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Title: Self-assembly of surface-modified clays for functional biomimetic materials
Author: Xu, Peicheng
ISNI:       0000 0004 7660 8709
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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Synthetic Laponite-clay particles with a platelet-like shape display strong gelation when dispersed in aqueous solutions because of their positively charged rims and negatively charged flat surfaces. In this thesis, my aim was to modify the surfaces of these clay particles such that we can both access their liquid crystalline (LC) discotic phase and further build transparent and mechanically resilient coatings with a 3D "brick-and-mortar" structure that is similar to that observed in natural mother of pearl (nacre). I first introduce a simple strategy that successfully suppresses Laponite's ageing phenomenon and enables the system's isotropic-to-LC phase transition. By grafting Laponite particle surfaces with comb-like polymers, poly (L-lysine)-g-poly (ethylene glycol) (PLL-PEG), I was able to screen negative surface charges and ensure steric stabilisation. Besides using long-chain polymers, I also coated the positively charged Laponite rims with small, barrel-shaped molecules cucurbit[7]uril (CB[7]). By carefully tuning the ratio between CB[7] and Laponite, the system experienced a macroscopic phase separation into a Laponite-poor suspension and a birefringent LC gel. Inspired by the hierarchical structure of nacre, here I also demonstrate a simple approach to fabricate polymer-clay hybrid films via a water-evaporation process. In this third method, Laponite platelets were bridged by natural abundant polymers (carboxymethyl cellulose) through hydrogen bonding. This hybrid material possesses high transparency, flexibility and an outstanding fire-retardant property. After Ca2+ ion-coordination of these cellulose-Laponite composite films, the interface between the polymers and clays was further strengthened, leading to enhanced mechanical properties along with improved thermal- and water-resistance. I also present that using Dextran as a depletant, sterically stabilised Laponite can access its liquid crystal phase under low clay concentration. Finally, I show that Laponite can be coated with various polymers (PEO, chitosan, sodium alginate) for the purpose of obtaining LC gels and hybrid films. I believe that our findings on surface-modification of clay particles can open new routes to large-scale and inexpensive production of bio-inspired functional materials.
Supervisor: Eiser, Erika Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: self-assembly ; liquid crystal ; nanocomposite