Recyclable low-temperature phase change microcapsules for cold storage

Journal Pre-proofs Recyclable Low-Temperature Phase Change Microcapsules for Cold Storage Jiaojiao Zhao, Jinling Long, Yongqiang Du, Jiankui Zhou, Yadong Wang, Zipeng Miao, Yingliang Liu, Shengang Xu, Shaokui Cao PII: DOI: Reference:

S0021-9797(19)31496-1 https://doi.org/10.1016/j.jcis.2019.12.037 YJCIS 25777

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

19 September 2019 7 December 2019 9 December 2019

Please cite this article as: J. Zhao, J. Long, Y. Du, J. Zhou, Y. Wang, Z. Miao, Y. Liu, S. Xu, S. Cao, Recyclable Low-Temperature Phase Change Microcapsules for Cold Storage, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.12.037

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© 2019 Published by Elsevier Inc.

Recyclable Low-Temperature Phase Change Microcapsules for Cold Storage

Jiaojiao Zhao,1 Jinling Long,1 Yongqiang Du,1 Jiankui Zhou,1 Yadong Wang,1 Zipeng Miao,1 Yingliang Liu,1,2* Shengang Xu,1,2 Shaokui Cao1,2* 1School

of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, P. R. China

2Henan

Key Laboratory of Advanced Nylon Materials and Application, Zhengzhou University, Zhengzhou 450001, P. R. China

ABSTRACT Recyclable low-temperature phase change microcapsules (LTPCMs) have the potential applications in the short-distance cold chain transportation due to their reliable reusability in cold storage. Herein, LTPCMs are synthesized via in-situ suspension copolymerization of styrene and methyl methacrylate in absence of harm substances, providing the non-crosslinking copolymer shells. n-Dodecane, n-tridecane and ntetradecane, inducing the microphase separation of non-crosslinking copolymers, are successfully encapsulated to achieve n-do-LTPCMs, n-tri-LTPCMs and n-tetraLTPCMs, which respectively bear the high phase change enthalpy of 110.53 J·g-1 at 8.69 oC, 38.33 J·g-1/93.71 J·g-1 at -17.61 oC/-4.96 oC and 166.79 J·g-1 at 8.59 oC and subsequently show the cold-discharging periods of 30 min, 40 min and 120 min. The multiple circulation of cold-discharging process indicates the excellent recyclability for cold storage owing to their unchanged cold-discharging period. Especially, n-tetraLTPCM-65 bears the best comprehensive cold-storing performance in all the previously reported LTPCMs, such as narrow cold-discharging temperature range of 3 ~ 4 oC, long cold-discharging period of 69 ~ 120 min and low cold-discharging capacity of 33.4 J·g-1·K-1. This work successfully provided the recyclable LTPCMs for cold

Correspondence to: Prof. Yingliang Liu, [email protected]; Prof. Shaokui Cao, [email protected]; Tel.&Fax: +86-371-6776-3561. Declarations of interest: none. *

1

storage in the short-distance cold chain transportation.

KEY WORDS: low-temperature phase change microcapsule; cold storage; cold chain transportation; recyclability; in-situ suspension copolymerization

1. Introduction Logistics and express delivery [1-4], especially supply chain between suppliers and customs [5-8], presently have become the important means of effective circulation of goods. With the help of single-operated electric vehicles [9], the distribution of all kinds of goods has deeply stepped into every family by an agile individual carryout on the online-to-offline (O2O) internet trading platform [5,10-12], whose whole transportation process is promised by Chinese companies to finish within one hour inside the same city. This new kind of fast and high-efficient distribution form has largely extended the logistics terminals and facilitated our daily life. At the same time, it also demands various portable thermal or cold environments for temperature-keeping or temperature-sensitive goods [5,9,12], in which cold storage and thermal storage are two significant aspects of energy conservation [13,14]. Except for traditional warmkeeping issue, the fresh-keeping or freezing issue to impede the decay and failure of goods should be particularly paid much more attention in the transportation process of food, medicine and biological products [13-16]. The main approach to solve this problem is the cold chain transportation [17]. In this occasion, the distribution approach with low-temperature phase change (LTPC) materials as cold sources is a promising transportation form, which allows the cold chain transportation to break away from expensive and complicated refrigeration systems [13,14] due to the high latent enthalpy (Hpc) and the constant phase change temperature (Tpc) of LTPC materials [18,19]. This approach can solve the “first mile”, “last mile”, multiple small-quantity distribution in the cold chain transportation by applying various portable cold-storing products [5,9]. Besides, the scale-up application of LTPC materials in the cold chain transportation can also alleviate the huge 2

fluctuation of power system between daytime and nighttime by storing the cold energy during the low power-loading nighttime and then releasing it during the high powerloading daytime [20-21], which makes us to more rationally apply the electrical power to meet various cooling requirements and to save the electricity costs [5]. The common solid-liquid LTPC materials for cold storage are mostly water, inorganic salts, fatty acids and n-alkanes [22], which have been applied in many fields such as air conditioners and refrigerators [23,24], buildings [25,26], electronic cooling [27] and transportation of temperature-sensitive products [28]. However, these LTPC materials are prone to leak and corrode [26], which limit their applications. To solve the above problems, it is necessary to seek an appropriate packaging method. The microencapsulated LTPC materials, named as “low-temperature phase change microcapsules (LTPCMs)”, are regarded to be a species of outstanding cold-storing materials [14,26] because of their circular reusability in the energy management. Similarly to the microencapsulated thermal-storing phase change microcapsules [18,21,28-30],

