Elucidation of Molecular Parameters Governing the Interactions and Properties of Brush Particles

Abstract

Elucidation of the mutual influence of the composition and architecture of polymer canopies on the assembly and physical properties of particle brush-based materials holds the promise of advancing the understanding of the governing parameters controlling interactions in hybrid materials and the development of novel functional materials. In this thesis, how the brush molecular parameters govern the collective interactions and properties was investigated. A number of precedent studies clarified the role of architecture (depending on chain length, particle size and particle size dispersity) on the interactions in brush particle assembly. Based on the previous reports, the present thesis further explored new parameters, the surface functionalizationdensity and dispersity in chain length governing the mechanical or thermal properties. First, the elastic properties of three series of brush particle systems were investigated, differentiated by grafting density as dense, intermediate and sparse brush systems. Dense and intermediate systems displayed uniform microstructures; the degree of order increased with grafting density. For dense and intermediate brush particle systems, instrumented indentation analysis revealed an increase ofthe elastic modulus and hardness with the degree of polymerization of tethered chains. Furthermore, the effectiveness of ligands to enhance interactions increased with decreasing grafting density. The results were rationalized as a consequence of more pronounced brushinterdigitation in the case of intermediate systems and the resulting increase of the dispersion interactions between ligands of adjacent particles. A reversed trend in modulus was observed in films of sparse brush particles that also featured the formation of string-like superstructures. Here,the elastic modulus and hardness were substantially increased for low molecular ligands and continuously decreased with increasing degree of polymerization of tethered chains along with a transition from string-like to uniform morphologies. Also, the molecular weight distribution of grafts was found to affect the structure formation and interparticle interactions. Wide-dispersedbrush particles assembled into a less-ordered superstructure due to the less-regular brush thicknessaround a core. The wide-distributed molecular weight practically encouraged entanglement formation in the system by thinning the concentrated brush layer. Toughness of the dispersed brush particle films was enhanced by the increased entanglement density. Glass transition was turnedout to be broadened as the molecular weights broke up.In addition, phase behavior driven by interparticle brush interactions in binary brush particles was also studied via collaborative works. Specifically, two types of binary brush materials were explored: binary mixtures of homopolymer-tethered particles and diblock copolymer-tetheredparticles. In the binary mixture materials, a LCST-type pair of ligands was found to induce the reversible phase separation throughout a relevant thermal processing. Diblock copolymer brush, on the other hand, led the particulate colloids to phase separated assemblies depending on grafting density. Dense BCP brush particles formed concentric particle distribution while sparser BCPsystems assembled into network structures in different levels subject to grafting density. The systematic investigations that demonstrated the importance of molecular design of particle brush and the pioneering works that explored the controllability of assembly structure ofheterogeneous particle brush system filled and widened the field of brush particle-based hybrid material. The results of this thesis identified the applicability and versatility of the building block materials with functionality and mechanical durability.