A New Approach To Well-controllable Hierarchical Structures Of Polymeric Materials Via RAFT Emulsion Polymerization And Its Applications

To balance the comprehensive properties, polymeric products are usually multi-component and multi-phase systems.The performances of the products are not only controlled by the microstructure of polymer chains, but also influenced by the morphology. The existing industrial products lack precise controllability on the chain microstructure and the phase morphology, thus limiting the high performance and functionalization of the polymeric materials, and also making it difficult to study the relationship between microstructures and macroscopic properties.Recently, some important progresses in reversible addition-fragmentation chain transfer (RAFT) emulsion polymerization have been made. Polymer latexes of high molecular weight block copolymers and gradient copolymers could be easily prepared, and the compositions and chain structures can be conveniently controlled in the polymerization process. In the current thesis, we proposed and explored a novel method to fabricate well-structured nano-hybrid polymeric materials with multiple components and multiple phases by taking advantage of the powerful controlling ability over the functional groups on the particle surface, the chain structure and the particle morphology in RAFT emulsion polymerization. Some applications were also demonstrated. The block copolymer latex particles of styrene and n-butyl acrylate with a large number of carboxylic groups bonded on the surface were synthesized by RAFT emulsion polymerization using an amphiphilic oligo(acrylic acid-block-styrene) macro-RAFT agent(named as PAA-b-PS-RAFT) as both mediator and surfactant. And on this basis, polymeric materials with multiple components and hierarchical structures were designed and controllably prepared, including polymer latexes of carboxy functionalized core-shell particles, polymer/polymer nanocomposites and polymer/inorganic nanocomposites. The relationship between the structure and the performance of these materials were then investigated systematically.The main research contents and innovative results achieved are as follows:1) With a carefully designed amphiphilic macroRAFT agent, polystyrene(PS), polystyrene-b-poly(n-butyl acrylate) diblock copolymer (PS-b-PnBA) and polystyrene-b-poly(n-butyl acrylate)-b-polystyrene triblock copolymer (PS-b-PnBA-b-PS) were prepared via RAFT emulsion polymerization with little coagulum, predicted molecular weight, and relatively narrow molecular weight distribution. In these cases, PAA-b-PS-RAFT was used as a reactive surfactant and polymerization mediator. During the polymerization, the ionized carboxyl functional groups stayed on the surface of particles and polymeric chains grew inwards in a living manner. As a result, the chain structure and core-shell nanostructures could be elegantly tuned by monomer feeding sequence.2) A coagulatable and redispersible copolymer latex system was developed by bonding a large number of carboxylic groups on the particle surface and the solf-core/hard-shell structure design. A series of PS-b-PnBA diblock copolymers with the PnBA content up to 70 wt% were prepared via RAFT emulsion polymerization with the amphipathic carboxy functionalized macroRAFT agent. By simply controlling monomer feeding sequence, the latex particles composed of a soft poly(n-butyl acrylate) core and a hard polystyrene shell could be synthesized. The hard plastic shell could prevent the elastomer core from deformation and fusion at room temperature. It was found that the latex particles with the nBA content≤60 wt%could be easily coagulated by HC1 and redispersed by NaOH and ultrasound. The coagulation and redispersion were repeatable. When the nBA content was over 70wt%, the plastic shell became too thin, resulting in collapsed sticky particles. The critical shell thickness for redispersible latexes was about 8 nm.3) A series of PS/PnBA block copolymer particles with different molecular chain structures were synthesized by RAFT emulsion polymerization. These copolymer latexes were blended with PS latex to prepare polymeric nanocomposites and study the influence of the chain structures of rubber particles on the mechanical properties of PS. It was found that nano-sized phases could be achieved by latex blending. With the increase of the rubber content, the nano-morphology of the composites changed from sea-island structure to co-continuous structure. Nano-sized morphology makes it possible to transform the mode of microscopic deformation from crazing to shear yielding. As a result, the intrinsic ductility of polystyrene could be transferred from the microscopic to the macroscopic level, which will result in a significant toughness improvement. Experiments showed that the modulus of the nanocomposites decreased linearly with the increase of the rubber content, and had little to do with the chain structure of the copolymer particles. However, the chain structure of the rubber particles had a great influence to the elongation at break and the notched impact strength. At the same rubber content, the toughening efficiency of the triblock copolymer was superior to those of PnBA homopolymer and diblock copolymer. In PS/SnBAS(25K-100K-25K) blending system, an obvious brittle-ductile transition (BDT) was observed when rubber content was about 30%. The notch impact strength reached 41.4kJ/m2 and the elongation at break reached 29.7% with a Young’s modulus of 1100MPa at the rubber composition of 33wt%.4) Polystyrene/poly(n-butyl acrylate)/montmorillonite (PS/PnBA/MMT) ternary hybrid films were fabricated by colloidal blending of oligo(acrylic acid) functionalized core-shell particles of PS/PnBA diblock copolymers and Na+ -MMT aqueous dispersions. The MMT platelets could be sequestered either to the PS or PnBA domains by changing the monomer adding sequence. Coherent hybrid films could be formed when the particles possessed a relatively thick soft PnBA shell (core/shell ratio=1:1 or 3:7) without adding any coalescing agent even when 15wt% MMT was added. TEM observations revealed that around 20nm thick clay stacks were well dispersed throughout the hybrid films. A strong interaction was observed between clay and the block copolymer via acrylic acid groups on the particle surface. Increasing the levels of PS or MMT could much increase the modulus and tensile strength of the hybrid films. However, PS composition would be limited by the crack formation and brittleness of the resultant films. The ternary hybrid design would allow one to gain good balance between mechanical properties and coherent film formation. The enhancing efficiency of MMT for the modulus and tensile strength was much higher when placing the clay stacks in PS domains than in PnBA domains, which was particularly obvious when MMT levels were higher than 10 wt%.