Polyphenol-Induced Phase Transition of Thermo-responsive Hydrogels
4.3 Phase Transition Behaviors of PNIPAM Microgels Induced by Ethyl Gallate
4.3.3 The Intact-to-Broken Transformation Behaviors of Core-Shell PNIPAM Microcapsules in Aqueous
Solution with Varying EG Concentration
The observation of change in size and structure of monodisperse core-shell PNIPAM microcapsules in EG aqueous solutions (10, 20, 30 mmol/L at 20ıC and 20 mmol/L at 25ıC) is carried out using a CLSM (TCS SP5-II, Leica) equipped with a ther- mostatic stage system (TSA02i, Instec). EG aqueous solutions are placed in water bath with a designed temperature for at least 2 h before measurement. Core-shell PNIPAM microcapsules are exposed to DI water at the same designed temperature controlled by the thermostatic stage for at least 30 min before measurement. The DI water around the core-shell PNIPAM microcapsules is replaced with EG aqueous solution with a preset concentration at the same temperature. The size and structure transformation behaviors of PNIPAM microcapsules are recorded by the CLSM.
The dynamic deswelling ratio of the PNIPAM microcapsules at time t (Rd,t) is calculated by
Rd;tD dt
d0
(4.5)
Fig. 4.14 CLSM micrographs of dynamic phase transition behaviors of PNIPAM microcapsules with colored oil core in aqueous solution with varying EG concentration and at different temperatures. The volume ratio between the inner core and the whole microcapsule is about 0.27. (a) TD20ıC, CEGD10 mmol/L; (b) TD25ıC, CEGD10 mmol/L; (c) TD20ıC, CEGD20 mmol/L; (d) TD20ıC, CEGD30 mmol/L. Scale bars are 200m (Reproduced with permission from Ref. [4], Copyright (2012), Elsevier)
where dtis the diameter (m) of the PNIPAM microcapsules at time t (s) and d0is the diameter (m) of the PNIPAM microcapsules initially in DI water just before the water is replaced by the EG aqueous solution.
Figure4.14shows the CLSM micrographs of dynamic phase transition behaviors of core-shell PNIPAM microcapsules in aqueous solution with varying EG concen- trations and at different temperatures. For the detailed phase transition behaviors corresponding to an EG concentration of 10 mmol/L at 20ıC, when the surrounding DI water is replaced by the EG solution, the PNIPAM microcapsules shrink slightly in the first few seconds and then keep unchanged afterwards. Figure4.14b shows that the PNIPAM microcapsules shrink slowly and squirt out the colored oil core when the temperature is increased to 25ıC. At time tD0, the diameters of the PNIPAM microcapsules in water at 25ıC are smaller than that at 20ıC, because the PNIPAM shells shrink slightly when the temperature rise from 20 ıC to 25ıC [13]. When the environmental water is replaced by aqueous solution with an EG concentration of 20 or 30 mmol/L at 20ıC, the shell of PNIPAM microcapsules
106 4 Polyphenol-Induced Phase Transition of Thermo-responsive Hydrogels
Fig. 4.15 Dynamic shrinking behaviors of PNIPAM microcapsules with a colored oil core in aqueous solution with varying EG concentration. The error bars represent the deviation from more than ten microcapsules (Reproduced with permission from Ref. [4], Copyright (2012), Elsevier)
shrinks rapidly within 12 s, and the core-shell microcapsules are transformed from the intact state to the broken state because the PNIPAM shell cannot contain the colored oil core anymore, as shown in Fig.4.14c, d.
The dynamic shrinking behaviors of PNIPAM microcapsules with a colored oil core in aqueous solution with varying EG concentration are shown in Fig.4.15. By comparing with the CLSM micrographs in Fig.4.14, it can be seen that the PNIPAM microcapsules can burst and release their oil core when the Rd,tvalue is reduced to a range between 0.88 and 0.7. When the concentration of EG aqueous solution is fixed at 10 mmol/L, the microcapsules can transform from the intact state to the broken state with increasing the temperature from 20ıC (below the VPTT, which is approximately 23ıC, as shown in Fig.4.13) to 25ıC (above the VPTT). When the EG concentration is 20 or 30 mmol/L, the environment temperature 20ıC is higher than the corresponding VPTT values (approximately 16 or 12 ıC, respectively, as shown in Fig. 4.13). Therefore, under these conditions the microcapsules can transform from the intact state to the broken state. The larger the difference between the operation temperature and the corresponding VPTT is, the faster the shrinkage of the core-shell microcapsules. For example, at 20ıC the PNIPAM microcapsules
Fig. 4.16 State diagram of the intact-to-broken transformation of core-shell PNIPAM microcapsules in aqueous solution as a function of temperature and EG concentration
(Reproduced with permission from Ref. [4], Copyright (2012), Elsevier)
in aqueous solution with EG concentration of 30 mmol/L shrink faster than that in aqueous solution with EG concentration of 20 mmol/L, because the difference between the operation temperature and the corresponding VPTT under the former condition is 8ıC while that under the later condition is just 4ıC.
Besides the EG concentration and temperature, the effect of the volume ratio between the inner core and the whole microcapsule on the intact-to-broken trans- formation behaviors as well as the Rd,t value when microcapsule bursts is also investigated. The volume ratio between the inner core and the whole microcapsule of the above-mentioned microcapsules in Figs.4.14and4.15is about 0.27. Another batch of microcapsules is prepared with the volume ratio between the inner core and the whole microcapsule being about 0.36. The experimental results show that, in the experimental range, the intact-to-broken transformation behaviors and the Rd,tvalue when microcapsule bursts of two batches of microcapsules are almost the same.
Figure4.16illustrates the state diagram of the intact-to-broken transformation of core-shell PNIPAM microcapsules in aqueous solutions as a function of temperature and EG concentration. The intact-state region is located in the lower left region and the broken-state region is located in the upper right region, which are separated by a narrow region near the CEG-VPTT curve. From the state diagram, the state of PNIPAM microcapsules is related to the temperature and the EG concentration.
The results indicate that the core-shell PNIPAM microcapsules may be highly potential to be used as sensors and/or indicators for some simple detection of EG concentration if the EG concentration falls in the range from 0 to 35 mmol/L. When the core-shell PNIPAM microcapsules are put in EG aqueous solution and are heated up gradually, their size and the structure will change and transform from an intact
108 4 Polyphenol-Induced Phase Transition of Thermo-responsive Hydrogels
state to a burst state. Therefore, the EG concentration can be determined simply by measuring the corresponding critical temperature for the microcapsules to squirt out their colored oil cores.