A Novel Method for the Preparation of Narrow-Disperse

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A Novel Method for the Preparation of Narrow-disperse Nanoencapsulated Phase Change Materials by Phase Inversion Emulsification and Suspension Polymerization Yan Wang, Jian-ping Wang, Guanghua Nan, He Wang, Wei Li, and Xingxiang Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01026 • Publication Date (Web): 25 Aug 2015 Downloaded from http://pubs.acs.org on September 7, 2015

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Industrial & Engineering Chemistry Research

A Novel Method for the Preparation of Narrow-disperse Nanoencapsulated Phase Change Materials by Phase Inversion Emulsification and Suspension Polymerization Yan Wang a, Jianping Wang a *, Guanghua Nan a, He Wang a, Wei Li a, Xingxiang Zhang a a

Tianjin Municipal Key Lab of Fiber Modification and Functional Fibers, Institute of Functional

Fibers, Tianjin Polytechnic University, Tianjin 300387, China

AUTHOR INFORMATION * Corresponding author Tel.: +86 22 83955368; fax: +86 22 83955282. E-mail address: [email protected]

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ACS Paragon Plus Environment


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Abstract Narrow-disperse nanoencapsulated phase change materials with methyl methacrylate as shell material and n-octadecane as core material were successfully prepared by phase inversion emulsification and suspension polymerization. We characterized the structure, surface morphology, particle size distribution and thermal properties of prepared Nano-capsules using some methods, such as the fourier infrared spectrum (FTIR), scanning electron microscopy analysis (SEM), transmission electron microscopy (TEM), differential scanning calorimetry analysis (DSC) and thermogravimetric analysis (TGA). The effects of surfactant concentration and monomers/n-Octadecane mass ratios on the average diameter and phase change properties of nano-encapsulated n-Octadecane were investigated. The average diameter of nano-capsules is about 300-500 nm. The surface is quite smooth and compact. An obvious core-shell structure can be observed from TEM images, and the thickness of the shell is about 50 nm. When the n-Octadecane/monomer mass ratio is 5:4, the encapsulation efficiency of the nano-capsules reaches the highest 52.9%.

Keywords: Narrow-disperse; nano-capsules; phase-inversion emulsification; suspension polymerization; n-Octadecane

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1. INTRODUCTION Phase change materials (PCMs) have great capacity to absorb and release a lot of latent heat during a phase change process while the temperature of the materials remains constant1-4, which can be applied in many fields for energy storage. However, the applications of such a PCMs are controlled by the greater volume expansion and the liquid leakage4. So, encapsulation of PCMs with proper materials as shells is required3. The encapsulated phase change materials (microcapsules or nano-capsules) have several advantages, such as controlling the volume changes of core material during the period of phase change, preventing PCMs from leakage during a phase change process, preventing PCMs from the influences of the external environment and increasing the heat-transfer area5, 6. Microencapsulated phase change materials (Micro-PCMs) have been widely studied since the late of 1970s. From then on, Micro-PCMs have been wildly used in coolants, fibers, fabrics, solar and nuclear heat storage systems and suspensions for heat transfer7, 8. There are various methods for encapsulation of PCMs, such as in situ polymerization3, 9-11, interfacial polymerization12 and complex coacervation13. Nano-capsule formulations are effectively based on nanometric-scaled emulsions, so-called nano-emulsions, which can be seen as a template for nano-capsule generation, even if these two steps can often be combined into one14. According to the literatures, nano-emulsions can be produced by high-energy15,

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or

low-energy emulsification methods17-19. High-energy emulsification methods are also called the conventional emulsification methods; they need high mechanical energy generated by ultrasound generators or high-pressure homogenizers to produce nano-emulsions18. Compare with a high-energy method, a low-energy emulsification methods is an energy-saving method. Nanometrics-scaled emulsion droplets can be obtained by diverting the intrinsic physicochemical properties of the surfactants, co-surfactants and ingredients of the system14. They can take advantage of the chemical energy stored in the ingredients18, 20; make use of the phase transitions taking place during the emulsification process17, 19, 21 and produce the nano-emulsions almost spontaneously. The phase transition can be achieved either by changing the temperature at a fixed composition known as the phase inversion temperature (PIT) method19, or by changing the volume fraction of the dispersed phase at the constant temperatures known as the emulsion inversion point (EIP) method17, 18. The method has enjoyed huge popularity both in theoretical study and practical application as an energy-saving process from the time of its introduction. 3



