pressure dependence of the conformational transition of linear chain compounds

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The oxyethylene chain usually takes a helical conformation in the solid state. On the other hand, a planar zigzag modification of the oxyethylene block is found in a series of alkyl-oxyethylene block oligomers (CィイD2nィエD2EィイD2mィエD2, CィイD2nィエD2EィイD2mィエD2CィイD2nィエD2) in the solid state. The planar zigzag structure of the oxyethylene block is stabilized by cohesive force of the crystal lattice of the planar zigzag alkyl block(s). Since external pressure influences the molecular conformation as a similar manner of the intermolecular cohesive force, we have tried to observe a pressure-induced crystallization and conformational transition of diblock oligdmers CィイD210ィエD2EィイD2mィエD2 by using high pressure.Pressure induced crystallization and conformation transition were studied by infrared spectroscopy equipped with a high-pressure diamond anvil cell. Calcium fluoride was selected as a pressure-transmitting medium. Ruby fluorescence was used to calibrate the in-situ pressure.The following results were obtained in this study :1. The molecular conformation of the CィイD210ィエD2EィイD2mィエD2 compounds depends on the block length. While the oxyethylene block assumes the planar zigzag conformation for m = 1-4, the oxyethylene block assumes the helical conformation for m = 7 and 8.2. In case of m = 5 and 6, the conformation of the oxyethylene block depends on the rate of compression.The planar zigzag conformation was obtained by a rate more than ca. 0.5 GPa/sec. On the other hand, the helical conformation was obtained by a rate less than 1 GPa/min. or by annealing the solid.3. Pressure-induced amorphization was observed for the solid of CィイD210ィエD2EィイD2mィエD2 having the helical oxyethylene block.