Synthesis, Characterizations and Mechanical Properties of

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Synthesis, characterizations and mechanical properties of microcapsules with dual shell of polyurethane (PU)/melamine formaldehyde (MF): effect of different chain extenders Jianfeng Hu, Xiaotong Zhang, Jinqing Qu, Yuliang Wen, and Weifu Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04973 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

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Industrial & Engineering Chemistry Research

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Synthesis, characterizations and mechanical properties of microcapsules with dual shell of polyurethane (PU)/melamine formaldehyde (MF): effect of different chain extenders Jianfeng Hua,*, Xiaotong Zhangb, Jinqing Qua, Yuliang Wenc, Weifu Sund,e,f, * a

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China b

School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK c

Guangzhou Goaland Energy Conservation Tech Co., Ltd, Luogang District, Guangzhou 510663, PR China d

School of Mechatronical Engineering, Beijing Institute of Technology, Beijing, 100081, P.R. China e

State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing, 100081, P.R. China f

Department of Chemistry, University College London, London, WC1E 6BT, U.K.

ABSTRACT: The mechanical properties and micro-structure of moisture-curing polyurethane (PU)/melamine formaldehyde (MF) microcapsules are not clear yet because of the complicated oil/water formulation process. PU/MF microcapsules with isophorone diisocyanate as core were formulated with butanediol (BDO), polyethylene glycol (PEG) 400, PEG 1000, and PEG 2000 using a two-step synthesis method in this work. The resulted microcapsules were characterized using various techniques. Furthermore, their mechanical properties were measured using a micromanipulation technique, showing that PU/MF microcapsules were successfully synthesized with the different alcohol chain extenders and the main groups have been revealed by their FTIR spectra. The morphologies of the microcapsules displayed the differences caused by the chain extenders. Size distributions and mechanical properties were measured to study the effects of the length of alcohol chain extenders. TEM imagines with sharp interfaces between PU and MF layers and data of SEM-EDS of the shell identified the mechanism of reactions in the shell. Keywords: chain extender, self-healing, microcapsule, moisture-curing, micromanipulation 1 ACS Paragon Plus Environment

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INTRODUCTION Moisture curing microcapsules are new research field in self-healing materials area

1,2,3,4

. Since

the core of such microcapsules has active reactivity with the water, even the moisture in the air when the capsules are broken, so that they can be widely used in the buildings or facilities adjacent to the water or in the water to resist corrosion5,6,7,8 . Normally, the core materials belong to isocyanate serials9,10, which requires a high-level rigorous hermetic seal. Shell play an important role in providing protection towards core and various kinds of shell have been developed, such as graphene oxide (GO)/poly (urea-formaldehyde) (PUF) hybrid shell,11 Fe3O4/SiO2 hybrid shell,12 etc. In order to protect the core agent, dual shells are often adopted to prepare microcapsules13,14,15,16. Polyurethane (PU)/melamine formaldehyde (MF) microcapsules probably are the most popularly studied system in literatures because of the simple formulation process, economical materials and low toxicity of the Oil/water (O/W) emulsion system17. However, O/W emulsion system tends to introduce water into the core materials and bring the risk to inactivate the core materials in a short time. In the previous work, researchers normally attempted to use scanning electron microscopy (SEM) to observe the shell10. But, there were no clear evidences to support the existence of dual shells. In our previous work, Hu, et al used transmission electron microscopy (TEM) to observe the shell thickness of MF, and established the relationship between mechanical property and reaction time of the shell18 based on single-shell microcapsules. In isocyanate-water emulsion system for preparing the moisture-curing microcapsules, longer reaction time implies higher risk to introduce water into the core18, which will result in the decreased efficiency of self-healing behavior. Although dual shell has been adopted, nonetheless attention has been paid to optimize the synthesized experimental conditions using amines instead of alcohols17 or mechanism,19 and the mechanical properties have not been touched upon.17 The properties of shell are the potential factors to affect the efficiency of self-healing behaviors. But the reactions that happened within the dual shells are complicated and related study is not so intensive and sufficient. Especially, it is difficult to distinguish PU from MF and there is a lack of direct and convincing evidence.17 Furthermore, the relationship between the mechanical properties of microcapsules and different chain extenders is not well understood. But this can offer insight into adjusting the mechanical properties by varying chain extender instead of increasing the reaction time under the risk of inactivating the 2 ACS Paragon Plus Environment


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core materials. One of the efficient approach is to adjust the mechanical properties of microcapsules by using different chain extenders for given identical reaction time. In this paper, PU/MF microcapsules formulated with different chain extenders1, 4-Butanediol (BDO), polyethylene glycol (but it has the smallest amount of reactive group. PEG) 400, PEG 1000 and PEG 2000 have been successfully prepared as confirmed by Fourier transform infrared (FTIR). Particle size analyser has been used to test the size distribution of these microcapsules. The dual layers of the shell structure of microcapsules has been characterized using transmission electron microscope (TEM) and the morphology and compositional analysis of the outer shell and inner shell of PU/MF microcapsules have been conducted with scanning electron microscopy-energy dispersive spectrum (SEM-EDS). Finally, the mechanical properties of the shell of microcapsules have been monitored by micromanipulation20,21,22.