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As we know, Micro-PCMs and Nano-PCMs have attracted more and more attention since 1980s as energy storage materials8. A lot of works have been done in fabricating and characterizing of nano-capsules with PMMA as shell materials using different types of high-energy emulsification methods, such as miniemulsion, high shear emulsification, high-speed stirring and so on. Wang22

introduced ultraviolet photoinitiated emulsion polymerization to

encapsulate eicosanoic–stearic acid using high-speed stirring emulsification. The average diameter and of latent heats prepared nanocapsules are 46 nm and 128.3 J/g, respectively. Tumirah23 used miniemulsionin-situ polymerization method to fabricate nanocapsulated n-octadecane with St-MMA copolymer shell. The average diameter and of latent heats prepared nanocapsules are 102 nm and 104.9 J/g, respectively. Alkan et al.

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prepared docosane/ PMMA

nanocapsules using high-speed stirring emulsification. The prepared nanocapsules have the diameter in range of 100-200 nm and latent heats of 54.6 J/g. Alay at al.25 prepared poly (methylmethacrylate-co-glycidyl methacrylate) nanocapsules containing n-hexadecane as phase change material using high-speed stirring emulsification. The average diameter and latent heats of obtained nanocapsules are 100-300 nm and 148.05 J/g, respectively. Chen5 synthesized nanocapsules containing n-dodecanol as core and polymethyl methacrylate as shell by miniemulsion polymerization with polymerizable emulsifier DNS-86 and co-emulsifier hexadecane. The mean diameter of nanocapsules are 150 nm and latent teats are 98.8 J/g. However, there is still little information available for the preparation of nano-capsules based on the method of phase inversion emulsification. In this study, we applied the method to prepare the nano-capsules, and successfully synthesized the Narrow-disperse nano-capsules containing n-octadecane as core material and poly (methylmethacrylate) (PMMA) as wall material. We adopted nonionic surfactant Span 80 and Tween 80 as emulsifiers. The effects of the concentration of the emulsifier on the size and the distribution of the capsules were investigated. The surface morphology, size distribution, chemical structure and thermal stability of the nano-capsules further studied.

2. EXPERIMENTAL SECTION 2.1 Materials. Methyl methacrylate (MMA, AR, Tianjin FuChen Chemical Reagent Factory) was used as the shell-forming monomer. n-Octadecane (95 wt%, Union Lab. Supplies Limited, Hong Kong) was used as core material. Span 80 (C.P. Tianjin Guangfu Fine Chemical

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Research Institute) and Tween 80 (C.P. Tianjin Beifang Tianyi Chemical Reagent Factory) were used as a co-emulsifier. Allyl methacrylate (ALMA, AR, Tianjin Chemical Reagent Research Institute) was used as a cross-linking agent. 2, 2-azodiisobutyronitrile (AIBN, AR, Tianjin Chemical Reagent Research Institute) was used as an initiator.

2.2 Preparation of nano-emulsions and nano-capsules. The emulsions were prepared using a mixture of the nonionic surfactants Span 80 and Tween 80; the mixing ration is Span 80:Tween 80=0.486:0.514 and the mixture HLB value is 9.8. A certain amount of monomer MMA, cross-linking agent ALMA, phase change material n-Octadecane and initiator AIBN were mixed in a three-neck flask as oil phase, the oil phase was dispersed for 2-3 min under the ultrasonic instrument to dissolve the initiator AIBN fully, then surfactants (the mixture of Span80 and Tween80) were added into the three-neck flask at 40 °C in water bath. The nano-emulsions were prepared by slowly adding water with a peristaltic pump with gentle mechanical agitation. The agitation rate was kept constant at approximately 90 rpm. The addition rate of water was 1.0 ml/min. And the surfactant concentration was changed from 5.0 to 9.0 wt%. Recipes for the preparation of nano-capsules showed in Table 1. Table 1 Recipes for the preparation of nano-capsules Nano-capsules were synthesized by a suspension polymerization. The O/W emulsions were formed above. Then the temperature was elevated to 68 °C to start a suspension polymerization, the mixture was stirred continuously at rates of 250 rpm with a nitrogen inlet and reflux condenser for 2h. The resultant nano-capsules were washed with hot water and hot alcohol to remove the unencapsulated n-Octadecane and residual monomer, then filtered and dried in an oven at about 50 °C until the mass was a constant. Figure 1 shows the preparation process of the nano-capsules. Figure 1 Preparation processes of nano-capsules.

