Hildebrandt M, Pham Thuy D, Kippenberger J, Wigger T L, Houston J E, Scotti A, Karg M
Institut für Physikalische Chemie I: Kolloide und Nanooptik, Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, D-40225 Düsseldorf, Germany.
European Spallation Source ERIC, Box 176, SE-221 00 Lund, Sweden.
Soft Matter. 2023 Sep 27;19(37):7122-7135. doi: 10.1039/d3sm01062g.
Microgels are often discussed as well-suited model system for soft colloids. In contrast to rigid spheres, the microgel volume and, coupled to this, the volume fraction in dispersion can be manipulated by external stimuli. This behavior is particularly interesting at high packings where phase transitions can be induced by external triggers such as temperature in the case of thermoresponsive microgels. A challenge, however, is the determination of the real volume occupied by these deformable, soft objects and consequently, to determine the boundaries of the phase transitions. Here we propose core-shell microgels with a rigid silica core and a crosslinked, thermoresponsive poly--isopropylacrylamide (PNIPAM) shell with a carefully chosen shell-to-core size ratio as ideal model colloids to study fluid-solid transitions that are inducible by millikelvin changes in temperature. Specifically, we identify the temperature ranges where crystallization and melting occur using absorbance spectroscopy in a range of concentrations. Slow annealing from the fluid to the crystalline state leads to photonic crystals with Bragg peaks in the visible wavelength range and very narrow linewidths. Small-angle X-ray scattering is then used to confirm the structure of the fluid phase as well as the long-range order, crystal structure and microgel volume fraction in the solid phase. Thanks to the scattering contrasts and volume ratio of the cores with respect to the shells, the scattering data do allow for form factor analysis revealing osmotic deswelling at volume fractions approaching and also exceeding the hard sphere packing limit.
微凝胶常被视为软胶体的理想模型体系。与刚性球体不同，微凝胶的体积以及与之相关的分散体系中的体积分数可通过外部刺激进行调控。在高堆积情况下，这种行为尤其有趣，因为对于热响应性微凝胶而言，外部触发因素（如温度）可诱导相变。然而，一个挑战在于确定这些可变形软物体实际占据的体积，进而确定相变的边界。在此，我们提出具有刚性二氧化硅核和交联热响应性聚异丙基丙烯酰胺（PNIPAM）壳且壳核尺寸比经过精心选择的核壳微凝胶，作为研究由毫开尔文温度变化诱导的流固转变的理想模型胶体。具体而言，我们在一系列浓度下利用吸收光谱确定结晶和熔化发生的温度范围。从流体态缓慢退火至晶态会形成在可见波长范围内具有布拉格峰且线宽极窄的光子晶体。随后使用小角X射线散射来确认流体相的结构以及固相中的长程有序、晶体结构和微凝胶体积分数。由于核与壳的散射对比度和体积比，散射数据确实能够进行形状因子分析，揭示在体积分数接近并超过硬球堆积极限时的渗透去溶胀现象。
