Polymer/graphene oxide composite microcapsules with greatly improved barrier properties

Abstract

Microcapsules with a polystyrene/graphene oxide composite shell were prepared by Pickering emulsion templating. Graphene oxide was modified using [2-(methacryloxy)ethyl]trimethylammonium chloride (METAC) or decyltrimethylammonium chloride (DTAC) to stabilize oil-in-water (O/W) Pickering emulsions. Scanning electron microscopy (SEM) was used to observe the morphology of the microcapsules. Thermal gravimetric analysis (TGA) was utilized to investigate the barrier property of the microcapsules. The activation energy of evaporation of the encapsulated material (n-hexadecane) is enhanced from 32.2 kJ mol⁻¹ to 46.2 kJ mol⁻¹, and even 60.9 kJ mol⁻¹ because of the restriction of the polymer/graphene oxide composite shells. These microcapsules with greatly enhanced barrier properties may be potential candidates in encapsulating gas- or moisture-sensitive substances and volatile materials, especially for long-term storage or application.

---

1. Introduction

Microencapsulation, a classic process in which droplets or tiny particles are coated with a polymer or inorganic shell,¹ is always used to protect the specific functional core materials from the outside environment.² Up to now, microencapsulation has been widely applied to the food industry,³ flavors and fragrances,⁴ drug delivery,^5,6 self-healing materials⁷ and coatings,⁸ energy storage,⁹ etc. It is necessary for microcapsules to protect core materials from leakage even if in some situations, the release of the payloads on demand is needed in response to external stimuli such as pH,¹⁰ mechanical stress¹¹ and temperature.¹² The barrier property of microcapsules is extremely important when encapsulated substances are chemically active and/or relatively volatile.³ In addition, good barrier property guarantees the long-term storage and practical applications of microcapsules.¹³ However, polymer microcapsules are commonly suffered from their relatively high permeability. Microcapsules with multi-layered structure¹⁴ and thicker shell¹⁵ have been prepared to improve their protection, whereas the encapsulation ratio of functional core materials decreases. The encapsulation of a moisture sensitive liquid within a metal shell was achieved via a multi-step process of emulsification, interfacial polymerization and electroless plating.¹⁶

Introduction of inorganic nanoparticles into polymer shells has also been proven to be an effective method to improve the protection of containers.¹⁷ Polymer/inorganic composite microcapsules were prepared via interfacial polymerization, which is a conventional method for the microencapsulation of functional materials.¹⁸ It can be speculated that the dispersion of inorganic nanoparticles in polymer matrix would affect the barrier property greatly. Thus, in situ sol–gel method¹⁸ and intercalative polymerization¹⁷ were utilized to achieve good dispersion of inorganic particles. Moreover, a stable emulsion is commonly required to serve as the template before the polymerization or reaction. Therefore, these methods are somewhat complicated. It is well known that Pickering emulsions can be stabilized by inorganic particles with suitable amphiphilicity¹⁹ and polymer/inorganic microcapsules are conveniently fabricated after the polymerization of Pickering emulsions.²⁰ Pickering emulsion templating has attracted increasing attentions to prepare polymer microcapsules due to its high stability,^21,22 low cost and toxicity.²³ However, inorganic particles normally locate on the surface of microcapsules after the polymerization since they are originally at the interface of Pickering emulsions and thus, the improvement of the barrier property is limited.²⁴

Graphene with a unique two dimensional structure shows excellent gas impermeability resistance and thus, it has been widely utilized as a barrier coating or film against oxygen, carbon dioxide and moisture.²⁵ Although graphene is only one atom thick, its p-orbitals forms a dense, delocalized cloud that blocks the gap within its aromatic rings. This creates a repelling field, which does not allow the smallest molecules to pass through even when ∼1–5 atm pressure difference is imposed across its atomic thickness at room temperature.²⁶ As the oxidized derivative of graphene,^27,28 graphene oxide (GO) displays the amphiphilicity due to its hydrophobic basal plane and some hydrophilic groups (e.g., carboxyl and hydroxyl groups) decorating the periphery,²⁹ which makes it assemble at the oil–water interface.³⁰ It is speculated that GO can be used as a Pickering stabilizer to prepare polymer composite microcapsules with excellent barrier property. Polymer/GO composite microcapsules have been prepared via Pickering miniemulsion polymerization utilizing GO as the stabilizer,^31,32 while the barrier property of microcapsules was not investigated. In our previous paper, polystyrene composite microcapsules were fabricated via GO stabilized Pickering emulsion templating.³³ However, the improvement of barrier property is limited because GO nanosheets are only covered on the surface of microcapsules.

