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.