LTPCMs

are

also

prepared

by

physically,

chemically

or

physicochemically encapsulating the LTPC materials to form the micron-sized coreshell capsules [28]. LTPCMs not only overcome the leakage and corrosion problems of conventional LTPC materials, but also possess the large specific surface area, the adjustable phase transition temperature and the fascinating recyclable characteristics [31-34]. Besides, LTPCMs also have a potential to be molded into various shaped polymer products, which might be the significant tools with single-operated electric vehicles to further extend the cold chain terminals. Therefore, LTPCMs are believed surely to be a kind of excellent candidate materials for cold storage in the short-distance cold chain transportation. However, as described in the supplementary materials (See. Table S1), it is difficult to synthesize LTPCMs with high Hpc value, which is to determine the cold-storing capacity/efficiency and the cold-discharging period in the practical applications. Additionally, many LTPCMs are synthesized using toxic melamine or formaldehyde as raw materials of crosslinking polymer shells, which are strictly prohibited in the package and transportation processes of food, medicine and biological products. Especially, the previously reported LTPCMs are scarcely studied 3

for the practical cold-storing measurements [35,36]. LTPCMs with a high Hpc value are synthesized in this work through in-situ suspension copolymerization for cold storage in the short-distance cold chain transportation of food, medicine and biological products. The non-crosslinking copolymers (PS-co-PMMA) of styrene (St) and methyl methacrylate (MMA) without harm formaldehyde and melamine are applied as shell materials to improve the toughness of LTPCMs. The LTPC materials, such as n-dodecane, n-tridecane and ntetradecane, are applied as core materials for cold storage, affording n-do-LTPCMs, ntri-LTPCMs and n-tetra-LTPCMs. Finally, their cold-discharging performance is studied in detail by measuring the cold-discharging capacity, the cold-discharging period/speed and the recyclable cold-storing reliability under the natural temperatureelevating environment of cold chain transportation.

2. Materials and Methods 2.1. Materials Styrene (St) and methyl methacrylate (MMA) were purchased from Tianjin Komiou Chemical Reagent Co. Ltd. St and MMA were washed with 10% sodium hydroxide (NaOH) solution to remove the inhibitor and dried with anhydrous calcium chloride (CaCl2) for 24 h. Then, the purification process is performed by distilling under the reduced pressure. The purified St and MMA were stored in a refrigerator. nDodecane (n-do), n-tridecane (n-tri), n-tetradecane (n-tetra) and the initiator benzoyl peroxide (BPO) were obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. Polyvinyl alcohol (PVA 1799) and tricalcium phosphate (TCP) were from Tianjin Komiou Chemical Reagent Development Center. Deionized water was purchased from Zhengzhou Senqi Drinking Water Co., Ltd. Except for St and MMA, all other chemicals were directly used without further purification.

2.2. Synthetic procedure of LTPCMs 4

The specific procedure to synthesize LTPCMs via in-situ suspension copolymerization is described as below: First of all, the discontinuous phase containing BPO, core materials and a certain ratio of copolymerized monomers for shell material were mixed on a magnetic stirrer at room temperature according to a fixed weight ratio. Then, the continuous phase including PVA, TCP and deionized water were added to a four-necked flask with a digital mechanical stirrer, a reflux condensation and a thermometer. Next, the continuous phase was heated to 80 oC at the stirring of 450 rpm for 10 minutes in order to sufficiently dissolve PVA and TCP. After the temperature was lowered to 60 oC, the above discontinuous phase was added dropwise at a constant rate using a dropping funnel. After the addition was completed, the suspension solution was kept at 60 oC for 10 minutes to make the size of suspension droplets uniform. Afterwards, the reaction temperature was raised to 80 oC to start the polymerization reaction for 6 hours. Subsequently, the as-synthesized LTPCMs were slowly cooled to room temperature, washed for three times with deionized water, filtered to remove the impurities and the unencapsulated core materials. The purified LTPCMs were placed in a petrie dish for air-drying. In the process of polymerization reaction, the mass ratio (RSt/MMA) was set to be 0/100, 25/75, 40/60, 60/40, 80/20 or 100/0 and the mass ratio (Rwater/oil) to be 1/1, 2/1, 3/1 or 4/1, to optimize the polymerization condition of LTPCMs at the n-alkane feed amount of 50wt%. Then, all the LTPCMs with different core materials, such as ndodecane, n-tridecane and n-tetradecane, were synthesized under the optimal polymerization condition, which were respectively named as n-do-LTPCMs, n-triLTPCMs and n-tetra-LTPCMs.

2.3. Cold-storing measurement Referring to the natural temperature-elevating environment of the low-temperature short-distance cold chain transportation of food, medicine and biological products, the cold-storing performance was investigated by testing the temperature change of a certain amount of LTPCMs from -60 oC to room temperature, to determine the cold5

discharging period/speed and the recyclable cold-storing reliability. The specific operation process was given as follows: Firstly, PS-co-PMMA particles or LTPCMs (10 mL, 30 mL or 50 mL) were put into a tube with an inner/outer diameter of 2.6/2.9 cm. Secondly, PS-co-PMMA particles or LTPCMs were cooled to -60 oC by liquid nitrogen and then placed into a 250 mL Erlenmeyer flask with a thermometer as shown in Fig. S1. The temperature-time curve was recorded at a one-minute interval within 180 min.

2.4. Characterization of LTPCMs LTPCMs were photographed on the polarizing optical microscope (POM, 59XC, Shanghai Optical Instrument Factory). The DSC curves were tested from -50 oC to 50 oC

using the thermal analyzer (STA 449 F3 NETZSCH, Germany) at a scanning rate of

2 oC/min under the nitrogen atmosphere [37]. The actual content of LTPC materials, i.e., encapsulation rate (Rencap), was determined by Formula (1) [38-40]: Rencap =

HLTPCM × 100% Hcore

(1)

The encapsulation efficiency (Eencap) was calculated by Formula (2) [41,42]: Eencap =

HLTPCM ×100% Rcore / shell )Hcore ( Rcore / shell  1

(2)

Where, HLTPCM is the melting enthalpy of LTPCMs,  H core is the melting enthalpy of pure core material, Rcore/shell is the mass ratio of core-to-shell materials. The thermal stability of LTPCMs was also evaluated from 30 oC to 600 oC at a scanning speed of 10 oC/min under the nitrogen atmosphere on the thermal analyzer (STA 449 F3 NETZSCH, Germany). The IR spectra were recorded from 400 cm-1 to 4000 cm-1 with a resolution of 2 cm-1 on the Fourier infrared spectrometer (TENSOR II, Bruker, Germany). The morphology of LTPCMs was observed on the scanning electron microscope (SEM, Phenom ProX, Phenom Scientific Instruments Co., Ltd.) to achieve the surface topography and the shell thickness, as well as the particle size/distribution. 6