Polymer/GO composite microcapsules with greatly enhanced protection for functional core materials are expected when GO is well dispersed in polymer shells. In the present work, [2-(methacryloxy)ethyl]trimethylammonium chloride (METAC) and decyltrimethylammonium chloride (DTAC) were used to functionalize GO nanosheets to improve the compatibility between the polymer and GO. Polymer composite microcapsules were obtained after the polymerization of Pickering emulsions stabilized by modified GO. As a kind of phase change materials (PCMs), n-hexadecane was selected as the core material. The microencapsulated PCM (Micro-PCM) showed a significant improvement of heat resistance. This paper develops a convenient and universal strategy to prepare polymer composite microcapsules for the efficient protection of functional core materials.

2. Experimental section

2.1 Materials

Adipic acid (99.0%), azo-bisisobutyronitrile (AIBN, 99%), diethanolamine (99.0%), hydrochloric acid (37%), hydrogen peroxide (30%), n-hexadecane (98%), potassium permanganate (99%), sodium nitrate (99%), sulfuric acid (98%) and styrene (99%) were purchased from Shanghai Chemical Reagent Co. (China). Graphite powders (99.95%), [2-(methacryloxy)ethyl]trimethylammonium chloride (METAC, 75%), decyltrimethylammonium chloride (DTAC, 99%) and divinylbenzene (80%) were supplied by Aladdin Chemistry Co. Ltd. Styrene and divinylbenzene were distilled under vacuum and AIBN was recrystallized prior to use. Deionized water was used throughout the experiments.

2.2 Preparation of GO

GO nanosheets were prepared using a modified Hummers method³⁴ from graphite. The GO suspension was dialyzed in deionized water for one week. GO was dried in a vacuum oven at 50 °C for 24 h, and was then dispersed in water and sonicated for 1–2 h with the power of 300 W to form the GO aqueous suspension with different GO concentrations (0.06, 0.12 and 0.24%).

2.3 Modification of GO

The polycondensate of diethanolamine and adipic acid was prepared according to the literature.^35,36 0.4 g polycondensate aqueous solution (10%) was added to 40.0 g GO aqueous suspension (0.12%), and the pH was adjusted to 2 by hydrochloric acid solution (10%). The mixture was stirred magnetically at room temperature for 20 min. 0.2 g METAC aqueous solution (10%) was added to 40.0 g GO aqueous suspension (0.12%), stirring magnetically at room temperature for 20 min. Similarly, 0.2 g DTAC aqueous solution (1%) was mixed with 40.0 g GO aqueous suspension (0.12%) under stirring magnetically at room temperature for 20 min. Besides, 0.1 g DTAC aqueous solution (1%) was mixed with 40.0 g GO aqueous suspension (0.06%) and 0.4 g DTAC aqueous solution (1%) was mixed with 40.0 g GO aqueous suspension (0.24%) in the same way.

2.4 Preparation of microcapsules

Microcapsules was fabricated using modified GO as Pickering stabilizer and n-hexadecane as porogenic agent. The modified GO aqueous dispersion was mixed with an oil phase containing 3.2 g n-hexadecane, 0.6 g styrene, 0.2 g divinylbenzene and 0.01 g AIBN. The mixtures were emulsified using an FJ200-S homogenizer at 16 000 rpm for 15 min. The obtained emulsions were polymerized at 68 °C for 16 h under nitrogen atmosphere. The microcapsules were isolated by filtration, rinsed 3 times with water and dried at 30 °C for 4 h till constant weight. The recipes for the microcapsules preparation are listed in Table 1. Microcapsules stabilized by various modified GO were labeled as Micro-hexadecane-X, where hexadecane represents the core materials and X represents the name of the modifier.

Table 1 Recipes for the preparation of microcapsules samples

Sample	Water phase (g)	Oil phase (g)
Micro-hexadecane-polycondensate	GO aqueous dispersion (0.12%) 40.0 + polycondensate aqueous solution (10%) 0.4 + HCl (10%) 1.0	n-Hexadecane 3.2 + styrene 0.6 + divinylbenzene 0.2 + AIBN 0.01
Micro-hexadecane-METAC	GO aqueous dispersion (0.12%) 40.0 + METAC aqueous solution (10%) 0.2
Micro-hexadecane-DTAC	GO aqueous dispersion (0.12%) 40.0 + DTAC aqueous solution (1%) 0.2
Micro-hexadecane-DTAC-L	GO aqueous dispersion (0.06%) 40.0 + DTAC aqueous solution (1%) 0.1
Micro-hexadecane-DTAC-H	GO aqueous dispersion (0.24%) 40.0 + DTAC aqueous solution (1%) 0.4

