Manipulation of Athermal Nuclei in Aqueous Poly(ethylene oxide) by Scanning Activity Gravimetric Analysis

Abstract

We show that polymer solutions exhibit memory effects as they undergo repeated dissolution (deliquescence) and crystallization (efflorescence) transitions, similar to those previously observed in polymer melts. These memory effects have been probed using scanning activity gravimetric analysis (SAGA) on aqueous poly(ethylene oxide). SAGA is a new method that enables manipulation of the thermodynamic state of a polymer/solvent system under isothermal conditions. The measurements have been performed on individual micron-sized (∼10 ng) polymer particles that are levitated electrostatically and subjected to programmed changes in the activity of solvent vapor. The particle mass rapidly changes due to sorption or evaporation of solvent in response to changes in vapor activity, providing insights into the changing state of the polymer/solvent system. Repeated cycling of solvent activity within a levitated particle reveals systematic shifts up and down in the deliquescence and efflorescence activities depending upon the extreme activities reached. Melt memory effects have previously been attributed to remnants of lamellar crystals that persist beyond the nominal melting temperature. The present observations suggest that the thickness of these remnants evolves in solution, changing the supersaturation at which they may serve as athermal nuclei. An extension to the classical thermodynamic model of lamellae that includes a reduced interfacial tension at the perimeter of the high energy fold surface predicts the existence of a metastable radius that allows remnants to slowly increase or decrease in thickness under certain conditions, thereby rationalizing the ability to manipulate efflorescence behavior. We also present an analysis that suggests that SAGA can be extended to other polymer/solvent systems with the appropriate choice of temperature.