Poly(Mono/Diethylene Glycol N-Tetradecyl Ether Vinyl Ether)S with Various Molecular Weights As Phase Change Materials

Article Poly(mono/diethylene glycol n-tetradecyl ether vinyl ether)s with Various Molecular Weights as Phase Change Materials

Dongfang Pei ID , Sai Chen, Wei Li and Xingxiang Zhang *

Tianjin Municipal Key Lab of Advanced Fiber and Energy Storage Technology, School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China; [email protected] (D.P.); [email protected] (S.C.); [email protected] (W.L.) * Correspondence: [email protected]; Tel.: +86-022-8395-5054

Received: 24 January 2018; Accepted: 13 February 2018; Published: 15 February 2018

Abstract: At present, research on the relationship of comb-like polymer phase change material structures and their heat storage performance is scarce. Therefore, this relationship from both micro and macro perspectives will be studied in this paper. In order to achieve a high phase change enthalpy, ethylene glycol segments were introduced between the vinyl and the alkyl side chains. A series of poly(mono/diethylene glycol n-tetradecyl ether vinyl ethers) (PC14EnVEs) (n = 1, 2) with various molecular weights were polymerized by living cationic polymerization. The results of PC14E1VE and PC14E2VE showed that the minimum number of carbon atoms required for side-chain crystallization were 7.7 and 7.2, which were lower than that reported in the literature. The phase change enthalpy 89 J/g (for poly(mono ethylene glycol n-tetradecyl ether vinyl ethers)) and 86 J/g (for poly(hexadecyl acrylate)) were approximately equal. With the increase of molecular weight, the melting temperature, the melting enthalpy, and the initial thermal decomposition temperature of PC14E1VE changed from 27.0 to 28.0 ◦C, from 95 to 89 J/g, and from 264 to 287 ◦C, respectively. When the number average molar mass of PC14EnVEs exceeded 20,000, the enthalpy values remained basically unchanged. The introduction of the ethylene glycol chain was conducive to the crystallization of alkyl side chains.

Keywords: comb-like polymers; phase change materials; thermal properties; side chain crystallization

1. Introduction Thermal energy storage including sensible heat and latent heat can bridge the thermal energy requirements and the supply in time and space [1–3]. Phase change materials (PCMs) are the most widely used thermal energy storage materials. During the phase change process, PCMs absorb/release heat to remain in an isothermal state for a period of time [4,5]. They contribute to the efﬁcient use of waste heat, solar energy, building applications, etc. Small molecule PCMs have low material costs, but high packaging costs. Currently, the research mainly focuses on polymeric phase change materials, such as comb-like polymer PCMs. Comb-like polymers, also called brush-like polymers, contain main chains (polymeric backbones), and side chains (e.g., alkyl chain, polyethylene glycol, etc.) which are chemically linked onto the backbones. The ﬁrst comb-like polymers, poly(n-alkyl acrylate)s, were synthesized by Fisher [6] using the radical polymerization method. Since then, poly(n-alkyl methacrylate)s, poly(n-alkyl vinyl ester)s, poly(n-alkyl vinyl ether)s, poly(n-alkyl acrylamide)s, poly(n-alkyl ethylene)s, poly(n-alkyl ethylene oxide)s, poly(polyethylene glycol octadecyl ether methacrylate), poly(diethylene glycol hexadecyl ether acrylate), and poly(n-alkyl itaconate)s [7–12] were synthesized in succession. These reports about comb-like polymers with different topologies, rigidity degrees of main chains, and the length scale of

Polymers 2018, 10, 197; doi:10.3390/polym10020197 www.mdpi.com/journal/polymers Polymers 2018, 10, 197 2 of 18 side chains have mainly included fabrication and chemical modification techniques, but rarely studied their phase change energy storage characteristics. Liu et al. [13] prepared a series of shape-stabilized comb-like polymeric phase change materials (poly(ethylene-graft-maleic anhydride)-g-alkyl alcohol) by the esterification reaction. When the side-chain length increased from 14 to 26, the substitution degree was changed from 67.3 to 32.6; the phase transition temperatures and the heat enthalpy were changed from 36.4 to 67.1 ◦C and from 125.7 to 146.2 J/g, respectively. Wang et al. [14] prepared a series of shape-stabilized comb-like polymeric phase change materials (poly(styrene-co-maleic anhydride)-g-alkyl alcohol). When the side-chain length of poly(styrene-co-maleic anhydride)-g-alkyl alcohol increased from 14 to 26, the phase transition temperature and the heat enthalpy were changed from 34.7 to 73.7 ◦C and from 37.9 to 101.7 J/g, respectively. Also, the substitution degree was changed from 56.1 to 25.0. Comparing the above two studies, it was easy to find that the graft ratio and the heat enthalpy of comb-like polymeric phase change materials decreased with the increase of the rigidity of the main chains. The graft ratio of comb-like polymeric phase change materials also decreased with the increase of the length of the alkyl side chains. Ahmet Sari et al. [15] synthesized a new kind of polymeric solid-solid phase change materials (SSPCMs) polystyrene-graft-PEG copolymers, in which polystyrene was used as a hard segment, and PEG6000 was used as a soft segment. By controlling the feed ratio of polystyrene and PEG6000 to 4:1, 2:1, and 1:1, the synthesized SSPCMs had typical solid-solid phase transition characteristics, with the phase transition temperatures in the range of 55–58 ◦C and the latent heat enthalpy in the range of 116–174 J/g. The reaction also belongs to the grafting reaction, in which the benzene ring connects the flexible backbone and the side chains. Shi et al. [16] synthesized poly(vinyl alcohol)-g-octadecanol copolymer-based solid-solid phase change materials through the “grafting to” method, and the heat enthalpy changed from 39 to 61 J/g under the grafting ratios of 283% and 503%. Between poly(vinyl alcohol) and octadecanol, 2, 4-toluene diisocyanate (TDI) was used as a linking group. Therefore, in order to overcome the shortcomings of the graft polymer phase change materials, we decided to prepare a new homopolymer comb-like phase change material, and our research group made the following attempts. Meng et al. [8] first designed and synthesized the monomer polyethylene glycol octadecyl methacrylate, then prepared poly(polyethylene glycol octadecyl ether methacrylate) (poly(C18E2MMA)) with flexible backbones through free radical polymerization. The melting and crystallizing enthalpy of poly(C18E2MMA) were 73 and 81 J/g, respectively, which started to melt at 41.1 ◦C and crystallize at 35.4 ◦C. Zhang et al. [7] synthesized diethylene glycol hexadecyl ether acrylate (C16E2AA), and then poly(diethylene glycol hexadecyl ether acrylate) (PC16E2AA) was prepared by ◦ ◦ free radical polymerization. PC16E2AA melted at 33.8 C and crystallized at 25.8 C, and the melting and crystallizing enthalpy were 90 and 85 J/g, respectively. However, free radical polymerization has the problems of uncontrollable molecular weights and wide molecular weight distribution. In order to obtain homopolymers with controllable molecular weights and narrow molecular weight distributions, we choose the method of cationic polymerization. The experimental process consists of two parts: the synthesis of suitable monomers and the cationic polymerization. At present, many researchers report the preparation of phase change materials by grafting, physical/chemical hybrid methods, and so on. However, there are too many uncertainties in these methods, resulting in different performances of the phase change materials produced in different batches. In order to reduce the uncertainty, our research group proposed the preparation of homopolymer comb-like phase change materials, and this article will focus on the impact of different molecular weights on the energy storage performances of homopolymer phase change materials. To date, we have successfully prepared poly(polyethylene glycol n-alkyl ether vinyl ether)s (PC16E1VE, PC16E2VE, PC18E1VE, PC18E2VE) as homopolymers comb-like phase change materials [17]. Their phase transition enthalpies were all over 100 J/g, but the phase transition temperatures were a little high. So, the regular flexible main chain is the best choice in order to obtain comb-like polymeric phase change materials with good thermal performance. Polymers 2018, 10, 197 3 of 18

At present, studies on the inﬂuence of the number average molar mass of polymers on homopolymer comb-like phase change materials’ energy storage are still rare. This article will focus on the inﬂuence of different molecular weights on the phase transition enthalpy of homopolymer phase change materials. In order to achieve appropriate phase transition temperatures and low production costs, we selected n-tetradecane as the phase change matrix. Mono/diethylene glycol n-tetradecyl ether vinyl ethers (C14EnVEs) and a series of comb-like polymers-poly(mono/diethylene glycol n-tetradecyl ether vinyl ethers) (PC14EnVEs) (n = 1, 2) with various molecular weights were designed, synthesized, characterized, and discussed.

