Dynamics of Dense Microgel Suspensions

Microgels are a multifaceted material with a broad range of applications including biomimicry and drug delivery. Aqueous microgels are micron-sized hydrogel particles composed of a swollen network of interconnected polymers. Based on the properties of the polymers, microgels can be designed to be stimuli-responsive or to act as model colloidal particles. Chapter 1 details the background and uses of microgel particles. As dense suspensions of microgels tend to form colloidal glasses at high concentrations, the opening chapter provides a strong focus on the glass transition and details certain aspects including glass fragility, dynamic heterogeneity, and the confinement effects. Starting in Chapter 2, common methods of microgel synthesis are introduced, with a focus on emulsion polymerization. This is the method used throughout this dissertation to synthesize microgels based on two polymer backbones: poly(N-isopropylacrylamide) (PNIPAM) and poly([2-(Methacyloyloxy)ethyl]-dimethyl-(3-sulfopropyl)] ammonium hydroxide) (PMSA). PNIPAM has a lower critical solution temperature (LCST) of 32oC, which has led to significant interest for biomedical applications. PMSA is a zwitterionic polymer derived from molecules found in the cell membrane, making it highly biocompatible. In Chapter 3, these microgels are employed to replicate the properties of lubricin, a globule protein, with the goal to develop a biomimetic super-lubricant inspired by the synovial fluids. It is demonstrated that the unique rheological properties and low friction behavior is replicated through mixtures of microgels and hyaluronic acid, a biopolymer also found in the synovial fluids. The remainder of this dissertation discusses a direct microscopic study of the glassy dynamics of microgel suspensions using particle-tracking algorithms. In Chapter 4, it is demonstrated that tuning the elasticity of microgel particles changes the glass fragility from fragile to strong glass-forming behavior. In Chapter 5, these glassy suspensions are shown to be spatially heterogeneous and links are developed between glass fragility and dynamic heterogeneity. The growth of dynamic heterogeneity is found to be directly related to the glass fragility. In Chapter 6, deviations in particle dynamics are examined for strong glass-forming suspensions under strong spatial confinement. Interestingly, the confinement length scale is shown to be less than the length scale for hard-sphere suspensions.