Spatially and Temporally Resolved Heterogeneities in a Miscible Polymer Blend

Abstract

Mapping the spatial and temporal heterogeneities in miscible polymer blends is critical for understanding and further improving their material properties. However, a complete picture on the heterogeneous dynamics is often obscured in ensemble measurements. Herein, the spatial and temporal heterogeneities in fully miscible polystyrene/oligostyrene blend films are investigated by monitoring the rotational diffusion of embedded individual probe molecules using defocused wide-field fluorescence microscopy. In the same blend film, three significantly different types of dynamical behaviors (referred to as modes) of the probe molecules can be observed at the same time, namely, immobile, continuously rotating, and intermittently rotating probe molecules. This reveals a prominent spatial heterogeneity in local dynamics at the nanometer scale. In addition to that, temporal heterogeneity is uncovered by the nonexponential characteristic of the rotational autocorrelation functions of single-molecule probes. Moreover, the occurrence probabilities of these different modes strongly depend on the polystyrene: oligostyrene ratios in the blend films. Remarkably, some probe molecules switch between the continuous and intermittent rotational modes at elevated temperature, indicating a possible alteration in local dynamics that is triggered by the dynamic heterogeneity in the blends. Although some of these findings can be discussed by the self-concentration model and the results provided by ensemble averaging techniques ( e.g. , dielectric spectroscopy), there are implications that go beyond current models of blend dynamics.

Validity of Single Fluorescent Molecule to Report Local Dynamics of a Polymer Matrix: The Effect of Molecular Architecture

“…It is observed that most fluorescent molecules are of doublelobe-shaped pattern, indicating their spatial orientation parallel to the film surface, with a small portion having the out-of-plane orientation. [18,21,27,[30][31][32][33] The videos recorded at different temperatures show that with the increase of temperature, more and more fluorescent molecules rotate and their rotating angle becomes larger. Dynamical heterogeneity is also clearly observed -the rotational rate of fluorescent molecules differs for probes at different locations and that of the same fluorescent molecule changes with time.…”

How Probes Affect the Local Dynamics in Supercooled Liquid Revealed by Single Molecule Fluorescence Microscopy

“…The correlation rotational time τ c could be calculated by $${\tau _{\mathrm{c}}} = ( {{{{\tau _{{\mathrm{KWW}}}}} \mathord{/ {\vphantom {{{\tau _{{\mathrm{KWW}}}}} {{\beta _{{\mathrm{KWW}}}}}}} \kern-\nulldelimiterspace} {{\beta _{{\mathrm{KWW}}}}}}} ) \cdot \Gamma ( {{{{1 \mathord{/ {\vphantom {1 \beta }} \kern-\nulldelimiterspace} \beta }}_{{\mathrm{KWW}}}}} )$$, where Γ denotes a gamma function. [ 26,45 ]…”

“…The correlation rotational time 𝜏 c could be calculated by 𝜏 c = (𝜏 KWW ∕𝛽 KWW ) ⋅ Γ(1∕𝛽 KWW ), where Γ denotes a gamma function. [26,45] The preciseness of measurement of the dynamics by doped fluorescent probe was depended on the length of rotation trajectory and the characteristic relaxation time varied with the trajectory length. Considering the existence of dynamic heterogeneity with a broad distribution of relaxation time, an alternative approach was to conduct analysis in the frequency domain, i.e., the power spectrum analysis.…”