Stimuli-responsive polymers are an important class of materials with many applications in nanotechnology and drug delivery. The most prominent one is poly- N -isopropylacrylamide (PNIPAm). The characterization of the kinetics of its change after a temperature jump is still a lively research topic, especially at nanometer-length scales where it is not possible to rely on conventional microscopic techniques. Here, we measured in real time the collapse of a PNIPAm shell on silica nanoparticles with megahertz x-ray photon correlation spectroscopy at the European XFEL. We characterize the changes of the particles diffusion constant as a function of time and consequently local temperature on sub-microsecond timescales. We developed a phenomenological model to describe the observed data and extract the characteristic times associated to the swelling and collapse processes. Different from previous studies tracking the turbidity of PNIPAm dispersions and using laser heating, we find collapse times below microsecond timescales and two to three orders of magnitude slower swelling times.

中文翻译：

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XFEL 脉冲驱动的纳米凝胶的实时膨胀-塌陷动力学

刺激响应聚合物是一类重要的材料，在纳米技术和药物输送方面有许多应用。其中最突出的是聚氮-异丙基丙烯酰胺（PNIPAm）。温度跃变后其变化动力学的表征仍然是一个活跃的研究课题，特别是在不可能依赖传统显微技术的纳米长度尺度上。在这里，我们在欧洲 XFEL 上使用兆赫兹 X 射线光子相关光谱法实时测量了二氧化硅纳米颗粒上 PNIPAm 壳的塌陷。我们将粒子扩散常数的变化描述为时间的函数，从而描述了亚微秒时间尺度上的局部温度。我们开发了一个唯象模型来描述观察到的数据并提取与膨胀和塌陷过程相关的特征时间。与之前跟踪 PNIPAm 分散体浊度并使用激光加热的研究不同，我们发现塌陷时间低于微秒时间尺度，膨胀时间慢了两到三个数量级。