2. Materials and Methods

2.1. Materials

1-Bromotetradecane (C14H29Br) and n-tetradecane (C14H30) were purchased from TCI and used as received. Ethylene glycol mono-vinyl ether (E1VE) (TCI; >95.0%), diethylene glycol mono-vinyl ether (E2VE) (TCI; >96.0%), and isobutyl vinyl ether (IBVE) (TCI; >99.0%) were washed with a 0.1 M aqueous alkaline solution and then with water, and monomers were then distilled twice over calcium hydride and stored in a brown ampule under dry nitrogen in a refrigerator. Solvents and added bases (hexane, dimethyl sulfoxide, and ethyl acetate) were puriﬁed by the usual methods and distilled at least twice over CaH2 and metallic sodium (for hexane) just before use. Et1.5AlCl1.5 (Aldrich; 0.4 M in toluene) was used as supplied. 1-(Isobutoxy) ethyl acetate (CH3CH(OiBu)OCOCH3 (IBEA)), as a cationogen, was prepared by the addition reaction of isobutyl vinyl ether and acetic acid, and then IBEA was distilled over CaH2 under reduced pressure [18]. Sodium hydroxide (NaOH), aqua ammonia, dimethyl sulfoxide (DMSO), and diluted hydrochloric acid (HCl) were purchased from Tianjin Sailboat Chemical Reagent Co., Ltd., (Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China) and used as received.

2.2. Fabrication of Mono/diethylene glycol n-tetradecyl ether vinyl ethers

Monomers C14E1VE and C14E2VE were fabricated according to the methods of the published literature [19–21]. Taking the fabrication process of C14E1VE as an example, the experimental process was as follows: 6 g of NaOH was added to 200 mL of anhydrous dimethyl sulfoxide, and the mixture solution was stirred at 30 ◦C for 3 h under nitrogen atmosphere. Then, 0.1 mol of ethylene glycol mono-vinyl ether (E1VE) was added using a dry medical syringe, and the compounds were continuously stirred for 2–3 h, followed by the addition of 0.1 mol of 1-bromotetradecane under stirring using a dry medical syringe. The reaction was maintained for 48 h, and then the mixed solution was washed with deionized water. Next, the crude C14E1VE was purified by column chromatography elution with a solution of petroleum ether and ethyl acetate. The purified product was verified by 1 13 H NMR, C NMR, and FTIR spectroscopy. The synthetic process of C14E2VE was similar to that of C14E1VE.

2.3. Synthesis of Poly(mono/diethylene glycol n-tetradecyl ether vinyl ethers)

The polymerization of C14EnVEs (n = 1, 2) was carried out according to references [22–24]. The obtained crude product was precipitated at least twice in a volume ratio of 95/5 mixture of methanol and methylene chloride, followed by ﬁltration and subsequent vacuum drying overnight to obtain the product. Scheme1 gives the chemical formula of the preparation process of C 14EnVEs and PC14EnVEs (n = 1, 2). Polymers 2018, 10, 197 4 of 18 Polymers 2018, 10, x FOR PEER REVIEW 4 of 18 Polymers 2018, 10, x FOR PEER REVIEW 4 of 18 H H H H NaH/NaOH H CCH H CCH + Br-C14H29 NaH/NaOHDMSO CC H CCO + Br-C14H29 H O ROC14H29 ROH DMSO H O H O ROH ROC14H29 IBEA/Et1.5AlCl1.5 R = CH2CH2 (E1VE); IBEA/Et1.5AlCl1.5 R = CH2CH2OCH2CH2 (E2VE) R = CH2CH2 (E1VE); ROC H R = CH2CH2OCH2CH2 (E2VE) 14 29 ROC14H29

ROC14H29 ROC14H29

O O X: Added Base H C 3 O O X: Added Base H H3C CH2 CH CH X O C CH3 C C + H CH CH CH X O C CH O 2 H2 H2 - 3 C C + O Et1.5AlCl1.5 O H2 H2 - CH3 O Et AlCl H2C CH3OH 1.5 1.5 CH CH3 H2C CH3OH CH CH3 ROC14H29 ROC14H29 CH3 ROC14H29 ROC14H29

O O H3C O O

H3C CH2 CH CH OCH3 C C CH CH CH OCH O 2 H2 H2 3 C C O H2 H2 CH3 H2C CH CH3 H2C CH CH 3 CH3 Scheme 1. The chemical formula of C14EnVEs and PC14EnVEs (n = 1, 2). Scheme 1. The chemical formula of C14EnVEs and PC14EnVEs (n = 1, 2). Scheme 1. The chemical formula of C14EnVEs and PC14EnVEs (n = 1, 2). PC14E1VE and PC14E2VE with various molecular weights were successfully prepared by living cationicPC14 polymerization,EE11VEVE and and PC PC1414EE 2andVE2VE with withtheir various variousthermal molecular molecular properties weights weights are analyzed were were successfully successfully below. Scheme prepared prepared 2 shows by living the cationicdiagram polymerization, polymerization,of the fabrication and and process their their thermal of thermal PC14 propertiesE1VE prop anderties PC are14 analyzedEare2VE analyzed with below. various below. Scheme molecular Scheme2 shows weights. 2 the shows diagram the ofdiagram the fabrication of the fabrication process of process PC14E 1ofVE PC and14E1 PCVE14 andE2VE PC with14E2VE various with various molecular molecular weights. weights.

Scheme 2. The preparation process illustration of PC14E1VE and PC14E2VE with various molecular Schemeweights. 2. The preparation process illustration of PC14E1VE and PC14E2VE with various molecular Scheme 2. The preparation process illustration of PC14E1VE and PC14E2VE with various molecularweights. weights. 2.4. Characterization 2.4. Characterization Fourier Transform Infrared spectroscopy (FTIR) measurements were performed on a Bruker TENSOR37Fourier spectrometer, Transform Infrared and the spectroscopy spectra were processed(FTIR) measurements with OMNIC were software. performed The resolution on a Bruker was TENSOR37 spectrometer, and the spectra were processed with OMNIC software. The resolution was

Polymers 2018, 10, 197 5 of 18

2.4. Characterization Fourier Transform Infrared spectroscopy (FTIR) measurements were performed on a Bruker TENSOR37 spectrometer, and the spectra were processed with OMNIC software. The resolution was 4 cm−1, and 32 scans were accumulated. FTIR spectra were recorded in the range of 4000 to 500 cm−1 at roomPolymers temperature. 2018, 10, x FOR PEER REVIEW 5 of 18 A Bruker DMX-300 1H MHz nuclear magnetic resonance spectrometer (NMR) was used to 4 cm−1, and 32 scans were accumulated. FTIR spectra were recorded in the range of 4000 to 500 cm−1 determineat room the temperature. molecular structures of C14EnVEs and PC14EnVEs in CDCl3 at room temperature. The numberA Bruker average DMX-300 molar 1H MHz mass nuclear (Mn) and magnetic molecular resonance weight spectrometer distribution (NMR) (MWD) was of used PC14 toEn VEs (n =determine 1, 2) were the measured molecular structures using gel of permeationC14EnVEs and chromatographyPC14EnVEs in CDCl3 (GPC)at room intemperature. tetrahydrofuran at room temperature.The number average molar mass (Mn) and molecular weight distribution (MWD) of PC14EnVEs A(n TA= 1, Q20002) were differential measured using scanning gel permeation calorimeter chromatography (DSC) was used(GPC) to in study tetrahydrofuran the thermal at room behavior temperature. of PC14EnVEs. Specimens of 3 to 5 mg were encapsulated in an aluminum pan under a nitrogen atmosphereA TA and Q2000 first differential heated from scanning−30 to calorimeter 80 ◦C at a(D rateSC) was of 10 used◦C/min to study and the kept thermal at 80 behavior◦C for of 2 min. Subsequently,PC14EnVEs. the Specimens specimen of was 3 to cooled 5 mg towere−30 encapsul◦C at −ated10 ◦inC/min an aluminum and maintained pan under for a 2 nitrogen min. Finally, atmosphere and first heated from −30 to 80 °C at a rate of 10 °C/min and kept at 80 °C for 2 min. the specimen was heated again from −30 to 80 ◦C at a rate of 10 ◦C/min. The DSC thermograms in Subsequently, the specimen was cooled to −30 °C at −10 °C/min and maintained for 2 min. Finally, the firstthe coolingspecimen and was second heated heatingagain from processes −30 to 80 were °C at recorded.a rate of 10 °C/min. The DSC thermograms in the Thermogravimetricfirst cooling and second analysis heating (TGA) processes was were performed recorded. using a NETZSCH STA 409 PC/PG TG-DTA ◦ ◦ from 25 toThermogravimetric 600 C with a heating analysis rate (TGA) of 10 wasC/min performed under using a nitrogen a NETZSCH atmosphere. STA 409 PC/PG TG-DTA from 25 to 600 °C with a heating rate of 10 °C/min under a nitrogen atmosphere. 3. Results and Discussion 3. Results and Discussion 3.1. Structure of Mono/diethylene glycol n-tetradecyl ether vinyl ether 3.1. Structure of Mono/diethylene glycol n-tetradecyl ether vinyl ether Figure1 presents the FTIR spectra of mono/diethylene glycol n-tetradecyl ether vinyl ether. Figure 1 presents the FTIR spectra of mono/diethylene glycol n-tetradecyl ether vinyl ether. In In Figure1A (C 14H29Br (a), C14E1VE (b)), the strong adsorption peaks located at approximately Figure−1 1A (C14H29−Br1 (a), C14E1VE− (b)),1 the strong adsorption−1 peaks−1 located at approximately 1621 cm−1, 1621 cm , 1204 cm , 1126 cm , and 730 cm /719 cm were assigned to C=C, C–O–C2H4, 1204 cm−1, 1126 cm−1, and 730 cm−1/719 cm−1 were assigned to C=C, C–O–C2H4, and C2H4–O–C14H29 and C2H4–O–C14H29 stretching vibration bands and the (CH2)n (n > 4) rocking bands in C14E1VE, stretching vibration bands and the (CH2)n (n > 4) rocking bands in C14E1VE, respectively. None of respectively. None of these specific peaks were observed in the spectrum of the C14H29Br, exceptfor these specific peaks were observed in the spectrum of the C14H29Br, exceptfor the absorption bands at the absorption bands at 730 cm−1/719 cm−1. In Figure1B (C H Br (a), C E VE (b)), the stretching 730 cm−1/719 cm−1. In Figure 1B (C14H29Br (a), C14E2VE (b)), the14 stretching29 vibration14 2 bands of C14E2VE −1 −1 −1 −1 vibrationat 1624 bands cm−1,of 1216 C14 cmE2−VE1, 1156 at 1624cm−1, cmand 1141, 1216 cm− cm1 were, 1156characteristic cm , and peaks 1141 of C=C, cm C–O–Cwere2H characteristic4, C2H4– peaksO–C of C=C,2H4, and C–O–C C2H42–O–CH4,C142HH294, –O–Crespectively,2H4, and and C 2theH4 –O–Crocking14 Hbands29, respectively, at 730 cm−1/719 and cm the−1 rockingalso were bands −1 −1 at 730assigned cm /719 to (CH cm 2)n also(n > were4). These assigned characteristic to (CH 2peaks)n (n >demonstrate 4). Thesecharacteristic that C14EnVEs peaks(n = 1,demonstrate 2) were that Csuccessful14EnVEs (synthesized.n = 1, 2) were successful synthesized.