2.3 Characterization of the samples. The surface morphology of nano-capsules was observed by using a field-emission scanning electronic microscope (FE-SEM; Hitachi S-4800). Transmission electron microscopy (TEM) was performed with an electron microscope (Hitachi H-7600, Japan) operating at an accelerating voltage of 100 kV. FTIR Spectra of n-Octadecane

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and nano-capsules were obtained using a Fourier Transformed Infrared-Spectroscopy (FTIR, wave number 4,000-400cm−1) at room temperature. The phase change properties of n-octadecane and capsules were measured using a Differential Scanning Calorimeter (DSC, NETZSCH DSC 200 F3, Germany) at a rate of ±10 °C/min in a nitrogen atmosphere. The content of n-Octadecane in the nano-capsule can be calculated according to the measured enthalpies as following Formula (1): P=

∆H ∆H o

(1)

× 100%

where, ∆H is the enthalpies of nano-capsule; ∆Ho is the melting enthalpy of n-Octadecane. The thermal stability of capsules was investigated by using a thermogravimetric analyzer (TG; NETZSCH STA 409 PC/PG TG-DTA, Germany) at a scanning rate of 10 °C/min in the range of room temperature to 600 °C in a nitrogen atmosphere.

3. RESULTS AND DISCUSSION 3.1 Optimum HLB values26. The emulsions were prepared using a mixture of the nonionic surfactants Tween 80 and Span 80. The mixing ratios were adjusted to satisfy the proper HLB values for optimum emulsification conditions. The mixed HLB values were calculated by the following Formula (2): HLB = HLB ∙ T% + HLB ∙ S%

(2)

Where, HLBT, HLBS and HLBmix are the HLB values of Tween 80 (15.0), Span 80 (4.3) and the mixed surfactants, and T% and S% are the mass percentages of Tween 80 and Span 80 in the mixed surfactants, respectively. All the HLB values used were obtained at 25 °C. To obtain the optimum HLB value, an emulsions for Sample S5 (without AIBN) in which contained n-octadecane, MMA and ALMA, and 5.0 wt% surfactants at different surfactant mixing ratios were prepared at 50 °C. The relationship between the droplet size of the obtained emulsions and the HLB values is shown in Figure 2. We can see that emulsions with droplet diameters below 500 nm were obtained at this temperature by adjusting the HLB values of the surfactants. And an optimum HLB in range of 9.6-10.2 was obtained. The optimum HLB of 9.8 was used in this experiment.

Figure 2 Average diameter of droplet as a function of the mixed HLB values for sample S5 6


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at 50 °C.

3.1 FTIR of the encapsulated PCMs. FTIR spectra of n-Octadecane, poly (methyl methacrylate) (PMMA) resin and nano-capsules containing n-Octadecane are presented in Figure 3. Multiple strong absorption peaks located at approximately 2,848 cm−1 and 2,912 cm−1 in the spectra of n-Octadecane are attributed to the asymmetric and symmetric –CH2 stretching vibrations. The peak at 1,471 cm−1, which is associated with the C-H bending, is also characteristic for n-octadecane. The peak at approximately 715 cm−1 in the spectra of n-octadecane is associated with the in-plane rocking vibration of the CH2 group. In addition, some of the specific peaks are only found in the spectra of the polymer shell PMMA and nano-capsules. For instance, the peak at 1730 cm-1 is assigned to the carbonyl group; the peaks at 1,220 cm-1 and 1,440 cm-1 are assigned to C-O stretching of an ester group in MMA. It shows that the n-octadecane was contained in the nano-capsules with PMMA resin as shell materials. Figure 3 FTIR spectra for n-Octadecane, nano-capsules and P(MMA-co-ALMA).

