Synthesis, characterization and testing of nano-structured particles for effective impact modification of glassy amorphous polymers

[摘要] ENGLISH ABSTRACT:The synthesis of structured nanoparticles, in particular core/shells, IS of greattechnological and economical importance to modem materials science. One of theadvantages of structured particles is that they can be synthesized with either a solid core(albeit soft or hard) or a liquid core (of varying viscosity). This adds to the versatility ofstructured particles and their relevance to a majority of industrial and commercial endapplications.The synthesis of core/shell particles with liquid cores was investigated for theeffective impact modification of glassy amorphous polymers. Polybutyl acrylate waschosen as the shell due to its rubbery nature. Hexadecane functioned as the core oil andfacilitated osmotic stability by being a suitable hydrophobe for the miniemulsionsynthesis. Polymer synthesis was preceded by the prediction of particle morphology byusing thermodynamic prediction models.Core/shell particles with liquid cores were synthesized via miniemulsionpolymerization. This resulted in the direct introduction of core-oil and monomer into theminiemulsion droplets. Polymerization was achieved in situ, resulting in the formation ofparticles with the desired morphology. For additional strength, stability and matrixmixing capabilities, methyl methacrylate (MMA) was grafted onto the initial core/shellparticles. The obtained morphology was in contradiction with the predicted morphology,thus pointing to strong kinetic influences during the polymerization process. Theseinfluences could be attributed to surface anchoring of polymer chains due to the initiator(KPS) used, the establishment of the polymerization locus as well as the increase inviscosity at the polymerization locus. To test these influences a surface-inactiveinitiating species (AIBN) and an interfacial redox initiating species (cumylhydroperoxide/Fe/) were used. Use of the former resulted in the formation of solidpolymer particles due to homogeneous polymerization throughout the droplet, thusleading to an inverse core/shell morphology as a result of thermodynamic considerations.The redox initiator promoted kinetic influences as a result of fast polymerization kineticsat the droplet/water interface. This, as well as the increase in viscosity, facilitated theproduction of core/shell particles.To obtain core/shell particles with the desired size, the influence of surfactantconcentration was investigated. Capillary hydrodynamic fractionation (CHDF) was usedto determine the particle size of the initial core/shell particles as well as the size of theMMA-grafted core/shell particles. The area stabilized per surfactant molecule wascalculated stoichiometrically and compared to classical miniemulsion results, i.e. datagenerated from the synthesis of polymeric latexes in the presence of a hydrophobe, but ata much lower hydrophobe:monomer ratio than was used here. The influence of methanolas well as the possibility of scaling-up the process was also investigated.The study was further expanded to the investigation of living miniemulsionpolymerization techniques to control the molecular architecture of synthesized core/shelllatexes. The influence of different RAFT agents, initiators and monomers wereinvestigated on the core/shell formation properties of the investigated systems. Thecombined effects of establishing the polymerization locus as well as increasedpolymerization kinetics, thus increasing the viscosity at the polymerization locus, lead tothe successful formation of liquid- filled core/shell particles.To conclude, the ability of the synthesized core/shell particles to induce impactmodification in glassy amorphous polymers was investigated. Results showed thatincorporation of these particles could effectively modify the intrinsic properties of theinvestigated polymers, resulting in a brittle-to-ductile transition. Improved impact resultsof the investigated glassy matrix were obtained.Keywords: core/shell, liquid-filled, RAFT, miniemulsion, impact modification

[发布机构] Stellenbosch University