Kenta Iwashima, Kenji Watanabe, and Jiro Kumaki
Department of Organic Materials Science, Yamagata University, Japan
Polymer monolayers spread on a water surface transform from an isolated chain to amorphous, then to a crystalline state upon compression, which can be transferred at each stage onto a substrate for observation by atomic force microcopy (AFM) . Previously, we successfully observed a folded chain crystal (FCC) of an isotactic poly(methyl methacrylate) (it-PMMA) , its melting behavior in situ at high temperature , and crystallization of single isolated chains  at a molecular level by AFM.
In contrast to the it-PMMA, polylactide (PLA) crystallizes into an extended chain crystal (ECC) on a water surface . Since the width of the crystal corresponds to the molecular weight (Mw), the chain packing in the crystal can be specifically identified. We studied the crystallization of a mixture of high and low Mw PLAs and found that the high Mw PLA first crystallized, followed by the low Mw PLA crystallization, indicating that the molecular weight recognition occurred during the crystallization process. We also studied the crystallization of a high Mw PLA monolayer highly diluted by a PLA oligomer, the Mw of which was too low to crystallize. As the crystallization of the high Mw PLA to ECC was disturbed, upon compression, FCC first formed, which then aggregated to transform into ECC; the transformation from FCC to ECC was clearly visualized. Compression of a D, L-PLA mixture forms a stereocomplex on a water surface . There is a controversy whether D- and L- PLA pack in a stereocomplex in a parallel or anti-parallel arrangement. We studied the crystallization of linear and cyclic stereoblock copolymers with the parallel and anti-parallel arrangements for the D- and L-PLA block segments , and found that the parallel arrangement more stably formed in the sterecomplex.
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