---

2.5 Characterization

Atomic force microscope (AFM) image of GO was acquired using a Multimode 8 in the tapping mode. Before AFM test, GO was dispersed in water by ultrasonication and then spin coated onto a freshly cleaved mica plate. Photographs of GO and modified GO dispersed in water and oil were taken by a Canon Ixus 310HS digital camera. FT-IR spectra (KBr) were collected with a Thermo Nicolet Nexus 6700 instrument. The microscopy images of emulsion droplets were observed by an EV5680 optical microscope after the emulsions were dropped on glass slides. A TESCAN 5136 MM scanning electron microscope (SEM) and a Zeiss Ultra-55 field-emission scanning electron microscope (FE-SEM) were used to observe the morphology of microcapsules. Before the test, the microcapsules were dispersed in deionized water and dried on mica substrates. All samples were sprayed with gold before observation. The glass transition temperature (T_g) of the shell of the microcapsules was tested by a TA Q2000 differential scanning calorimeter (DSC) with a heating rate of 20 °C min⁻¹ under nitrogen atmosphere. T_g was measured via TA Universal Analysis 2000 software. Isothermal evaporation of bulk n-hexadecane and microencapsulated n-hexadecane were characterized by a Pyris-1 thermal gravimetric analysis (TGA) under a nitrogen atmosphere with a flow rate of 40 mL min⁻¹ at 90, 100, 110 and 120 °C, respectively. Melting and crystallization behaviors of microencapsulated n-hexadecane were investigated by a Mettler DSC-1 differential scanning calorimeter (DSC) with a heating or cooling rate of 10 °C min⁻¹ under nitrogen atmosphere. The phase change enthalpy values of bulk n-hexadecane and microencapsulated n-hexadecane were calculated via STARe thermal analysis software of Mettler Toledo.

3. Results and discussion

3.1 Surface modification of GO

The full exfoliation of GO nanosheets is achieved with the thickness of about 1 nm and an average size of 450 nm (Fig. S1 and S2†). With some hydrophilic groups (e.g., carboxyl and hydroxyl groups) decorating the periphery,²⁹ GO can well disperse in water. The hydrophobic basal plane and the hydrophilic edge of GO endows it with amphiphilicity, and thus it can be used as a Pickering stabilizer.³⁷ The prepared GO has good hydrophilicity seen from sample A1 in Fig. 1a. In our previous paper,³³ hydrophilic GO nanosheets were modified by the polycondensate of diethanolamine and adipic acid for the preparation of stable Pickering emulsions because solid particles with extreme hydrophilicity are not suitable to stabilize Pickering emulsions.³⁸ The polycondensate modified GO shows the characteristic absorption peaks of both the polycondensate and GO (Fig. S3†). The peak at 1625 cm⁻¹ corresponding to the N–H bending of the polycondensate is shifted to 1644 cm⁻¹ for the polymer modified GO, which confirms the interaction between GO and the polymer.^39,40 Herein, DTAC with a lipophilic alkyl chain and METAC with a double bond are also utilized to functionalize the surface of GO nanosheets (Fig. 1a). It is obvious that the three kinds of modified GO become more hydrophobic and the flocculation appears due to the strong electrostatic attraction between organic molecules and GO.⁴¹

Fig. 1 Photographs of (A1, A2) GO, (B1, B2) DTAC-modified GO, (C1, C2) METAC-modified GO and (D1, D2) polycondensate-modified GO dispersed in (a) water and (b) styrene.

Since the polar GO sheets have poor compatibility with the majority of nonpolar polymers and can hardly achieve polymer/GO composites with good dispersion of GO particles, it is essential to modify the hydrophilic GO surface.⁴² Recently, it has been proven that the modification of GO with chemicals containing the long alkyl chain is an effective way to make GO lipophilic for the preparation of polymer/GO composite materials.^43,44 In the present work, the lipophilicity of modified GO is studied by dispersing the dried samples into styrene (Fig. 1b). It is found that pure GO and polycondensate-modified GO have poor lipophilicity, and thus they tend to accumulate at the bottom. However, DTAC-modified GO can be well dispersed in styrene because of the graft of lipophilic hydrocarbon chains. The loose precipitate is clearly seen when GO is functionalized by METAC. Therefore, three modified GO samples have obviously different lipophilicity and the structure of polymer/GO composite microcapsules may be tailored when they are used as Pickering stabilizer.