EXPERIMENTAL SECTION Materials Desmodur L-75 (toluene diisocyanate, TDI) and Desmodur I (isophorone diisocyanate, IPDI) were obtained from Bayer Materials Science (Bayer, Germany). Melamine, formaldehyde solution (37.0 wt%), gum arabic (GA), triethylamine (TEN), BDO, PEG 400, PEG 1000 and PEG 2000, hydrochloric acid (37.0 wt%), and acetone were purchased from Sigma-Aldrich (St. Louis, MO). All these products were used without further purification. Preparation of Prepolymer of MF (P-MF) P-MF was prepared following the method reported by Ming et al

18

. Briefly, 0.75 g melamine,

1.45 g formaldehyde solution (molar ratio of melamineto formaldehyde is 3.0:1.0) and 10 mL water were put in a 25-mL three-neck flask to prepare P-MF at 65 oC for 40 min under vigorous stirring. TEN was used to adjust the pH value to 8~10. Synthesis of IPDI-filled PU/MF double-layered microcapsules

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PU/MF microcapsules were prepared following the method reported elsewhere.18 Briefly, 60 mL aqueous GA (3wt %) and the above-mentioned P-MF solution were put in a 250-mL beaker. Mixture of TDI pre-polymer (3.00g) and IPDI (9.00g) was joined into the system. Under a homogenizer (FM20-D, FLUKO, Germany) at the speed of 2000 rpm for 30mins, a uniform emulsion was generated at the first emulsion step. Then, it was transferred to a digital mixer (JB90-SH, Shanghai specimen model factory, Shanghai) with a three-bladed propeller at 300 rpm for 30 mins under ice bath. 30 mL aqueous chain extender (1.5wt% BDO, PEG 400, PEG 1000 and PEG 2000) was added into the beaker and the pH value of mixture was adjusted to 4.0 with aqueous HCl (37.0 wt%) under ice bath for another 30 mins. The aim of ice bath is used to slow the diffusion of core materials for preparing microcapsules with good spherical shape and slow the speed of those reactions to avoid the inactivation of core materials. Subsequently, the mixed solution was heated to 65℃ at a speed of 10 k min-1 and kept it at such temperature for 2h to swell. Finally, the synthesized microcapsules were separated with centrifuge and washed them for a couple of times with deionized water. Particles were dried in a freeze-dryer (ScanVac CoolSafe, UK) in -60℃ for 48 hours. The dried particles were kept for further testing.



Scheme 1 The formulation process and schematic structure of the prepared PU/MF dual shelled microcapsules. The prepare process is shown in Scheme 1, the mixture of IPDI and Desmodur L-75(TDI prepolymer) is added into gum Arabic aqueous solution. TDI pre-polymer is used to form PU shell and IPDI is used for core materials because of the priority reactivity to H2O during ice-bath process. Then, PEG reacts with TDI pre-polymer in the interfacial polymerisation process. MF shell is produced with in situ polymerisation of P-MF in the swelling time. Characterizations 3.0 g microcapsules were crushed in motar for 5min, and the shell debris was washed with acetone for 3~4 times, and dried in drier. Then, the shell debris together with KBr pellets were detected with a Fourier transforms infrared spectrometer (FTIR) (Bruker 550, Bruker, Germany).


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The number of scan and resolution of FTIR are 32 and 4 cm-1, respectively. The spectra between 4000–400 cm-1 of the samples were monitored. Scanning electron microscopy (JEOL 6060, Tokyo, Japan) & energy dispersive spectrometer (Inca 300, Oxford instruments, UK) (SEM-EDS) were chosen to observe the morphologies of the surfaces of microcapsules at 10, 15, and 30KV. The shell thickness was quantified using transmission electron microscopy (TEM) (JEOL 1200EX, Tokyo, Japan) following a standard methodology described in Zhang et al. 23. Some microcapsules samples were dispersed in epoxy resin. After the resin cured, the samples for TEM were sliced into pieces for testing. Direct measurements were performed with TEM for 3 times in random24. Some dry particles were dispersed in water with an ultrasonic cleaner (VWR, UK) for 30 mins. The size distribution of the microcapsules was evaluated with a particle size analyser (Mastersizer 3000, Malvern, IL) based on laser diffraction. The absolute span (AS) and relative span (RS) values are defined by the follows equations17 :

AS = D90 − D10

(1)

D90 − D10 D50

(2)

RS =

where D90, D50 and D10 represent the maximum particle diameter below which 90%, 50% and 10%, respectively, of the sample volume exists.



Scheme 2 Illustrated structure of micromanipulation device.

