| Block copolymers(BCPs),comprised of two or more covalently connected segments with different chemical compositions,have received widespread attention in the past decades due to their fascinating property of spontaneously self-assembling into various nanoscale structures in selected solvents or bulk.As an advanced tool for fabricating novel nanomaterials,BCPs are leading to promising applications in biomedical diagnostics,environmental sciences and sensing chemical analytes.Investigations of the solution self-assembly for simple AB diblock or ABC triblock copolymers and the phase behavior for common BCPs bulk have been persisted extensively for several decades.Based on enormous amount of works,a relatively mature system has been established both in experimental study and theoretical analysis.However,some pressing shortages,such as simple structure of aggregates,few methods of adjustment and single transition of phase structure,emerge with the applications of BCPs in practical applications products.Spurred by the requirement of multicompartment nanostructures of BCPs micelles and novel phase behavior of BCPs bulk,the studies of the solution self-assembly for multiblock copolymers and the preparation of BCPs bulk with abnormal phase behavior have inspired interests.In this dissertation,the morphologies,nanostructures,mechanisms and responsiveness of the solution self-assembly for the stimuli responsive ABCBA pentablock copolymers were systematically studied.By introducing the hydrogen(H)-bonding interaction into the ABA terblock copolymers,the abnormal temperature-dependent phase transition processes of BCPs bulk were investigated.Firstly,a series of PNIPAM-b-PtBA-b-PPO-b-PtBA-b-PNIPAM BCPs(NTOTN)with various lengths of PNIPAM block were synthesized via ATRP.These pentablock terpolymers self-assembled into thermosensitive micelles with PNIAPM blocks as corona and PtBA,PPO blocks as core in aqueous solutions at low temperature.With decreasing the length of coronal chains,the nanostructures of micelles transformed from corona-core spheres,through corona-core cylinders and corona-shell-core spheres,to large compound spherical micelles(LCMs).This transformation could be attributed to the decrease of coronal free energy and the increase of core free energy.With increasing temperature,the thermosensitive PNIPAM coronal chains collapsed onto the core,causing the decreased transmittance of micelle solutions and the transformation of micelle structures into uniform spheres at high temperature.Subsequently,in order to expand the functionality of ABCBA micelles,the PtBA blocks of NTOTN pentablock terpolymers were hydrolyzed into PAA with varying degrees of hydrolysis.After self-assembly in aqueous solutions,the both pH and thermo sensitive pentablock multicomponent micelles were obtained.The adjustment of pH or temperature would cause the assembly/disassembly,aggregation/discretization of micelles,leading the obvious change of the macro-properties of micelle solutions and the evident transformation of the micro-nanostructures of micelles.At low pH and low temperature,with increasing hydrophilicity of BCPs,the micelles transformed from LCMs with bent corona,through LCMs with extended corona,corona-core irregularities,corona-shell-core spheres,coexistence of corona-shell-core spheres and corona-core cylinders,corona-core cylinders,coexistence of corona-core spheres and corona-core cylinders,to corona-core spheres finally.After introducing electrostatic repulsion by increasing pH,four micelles,i.e.,inhomogeneous LCMs with rolling surface,corona-core spheres,corona-core cylinders and corona-shell-core spheres were observed.Moreover,in order to enhance the core microphase separation of ABCBA micelles,the fluorocarbon block was introduced into the BCPs.A series of PEGMA-b-PFMA-b-PPO-b-PFMA-b-PEGMA BCPs(EFOFE)with various lengths of PEGMA and PFMA blocks were synthesized.These pentablock terpolymers were successively self-assembled into the precursor micelles with PFMA block as core,PEGMA,PPO blocks as corona in methanol and the final micelles with PFMA,PPO blocks as core,PEGMA block as corona in aqueous solutions.The nanostructures of precursor micelles were depended on the volume fraction of PFMA blocks(f _F),with increasing f _F,the precursor micelles transformed from porous ellipsoids,through hollow ellipsoids,hollow cylinders,to lamellas.The core structures of final micelles were depended on the volume fraction of PEGMA blocks(f _E)and the core volumefraction of PFMA block(Φ_F),with increasingΦ_F,the core transformed form lamellar structure,through bicontinuous structure,to homogeneous structure.The increase of f _E would promote the core transform into the structure corresponding to the lowerΦ_F.Finally,a series of PAA-b-PPO-b-PAA BCPs(AOA)with various lengths of PAA and PPO blocks were synthesized to investigate the temperature-dependent phase behavior of BCPs bulk controlled by the combined interaction of van der Waals force and hydrogen bonds(H-bonds).The enhanced phase separation upon increasing temperature was identified in AOA BCPs.This abnormal phase behavior was analyzed from the standpoints of the transformation of H-bonding interaction,the change of interaction parameter and the thermodynamic viewpoint.With increasing temperature,the weakening and transformation of H-bonding interaction caused the firstly fast increase and then slow increase of interaction parameter,leading to the increase of mixing free energy upon heating.As a result,the disordered structures of BCPs tended to become ordered structures.The ordered structures improved their structural regularity and might exhibit an inverse order-to-order phase transition(IOOT)at higher temperature. |