3.2 Pickering emulsions stabilized by modified GO

The appropriate amphiphilicity of solid particles is necessary for the preparation of a stable Pickering emulsion.³⁸ The obtained three kinds of modified GO were employed to prepare oil-in-water (O/W) Pickering emulsions according to the formulations listed in Table 1. The morphology of the Pickering emulsion droplets was investigated via an optical microscope, as shown in Fig. 2. In general, three types of modified GO can all stabilize Pickering emulsions effectively and the size of emulsion droplets is about 30 μm. However, the Pickering emulsion stabilized by DTAC-modified GO has a relatively wider size distribution and the diameter of some droplets is even larger than 50 μm, which may be induced by the high lipophilicity of Pickering stabilizer. From Fig. S4 and S5,† a contradiction between the dissolubility of DTAC-modified GO in styrene and the stability of Pickering emulsions is clearly found when different amount of DTAC is grafted on the surface of GO. Therefore, sample B2 in Fig. 1b is selected to prepare polymer composite microcapsules because it is highly lipophilic but can still well stabilize a Pickering emulsion for further polymerization.

Fig. 2 Optical microscopy images of Pickering emulsions stabilized by (a) polycondensate-modified GO, (b) METAC-modified GO and (c) DTAC-modified GO.

We furtherly investigate the stability of above Pickering emulsions (Fig. 2) by a rotary rheometer. Fig. 3 shows double-log plots of the shear viscosity (η) as a function of shear rate (γ) for emulsions stabilized by different modified GO. All emulsions show similar shear-thinning behavior. As the shear rate increases, the hydrodynamic forces become large enough for droplet flocs to be deformed, elongated and aligned with the shear field, eventually being disrupted and leading to a rapid decrease in viscosity. It has been proven that slightly flocculated particles are beneficial for the long term stability of the emulsions.³³ More particle aggregates adsorbed on the droplets surface will result in higher viscosity of the emulsions. As the emulsion viscosity increases, the probability of the coalescence decreases and thus, the emulsion stability is improved. At low shear rate, the viscosity of emulsion stabilized by DTAC-modified GO (curve C in Fig. 3) is a little less than those of emulsions stabilized by polycondensate and METAC-modified GO (curves A and B in Fig. 3), indicating the lower stability of emulsion stabilized by DTAC-modified GO. Because of the high lipophilicity of DTAC-modified GO, less GO aggregates adsorbed on the droplets surface lead to the decrease in emulsion viscosity, which should be responsible for the reduced stability of the emulsion. In spite of that, the stability of the O/W Pickering emulsion stabilized by DTAC-modified GO is still high enough to undergo the following polymerization.

Fig. 3 Viscosity (η) as a function of shear rate (γ) for Pickering emulsions stabilized by (A) polycondensate-modified GO, (B) METAC-modified GO and (C) DTAC-modified GO at 25 °C.

3.3 Morphology of microcapsules

After polymerization of the Pickering emulsions in Fig. 2, we successfully fabricated microencapsulated n-hexadecane. Since n-hexadecane is the non-solvent for the polymer, styrene-DVB copolymer phase separates towards the interface of the colloidosomes under the drive force of interfacial tensions during polymerization.²⁰ Fig. 4 shows SEM images of the three types of microcapsules. The sizes of microcapsules are approximately 20–30 μm, which are corresponding to those of the emulsion droplets in Fig. 2. It is obvious that the surface of many microcapsules is sunken. This may be caused by the shrinkage of microcapsules during Pickering polymerization at a relatively high temperature.⁴⁵ The uneven surface also provides a proof of the hollow structure of microcapsules.

Fig. 4 SEM images of (a) Micro-hexadecane-polycondensate, (b) Micro-hexadecane-METAC and (c) Micro-hexadecane-DTAC.

The surface morphology of a single microcapsule is observed by FE-SEM (Fig. 5). The rough surface of polymer microcapsules is clearly seen when the polycondensate and METAC-modified GO are used as Pickering stabilizer. Polymer microcapsules are fully covered by wrinkled GO flakes. However, the surface of Micro-hexadecane-DTAC is smoother and GO sheets are not so obvious. From the FE-SEM image with large magnification (Fig. 5c), GO nanosheets on the surface of the microcapsules are also found, but the amount of GO is much less. The difference in the surface morphology among the samples may be caused by the lipophilicity of modified GO. As mentioned above, DTAC-modified GO has high lipophilicity and can be well dispersed in styrene. The Pickering emulsion is only stabilized by a certain proportion of functionalized GO and another part of solid particles with higher lipophilicity dissolves in oil phase, which results in less GO on the surface of microcapsules. Furthermore, less GO used as Pickering stabilizer should also be responsible for the broader size distribution of Pickering emulsion droplets and polymer microcapsules. Further increasing the lipophilicity of modified GO results in an unstable Pickering emulsion as shown in Fig. S4† due to too little GO with appropriate amphiphilicity stabilized at the surface of droplets.

