Selüloz nanokristal süspansiyonlarının misel oluşturan sürfaktanlar aracılığılya sıvıdan yumuşak katı formuna geçmesi

Cellulose nanocrystals (CNC) are versatile nanomaterials benefited widely for both industrial and scientific purposes. Their abundance, non-toxicity, biodegradability, extraordinary mechanical, optical, and thermal properties make them suitable for a wide range of applications. Particularly, high aspect ratio, hydrophilicity, easy modification of surface properties make CNC ideal rheology modifiers. In presence of salts, polymers and surfactants, thixotropic features of aqueous CNC suspension alter drastically, and liquid to soft solid phase transition is often observed. Even though rheological properties of neat aqueous CNC suspensions are widely studied and understood, there is an undiscovered area about gelation in presence of macromolecules, like polymers and surfactants. Understanding gelation mechanism is crucial to tailor mechanical properties of the complex CNC mixtures. Our study is focused on surfactant – CNC interactions especially when the surfactant concentration is higher than critical micelle concentration. This thesis is the result of two different studies. In the first one, the bulk rheology (performed in both linear and transition from linear to non-linear viscoelastic region) of aqueous CNC suspensions in the presence of cationic surfactants of 1-decyl-3-methyl imidazolium chloride and 1-decyl-3-methyl imidazolium ferric tetrachloride was studied separately. The head and tail groups of these surfactants are identical, the only difference is the counterions, which led to formation of identical micelles of identical size and shape but with varying interparticle interactions. Micelles of [C10mim][Cl] are strongly positively charged and stabilized via long-range Coulomb repulsion whereas [C10mim][FeCl4] surfactants form nearly neutral micelles interacting with short-range hard-sphere type interaction. We, therefore, evaluate directly, for the first time, the effect of micelle charge on the gelation of CNC suspensions using the same surfactant system. At intermediate surfactant concentrations (slightly above CMC), positively charged micelles electrostatically interact with negative CNC and adhere to their surface; therefore, micelles behave as crosslinking points and gel network becomes strong. However, at high surfactant concentrations well above critical micelle concentration, the charged micelles repel each other and result in much weaker gel networks. Such a drastic weakening effect was absent in presence of neutral micelles. In the second study, thermo-responsive block copolymer with the molecular structure of (PEO)100-b-(PPO)65-b-(PEO)100 (which is also a non-ionic surfactant commercially named Pluronic F127) is mixed with aqueous CNC suspensions and liquid to soft solid phase transition was observed as micellization takes place. The micelles are neutral due to nature of block copolymer. Pluronic F127's aqueous solutions are thermo-responsive; they undergo gelation due to Pluronic F127 micelle formation taking place at above critical micelle temperature (CMT), near the room temperature. CMT and therefore gelation temperature go down as Pluronic F127 concentration increases. Since the critical temperature is lower than body temperature yet still high enough for processibility at room temperature, Pluronic F27 is suitable for biomedical applications. Gel point of the complex is increased with increasing CNC concentration and thermal and mechanical reversibility of the complex is also preserved. The resultant Pluronic F127 / CNC solutions are promising materials for especially dual drug therapy in which both Pluronic F127 and CNC can be utilized as drug carriers. Our studies demonstrate that addition of ionic and non-ionic surfactants and polymers to aqueous CNC suspensions can have a dramatic influence on the phase change due to network formation by electrostatic, hydrophilic, or hydrophobic interactions. Designed by manipulating these interactions, CNC-based hydrogels can be used in tissue engineering, drug and gene delivering systems in medicine as well as protective coatings in the industry.