60	1.250	0.375
F₄	60	1.250	0.625

Characterization

Fourier transformed infrared spectra (FTIR) of C16E2, C16E2AA and PC16E2AA were obtained using a spectrometer (Bio-RadFT S6000, USA) over the range of 4000–750 cm⁻¹. The resolution was 4 cm⁻¹.

¹H nuclear magnetic resonance (NMR) spectra of C16E2, C16E2AA and PC16E2AA were obtained using a Varian UNITY Plus 400 MHz (Bruker, Germany), with CDCl₃ as the solvent.

The number average molecular weight (M_n) and weight average molecular weight (M_w) of PC16E2AA were measured via gel permeation chromatography (GPC, Malvern, Viscotek 270) with differential and viscosity detectors at a flow rate of 1 mL min⁻¹ at 35℃ in tetrahydrofuran (THF). The polydispersity index of PC16E2AA was defined as the ratio M_w/M_n.

The phase change properties of the samples were characterized using a differential scanning calorimeter (DSC, NETZSCH 200 F3, Germany) from −20℃ to 60℃. The sample was heated from −20℃ to 60℃ at a rate of 10℃/min under a nitrogen atmosphere and was maintained at 60℃ for 2 min. The sample was then cooled to −20℃ at a rate of −10℃/min and was maintained at −20℃ for 2 min. The sample was heated again to 60℃ at a rate of10℃/min. The cooling scan and second heating scan data were recorded.

The thermal stability of the samples was investigated using a thermogravimetric analyzer (TG NETZSCH STA409PC, Germany) from 25℃ to 600℃ at a heating rate of 10℃/min under a nitrogen atmosphere. The 5% mass-loss temperature was recorded as the thermal stability (T_0.05).

The crystallization behavior of these samples was characterized via X-ray diffraction (XRD, BRUKER AXS,D8 DISCOVER with GADDS, USA) at room temperature, and the scan range was 5–40° (2θ). The scan step size was 0.050°.

The fiber morphologies were characterized using a field emission scanning electron microscope (SEM, Hitachi S-4800, Japan), and the fiber diameter distribution was measuring using Image-Pro Plus software to process the SEM images.

The fiber microstructures were observed using a transmission electron microscope (TEM, Hitachi H-7650, Japan).

Results and discussion

FTIR

The FTIR spectra obtained for C16E2, C16E2AA and PC16E2AA at 25℃ are shown in Figure 1. For the C16E2 (A) spectrum, the absorbing peak at 3408 cm⁻¹ was assigned to the -OH stretching vibration. The absorbing peaks at 2850–2918 cm⁻¹ represent the asymmetric and symmetric stretching C-H vibrations, respectively, and were also observed in the spectra obtained for C16E2AA (B) and PC16E2AA (C). The absorbing peak at 1468 cm⁻¹ was assigned to the in-plane angular -CH₂- vibration. The peak at 1126 cm⁻¹ was assigned to the aliphatic ether bond (C-O-C) stretching vibration. The band (absorbing peak) at 722 cm⁻¹ was attributed to the rocking vibration of the -(CH₂) _n - plane with more than four units. For C16E2AA, the characteristic R-OH peak disappeared, which indicates that the R-OH was depleted via a chemical reaction; the strong peak appearing at 1718 cm⁻¹ was assigned to the ester carbonyl (C=O) stretching vibration. The peak at 1636 cm⁻¹ was assigned to the stretching vinyl bond (C=C) vibration. The absence of a hydroxyl group and the presence of the vinyl indicate that C16E2AA was successfully synthesized. The characteristic absorbing peak for the vinyl bond (C=C) at 1635 cm⁻¹ disappeared from the spectrum in Figure 1 (C). The peak at 810 cm⁻¹, which was assigned to the C=C-H vibration, was clearly weakened. This result indicates that PC16E2AA was probably synthesized via free radical polymerization.

¹H NMR characterization

The ¹H NMR spectra obtained for C16E2, C16E2AA and PC16E2AA in CDCl₃ are shown in Figure 2. Compared to the those in the C16E2 spectrum, the peaks at 6.13 and 5.57 ppm in the C16E2AA spectrum are characteristic of hydrogen atoms linked to the vinyl bond (C=C). The peaks assigned to the methyl hydrogen atoms in the vinyl bond (C=C) appear at 1.94 ppm. The methylene peak from the alkyl side chain connected to the oxygen atom falls in the range of 0.87–1.94 ppm.²² The carbon–carbon double bonding electron is closer to the carbon nucleus in PC16E2AA; therefore, the proton shielding effect in the spectrum is weakened. In turn, the resonance occurs downfield. Accordingly, the C=C disappeared after polymerization, and the chemical shift moved up field and was diminished, which indicates that PC16E2AA was fabricated. Based on the FTIR and ¹H NMR spectra for C16E2 (A) and C16E2AA (B), the target product, PC16E2AA, was fabricated.