The mechanical properties of single microcapsule were measured by a micromanipulation technique. The principle of this technique is to compress single microcapsule between two parallel surfaces. These two surfaces were made of a glass probe pasted on a force transducer with super glue and sample stage. Two microscopes were used to monitor the movement of microcapsule on the stage. The single particle was deformed with the compression at a pre-set speed and displacement, the data of that process was recorded by the computer linked with force transducer. The details were introduced elsewhere

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. The diameter of each particle was

determined from its image under the microscope. A commercial force transducer (Model 403A, Auora Scientific Inc., Canada), which has the maximum scale of 10 mN and resolution of 0.1µN, was used in this work. The compression speed was set at 2 µm·s−1 and the micromanipulation measurements were carried out at room temperature (23±0.5 ◦C). More information about this technique is described elsewhere25. Determination of the microcapsules’ core fraction: the core fraction of the resultant microcapsules was determined by extracting oil in a soxhlet apparatus. A certain amount of dried microcapsules was sealed in a known weight of filter paper bag (W0). Then, the bag was precisely weighed (W1). Afterwards, the sample bag was placed in a Soxhlet apparatus, extracted with ethyl acetate at 90 oC for 2 h, and dried in a vacuum oven at 60 oC. After cooling in a desiccator, the sample bag was weighed again (W2). Eventually, the core fraction of the 7 ACS Paragon Plus Environment

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microcapsules, α, was calculated by:

α=

W1 − W2 ×100% W1 − W0

(3)

RESULTS AND DISCUSSION FTIR of shells of microcapsules a: BDO

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b: PEG 400

c: PEG 1000 d: PEG 2000

4000 3500 3000 2500 2000 1500 1000

500

-1

Wavenumber (cm ) Figure 1 FTIR spectra of the shells of microcapsules.

FTIR spectra of shells made of BDO, PEG 400, PEG 1000, and PEG 2000 were shown in Figure 1. These four curves almost are consistent with each other. Different chain extenders do not introduce other groups in molecular structure. Only the distance changed between two sides of chain extender. The stretching vibration peak at 3375 cm-1 would be N-H and –HO. However, there exist difficulty to identify the sources of N-H and hydroxyl group (-OH) during present emulsion method. Because during the process of shell formation, there are several reactions that can produce -NH and- OH simultaneously. In future, micro-fluid technology will be employed to synthesize this kind of microcapsules and to control the emulsion process in the hope of separately identifying the individual components. Stretching vibration peak of C=O and C-N 8 ACS Paragon Plus Environment


were about 1655 cm-1 and 1225 cm-1, which can be ascribed to the amide part of PU. Besides, the stretching vibration peaks of C-H at about 2955 cm-1, the peaks of C-O-C at about 1067 cm-1 can be attributed to the methylene group and ether bond (some belonged to P-MF, some existed in chain extenders which contain C-O-C) in shell. Triazine ring, which belongs to P-MF, was presented by the stretching vibration of C=N at about 1543 cm-1 and the bending vibration peak of C=N nearby 766 cm-1. The characteristic peak at 2264 cm-1 indicated the existence of –NCO from the core materials. (During the preparation of specimen for FTIR tests, the microcapsules were broken and then washed using acetone for a couple of times, but some remain left to stick on the shell.)

Morphology and size distributions of the microcapsules Figures 2 and 3 show the size distribution and SEM images of PU/MF microcapsules with different alcohol chain expanders, respectively. As observed from Figure 2, a broad grain size distribution can be observed. The finally resulted microcapsules have different particle size as shown in SEM images in Figure 3. Note that in carrying out the measurement of size distribution, the dry particles were dispersed in water with an ultrasonic cleaner for 30 mins. However, because of the hydrophobility of microcapsules, the particles cannot be stably dispersed in the water after ultra-sonication and they are prone to re-unite together. This can aggravate the broad distribution of particle size. The surfaces of microcapsules prepared with BDO were the smoothest among those samples, while those with PEG 2000 exhibit the roughest surfaces. The dimples on the microcapsules PEG 2000 indicate the reaction of the core materials with H2O might have happened when the microcapsules were prepared. The dimples are also present for particles resulting from the reaction with PEG 400, PEG 1000, although not so pronounced. The core materials contain IPDI and TDI pre-polymer. –NCO is very active to react with H2O. During this process, the core materials was consumed and the resulted CO2 was released during the microencapsulation, thus leading to the formation of the dimples. As shown in Table 1, some microcapsules prepared under the same conditions but with different chain extenders, such as BDO, PEG 400, PEG 1000 and PEG 2000 showed small difference in 9 ACS Paragon Plus Environment

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D10, D50, D90, AS and RS. This indicates that the chain extender does not affect the size distribution obviously in this process. (b) 16

BDO

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14 12 10 8 6 4 2 0 0

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Figure 2 The size distribution of PU/MF microcapsules made with different chain expanders of (a) BDO, (b)PEG 400, (c) PEG 1000 and (d) PEG 2000.



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Figure 3 SEM images of PU/MF microcapsules made with different chain expanders of (a) BDO, (b)PEG 400, (c) PEG 1000 and (d) PEG 2000.

