Responsive micro- and nano-structures through interfacial assembly of star polymers

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Xu, Weinan
Tsukruk, Vladimir V.
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Responsive polymeric nanostructures have attracted much attention in recent years due to their abilities to adapt and respond to external stimuli, and potential applications in bio-sensing, self-healing coatings, drug delivery, tunable catalysis, and bio-imaging. Star polymers have emerged as novel building blocks for such assembled structures due to their unique architectures and multiple responsive properties. A challenging task in this filed is how to precisely control the interactions between star polymers and with other components, and maintain the responsive properties of the functional stars in the assembled nanostructures. Therefore, the goal of the proposed work is to understand the responsive properties and interactions of star polymers in different conditions, including solution and interfaces, and utilize them as building blocks for polymeric micro- and nano-structures such as polymersomes, ultrathin films and microcapsules, which have intriguing properties in terms of stability, responsiveness and functionalities compared with conventional linear polymers based structures. Specifically, in the first place, we studied the solution phase behavior of responsive star polymers by using in situ (small angle neutron scattering) SANS, and showed that in semidilute solution, the temperature induced phase separation for thermo-responsive star polymers are significantly different from that of their linear counterparts. The star polymers show limited microphase separation with aggregates composed of several molecules, while the corresponding linear polymers have LSCT (low critical solution temperature) type phase separation. Secondly, we studied the responsive properties and assembly of amphiphilic star polymers at the air/water interface and in Langmuir-Blodgett monolayer. We found that the confined interface environment leads to different conformational changes and assembly behaviors of the star polymers compared with those in solution state. For instance, when there is a hydrophilic to hydrophobic transition, the polymers tend to go from water subphase to the air/water interface, rather than showing coil to globule transition in aqueous solution. Thirdly, we utilized the star polymers as major component to fabricate 3D responsive microstructures such as thin shell microcapsules, by using layer-by-layer (LbL) assembly technique, which has rarely been explored before, especially for complex star block copolymers. The assembly microcapsules have hierarchical multicompartmental structure, which enables the encapsulation and release of multiple molecules simultaneously. The shell of the multilayer microcapsules has porous 3D network structure, with fine controlled permeability. Lastly, for star polymers with multiple responsive properties, we found that their responsiveness is well maintained after being assembled into microstructures, so that the microcapsules have multiple responsive properties. The multiple responses in structure and permeability to external stimuli enable the controlled and programmable delivery of multiple cargo molecules, such as those we demonstrated in this study: microcapsules with pH and temperature dual responsiveness, as well as ionic conditions and UV dual responsive properties.
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