Adaptive hydrogels, often termed smart materials, are macromolecules whose structure adjusts to external stimuli. Responsive micro- and nanogels are particularly interesting because the small length scale enables very fast response times. Chemical cross-links provide topological constraints and define the three-dimensional structure of the microgels, whereas their porous structure permits fast mass transfer, enabling very rapid structural adaption of the microgel to the environment. The change of microgel structure involves a unique transition from a flexible, swollen finite-size macromolecular network, characterized by a fuzzy surface, to a colloidal particle with homogeneous density and a sharp surface. In this contribution, we determine, for the first time, the structural evolution during the microgel-to-particle transition. Time-resolved small-angle x-ray scattering experiments and computer simulations unambiguously reveal a two-stage process: In a first, very fast process, collapsed clusters form at the periphery, leading to an intermediate, hollowish core-shell structure that slowly transforms to a globule. This structural evolution is independent of the type of stimulus and thus applies to instantaneous transitions as in a temperature jump or to slower stimuli that rely on the uptake of active molecules from and/or exchange with the environment. The fast transitions of size and shape provide unique opportunities for various applications as, for example, in uptake and release, catalysis, or sensing.

中文翻译：

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响应性水凝胶塌陷过程中的时间分辨结构演化：微凝胶到颗粒的转变。


适应性水凝胶通常被称为智能材料，是一种结构可根据外部刺激进行调整的大分子。响应性微米凝胶和纳米凝胶特别令人感兴趣，因为较小的长度尺寸可以实现非常快的响应时间。化学交联提供了拓扑约束并定义了微凝胶的三维结构，而它们的多孔结构允许快速传质，使微凝胶能够非常快速地适应环境的结构。微凝胶结构的变化涉及从以模糊表面为特征的柔性、膨胀的有限尺寸大分子网络到具有均匀密度和尖锐表面的胶体颗粒的独特转变。在这篇文章中，我们首次确定了微凝胶到颗粒转变过程中的结构演化。时间分辨小角度 X 射线散射实验和计算机模拟明确揭示了一个两阶段过程：在第一个非常快的过程中，在外围形成塌缩的团簇，形成中间的中空核壳结构，该结构缓慢转变到一个小球。这种结构演化与刺激类型无关，因此适用于温度跳跃等瞬时转变或依赖于从环境中吸收活性分子和/或与环境交换的较慢刺激。尺寸和形状的快速转变为各种应用提供了独特的机会，例如在吸收和释放、催化或传感方面。