Ren Hao, Qiu Xing-Ping, Shi Yan, Yang Peng, Winnik Françoise M
Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an 710119, China.
Department of Chemistry, University of Montreal, CP 6128 Succursale Centre Ville, Montreal, Quebec H3C 3J7, Canada.
Macromolecules. 2019 Apr 23;52(8):2939-2948. doi: 10.1021/acs.macromol.9b00193. Epub 2019 Apr 1.
A series of azopyridine-terminated poly(-isopropylacrylamide)s (PNIPAM) (C12-PN-AzPy) (∼5000 < < 20 000 g mol, polydispersity index 1.25 or less) were prepared by reversible addition-fragmentation chain-transfer polymerization of NIPAM in the presence of a chain-transfer agent that contains an AzPy group and an -dodecyl chain. In cold water, the polymers form nanoparticles (5.9 nm < < 10.9 nm) that were characterized by light scattering (LS), H NMR diffusion experiments, and high-resolution transmission electron microscopy. We monitored the pH-dependent photoisomerization of C12-PN-AzPy nanoparticles by steady-state and time-resolved UV-vis absorption spectroscopy. Azopyridine is known to undergo a very fast cis-to-trans thermal relaxation when the azopyridine nitrogen is quaternized or bound to a hydrogen bond donor. The cis-to-trans thermal relaxation of the AzPy chromophore in an acidic nanoparticle suspension is very fast with a half-life τ = 2.3 ms at pH 3.0. It slows down slightly for nanoparticles in neutral water (τ = 0.96 s, pH 7.0), and it is very slow for AzPy-PNIPAM particles in alkaline medium (τ > 3600 s, pH 10). The pH-dependent dynamics of the cis-to-trans dark relaxation, supported by Fourier transform infrared spectroscopy, H NMR spectroscopy, and LS analysis, suggest that in acidic medium, the nanoparticles consist of a core of assembled C12 chains surrounded by a shell of hydrated PNIPAM chains with the AzPy end groups preferentially located near the particle/water interface. In neutral medium, the shell surrounding the core contains AzPy groups H-bonded to the amide hydrogen of the PNIPAM chain repeat units. At pH 10.0, the amide hydrogen binds preferentially to the hydroxide anions. The AzPy groups reside preferentially in the vicinity of the C12 core of the nanoparticles. The morphology of the nanoparticles results from the competition between the segregation of the hydrophobic and hydrophilic components and weak attractive interactions, such as H-bonds between the AzPy groups and the amide hydrogen of the PNIPAM repeat units.
通过在含有偶氮吡啶基团和十二烷基链的链转移剂存在下，对N - 异丙基丙烯酰胺进行可逆加成 - 断裂链转移聚合反应，制备了一系列以偶氮吡啶封端的聚（N - 异丙基丙烯酰胺）（PNIPAM）（C12 - PN - AzPy）（5000＜ ＜20000 g/mol，多分散指数为1.25或更低）。在冷水中，这些聚合物形成纳米颗粒（5.9 nm＜ ＜10.9 nm），通过光散射（LS）、1H NMR扩散实验和高分辨率透射电子显微镜对其进行了表征。我们通过稳态和时间分辨紫外 - 可见吸收光谱监测了C12 - PN - AzPy纳米颗粒的pH依赖性光异构化。已知当偶氮吡啶氮被季铵化或与氢键供体结合时，偶氮吡啶会发生非常快速的顺式到反式热弛豫。在酸性纳米颗粒悬浮液中，偶氮吡啶发色团的顺式到反式热弛豫非常快，在pH 3.0时半衰期τ = 2.3 ms。对于中性水中的纳米颗粒，其速度略有减慢（τ = 0.96 s，pH 7.0），而对于碱性介质中的AzPy - PNIPAM颗粒则非常缓慢（τ＞3600 s，pH 10）。由傅里叶变换红外光谱、1H NMR光谱和LS分析支持的顺式到反式暗弛豫的pH依赖性动力学表明，在酸性介质中，纳米颗粒由组装的C12链核心组成，周围是水合的PNIPAM链壳，偶氮吡啶端基优先位于颗粒/水界面附近。在中性介质中，围绕核心的壳包含与PNIPAM链重复单元的酰胺氢形成氢键的偶氮吡啶基团。在pH 10.0时，酰胺氢优先与氢氧根阴离子结合。偶氮吡啶基团优先位于纳米颗粒的C12核心附近。纳米颗粒的形态是由疏水和亲水成分的分离以及弱吸引相互作用（如偶氮吡啶基团与PNIPAM重复单元的酰胺氢之间的氢键）之间的竞争导致的。