3. Results and discussion 3.1. Recyclable LTPCM Design The low-temperature short-distance cold chain transportation of food, medicine and biological products is different from the transportation of common goods. It especially demands the reliable recyclability of cold-storing materials in the energy management. In this work, the recyclable cold-storing ability of LTPCMs is mainly reflected on the reliable reusability in the cold storage, which will be proved by the following cold-storing cyclic reliability. Besides, LTPCMs have a certain recyclable characteristics in the material reusability, which is from the facile separation of shell materials (non-crosslinking PS-co-PMMA) and core materials (n-alkanes) through simple evaporation and condensation under the reduced pressure. Subsequently, the core materials can be applied again as core materials. The non-crosslinking copolymers, which are usually called as thermoplastic polymers, can be molded into various shaped polymer products through the processing methods of thermoplastic polymers. Even, the non-crosslinking copolymers have a potential to be biodegraded like polyethylene [43], polystyrene [44,45] and poly(ethylene terephthalate) [46], which were respectively reported by Science (Title: One way to get rid of Styrofoam- Feed it to mealworms) [47] and Nature Materials (Title: Plastics on the menu) [48]. All these characteristics of LTPCMs, together with nontoxic feature without harm formaldehyde and melamine, also afford the environmental-friendly feature to a certain extent in the material field.

3.2. Synthesis of LTPCMs LTPCMs are synthesized in this work through in-situ suspension copolymerization in absence of harm formaldehyde or melamine. As mentioned above, they are expected to be suitable for cold storage in the low-temperature short-distance cold chain transportation of food, medicine and biological products. The specific synthetic procedure is illustrated in Fig. 1. The non-crosslinking copolymer of styrene (St) and methyl

methacrylate

(MMA),

instead 7

of

formaldehyde/melamine-containing

crosslinking polymers, is applied as shell material. The mass ratio RSt/MMA is adjusted to improve the toughness of LTPCMs and then to hold the low saturated vapor pressure of LTPC core materials. As a result, the encapsulation rate (Rencap) and the encapsulation efficiency (Eencap) will be increased. The LTPC materials, such as ndodecane, n-tridecane and n-tetradecane, are applied as core materials for cold storage. Their content might be tuned to increase the Hpc value of LTPCMs. The copolymerization condition is firstly optimized by changing the RSt/MMA and Rwater/oil values when the n-alkane feed amount is set to be 50wt%.

Fig. 1. Schematic illustration to synthesize LTPCMs via in-situ suspension copolymerization. (to be reproduced in color on the Web and in black-and-white in print)

The RSt/MMA effect on LTPCMs is investigated in detail taking n-tri-LTPCMs as an example. The POM images of n-tri-LTPCMs with different RSt/MMA values, together with the change trend of Rencap and Eencap, are shown in Fig. 2. Fig.s 2a-2e indicate that the microcapsule size firstly decreases and then increases as the RSt/MMA value decreases. At RSt/MMA=60/40, the microcapsule size is small and uniform as shown in Fig. 2c. This point indicates that the appropriate addition of hydrophilic MMA improves the dispersity of hydrophobic St and n-alkanes in the water by enhancing the surface tension, hinting the toughness increase of LTPCMs. However, at RSt/MMA=20/80, the microcapsule size is mostly ellipsoidal as shown in Fig. 2e. When the RSt/MMA value is 8

further decreased to 0/100, the agglomeration phenomenon happens as shown in Fig. S2 because the MMA monomer tends to more fastly polymerize in the individual droplet due to its hydrophilicity [38, 49]. In other words, the existence of St can effectively slow down the polymerization rate of MMA allowing the microcapsules to adjust the shape in the sufficient time. With the amount increase of MMA, the surfactant at the droplet surface can be kept at a higher level. However, when the MMA amount reaches a certain value, the surfactant molecules on the droplet will be reduced so that the stabilized droplet size is no longer consistent [20]. In addition, Fig.s 2f and S3 show that Rencap, Eencap and yield of LTPCMs respectively reach the highest values of 43.85%, 87.71% and 82.83% at RSt/MMA=40/60, implying the high Hpc value. All the above results indicate that the copolymerization reaction for LTPCMs is ongoing very well at RSt/MMA=40/60. The Rwater/oil effect on LTPCMs is also investigated in the supplementary materials. (a)

(c)

(b)

100

(f) Rencap Eencap

500μm

(d)

500μm

500μm

(e)

(a) RSt/MMA = 100/0 (b) RSt/MMA = 75/25 (c) RSt/MMA = 60/40 (d) RSt/MMA = 40/60

500μm

500μm

(e) RSt/MMA = 20/80

Percentage (%)

80 60 40 20 0

100/0

75/25

60/40

40/60

20/80

RSt/MMA

Fig. 2. POM images (a-e) and Rencap/Eencap change trend (f) of n-tri-LTPCMs with differet RSt/MMA values. (to be reproduced in color on the Web and in black-and-white in print)

3.3. Characterization of LTPCMs 3.3.1. FT-IR analysis The FT-IR spectra of n-dodecane, PS-co-PMMA and n-do-LTPCM are shown in Fig. S6a. In the FT-IR spectrum of n-do-LTPCM, two peaks at 2958/2925 cm-1 and one peak at 2854 cm-1 are respectively assigned to the asymmetric stretching vibration and the symmetric stretching vibration of C-H in the aliphatic group. The peak at 1466 cm-1

9

is attributed to the bending vibration of C-H in the CH2 and CH3 groups. All the above signals are mostly derived from the characteristic signals of n-dodecane. The stretching vibration of C=O group at 1730 cm-1 [50] also appears in the n-do-LTPCM spectrum. Besides, the multiple peaks at 1274 cm-1, 1197 cm-1 and 1136 cm-1 are attributed to the C-H stretching vibration of COOCH3 [51]. These peaks are the characteristic signals of PMMA. In addition, the characteristic signal of benzene ring also appears at 1607 cm1,

including two peaks at 762 cm-1 and 703 cm-1 caused by the out-of-plane bending

vibration of benzene ring [52,53]. In the FT-IR spectrum of n-dodecane or PS-coPMMA, there are respectively the above corresponding peaks in existence, which sufficiently proves that n-dodecane is successfully encapsulated by the PS-co-PMMA copolymer. Similarly, the characteristic signals of n-tridecane, n-tetradecane and PSco-PMMA exist in the FT-IR spectra of n-tri-LTPCM and n-tetra-LTPCM of Fig.s S6b and S6c, which prove that n-tridecane and n-tetradecane are also successfully encapsulated by the PS-co-PMMA copolymer.

