Fabrication Of Stimuli-Responsive Polymeric Hydrogel Photonic Crystals

Colloidal photonic crystals have been investigated significantly over the pastdecade, together with the synthesis of polymeric colloidal microspheres and theirself-assembly leading to3-dimentional ordered arrays. The variation of the periodicstructures can tune light propagation, which holds technological implications invarious areas, such as telecommunication, switch, optical fibres, display devices,biological detection, and chemical sensors. Much progress has been made in thefabrication of polymeric colloidal crystal arrays that are responsive to external stimuli,which result in the modification of the lattice spacing of the opal structures.Accordingly, the wavelength of diffraction of these photonic colloidal crystals (PCCs)shifts due to the change of photonic band gap (PBG). For diffraction-basedapplications, it is critical to engineer PCCs to exhibit bright and structure-based color.Up to now, the close-packed microgel opals with variable structures arecommonly constructed from the self-assembly of poly(N-isopropylacrylamide)(PNIPAM) microspheres. Most PNIPAM-based PCCs experience an unavoidablevolume change around PNIPAM lower critical solution temperature (LCST).Consequently, their crystal lattices are vulnerable to collapse and become disordered.During responses to external stimuli, it is difficult for the PNIPAM-based PCCs tomaintain their narrow bandwidth diffraction and thus color purity. In the thesis, wehave successfully prepared a novel smart hydrogel composed of polystyrene-co-poly(N,N-dimethylacrylamide)(PS-co-PDMAA) microspheres withinner regions rich in hard and hydrophobic PS and outer regions of soft andhydrophilic PDMAA. With narrow size distribution, the synthesized PS-co-PDMAAhydrogel microspheres self-assembled readily, during centrifugation or under anelectric field, leading to the formation of opal hydrogels. Due to their PBG could beenturned with the changing external stimuli in a broad range, it should bring insightsinto a universal approach to functional polymeric colloidal crystal, such as switch,display devices, biological detection, and chemical sensors.In the second chapter, we have successfully prepared monodispersed core-shellhydrogel PS-co-PDMAA microspheres with inner regions rich in hard andhydrophobic PS and outer regions of soft and hydrophilic PDMAA. Thesubmicrometer-sized monodispersed PS-co-PDMAA microspheres were prepared byusing emulsion polymerization method. In the preparation of PS-co-PDMAAmicrospheres, with increasing the monomer ratio of DMAA to St, the mean diameterbecame smaller simply. With increasing the initiator dosage, the mean diametersdecreased drastically; however, when the initiator dosage increased further to acritical amount, the mean diameters decreased slowly. With increasing the surfactantdosage, the mean diameters became smaller simply, the monodispersity always good.With increasing the reaction time, the particle sizes became larger simply, meanwhilethe monodispersity still good. The monodispersed PS-co-PDMAA microspheres weregood at to be the units of the colloidal crystals with a tunable bandgap position.In the third chapter, with narrow size distribution, the synthesizedPS-co-PDMAA hydrogel microspheres self-assembled readily, during centrifugation,leading to the formation of closed-packed opal hydrogels. With a lot of acrylamidegroups on the surface of the microspheres resulting in high surface hydrophilicity, theopal hydrogels responded rapidly to water sensitively with high diffraction colorpurity. The volume of the opal hydrogels increased continuously with the increase ofthe amount of water and decreased with the decrease of the amount of water. Such areversible swell-shrink behaviour induced by the change of the amount of water wasaccompanied by the rapid PBG alteration of the opal hydrogels exhibiting corresponding red-blue shift of their diffraction. The diffraction could shift its peakposition larger than500nm, maintaining very much narrow full width at half maxima(FWHM) in the range of20to40nm. The opal hydrogels are sensitive to the presenceof SCN~-selectively and quantitatively; the interaction between SCN~-ions and theDMAA repeat units suppressed the interaction between the DMAA repeat units andwater molecules. These novel and smart opal hydrogels with sensitive, reversible, andrepeatable responses to external stimuli should be applicable to the construction ofdiffraction-based sensors.In the last chapter, direct assembly of colloids under an electric field has beenused to create three-dimensional colloidal crystals on electrodes. Actually, large-areaordered colloidal crystals on the surface of an electrode using electrical forces is amore intelligent than any other methods. It’s more cheap and easy. In polymerizedcolloidal crystals (PCCAs), the bandgap position can be controlled by swelling orcompressing the gel matrix with specific molecules or external pressure, respectively.In these systems, the electrokinetic force on the PS-co-PDMAA microspheres inducedby the electric field causes the compression or relaxation of the colloidal lattices, inwhich the constituent PS-co-PDMAA microspheres additionally experience strongelectrostatic repulsive interactions. Therefore, the photonic bandgap of PCCs ismodulated dynamically in response to the electric field. In addition, the colors werealso can be achieved through loading different times. With the loading time increasingthe PBG of PCCs has a blue-shift. When an electric field was applied to this latex, themicrospheres self-assembled due to the electro kinetic force exerted on themicrospheres. As a result, the reflection color of the PCCs was modulated insynchrony with changes in the applied field. Moreover, a DC electric field enables tomaintain the tuned bandgap positions of PCCs for at least a few minutes withoutdeteriorating after remove the DC field. The high stability and reflectivity of the PCCs,combined with the ability to tune their colors over localized areas, enabled us tofabricate a reflective mode display device capable of repeatedly changing the color ofa pattern of characters with fast response and clear boundaries. Our electro-responsivePCCs have potential importance in a wide range of optical applications, including optical switches, tunable mirrors and display devices due to their fast response andlow actuation voltage.