A B b C E VE 14 1 b C E VE 14 2

C H Br C H Br a 14 29 a 14 29 Transmitance (%) Transmitance (%) Transmitance

3000 1500 1000 3000 1500 1000 -1 -1 Wavenumber (cm ) Wavenumber (cm )

Figure 1. FTIR spectra of C14E1VE (A) and C14E2VE (B): C14E29Br (a) and corresponding C14EnVEs (b) Figure 1. FTIR spectra of C E VE (A) and C E VE (B): C E Br (a) and corresponding C E VEs (n = 1, 2). 14 1 14 2 14 29 14 n (b)(n = 1, 2).

The 1H NMR and 13C NMR spectra of C14EnVEs are shown in Figure 2. As shown in Figure 2a, 1 13 Thefor C14HE1 NMRVE, the and chemicalC NMR shift spectraat 0.88 ppm of C (a)14E wasnVEs characteristic are shown of in the Figure H atoms2. As in shown the CH in3 group, Figure 2a, and the H atoms of methylene in C13H26 were located at 1.26 (b), 1.60 (c), and 3.47 (d) ppm [25]. The for C14E1VE, the chemical shift at 0.88 ppm (a) was characteristic of the H atoms in the CH3 group, chemical shifts of H atoms at 3.65 ppm (e) and 3.83 ppm (f) were characteristic of the CH2 groups in O–CH2CH2–O; the maximum chemical shift at 6.50 ppm (i) was characteristic of the H atom of C=CHO; and 4.20 ppm (h) and 4.01 ppm (g) were characteristic of the H atoms in CH2=C. As shown

Polymers 2018, 10, 197 6 of 18

and the H atoms of methylene in C13H26 were located at 1.26 (b), 1.60 (c), and 3.47 (d) ppm [25]. The chemical shifts of H atoms at 3.65 ppm (e) and 3.83 ppm (f) were characteristic of the CH2 groups in O–CH2CH2–O; the maximum chemical shift at 6.50 ppm (i) was characteristic of the H atom of C=CHO; and 4.20 ppm (h) and 4.01 ppm (g) were characteristic of the H atoms in CH2=C. As shown in Figure2b, the chemical shift of 14.1 ppm (a) was assigned to the C atom of the CH 3 group in the side Polymers 2018, 10, x FOR PEER REVIEW 6 of 18 chain, and the peaks at 22.7 (b), 26.1 (c), 29.3 (d), 29.6 (e), 31.9 (f), and 70.1 (i) ppm were characteristic of theinC Figure atoms 2b, of the the chemical CH2 groups shift of in14.1 the pp alkylm (a) sidewas assigned chain. The to the chemical C atom of shifts the CH at3 67.3 group ppm in the (g) and 69.0 ppmside chain, (h) were and characteristic the peaks at of22.7 methylene (b), 26.1 carbon(c), 29.3 atoms (d), 29.6 between (e), 31.9 two (f), ether and bonds,70.1 (i) and ppm the were C atoms 1 of C=Ccharacteristic were located of the at C 86.5 atoms ppm of (j)the and CH2 151.8 groups ppm in the (k). alkyl The sideH chain. NMR The spectrum chemical of shifts C14E 2atVE 67.3 is ppm shown in Figure(g)2 c.and As 69.0 shown ppm in (h) the were ﬁgure, characteristic the chemical of methylene shift at 0.88carbon ppm atoms (a) wasbetween characteristic two ether bonds, of the Hand atoms the C atoms of C=C were located at 86.5 ppm (j) and 151.8 ppm (k). The 1H NMR spectrum of C14E2VE in the CH3 group derived from the alkyl side chain, and the characteristic H atoms of methylene in is shown in Figure 2c. As shown in the figure, the chemical shift at 0.88 ppm (a) was characteristic of C13H26 were located at 1.26 (b), 1.60 (c), and 3.47 (d) ppm. The chemical shifts at 3.65 ppm (e) and the H atoms in the CH3 group derived from the alkyl side chain, and the characteristic H atoms of 3.83 ppm (f), 3.89 ppm (g) and 3.91 ppm (h) were the characteristic of the H atoms of CH2 groups in methylene in C13H26 were located at 1.26 (b), 1.60 (c), and 3.47 (d) ppm. The chemical shifts at 3.65 –O–CH CH –O–CH CH –O–; 6.50 ppm (k) represented the H atom of C=CHO–; and the 4.20 ppm (i) ppm2 (e)2 and 3.832 ppm2 (f), 3.89 ppm (g) and 3.91 ppm (h) were the characteristic of the H atoms of and 4.01 ppm (j) shifts were the characteristic of the H atoms in CH =C. Furthermore, Figure2d shows CH2 groups in –O–CH2CH2–O–CH2CH2–O–; 6.50 ppm (k) represented2 the H atom of C=CHO–; and 13 the theCNMR 4.20 ppm spectrum (i) and 4.01 of C ppm14E2 VE.(j) shifts The were chemical the characteristic shift at 14.1 of ppm the H (a) atoms was assignedin CH2=C. toFurthermore, the C atom of a CH3 Figuregroup 2d in shows the side the chain, 13C NMR and spectrum the peaks of C at14E 22.72VE. (b), The 26.1 chemical (c), 29.3 shift (d), at 14.1 29.6 ppm (e), (a) 31.9 was (f), assigned and 70.1 (k) ppmto were the C the atom characteristic of a CH3 group of thein the C side atoms chain, of theand CHthe 2peaksgroups at 22.7 in the(b), 26.1 alkyl (c), side 29.3 chain. (d), 29.6 The (e), chemical31.9 shifts(f), at and approximately 70.1 (k) ppm were 67.3 the ppm characteristic (g) and 69.0 of the ppm C atoms (h, i, of j, g)the were CH2 groups characteristic in the alkyl of side the methylenechain. carbonThe atoms chemical between shifts at the approximately two ether bonds, 67.3 ppm and (g) the and C atoms 69.0 ppm of C=C(h, i, werej, g) were located characteristic at 86.5 ppm of the (l) and 151.8methylene ppm (m). carbon Evidence atoms from between the NMRthe two measurements ether bonds, and along the C with atoms the of FTIRC=C were spectra located indicated at 86.5 that ppm (l) and 151.8 ppm (m). Evidence from the NMR measurements along with the FTIR spectra C14EnVEs (n = 1, 2) were synthesized successfully. indicated that C14EnVEs (n = 1, 2) were synthesized successfully.