3.3.2. Surface morphology Fig. 3 shows the SEM images of LTPCMs containing different n-alkanes. It can be seen from Fig.s 3a, 3e and 3i that n-do-LTPCM, n-tri-LTPCM and n-tetra-LTPCM take on the regular spherical structure. The statistical results in Fig. S7 and Table S2 indicated that the size distribution of n-do-LTPCM and n-tetra-LTPCM are wider and their mean diameter is larger, compared with n-tri-LTPCM. Notably, the magnified surfacial SEM images in Fig.s 3d, 3h and 3l display that LTPCMs have a lot of small pores on the surface, which are not found on the surface of PS-co-PMMA particles in Fig. S8. The pore amount increases as the carbon atom number is enhanced from ndodecane to n-tetradecane as shown in Fig.s 3d, 3h and 3l. The statistical results in Fig. S9 and Table S3 indicated that the average pore diameters on the surfaces of n-doLTPCM, n-tri-LTPCM and n-tetra-LTPCM are respectively 1.61 μm, 1.77 μm, 1.92 μm. The pore area ratio on the surface is gradually increased with the number increase of carbon atom. The above phenomenon might be induced by the microscopic phase 10

separation of PS-co-PMMA shell. Relatively, the PMMA segments are hydrophilic and the PS segments are hydrophobic. This induces that the PMMA segments are inclined to distribute in the outer shell while the PS segments distribute in the inner shell. Additionally, the number increase of carbon atom enhances the affinity of n-alkane with the PS segments, leading to the much larger trend for the PS segments to distribute in the inner shell. As a result, the more PMMA segments distributes in the outer shell. Due to the evaporation of water molecules inside the hydrophilic PMMA segments, the pores are formed on the outer surface of LTPCMs. The pore amount is also enhanced with the number increase of carbon atom in n-alkane. In other words, the interaction change of internal n-alkane/copolymer interface in LTPCMs causes the affinity transition of copolymer shell and water medium at the external interface. Finally, the different amount of pores appears on the outer surface of LTPCMs. To the best of our knowledge, it is for the first time that this interesting synergetic transition of internal/external interfacial interaction is visually found in the phase change microcapsules. This point is also the reason why the in-situ suspension polymerization of pure MMA monomer is not able to achieve LTPCMs in our polymerization condition as shown in Fig. S2. The above microphase separation of hydrophilic PMMA segments and hydrophobic PS segments is well proved by the EDS results of oxygen content from the PMMA segments in Fig. S10 and Table S4. Where, the oxygen content on the outer surface is increased from 20.39atom% to 24.19atom% with the number increase of carbon atom while the oxygen content in the inner surface is decreased from 18.69atom% to 15.21atom%. At the same time, it was found in Fig.s 3b/3c, 3f/3e and 3j/3k that the smooth inner surface of n-do-LTPCMs, n-tri-LTPCMs and n-tetra-LTPCMs has no any pore and their shell thicknesses are respectively 24.00−42.10 μm, 16.00−52.20 μm and 28.60−64.30 μm, suggesting the fine core-shell structure. Additionally, Fig. S8 shows that the PS-co-PMMA particle is much smaller in the size than LTPCMs, indicating the successful encapsulation of LTPC materials.

11

(b)

(a)

(d)

(c)

41.20μm

38.20μm

24.00μm 42.10μm

300μm

300μm (e)

(g)

(f) 21.50μm

20μm

100μm (h)

16.00μm 19.00μm

43.90μm 52.20μm

100μm

300μm (i)

(l)

(k)

(j)

20μm

100μm

28.60μm

40.20μm 33.90μm

300μm

40.40μm

64.30μm

300μm

100μm

20μm

Fig. 3. SEM images of LTPCMs (a-d: n-do-LTPCM; e-h: n-tri-LTPCM; i-l: n-tetraLTPCM). (to be reproduced in color on the Web and in black-and-white in print)

3.3.3. Thermal analysis The DSC curves of n-dodecane, n-tridecane, n-tetradecane and LTPCMs are shown in Fig. 4. Evidently, LTPCMs have a similar phase change process to n-alkanes. Amongst, there is one solid-liquid phase change peak in the DSC curves of Fig.s 4a and 4g, in which the Hpc values of n-do-LTPCMs and n-tetra-LTPCMs are respectively 110.53 J·g-1 at -8.69 oC and 116.19 J·g-1 at 6.65 oC, suggesting the high cold storage capacity. However, two DSC curves in Fig. 4d have two peaks, i.e., transition and -transition, in which -transition is caused by the solid-solid phase change of n-tridecane and -transition by its solid-liquid phase change. The similar  phase change process was reported when n-heptadecane is encapsulated by polystyrene [54]. Herein, the --transition H value of n-tri-LTPCM is respectively 30.39 J·g-1 at -18.19 oC and 63.67 J·g-1 at -4.90 oC, suggesting that n-tri-LTPCM has a two-step 12

cold-storing process. Besides, Fig.s 4a, 4d and 4g show that the Tph value of LTPCMs are lower than the pure core materials, indicating LTPCMs more early step into the phase change process than pure n-alkanes. This point is caused by the huge specific surface area of LTPCMs, which provides the more interface for thermal transportation. Based on the DSC results of n-do-LTPCMs, n-tri-LTPCMs and n-tetra-LTPCMs, their Rencap values calculated by Formula (1) are respectively 42.82%, 35.49% and 42.02% while their Eencap values calculated by Formula (2) are respectively 85.64%, 70.98% and 84.04%. This suggested that n-dodecane and n-tetradecane can be more efficiently encapsulated than n-tridecane. n-dodecane n-do-LTPCM