3.2 The size distributions and the polydispersity of nano-capsules. The size distributions of nano-capsules with different surfactant concentration are given in Figure 4. The diameters of nano-capsules with different surfactant concentration are in the range of 300-500nm, and with a narrow size distribution. The parameters of size distribution in Table 2 show the average diameters are 466, 420, 395, 338 and 328 nm with PDI (polydispersity index) of 0.085, 0.039, 0.068, 0.039 and 0.075 for as the surfactant concentration increases from 5g to 9g, respectively. It indicates that the average diameters of the nano-capsules show a little decrease trend with the increase of the surfactant concentration. This is why the amount of surfactant determines the total interfacial area. As the amount of surfactant is increasing, the oil phase can be dispersed into smaller droplets, in which polymerization occurs and nano-capsules form subsequently, leading to the decrease in the average diameter of nano-capsules5. We can also know the nano-capsules have a narrow size distribution with the value of the standard deviation. All of PDI are lower than 0.1, means narrow-disperse nanoencapsulated phase change materials were successfully prepared. Figure 4 Size distributions of nano-capsules with different surfactant concentration: (a) S4; 7



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(b) S6; (c) S7; (d) S8; (e) S9. Table 2 Size distribution parameters of samples. From the experimental data as shown in Table 2, the average diameter of nano-capsules tends to decrease with the increase in the total amount of surfactant. According to reviewer recommendations, a liner fitting was used to determine the relationship between the average diameter of nano-capsules and the total amount of surfactant. Figure 5 shows the relationship between the average diameter of nano-capsules and the total amount of surfactant, in which line and symbols represent values fitted by the linear regression and experimental results, respectively. It can see that the square of the correlation coefficient (R2) is greater than 0.92. This indicates that there is a good correlation between the average diameter of nano-capsules and the total amount of surfactant.

Figure 5 Fitted line plot of the average diameter vs. the total amount of surfactant.

3.3 Morphology and microstructure of nano-capsules. SEM micrographs of nano-capsules synthesized with various surfactant concentrations are given in Figure 6. The capsules show a regular spherical shape, and their sizes are in agreement with the results obtained from particle size distribution analysis. The average diameter is about 400 nm. It clearly shows the surface morphology of the nano-encapsulated PCMs is relatively smooth and compact with almost no defects, that the shell material has played a very good protective effect. Figure 7 indicates the TEM images of nano-capsules. We can clearly see the nano-capsules are of core/shell structure and spherical profile, and the maximum thickness (Tmax) of the shell is about 50 nm. It means that the n-octadecane was successfully encapsulated with PMMA resin as shell materials.

Figure 6 SEM images of nano-capsules with different surfactant: (a) S4; (b) S6; (c) S7; (d) S8; (e) S9 Figure 7 TEM micrographs of nano-capsules: S7

3.4 The effects of core/shell ratio on the phase change behavior of nano-capsules. In order to explore the effects of core/shell (n-Octadecane/ monomer mass) 8


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ratio on the encapsulation efficiencies of nano-capsules, a series of experimental samples with different core/shell ratios were carried out. The melting and crystallization properties of nano-capsules with different core/shell ratios at a heating and cooling rate of 10.0 °C/min are presented in Table 3, and the DSC curves are shown in Figure 8. The encapsulation efficiencies, shown in Tables 3, were calculated according to the above-mentioned Formula (1). The results show that prepared nano-capsules have strong endothermic peak and the exothermic peak in the process of heating and cooling, and is the same as n-Octadecane. From Table 3, it can also see that the encapsulation efficiency increased with the increase of core/shell ratio in the range from 3:5 to 5:4, and then decreased. That is to say, when the core/shell ratio is 5:4, the encapsulation efficiency is the maximum at 52.9% among all. A higher core/shell ratio will lead a thinner nano-capsule shell, and nanocapsules are fragile, lower the encapsulation efficiency and oil loading capacity are ascribed to an easy leakage of core material.

Figure 8 DSC curves of nano-capsules with different core/shell ratios: (a) S1; (b) S2; (c) S3; (d) S4; (e) S5. Table 3 Enthalpies and estimated thicknass of nano-capsules with different core/shell ratios.

3.5 The effects of core/shell ratio on the shell thickness of nano-capsules. For a sphere, there are some basic formulas as flowing:

=

(3)

= ∙

(4)

Where, V, R, W, ρ are volume, radius, weight and material density of the sphere, respectively.

