Single-Molecule Tracking in Poly(Ethylene Oxide) Films: Revealing the Effects of Molecular Weight, Network Plasticization, and Thermal Annealing on Anionic Dye Diffusion

Abstract

This paper reports single-molecule tracking (SMT) measurements of the diffusion behaviors of individual, anionic sulforhodamine B (SRB) dye molecules in a series of poly­(ethylene oxide) (PEO) films, aimed at clarifying the influences of the molecular weight, network plasticization, and thermal annealing on such dynamics. Micrometer-thick PEO films were prepared by drop-casting from its aqueous (0.2%, 1 nM SRB) solution, followed by drying in air and thermal annealing at 90 °C for 36 h. The diffusion of individual SRB occurring within the amorphous domains was recorded at different relative humidities (5–95%) to characterize the microscale domains’ local aspect-ratio, orientation, and molecular permeability at high spatial resolution. The results revealed the involvement of crystalline phases in confining SRB diffusion to submicron distances and guiding longer-range diffusion along one-dimensional-like amorphous morphologies. Upon annealing, amorphous domains were wider, more continuous, and more permeable to SRB probes. The enhanced transport in plasticized PEO, as reflected by the higher SRB mobility and diffusivity, was linked to the polymer’s higher chain and segmental mobilities and reduced hydrogen-bonding interactions. This work has demonstrated the usefulness of SMT for an advanced characterization of solid polymer electrolytic films, highly beneficial for the development of safer lithium-ion batteries.

Solvent-Induced Swelling Behaviors of Microphase-Separated Polystyrene-block-Poly(ethylene oxide) Thin Films Investigated Using In Situ Spectroscopic Ellipsometry and Single-Molecule Fluorescence Microscopy

“…PEO (3.8 k) films were dissolved under water vapor (data not shown). 68 In contrast, PS (15 k) films exhibited negligible thickness change under water vapor, as expected from their poor compatibility (Figures 2c and 3c and Table 2). Of note, the maximum thickness gain of PS-b-PEO films (PEO volume fraction ≈22%) was 6−9%, much smaller than that expected from the result of PEO films (140%) (Figure 3abd and Table 2).…”

“…Indeed, PEO (12 k) films exhibited a gradual thickness increase upon exposure to water vapor (Figure d), even though changes in their ellipsometric thickness and n were not reversible owing to partial dissolution under water vapor. PEO (3.8 k) films were dissolved under water vapor (data not shown) . In contrast, PS (15 k) films exhibited negligible thickness change under water vapor, as expected from their poor compatibility (Figures c and c and Table ).…”

“…PEO films exhibited the enhancement of SRB diffusion under EtOH vapor but did not clearly show the subsequent reduction of SRB diffusion in contrast to the PS-b-PEO films (Figure 7ab). The D SMT values obtained from the PEO films were smaller than those from the PS-b-PEO films, possibly reflecting their higher crystallinity 68 7b), which could reflect the larger contributions of PS−PEO interfacial region having lower solute diffusivity owing to the larger interface/volume ratio in the former. 22,25 The diffusion of SRB molecules in PS-b-PEO and PEO films was enhanced by the longer duration of water vapor exposure, as shown by trajectory data acquired at 65% RH and 80% RH (Figure 6), which corresponded to exposure times of 3 and 7 min in Figure 2ab.…”

“…On the other hand, PS-b-PEO films showed a much smaller change in E under water vapor, as with PS films and in contrast to PEO films, in spite of the high water−PEO compatibility (Figures S6 and4b). 68 NR fluorescence at 580 nm in water vapor-exposed PS-b-PEO films was as intense as that in PS-b-PEO films under N 2 (Figure S6ab). These observations support the preferential distribution of NR to PS microdomains in the PS-b-PEO films.…”

Impact of Chain Conformation on Structural Heterogeneity in Polymer Network

“…Inducing solvent vapor swelling (SVS) of thin films in combination with single-molecule tracking (SMT) affords an opportunity to investigate the network structure of polymers in crowded environments. Previously, the heterogeneous dynamics of small-molecular dyes in water−vapor−swollen films of poly(vinylpyrrolidone) 53 and poly(ethylene oxide) 54 have been studied. However, because of technical difficulties, it is still challenging to unravel the essential structural characteristics of a solvent−swollen film, including critical length scales such as the tube diameter and correlation length, which obscure the physical origin of network heterogeneity.…”