120

(a)

100

2.0

o

o

-8.69 C

-6.41 C

110.53 J/g

258.12 J/g

0.5

-15

-10 -5 o 0 Temperature ( C)

n-tridecane n-tri-LTPCM

o

-18.19 C 30.39 J/g

o

-4.90 C 63.67 J/g

0.5

(g)

n-tetradecane n-tetra-LTPCM

o

300 400 o Temperature ( C)

500

o

8.92 C 276.51 J/g

0.5

0

5 10 o 15 Temperature ( C)

20

25

60

50

60

PS-co-PMMA particle 10 mLn-tri-LTPCM 30 mLn-tri-LTPCM 50 mLn-tri-LTPCM

-40

0

10

20

30

40

(i)

20

60

0

40

-20 PS-co-PMMA particle 10 mLn-tetra-LTPCM 30 mLn-tetra-LTPCM 50 mLn-tetra-LTPCM

-40

0

-5

50

Time (min)

20

0.0

40

-20

600

n-tetra-LTPCM PS-co-PMMA particle n-tetradecane

(h)

100

o

6.65 C

30

o

200

80

116.19 J/g

20

0

-60 100

120

1.5

-10

40

10

5

Weight loss (%)

Heat flow (mW/mg)

0 -5 -10 o Temperature ( C)

2.0

1.0

60

0 -15

-20

10

(f)

20

20

0.0

0

Time (min)

n-tri-LTPCM PS-co-PMMA particle n-tridecane

(e)

PS-co-PMMA particle 10 mLn-do-LTPCM 30 mLn-do-LTPCM 50 mLn-do-LTPCM

-40

600

80

179.39 J/g

55.76 J/g

500

Temperature ( C)

-17.52 C

1.0

2.5

300 400 o Temperature ( C)

o



-25

200

100

-2.92 C

-20

o

Heat flow (mW/mg)

120

(d)

0

-60

100

10



o

1.5

5

Temperature ( C)

2.0

40

0

-20

Weight loss (%)

-25

60

20

0.0

(c)

20

Temperature ( C)

1.5 1.0

n-do-LTPCM PS-co-PMMA particle n-dodecane

(b)

80

Weight loss (%)

Heat flow (mW/mg)

2.5

-60 100

200

300 400 o Temperature ( C)

500

600

0

20

40

60

80

100

Time (min)

Fig. 4. DSC (a, d, g), TG (b, e, h) and temperature-elevating (c, f, i) curves of nalkanes, PS-co-PMMA particles and LTPCMs. (to be reproduced in color on the Web and in black-and-white in print)

The TG curves of n-alkanes, PS-co-PMMA and LTPCMs are shown in Fig.s 4b, 4e and 4h. Evidently, the weight-losing of n-alkanes and PS-co-PMMA are respectively completed in a one-step process. However, there are two-step weight-losing processes 13

in all the TG curves of LTPCMs, resulting from the evaporation of core materials in the first step and the copolymer decomposition in the second step. As shown in Table 1, LTPCMs have a higher gasification onset temperature (Tonset) than n-alkanes due to the encapsulation effect. For example, n-do-LTPCM shows a higher gasification Tonset of 149.10 oC than 93.60 oC of n-dodecane, suggesting the protective effect of shell material for n-dodecane. Similarly, n-tri-LTPCM displays a higher gasification Tonset of 130.90 oC than 106.90oC of n-tridecane and n-tetra-LTPCM reveals a higher gasification Tonset of 187.20 oC than 110.90 oC of n-tetradecane. In addition, the firststep weight-losing ratio for n-do-LTPCMs, n-tri-LTPCM and n-tetra-LTPCM are respectively 45.08%, 47.21% and 49.73%, which are much higher than the Rencap values (42.82%, 35.49% and 42.02%). This result indicated that the first weight-losing process is caused not only by the gasification of core materials, but also by the partial decomposition of PS-co-PMMA copolymer shell [51].

Table 1. TG data of n-alkanes, PS-co-PMMA and LTPCMs First step Samples

Tonset a

Tend b

(oC)

(oC)

n-dodecane

93.60

151.30

n-tridecane

106.90

n-tetradecane

Second step Tonset a

Tend b

(oC)

(oC)

96.03

---

---

---

182.79

99.06

---

---

---

110.90

184.52

99.13

---

---

---

PS-co-PMMA

---

---

---

361.87

419.82

94.43

n-do-LTPCM

149.10

161.11

45.08

359.80

414.25

46.23

n-tri-LTPCM

130.90

181.24

47.21

361.65

421.90

52.77

187.20

210.95

49.73

359.25

413.66

48.75

n-tetraLTPCM

c

Rloss (%)

Rlossc (%)

Note: a Tonset is the decomposition temperature when the weight-losing ratio is 5%. bT end

c

is the decomposition temperature when the decomposition is ended.