Figure 9 Schematic diagram for shell thickness of nano-capsules. Similarly, for a capsule (as shown in Figure 9), we can write the following:

=

(5)

= ∙

(6)

=

(7)

= ∙

(8)

= −

(9) 9



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= ∙ = ∙ ( − )

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(10)

By the derivation, we get Formula (11)： &'()*

$ = − ∙ %&

+,*--

∙

.+,*-.'()*

+ 10

1

2 3

(11)

where, T and RMC are the shell thickness and radius of the capsule, respectively. In the Formula (11), all items of on the right-hand side are known, in which ρcore=ρn-octadecane=0.777 g/ml and ρshell≈ρPMMA=1.19 g/ml. Therefore, we can estimate the capsule thickness of different ratio of core to wall by Formula (11). The results are shown in Table 3. From Table 3, the shell thickness significantly reduce with increasing of the core to shell ratios for a nano-capsule in average diameter of 466 nm. When the core to shell ratios is increased to 5:3, the shell thickness of the nano-capsule was only 27.5 nm. The shell thickness of the nano-capsule is so thin that it may not be able to undergo the volume change due to the phase change process of core. So, this will lead to a lower the encapsulation efficiency. Also, we can predict that such nano-capsules may be more suitable for the application of drug release, which is not suitable for the protection of solid-liquid phase change materials.

3.5 Thermal stability of nano-capsules. TG curves of n-Octadecane, P (MMA-co-ALMA) and nanocapsules with different core/shell ratios are presented in Figure 10. The first mass-loss stage of nano-capsules is corresponding to the n-Octadecane diffused out of the shell, and the subsequent mass-loss stage is caused by the decomposition of shell materials. The initial mass-loss temperature (the temperature of the weightlessness 5%) increases with the increases of shell content. We can see the initial mass-loss temperatures are 164 oC, 176 oC and 185 oC, with the methyl methacrylate contents are 6g, 8g and 10g from the detail with an enlarged scale. This means the stability of the nanocapsules increased with the increases of shell content. The mass-loss of the first stage is equivalent to the content of n-Octadecane in the nanocapsules. From the TG curves, we can see that S5 has the largest weightlessness, followed by S3, and S4 is the smallest. The result is in agreement with the result of DSC above. Figure 10 TG curves of P(MMA-co-ALMA), n-Octadecane and nanocapsules with different core/shell ratios.

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3.7 The formation mechanism of nano-capsules. In this experiment, the formation mechanism of the nano-emulsion and nano-capsules can be described as follows: The formation mechanisms of the nano-emulsion27: As the water phase is poured into the oil phase containing n-octadecane, MMA, ALMA, and AIBN, the system starts as a W/O micro-emulsion. Upon increasing the volume fraction of water, water droplets merge together and bicontinuous or lamellar structures are formed, which, after the Emulsion inversion point is passed, decompose into smaller oil droplets upon further increasing the water content. Further dilution with water does not change the droplet size at this stage of droplet formation in the presence of a high surfactant concentration. The o/w nano-emulsion was obtained. The formation mechanisms of the nano-capsules28: After an o/w nano-emulsion is prepared which contains, as n-octadecane, MMA, ALMA, and AIBN using phase inversion emulsification, the temperature of the system was raised and the polymerization started. When the droplet composition reaches the binodal boundary, the polymer phase separates as small coacervate phase within the emulsion droplets because n-octadecane is a poor solvent for generated P (MMA-co-ALMA) polymer. These coacervate droplets are mobile and migrate to the oil/water interface. Further molecular weight polymer increasing causes the polymer to precipitate at the interface, forming the shell. The process is shown schematically in Figure 11.

Figure 11 Schematic representation of the mechanism of shell formation.

4. CONCLUSION Narrow-disperse

nano-capsules

containing

n-Octadecane

were

synthesized

with

P

(MMA-co-ALMA) as shell and with Span80 and Tween80 as the compounded emulsifier based on the low-energy emulsification method. The average diameter of nano-capsules is about 400 nm with the polydispersity indexes (PDI) are less than 0.1, and the thickness of the shell is about 50nm. The surface is relatively smooth and density with almost no defects. The average diameter decreased with the increase of the surfactant concentration. When the n-Octadecane/monomer mass ratio is 5:4, the encapsulation efficiency of the nano-capsules reaches the highest 52.9%. The result of TG shows the stability of the nano-capsules increased with the increases of shell content.

