Chain-length-dependent conformational transformation and melting behaviour of alkyl/oligo(oxyethylene)/alkyl triblock compounds: α-octyl-ω-octyloxyoligo(oxyethylene)s

Abstract

The chain-length-dependent conformational transformation and the melting behaviour of triblock compounds α-octyl-ω-octyloxyoligo(oxyethylene)s, H(CH₂)₈(OCH₂CH₂)_mO(CH₂)₈H (abbreviated as C₈E_mC₈) (m = 1–8), have been studied by infrared spectroscopy and differential scanning calorimetry. The compounds with m = 1–5 assume the all-trans planar form (γ-form) in the solid state, while those with m = 7 and 8 assume the planar/helical/planar form with conformational defects in the alkyl chain (β′-form). Conformational polymorphism was observed for C₈E₆C₈: the γ-form for the annealed solid and the planar/helical/planar form without conformational defects (β-form) for the unannealed solid. The conformational transformation from the planar form into the planar/helical/planar form takes place at a length of the oligo(oxyethylene) chain m = 6. This result for C₈E_mC₈ and a similar conformational transformation for C₆E_mC₆ at m = 5 (previous work) demonstrate that the conformation of the C_nE_mC_n triblock compounds in the solid state is determined by intramolecular conformational restoring force in the central oligo(oxyethylene) block, intermolecular dipole–dipole interaction of the C–O bonds and intermolecular packing force in the end alkyl blocks. The melting points of the γ-form solid of C₈E_mC₈ are much lower than the melting points of n-alkanes with similar molecular masses. The observed thermodynamic quantities show that the planar structure of the oligo(oxyethylene) chain is stabilized by the force of the magnitude that maintains the rotator phase of n-alkanes. For the β′-form solid of C₈E_mC₈, the alkyl blocks, which are partially noncrystalline, and the oligo(oxyethylene) block melt together at the melting point, unlike the β-form solid of C₆E_mC₆, for which the melting of the alkyl blocks takes place before the melting of the oligo(oxyethylene) block. The β-form solid of C₈E₆C₈ (unannealed) melts via the γ-form solid.