| Fluorescence spectroscopy has been widely used in polymer science. Recently, aggregation-induced emission (AIE) had become an emerging research topic. AIE mechanism is usually considered to be restricted intramolecular rotation (RIR). Therefore, it is feasible to use AIE probes in polymer chain dynamics studies. However, the application of AIE probes in polymer system is still limited. In this thesis, we introduced a typical AIE chromophore tetraphenylethylene (TPE) into some water-soluble polymers, and explored the relationship between the light-emitting behavior of TPE unit and the intrinsic properties of these polymers.Firstly, by attaching two short polyethylene glycol (PEG) chains to one TPE molecule, we obtained three amphiphilic oligomers TPE-2PEGx (x=350,550 and 750). TPE-2PEGx shows different AIE behavior in tetrahydrofuran (THF)/hexane and THF/water mixtures, indicating that PEG exists in different comformations in these three solvents:random coil in THF, compact stacking in hexane, and twisted helix in water. In aqueous solution, TPE-2PEGx tends to form aggregates through hydrophobic interaction; and because of the affinity between -CH2CH2- units and phenyl rings, the PEG segments tend to draw near to the TPE group. All of the above processes can be observed from fluorescence signals. The fluorescence intensity of TPE-2PEG350 aqueous solution is sharply reduced at elevated temperature, due to the collapse of its aggregates. The fluorescence behavior of TPE unit is very sensitive to the conformation and movement of its adjacent chain segments.Secondly, by covalently connecting PEG chains (molecular weights:750,1900 and 5000) to a TPE group in different manners, we obtained nine amphiphilic macromolecules TPE@PEG. TPE unit is attached at one end of PEG chain in TPE-PEGx, connected with one alkyl chain and one PEG chain in RTPE-PEGx, and connected with two PEG chains in TPE-2PEGx. In aqueous solution, RTPE-PEGx forms stable and ordered micelles at low concentration, result in a strong emission. TPE-PEGx and TPE-2PEGx form unstable and disordered aggregates above the first critical aggregation concentration (CAC1), which can be observed from the fluorescence intensity-concentration curve. A small PEG molecular weight leads to a lower CAC1 value, and TPE-PEGx has a great tendency to aggregate than TPE-2PEGx. The thermosensibility of TPE@PEG is a measure of its aggregate stability.Thirdly, we studied the hydrogen-bond-driven complex formation between TPE@PEG and poly(acrylic acid) (PAA). The fluorescence emission of TPE@PEG is enhanced by adding PAA. The fluorescence intensity reaches its maximum when the molar ratio of carboxyl group to ether group reaches 1:1, indicating that PAA and TPE@PEG form a stable complex. According to the variation of fluorescence intensity with PAA amount, we can infer the morphology of PAA/TPE@PEG complex:PAA/TPE-PEG750 forms interchain aggregates; PAA/TPE-PEG1900 and PAA/ TPE-PEG5000 form coils; PAA/RTPE-PEGx forms "flower-like" micelles; and PAA/ TPE-2PEGx froms network. The dissociation of PAA/TPE@PEG complex at elevated pH and temperature can be observed from the reduction in fluorescence intensity.At last, we synthesized two cationic AIE probes:trans-and cis-TPE-DPy-Mel, and introduced them into three polyanion systems:heparin, chondroitin 4-sulfate, and hyaluronic acid. Upon binding with polyanions through electrostatic interation, both trans- and cis-TPE-DPy-Mel show an enhanced fluorescence intensitiy and a blue-shift in emission maximum. The charge density, hydrophobility and chain-flexibility of polyanions all affect the sensing behavior of these two probes. The fluorescence intensity of hyaluronic acid/trans-TPE-DPy-MeI complex is sharply reduced at elevated temperature, according to the dissociation of the electrostatic complex. The interaction between heparin/trans-TPE-DPy-MeI complex and metal ions follows the "polyelectrolyte effect" principle. |
PDF Full Text Request