Controlling the mobility in nanostructured environments by stimuli-responsive polymers
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Abstract
Understanding and controlling the diffusion of small molecules, macromolecules and nanoparticles in solution and complex, nanostructured environments is of paramount fundamental and technological importance. Often, diffusion is the dominant mechanism for the transport of such species in e.g. solid nanoporous structures, polymer solutions and gels, or in living cells. Thus, it is relevant for many processes and applications including drug delivery, cell nutrition, porous chromatography, polymer synthesis and separation, treatment of waste water, oil recovery, etc. For all of those applications, one prerequisite is that the mobility of the species needs to be controllable. To control the mobility of the species, the size of the species itself or the size and/or density of the nanopores of surrounding environment can be changed. Stimuli-responsive polymers are ideal candidate materials to construct both systems, as they are capable of conformational changes when they are exposed to external stimuli. In addition, they can form versatile configurations such as mixed polymer brushes, micelles, vesicles, layer-by-layer films, and so on, which provides a feasible way to construct the responsive species or the responsive environment for mobility control.
For observation of species’ mobility, fluorescence correlation spectroscopy (FCS) technique is a well-developed technique. The fluorescent intensity fluctuations caused by the diffusion of the species through a very small confocal detection volume are recorded and the change process can be traced. Because of its extremely small detection volume (V< 10-15 L), high sensitivity is reached and even single molecule can be traced in the solution. Nowadays FCS has been developed as a powerful technique for studying the dynamics of fluorescent species such as small molecules, macromolecules, or nanoparticles in various environments in polymer and colloid science.
In this thesis, the species’ mobility has been controlled by combining stimuli-responsive polymers. For the stimuli-responsive polymers, I choose a typical thermo-responsive polymer and a pH-responsive polymer, as they are the most classic and widely used polymers in material science and biology. FCS is used to monitor the mobility of the species in solution and also in porous media.
In the first part of this thesis, the species mobility has been controlled by changing the pore size of the surrounding medium. Poly(N-isopropylacrylamide) (PNIPAM) is grafted onto the well-defined, highly ordered porous network-silica inverse opal. When temperature increases above the lower critical solution temperature of PNIPAM, PNIPAM chains collapse accordingly, resulting in an increase of the mobility of the penetrant in the system, and vice versa.
In the second part of this thesis, the mobility of the nanoparticles has been controlled by changing the size of the nanoparticle itself. I prepare pH-responsive nanoparticles, which are composed of pH-insensitive polystyrene cores and pH-responsive poly (2-diethylaminoethyl methacrylate) (PDEA) hairs. When the pH increases, the PDEA hairs collapse and the size of the hairy nanoparticle will decrease, which results in faster mobility of the nanoparticles in the solution. The effect is particularly evident when the nanoparticles are dispersed in a nanostructured environment.