Table 1 Information of size distribution of microcapsules prepared with different chain extenders. D50

BDO 66.4±0.1

PEG 400 66.5±0.2

PEG 1000 66.2±0.4

PEG 2000 68.8±0.3

D10

29.8±0.1

25.4±0.1

21.2±0.1

22.1±0.2

D90

118.4±0.4

129.5±0.7

134.7±0.8

138.2±0.4

Absolute span(µm) Relative span (µm)

88.6±0.4

104.1±0.6

113.5±0.7

116.1±0.5

1.3±0.01

1.6±0.01

1.7±0.01

1.7±0.01


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Shell thickness of microcapsules As shown in Figure 4, one sharp interface between two layers in shell was clearly observed in each kind of microcapsules. Considering the formulation process, the main reactions that has happened in shell layers are the interfacial polymerization and in situ polymerization. The different colors of two layers indicate that the reaction between TDI pre-polymer and chain extender was dominant during ice-bath time whereas the reaction of MF polymerization was dominant during swelling time. However, as observed from Figure 4d, different from the other three chain extenders, some small particles (black dots) are attached to the inner layer in microcapsule’s shell made with PEG 2000. It was because the molecular structure of PEG 2000 bears the longest among those chain and the structure of shell layer is not compact enough to defend the permeation of H2O. The black dots are the products of the reaction between IPDI and H2O. At least 5 different places have been taken to measure the thickness of shells and the results were shown in Table 2. The data shown in Table 2 reveal that the microcapsules made from PEG2 000 has the largest thickness of inner shell and outer shell, which are estimated to be 0.9±0.2, 1.6±0.2 µm, respectively. It would be interesting to look at the effect of molar ratio of NCO/OH or hydroxyl value on the shell thickness, however, there exist difficulty in accurately controlling the amounts of -OH, which might come from chain extenders, Arab gum and even water. Since -OH of the used extenders was used to react with –NCO of TDI-prepolymer to form the shell. Theoretically speaking, the reaction between –NCO and –OH is electrophilic addition. The greater dielectric coefficient, the more active the extender becomes. It is worth mentioning that, apart from the molar ratio NCO/OH, the mass matter is also another factor we can’t ignore. In practical application, one compromise needs to be made for the convenience. Besides, it can be observed that core contents do not display significant difference, most of which are around 73-74%, largely independent of chain extenders. Similar approach has been employed to study the core content during the preparation previously.17 The results show that the core content is sensitively dependent upon the reaction conditions, such as ice bath, with or without P-MF and the agitation speed.



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Figure 4 TEM imagines of PU/MF microcapsules made with different chain expanders of (a) BDO, (b) PEG 400, (c) PEG 1000 and (d) PEG 2000. Table 2 Thicknesses of shell and core fraction of microcapsules made with different chain extenders. Core fraction Thickness of Materials Thickness of h:H (%) Inner shell h outer shell H (µm) (average) (µm) BDO

0.8±0.2

1.3±0.07

0.6

73.83±0.35

PEG 400

0.3±0.03

1.4±0.04

0.2

73.74±0.38

PEG 1000

0.4±0.1

0.6±0.03

0.6

74.50±0.42


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PEG 2000

0.9±0.2

1.6±0.02

0.6

72.91±0.50

Elemental analysis of shell Since it is difficult to in-situ monitor the reaction process of shell formation. Here we made an attempt to use the EDS data as an indirect tool to probe into the underlying reaction process. Although the evidences are not direct, the resulted results still could give a hint of the proposed reaction mechanism. In order to probe into the chemical reactions that happened in the shell, SEM-EDS was used to perform elemental analysis of C, N, O on the surfaces of inner shell and outer shell. Three different spots (blue spots on the inner shell and red spots on the outer shell as shown in Figure 5) were chosen randomly during EDS analysis. As illustrated from Scheme 1, N was majorly introduced during the last step to form MF shell, even a little bit of N was generated in the inner shell by the chemical reaction of P-MF and TDI at the first stage at TEN’s existence. The particles prepared with BDO and PEG 400 have more N on the outer shell than the inner shell. According to the reactions happened during the process of preparing PU/MF microcapsules, with the increase of the length of chain extenders, the molecular chain containing N was gradually dragged into the inner shell. This could be evidenced by the data of microcapsules prepared with PEG 1000, and PEG 2000, as listed in Table 3.



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Figure 5 Morphologies of microcapsules for element analysis.

Table 3 Molar fractions of C, N, and O elements in the outer and inner shells of microcapsules prepared with BDO, PEG 400, PEG 1000 and PEG 2000.

BDO

PEG400

Outer shell 1 Outer shell 2 Outer shell 3 Inner shell 1 Inner shell 2 Inner shell 3 Outer shell 1 Outer shell 2 Outer shell 3

C (mole%)

N (mole%)

O (mole%)

86.4 96.1 83.2 96.9 93.9 95.2 87.1 81.7 88.0

6.9 0.00 6.0 0.00 2.1 0.00 0.00 4.2 3.5

6.8 3.9 10.9 3.1 4.0 4.9 12.9 14.2 8.4


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PEG1000

PEG2000

Inner shell 1 Inner shell 2 Inner shell 3 Outer shell 1 Outer shell 2 Outer shell 3 Inner shell 1 Inner shell 2 Inner shell 3 Outer shell 1 Outer shell 2 Outer shell 3 Inner shell 1 Inner shell 2 Inner shell 3

