The Shape And Structure Of Block Copolymer Assemblies Via Confined Emulsion Self-assembly Tuned By External Stimuli

Block copolymers(BCP)is formed by polymer chain segments with different physical and chemical properties connected by covalent bonds.Due to the different physical and chemical properties,each block is incompatible with each other,resulting in micro-phase separation,and nanoparticles with different morphologies are assembled,which has potential application prospects in catalysis,devices,diagnostics and drug delivery.The traditional BCP self-assembly methods,such as bulk selfassembly and solution self-assembly,can obtain nanoparticles with different morphologies,such as spherical particles,worm-like particles,rod-like particles,etc.However,due to the numerous factors affecting the assembly process and the unstable assembly environment,the traditional BCP assembly method is difficult to form a regular and controllable assembled structure,which limits the practical application of block copolymer assembly.In recent years,block copolymer emulsion-confined selfassembly has attracted extensive attention.That is,the BCP is firstly confined within the emulsion droplet,then,with the evaporation of the organic solvent,microphase separation of BCP occurs,resulting in the formation of ordered structures.Since the emulsion droplets provide a closed and stable environment for the block copolymer assembly as well as a soft emulsion template,some novel self-assembled nanostructures can be obtained,such as pupa-like structure,onion-like structure,bud-like structure,etc.If external stimuli such as temperature,pH and light are introduced in the process of BCP emulsion-constrained assembly,due to the characteristics of weak stimuli and strong response of the functional components to external stimuli,as well as the controllable adjustment of the interfacial properties of emulsion droplets by external stimuli,the assembly structure can be adjusted and the assembly morphology can be enriched,thus broading their applications in design the intelligent responsive materials.However,at present,emulsion-constrained assembly morphology of BCP under external stimuli is irreversible,and the external stimulation is single,so it is difficult to achieve more complex assembly structure.In this thesis,the temperature-sensitive poly(N-isopropylacrylamide)(PNIPAM)is introduced into the emulsion confined self-assembly of BCP.Based on the reversible change of the hydrophobicity of PNIPAM during the temperature switching process,the BCP assembly structure can be reversible and repeatedly changed by switching the external temperature,realizing the temperature-controlled accurate release of the internal loaded drug.Furthermore,we introduce light-responsive azobenzene(AZO)molecules into the emulsion constrained assembly of BCP.Based on the light responsiveness and pH sensitivity of azobenzene group,both the assembly morphology and structure of BCP are responsive to light and pH.Thus,diversified morphorlogies of the BCP are obtained.The main research contents and conclusions are as follows:(1)Temperature stimulation modulates the reversible transition of block copolymer morphology in emulsion-constrained assembly.As a typical temperature-sensitive polymer,PNIPAM will correspondingly change its hydrophilicity and hydrophobicity when switching the ambient temperature above or below its critical transition temperature point.Based on this,the emulsion-constrained co-assembly of PNIPAM and polystyrene-b-poly(4-vinylpyridine)(PS-b-P4VP)BCP is conducted.As a result,nanoparticles with different morphology and structure are prepared by changing the ambient temperature during the block copolymer assembly process.In addition,through varying the ambient temperature under solvent adsorption annealing,the hydrophobic-hydrophilic transition of PNIPAM results in the change of the interface interaction,which results in multiple reversible cyclic transformations of the assembled morphology of PS-b-P4 VP nanoparticles from raspberry-like particle to onion-like particle,raspberry-like particle,onion-like particle,and finally to raspberry-like particle.Finally,the temperature-controlled reversible transformation of the polymer nanoparticle structure is utilized to achieve temperature-controlled precise release of its internally loaded drug.(2)Dual light/pH stimulation modulates the transformation of block copolymer emulsion-constrained assembly morphology.The photo-responsive azobenzene small molecule is co-assembled with PS-b-P4 VP under emulsion-constrained conditions,and the azobenzene group undergoes a trans-cis conformational isomerization transition under 365 nm UV irradiation,resulting in changes of its hydrophilicity and hydrophobicity.As a result,morphological and structural transformation of the polymer nanoparticles from chrysalis-like particles to onion-like particles with P4 VP in the outermost layer is generated.In addition,the hydrogen bond formed by P4 VP phase in PS-b-P4 VP and azobenzene small molecule is sensitive to pH,and the co-assembled morphology and structure can be controlled by adjusting the pH of the solution.Specifically,in acidic environment,the P4 VP phase is protonated,resulting in the breakage of hydrogen bond,and the co-assembled nanoparticles are transformed from chrysalis-like to onion-like.In the alkaline environment,azobenzene is neutralized,inducing the increase of its hydrophilicity.Therefore,hydrogen bonds between P4 VP chains and azobenzene molecules are broken,and polymer nanoparticles transform into onion-like particles.