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ACKNOWLEDGMENT The authors are thankful to the National Natural Science Fund of China (No. 50573058), the National Natural Science Foundation of China (Grant No.51203113), Aeronautical Science Foundation of China (Grant No.201229Q2002), and the Science and Technology Development Plan of Tianjin Municipal (09ZCKFGX02200) for their financial support.

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COLLOIDS AND SURFACES A-PHYSICOCHEMICAL AND ENGINEERING ASPECTS 2011, 375, 102. (20) Mei, Z.; Liu, S.; Wang, L.; Jiang, J.; Xu, J.; Sun, D., Preparation of positively charged oil/water nano-emulsions with a sub-PIT method. J. Colloid Interface Sci. 2011, 361, 565. (21) Sole, I.; Pey, C. M.; Maestro, A.; Gonzalez, C.; Porras, M.; Solans, C.; Gutierrez, J. M., Nano-emulsions prepared by the phase inversion composition method: Preparation variables and scale up. J. Colloid Interface Sci. 2010, 344, 417. (22) Wang, Y.; Zhang, Y.; Xia, T.; Zhao, W.; Yang, W., Effects of fabricated technology on particle size distribution and thermal properties of stearic–eicosanoic acid/polymethylmethacrylate nanocapsules. Sol. Energy Mater. Sol. Cells 2014, 120, 481. (23) Tumirah, K.; Hussein, M.; Zulkarnain, Z.; Rafeadah, R., Nano-encapsulated organic phase change material based on copolymer nanocomposites for thermal energy storage. Energy 2014, 66, 881. (24) Alkan, C.; Sarı, A.; Karaipekli, A.; Uzun, O., Preparation, characterization, and thermal properties of microencapsulated phase change material for thermal energy storage. Sol. Energy Mater. Sol. Cells 2009, 93, 143. (25) Alay,

S.;

Göde,

F.;

Alkan,

C.,

Preparation

and

characterization

of

poly

(methylmethacrylate-coglycidyl methacrylate)/n-hexadecane nanocapsules as a fiber additive for thermal energy storage. Fibers and Polymers 2010, 11, 1089. (26) Liu, W.; Sun, D.; Li, C.; Liu, Q.; Xu, J., Formation and stability of paraffin oil-in-water nano-emulsions prepared by the emulsion inversion point method. J. Colloid Interface Sci. 2006, 303, 557. (27) Fernandez, P.; André, V.; Rieger, J.; Kühnle, A., Nano-emulsion formation by emulsion phase

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inversion. Colloids Surf. Physicochem. Eng. Aspects 2004, 251, 53. (28) Dowding, P. J.; Atkin, R.; Vincent, B.; Bouillot, P., Oil core-polymer shell microcapsules prepared by internal phase separation from emulsion droplets. I. Characterization and release rates for microcapsules with polystyrene shells. Langmuir 2004, 20, 11374.

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Figure captions

Figure 1 Preparation processes of nano-capsules. Figure 2 Average diameter of droplet as a function of the mixed HLB values for sample S5 at 50 °C. Figure 3 FTIR spectra for n-Octadecane, nano-capsules and P (MMA-co-ALMA). Figure 4 Size distributions of nano-capsules with different surfactant concentration: (a) S4; (b) S6; (c) S7; (d) S8; (e) S9.

Figure 5 Fitted line plot of the average diameter vs. the total amount of surfactant. Figure 6 SEM images of nano-capsules with different surfactant: (a) S4; (b) S6; (c) S7; (d) S8; (e) S9.

Figure 7 TEM micrographs of nano-capsules: S7.

Figure 8 DSC curves of nano-capsules with different core/shell ratios: (a) S1; (b) S2; (c) S3; (d) S4; (e) S5.

Figure 9 Schematic diagram for shell thickness of nano-capsules.

Figure 10 TG curves of P (MMA-co-ALMA), n-Octadecane and nanocapsules with different core/shell ratios.


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Table captions Table 1 Recipes for the preparation of nano-capsules. Table 2 Size distribution parameters of samples.

Table 3 Enthalpies and estimated thicknass of nano-capsules with different core/shell ratios.