Fig. 5 FE-SEM images of (a) Micro-hexadecane-polycondensate, (b) Micro-hexadecane-METAC and (c) Micro-hexadecane-DTAC. The insets show the corresponding surface morphology in a large magnification.

Fig. 6 shows the inner wall of microcapsules via a broken microcapsule. Different from the outer wall (Fig. 5a), the inner surface of Micro-hexadecane-polycondensate is quite neat only with many small “bubbles” (Fig. 6a), which is caused by the phase separation of poly(styrene-co-divinylbenzene) during polymerization.²⁰ The small “bubbles” are also seen on the inner surface of Micro-hexadecane-METAC and some GO flakes are attached on the inner wall (Fig. 6b). METAC-modified GO has an element of lipophilicity, and thus the loose precipitate is formed when it is dispersed in styrene seen from Fig. 1. It is speculated that a small amount of functionalized GO flakes with double bonds on the surface are attached on the inner surface of polymer microcapsules via covalent interaction because they can copolymerize with the monomer. When DTAC-modified GO is used as Pickering stabilizer, the inner wall of microcapsules is fully covered by GO nanosheets, which results from the good lipophilicity of functionalized GO. The morphology of the inner wall is similar with that of the outer surface (Fig. 5c and 6c), indicating the good dispersion of GO nanosheets in polymer shell.

Fig. 6 FE-SEM images of the inner surface morphology of (a) Micro-hexadecane-polycondensate, (b) Micro-hexadecane-METAC and (c) Micro-hexadecane-DTAC. The inset shows the corresponding sample's inner surface in a large magnification.

As we know, dispersing inorganic particles into polymer matrix and the covalent interaction between polymer chains and inorganic particles may restrict the segment motion of polymer chains, and are expected to raise the glass transition temperature (T_g).⁴⁶ Since the cross-linking agent would make T_g of the polymer difficult to be detected, we prepared Micro-hexadecane without divinylbenzene and then washed away the inside n-hexadecane with ethanol. A sample stabilized by unmodified GO for comparison is also fabricated in the same way. As shown in Fig. 7, it is proved that T_g of Micro-hexadecane-METAC (100 °C) and Micro-hexadecane-DTAC (101 °C) are much higher than that of polymer microcapsules stabilized by pure GO (80 °C) and polycondensate-modified GO (85 °C). Although the covalent interaction between polymer chains and inorganic particles is more efficient to constrain the segment motion of polymer chains, T_g of Micro-hexadecane-DTAC is still a bit higher than that of Micro-hexadecane-METAC. This can be explained by more GO nanosheets dispersed in polymer matrix when DTAC-modified GO is used as Pickering stabilizer due to its high lipophilicity, which has been found in Fig. 5 and 6.

Fig. 7 DSC thermograms of polymer microcapsules stabilized by (A) pure GO, (B) polycondensate-modified GO, (C) METAC-modified GO and (D) DTAC-modified GO.

3.4 Barrier property of microcapsules

The barrier property of the polymer/GO composite microcapsules with GO covered on the surface via covalent interaction (Micro-hexadecane-METAC) and well dispersed GO in the matrix (Micro-hexadecane-DTAC) were investigated by TGA (Fig. S6†). Microcapsules in our previous work (Micro-hexadecane-polycondensate) were employed as a comparison sample. The isothermal release of hexadecane encapsulated in microcapsules is almost linear (in a one-step process) at different temperatures. The activation energy of evaporation of the core material is calculated via the Arrhenius equation,¹ and the higher activation energy means the slower release rate. The napierian-logarithm of release rate constant as a function of the reciprocal of temperature is shown in Fig. 8. The activation energy of evaporation of unencapsulated n-hexadecane is calculated as 32.2 kJ mol⁻¹. Micro-hexadecane-polycondensate displays a little higher activation energy (36.8 kJ mol⁻¹). Interestingly, the activation energies of Micro-hexadecane-METAC and Micro-hexadecane-DTAC are as high as 46.2 and 60.9 kJ mol⁻¹, respectively. The protection of the core material from leakage is greatly improved, which can only be ascribed to the structure of polymer microcapsules. The covalent interaction between GO and the polymer (Micro-hexadecane-METAC) and the good dispersion of GO nanosheets in polymer matrix (Micro-hexadecane-DTAC) should be responsible for the obviously enhanced barrier property. It can be speculated that the amount of modified GO will have great effect on the structure and property of resulted polymer microcapsules since the dispersion of GO nanosheets in polymer matrix promotes the barrier property obviously. Polymer microcapsules with a lower GO concentration of 0.6% (Micro-hexadecane-DTAC-L) show a less activation energy of hexadecane evaporation of 47.2 kJ mol⁻¹, confirming that the significant improvement of the protection for the core material stems from the good dispersion of GO nanosheets in polymer shells. Micro-hexadecane-DTAC-H exhibits a close activation energy of hexadecane evaporation to Micro-hexadecane-DTAC, indicating that further increase in the amount of GO is not significant for the improvement in the performance. This may be induced by the aggregation of GO sheets in polymer matrix.

