Structure and physical properties study on saccharide conjugated block copolymers and their composites

Abstract

Hybrid materials containing both synthetic polymers and naturally occurring saccharide molecules have been widely studied due to their environmental friendliness derived from saccharides with maintaining the advantages of synthetic polymers. Saccharide-conjugated block copolymers are expected to have high χ capable of microphase separation at small molecular weights. In this thesis, fundamental morphology behavior of saccharide-based block copolymers are studied and utility of saccharide-conjugated composite. In Chapter 3, ABA type triblock-like polymer conjugates are synthesized with polystyrene (PS) as the mid-block and either maltotriose (MT) or maltose (Mal) as the end units. PSs with different molecular weight are prepared by atom transfer radical polymerization (ATRP) and block copolymers are prepared by click reaction between saccharide blocks and PS. The morphologies are confirmed by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). MT-PS-MTs are found to form highly ordered domain structures with sub-10 nm domain size, and the morphology changes from hexagonally packed cylinders to body-centered cubic spheres to disordered spheres with decrease of saccharide weight fraction. Mal-PS-Mals also could self-assemble into microphase separated structures, but lack of uniform packing owing to the close proximity of polymer glass transition temperature (T$_g$) and order-disorder transition temperature (T$_{ODT}$). Especially, irreversible morphology change occurs at high temperature due to the saccharide block caramelization derived pentablock formation. In Chapter 4, diblock copolymers are synthesized with oligosaccharide, malto-octaose (MO) and poly(ethylene glycol) (PEG). The morphologies are confirmed by SAXS, and the Flory-Huggins interaction parameters (χ) are estimated based on mean-field theory modified with Fredrickson and Helfand fluctuation model and polynomial equation fitted to phase boundaries of self-consistent mean-field theory (SCFT). MO-b-PEGs are found to form microphase separated structures, and the symmetric BCPs show lamellar morphology. The asymmetric MO-b-PEG self-assemble into hexagonally packed cylinder morphology, however, the crystallization of PEG block makes the morphology be destroyed and induce repacking to new structure. The χ(T) of MO-b-PEG yields as χ(T) = 13.416/T + 0.384, indicating high χ despite the chemical similarity between MO and PEG. In Chapter 5, maltose-polybutadiene and maltose-disulfide composite are synthesized by mimicking the typical compatibilizer and conventional coupling agent, respectively. Maltose-polybutadiene (Mal-PB) is prepared by coupling reaction between alkyne-functionalized maltose and azide-functionalized polybutadiene, and maltose-cystamine-maltose (Mal-Cys-Mal) as maltose-disulfide composite, is prepared by direct reductive amination of the reducing end of maltose with amine groups of cystamine. Mal-PB is found to be a good compatibilizer between silica and rubber matrix, however, not suitable as coupling agent due to lack of sulfur. According to the results of increased bound rubber content and improved silica dispersion of rubber compound prepared with Mal-Cys-Mal, and the improved viscoelastic properties and tensile properties of the vulcanizate, it could be said that Mal-Cys-Mal has potential as a coupling agent.