a b

g H H i H H CC CC H  CH CH OCH CH (CH ) CH h 2 2 2 2 2 11 3 H  CH2CH2OCH2CH2CH2(CH2)8CH2CH2CH3 f e d c b a j k g h i f c e d b a

f c b a i g k j d,e d b a h f e c i h g

876543210160 140 120 100 80 60 40 20 0  (ppm) ppm

d c H H i H H k CC CC H  CH2CH2OCH2CH2OCH2CH2CH2(CH2)8CH2CH2CH3 j m H  CH2CH2OCH2CH2OCH2CH2(CH2)11CH3 l g j i h k f c e d b a h g f e d c b a

h,i,j f c b a k g d,e m l f e ihg d b a k j c

876543210140 120 100 80 60 40 20 0 ppm ppm

Figure1 2. 1H NMR spectra of C14EnVEs (a,c) and 13C NMR spectra of C14EnVEs (b,d) (n = 1, 2). Figure 2. H NMR spectra of C14EnVEs (a,c) and C NMR spectra of C14EnVEs (b,d)(n = 1, 2). 3.2. Thermal Behavior of Mono/diethylene glycol n-tetradecyl ether vinyl ether

Polymers 2018, 10, 197 7 of 18

3.2. Thermal Behavior of Mono/diethylene glycol n-tetradecyl ether vinyl ether Polymers 2018, 10, x FOR PEER REVIEW 7 of 18 Figure3 shown the DSC curves of C 14EnVEs (n = 1, 2) in the heating and cooling process. The phaseFigure change 3 shown behaviors the DSC of C 14curvesH30 and of C C1414EHnVEs29Br (n are = 1, also 2) in presented the heating in Figureand cooling3 for comparison.process. The Thephase thermal change parameters behaviors of C14HH3030 and,C14 CH1429HBr,29Br and are C also14En VEspresented are also in listedFigure in 3 Tablefor comparison.1. The melting The andthermal crystallization parameters temperatures of C14H30, C14 ofH29 CBr,14H and29Br, C C1414EnHVEs30,C are14 Ealso1VE, listed and Cin14 TableE2VE were1. The 8.6, melting 10.1, 17.9,and ◦ ◦ andcrystallization 21.9 C, andtemperatures−11.1, − of3.4, C14−H2.529Br, and C14H 0.530, CC,14E respectively.1VE, and C14E2VE For were C14H 8.6,29Br, 10.1, the 17.9, presence and 21.9 of bromine°C, and − atoms11.1, −3.4, greatly −2.5 affectsand 0.5 the °C, regularityrespectively. of theFor arrangementC14H29Br, the ofpresence the 14 carbonof bromine atoms atoms during greatly the phaseaffects changethe regularity process. of the By arrangement contrast, the of effects the 14 of carbon the substituted atoms during groups the phase CH2=CH–OCH change process.2CH2 O–By andcontrast, CH2 =CH–OCHthe effects 2ofCH the2O–CH substitute2CH2dO– groups on the CH movement2=CH–OCH of2CH the2O– 14 carbonand CH atoms2=CH–OCH in the2CH phase2O– transitionCH2CH2O– process on the movement were less thanof the that 14 carbon of the atoms bromine in the atom. phase The transition phase transitionprocess were enthalpy less than of Cthat14H of29 Brthe wasbromine the smallest,atom. The but phase the transition phase change enthalpy enthalpy of C14H of29Br C 14wasE1 VEthe wassmallest, closest but tothe that phase of nchange-tetradecane. enthalpy In of addition, C14E1VE whenwas closest the substituents to that of n-tetradecane. were changed In from addition, CH2 =CH–OCHwhen the substituents2CH2O– to CHwere2=CH–OCH changed 2CHfrom2O–CH CH2=CH–OCH2CH2O–, the2CH thermal2O– to enthalpy CH2=CH–OCH values decreased2CH2O–CH from2CH 2452O–, to the 201 J/g.thermal The numberenthalpy averagevalues decreased molar mass, from steric 245 hindrance,to 201 J/g. The and number other factors average of themolar substituents mass, steric all hindrance, affect the arrangementand other factors of the of longthe substituents alkyl chains. all As affect the numberthe arrangement average molarof the masslong alkyl of the chains. substituent As the increases, number theaverage proportion molar ofmass the amorphousof the substituent content alsoincreases, increases, the making proportion it harder of the for someamorphous of the carboncontent atoms also nearincreases, the substituent making it harder to enter for the some lattice. of the The carbon volume atoms of bromine near the atoms substituent is larger to enter than thatthe lattice. of oxygen The atoms.volume Comparedof bromine with atoms the is phase larger change than that enthalpy of oxygen of C 14atoms.H29Br Compared and C14E1 VE,with it the is not phase difficult change to findenthalpy that theof C steric14H29Br effect and is C more14E1VE, affected it is not than difficult the number to find averagethat the steric molar effect mass is of more the substituent affected than on thethe number alkyl chain average crystallization. molar mass The of th othere substituent factors containon the alkyl the intermolecularchain crystallization. interactions The other and factors so on. Furthermore,contain the intermolecular the supercooling interactions phenomenon and so of on. C14 Furthermore,EnVEs is more the obvious supercooling than of phenomenon the brominated of tetradecaneC14EnVEs is more and nobvious-tetradecane. than of the This brominated may be because tetradecane brominated and n-tetradecane. tetradecane This and mayn-tetradecane be because werebrominated used as tetradecane commercially and n supplied,-tetradecane so theywere may used contain as commercia a fewlly impurities, supplied,while so they the may C14 containEnVEs monomersa few impurities, were obtained while the after C14 repeatedEnVEs monomers purifications. were Theobtained impurities, after repeated which may purifications. act as nuclei The in hetero-nucleation,impurities, which may were act removed as nuclei in in the hetero-nucleation, purifying process, were so theremoved crystallization in the purifying of C14E nprocess,VEs were so dominatedthe crystallization by homogeneous of C14EnVEs nucleation. were dominated by homogeneous nucleation.

C H 14 30 Endo

C E VE 14 2

C H Br C E VE 14 29 14 1 Heat Flow (W/g)

-20 -10 0 10 20 30 Temperature (C)

FigureFigure 3. 3.DSC DSC curves curves of of C C1414EEnnVEs,VEs, CC1414H3030,, and and C C1414HH2929Br.Br.

Figure 4 presents the TGA curves of C14EnVEs, C14H30, and C14H29Br. It can be seen that when the Figure4 presents the TGA curves of C 14EnVEs, C14H30, and C14H29Br. It can be seen that when theterminal terminal hydrogen hydrogen atoms atoms of n of-tetradecanen-tetradecane were were replaced replaced by other by other groups, groups, the thethermal thermal stability stability of the of thecorresponding corresponding materials materials increased increased as asthe the volume volume of of the the substituen substituentt increased. increased. According According to to thethe references, thethe thermal thermal stability stability of theof the experimented experimented materials materials was characterizedwas characterized by the initialby the thermal initial thermal decomposition temperature T5wt %, when the weight loss rate of materials reached 5%. The T5wt % of C14H30, C14H29Br, C14E1VE, and C14E2VE were 130, 174, 191 and 214 °C, respectively.

Polymers 2018, 10, 197 8 of 18

decomposition temperature T5wt %, when the weight loss rate of materials reached 5%. The T5wt % of ◦ C14H30,C14H29Br, C14E1VE, and C14E2VE were 130, 174, 191 and 214 C, respectively.

Table 1. DSC and TG measurement results of C14H30,C14H29Br, and C14EnVE (n = 1, 2).

◦ ◦ Tm ( C) Tc ( C) ◦ 5 ◦ 6 Specimen ∆Hm (J/g) ∆Hc (J/g) ∆T ( C) T5wt % ( C) 1 2 3 4 Tmo Tmp Tco Tcp

C14H29Br 1.2 8.6 −8.2 −11.1 138 −136 9.4 174 C14H30 3.6 11.7 3.8 −2.8 210 −209 −0.2 130 C14E1VE 11.8 19.6 1.1 −2.3 193 −193 10.7 191 C14E2VE 15.6 24.2 2.5 −0.4 199 −197 13.1 214 1 Onset temperature of melting point; 2 peak temperature of melting point; 3 onset temperature of crystallization 4 5 6 point; peak temperature of crystallization point; ∆T = Tmo−Tco; the initial thermal decomposition temperature. Polymers 2018, 10, x FOR PEER REVIEW 8 of 18

100 C E VE (T = 214 C) 14 2 5wt%  80

60 C E VE(T = 191 C) 14 1 5wt%  40 C H (T = 130 C) 14 30 5wt%  Mass (%)

20 C H Br (T = 174 C) 14 29 5wt%  0

100 200 300 400 500 600 Temperature (C)

FigureFigure 4. 4.TGA TGA curves curves of of C C1414EEnnVEs, C1414HH3030, ,and and C C1414HH2929Br.Br.

3.3. StructureTable of Poly(mono/diethylene 1. DSC and TG measurement glycol n-tetradecyl results of C ether14H30, vinyl C14H29 ether)Br, and C14EnVE (n = 1, 2).