Fig. 8 Napierian-logarithm of release rate constant of n-hexadecane (ln k) as a function of the reciprocal of temperature (T⁻¹) for (A) n-hexadecane, (B) Micro-hexadecane-polycondensate, (C) Micro-hexadecane-DTAC-L, (D) Micro-hexadecane-METAC, (E) Micro-hexadecane-DTAC-H and (F) Micro-hexadecane-DTAC.

3.5 Thermal properties of microcapsules

Since n-hexadecane has been well protected by polymer/GO composite microcapsules, its phase change behavior was also characterized by DSC, as shown in Fig. 9. The results are listed in Table 2. The encapsulation ratio of n-hexadecane can be calculated with the following equation based on enthalpy values.⁴⁷
where η is the encapsulation ratio of PCM. ΔH_Micro-PCM and ΔH_PCM are measured enthalpies of Micro-PCM and bulk PCM, respectively. With an average enthalpy of 185.8 J g⁻¹, the calculated encapsulation ratio of Micro-hexadecane-polycondensate is 78.0%. Accordingly, the encapsulation ratios of Micro-hexadecane-METAC and Micro-hexadecane-DTAC are 80.0% and 83.9%. The relatively high encapsulation ratios are attributed to the efficient stabilization of modified GO for Pickering emulsions. In addition, the exothermic and endothermic peaks of Micro-PCMs are wider than those of bulk n-hexadecane because of the poor thermal conductivity of the polymer.⁴⁸ However, Micro-hexadecane-DTAC has narrower exothermic and endothermic peaks than those of Micro-hexadecane-polycondensate and Micro-hexadecane-METAC. The improved thermal conductivity of Micro-hexadecane-DTAC is induced by the good dispersion of GO sheets in polymer microcapsules.

Fig. 9 DSC curves of (A) n-hexadecane, (B) Micro-hexadecane-polycondensate, (C) Micro-hexadecane-METAC and (D) Micro-hexadecane-DTAC.

Table 2 Melting and crystallization of n-hexadecane and Micro-hexadecane^a

Sample	ΔHm (J g^−1)	ΔHc (J g^−1)	Tm peak (°C)	Tc peak (°C)	ΔT (°C)
a ΔHm = enthalpy on DSC melting curve, ΔHc = enthalpy on DSC crystallization curve, Tm peak = melting peak temperature, Tc peak = crystallization peak temperature, ΔT = degree of supercooling.
n-Hexadecane	237.5	−238.7	20.9	14.1	6.8
Micro-hexadecane-polycondensate	184.7	−186.8	29.9	5.6	24.3
Micro-hexadecane-METAC	189.6	−191.4	27.3	6.9	20.4
Micro-hexadecane-DTAC	198.5	−200.9	25.4	9.1	16.3
Micro-hexadecane-DTAC-L	196.8	−199.1	28.4	1.7	26.7
Micro-hexadecane-DTAC-H	191.5	−194.0	25.2	9.5	15.7

---

Supercooling is the process of lowering the temperature of a liquid below its freezing point without becoming a solid.⁴⁹ The degree of supercooling (ΔT) can be determined according to the difference between melting peak temperature (T_{m peak}) and crystallization peak temperature (T_{c peak}). An effective way for decreasing supercooling is the addition of solid nucleating agents, such as nanoparticles and impurities, to PCM liquids as seeds and catalysts for nucleation and crystal growth. It has been reported that well dispersed multi-wall carbon nanotubes in hexadecane acted as nucleating agent for decreasing of supercooling.⁵⁰ From Fig. 9, it is clearly found that the supercooling of Micro-hexadecane-DTAC (16.3 °C) is much lower than that of Micro-hexadecane-METAC (20.4 °C) and Micro-hexadecane-polycondensate (24.3 °C) even though they have similar particle size. It is proved that DTAC-modified GO can also be well dispersed in n-hexadecane due to its high lipophilicity (Fig. S7†). GO nanosheets dissolved in n-hexadecane which acts as nucleating agent may be responsible for the decrease of supercooling of Micro-hexadecane-DTAC.

