In-Situ Real-time AFM Observation of Folded-Chain Crystallization of Single Isolated Isotactic Poly(methyl methacrylate) Chains in a Langmuir-Blodgett Monolayer at the Molecular Level

The crystallization process of a single isolated polymer chain into a folded-chain crystal (FCC) was clearly visualized at the stem level in situ and in real time by atomic force microscopy (AFM). The sample was a single Langmuir monolayer deposited on mica, in which isotactic poly(methyl methacrylate) (it-PMMA) was solubilized as isolated chains in an it-oligo(MMA) monolayer, the molecular weight of which was too small to crystallize.. The crystallization of an isolated it-PMMA chain into a FCC composed of a double-stranded helix of the single chain occurred under high humidity and was followed by AFM. The nucleation occurred at any point of the chain and the crystal grew by winding the residual part of the amorphous chain. Under the humidity condition at which the nucleation occurred frequently, multiple crystals were formed in a single chain to form a necklace crystal. Further, the growth of a crystal formed at the end of an amorphous chain was observed at the stem level. Unexpectedly, the crystal grew not only on the side where the amorphous chain was attached but also significantly on the opposite side where no amorphous chain was attached, clearly indicating that the chains significantly slipped inside the folded double-stranded-helix crystal and crystallized on the opposite side. The crystallization behavior of a single isolated chain provides new insight in understanding the polymer crystallization process.