−1 The FTIR spectraT ofm (°C) PC14 EnVEsTc are (°C) shown in Figure5. No peaks appeared near 1621 cm in Specimen ΔHm (J/g) ΔHc (J/g) ΔT (°C) 5 T5wt % (°C) 6 1 2 3 4 the spectra of PC14ETmonVEs,T whichmp T suggestedco Tcp that the double bonds changed to single bonds. From −1 the FTIRC14 spectrumH29Br of1.2 PC 14E8.61VE (a),−8.2 the − single11.1 ether138 bond at 1126−136 cm replaced9.4 the sharp 174 bands at −1 −1 1204 andC 112614H30 cm 3.6for C1411.7E1VE, 3.8 and the−2.8 band in210 the spectrum−209 of PC14E−0.22VE (b) at 1123130 cm also −1 replacedC the14E1VE bands at11.8 1216, 19.6 1156, and1.1 1141−2.3 cm for193 C 14E2VE.− These193 results10.7 proved that 191 the cationic −1 polymerizationsC14E2VE were15.6 successfully 24.2 2.5 completed. −0.4 The199 characteristic −197 absorption13.1 band at 214 720 cm was attributed1 Onset to temperature the rocking of vibrations melting point; of (CH 2 peak2)n groupstemperature (n > 4)of inmelting PC14 Epoint;nVEs, 3 whichonset temperature indicated thatof the crystalcrystallization form of the point; alkyl side4 peak chain temperature was changed of crystallization from the orthorhombic point; 5 ΔT = T crystalmo−Tco; 6 in the the initial monomers thermal to the hexagonaldecomposition crystal intemperature. the polymers. 13 Figure6 shows the C NMR spectra of PC14E1VE and PC14E2VE. Taking PC14E2VE as an example, 3.3.it was Structure easily determined of Poly(mono/diethylene that the chemical glycol shift n-tetradecyl values of ether C=C vinyl changed ether) from 86.5 ppm (l) and 151.8 ppm

(m) toThe 31.9 FTIR (g) andspectra 74.1 of (m), PC by14En comparingVEs are shown Figures in 2Figured and 65.. TheNo peaks chemical appeared shift values near of1621 other cm− carbon1 in the spectraatoms in of the PC14 sideEnVEs, chains which showed suggested almost that no the change. double bonds changed to single bonds. From the FTIR spectrum of PC14E1VE (a), the single ether bond at 1126 cm−1 replaced the sharp bands at 1204 and 1126 cm−1 for C14E1VE, and the band in the spectrum of PC14E2VE (b) at 1123 cm−1 also replaced the bands at 1216, 1156, and 1141 cm−1 for C14E2VE. These results proved that the cationic polymerizations were successfully completed. The characteristic absorption band at 720 cm−1 was attributed to the rocking vibrations of (CH2)n groups (n > 4) in PC14EnVEs, which indicated that the crystal form of the alkyl side chain was changed from the orthorhombic crystal in the monomers to the hexagonal crystal in the polymers. Figure 6 shows the 13C NMR spectra of PC14E1VE and PC14E2VE. Taking PC14E2VE as an example, it was easily determined that the chemical shift values of C=C changed from 86.5 ppm (l) and 151.8 ppm (m) to 31.9 (g) and 74.1 (m), by comparing Figure 2d and Figure 6. The chemical shift values of other carbon atoms in the side chains showed almost no change.

Polymers 2018, 10, 197 9 of 18 Polymers 2018, 10, x FOR PEER REVIEW 9 of 18

Polymers 2018, 10, x FOR PEER REVIEW 9 of 18

a 720

b 720 1126

b 1126 721 Transmittance (%)

721 Transmittance (%) 1123

3500 3000 2500 2000 1500 1000 500 1123 Wavenumber (cm-1) 3500 3000 2500 2000 1500 1000 500 Figure 5. FTIR spectra of PC14E1VE (a)-1 and PC14E2VE (b). Figure 5. FTIR spectraWavenumber of PC14E1VE (cm (a) and) PC14E2VE (b). H H Figure 5. FTIR spectra of PC14E1VE (a) and PC14E2VE (b). H H * CC * * CC * H H H g l  CH2CH2OCH2CH2CH2(CH2)7CH2CH2CH2CH3 H H CH CH OCH CH OCH CH CH (CH ) CH CH CH CH * CC * ij kf cde h b a g m 2 2 2 2 2 2 2 2 7 2 2 2 3 d,e,f,g * CC * ijk j l f c e d h b a AH g l  CH2CH2OCH2CH2CH2(CH2)7CH2CH2CH2CH3 ij kf cde h b a BH g m  CH2CH2OCH2CH2OCH2CH2CH2(CH2)7CH2CH2CH2CH3 ijk b a d,e,f,g j l f d,e,fc e d h A h B g,h d,e,f h b b a l g,h k,l c c a j,k b b i j i a m l k,l c c a j,k 80 70j 60i 50 40 30 20 10 0 80m 70 60i 50 40 30 20 10 0

 (ppm) (ppm) 80 70 60 50 40 30 20 10 0 80 70 60 50 40 30 20 10 0

13 Figure (ppm) 6. C NMR spectra in CDCl3 of PC14E1VE (A) and PC14(ppm)E2VE (B).

1313 In order to systematicallyFigureFigure 6. 6. CC NMR NMR study spectra spectra the ininfluence in CDCl CDCl3 3of ofof PC PC the1414EE 1number1VE (A)) andand average PCPC1414EE 22molecularVEVE ( (BB).). weight on the phase transition enthalpy of homopolymer type phase change materials, a series of comb-like In order to systematically study the influence of the number average molecular weight on the homopolymersIn order to with systematically different mole studycular the weights inﬂuence used of as the phase number chan averagege materials molecular were prepared weight onby phase transition enthalpy of homopolymer type phase change materials, a series of comb-like theliving phase cationic transition polymerization. enthalpy of The homopolymer number average type phasemolecular change weight materials, (Mn) and a series molecular of comb-like weight homopolymers with different molecular weights used as phase change materials were prepared by homopolymersdistribution (MWD) with differentof a series molecular of PC14E weights1VEs and used PC14 asE phase2VEs with change various materials molecular were preparedweights are by shownliving cationic in Figures polymerization. 7 and 8, respectively. The number These averagepolymers molecular PC14EnVEs weight (n = 1, (2)M weren) and measured molecular using weight gel living cationic polymerization. The number average molecular weight (Mn) and molecular weight permeationdistribution chromatography(MWD) of a series (GPC) of PCin THF14E1VEs at room and PCtemperature.14E2VEs with The various results molecularare listed in weights Table 2. are distribution (MWD) of a series of PC14E1VEs and PC14E2VEs with various molecular weights are shown in Figures 7 and 8, respectively. These polymers PC14EnVEs (n = 1, 2) were measured using gel shown in Figures7 and8, respectively. These polymers PC 14EnVEs (n = 1, 2) were measured using gel permeation chromatography (GPC) in THF at room temperature. The results are listed in Table 2. permeation chromatography (GPC) in THF at room temperature. The results are listed in Table2.

Polymers 2018, 10, 197 10 of 18 PolymersPolymers 2018 2018, 10, 10, x, xFOR FOR PEER PEER REVIEW REVIEW 1010 of of 18 18

-4 M 10-4 -4-4 M n10 MM 1010 n nn (M /M ) (M /wM )n (M(M/M/M) ) w n w w n n

2.5 1.7 1.7 2.5 (1.40) (1.59)(1.59) (1.40)

1.3

1.3 2.3 2.3 (1.42)(1.42) (1.46)(1.46)

1.1

1.1 2.2 2.2 (1.21)(1.21) (1.58)(1.58)

0.7 0.7

(1.18)(1.18) 1.9 1.9 (1.45)(1.45)

1515 16 16 17 17 18 18 19 19 20 20 1515 16 16 17 17 18 18 19 19 20 20 EV(mL) EV(mL) EV(mL) EV(mL)

FigureFigure 7. 7. M Mn nand and MWD MWD of of PC PC1414EE1VE1VE obtained obtained using using IBEA/Et IBEA/Et1.51.5AlClAlCl1.51.5 in in n -hexane,n-hexane, with with ethyl ethyl acetate acetate Figure 7. Mn and MWD of PC14E1VE obtained using IBEA/Et1.5AlCl1.5 in n-hexane, with ethyl acetate added as a base at 30◦ °C: [monomer]0 0 = (0.1, 0.15, 0.19, 0.24, 0.27, 0.33, 0.35, 0.37); [IBEA]0 0 = 4.0 mM; addedadded as as a base at 30 °C:C: [monomer]0 == (0.1, (0.1, 0.15, 0.15, 0.19, 0.19, 0.24, 0.24, 0.27, 0.27, 0.33, 0.35, 0.37); [IBEA] 0 == 4.0 4.0 mM; mM; [Et1.5AlCl1.5]0 = 20 mM; [added base]0 = 1.0 M, in n-hexane, with various reaction times. [Et[Et1.51.5AlClAlCl1.51.5]0] =0 20= 20 mM; mM; [added [added base] base]0 =0 1.0= 1.0 M, M, in inn-hexane, n-hexane, with with various various reaction reaction times. times.

