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 alpha-octyl-omega-octyloxyoligo(oxyethylene)s, H(CH2)8(OCH2CH2)mO(CH2)8H (abbreviated as C8EmC8) (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 (gamma-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 (beta'-form). Conformational polymorphism was observed for C8E6C8: the gamma-form for the annealed solid and the planar/helical/planar form without conformational defects (beta-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 C8EmC8 and a similar conformational transformation for C6EmC6 at m = 5 (previous work) demonstrate that the conformation of the CnEmCn 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 gamma-form solid of C8EmC8 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 beta'-form solid of C8EmC8, the alkyl blocks, which are partially noncrystalline, and the oligo(oxyethylene) block melt together at the melting point, unlike the beta-form solid of C6EmC6, for which the melting of the alkyl blocks takes place before the melting of the oligo(oxyethylene) block. The beta-form solid of C8E6C8 (unannealed) melts via the gamma-form solid.

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

“…The above consequences indicate that the stable molecular form of C 7 E 4 C 5 and C 8 E 4 C 4 is the γ form and the metastable form is the β form, whereas the stable form of C 9 E 4 C 3 and C 11 E 4 C 1 is the β form and the metastable form is the γ form or the γ‘ form. Although the metastable β form for C 7 E 4 C 5 and C 8 E 4 C 4 conforms to the previous results for other C n E m C n ‘ s, − ,, the metastable γ or γ‘ form with a planar oligo(oxyethylene) block is a new finding in the present work.…”

“…The values of ∆H m for C k E 4 C 12-k s are close to the value of ∆H m(ro) for n-C 25 H 52 (Figure 10). As pointed out in our previous studies, 24,25 the planar structure of the oligo(oxyethylene) block in the γ-form solid is stabilized by the force of a magnitude that maintains the rotator phase of n-alkanes. If the values of ∆H m for C k E 4 C 12-k s are lower than the value of ∆H m(ro) for n-C 25 H 52 , then the planar structure of the oligo(oxyethylene) block is not stabilized, and the β form in turn becomes more stable.…”

“…24,25,27 We have interpreted the lower melting points of the γ-form C n E m C n s as due to the higher Gibbs energy of crystal arising from intermolecular repulsive interactions, the conformational restoring force acting in the central oligo(oxyethylene) block, and the electrostatic interaction between ether oxygen atoms. 24,25 This elucidation is confirmed in this work by the fact that the values of ∆H m for the γ-form C k E 4 C 12-k s (k ) 6-8) are about 30% smaller than the value of ∆H sum for n-C 25 H 52 , and the values of ∆S m for the γ-form C k E 4 C 12-k s are only about 10% smaller than the value of ∆S sum for n-C 25 H 52 . We used here ∆H sum and ∆S sum , rather than ∆H m(ro) and ∆S m(ro) , for n-C 25 H 52 , because the stable solid of this compound melts via the rotator phase.…”