4. Conclusion

In summary, METAC and DTAC were used to modify the surface structure of GO via the strong electrostatic attraction and the functionalized GO was utilized as Pickering stabilizer to prepare polymer microcapsules. After polymerization of Pickering emulsion templates, METAC-modified GO is mainly covered on the surface of microcapsules via covalent interaction, while GO nanosheets with DTAC are well dispersed in polymer shells due to their high lipophilicity. Therefore, these polymer composite microcapsules have elevated glass transition temperature. The activation energies of evaporation of Micro-hexadecane-METAC and Micro-hexadecane-DTAC are as high as 46.2 and 60.9 kJ mol⁻¹, respectively, while that of unencapsulated n-hexadecane is only 32.2 kJ mol⁻¹. Furthermore, Micro-PCM stabilized by DTAC-modified GO shows a decreased supercooling degree due to the nucleation effect of dissolved GO sheets in the core material. Polymer/GO composite microcapsules with greatly improved barrier property may have promising applications to microencapsulate functional materials with the requirement of long-term storage and application, especially for the encapsulation of chemically or physically active substances.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NSFC) (No. 51373038).

References

1. Y. Zhao, Y. Li, D. E. Demco, X. Zhu and M. Moller, Langmuir, 2014, 30, 4253–4261.
2. J. van Wijk, J. W. O. Salari, N. Zaquen, J. Meuldijk and B. Klumperman, J. Mater. Chem. B, 2013, 1, 2394–2406.
3. Y. Serfert, C. Lamprecht, C. P. Tan, J. K. Keppler, E. Appel, F. J. Rossier-Miranda, K. Schroen, R. M. Boom, S. Gorb, C. Selhuber-Unkel, S. Drusch and K. Schwarz, J. Food Eng., 2014, 143, 53–61.
4. R. Ciriminna and M. Pagliaro, Chem. Soc. Rev., 2013, 42, 9243–9250.
5. Y. F. Zhu, W. J. Meng, H. Gao and N. Hanagata, J. Phys. Chem. C, 2011, 115, 13630–13636.
6. C. Li, G. F. Luo, H. Y. Wang, J. Zhang, Y. H. Gong, S. X. Cheng, R. X. Zhuo and X. Z. Zhang, J. Phys. Chem. C, 2011, 115, 17651–17659.
7. H. H. Jin, C. L. Mangun, A. S. Griffin, J. S. Moore, N. R. Sottos and S. R. White, Adv. Mater., 2014, 26, 282–287.
8. A. Latnikova, D. O. Grigoriev, H. Möhwald and D. G. Shchukin, J. Phys. Chem. C, 2012, 116, 8181–8187.
9. H. Zhang and X. Wang, Colloids Surf., A, 2009, 332, 129–138.
10. I. Hofmeister, K. Landfester and A. Taden, Macromolecules, 2014, 47, 5768–5773.
11. T. S. Coope, U. F. J. Mayer, D. F. Wass, R. S. Trask and I. P. Bond, Adv. Funct. Mater., 2011, 21, 4624–4631.
12. M. Prevot, C. Dejugnat, H. Mohwald and G. B. Sukhorukov, ChemPhysChem, 2006, 7, 2497–2502.
13. G. Wu, R. Y. Dai, W. G. Li, P. P. Yin, H. Z. Chen and M. Wang, Curr. Appl. Phys., 2011, 11, 321–326.
14. Y. Yang, Z. J. Wei, C. Y. Wang and Z. Tong, ACS Appl. Mater. Interfaces, 2013, 5, 2495–2502.
15. Q. Li, A. K. Mishra, N. H. Kim, T. Kuila, K. Lau and J. H. Lee, Composites, Part B, 2013, 49, 6–15.
16. M. W. Patchan, L. M. Baird, Y. R. Rhim, E. D. LaBarre, A. J. Maisano, R. M. Deacon, Z. Y. Xia and J. J. Benkoski, ACS Appl. Mater. Interfaces, 2012, 4, 2406–2412.
