Three-dimensional Confined Self-assembly Of Block Copolymers Directed By Surfactants

Diblock copolymers exhibit abundant self-assembly behaviors due to their thermodynamic incompatibility.In the three-dimensional confined state,the complexity of the self-assembly process of block copolymers will increase,and polymer microspheres with various structures can be formed.These microspheres have shown great potential in the fields of optics,electronics,biomedicine and so on.The emulsion droplets have unique three-dimensional dynamic confined environment.It is often used as soft template to develop three-dimensional confined self-assembly of linear diblock copolymers.In this system,researchers can control the structure of the constrained assembly by adjusting the interface interaction,the degree of confinement,the properties of the solvent and the composition of the polymer.Among them,the interfacial interaction has a significant effect on the assembly results,and the interfacial interaction is mainly controlled by changing the properties of surfactants.At present,there are three types of surfactants used to control the interfacial interaction of emulsion droplets:ionic small molecule surfactants,non-ionic polymer surfactants and nanoparticle hybrid surfactants.The development of new surfactants is of great significance to expand the confined assembly structure of block copolymers.Based on the above research background,we developed a series of new polymer surfactant systems from the design of new surfactants and the control of confined assembly,by introducing inorganic hydrophilic head group and using controlled polymerization to improve the hydrophobic tail chain,and successfully used them to control the confined self-assembly of diblock copolymers to obtain polymer colloidal particles with untraditional structures.The main research progress includes the following three parts:Firstly,a cationic polymer surfactant N-PS_n with quaternary ammonium salt as hydrophilic head group and polystyrene as hydrophobic tail chain was synthesized by reversible addition fragmentation chain transfer(RAFT)polymerization.The three-dimensional confined self-assembly of symmetrical block copolymer PS-b-P2VP was successfully controlled by this surfactant.During the assembly process,N-PS_n exibited unique properties.It can not only play the role of traditional surfactant,but also stabilize the emulsion droplet,and its PS tail chain can be inserted into the PS phase region of PS-b-P2VP to regulate the volume fraction of PS block.By using N-PS_n/PVA binary surfactant system,we prepared a series of submicron PS-b-P2VP colloidal particles with specific internal structure,such as spherical particles with onion like stacking structure and ellipsoidal particles with axial layered stacking structure,especially the ellipsoidal PS-b-P2VP particles embedded with axial stacking P2VP ring.The effects of the amount of N-PS_n,the hydrophobic chain length of N-PS_n and the molecular weight of PS-b-P2VP on the assembly structure were studied.Because RAFT polymerization method is generally suitable for common monomer molecules,in the synthesis of this kind of cationic polymer surfactants,we can obtain a variety of polymer tail chains by introducing different monomers,so this kind of surfactants can be used to regulate a variety of block copolymer confined assembly systems,so as to develop polymer colloidal particles with rich structure.Secondly,we extended the N-PS_n/PVA binary surfactant system to regulate the three-dimensional confined self-assembly of PS-b-P2VP/PMMA blends.In this system,the macro phase separation occurs between PS-b-P2VP and PMMA,and the micro phase separation occurs in PS-b-P2VP itself.Therefore,the colloidal particles formed by the blending of PS-b-P2VP and PMMA can exhibit different internal structures.By adjusting the parameters such as the ratio of N-PS_n to PVA,the molecular weight of N-PS_n,the molecular weight of PS-b-P2VP,the mass ratio of PS-b-P2VP to PMMA,and the size of emulsion droplets,we systematically studied the three-dimensional confined assembly structure and evolution behavior of PS-b-P2VP/PMMA,and obtained Janus type colloidal particles with multi-level structure.This work provides a simple and effective method for the preparation of Janus colloidal particles with hierarchical structure.Thirdly,we prepared a kind of POM hybrid surfactants by replacing the counter anions of cationic polymer surfactant N-PS_n with polyoxometalates with 1 nm in size.Due to the large size and multi charge characteristics of the polyoxometalate anions,the surface of the polyoxometalate clusters can interact with many N-PS_n chains at a long distance,so that POM hybrid surfactants exhibit strong aggregation at the interface of the emulsion droplets.Based on this feature,we propose an interfacial phase separation strategy to control the three-dimensional confined self-assembly of block copolymers.The asymmetric assembly of PS-b-P2VP in microemulsion droplets was achieved by POM hybrid surfactant and PVA binary surfactant system.The onion like polymer colloidal particles with Janus structure were prepared for the first time,and the Janus ratio of the surface of the particles can be adjusted.These Janus onion-like particles can be further used as solid surfactants to prepare Pickering emulsions.The formation mechanism of Janus onion particles was studied in detail by experiment and computer simulation.The results show that the formation of this unique Janus structure is due to two special roles of POM hybrid surfactants on the surface of emulsion droplets:one is the strong affinity for PS components in the droplets;the other is the strong interfacial aggregation ability resulting in the phase separation between PVA and the surface of emulsion drops.Under the above action,POM hybrid surfactants successfully induced large aggregation of PS components on the surface of emulsion droplets,thus forming a unique Janus structure.These results provide a new strategy for regulating the asymmetric confined self-assembly of block copolymers.In conclusion,two novel surfactant systems were designed and synthesized in this work,which realized the controllable design of three-dimensional confined interface,regulated the three-dimensional confined self-assembly behavior of block copolymers and their blends,and successfully prepared a variety of polymer colloidal particles with novel nanostructures.Combined with theoretical simulation,the related confined assembly mechanism was discussed in depth.This work enriches the kinds of surfactants in confined assembly,and provides a new platform for the development of polymer colloidal particles with rich structure and function.