Thermoresponsive Nanospheres with a Regulated Diameter and Well-Defined Corona Layer

In the present work, we prepared core–corona-type nanospheres bearing a thermoresponsive polymer with a controlled chain length on their surface. The corona layers were composed of poly­(<i>N</i>-isopropylacrylamide) (PNIPAAm) chains (<i>M</i>_n = 3000–18 000) with a narrow polydispersity index prepared by atom-transfer radical polymerization (ATRP). Nanospheres were prepared by dispersion copolymerization of styrene with the PNIPAAm macromonomer in a polar solvent. The obtained nanospheres were monodisperse in diameter. The diameter of the nanospheres was regulated either by the number or chain length of the PNIPAAm macromonomers. In fact, the nanosphere diameter was regulated from ca. 100 to 1000 nm. When two types of PNIPAAm macromonomers are used, the obtained nanospheres have two different kinds of PNIPAAm on their surface. The surface of the nanospheres was observed to be thermoresponsive nanosphere in 0, 50, 100 mmol L^–1 NaCl aqueous solution. The nanosphere diameter and the surface-grafted polymer were concurrently adjusted for use in biomedical applications

Effects of Syndiotacticity on the Dynamic and Static Phase Separation Properties of Poly(<i>N</i>‑isopropylacrylamide) in Aqueous Solution

The dynamic and static phase separation behavior in aqueous poly­(<i>N</i>-isopropylacrylamide) (PNIPAM) solutions is highly sensitive to the tacticity of PNIPAM. We investigated the phase separation dynamics of aqueous solutions of PNIPAM with different tacticities (atactic and syndiotactic-rich types) and found that the phase separation dynamics of syndiotactic-rich PNIPAM was much different from that of atactic-type PNIPAM. First, phase separation in syndiotactic-rich PNIPAM was faster. Second, there was a critical point (<i>C</i>_cp) in the concentration dependence of the phase separation rate: the phase separation accelerated dramatically when the solution concentration was higher than 2.0 wt % (= <i>C</i>_cp). Third, syndiotactic-rich PNIPAM required a higher thermal energy for phase separation compared to atactic PNIPAM. Such behavior can be explained on the basis of the high hydrophobicity of syndiotactic-rich PNIPAM in a dehydrated state and a diffusion-controlled aggregation model. The present study shows that precise control of the stereoregularity will open new channels toward the design and development of stimuli-responsive-polymer-based smart materials