Single-molecule probes revealed dynamics of confined nano-regions in miscible polymer blends

Abstract

Miscible mixtures of polymer blends have physical properties that are often linked simply to the blend composition, thus offering an inexpensive and convenient method to achieve new high performance polymers. Confinement effect has been found in various polymer blend systems by the ensemble methods, but further understanding the confinement effect still requires large efforts both in experiment and in theory. Single molecule spectroscopy has the potential to provide an in-depth insight to the dynamic information by directly coupling their reorientation to the segmental relaxation of the surrounding polymer matrix. We investigate the confinement effects in polystyrene and oligostyrene blend films by using single-molecule defocused wide-field fluorescence microscopy. According to the observation for dynamic behaviors of probe molecules in the blend films of 75 wt.% and 25 wt.% polystyrene, we find that there are two types of single molecules in the blend films: rotational molecules and immobile molecules. The experimental temperature of 296 K is between the glass transition temperature (<i>T</i>_g) values of two pure components and also is far from the two <i>T</i>_g values. At the temperature, oligostyrene component is trapped by the frozen polystyrene component, but they still move locally. Therefore, the rotational and immobile molecules should couple to the oligostyrene component and polystyrene component, respectively. The distribution of rotational single molecules reveals that the confined regions randomly distribute across miscible polymer blends. The length scale of confined region is estimated to be close to that of the probe molecule by taking into account the rotational dynamics of single molecules. The local relaxation of blend film is also investigated by the rotational correlation time which can be estimated by fitting the autocorrelation curve of 〈cos(<i>Φ</i>)〉 with a Kohlrausch-Williams-Watts stretched exponential function. The histograms of the rotational correlation times in the blend films of 75 wt.% and 25 wt.% polystyrene are obtained respectively, which reveal the characteristic of local dynamic distribution in the confined nano-regions. We find that the dynamic behavior in the blend film of 75 wt.% polystyrene is faster than that of 25 wt.% polystyrene, indicating there is a confinement effect in the blend due to the increased constraints imposed by the polystyrene component at a higher concentration of polystyrene. All results observed in the experiment can be explained qualitatively by the self-concentration model. Our work indicates that the single molecule defocused wide-field fluorescence microscopy is a powerful tool to study the complex dynamic features in the polymer blends.