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Zwitterionic Poly(vinylidene fluoride) Graft Copolymer with Unexpected Fluorescence Property
journal contribution
posted on 2019-03-20, 00:00 authored by Mahuya Pakhira, Radhakanta Ghosh, Santi P. Rath, Dhruba P. Chatterjee, Arun K. NandiRecently, there has
been a growth of research on the nonconjugated
polymer exhibiting fluorescence property and it would be exciting
if fluorescence property is developed in zwitterionic polymers because
of their good water solubility. Poly(vinylidene fluoride) (PVDF) grafted
with poly(dimethyl amino ethyl methacrylate) (PDMAEMA) is fractionated
and a highly water-soluble fraction (PVDM-1) is quaternized with 1,3-propane
sultone, producing a zwitterionic polymer, PVDF-g-PDMAEMA-sultone (PVDMS). PVDM-1 shows the fluorescence property
with very low quantum yield (1%) in water, but on quaternization,
fluorescence quantum yield increases to 8%. Transmission electron
microscopy results indicate that the PVDM-1 cast from water has vesicular
morphology, whereas PVDMS exhibits aggregated vesicular morphology.
The 1H NMR spectra indicate the presence of 72 mol % DMAEMA
in PVDM-1 wherein 66% of -NMe2 groups is quaternized upon postpolymerization
modification. PVDM-1 exhibits absorption peaks at 210, 276, and 457
nm with a hump at 430 nm, whereas PVDMS exhibits two absorption peaks
at 203 and 297 nm. PVDM-1 exhibits a broad emission peak at 534 nm,
whereas PVDMS exhibits a sharp emission peak at 438 nm. An attempt
has been made from density functional theory calculations to shed
light on the origin of fluorescence in both PVDM-1 and in the zwitterionic
PVDMS. The excitonic decay occurs from the lowest unoccupied molecular
orbital (LUMO) of carbonyl group to the highest occupied molecular
orbital (HOMO) of tertiary amine group for PVDM-1, whereas in PVDMS,
the excitonic transition occurs from the LUMO situated over the quaternary
ammonium group to the HOMO located on the electron-rich terminal sulfonate
group.