Heterogeneous diffusion in thin polymer films as observed by high-temperature single-molecule fluorescence microscopy

Area of Science:

• Materials Science
• Polymer Physics
• Spectroscopy

Background:

• Understanding polymer dynamics near the glass transition temperature (Tg) is crucial for material processing and applications.
• Previous single-molecule spectroscopy studies were limited to temperatures below those relevant for thermoplastic processing.
• Polystyrene (PS) films with low molecular weight perylene diimide (PDI) probes were used to study dynamics.

Purpose of the Study:

• To investigate the temperature-dependent dynamics of single perylene diimide (PDI) molecules in thin polystyrene (PS) films.
• To explore polymer dynamics at temperatures up to 135 °C, relevant for thermoplastic processing.
• To understand the influence of film thickness and polymer density profiles on molecular mobility.

Main Methods:

• Utilized a custom-built single-molecule fluorescence microscopy setup capable of reaching high temperatures (up to 135 °C).
• Studied PDI molecules in thin PS films with a molecular weight below the entanglement length to ensure relaxed polymer chains.
• Analyzed single-probe molecule tracks to distinguish between mobile and immobile molecules based on track radius.
• Employed Monte Carlo random walk simulations to model and rationalize observed heterogeneities.

Main Results:

• Significant heterogeneity in the motion of single PDI probe molecules was observed near the glass transition temperature (Tg).
• PDI probes were immobile below the bulk glass transition temperature (Tg,bulk) and mobile above Tg,bulk + 40 K.
• The fraction of mobile probes in the range of 0–40 K above Tg,bulk was strongly dependent on film thickness, with thinner films showing mobility at lower temperatures.
• Simulations supported a model involving a Tg profile and surface effects due to a decreasing polymer density profile at the polymer-air interface.

Conclusions:

• High-temperature single-molecule fluorescence microscopy enables the study of polymer dynamics in a temperature range relevant to material processing.
• Heterogeneous dynamics near Tg in thin PS films are influenced by film thickness and polymer density variations.
• The observed mobility differences can be explained by a model incorporating a gradient in Tg and preferential probe localization at the film surface.