Molecular Aggreagtion Structure and Molecular Motion of Ultrathin Polymer Films

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Molecular aggregation structure of crystalline polymers depends on their chemical structure as well as crystallization conditions. In the case of crystalline polymers containing polar groups such as polyesters and nylons, the van der Waals interaction, the dipole-dipole interaction, the static electric interaction, and the hydrogen bonding are induced between the polymer chains. These inter-chain interactions strongly influence chain conformation, crystal structure, chain folding structure on the lamellar surface, and lamellar stacking structure. It is considered that these polar groups as structural defects in chains stay rather at the chain-folding surface than in the crystal. However, the detail of spatial distribution of the polar group in the higher-order structure has not experimentally been revealed yet. Clarification of the relationship between the molecular aggregation structure and the chemical structure of aliphatic polyesters provides significant information regarding molec … More ular design of new biodegradable polymers controlled their structural and physical properties.In this study, molecular aggregation structure in thin films of linear aliphatic polyesters containing long methylene and short fluoromethylene sequences was investigated by atomic force microscopy (AFM), polarized optical microscopy (POM), attenuated total reflection (ATR) FTIR spectroscopy, and static contact angle measurements.Linear aliphatic polyesters [O(CH2)iOCO(CF2)jCO]n (i=12, 22, j=2,8,10 : iH-jF) used as samples were synthesized by melt-polycondensation of short perfluorinated dicarboxylic acids and long α-ω alkane diols. The weight-averaged molecular weight, Mw, of the polyesters was ca. 15,000. Thin films of the polyesters were prepared by the solvent-casting method, and then were isothermally crystallized from the melt. Thickness of the films was ca. 1.5 micrometers.For all the polyesters, negative spherulites were observed in their POM images. The extinction ring was also observed for the 22H-series polyesters, which indicated that chain orientation was periodically changed from the center of a spherulite. Based on the surface morphology observed by AFM, it was suggested that crystalline lamellae were formed for all the polyesters. From ATR-FTIR spectral measurements, it was revealed that methylene sequences of the polyesters became conformationally order with increasing the number of methylene groups. The methylene sequences, on the contrary, became disorder containing the gauche bonds with increasing the number of fluoromethylene groups. The inter-chain interactions around the CO groups became weaker with increasing the ratio of the number of methylene groups to that of fluoromethylene groups. The surface free energy of the films estimated from the static contact angle data implied that the fluoromethylene sequences were relatively concentrated in the surface region compared with the methylene sequences. Less