The correlation between thermally induced precipitate‐to‐coacervate transition and glass transition in a polyelectrolyte‐bolaamphiphile complex

Abstract

The precipitate and the coacervate are two aggregated states in the polyelectrolyte complexes (PECs). The precipitate‐to‐coacervate transition and glass transition in PECs have been widely reported in the past. In many cases, the two phenomena are studied independently, although both of them are apparently affected by water and small ions. Here, utilizing a PEC system consisting of poly(acrylic acid) (PAA) and a cationic bolaamphiphile (DBON), we explore the states of PECs as a function of salt, temperature, and the molecular weight of PAAs. By a combination of microscopic observation, time‐resolved fluorescence measurements, and differential scanning calorimetry, we identify salt/temperature driven precipitate‐to‐coacervate transitions of the complexes. The thermally induced morphology transformation from the precipitate to coacervate occurs around the glass transition temperature, indicating a strong correlation between the two processes. As the molecular weight of the PAA increases, the thermal transition temperature becomes higher. This finding offers new insights on the mechanistic interactions that dictate the aggregated states of PECs. Based on the photothermal effect of DBON, we also develop a UV light‐induced strategy to mediate the precipitate‐to‐coacervate transition, providing a fantastic platform to create functional PEC materials.

Coacervate or precipitate? Formation of non-equilibrium microstructures in coacervate emulsions

“…These flow-induced flocs can coexist with coacervate droplets. Based on the microstructural criterion for differentiating coacervate and precipitate morphologies, 57–65 the flow-induced transition from droplets to amorphous aggregates indicates a transition from coacervate to precipitate initiated entirely by flow. This result constitutes the first demonstration of modulating the PEC condensate morphology using a non-chemical handle: prior works assume chemistry or component composition are the primary driver for initiating the coacervate-precipitate transition.…”

“…We have observed flow-induced, floc-like, micron-scale PEC aggregates in the PAA–PAH system over a wide range of compositions. Using classifications established by a consensus of prior literature based on polyelectrolyte complex structure, 57–65 one could call these “precipitates,” which might imply that they are composed of microscopic solids or have solid-like rheology at equilibrium. 20–22,51–56 Instead, we find that “precipitates” of PAA–PAH comprise aggregates of viscoelastic liquid-like colloidal PEC coacervate droplets, and they coexist with smaller spherical droplets.…”

“…20–22,56 Their microstructure forms an aggregated or “flaky” morphology at the microscale. 57–65 By contrast, PECs classified as coacervates exhibit viscoelastic liquid-like rheology, which accompanies an aggregate-free droplet morphology. Current experimental literature distinguishes precipitate PECs from coacervate PECs using microscopy and rheological measurement as equally valid methods.…”

“…Current experimental literature distinguishes precipitate PECs from coacervate PECs using microscopy and rheological measurement as equally valid methods. 60–65 The implicit assumption that enables this equivalence is that qualitatively distinct rheological behavior uniquely determines PEC morphology. This is an assumption we aim to test herein.…”