Nick Mitchell, PhD
Thesis title:
"RATIONAL DESIGN OF POLYMERS FOR SPECIFIC INTERACTIONS WITH LIQUID AND SOLID STATE WATER"
NICK'S CV
Bio
Nick grew up in Alabama. Roll Tide!!!
Etc.
Project 1: Ice Nucleation Promotion Impact on Ice Recrystallization Inhibition Activity of Polyols
Heterogenous ice nucleation occurs vis-à-vis nucleating agents already present in solution yet can occur within a rather broad range of temperature (0 to circa -38 C). Controlling this temperature and the subsequent growth of resulting ice crystals is crucial for the survival of biological organisms (certain insects, fish, plants that endure sub-zero temperatures); as well as in the context of medical cryopreservation and food science. In these environments, uncontrolled crystal shape and size can rupture the cell membrane causing irreversible and catastrophic damage. Antifreeze (AF) proteins and synthetic AF analogs address this role to restrict crystal growth and to shape ice crystals. Yet, if the nucleation temperature is not controlled and occurs in the lower temperature range, nascent ice crystals will have grown to a significantly larger size before the AF agents can be active on their surface to halt or slow the Ostwald ripening process during recrystallization. At higher nucleation temperature diffusion of AF macromolecules is enhanced, and dynamic crystal shaping can start earlier, producing smaller crystals overall. While antifreeze proteins, the inspiration for these synthetic analogs, are always applied in a salt buffer aqueous environment (most typically PBS buffer), the heterogeneous nucleation events are stochastic and occur within a wide temperature range. Silver iodide (AgI), however is a highly effective ice nucleation promoter as its crystal lattice structure is a 98% lattice match to the basal plane of hexagonal ice (Ih) crystals acting as a template for water molecule orientation and decreasing the interfacial free energy. Here, we expose the advantage of purposely seeding such nascent ice crystals with AgI ata defined and higher temperature (-7 C), in ultrapure water (UPW) such that nucleation can only come from AgI (and also in AgI/PBS), resulting in the most potent synthetic IRI observed to date (at concentrations as low as 0.001 mg.ml-1).
Project 2: Structure function relationships for synthetic cryogenic protectants
One polymer that does exhibit similar IRI, THA, and DIS potency to anti-freeze proteins is glycerol-grafted-poly(vinyl alcohol) (G-g-PVA; from our lab). While G-g-PVA has been demonstrated to show remarkable potency in these key areas, owing to the presence of a 1,2 diol in its pendant group, its widescale implementation has proven to be difficult resulting from the unreliable synthetic method utilized for its production. To this end we have developed a series of diol containing monomers that mimic the monomeric binding structure of G-g-PVA; allowing for direct synthesis of a variety of tailored and reproducible co-polymerizations. We have discovered here the critical role of pendent group tether length, through the incorporation of ethylene glycol spacers, on cooperative polymer ice-surface interaction
Project 3: Insights into the Role of End-Groups & Molecular Assembly on the phase Transition Temp. (LCST)
The effect of end group chemistry on the thermal phase transitions (critical solution temperature) of aqueous solutions of pNIPAM was studied by differential scanning calorimetry (DSC) as a function of both molecular weight and concentration. We observed that increasing the hydrophilicity of the end-group significantly increased the phase transition temperature of low molecular weight polymer (10kDa) but did not increase the transition temperature of the larger polymers (40kDa) at lower concentrations. However, the presence of two terminal, hydrophilic groups significantly increased the concentration dependence of polymers at both molecular weights. This, we found was influenced by the macromolecular self-assembly of this type of system where the resulting morphology influenced the nature of the phase transition and its concentration dependence. We found that the assembled structures exhibited a bimodal thermal transition owing to the two regions of differing polymer chain density.