Volatilization

As shown in Fig. 1, the preparation of the dual shell microcapsules includes three steps. Firstly, a micron-sized emulsion was firstly obtained after stirring the mixture of STF, liquid paraffin, and Span80 (as emulsifier) at a low rotation speed (Fig. 1(a)). Then, the polycondensation occurs between the PEG in the STF droplets and CD-MDI at the surface of the emulsion to form a preliminary polyurethane shell layer (Fig. 1(b)). Finally, the unreacted isocyanate on the surface of polyurethane shell layer reacts with DETA to form a dense polyurea shell layer (Fig. 1(c)). As a result, dual shell microcapsules are formed.
The dispersed particles adopted for the preparation of STF are solid silica microspheres with a particle size of about 150 nm, which plays a decisive role in the shear thickening performance of STF, as shown in Fig. 1(e). To investigate the shear thickening property of the STF, the rheological tests of STF with different silica concentrations are carried out (details in ESI†). As shown in Fig. 1(f), the viscosity of SiO₂/PEG200 fluids firstly decreases with the increase in the shear rate, then increases rapidly after a critical shear rate is reached. The higher the concentration of silica, the lower the critical shear rate and the faster the viscosity mutation. When the concentration of silica is 68.5%, after a critical shear rate at 60 s⁻¹ was reached, the viscosity increases rapidly and the value at the peak was 28 times larger than the initial value. To make sure that STF can be suspended in the solvent, the STF with lower concentration (62.0%) is chosen. Nevertheless, the consumption of PEG during the following reaction process will increase the concentration of silica, which ensures the good shear thickening performance (details in ESI†). This ingenious design not only ensures the dispersion of STF but also maintains good shear thickening performance.
The emulsification effect of STF in liquid paraffin was observed by optical microscopy and the prepared double-layered microcapsules, and the cross-sections of composites were observed by SEM, as shown in Fig. 2(a). It can be seen from Fig. 2(a1) and (a4) that STF emulsification in liquid paraffin is well dispersed. The average droplet diameter is 100 μm with an agitation rate of 800 rpm. As shown in Fig. 2(a2) and (a3), the spherical particle size and double layered microcapsule wall are 190 μm and 14.31 μm, respectively. The surface of the microcapsules has a certain roughness, which is believed to be caused by the uneven shrinkage of wall materials caused by the rapid evaporation of solvent in the drying process and the certain adhesion between microcapsules in the emulsion reaction. We also used drop addition to prepare STF capsules for comparison (details in ESI†).
To investigate the structure of the core material, pure wall material, and microcapsules, the FTIR test was carried out, and the results are shown in Fig. 2(b). The peak at 1082 cm⁻¹ corresponds to the asymmetric and symmetric vibrations of the Si–O–Si groups of the silica microspheres in the core material STF, which could also be observed in the spectra of the microcapsules. In the spectra of b2 and b3, the carbonyl peaks in the range of 1646–1543 cm⁻¹ and the peak of the stretching vibration of –NH at 3279 cm⁻¹ are observed. The same absorption peak also appears in the spectra of microcapsules, which confirms the formation of polyurea and polyurethane. By comparing the spectra of b3 and b4, the microcapsules and polyurea have the same characteristic absorption peaks at 2922 cm⁻¹ and 2854 cm⁻¹, which further indicates that the outermost layer of the microcapsules is the polyurea shell. According to the infrared spectrum, the absorption characteristic peaks of the STF and the polyurea-polyurethane shell can be observed, which confirms the successful encapsulation of STF within the microcapsules.
Besides, the thermogravimetric analysis of the double-layered microcapsules, pure core material, and pure wall material are shown in Fig. 2(c). According to Fig. 2(c1), the STF shows only one thermal degradation stage from 150 °C to 370 °C, which corresponds to the thermal decomposition process of the PEG contained in it. The weight of the pure core material (STF) decreases rapidly at 225 °C. In comparison, the microcapsule with STF as the core shows two weight loss stages (Fig. 2(c4)), indicating the successful encapsulation of STF in the PU/PUA shell. Moreover, the initial decomposition temperature of the microcapsule is quite close to that of the STF, which indicates that the weight loss of the first stage at 240 °C mainly arises from the volatilization and decomposition of the STF. By comparing Fig. 2(c2–c4), it clearly shows that the decomposition temperature of the polyurethane shell and polyurea shell is 320 °C, proving that the core material has a good coating effect under PU and PUA shell. Compared to Fig. 2(c1), the thermal weight loss temperature point of STF in microcapsule increases from 225 °C to 240 °C and the weight loss speed of STF slows down. This indicates that the polyurea polyurethane double-layered microcapsules have good thermal protection to the core material. The polyurea polyurethane shell can not only improve the service temperature of STF but also slows down the leakage of STF.