From Single Chains to Aggregates, How Conjugated Polymers
Behave in Dilute Solutions
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Abstract
Conjugated polymers offer unique
combination of easily tailored mechanical, electrical and optical
properties that makes them perfect materials for the preparation of
various devices such as light-emitting diodes, photovoltaic cells
or field-effect transistors. However, the design and fabrication of
such devices in a controlled and reproducible way are possible only
if the behavior and the properties of individual polymer chains are
well understood. One major problem in this respect is that aggregation
often occurs even in dilute solutions and prevents the single polymer
chain studies. To address this issue, in this work we employed fluorescence
correlation spectroscopy (FCS) to study the behavior of a model conjugated
polymer, poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene)
(MEH-PPV) in several commonly used solvents. The very high sensitivity
of FCS allowed measurements in ultradilute solutions and thus unambiguous
determination of the hydrodynamic radius of single polymer chains.
The solvent quality for MEH-PPV was then quantitatively evaluated
from the measured logarithmic scaling of the single chain hydrodynamic
radius versus the polymer molecular weight. Scaling exponents of 0.40,
0.41, and 0.43 were found in toluene, chloroform and 1,2-dichlorobenzene,
respectively. These values are well below the θ-condition, emphasizing
poor solvent quality for MEH-PPV, despite the fact that all studied
solvents are commonly regarded as “good” solvents. In
addition, by investigating the aggregation behavior of MEH-PPV at
higher polymer concentrations, we found a clear relation between aggregates
size and solvatochromism that indicates more extended chain conformation
in larger aggregates.