Molecular Conformation and Melting Behavior of Alkyl/Oligo(oxyethylene)/Alkyl Triblock Position Isomers: Effect of the Position of an Oligo(oxyethylene) Block

The molecular conformation and melting behavior of triblock position isomers H(CH 2 ) k (OCH 2 CH 2 ) 4 O(CH 2 ) 12-k H (abbreviated as C k E 4 C 12-k ) with k = 6-11 have been studied by infrared spectroscopy and differential scanning calorimetry (DSC), with focus on the effect of the position of an oligo(oxyethylene) block in the molecule. The analysis of infrared spectra has revealed that the stable molecular form changes from a fully planar structure (y form) to a planar/helical/planar structure (β form) with a change of the position of the tetrakis-(oxyethylene) block from the center to the end of the molecule. The DSC measurements have shown that the melting points of the γ-form solid decrease and the melting points of the /?-form solid increase with a shift of the tetrakis(oxyethylene) block toward the terminal of the molecule. The stabilities of the two molecular forms change over between k = 8 and 9. C 8 E 4 C 4 and C 9 E 4 C 3 exhibit contrasting conformational behavior with temperature; when the temperature is increased, the metastable β form of C 8 E 4 C 4 transforms into the stable y form, while the metastable y form of C 9 E 4 C 3 transforms into the stable β form. The metastable y form with a planar oligo(oxyethylene) block is a new finding in the present work. The experimental results of the stabilities of molecular forms are explained by the relative stabilities of partial crystal lattices formed by the alkyl and oligo(oxyethylene) blocks.