-4 M 10-4 -4-4 M n10 MM 1010 n nn (M /M ) (M /wM )n (M(M/M/M) )

w n w n

w n

1.2 1.2 2.3 2.3 (1.13)(1.13) (1.55)(1.55)

1.0 1.0

(1.19)(1.19) 2.0 2.0 (1.49)(1.49)

0.9 0.9

(1.31)(1.31) 1.7 1.7 (1.38)(1.38)

0.6 0.6 1.4 1.4

(1.14)(1.14) (1.14)(1.14)

1515 16 16 17 17 18 18 19 19 20 20 1515 16 16 17 17 18 18 19 19 20 20 EV (mL) EVEV (mL) (mL) EV (mL)

Figure 8. Mn and MWD of PC14E2VE obtained using IBEA/Et1.5AlCl1.5 in n-hexane, with ethyl acetate FigureFigure 8. Mn andand MWD of PC1414EE22VEVE obtained obtained using IBEA/Et1.51.5AlClAlCl1.51.5 inin nn-hexane,-hexane, with with ethyl ethyl acetate acetate added as a base at 30◦ °C: [monomer]0 = (0.07, 0.11, 0.13, 0.15, 0.17, 0.21, 0.26, 0.3); [IBEA]0 = 4.0 mM; addedadded as as a base at 30 °C:C: [monomer]0 == (0.07, (0.07, 0.11, 0.11, 0.13, 0.13, 0.15, 0.15, 0.17, 0.17, 0.21, 0.26, 0.3); [IBEA] 00 == 4.0 4.0 mM; mM; [Et1.5AlCl1.5]0 = 20 mM; [added base]0 = 1.0 M, in n-hexane, with various reaction times. [Et[Et1.51.5AlClAlCl1.51.5]0] =0 20= 20 mM; mM; [added [added base] base]0 =0 1.0= 1.0 M, M, in inn-hexane,n-hexane, with with variou variouss reaction reaction times. times.

Polymers 2018, 10, 197 11 of 18

Table 2. PC14EnVE (n = 1, 2) obtained by living cationic polymerization.

−4 Specimen Mn × 10 Dispersity ∆Hm (J/g) Xc (%) Nc

PC14E1VE 0.7 1.18 95 33.5 6.8 PC14E1VE 2.5 1.40 89 31.4 6.3 PC14E2VE 0.6 1.14 93 32.8 7.6 PC14E2VE 2.3 1.55 83 29.3 6.8

In order to further analyze the impact of different molecular weights on the crystallization properties of polymers, the crystallinity of polymers and the number of crystallizable carbon atoms per side chain were calculated by Equations (1)–(3) [26,27]:

∆Hm = nk + ∆Hm,e, (1)

Nc = ∆Hm/k, (2)

Xc = (Nc × 14.026)/Munit, (3) where k is the contribution of each added CH2 group to the enthalpy; ∆Hm,e is a constant reﬂecting the contribution of the chain end to the enthalpy; Nc is the number of crystallizable CH2 groups; and Xc is the crystallinity of the polymers PC14EnVEs. The number of crystallizable CH2 groups (Nc) and the crystallinity (Xc) of the polymers PC14EnVEs are listed in Table2. The Nc of PC14E1VE decreased from 6.8 to 6.3 as the Mn increased from 7000 to 25,000; and the Nc of PC14E2VE decreased from 7.6 to 6.8 as the Mn increased from 6000 to 23,000. In other words, during the crystallization at 10 ◦C/min, the number of amorphous carbon atoms next to the main chains of PC14E1VE and PC14E2VE were 7.7 and 7.2, respectively; while, for poly(n-alkyl acrylates) and poly(n-alkyl methacrylate), the number of amorphous carbon atoms in the side chains was over 8 and 12, respectively [9]. In addition, the Xc decreased from 33.5% to 31.4% when the Mn of PC14E1VE changed from 7000 to 25,000. As the number average molar mass increased, the crystallinity of the polymers and the number of crystallizable carbon atoms in the side chains of PC14E1VE and PC14E2VE were all reduced. The results were consistent with the previously reported literature [26,28]. For PC14EnVE, when n changed from 1 to 2, the number of crystallizable CH2 groups Nc were increased but the crystallinity of the polymers Xc was decreased simultaneously. The introduction of a ﬂexible ethylene glycol segment between the main chains and side chains can decrease the degree of the frustrated crystallization of side chains to a certain extent.

3.4. Thermal Behavior of Poly(mono/diethylene glycol n-tetradecyl ether vinyl ether)

The DSC curves of PC14E1VE and PC14E2VE with various molecular weights in the heating and cooling processes are shown in Figures9 and 10, respectively. Obvious endothermic and exothermic peaks, which were assigned to the melt and crystallization of part of the carbon atoms away from the main chains in the side chains, appeared in the DSC curves. The energy storage behavior parameters of PC14E1VE and PC14E2VE are listed in Tables3 and4, respectively. With the increase of the polymers’ molecular weight, the melting temperature (Tm) and freezing temperature (Tc) rose at the beginning and then tended toward a constant. When the number average molar mass of PC14E1VE and PC14E2VE were more than 20,000, the final melting temperature (Tm) and freezing temperature (Tc) were 28.0, 17.5 ◦C and 24.4, 16.3 ◦C, respectively. Due to the insertion of a flexible ethylene glycol segment between the main chains and side chains, the frustrated crystallization phenomenon of the side chains was somewhat alleviated. The movement of the side chains of PC14E2VE was more flexible than that of PC14E1VE. A total of 6.8 carbon atoms in the side chain of PC14E2VE can form crystals, while the number of crystallizable carbon atoms in the side chain of PC14E1VE was 6.3. PC14E2VE melted and froze at lower temperatures than those of PC14E1VE. Because the flexibility of the side chains was Polymers 2018, 10, 197 12 of 18 increased with the introduction of the diethylene glycol segment, the molecular thermal movement Polymers 2018, 10, x FOR PEER REVIEW 12 of 18 was more pronounced, and the side chain crystallization required a lower temperature. When the number average molar mass of PC EnVEs increased, the melting enthalpy (∆Hm) and crystallization number average molar mass of PC1414EnVEs increased, the melting enthalpy (ΔHm) and crystallization enthalpy (∆Hc) decreased at the beginning. When the number average molar mass of the polymer enthalpy (ΔHc) decreased at the beginning. When the number average molar mass of the polymer exceeded 20,000, the melting enthalpy values of PC E VE and PC E VE tended toward a constant of exceeded 20,000, the melting enthalpy values of PC1414E11VE and PC1414E2VE tended toward a constant of 89 and 83 J/g, respectively. respectively. Therefore, Therefore, in order to obtain homopolymer phasephase change materials with stable thermal performances, the number average mo molarlar mass of the polymers needs to be strictly controlled. The phase change enthalpy of poly(mono ethylene glycol n-tetradecyl ether vinyl ethers) (89 J/g)J/g) was was approximately approximately equal to that of poly(hexadecylpoly(hexadecyl acrylate)acrylate) (86(86 J/g)J/g) [29]. [29]. In the DSC curves of PC EnVEs (n = 1, 2), the single peak demonstrated that there was only one packing form for of PC14EnVEs (n = 1, 2), the single peak demonstrated that there was only one packing form for the theside side chains, chains, which which was wasthe hexagonal the hexagonal structure structure [8,30,31]. [8,30 In,31 order]. In orderto examine to examine the reliability the reliability of phase of phase change materials in the process of actual use, PC E VE (M = 2.3 × 10−4) and PC E VE change materials in the process of actual use, PC14E1VE (M14n = 12.3 × 10−n4) and PC14E2VE (Mn = 2.314 × 102 −4) × −4 (wereMn = chose 2.3 for10 a )thermal were chose cycling for atest. thermal After cycling a test of test. 200 After thermal a test cycles, of 200 the thermal shapes cycles, of the the DSC shapes curves of the DSC curves of PC E VEs were almost unchanged; the melting enthalpy (∆H ), crystallization of PC14EnVEs were almost14 n unchanged; the melting enthalpy (ΔHm), crystallizationm enthalpy (ΔHc), enthalpy (∆H ), melting temperature (T ), and freezing temperature (T ) for PC E VE and PC E VE melting temperaturec (Tm), and freezingm temperature (Tc) for PC14E1VEc and PC1414E2VE1 were 89,14 882 J/g, ◦ ◦ were28.0, 17.7 89, °C 88 and J/g, 82, 28.0, 83 J/g, 17.7 24.5,C and16.4 °C, 82, respectively. 83 J/g, 24.5, The 16.4 changesC, respectively. of these phase The change changes parameters of these phase change parameters were not significant, indicating that PC E VE and PC E VE as green were not significant, indicating that PC14E1VE and PC14E2VE as green14 and1 sustainable14 2materials can andbe recycled sustainable several materials times. can be recycled several times.

h B A g f e

Endo d c h b a

g Heat flow (W/g) f e Exo d Heat flow (W/g) c b a 10 20 30 40 0102030 Temperature (C) Temperature (C)

Figure 9. DSC thermograms of PC14E1VEs: (A) heating curves and (B) cooling curves. Figure 9. DSC thermograms of PC14E1VEs: (A) heating curves and (B) cooling curves.

