Investigating polymer crystallization using single molecule tracking and Förster resonance energy transfer

Despite the importance of understanding semicrystalline polymers, a consensus on the mechanism of polymer crystallization remains elusive. We implement single molecule tracking and Förster resonance energy transfer (FRET) to investigate this process. FRET can be used to determine the distance between a donor and an acceptor, on the scale of 1-10 nm. Single molecule FRET (SM-FRET) enables the identification of individual molecular conformations and intermediate states often undetectable in ensemble studies. Using a model 5 kg/mol poly(ethylene glycol) system labeled with FRET donor and acceptor dyes at either end of the polymer chain, we apply SM-FRET microscopy to explore polymer crystallization. The significant variation in FRET signals from labeled PEG chains, based on their conformation, enables a distinction of a variety of states. Utilizing SM-FRET, we demonstrate different conformations of PEG chains within crystals grown isothermally at different temperatures. Crystals grown with a small degree of subcooling displayed fully extended conformations and near-zero FRET efficiency, while crystals grown with a slightly higher degree of subcooling had singly folded chains with a large FRET efficiency, in addition to some fully extended chains, likely due to lamellar thickening. Additionally, we used SM-FRET to shed light on the dynamic behavior of single chains during diffusion and adsorption. This approach has the potential to directly monitor single chains during crystallization.