99.0 94.2 94.4 92.1 87.7 92.1 88.6 80.6 86.0

0.00 0.00 0.00 0 3.3 0 0 8.3 4.9

1.1 5.8 5.6 7.9 9.1 7.9 11.4 11.1 9.2

85.4 89.6 90.1 91.0 92.2 82.7

7.1 2.7 3.6 3.6 2.1 10.6

7.6 7.7 6.3 5.4 5.8 6.8

Mechanical properties of microcapsules Unaxial compression tests were performed to study the mechanical properties of the prepared microcapsules using micromanipulations. The micromanipulation device structure was displayed in Scheme 2. A probe made of glass with 95-µm diameter was stick to the tube of transducer using superglue. Particles were dispersed on an optical glass slide set on the stage. 50 microcapsules were chosen randomly to be compressed until rupture. The nominal rupture stress calculation equation is:

p = fr π R2

(4)

where p is nominal rupture stress, fr is the rupture force, and R is initial radius of the microcapsule. The nominal rupture tension (Tr) is defined by

Tr = fr 2R

(5)

As shown in Figure 6, O is the starting point where the probe contacts the microcapsule. And Y is the yield point of the first layer, OY shows elastic behavior. Y’ is the yield point of the second layer, YY’ is the second elastic behavior, which can be correlated with Young’s modulus 16 ACS Paragon Plus Environment


according to the conventional Hertz model.26,27 The mechanical property (i.e., Young’s modulus) is clearly different between OY and YY’, revealing that there are two different layers in the shell of the same microcapsule. R is the rupture point, and Y’R is viscoelastic behavior. D is the point when probe retouched the microcapsule after the microcapsule was compressed to rupture, and E is the point when the probe touched the stage.18 10 8

Force (mN)

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Displacement (µm) Figure 6 Compressive force as a function of displacement. The size of microcapsule is 24 µm in diameter.

The mechanical properties of microcapsules with different chain expenders of BDO, PEG 400, PEG 1000, PEG 2000 are presented in Figure 7. Generally speaking, both the rupture force of microcapsules (Figure 7a) and the displacement at rupture (Figure 7b) increase with the increase of their diameters. On the contrary, the nominal rupture stress (Figure 7c) decreases with the increase of the diameter. The trend of the nominal rupture tension (Figure 7d) as a function of the diameter exhibits complicate cases. In the cases of PEG 1000 and PEG 2000, the nominal rupture tension first increases with the increase of the diameter and then reaches a peak located at D=33 and 32 µm, respectively, followed by gradually decreasing, mainly due to the long length of chain extender; in contrast, in the cases of BDO and PEG 400, the nominal rupture tension


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always increases with the increase of the diameter. Such difference could be ascribed to the different length of molecular chain. Special attention might be paid to the displacement at rupture in Figure 7b, in which the displacement at rupture are almost independent of the particle diameter. This is associated with the molecular structure of PEG 2000, in which the content of hydroxy group (-OH) is relatively smaller as compared with PEG 400 and PEG 1000 at given weight. The lower content of hydroxy group (-OH) leads to the relatively loose structure of the shell and the moisture can penetrate into the core acting as somewhat role of extender (as confirmed by the TEM image in Figure 4d). The higher the microcapsule’s diameter, the higher contents of moisture and thus the much more rigid the shell becomes. Therefore, under given elastic limit, the microcapsules using PEG 2000 are prone to rupture and this may explain why the displacement at rupture does change much with the further increase of the microcapsules (Figure 7b). The mechanical properties of microcapsules made with different chain extenders have been summarized in Table 4. The rupture force firstly decreases from 2.4±1.2 to 1.8±0.8 and 1.7±0.9 mN, then increases to 2.2±0.8 mN, corresponding to BDO, PEG 400, PEG 1000 and PEG 2000, respectively. The corresponding deformation at rupture and nominal rupture stress are 27.2±10.1% and 3.4±1.0 MPa, 35.8±11.6% and 3.7±0.7 MPa, 29.2±13.9% and 3.4±1.6 MPa, and 21.5±6.6% and 3.2±1.6 MPa, respectively. Compared to single-layer shell microcapsules whose nominal fracture stress is typically around 1 MPa,18 the mechanical properties have been significantly improved for given similar reaction time (2-3h). As observed from Table 2 and Table 4, the shell thickness of microcapsules prepared with different chain extenders exhibits somewhat regular trend. But the relationship between dual shell thickness mechanical properties becomes complicated except rupture force. PEG 1000 has the lowest shell thickness (the total thickness of inner and outer shells) and also delivers the lowest rupture force. Different from the single shell microcapsule, the dual shell microcapsules will need to consider different parameters such as the thickness ratio of outer shell to inner shell, the total thickness of dual shells and also the ratio of individual thickness or total thickness to the whole microcapsule size, etc. But this work will need laborious and rigorous efforts. For example, the thickness characterization by TEM will need to damage the microcapsules whereas the mechanical test requires intact and complete microcapsules before tests. Besides, we note that the higher the average nominal rupture tension, 18 ACS Paragon Plus Environment


the stronger the microcapsules. For microcapsules made with the chain extender of BDO, the average nominal rupture tension is 75.7±19.0 N/m, which is the highest one among these 4 types of chain extenders. It is supposed that PEG 2000 should have the weakest rupture tension since it has the biggest molecular weight; actually, the one with PEG 1000 delivers the weakest rupture tension. This is because that PEG 2000 has the relatively smallest amount of reactive group for given mass. From the element analysis results (Table 3), more P-MF reacted with TDI prePolymer during the shell formulation in the microcapsules when using PEG 2000 than using the other chain extenders. This finally strengthens the shell of the microcapsules using PEG 2000. In generally, the one with PEG 1000 exhibits the weakest strength as the average nominal rupture tension is 62.2±19.0 N/m.


