We investigate the local viscosity of a polymer glass around its glass transition temperature using environment-sensitive fluorescent molecular rotors embedded in the polymer matrix. The rotors' fluorescence depends on the local viscosity, and measuring the fluorescence intensity and lifetime of the probe therefore allows to measure the local free volume in the polymer glass when going through the glass transition. This also allows us to study the local viscosity and free volume when the polymer film is put under an external stress. We find that the film does not flow homogeneously, but undergoes shear banding that is visible as a spatially varying free volume and viscosity.
Environmentally sensitive molecular rotors are widely used to probe the local molecular environment in e.g. polymer solutions, polymer glasses, and biological systems. These applications make it important to understand its fluorescence properties in the vicinity of a solid surface, since fluorescence microscopy generically employs cover slides, and measurements are often done in its immediate vicinity. Here, we use a confocal microscope to investigate the fluorescence of (4-daspi) in glycerol/water solutions close to the interface using hydrophilic or hydrophobic cover slips. Despite the dye’s high solubility in water, the observed lengthening of the fluorescence lifetime close to the hydrophobic surface, implies a surprising affinity of the dye with the surface. Because the homogeneous solution and the refractive index mismatch reduces the optical sectioning power of the microscope, we quantify the affinity with the help of a simple model of the signal vs. depth of focus, exhibiting surface and bulk contributions. The model reduces artefacts due to refractive index mismatch, as supported by Monte Carlo ray tracing simulations.
Emulsions often act as carriers for water-insoluble solutes that are delivered to a specific target. The molecular transport of solutes in emulsions can be facilitated by surfactants and is often limited by diffusion through the continuous phase. We here investigate this transport on a molecular scale by using a lipophilic molecular rotor as a proxy for solutes. Using fluorescence lifetime microscopy we track the transport of these molecules from the continuous phase toward the dispersed phase in polydisperse oil-in-water emulsions. We show that this transport comprises two time scales, which vary significantly with droplet size and surfactant concentration, and, depending on the type of surfactant used, can be limited either by transport across the oil–water interface or by diffusion through the continuous phase. By studying the time-resolved fluorescence of the fluorophore, accompanied by molecular dynamics simulations, we demonstrate how the rate of transport observed on a macroscopic scale can be explained in terms of the local environment that the probe molecules are exposed to.
Pea protein isolate (Pisum sativum L., PPI) has been much studied in the last decade because of its potential as a bio-based alternative for surfactants to produce innovative and environmentally friendly emulsion products. PPI is ideal due to its favorable nutritional properties, low allergenicity and low environmental impact. Despite its growing popularity, understanding the stabilisation mechanism of emulsions stabilized with PPI remains a key question that requires further investigation. Here, we use fluorescence lifetime microscopy with molecular rotors as local probes for interfacial viscosity of PPI stabilized emulsions. The fluorescence lifetime correlates to the local viscosity at the oil-water interface allowing us to probe the proteins at the interfacial region. We find that the measured interfacial viscosity is strongly pH-dependent, an observation that can be directly related to PPI aggregation and PPI reconformation. By means of molecular rotor measurements we can link the local viscosity of the PPI particles at the interface to the Pickering-like stabilisation mechanism. Finally, this can be compared to the local viscosity of PPI solutions at different pH conditions, showing the importance of the PPI treatment prior to emulsification.
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