Fabrication Of PNIPAm-based Colloidal Photonic Crystals And Synthesis Of Temperature-induced Physically Crosslinked Hydrogels

Stimuli-responsive nanogels are three-dimensional,hydrophilic,cross-linked elastic network.Their chemical/physical properties(e.g.,hydrophilicity,permeability,and viscoelasticity)can be manipulated by external triggers such as p H,temperature,magnetic field,and so on.Among them,the temperature-responsive poly(Nisopropylacrylamide)(PNIPAm)is one of the most widely studied smart nanogels.On one hand,the monodisperse PNIPAm-based nanogels can self-assemble into colloidal photonic crystals(CPCs)that exhibit brilliant structural colors regulated by the change of temperature.However,the CPCs currently have two issues: First,the ordered crystal structure of the hydrogel photonic crystals disappears above the phase transition temperature(Tp)due to the thermally induced shriankage of the microgels.Secondly,the hydrogel photonic crystals easily lose their ordered structure in response to small disturbance because of their extremely fragile nature of the crystalline structure.On the other,the PNIPAm-based polymers can transform into physically crosslinked hydrogels through temperature triggered in-situ sol-gel transition.However,most of these thermally gelable PNIPAm-based polymers are focused on the design of linear,grafted or block polymers which undergo complex preparation process,perform high viscosity and gelate only at high concentrations.In view of the above research status and development bottlenecks,in this thesis,multi-responsive PNIPAm-based nanogels are prepared with various structures and functional groups.And the potential applications of these PNIPAm-based nanogels are discussed.The specific content is as follows:1.A series of temperature/p H sensitive PNIPAm/PAA IPN nanogels is created by the in-situ polymerization of AA monomer in the PNIPAm network.The IPN nanogels with various amounts of PAA component are used as the nano-carrier systems for protein adsorption and superparamagnetic iron oxide nanoparticles(Fe₃O₄ NPs)loading.On one hand,it is more conductive for the IPN nanogels to capture proteins as the temperature is increased or as the p H value is decreased.On the other,Fe₃O₄ NPs are encapsulated by in-situ redox of iron cations in the interior of IPN nanogels which provid the space for Fe₃O₄ NPs to nucleate and grow up.The increase of both amounts of carboxyl groups and iron cations results in the increase of loaded Fe₃O₄ NPs.2.The monodisperse IPN/Fe₃O₄ nanogels and(IPN/Fe₃O₄)@PNIPAm nanogels with different refractive index and structure are prepared,which can selfassemble into colloidal photonic crystals(CPCs)and show brilliant structural colors.The reflection peak of CPCs along with the structural colors show the red-shift with the increase of the number of loaded-Fe₃O₄ nanoparticles.In addition,the CPCs with “defect” can be formed by mixing different nanogels with various structures and refractive indexes.These defected-CPCs can also be reproducibly and reversibly tuned in minutes using external stimuli as a result of the temperature-responsive property of the PNIPAm.3.In order to prepare the CPCs which can still maintain crystalline arrays above the Tp,the IPN-BAC nanogels which crosslinked with N,N'-Bis(acryloyl)cystamine(BAC)are synthesized.The IPN-BAC CPCs still show striking structural colors even as the temperature is above the Tp,while the IPN-BIS CPCs are quickly disrupted.Furthermore,the thermostable IPNBAC nanogels as the core material are encapsulated by glucose-sensitive NIPAM/VPBA copolymer shell.The resulting core/shell nanogels still selfassembled into glucose-sensitive CPCs at a temperature even above its PTT.The CPCs demonstrate striking colors change from blue to green in response to low glucose concentration.4.The IPN nanogels are well designed by adjusting the particle sizes of PNIPAm and the amounts of loaded-PAA,which can undergo reversible temperaturetriggered in-situ sol-gel transition above the Tp.The IPN CPCs still show brilliant colors and the crystalline arrays can be immobilized by the sol-gel transition process above the Tp.This outstanding result is believed to be a significant advance in the related field since thermosensitive crystals are known to melt into disordered state at elevated temperature,due to dramatic shrinkage of the thermosensitive nanogels.5.In order to prepare the thermally gelable PNIPAm-based copolymers,the hydrophobic monomer N-tert-butyl acrylamide(TBA)is copolymerized with NIPAm and the resultant is abbreviated as PNT NPs.The PNT NPs can achieve the sol-gel transition above the Tp but lose their structural colors due to the shrinkage of nanogels.Thus the hydrophilic monomer N-acrylamido-Lphenylalanine(APhe)is incorporated into the PNT NPs,and the resultant is abbreviated as PNTA NPs.These PNTA NPs exhibit tunable striking structural colors during sol-gel transition above the Tp,which is rarely reported in soft nanoparticle field.