Rloss is the weight-lossing ratio. 14

3.4. Cold-Storing Evaluation The cold-storing performance was evaluated by measuring the temperature change curve of LTPCMs in the naturally temperature-elevating environment from -60 oC to room temperature, which is the practical condition of the low-temperature shortdistance cold chain transportation of food, medicine and biological products. It was well known that the cold energy is also the enthalpy change similarly to the thermal energy. The cold energy is released when the cold-storing system absorbs the heat enthalpy from the external environment. Similarly, the thermal energy is released when the thermal-storing system gives out the heat enthalpy. The same thing of cold/thermal energy is that the beforehand emission/absorption of heat enthalpy, i. e., cold/thermal storage, is both finished by externally working such as mechanical work or heat transportation. Therefore, the cold energy is beforehand stored into LTPCMs in our experiments through a freezing process in the liquid nitrogen. The temperatureelevating curves of n-do-LTPCM, n-tri-LTPCM and n-tetra-LTPCM are illustrated in Fig.s 4c, 4f and 4i. Evidently, the PS-co-PMMA particles in Fig. S11 have not shown any stable cold-discharging period, indicating no cold-storing performance. However, the temperature-elevating curves of n-do-LTPCM, n-tri-LTPCM and n-tetra-LTPCM in Fig.s 4c, 4f and 4i have a three-step temperature-elevating process with the phase change of core materials as a reference [55]: before the phase change, in the phase change and after the phase change. In the first stage, the temperature rapidly rises by absorbing the external heat enthalpy in the form of sensible heat. Then, in the melting stage of core materials, the temperature is maintained in a relatively stable colddischarging state by absorbing the external heat in the form of latent heat, which could be provided by the phase change of core materials. This constant temperature stage will be applied as an effective cold-storing process for the low-temperature short-distance cold chain transportation of food, medicine and biological products. Finally, the temperature rapidly rises again to room temperature because the external heat enthalpy is absorbed once again in the form of sensible heat.

15

As seen from Fig. 4c, the cold-discharging period of n-do-LTPCM in the second stage can last 30 min between -14 oC ~ -8 oC, corresponding to the phase transition at 8.69 oC in Fig. 4a. According to Fig. 4f, the cold-discharging period of n-tri-LTPCM shows two stages: -20 oC ~ -15 oC and -10 oC ~ 0 oC, which are respectively derived from the / phase transition at -18.19 oC/-4.90 oC in Fig. 4d. Their cold-discharging period can respectively last 15 min and 20 min. The cold-discharging period from  phase change enthalpy is shorter than that from  phase change. This indicates that the  solid-solid phase change can also provide the heat enthalpy to prolong the colddischarging period. Fig. 4i displays that the cold-discharging period of n-tetra-LTPCM reaches 64 min between 1.9 oC ~ 6.2 oC, attributing to the phase change at 6.65 oC in Fig. 4g. 20

-20

-40

0

500

1000 Time (min)

1500

(b)

0 o

10 20 30 40 50 60 70 80 90 100 2000

Temperature ( C)

o

Temperature ( C)

0

-60

20

(a)

-20 10 cycle 50 cycle 100 cycle

-40

-60

0

20

40

60 Time (min)

80

100

Fig. 5. 100-Cycle temperature-elevating curves of n-tetra-LTPCMs at the feed amount of 50wt% (a: individual curves; b: overlapped curves). (to be reproduced in color on the Web and in black-and-white in print)

All these results suggested that the PS-co-PMMA copolymer can well encapsulate n-dodecane, n-tridecane and n-tetradecane for cold storage in spite of the existence of microphase separation. Fig.s 4c, 4f and 4i evidently indicated that the cold-discharging period is increased as the amount increase of LTPCMs, hinting the positive correlation between the microcapsule amount and the cold-discharging period, i.e., the larger the volume and the longer the cold-discharging period. The as-prepared LTPCMs in this work have the potential applications for the short-term cold chain transportation of food, 16

medicine or biological products. (b) 5

20

t / min

o

4 3

o

10

-20

55

0

20

40

60

-1

30

40

50

Volume / mL

90 75 60

10

20

50

30

40

50

Volume / mL

30 -1

Cp / J،¤g ،K

-1

PS-co-PMMA 10 mLn-tetra-LTPCM 30 mLn-tetra-LTPCM 50 mLn-tetra-LTPCM

-40

20

v / J،min

Temperature ( C)

0

-60

120 105

T / C

(a)

45 40 35

10 10

80 100 120 140 160 180

20

20

30

40

50

10

20

Time (min)

30

40

50

Volume / mL

Volume / mL

Fig. 6. Temperature-elevating curves (a) and change trend of cold-storing data (b) for n-tetra-LTPCMs at the feed amount of 65wt%. (to be reproduced in color on the Web and in black-and-white in print)

Table 2. Cold-storing data for practical applications Samples

V (cm3)

m (g)

T (oC)

t (min)

Cp (J·g-1·K-1)

v (J·min-1)

PCL-RT-5[35]

/

/

-2 ~ 10

16.7

/

/

MPCM6D[36]

/

/

0 ~ 10

33 ~ 87

/

/

n-do-LTPCM-50

50

17.3

-14 ~ -8

30

18.4

63.6

10

4.6

4.5 ~ 8.3

24

43.9

31.8

30

13.7

1.5 ~ 5.5

48

41.7

47.7

50

22.8

1.9 ~ 6.2

64

38.8

59.5

10

4.7

8 ~ 11

69

55.6

11.4

30

14.0

8 ~ 12

95

41.7

24.6

50

23.3

7 ~ 12

120

33.4

32.4

n-tetra-LTPCM-50

n-tetra-LTPCM-65

Note: m, the mass of LTPCMs; V, the volume of LTPCMs; T, the cold-discharging temperature range; t, the cold-discharging period; Cp, the cold-discharging capacity calculated by Cp=pc/T; v, the cold-discharging speed calculated by v=pc·m/t.

3.5. Cyclic reliability of Cold Storage 17

The cyclic reliability of cold storage is another attractive subject in the practical applications, indicating the recyclable reusability of LTPCMs. The temperature change curve of n-tetra-LTPCM is measured for 100 cycles as shown in Fig. 5a. The identical overlapped temperature-elevating curves for 10/50/100-cycle measurements in Fig. 5b indicated that the cold-storing performance of n-tetra-LTPCM has an excellent repeating ability due to its cyclic reliability of cold storage. The reusability of n-tetraLTPCM in the material field is also proved by the unchanged or undestroyed sphere shape after the cold-storing cycling measurement as shown in Fig. S12. This result means the as-prepared LTPCMs, even shape-molding products, have the potential recyclability in the cold energy management. Besides, when the feed amount of ntetradecane is increased to the optimal content of 65wt% in Fig.s S13/S14 and Table S5, Hph, yield and Rencap of n-tetra-LTPCM are respectively enhanced to 166.79 J·g-1, 88.48% and 60.32%. In this case, the cold-discharging period reach 120 min between 7 oC ~ 12 oC as shown in Fig. 6a owing to its high Hph value. Its long cold-discharging period has met the demand for the practical applications for the short-term cold chain transportation of food, medicine or biological products. The cold-storing performance might be further enhanced by adjusting the encapsulation rate of core materials.

