Scanning Activity Gravimetric Analysis (SAGA) of Aqueous Polyethylene Oxide

Citation

Olsen, Adam Paul (2006) Scanning Activity Gravimetric Analysis (SAGA) of Aqueous Polyethylene Oxide. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/5X91-2F81. https://resolver.caltech.edu/CaltechETD:etd-05262006-133416

Abstract

This thesis introduces a new technique—scanning activity gravimetric analysis (SAGA)—for investigating phase transitions in semicrystalline polymers. Isothermal growth and dissolution of polymer crystallites within picogram to milligram samples are manifested by mass changes in response to changes in the activity of sorbed solvent vapor. Single charged particles are levitated and weighed in an electrostatic field, providing access to highly supersaturated states. Phase transitions are inferred from simultaneous equilibrium sorption and light scattering measurements. Analogous to differential scanning calorimetry, scanning solvent activity up and down exposes broad transitions between the semicrystalline solid state and the dissolved state, which are influenced by sample history. We demonstrate dissolution and crystallization of nanogram samples of polyethylene oxide by controlling the activity of sorbed water vapor and observe self-nucleation of crystallites from partially states and fully dissolved states.

Memory effects in semicrystalline polymers have been documented for a variety of systems in which the rate of crystallization depends on the temperature and duration of a prior melting step. We report analogous observations of memory effects in aqueous solution droplets of polyethylene oxide. Remnants of the crystalline phase—clusters of chain folded molecules—that are too small to be detected by gravimetric or optical means persist for long times in solution after macroscopic crystals have been dissolved, and they then serve as athermal nuclei when the concentration of the polymer solution droplet is increased. These remnants evolve under certain conditions toward increasing or decreasing thickness, as indicated by a shift in the conditions at which they effectively catalyze crystal growth. Although memory effects in semicrystalline polymers have usually been attributed exclusively to sluggish kinetics of melting or dissolution, thermodynamic considerations may also play an important role. A simple model describing the free energy of a lamellar crystal based on insights of Janeschitz-Kriegl captures the metastability of lamellar remnants and their evolution in thickness. The qualitative successes of the model lend support to the thermodynamic rationalization of memory effects in semicrystalline polymers.

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