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7 6

Rupture force (mN)

b

BDO PEG 400 PEG 1000 PEG 2000

5 4 3 2 1 0 5

10

15

20

25

30

35

40

45

50

55

Displacement at rupture (µm)

a

24 BDO PEG 400 PEG 1000 PEG 2000

20 16 12 8 4 0 5

10

15

Diameter (µm) BDO PEG 400 PEG 1000 PEG 2000

10 8 6 4 2 0 5

10

15

20

25

30

35

40

20

25

30

35

40

45

50

55

45

50

55

Diameter (µm)

12

45

50

55

d

160

Nominal rupture tension (N/m)

c Nominal reputure stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


140


120 100 80 60 40 20 5

10

15

20

25

30

35

40

Diameter (µm)

Diameter (µm)

Figure 7 Mechanical properties of microcapsules prepared with various chain extenders of BDO, PEG 400, PEG 1000 and PEG 2000 (a) the rupture force versus diameter; (b) displacement at rupture versus diameter; (c) nominal rupture stress versus diameter and (d) nominal rupture tension versus diameter.

Table 4 Mechanical data of microcapsules made with different chain extenders (mean ± standard error). 20 ACS Paragon Plus Environment


2R (µm)

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Displacement at rupture (µm)

fr (mN)

Deformation at rupture %

p (MPa)

Tr (N/m)

8.0±3.9 8.9±3.6 7.4±4.2 6.8±4.0

2.4±1.2 1.8±0.8 1.7±0.9 2.2±0.8

27.2±10.1 35.8±11.6 29.2±13.9 21.5±6.6

3.4±1.0 3.7±0.7 3.4±1.6 3.2±1.6

75.7±19.0 69.6±14.7 62.2±19.0 70.6±20.8

BDO 29.6±8.9 PEG 400 25.1±6.8 PEG1000 26.7±11.5 PEG2000 31.4±10.3

As observed from Figure 6, there exist two yield points. For each yield point, yield force, the corresponding displacement, nominal yield strength σ and the deformation at yield point are displayed in Figures 8 and 9. The nominal yield strength is calculated according to, σ = Y π R2

(6)

where Y is the yield force. The yield results calculated from 50 microcapsules are listed in Tables 5 and 6 for the first and second yield points, respectively. It can be observed that microcapsules prepared from PEG 400 and PEG 1000 exhibit almost the best and comparable nominal yield strength in both yield points. The nominal yield strength at the first yield point increases from 1.3±0.8, 1.5±0.7 to 1.7±1.0 MPa, followed by a decrease to 1.4±0.8 MPa, corresponding to the cases of BDO, PEG 400, PEG 1000 and PEG 2000, respectively. Likewise, the corresponding nominal yield strength at the second yield point is estimated to be 2.7±1.0, 2.9±0.8, 2.9±1.3 and 2.7±1.4 MPa, respectively.


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4.0

b BDO PEG 400 PEG 1000 PEG 2000

1st Yield force (mN)

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0

5

10 15 20 25 30 35 40 45 50 55

Displacement at 1st yield (µm)

a


14 12 10 8 6 4 2 0 0

5

10 15 20 25 30 35 40 45 50 55

Diameter (µm)

Diameter (µm) 5.0 4.5

d


4.0 3.5

Deformation at 1st yield

c Nominal yield strength (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.0 2.5 2.0 1.5 1.0 0.5 0.0

0.7


0.6 0.5 0.4 0.3 0.2 0.1 0.0

0

5

10 15 20 25 30 35 40 45 50 55

0

5

10 15 20 25 30 35 40 45 50 55

Diameter (µm)

Diameter (µm)

Figure 8 Yield strength properties of microcapsules prepared with various chain extenders of BDO, PEG 400, PEG 1000 and PEG 2000 at the first yield point: (a) the 1st yield force versus diameter; (b) displacement at first yield point versus diameter; (c) nominal yield strength versus diameter and (d) deformation at 1st yield point versus diameter.



6 5

2nd yield force (mN)

b


4 3 2 1 0 0

5

10 15 20 25 30 35 40 45 50 55

Displacement at 2nd yield (µm)

a

20


18 16 14 12 10 8 6 4 2 0 5

10

15

Diameter (µm) 9

d BDO PEG 400 PEG 1000 PEG 2000

8 7 6

20

25

30

35

40

45

50

55

Diameter (µm) Deformation at 2nd yield

c

10

Nominal yield strength (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5 4 3 2 1 0


0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

5

10

15

20

25

30

35

40

Diameter (µm)

45

50

55

0

5

10 15 20 25 30 35 40 45 50 55

Diameter (µm)

Figure 9 Yield strength properties of microcapsules prepared with various chain extenders of BDO, PEG 400, PEG 1000 and PEG 2000 at the second yield point (a) the 2nd yield force versus diameter; (b) displacement at 2nd yield point versus diameter; (c) nominal yield strength versus diameter and (d) deformation at 2nd yield point versus diameter.


