Stimuli-responsive materials are of immense importance because of their ability to undergo alteration of their properties in response to their environment. The properties of such materials can be tuned by subtle adjustments in temperature, pH, light, and so forth. Among such smart materials, multi-stimuli-responsive polymeric materials are of pronounced significance as they offer a wide range of applications and their properties can be tuned through several mechanisms. Here, we aim to highlight some recent studies showcasing the multi-stimuli-responsive character of these polymers, which are still relatively little known compared to their single-stimuli-responsive counterpart.

An asymmetric triblock copolymer, poly(styrene-b-acrylic acid-b-ethylene glycol) (PS-b-PAA-b-PEG), was synthesized via reversible addition-fragmentation chain transfer controlled radical polymerization. Micelles of PS-b-PAA-b-PEG with PS core, PAA shell, and PEG corona were then prepared in aqueous solutions, followed by extensive characterization based on dynamic light scattering, zeta-potential, and transmission electron microscopy (TEM) measurements. The well-characterized micelles were used to fabricate hollow nanospheres of CaCO(3) as a template. It was elucidated from TEM measurements that the hollow nanospheres have a uniform size with cavity diameters of ca. 20 nm. The X-ray diffraction analysis revealed a high purity and crystallinity of the hollow nanospheres. The hollow CaCO(3) nanospheres thus obtained have been used for the controlled release of an anti-inflammatory drug, naproxen. The significance of this study is that we have overcome a previous difficulty in the synthesis of hollow CaCO(3) nanospheres. After mixing of Ca(2+) and CO(3)(2-) ions, the growth of CaCO(3) is generally quite rapid to induce large crystal, which prevented us from obtaining hollow CaCO(3) nanospheres with controlled structure. However, we could solve this issue by using micelles of PS-b-PAA-b-PEG as a template. The PS core acts as a template that can be removed to form a cavity of hollow CaCO(3) nanospheres, the PAA shell is beneficial for arresting Ca(2+) ions to produce CaCO(3), and the PEG corona stabilizes the CaCO(3)/micelle nanocomposite to prevent secondary aggregate formation.

Core-corona inversion of micelles of diblock copolymer poly(acrylic acid)-block -poly(N-isopropylacrylamide) (PAA-b -PNIPAM), has been successfully realized by switching either pH or temperature. The strong interaction of doxorubicin with the PAA block and the pH-sensitive drug release from the polymer make the system very useful as a controlled drug delivery system. The encapsulation of hydrophobic Nile Red molecules above the lower critical solution temperature of PNIPAM suggests that this polymer may be useful for removing hydrophobic pollutants.

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Hollow barium sulfate (BaSO 4 ) nanospheres were synthesized by templating a polymeric micelle of a triblock copolymer poly(styrene-b-acrylic acid-b-ethylene glycol) (PS-b-PAA-b-PEG). This polymer is known to form a micelle with a PS core, a PAA shell and a PEG corona in aqueous solutions. Barium chloride and sodium sulfate were used as precursors of BaSO 4 . In the synthesis, the PS core acts as a template for cavities of the hollow particles, the PAA shell is beneficial for arresting Ba 2+ ions to produce BaSO 4 , and the PEG corona stabilizes the BaSO 4 /polymer nanocomposite to prevent secondary aggregate formation. Hollow BaSO 4 nanospheres were obtained by removing the polymeric template from the BaSO 4 /polymer nanocomposite by calcination. The hollow BaSO 4 nanospheres thus fabricated were characterized by various techniques including transmission electron microscopy and X-ray diffraction analysis. The average diameter of the spheres is around 25 nm and the average cavity diameter is around 16 nm. The significance of the present method is that it can avoid formation of large crystals which is generally unavoidable in other methods.

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