Nano-gel containing thermo-responsive microspheres with fast response rate owing to hierarchical phase-transition mechanism

A new strategy is developed in this study to achieve thermo-responsive microspheres with fast response rates by designing a hierarchical phase-transition mechanism. The proposed thermo-responsive microspheres are composed of poly(N-isopropylacrylamide-co-acrylic acid) (PNA) microsphere matrixes and embedded poly(N-isopropylacrylamide) (PNIPAM) nano-gels, which have different volume phase-transition temperatures (VPTTs). The VPTT of PNIPAM nano-gels (VPTT(1)) is lower than that of PNA microsphere matrixes (VPTT(2)). Upon heating-up, the temperature increases across the VPTT(1) first and then the VPTT(2), as a result the PNIPAM nano-gels shrink earlier than the PNA microsphere matrixes. Upon cooling-down, the temperature decreases across the VPTT(2) first and then the VPTT(1), as a result the PNA microsphere matrixes swell earlier than the PNIPAM nano-gels. Consequently, large amounts of voids and channels form around the nano-gels inside the microsphere matrixes when the temperature changes across the range between VPTT(1) and VPTT(2), which are beneficial to the enhancement of water transport rate inside the microsphere matrixes. The experimental results show that, compared with normal homogeneous PNA (N-PNA) microspheres, the nano-gel containing PNA (C-PNA) microspheres exhibit remarkably fast response rate due to the hierarchical phase-transition mechanism attributed to different VPTT values of the embedded nano-gels and the microsphere matrixes.