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Table 5 Mechanical data of microcapsules made with different chain extenders (mean ± standard error) at 1st yield point. 2R Displacement at 1st yield Y1 σ1 Deformation at 1st (µm)

point

(mN)

(µm)

yield

(MPa)

%

BDO

29.6±8.9

3.3±1.9

0.9±0.7

11.5±6.3

1.3±0.8

PEG400

25.1±6.8

5.0±2.9

0.7±0.4

20.2±11.0

1.5±0.7

PEG1000 26.7±11.5

4.0±2.2

0.8±0.6

16.8±11.1

1.7±1.0

PEG2000 31.4±10.3

3.0±1.7

1.0±0.6

9.5±3.7

1.4±0.8

Table 6 Mechanical data of microcapsules made with different chain extenders (mean ± standard error) at 2nd yield point. 2R Displacement at 2nd yield Y2 Deformation at 2nd σ2 (µm)

point

(mN)

(µm)

yield

(MPa)

%

BDO

29.6±8.9

6.0±2.8

1.8±1.0

20.8±8.3

2.7±1.0

PEG400

25.1±6.8

7.5±3.4

1.4±0.6

30.5±11.8

2.9±0.8

PEG1000

26.7±11.5

6.2±4.0

1.4±0.8

25.1±13.7

2.9±1.3

PEG2000

31.4±10.3

5.6±3.6

1.8±0.7

17.7±6.2

2.7±1.4

CONCLUSIONS Dual shelled PU/MF microcapsules with different chain extenders (BDO, PEG 400, PEG 1000, and PEG 2000) have been successfully formulated as confirmed by FTIR spectra and SEM images. The results show that the length of alcohol chain extenders does not affect the size of microcapsules too much. The sharp interface between PU layer and MF layer can be clearly observed from TEM images for these 4 types of microcapsules using BDO, PEG 400, PEG 1000, and PEG 2000. The shell thickness of microcapsules was related to the chain extender’s length. As confirmed by TEM images of microcapsules using PEG 2000 acting as chain extender, H2O can permeate into the shell and react with TDI or IPDI during the process of preparing the particles. 24 ACS Paragon Plus Environment


The mechanical properties of microcapsules made with different chain extenders were characterized by uniaxial compression through micromanipulation. The results show that the microcapsules made with BDO demonstrate the strongest mechanical characteristics whereas those using PEG 1000 deliver the weakest mechanical property in terms of the nominal rupture tension. But microcapsules prepared from PEG 400 and PEG 1000 deliver relatively good and comparable nominal rupture and yield strengths in both yield points. The elemental analysis indicates that P-MF mostly involved to form the inner shell and outer shell of microcapsules using PEG 2000 among those chain extenders. This work provides insights into the correlation between mechanical properties of microcapsules and the molecular structure of chain extenders during emulsion synthesis, and also offer opportunity to engineer the mechanical strength of microcapsules to suit the needs of their industrial applications. AUTHOR INFORMATION Corresponding Authors *E-mail: (J.H) [email protected]; *E-mail: (W.S.) [email protected]; [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT The authors report no declarations of interest. The authors alone are responsible for the content and writing of the article. The research was supported by Guangzhou Science Technology and Innovation Commission (201508030019), China Scholarship Council (201506155073). REFERENCES (1)

(2)

(3)

White, S. R.; Sottos, N. R.; Geubelle, P. H.; Moore, J. S.; Kessler, M. R.; Sriram, S. R.; Brown, E. N.; Viswanathan, S. Autonomic healing of polymer composites. Nature 2001, 409, 794. Blaiszik, B. J.; Caruso, M. M.; McIlroy, D. A.; Moore, J. S.; White, S. R.; Sottos, N. R. Microcapsules filled with reactive solutions for self-healing materials. Polymer 2009, 50, 990. Blaiszik, B. J.; Jones, A. R.; Sottos, N. R.; White, S. R. Microencapsulation of galliumindium (Ga-In) liquid metal for self-healing applications. J. Microencapsul. 2014, 31, 350.


