Fluorene Based Rod-coil Block Copolymers:Synthesis, Morphology and Photophysical Properties
Date Issued
2009
Date
2009
Author(s)
Tung, Yi-Chih
Abstract
Polyfluorene based rod-coil block copolymers have been widely studied recently due to their potential applications on optoelectronic devices. Such block copolymers could not only manipulate the electron and optoelectronic properties but also provide the flexibility on morphology control. However, the effects of molecular architecture and phase behaviors on photophysical properties of such polymers have not been fully explored. In this thesis, both main chain and side chain fluorene based block copolymers were used to explore the effects of molecular architecture and morphology on photophysical properties. n the first part of this thesis (chapter 2), two kinds of rod-coil block copolymers, poly[2,7-(9,9-dihexylfluorene)]-block-poly(acrylic acid) (PF-b-PAA) and poly[2,7-(9,9-dihexylfluorene)]-block-poly[3-(trimethoxysilyl)propyl methacrylate] (PF-b-PTMSPMA) with different coil lengths in dilute solution are reported. Tape-like lamellar morphology was observed at a short coil length of PF-b-PAA. As the coil length increased, the large compound micelle, sphere, or vesicle was observed with different methanol contents due to the enhancement on the PAA swelling with methanol and the interfacial tension between the PF core and PAA corona. A further increase on the coil length, an inverted morphology of sphere or rod with PF corona and PAA core was shown first but the core/corona was reversed at a high methanol content resulted from the enhanced solubility of PAA. The morphological transformation led to a significant variation on optical absorption or fluorescence characteristics due to the possible H-aggregation formation. On the other hand, micelle morphologies of the PF-b-PTMSPMA in THF/MeOH or THF/EA mixed solvents are characterized by transmission electron microscopy (TEM) and scanning force microscopy (SFM). Ethyl acetate (EA) and methanol (MeOH) were used as selective solvents for the PF rod and PTMSPMA coil blocks, respectively, while THF as the common solvent. In the THF/MeOH mixed solvent, PF-b-PTMSPMA assembled into diverse morphologies of sphere, short cylinder, cylinder, and cylinder bundles. Besides the selective effects, the strong π-π interaction of PF contributed partially to the above morphologies. In the THF/EA mixed solvent, morphologies of the PF-b-PTMSPMA changed from large compound micelles (LCM) to hollow spherical micelles due to the strong core chain stretching. Stable micelles were obtained by crosslink -Si(OR)3 groups of the PTMSPMA block by triethylamine (TEA). The micellar morphology significantly affected the photophysical properties. In the THF/MeOH mixed solvent, blue shifts on the UV-vis and fluorescence spectra were observed, probably attributed to the formation of the H-aggregation in the PF core. However, the photophysical properties were insensitive to the different ratios of THF/EA, due to the insignificant aggregation of short PF corona. The present study revealed that the morphology and photophysical properties of fluorene based rod-coil polymers could be significantly manipulated through solvent, rod/coil ratio, and
Subjects
rod-coil
polyfluorene
electrospun
Type
thesis
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