Molecular weight and polydispersity index of PC16E2AA

The M_n and M_w of PC16E2AA are measured to be 29,800 and 94,500 g/mol from Figure 3, respectively. The polydispersity index was 3.17. PC16E2AA was synthesized via free radical polymerization. The molecular weight of methyl acrylate is 86.09 g/mol and the molecular weight of C16E2 is 330.546 g/mol. When C16E2AA is synthesized, a hydroxyl group with a molecular weight 16.999 g/mol is removed from the system. The ratio of the molecular weight of “main chain” in the whole polymer is approximately 0.215. If only the main chain of poly(methyl acrylate) is considered, the corresponding M_n and M_w are approximately 6400 and 20,300 g/mol, respectively. They are approximately 25 and 78 times that of n-octadecane, respectively. The high molecular weight, which suggests a high melt viscosity, protects the PCM from leakage during the phase change process.

Thermal properties of C16E2, C16E2AA and PC16E2AA

The phase change behavior and thermal stability of C16E2, C16E2AA and PC16E2AA are shown in Figures 4 and 5, respectively. The crystallized portion of C16E2, C16E2AA and PC16E2AA is the hexadecyl segment.¹³ During the heating process, C16E2 absorbs 127 J/g of heat at 27.6℃, and the phase changes from solid to liquid. During the cooling process, the material releases 121 J/g of heat at 21.2℃, and the phase changes from liquid to solid. This material is thermally stable up to 194℃. The melting temperature, crystallization temperature and enthalpy increase as the number of repeating units increases.²⁵ Reacting with AC yields C16E2AA. The melting temperature, crystallization temperature, melting enthalpy and crystallization enthalpy of C16E2AA were lower than those of C16E2 and were measured to be 22.6℃, 26.8℃, 101 J/g and 101 J/g, respectively. This phenomenon indicates that the crystallinity of C16E2AA decreased due to a decrease in molecular chain symmetry. In the cooling process, double exothermic peaks can be observed on the DSC curves for C16E2 (A) and C16E2AA (B), corresponding to melt to rotator phase transition and rotator phase to stable triclinic crystalline phase transition, respectively.²⁶ By contrast, the thermal stability temperature of C16E2AA was as high as 303℃, as shown in Figure 5 and Table 2, which is approximately 100℃ higher than that of C16E2. Both the melting and crystallization temperatures of PC16E2AA increased as the polymer formed during free radical polymerization. Meanwhile, the melting and crystallization enthalpies decreased further. PC16E2AA absorbed 90 J/g of heat at 33.8℃ and released 85 J/g of heat at 25.8℃. These variations following polymerization indicate the restriction of the polymeric main chain due to side-chain crystallization.²⁷ Both the melting and crystallization enthalpies were higher than those for a comb-like polymer with a rigid main chain.²⁸ Molecular structure is a key factor that determines the melting and crystallization behavior of polymers. The number of crystallizable carbon atoms decreases in hexadecyl, which is detrimental to energy-storage applications. In addition, when the temperature increased above the side-chain melting point 33.8℃, the PC16E2AA became a transparent solid gel with low mechanical strength. The thermally stable temperature for PC16E2AA was as high as 326℃, which is favorable for PCM applications, particularly those involving melt-spun thermo-regulated fibers formed using polypropylene,⁶ polyamide or polyester as the sheath component.

Table 2.

Phase change property and thermal stability ofC16E2, C16E2AA and PC16E2AA

Monomer or polymer	T_m (^oC)		ΔH_m^c (J/g)	T_c (^oC)		ΔH_c^f (J/g)	ΔH_a^g (J/g)	T_d^h (^oC)
Monomer or polymer	T_mo ^a	T_mp ^b	ΔH_m^c (J/g)	T_co ^d	T_cp ^e	ΔH_c^f (J/g)	ΔH_a^g (J/g)	T_d^h (^oC)
C16E2	27.6	36.1	127	25.2	21.2	121	124	194
C16E2AA	22.6	28.2	101	26.8	24.7	101	101	303
PC16E2AA	33.8	37.8	90	25.8	18.9	85	87	326

Onset temperature of the melting point.

Peak temperature of the melting point.

Enthalpy on the differential scanning calorimeter (DSC) heating run.

Onset temperature of the crystallization point.

Peak temperature of the onset crystallization point.

Enthalpy on the DSC cooling run.

ΔH_a = (|ΔH_m| + |ΔH_c|)/2.

The temperature on thermogravimetric analyzer measurement where the mass loss is 5%.

Crystallization properties

The XRD curves obtained for C16E2 (A), C16E2AA (B) and PC16E2AA (C) at room temperature are shown in Figure 6. C16E2 exhibits two characteristic diffraction peaks at 21.2° (110) and 23.5° (2Θ),²⁹ which correspond to inter-planar crystal spacings of 0.42 and 0.38 nm, respectively. Both C16E2AA and PC16E2AA exhibit a similar characteristic diffraction peak at 21.7° (2Θ) (d = 0.41 nm), which demonstrates that the alkyl side chains in the polymers crystallize in a hexagonal lattice analogous to that of conventional comb-like polymers.²⁷ The diffraction peak intensities of both C16E2AA and PC16E2AA are lower than those of C16E2, and their half-width is wider. This phenomenon indicates that introducing polar acrylic groups disturbs the molecular chain symmetry of the polymers and thus reduces their crystallizability.

