Fluctuation of Time-Resolved X-ray Diffraction Reveals the Rotational Dynamics of Nanoparticles in Polymer Materials

Polymer nanocomposites, consisting of a polymer matrix and nanoparticles, are key materials in modern soft matter science. Because their macroscopic properties are closely linked to nanoscale dynamics, developing techniques to observe nanoparticle motion is essential. Here, we present novel methods to analyze the rotational dynamics of crystalline nanoparticles in various polymers using conventional time-resolved X-ray diffraction (TR-XRD). In the first approach, diffraction intensity fluctuations caused by nanoparticle rotation were analyzed via autocorrelation functions to extract relaxation times, defined as the duration for a diffraction spot to traverse a single pixel of the detector. The relaxation times varied significantly depending on the material's rigidity and temperature. Second, two-time correlation analysis applied to TR-XRD data revealed time-dependent changes in nanoparticle dynamics during polymer phase transitions. These methods expand the utility of TR-XRD beyond structural analysis, offering a powerful tool to probe nanoscale rotational motion across a wide range of materials from liquids to solids.