Table 3. Phase change properties of PC14E1VEs with various molecular weights. Table 3. Phase change properties of PC E VEs with various molecular weights. Tm (°C)14 1 Tc (°C) Specimen Mn(GPC) × 10−4 Dispersity ΔHm (J/g) ΔHc (J/g) T5wt % (°C) mo mp co cp T ◦T T T◦ − Tm ( C) Tc ( C) ∆Hm ∆Hc T5wt % Specimen14 1 × 4 Dispersity PC E VE Mn(GPC) 0.7 10 1.18 23.1 27.0 20.3 17.3 95 (J/g) −96(J/g) 264◦ T T T T ( C) PC14E1VE 1.1 1.21 23.5mo 27.7mp 20.6 co17.1 cp 93 −92 267 PC1414E11VE 0.7 1.3 1.421.18 23.1 27.5 27.0 20.3 20.3 16.6 17.3 91 95 −92− 96269 264 PC E VE 1.1 1.21 23.5 27.7 20.6 17.1 93 −92 267 PC1414E11VE 1.7 1.59 23.8 27.9 21.0 17.2 90 −91 268 PC14E1VE 1.3 1.42 23.1 27.5 20.3 16.6 91 −92 269 PC14E1VE 1.9 1.45 23.2 27.7 20.7 16.9 89 −90 271 PC14E1VE 1.7 1.59 23.8 27.9 21.0 17.2 90 −91 268 14 1 PC14E1VE 1.9 2.2 1.581.45 23.723.2 27.8 27.7 20.6 20.7 17.1 16.9 89 89 −89− 90272 271 PC1414E11VE 2.2 2.3 1.461.58 23.823.7 28.1 27.8 21.0 20.6 17.6 17.1 89 89 −89− 89280 272 − PC1414E11VE 2.3 2.5 1.401.46 24.123.8 28.0 28.1 20.9 21.0 17.5 17.6 89 89 −89 89287 280 PC14E1VE 2.5 1.40 24.1 28.0 20.9 17.5 89 −89 287

Polymers 2018, 10, 197 13 of 18 Polymers 2018, 10, x FOR PEER REVIEW 13 of 18

A Endo h g

f e d c Heat flow (W/g) b a

10 20 30 40 Temperature (C)

h B g f e d c b a Heat flow (W/g) Exo

0102030 Temperature (C)

Figure 10. DSC thermograms of PC14EE22VEs:VEs: (A (A) )heating heating curves curves and and ( (BB)) cooling cooling curves. curves.

Table 4. Phase change properties of PC14E2VEs with various molecular weights.

Table 4. Phase change properties of PC14E2VEs with various molecular weights. Tm (°C) Tc (°C) Specimen Mn(GPC) × 10−4 Dispersity ΔHm (J/g) ΔHc (J/g) T5wt % (°C) Tmo ◦Tmp Tco Tcp◦ Tm ( C) Tc ( C) ∆Hm ∆Hc T Specimen M (GPC) × 10−4 Dispersity 5wt % PC14E2VE n 0.6 1.14 20.6 24.0 18.6 15.6 93 (J/g) -94(J/g) 228(◦C) Tmo Tmp Tco Tcp PC14E2VE 0.9 1.31 20.7 24.3 18.4 15.6 86 -86 230 PCPC14EE2VEVE 0.6 1.0 1.19 1.14 20.8 20.6 24.2 24.0 18.9 18.615.9 15.6 85 93 -85 -94 247 228 PC14E2VE 0.9 1.31 20.7 24.3 18.4 15.6 86 -86 230 PC14E2VE 1.2 1.13 19.5 24.0 18.9 15.3 85 -85 247 PC14E2VE 1.0 1.19 20.8 24.2 18.9 15.9 85 -85 247 PC14E2VE 1.4 1.14 20.1 24.4 18.9 16.3 84 -84 254 PC14E2VE 1.2 1.13 19.5 24.0 18.9 15.3 85 -85 247 PCPC14EE2VEVE 1.4 1.7 1.38 1.14 20.2 20.1 24.4 24.4 18.9 18.916.2 16.3 83 84 -83 -84 279 254 PCPC14EE2VEVE 1.7 2.0 1.49 1.38 20.7 20.2 24.6 24.4 19.0 18.916.3 16.2 83 83 -83 -83 284 279 PC E VE 2.0 1.49 20.7 24.6 19.0 16.3 83 -83 284 PC14E2VE 2.3 1.55 20.7 24.4 19.0 16.3 83 -83 286 PC14E2VE 2.3 1.55 20.7 24.4 19.0 16.3 83 -83 286 The relationship curves of different molecular weights, melting enthalpy, and phase transition temperaturesThe relationship of PC14E curvesnVEs are of shown different in Figure molecular 11. The weights, polyme meltingr backbone enthalpy, limited and the phase movement transition of thetemperatures alkyl side chains of PC14 soEn thatVEs only are shown part of in the Figure carbon 11 .atoms The polymer away from backbone the main limited chain the could movement enter into of thethe lattice alkyl side. As chains the linking so that group only partbetween of the the carbon main atomschain and away the from side the chain main changed chain could from enterethylene into glycolthe lattice. to diethylene As the linking glycol, groupthe phase between change the enth mainalpy chain and andphase the change side chain temperatures changed of from the polymer ethylene decreased.glycol to diethylene Because the glycol, molecular the phase thermal change motion enthalpy was and strengthened, phase change the temperatures side chain ofcrystallization the polymer occurreddecreased. at a Because lower temperature. the molecular As thermal can be seen motion from was Table strengthened, 2, the Nc of PC the14E side2VE chainwas larger crystallization than that ofoccurred PC14E1VE, at abut lower the enthalpy temperature. was lower As can than be seenthat of from PC14 TableE1VE,2 ,because the Nc inof PC PC1414EE2VE2VE the was mass larger fraction than ofthat the of amorphous PC14E1VE, butportion the enthalpy increased. was As lower shown than in thatFigure of PC11A-a,14E1 VE,when because the number in PC14 averageE2VE the molar mass massfraction of PC of14 theE1VEs amorphous was less portionthan 15,000, increased. the melt As enthalpy shown in decreased Figure 11 A-a,rapidly when with the the number increase average of the molecular weight; when the number average molar mass changed from 15,000 to 20,000, the melt enthalpy decreased slowly; and, lastly, when the number average molar mass exceeded 20,000, the

Polymers 2018, 10, x FOR PEER REVIEW 14 of 18

melt enthalpy was basically a constant. Meanwhile, the phase transition temperatures of PC14E1VEs (A-b (melting temperature Tm), A-c (freezing temperature Tc)) increased slowly with the increase of the molecular weight and finally also tended to a constant. The changes of melting enthalpy and phase transition temperatures of PC14E2VEs were similar to those of PC14E1VEs. The melt enthalpy of PC14E2VEs decreased quickly when the number average molar mass increased to 10,000; and the enthalpy decreased slowly when the number average molar mass changed from 10,000 to 15,000; when the number average molar mass exceeded 20,000, the melt Polymers 2018, 10, 197 14 of 18 enthalpy was basically a constant. The phase transition temperatures of PC14E2VEs also increased with the increase of the molecular weight, but the changes were not obvious. With the increase of molarmolecular mass weight, of PC14 Ethe1VEs entanglement was less than between 15,000, themolecules melt enthalpy became decreased more and rapidly more obvious, with the increaseand the ofmolecular the molecular thermal weight; motion when became the number slower. average Theref molarore, massmore changed energy fromwas 15,000needed to 20,000,to change the meltthe enthalpymolecular decreased motion state. slowly; For and,phase lastly, change when materials, the number the larger average the molarnumber mass average exceeded molar 20,000, weight the of meltthe polymer, enthalpy the was higher basically the aphase constant. transition Meanwhile, temper theature. phase In the transition polymeric temperatures phase change of PC materials,14E1VEs (A-bthe phase (melting change temperature enthalpy Twasm), clearly A-c (freezing affected temperature by the numberTc)) average increased molar slowly mass, with while the increase the effects of theon the molecular phase transition weight and temperatures ﬁnally also were tended not to obvious. a constant.