Page 26 of 28

Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(4)

(5) (6) (7) (8)

(9)

(10) (11)

(12)

(13)

(14) (15)

(16)

(17)

(18) (19)

(20) (21)

Cass, P.; Knower, W.; Hinton, T.; Shi, S. N.; Grusche, F.; Tizard, M.; Gunatillake, P. Synthesis and evaluation of degradable polyurea block copolymers as siRNA delivery agents. Acta. Biomater. 2013, 9, 8299. He, Y.; Zhang, X. Y.; Runt, J. The role of diisocyanate structure on microphase separation of solution polymerized polyureas. Polymer 2014, 55, 906. Khun, N. W.; Sun, D. W.; Huang, M. X.; Yang, J. L.; Yue, C. Y. Wear resistant epoxy composites with diisocyanate-based self-healing functionality. Wear 2014, 313, 19. Koh, E.; Kim, N. K.; Shin, J.; Kim, Y. W. Polyurethane microcapsules for self-healing paint coatings. RSC Adv. 2014, 4, 16214. Koh, E.; Lee, S.; Shin, J.; Kim, Y. W. Renewable Polyurethane Microcapsules with Isosorbide Derivatives for Self-Healing Anticorrosion Coatings. Ind. Eng. Chem. Res. 2013, 52, 15541. Sardon, H.; Irusta, L.; Fernandez-Berridi, M. J. Synthesis of isophorone diisocyanate (IPDI) based waterborne polyurethanes: Comparison between zirconium and tin catalysts in the polymerization process. Prog. Org. Coat. 2009, 66, 291. Yang, J. L.; Keller, M. W.; Moore, J. S.; White, S. R.; Sottos, N. R. Microencapsulation of Isocyanates for Self-Healing Polymers. Macromolecules 2008, 41, 9650. Fan, C. J.; Wei, T.; Hu, C. Y.; Zhou, X. D. Preparation of GO/PUF hybrid shell microcapsules using GO sheets as the particulate emulsifier. Micro Nano Lett. 2016, 11, 207. Jiang, F. Y.; Wang, X. D.; Wu, D. Z. Design and synthesis of magnetic microcapsules based on n-eicosane core and Fe3O4/SiO2 hybrid shell for dual-functional phase change materials. Appl. Energy 2014, 134, 456. Di Credico, B.; Levi, M.; Turni, S. An efficient method for the output of new selfrepairing materials through a reactive isocyanate encapsulation. Eur. Polym. J. 2013, 49, 2467. Wang, W.; Xu, L. K.; Li, X. B.; Lin, Z. F.; Yang, Y.; An, E. P. Self-healing mechanisms of water triggered smart coating in seawater. J. Mater. Chem. A 2014, 2, 1914. Wang, W.; Xu, L. K.; Li, X. B.; Yang, Y.; An, E. P. Self-healing properties of protective coatings containing isophorone diisocyanate microcapsules on carbon steel surfaces. Corros. Sci. 2014, 80, 528. Yi, H.; Yang, Y.; Gu, X. Y.; Huang, J.; Wang, C. Y. Multilayer composite microcapsules synthesized by Pickering emulsion templates and their application in self-healing coating. J. Mater. Chem. A 2015, 3, 13749. Ming, Y. Q.; Hu, J. F.; Xing, J. H.; Wu, M. H.; Qu, J. Q. Preparation of polyurea/melamine formaldehyde double-layered self-healing microcapsules and investigation on core fraction. J. Microencapsul. 2016, 33, 307. Hu, J. F.; Chen, H. Q.; Zhang, Z. B. Mechanical properties of melamine formaldehyde microcapsules for self-healing materials. Mater. Chem. Phys. 2009, 118, 63. Nam, Y. S.; Bae, M. S.; Kim, S.; Noh, I.; Suh, J. K. F.; Lee, K. B.; Kwon, I. K. Mechanism of albumin release from alginate and chitosan beads fabricated in dual layers. Macromol. Res. 2011, 19, 476. Zhang, Z.; Saunders, R.; Thomas, C. R. Mechanical strength of single microcapsules determined by a novel micromanipulation technique. J. Microencapsul. 1999, 16, 117. Sun, G.; Zhang, Z. Mechanical properties of melamine-formaldehyde microcapsules. J. Microencapsul. 2001, 18, 593. 26 ACS Paragon Plus Environment


(22) (23)

(24)

(25) (26) (27)

Sun, G.; Zhang, Z. Mechanical strength of microcapsules made of different wall materials. Int. J. Pharm. 2002, 242, 307. Zhang, F.; Wu, Q.; Chen, Z. C.; Zhang, M.; Lin, X. F. Hepatic-targeting microcapsules construction by self-assembly of bioactive galactose-branched polyelectrolyte for controlled drug release system. J. Colloid. Interf. Sci. 2008, 317, 477. Mercade-Prieto, R.; Nguyen, B.; Allen, R.; York, D.; Preece, J. A.; Goodwin, T. E.; Zhang, Z. B. Determination of the elastic properties of single microcapsules using micromanipulation and finite element modeling. Chem. Eng. Sci. 2011, 66, 2042. Feng, W.; Patel, S. H.; Young, M. Y.; Zunino, J. L.; Xanthos, M. Smart polymeric coatings - Recent advances. Adv. Polym. Tech. 2007, 26, 1. Sun, W. F.; Zeng, Q. H.; Yu, A. B.; Kendall, K. Calculation of Normal Contact Forces between Silica Nanospheres. Langmuir 2013, 29, 7825. Sun, W. F. Interaction forces between a spherical nanoparticle and a flat surface. Phys. Chem. Chem. Phys. 2014, 16, 5846.


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Synthesis, Characterizations and Mechanical Properties of

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