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Figure n-Octadecane(10.0g) MMA(10.0g) AMA (1.0g) AIBN (0.100g) Surfactants

Water(200.0g) 1.0ml/min

90rpm

250rpm 40 oC

40oC

N 2, 68 o C

Oil phase W/O emulsion

O/W Nano-emulsion

Phase inversion emulsification

Nanocapsules

Free radical polymerization

Figure 1 Preparation processes of nano-capsules.

1.6 1.4

Average diameter (µm)

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


1.2 1.0 0.8 0.6 0.4 0.2 9.5

10.0

10.5

11.0

Mixed HLB value

Figure 2 Average diameter of droplet as a function of the mixed HLB values for sample S5 at 50 °C.

19


4000

P(MMA-co-AMA) Nanocapsules n-Octadecane

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber(cm )

Figure 3 FTIR spectra for n-Octadecane, nano-capsules and P (MMA-co-ALMA).

(a) (b) (c) (d) (e)

30

Volume (%)

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

Transmittance(%)


20

10

0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Diameter (µm)

Figure 4 Size distributions of nano-capsules with different surfactant concentration: (a) S4; (b) S6; (c) S7; (d) S8; (e) S9.

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500

Average diameter (nm)

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


450

y=640-35.8x R2=0.9566

400

350

300 5

6

7

8

9

Total amount of surfactant (g)

Figure 5 Fitted line plot of the average diameter vs. the total amount of surfactant.

21



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

Figure 6 SEM images of nano-capsules with different surfactant: (a) S4; (b) S6; (c) S7; (d) S8; (e) S9.

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Figure 7 TEM micrographs of nano-capsules: S7.

Heat flow(mv/mg) Endo

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


e d c b a a b c d e

-10

0

10

20

30

40

50

60

o

Temperature( C)

Figure 8 DSC curves of nano-capsules with different core/shell ratios: (a) S1; (b) S2; (c) S3; (d) S4; (e) S5.

23



Figure 9 Schematic diagram for shell thickness of nano-capsules.

100 Mass(%)

98

80

Mass(%)

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

96 94 92 140

60

160 180 200 o Temperature( C)

P(MMA-co-AMA) S5 S4 S3 n-Octadecane

40 20 0 100

200

300

400

500

600

o

Temperature( C) Figure 10 TG curves of P(MMA-co-ALMA), n-Octadecane and nanocapsules with different core/shell ratios.


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Table 1 Recipes for the preparation of nano-capsules. Sample

n-Octadecane (g)

MMA(g)

S1 S2 S3 S4 S5 S6 S7 S8 S9

6 8 10 10 10 10 10 10 10

10 10 10 8 6 8 8 8 8

ALMA (g)

AIBN (g)

1 1 1 1 1 1 1 1 1

Emulsifier (g)

Water (g)

5 5 5 5 5 6 7 8 9

200 200 200 200 200 200 200 200 200

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

Table 2 Size distribution parameters of samples. Sample

The surfactant mass(g)

The average diameter(nm)

The median diameter(nm)

The standard deviation(nm)

The polydispersity index(PDI)

S4 S6 S7 S8 S9

5.0 6.0 7.0 8.0 9.0

466 420 395 338 328

450 415 387 340 322

136 83 103 67 90

0.085 0.039 0.068 0.039 0.075

Table 3 Enthalpies and estimated thicknass of nano-capsules with different core/shell ratios. Sample

n-Octadecane/ monomer

Encapsulation Efficiency (%)

Tmo (oC)

Tmp (oC)

∆Hm (J/g)

Tco (oC)

Tcp (oC)

∆Hc (J/g)

Estimated thicknass (nm)

S1

3:5

32.2

23.5

32.8

70

25.4

19.6

75

53.8

S2

4:5

38.9

25.6

32.0

87

26.0

20.9

88

44.8

S3 S4 S5

5:5 5:4 5:3

42.2 52.9 35.8

22.2 24.2 21.6

34.4 33.2 33.0

93 116 78

24.6 25.4 23.8

16.3 15.7 17.6

97 122 83

38.5 33.3 27.5

Tmo, Tmp and ∆Hm are onset temperature, peak temperature and enthalpy on DSC heating curve; Tco, Tcp and ∆Hc are onset temperature, peak temperature and enthalpy on DSC cooling curve, respectively.

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