100 35 100 30

A temperature change phase B

30 95 temperature change phase a T a T 95 mp b mp b 25 90 25 90 85 20 20 T T cp c cp c 80

85 Melt enthalpy (J/g) 15

15 ( Melt enthalpy (J/g) enthalpy Melt 

75 C ( )  C

80 10 ) 70 10 0.5 1.0 1.5 2.0 2.5 0.5 1.0 1.5 2.0 2.5 -4 -4 M 10 M 10 n n

Figure 11. Melt enthalpy (a) and phase changechange temperaturestemperatures (b,(b, c)c) vs.vs. molecularmolecular weightsweights of of PC PC1414EE11VEs ((AA)) andand PCPC1414E22VEsVEs ( (BB).).

The thermal stability of PC14EnVEs was important in thermal energy storage applications, and The changes of melting enthalpy and phase transition temperatures of PC14E2VEs were similar to the thermal degradation temperature of PCMs (Td) was one of the key parameters for determining those of PC14E1VEs. The melt enthalpy of PC14E2VEs decreased quickly when the number average molarthe practical mass increased environment to 10,000; and processing and the enthalpy techniques decreased. Here, slowlyaccording when to the numberreferences, average we selected molar massthe temperature changed from at 10,000 which to 15,000;the weight when loss the numberrate of average materials molar was mass 5% exceeded as the 20,000,initial thethermal melt decomposition temperature T5wt %. The TGA curves of PC14E1VE (A) and PC14E2VE (B) with various enthalpy was basically a constant. The phase transition temperatures of PC14E2VEs also increased withmolecular the increase weights of are the shown molecular in Figure weight, 12. butIt was the clear changes that were the T not5wt % obvious. of PC14E With1VEs and the increasePC14E2VEs of molecularincreased from weight, 264 theto 287 entanglement °C and from between 228 to 286 molecules °C, respectively, became more when and the morenumber obvious, average and molar the molecularmass increased. thermal Because motion the became interactions slower. Therefore,between molecules more energy gradually was needed increased, to change the theintertwining molecular motionphenomenon state. Forbecame phase more change and materials, more obvious, the larger resulting the number in fewer average and fewer molar defects weight in molecules, of the polymer, and thethe thermal higher the stability phase of transition the polymer temperature. gradually increased In the polymeric with the phaseincrease change of the materials, molecular theweight phase of changethe polymers. enthalpy These was results clearly indicated affected that by the thenumber increase average of polymers’ molar molecular mass, while weights the effects can improve on the phasethe thermal transition stability temperatures to a certain were extent. not obvious. The thermal stability of PC14EnVEs was important in thermal energy storage applications, and the thermal degradation temperature of PCMs (Td) was one of the key parameters for determining the practical environment and processing techniques. Here, according to the references, we selected the temperature at which the weight loss rate of materials was 5% as the initial thermal decomposition temperature T5wt %. The TGA curves of PC14E1VE (A) and PC14E2VE (B) with various molecular weights are shown in Figure 12. It was clear that the T5wt % of PC14E1VEs and PC14E2VEs increased from 264 to 287 ◦C and from 228 to 286 ◦C, respectively, when the number average molar mass increased. Because the interactions between molecules gradually increased, the intertwining phenomenon became more and more obvious, resulting in fewer and fewer defects in molecules, and the thermal stability of the polymer gradually increased with the increase of the molecular weight of the polymers. These results indicated that the increase of polymers’ molecular weights can improve the thermal stability to a certain extent. Polymers 2018, 10, 197 15 of 18 Polymers 2018, 10, x FOR PEER REVIEW 15 of 18

100 100

A 98 80

96 )

% 60 ( 200 220 240 260 280 300

Mass 40

0 100 200 300 400 500 600

Temperature (C)

100 100 98 B 80 96 94

92 60 220 240 260 280 300 (%) ass

M 40

0 100 200 300 400 500 600 Temperature (C)

FigureFigure 12. 12.TGA TGA curves curves of of C C1414EE11VE (A)) andand PCPC1414EE22VEVE ( (BB)) with with various various molecular molecular weights. weights.

To determine a clear relationship between T5wt % and the number average molar mass of To determine a clear relationship between T5wt % and the number average molar mass of PC14EnVEs, the relationship lines of T5wt % and molecular weights are plotted in Figure 13. With the PC14EnVEs, the relationship lines of T5wt % and molecular weights are plotted in Figure 13. With increase of molecular weights, the thermal stability of PC14EnVEs steadily improved. the increase of molecular weights, the thermal stability of PC14EnVEs steadily improved.

Polymers 2018, 10, 197 16 of 18 Polymers 2018, 10, x FOR PEER REVIEW 16 of 18

285

PC E VE 270 14 1 C) 

( 255 PC E VE 14 2 5wt% T 240

225 0.51.01.52.02.5 -4 M 10 n

Figure 13. T5wt % vs. the number average molar mass of PC14E1VE and PC14E2VE. Figure 13. T5wt % vs. the number average molar mass of PC14E1VE and PC14E2VE.

4. Conclusions 4. Conclusions

In this paper, C14EnVEs and a series of PC14EnVEs (n = 1, 2) with various molecular weights were In this paper, C14EnVEs and a series of PC14EnVEs (n = 1, 2) with various molecular weights were successfully synthesized through the substitution reaction and living cationic polymerization. Due to successfully synthesized through the substitution reaction and living cationic polymerization. Due to the introduction of a flexible ethylene glycol segment, the phase change enthalpy of poly(mono the introduction of a ﬂexible ethylene glycol segment, the phase change enthalpy of poly(mono ethylene ethylene glycol n-tetradecyl ether vinyl ethers) (89 J/g) was approximately equal to that of glycol n-tetradecyl ether vinyl ethers) (89 J/g) was approximately equal to that of poly(hexadecyl poly(hexadecyl acrylate) (86 J/g). The effects of the molecular weights of PC14EnVEs on the thermal acrylate) (86 J/g). The effects of the molecular weights of PC14EnVEs on the thermal properties were properties were investigated. The phase change enthalpy of the homopolymer comb-like phase investigated. The phase change enthalpy of the homopolymer comb-like phase change materials change materials PC14EnVEs were clearly affected by the number average molar mass, while the effect PC14EnVEs were clearly affected by the number average molar mass, while the effect of the molecular of the molecular weight on the phase change temperatures was not obvious. When the number weight on the phase change temperatures was not obvious. When the number average molar mass of average molar mass of PC14EnVEs exceeded 20,000, the enthalpy values were almost a constant at 89 PC14EnVEs exceeded 20,000, the enthalpy values were almost a constant at 89 and 83 J/g for PC14E1VE and 83 J/g for PC14E1VE and PC14E2VE, respectively. The melting temperatures of PC14E1VE changed◦ and PC14E2VE, respectively. The melting temperatures of PC14E1VE changed from 27.0 to 28.0 C from 27.0 to 28.0 °C and the crystallization temperature changed from 17.3 to 17.5 °C as the number and the crystallization temperature changed from 17.3 to 17.5 ◦C as the number average molar mass average molar mass increased. For PC14E2VE, the melting temperature changed◦ from 24.0 to 24.4 °C, increased. For PC14E2VE, the melting temperature changed from 24.0 to 24.4 C, and the crystallization and the crystallization temperature changed from 15.6 to 16.3 °C when the number average molar temperature changed from 15.6 to 16.3 ◦C when the number average molar mass increased. In addition, mass increased. In addition, the thermal stability of PC14E1VE and PC14E2VE also increased with the the thermal stability of PC14E1VE and PC14E2VE also increased with the increase of the number increase of the number average molar mass, and the initial thermal decomposition temperature T5wt average molar mass, and the initial thermal decomposition temperature T5wt % of PC14E1VE and % of PC14E1VE and PC14E2VE◦ was 287 and 286 °C, respectively. Thus, PC14EnVEs can be widely used PC14E2VE was 287 and 286 C, respectively. Thus, PC14EnVEs can be widely used as energy storage as energy storage materials based on their excellent thermal stability and thermal properties. After a materials based on their excellent thermal stability and thermal properties. After a test of 200 thermal test of 200 thermal cycles, the thermal performance parameters of PC14E1VE and PC14E2VE remained cycles, the thermal performance parameters of PC14E1VE and PC14E2VE remained almost unchanged. almost unchanged. The new homopolymer comb-like phase change materials PC14EnVEs can be The new homopolymer comb-like phase change materials PC14EnVEs can be applied in heat storage, applied in heat storage, energy conservation, and environmental protection. energy conservation, and environmental protection.

Acknowledgments:Acknowledgments:The The authors authors gratefully gratefully acknowledge acknowledge the financialthe financial support support for this for research this research from the Nationalfrom the NaturalNational Science Natural Foundation Science Foundation of China (No. of China 51573135 (No. and51573135 No. 51203113). and No. 51203113). AuthorAuthor Contributions: Contributions:Dongfang Dongfang Pei Pei and and Xingxiang Xingxiang Zhang Zhang conceived conceived and and designed designed the the experiments; experiments; Sai Sai Chen Chen andand Wei Wei Li Li contributed contributed to to the the characterization; characterization; Xingxiang Xingxiang Zhang Zhang revised revised the the manuscript. manuscript. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.

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