Fluorene Based Stimuli Responsive Rod-coil Block Copolymers:Syntheses, Morphologies and Photophysical Properties
Date Issued
2009
Date
2009
Author(s)
Lin, Sung-Tso
Abstract
Stimuli responsive block copolymers can provide a variety of applications due to their high sensitivity in detecting targeted species. Among of these studies, conjugated rod-coil block copolymers have attracted considerable attention because of their electronic and optoelectronic properties could be efficiently manipulated by minor perturbations of environmental stimuli. In addition, self-assembly of π-conjugated polymer based block copolymers lead to various nanoscale morphologies driven by inherent immiscibility between different blocks and the packing constrains imposed by the π-π interaction of the conjugated segment. Such morphological transformation can lead to the variation of their physical properties. Nevertheless, the effects of molecular architecture and phase behaviors on photophysical properties of rod-coil block copolymers with multifunctional sensory characteristics have rarely been explored. In this thesis, new fluorene based rod-coil block copolymers bearing stimuli responsive coil blocks were synthesized to explore the effects of polymer structure and morphology on photophysical properties.n the first part of this thesis (chapter 2), the synthesis, structures and multifunctional sensory properties of amphiphilic poly[2,7-(9,9-dihexylfluorene)]-block-poly[2-(dimethylamino)ethyl methacrylate] (PF-b-PDMAEMA) rod-coil diblock copolymers are reported. The new copolymers, with PDMAEMA coil lengths of 31, 45, 93 and 185 repeating units, were synthesized by atom transfer radical polymerization (ATRP). The surface structures and photophysical properties of the synthesized polymers were studied through the variation of solvent composition (THF/water), temperature, and pH. The PF7-b-PDMAEMA45 structure changed from spheres to separate cylinders, bundles of cylinders and spiral-shaped micelles as the solvent composition changed from 0 to 90 wt% water in THF. However, the long-range order structure of spiral-shaped loops was not observed at a long coil length. The micellar aggregates of PF7-b-PDMAEMA45 in water showed a reversible surface structure transformation from cylinder-bundles to spheres on heating from 25 to 75 oC. The variation of the micelle size with temperature was judged to be similar from both atomic force microscopy (AFM) and dynamic light scattering (DLS) measurements. The intermolecular PF aggregations led to fluorescence quenching and a blue-shift in the absorption spectra of the block copolymer as the water content increased. The photoluminescence (PL) intensity of PF7-b-PDMAEMA45 in water was thermoreversible based on its lower critical solution temperature. The PL characteristics suggested the new copolymers behave as an on/off fluorescence indicator of temperature or pH, with a reversible “on-off” profile at an elevated temperature in water: the pH-fluorescence intensity profile switched from “off-on” to “on-off” as the temperature increased.n the second part of this thesis (Chapter 3), the synthesis, structures and thermoresponsive properties of amphiphilic poly[2,7-(9,9-dihexylfluorene)]-b-poly(N-isopropylacryamide) (PF-b-PNIPAAm) rod-coil and coil-rod-coil block copolymers are reported. The new copolymers containing variant PF units of 7, 15 and 25 as the rigid segment and PNIPAAm as the flexible block were successfully synthesized via combining coupling reaction and ATRP. The surface structures and photophysical properties of the synthesized polymers were studied through the variation of solvent composition (THF/water), and temperature. Micellar structures with spherical, cylindrical, worm-like, bundles or honeycomb shape were particularly noticed. For the longer coil length of block copolymers, the structural transformation grew from sphere into cylinders and bundles at lower water compositions and changed into changed into smaller aggregates at higher water composition. A comparison between diblock and triblock copolymer, the micellar aggregates of triblock copolymer developed into cylinders and bundle micelles at higher water composition than the diblock copolymers with similar rod/coil ratio. The micellar aggregates in water also showed a reversible surface structure transformation on heating from 20 to 40 oC. The transformation of the micellar structure with different solution compositions and temperatures was judged to be similar from AFM, transmission electron microscopy (TEM) and DLS measurements. In addition, the intermolecular PF aggregations of the diblock copolymer led to fluorescence quenching and a blue-shift in the absorption spectra and PL spectra as the water content increased, suggesting an H-type aggregation. However, triblock copolymer exhibited a red shift in both absorption and PL spectra by increasing the water content, which reflected the J-type aggregation.n the third part of this thesis (Chapter 4), new thermoresponsive conjugated rod-coil-coil triblock copolymers were successfully synthesized from terminal azido functionalized poly(N-isopropylacrylamide)-b-poly(N-hydroxyethylacrylamide) (PNIPAAm-b-PHEAA) and alkynyl functionalized PF via click reaction. The azido functionalized PNIPAAm-b-PHEAA copolymers with different block ratio was prepared by atom transfer radical polymerization from an initiator bearing the azide group, whereas alkynyl functionalized PF was synthesized by Suzuki coupling reaction. The lower critical solution temperature (LCST) of the block copolymers increased with an enhanced hydrophilic PHEAA block ratio, since the longer PHEAA segment facilitated the copolymer chains to stretch at an elevated temperature. The micelles of PNIPAAm-b-PHEAA with different block ratio changed into spheres, aggregate spheres, vesicles, and wormlike micelles as the temperature was increased, due to the variation on the hydrophilic/hydrophobic characteristic of PNIPAAm. However, the micellar morphologies became cylinders, bundles, and hollow cylinders in the triblock PF-b-PNIPAAm-b-PHEAA, which were probably induced by the π-π interaction among the fluorene segments. The variation of the micelle morphology with temperature was consistent from the results of TEM, AFM, and DLS. Also, the micelle morphologies of PF-b-PNIPAAm-b-PHEAA showed a thermoreversible property based on its LCST. The PL characteristics behaved as an on/off fluorescence indicator of temperature, showing an “on-off-on” profile at an elevated temperature in water at a higher block ratio of PNIPAAm and switching to “on-off” as the block ratio of PNIPAAm decreased.The above results suggested that various morphologies of conjugated fluorene-coil block copolymers could be efficiently manipulated by block ratio, solvent, pH, or temperature and lead to significantly tuning on the photophysical properties.
Subjects
Stimuli responsive
Rod-coil
Block copolymers
Micelle morphologies
Photophysical properties
Type
thesis
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