A Synergistic Coassembly
of Block Copolymer and Fluorescent
Probe in Thin Film for Fine-Tuning the Block Copolymer Morphology
and Luminescence Property of the Probe Molecules
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Abstract
Here, we investigate a synergistic coassembly of a block
copolymer,
polystyrene-<i>b</i>-poly(4-vinylpyridine) (PS-<i>b</i>-P4VP), and a fluorescent probe molecule, pyrenebutyric acid (PBA),
in thin film using block copolymer supramolecular assembly (SMA) strategy
for a wide range of compositions tuned by varying the molar ratio
(<i>r</i>) of PBA and 4VP units. The PBA molecules form
supramolecules with PS-<i>b</i>-P4VP through H-bonding between
the carboxylic acid group of 1-pyrenebutyric acid and pyridine ring
of P4VP. For compositions <i>r</i> = 0, 0.1, 0.25, and 0.5,
the SMAs exhibit cylindrical morphology, whereas for <i>r</i> = 0.75 and 1, the SMAs generate lamellar morphology. Interestingly,
it has been observed that the orientation of the microdomains depends
on the solvent used for annealing and can be switched reversibly on
exposing the SMA films to corresponding solvent. In a nonselective
solvent like chloroform, the microdomains are oriented normal to the
substrate, whereas in a selective solvent like 1,4-dioxane, the microdomains
are oriented parallel. The synergistic coassembly of PS-<i>b</i>-P4VP and PBA in SMAs with higher molar ratio results in a structure-within-structure
pattern characterized by two length scales from phase separation of
block copolymer and parallel π–π stacking of the
pyrene moiety of PBA molecules inside the comb block. The photophysical
properties of PBA in different SMAs of varying composition were studied
both in solution and in thin film state and compared to pure PBA.
The UV–vis study shows the H type of aggregation of PBA molecules
inside the comb block by parallel stacking of the pyrene units, and
the PBA molecules orient parallel to the substrate when the microdomains
are oriented normal to the substrate. The pure PBA molecules in thin
film exhibit excimer emission extensively, whereas the PBA molecules
in different supramolecular assemblies exhibit emission ranging from
monomer to mixture of monomer and excimer. The SMA shows more intense
fluorescence emission compared to pure PBA both in solution and in
thin film