17. C. J. Fan and X. D. Zhou, Polym. Adv. Technol., 2009, 20, 934–939.
18. G. Wu, J. L. An, D. W. Sun, X. Z. Tang, Y. Xiang and J. L. Yang, J. Mater. Chem. A, 2014, 2, 11614–11620.
19. N. P. Ashby and B. P. Binks, Phys. Chem. Chem. Phys., 2000, 2, 5640–5646.
20. T. Chen, P. J. Colver and S. A. F. Bon, Adv. Mater., 2007, 19, 2286–2289.
21. B. P. Binks and S. O. Lumsdon, Langmuir, 2001, 17, 4540–4547.
22. S. A. F. Bon and P. J. Colver, Langmuir, 2007, 23, 8316–8322.
23. C. Wang, C. Zhang, Y. Li, Y. Chen and Z. Tong, React. Funct. Polym., 2009, 69, 750–754.
24. R. K. Bharadwaj, Macromolecules, 2001, 34, 9189–9192.
25. Y. Ahn, Y. Jeong and Y. Lee, ACS Appl. Mater. Interfaces, 2012, 4, 6410–6414.
26. V. Berry, Carbon, 2013, 62, 1–10.
27. Y. He, F. Wu, X. Sun, R. Li, Y. Guo, C. Li, L. Zhang, F. Xing, W. Wang and J. Gao, ACS Appl. Mater. Interfaces, 2013, 5, 4843–4855.
28. P. Guo, H. Song and X. Chen, J. Mater. Chem., 2010, 20, 4867–4874.
29. J. Kim, L. J. Cote, F. Kim, W. Yuan, K. R. Shull and J. X. Huang, J. Am. Chem. Soc., 2010, 132, 8180–8186.
30. Z. Sun, T. Feng and T. P. Russell, Langmuir, 2013, 29, 13407–13413.
31. S. C. Thickett, N. Wood, Y. H. Ng and P. B. Zetterlund, Nanoscale, 2014, 6, 8590–8594.
32. G. H. Teo, Y. H. Ng, P. B. Zetterlund and S. C. Thickett, Polymer, 2015, 63, 1–9.
33. Y. Zhang, X. H. Zheng, H. T. Wang and Q. G. Du, J. Mater. Chem. A, 2014, 2, 5304–5314.
34. W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339–1399.
35. H. Hu, H. Wang and Q. Du, Soft Matter, 2012, 8, 6816–6822.
36. M. H. Wu, Y. Luo and S. L. Chen, Int. J. Polym. Mater., 2004, 53, 119–129.
37. P. Xie, X. Ge, B. Fang, Z. Li, Y. Liang and C. Yang, Colloid Polym. Sci., 2013, 291, 1631–1639.
38. B. P. Binks and S. O. Lumsdon, Langmuir, 2000, 16, 8622–8631.
39. M. M. Gudarzi and F. Sharif, Soft Matter, 2011, 7, 3432–3440.
40. B. Shen, W. T. Zhai, C. Chen, D. D. Lu, J. Wang and W. G. Zheng, ACS Appl. Mater. Interfaces, 2011, 3, 3103–3109.
41. Z. Zheng, X. H. Zheng, H. T. Wang and Q. G. Du, ACS Appl. Mater. Interfaces, 2013, 5, 7974–7982.
42. H. Pang, Y. Y. Piao, C. H. Cui, Y. Bao, J. Lei, G. P. Yuan and C. L. Zhang, J. Polym. Res., 2013, 20, 304.
43. W. J. Li, X. Z. Tang, H. B. Zhang, Z. G. Jiang, Z. Z. Yu, X. S. Du and Y. W. Mai, Carbon, 2011, 49, 4724–4730.
44. M. K. Shin, B. Lee, S. H. Kim, J. A. Lee, G. M. Spinks, S. Gambhir, G. G. Wallace, M. E. Kozlov, R. H. Baughman and S. J. Kim, Nat. Commun., 2012, 3, 650.
45. N. Nakagawa and Y. Nonomura, Chem. Lett., 2011, 40, 818–819.
46. M. A. Aldosari, A. A. Othman and E. H. Alsharaeh, Molecules, 2013, 18, 3152–3167.
47. A. Sarı, C. Alkan, A. Karaipekli and O. Uzun, Sol. Energy, 2009, 83, 1757–1763.
48. W. Li, X. X. Zhang, X. C. Wang, G. Y. Tang and H. F. Shi, Energy, 2012, 38, 249–254.
49. X. X. Zhang, Y. F. Fan, X. M. Tao and K. L. Yick, J. Colloid Interface Sci., 2005, 281, 299–306.
50. S. Zhang, J. Y. Wu, C. T. Tse and J. Niu, Sol. Energy Mater. Sol. Cells, 2012, 96, 124–130.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra18297b

‡ These authors contributed equally to this manuscript.

---