Scanning Activity Gravimetric Analysis

Abstract

Supporting InformationThe relative humidity, or activity of water (a w ), inside the particle levitation chamber is controlled by modulating the pressure of a nitrogen stream that contains a fixed mole fraction of water vapor such that:where a w,o is the activity of water in the nitrogen stream recorded at atmospheric pressure (p o ) by a humidity sensor, and p is the absolute pressure inside the chamber recorded by a pressure transducer. Accuracy of eq S1 requires that the mole fraction of water vapor inside the chamber is identical to that at the humidity sensor. As humidity increases, however, there is an increasing likelihood that water is sorbed by surfaces within the apparatus, introducing capacitance that prevents very rapid changes in a w . The actual instrument response to imposed changes in a w was measured during continuous ( Figure S1) and stepwise ( Figure S2) scans with an aqueous sodium chloride droplet, which maintains equilibrium with the surrounding atmosphere. Discrepancy in the measured mass between continuous scans up and down in a w was used to quantify the lag and derive a correction that was applied to the PEO data in the main text. The difference in water activity between that recorded during the up scan (a w,up ) and down scan (a w,dn ) at a given m/m o for 4 < m/m o < 11 was determined from the data in Figure S1. Since the equilibration time exhibited in Figure S2 is symmetric with respect to a steps up and down in activity, we assume that the actual activity is a w = (a w,up + a w,dn )/2, and, therefore, the data requires a lag correction of ∆a w = (a w,up -a w,dn )/2; ∆a w increases with increasing a w for a w > 0.79 ( Figure S3). A linear fit to the data in Figure S3 was used to correct for the hysteresis exhibited in the continuous scan at high activity ( Figure S4). The corrected data displays no hysteresis and agrees well with the 4 equilibrium data points determined from the stepwise scan.

Humidity-Dependent Wetting Properties of High Hysteresis Surfaces

“…These results can be interpreted in light of previous studies of swelling of PEG and thin Rf-PEG films. 14,20 For 6KC8 samples, absorption of water into the film was found to increase dramatically at approximately 85% RH. Similar behavior is expected in the present polymer (6KC10), despite the difference in fluoroalkyl group (C8 vs C10): studies of 10KC8 and 10KC10 samples indicate that the midblock size, and not the endgroup size, determined the swelling characteristics of the sample.…”

“…SAGA (scanning activity gravimetrical analysis) studies of PEG have shown that at 21 °C the isothermal dissolution of crystallites in 100 kg/mol PEG occurs in response to an increase in RH above 89%. 20 The transition from a semicrystalline material with absorbed water to a concentrated solutionstermed deliquescencesoccurs over a range of activities for PEG. Water is absorbed into the amorphous regions of the solid polymer for activities up to 86% RH, and then dissolution of the crystallites occurs between 86% RH and 89% RH.…”

Crystallite dissolution in PEO-based polymers induced by water sorption

“…Apparent non-Fickian diffusion is known to be caused by polymer relaxation [98][99][100] and even penetrant reaction 1,56 , but in PEO and PS-b-PEO the non-Fickian behavior during sorption appears to be due to crystallite dissolution. We are aware of only one other report of PEO crystallite dissolution by water sorption inferred from gravimetric measurements on exceptionally small samples 101 . Our results are in agreement with this report.…”

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

“…A parametric plot of the recorded mass as a function of a w clearly illustrates hysteresis between dissolution and crystallization (Figure 4a): the activity of water required to dissolve the crystallites during the upward scan was significantly greater than the activity of water at which nucleation and growth of crystallites occurred during the downward scan. Since the solution-vapor isotherm represents a state of zero (undetectable) crystallinity, it is useful to transform the relative mass into an index (I c ) that increases from 0 in the fully dissolved solution to 1 as crystallinity increases: 19 where m 0 is the mass of the dry PEO particle, m(a w ) is the actual (a) A single charged particle is levitated using electrostatic forces that can be dynamically adjusted using an electrode cage to hold the particle at the geometric center in the environmental control chamber that imposes a programmed history of conditions of water activity (relative humidity) and temperature. A laser beam (532 nm, 25 mW) is incident on the particle, and the scattered intensity is observed over a selected angular range.…”

“…[16][17][18] We previously introduced scanning activity gravimetric analysis (SAGA) as a novel technique for investigating polymer crystallization from concentrated polymer solution under isothermal conditions. 19 This paper reports memory effects observed by SAGA in aqueous poly(ethylene oxide) (PEO). The present results suggest that small remnants of lamellae persist beyond the apparent dissolution point.…”