3.6. Cold-Storing Applications The cold-storing applications for the short-term cold chain transportation of food, medicine or biological products, demand the narrow cold-discharging temperature range (T) and the long cold-discharging period (t). The T value should be usually within several degrees centigrade to keep the food fresh or to hinder the bioproducts uncorrupt. The t value should be longer than one hour, which is the solemn promise on the Chinese urban online-to-offline (O2O) internet trading platform. Our coldstoring data, in which T is about 3 oC ~ 6 oC and t is longer than 30 min, are summarized in Table 2. Only two cold-storing reports about LTPCMs [35,36] are also listed in Table 2 for comparison. The change trend of cold-storing data of n-tetraLTPCM-65 is plotted in Fig. 6b. Evidently, the T value of n-tetra-LTPCM-65 is 18

narrower and its t value is longer, compared with two reported LTPCMs in Table 2. Additionally, its T and t values including the cold-discharging speed (v) increase with the volume enhancement, suggesting that the above cold-storing performance might be improved by enhancing the amount of LTPCMs or by designing the larger cold-storing product. The cold-discharging capacity (Cp) decreases with the volume enhancement, indicating that a small quality of cold energy discharge could keep the T value within 6 oC in the large cold-storing products. Amongst our as-prepared LTPCMs, n-tetra-LTPCM-65 is the most promising cold-storing materials for the shortterm cold chain transportation of food, medicine or biological products due to narrow cold-discharging temperature range (3 oC ~ 4 oC) and long cold-discharging period (69 min ~ 120 min), including low cold-discharging capacity of 33.4 J·g-1·K-1. Its coldstoring period has remarkably exceeded the one-hour promise on the Chinese urban online-to-offline (O2O) internet trading platform.

4. Conclusions The most of previously reported LTPCMs were synthesized by the crosslinking polymers using toxic melamine or formaldehyde as raw materials (See. Table S1), which is strictly prohibited in the package and transportation processes of food, medicine and biological products. Especially, only two previously reported LTPCMs (See. Table 2) are studied for the practical cold-storing measurement [35,36]. In this work, recyclable LTPCMs for cold storage are synthesized via in-situ suspension copolymerization for non-crosslinking PS-co-PMMA copolymer in absence of harm formaldehyde or melamine. n-Alkanes are successfully encapsulated by the PS-coPMMA copolymer, presenting n-do-LTPCM, n-tri-LTPCM and n-tetra-LTPCM. Through the SEM and EDS characterization on the internal/external surface of LTPCMs, it was visually found for the first time that the microphase separation of PS and PMMA segments causes the change of shell/water interfacial interaction and finally induces the outer porous surface. The DSC results suggested that the Hph values can reach 110.53 J·g-1 at -8.69 oC for n-do-LTPCM, 38.33 J·g-1/93.71 J·g-1 at -17.61 oC/19

4.96 oC for n-tri-LTPCM and 166.79 J·g-1 at 8.59 oC for n-tetra-LTPCM. Their colddischarging period reaches 30−120 min and takes on an excellent cyclic reliability. Simultaneously, a small quality of cold energy discharge could keep the narrow colddischarging temperature range. One of them, n-tetra-LTPCM-65 is the most promising cold-storing materials in the short-term cold chain transportation due to narrow colddischarging temperature range of 3 oC ~ 4 oC and long cold-discharging period of 69 min ~ 120 min, including low cold-discharging capacity of 33.4 J·g-1·K-1. The colddischarging performance of n-tetra-LTPCM-65 is much better than two previously reported cold-storing LTPCMs in Table 2 [35,36]. Its cold-storing period can completely meet the demand in the practical applications for the short-term cold chain transportation of food, medicine or biological products because it has remarkably exceeded the one-hour promise on the Chinese online-to-offline (O2O) internet trading platform. We believed that the cold-storing performance of LTPCMs might be further improved by adjusting the content and class of core materials, even designing the shape of cold-storing products.

Acknowledgement The authors greatly acknowledge the financial supports from the National Natural Science Foundation of China (NSFC, No. U1304212) and the Development Foundation for Distinguished Junior Researchers at Zhengzhou University (No. 1421320043).

Declarations of interest: none.

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Captions for Figures and Tables Fig. 1. Schematic illustration to synthesize LTPCMs via in-situ suspension 25

copolymerization. Fig. 2. POM images (a-e) and Rencap/Eencap change trend (f) of n-tri-LTPCMs with differet RSt/MMA values. Fig. 3. SEM images of LTPCMs (a-d: n-do-LTPCM; e-h: n-tri-LTPCM; i-l: n-tetraLTPCM). Fig. 4. DSC (a, d, g), TG (b, e, h) and temperature-elevating (c, f, i) curves of n-alkanes, PS-co-PMMA particles and LTPCMs. Table 1. TG data of n-alkanes, PS-co-PMMA and LTPCMs Note:

a

Tonset is the

decomposition temperature when the weight-losing ratio is 5%.

b

Tend is the

decomposition temperature when the decomposition is ended.

c

Rloss is the weight-

lossing ratio. Fig. 5. 100-Cycle temperature-elevating curves of n-tetra-LTPCMs at the feed amount of 50wt% (a: individual curves; b: overlapped curves). Fig. 6. Temperature-elevating curves (a) and change trend of cold-storing data (b) for n-tetra-LTPCMs at the feed amount of 65wt%. Table 2. Cold-storing data for practical applications. Note: m, the mass of LTPCMs; V, the volume of LTPCMs; T, the cold-discharging temperature range; t, the colddischarging period; Cp, the cold-discharging capacity calculated by Cp=pc/T; v, the cold-discharging speed calculated by v=pc·m/t.