Morphology of the fibers

Scanning electron micrographs of the electrospun submicron fibers, the diameter distributions and the average diameters (inset diagrams) for samples F₁–F₄ are shown in Figure 7. The fiber surface is smooth with no beads observed in any of these micrographs. The average diameter of the resultant PAN-co-VDC fiber was around 266 nm, and the average diameter of these sheath/core composite fibers increased from 341 to 371 nm as the feed rate of the core solution increased. The average diameter of sheath/core composite fibers was affected significantly by the sheath/core rate.

Sheath/core structure of the fibers

Transmission electron micrographs of the F₂ fiber, F₃ fiber and F₄ fiber are shown in Figure 8. A clear sheath/core composite structure was observed for all samples, which indicates that PC16E2AA was encapsulated with P(AN-co-VDC). Both of the sheath and core diameters can be measured from the TEM micrographs. The core diameters of samples F₂, F₃ and F₄ could be measured to be around 131, 260 and 250 nm, respectively. With the core feed rate increased, the core diameter increased firstly. While the core feed rate enhanced from 0.375 to 0.625 mL/h, the core diameter decreased to 250 nm, approximately, which was close to 260 nm approximately at 0.375 mL/h, however. When the core/sheath solution splits under high voltage, the increased core material cannot be enveloped completely by sheath material because of its specific surface energy affected by the molecular structure, solvent type and temperature.

Thermal properties of the fibers

The phase change properties of samples F₂, F₃ and F₄ are listed in Table 3. The fibers began absorbing heat at approximately 37℃ and released heat at approximately 32℃. Both the melting and crystallization enthalpies were affected by the feed rate of the core component. The melting enthalpy increased from 38 to 51 J/g as the core component feed rate increased from 0.125 to 0.375 mL/h, which is significantly higher than the melting enthalpies of polypropylene/microPCMs⁶ and polyacrylonitrile/MicroPCMs;²³ however, the value is lower than that of electrospun sheath/core fibers featuring an n-eicosane core.²² The enthalpy decreased slightly as the feed rate was increased further. Similar behavior was observed during the cooling process. The crystallization enthalpy increased from 34 to 48 J/g as the feed rate of the core component increased from 0.125 to 0.375 mL/h and declined with further increases in the feed rate. Therefore, the optimum core component feed rate for fabricating thermo-regulated submicron fibers were determined to be 0.375 mL/h.

Table 3.

Phase change properties and thermal stability of the fibers

Sample No.	T_m (℃)		ΔH_m (J/g)	T_c (℃)		ΔH_c (J/g)
Sample No.	T_mo	T_mp	ΔH_m (J/g)	T_co	T_cp	ΔH_c (J/g)
F₂ fiber	36.9	39.8	38	31.9	29.8	34
F₃ fiber	36.8	39.7	51	31.8	29.9	48
F₄ fiber	37.6	40.6	50	31.8	29.8	46

Conclusions

Diethylene glycol hexadecyl ether acrylate was successfully synthesized using sodium alkoxide with diethylene glycol hexadecyl ether and AC as raw materials. A comb-like PPCM, that is, poly(diethylene glycol hexadecyl ether acrylate) (PC16E2AA) was obtained via free radical polymerization. The polymer melts at 33.8℃ and crystallizes at 25.8℃ with enthalpies of 90 and 85 J/g, respectively. It was thermally stable below 326℃, which implies that it can be submitted to high-temperature processing, that is, as melt-spun thermo-regulated fibers. Coaxial submicron fibers with average diameters ranging from 341 to 371 nm were successfully electrospun. The fiber surface was smooth. The optimum feed rate for using the core component to fabricate thermo-regulated submicron fibers with high enthalpy was 0.375 mL/h. The core component of fiber melts at 37℃ and crystallizes at 32℃ with enthalpies of 48 and 50 J/g, respectively. It is estimated that the submicron fibers have a wider application area and promising prospect in the field of thermal-regulated fabrics, including underclothes, mittens and shoe-pads.

Footnotes

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

This work was supported by the National Natural Science Foundation of China (No. 21174105, No.51203113).

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Thermo-regulated sheath/core submicron fiber with poly(diethylene glycol hexadecyl ether acrylate) as a core

Abstract

Keywords

Experimental details

Experimental materials

Synthesis of diethylene glycol hexadecyl acrylate (C16E2AA)

Synthesis of poly (diethylene glycol hexadecyl acrylate) (PC16E2AA)

Coaxial electrospinning of sheath/core composite submicron fibers

Characterization

Results and discussion

FTIR

1H NMR characterization

Molecular weight and polydispersity index of PC16E2AA

Thermal properties of C16E2, C16E2AA and PC16E2AA

Crystallization properties

Morphology of the fibers

Sheath/core structure of the fibers

Thermal properties of the fibers

Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

References

¹H NMR characterization