Enhancing the mechanical properties of MgO-based formulations

Reactive magnesia cement (RMC) has seen an increase in studies in recent years to gauge and increase its feasibility to be the next concrete binding material. In this project, the self-healing properties of RMC-based composites have been explored. The microbial induced healing method used in Portland cement was modified and attempted on crack healing in RMC-based composites. This project showed that microbial induced healing is feasible and capable of completely sealing cracks that were up to 15 times wider than the maximum crack recoverable by water-air cycles. Besides sealing of crack surface, microbial healing has also been shown to produce a significantly higher amount of healing products throughout the depth of the cracks, which translate to better quality of crack healing. It is evident that hydrated magnesium carbonates are the main healing products formed, and different types of hydrated magnesium carbonates form in cracks of different width and curing conditions. The morphology of these hydrated magnesium carbonates depends on the pH, crack width and urease activity of bacteria. Lower pH, larger crack width and high urease activity curing solution will tend to favor production of needle-shaped nesquehonite crystals. Higher pH, smaller crack width and low urease activity curing solution will favor production of rosette-shaped hydromagnesite/dygingite crystals.Bachelor of Engineering (Civil

Biopolymer toughening with chain extenders

Polylactic acid (PLA) is one of the most popular biodegradable polymers today. However, it has its drawbacks, such as poor melt strength and narrow processing window. In this study, PLA 2003D was compounded with chain extenders (CE) at weight percentages of 0.2%, 0.5%, 0.75% and 1.0%, using two co-rotating screws. Two types of multifunctional epoxy extenders were used, Joncryl® ADR-4300 and Joncryl® ADR-4370F. The thermal, mechanical, chemical and rheological properties of the PLA blends were investigated. The DSC results showed that there was a lower crystallinity with increasing concentrations of CE, suggesting that the blends increased in ductility. The results showed that the mechanical (i.e. impact resistance, ductility) and rheological (i.e. melt strength) properties improved with increasing concentrations of CE. The impact resistance and ductility of the blends increased with increasing concentrations of CE. When compared to pure PLA, the PLA blends exhibit enhanced melt strength and strain-hardening behaviour. These results are supported by results of the FTIR and GPC, which indicated that the PLA blends had an increase in molecular weight. The change in molecular weight and molecular architecture due to the chain extender plays an important role in the enhancement of the properties. Lastly, a blown film application was used to demonstrate the improvements in melt strength and processing window. This experiment allows for future research to explore other possible applications of PLA blends or other polymer blends.Bachelor of Engineering (Materials Engineering

Improving printability of hydrogel-based bio-inks for thermal inkjet bioprinting applications via saponification and heat treatment processes

Material jetting bioprinting is a highly promising three-dimensional (3D) bioprinting technique that facilitates drop-on-demand (DOD) deposition of biomaterials and cells at pre-defined positions with high precision and resolution. A major challenge that hinders the prevalent use of the material jetting bioprinting technique is due to its limited range of printable hydrogel-based bio-inks. As a proof-of-concept, further modifications were made to gelatin methacrylate (GelMA), a gold-standard bio-ink, to improve its printability in a thermal inkjet bioprinter (HP Inc. D300e Digital Dispenser). A two-step modification process comprising saponification and heat treatment was performed; the GelMA bio-ink was first modified via a saponification process under highly alkali conditions to obtain saponified GelMA (SP-GelMA), followed by heat treatment via an autoclaving process to obtain heat-treated SP-GelMA (HSP-GelMA). The bio-ink modification process was optimized by evaluating the material properties of the GelMA bio-inks via rheological characterization, the bio-ink crosslinking test, nuclear magnetic resonance (NMR) spectroscopy and the material swelling ratio after different numbers of heat treatment cycles (0, 1, 2 and 3 cycles). Lastly, size-exclusion chromatography with multi-angle light scattering (SEC-MALS) was performed to determine the effect of heat treatment on the molecular weight of the bio-inks. In this work, the 4% H2SP-GelMA bio-inks (after 2 heat treatment cycles) demonstrated good printability and biocompatibility (in terms of cell viability and proliferation profile). Furthermore, thermal inkjet bioprinting of the modified hydrogel-based bio-ink (a two-step modification process comprising saponification and heat treatment) via direct/indirect cell patterning is a facile approach for potential fundamental cell-cell and cell-material interaction studies.Submitted/Accepted versionThis study was supported by the RIE2020 Industry Alignment Fund – Industry Collaboration Projects (IAF-ICP) Funding Initiative, as well as cash and in-kind contribution from the industry partner, HP Inc., through the HP-NTU Digital Manufacturing Corporate Lab

Conjugated, trans-Spanning Ligands as Models for Multivalent p-Phenyleneethynylenes

A conjugated, pyridine-containing, phenylethynyl ligand that forms complexes with AgI and PdII has been developed. NMR titration studies with PdII reveal a stoichiometric binding of the ligand to the metal atom, while similar studies with AgI reveal a binding that is dynamic on the NMR timescale. Analysis of the NMR spectroscopic data by Job\u27s plot analysis and non-linear curve fitting of a titration curve reveals a 1:1 binding ratio of ligand/silver cation and an association constant of Ka = 53 M “1. X-ray crystal structures of the ligand “metal complexes suggest ample room for the nearly barrierless rotation of the unsubstituted central benzene ring of the para-phenylethynyl chain. Subtle electronic differences in substituted systems provide some evidence of impaired rotation

The Influence of Secondary Interactions on the [N−I−N]+ Halogen Bond

[Bis(pyridine)iodine(I)]+ complexes offer controlled access to halonium ions under mild conditions. The reactivity of such stabilized halonium ions is primarily determined by their three-center, four-electron [N−I−N]+ halogen bond. We studied the importance of chelation, strain, steric hindrance and electrostatic interaction for the structure and reactivity of halogen bonded halonium ions by acquiring their 15N NMR coordination shifts and measuring their iodenium release rates, and interpreted the data with the support of DFT computations. A bidentate ligand stabilizes the [N−I−N]+ halogen bond, decreasing the halenium transfer rate. Strain weakens the bond and accordingly increases the release rate. Remote modifications in the backbone do not influence the stability as long as the effect is entirely steric. Incorporating an electron-rich moiety close by the [N−I−N]+ motif increases the iodenium release rate. The analysis of the iodine(I) transfer mechanism highlights the impact of secondary interactions, and may provide a handle on the induction of stereoselectivity in electrophilic halogenations