26

Figures in the black-and-white version

Fig. 1. Schematic illustration to synthesize LTPCMs via in-situ suspension copolymerization. (a)

(c)

(b)

100

(f) Rencap Eencap

500μm

(d)

500μm

500μm

(e)

(a) RSt/MMA = 100/0

Percentage (%)

80

(b) RSt/MMA = 75/25 (c) RSt/MMA = 60/40 (d) RSt/MMA = 40/60 500μm

500μm

(e) RSt/MMA = 20/80

60 40 20 0

100/0

75/25

60/40

40/60

20/80

RSt/MMA

Fig. 2. POM images (a-e) and Rencap/Eencap change trend (f) of n-tri-LTPCMs with differet RSt/MMA values.

27

(b)

(a)

(d)

(c)

41.20μm

38.20μm

24.00μm 42.10μm

300μm

300μm (e)

(g)

(f) 21.50μm

20μm

100μm (h)

16.00μm 19.00μm

43.90μm 52.20μm

100μm

300μm (i)

(l)

(k)

(j)

20μm

100μm

28.60μm

40.20μm 33.90μm

300μm

40.40μm

64.30μm

300μm

100μm

20μm

Fig. 3. SEM images of LTPCMs (a-d: n-do-LTPCM; e-h: n-tri-LTPCM; i-l: n-tetraLTPCM).

28

n-dodecane n-do-LTPCM

120

(a)

100

2.0

o

1.0

o

-8.69 C

-6.41 C

110.53 J/g

258.12 J/g

0.5

-15

-10 -5 o 0 Temperature ( C)

n-tridecane n-tri-LTPCM

120

(d)

200

300 400 o Temperature ( C)

o

-18.19 C

1.0

o

-4.90 C

30.39 J/g

63.67 J/g

0.5

0 -5 -10 o Temperature ( C)

40

120

200

400 300 o Temperature ( C)

500

6.65 C 1.0

o

8.92 C

116.19 J/g

276.51 J/g

0.5

0

10

-5

0

5 10 o 15 Temperature ( C)

20

25

30

40

50

60

0

o

60 40

-20 PS-co-PMMA particle 10 mLn-tetra-LTPCM 30 mLn-tetra-LTPCM 50 mLn-tetra-LTPCM

-40

0

-10

20

(i)

20

20

0.0

60

Time (min)

Temperature ( C)

Weight loss (%)

o

50

PS-co-PMMA particle 10 mLn-tri-LTPCM 30 mLn-tri-LTPCM 50 mLn-tri-LTPCM

-40

80

1.5

40

-20

600

n-tetra-LTPCM PS-co-PMMA particle n-tetradecane

(h)

100

2.0

30

0

-60 100

(g)

n-tetradecane n-tetra-LTPCM

2.5

10

5

20

o

60

0 -15

-20

-25

10

(f)

20

20

0.0

0

Time (min)

80



PS-co-PMMA particle 10 mLn-do-LTPCM 30 mLn-do-LTPCM 50 mLn-do-LTPCM

-40

600

Temperature ( C)

179.39 J/g

Weight loss (%)

-2.92 C

55.76 J/g

500

n-tri-LTPCM PS-co-PMMA particle n-tridecane

(e)

100

o

-17.52 C

-20

-60

100

10



o

1.5

5

0

o

40

0

-20

2.0

Heat flow (mW/mg)

60

20

-25

(c)

20

Temperature ( C)

1.5

0.0

Heat flow (mW/mg)

n-do-LTPCM PS-co-PMMA particle n-dodecane

(b)

80

Weight loss (%)

Heat flow (mW/mg)

2.5

-60 100

200

300 400 o Temperature ( C)

500

600

0

20

40

60

80

100

Time (min)

Fig. 4. DSC (a, d, g), TG (b, e, h) and temperature-elevating (c, f, i) curves of nalkanes, PS-co-PMMA particles and LTPCMs.

20

-20

-40

0

500

1000 Time (min)

1500

(b)

0 o

10 20 30 40 50 60 70 80 90 100 2000

Temperature ( C)

o

Temperature ( C)

0

-60

20

(a)

-20 10 cycle 50 cycle 100 cycle

-40

-60

0

20

40

60 Time (min)

80

100

Fig. 5. 100-Cycle temperature-elevating curves of n-tetra-LTPCMs at the feed amount of 50wt% (a: individual curves; b: overlapped curves).

29

(b) 5

20

t / min

o

4 3

o

10

-20

0

20

40

60

-1

30

40

50

Volume / mL

75 60

10

45 40

30

40

50

35

20 10

20

30

40

50

10

Volume / mL

Time (min)

20

Volume / mL

30

50

10

80 100 120 140 160 180

20

90

-1

Cp / J،¤g ،K

PS-co-PMMA 10 mLn-tetra-LTPCM 30 mLn-tetra-LTPCM 50 mLn-tetra-LTPCM

-40

-1

55

v / J،min

Temperature ( C)

0

-60

120 105

T / C

(a)

20

30

40

50

Volume / mL

Fig. 6. Temperature-elevating curves (a) and change trend of cold-storing data (b) for n-tetra-LTPCMs at the feed amount of 65wt%.

30

Graphical Abstract 20

0 o Temperature ( C)

Solid-Liquid

Core

-20

-40

-60

Core

Liquid

0

500

1000 Time (min)

1500

20

0 o

Core

Temperature ( C)

Solid

Shell

10 20 30 40 50 60 70 80 90 100 2000

Core

Liquid-Solid

-20

-40

-60

31

After 10 cycling After 50 cycling After 100 cycling

0

20

40

60 Time (min)

80

100

Author Contribution Statement Jiaojiao Zhao performs all the experiments and prepares the manuscript.

Jinling Long performs the preliminary experiments.

Yongqiang Du, Jiankui Zhou and Yadong Wang help Jiaojiao Zhao to characterize all the materials.

Zipeng Miao helps Jiaojiao Zhao to perform the experiments.

Yingliang Liu, Shengang Xu and Shaokui Cao supervise the ongoing of this work and revise the manuscript.

32