Chain-Length-Dependent Conformational Transformation and Melting Behavior of Alkyl/Oligo(oxyethylene)/Alkyl Triblock Compounds:  α-Hexyl-ω-hexyloxyoligo(oxyethylene)s

Abstract

The chain-length-dependent conformational transformation and melting behavior of triblock compounds α-hexyl-ω-hexyloxyoligo(oxyethylene)s, H(CH2)6(OCH2CH2) m O(CH2)6H (abbreviated as C6E m C6s) (m = 1−7), have been studied by differential scanning calorimetry and infrared spectroscopy. With an increase in the number of oxyethylene units (m), the molecular form of the triblock compounds in the solid state changes from an all-trans planar form (γ form) to a planar/helical/planar triblock form (β form) at m = 5. The melting points of the γ form C6E m C6s are much lower than the melting points of n-alkanes with similar molecular masses. This result is interpreted as due to the higher Gibbs energy of crystal for C6E m C6s. The observed thermodynamic quantities indicate 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. The β-form C6E m C6s melt stepwise through a solid−solid transition, at which the melting of only the alkyl blocks of the molecule occurs. The alkyl chains in the β-form C6E m C6s are loosely packed in the crystal even below the solid−solid transition temperature, while the crystallinity of the oligo(oxyethylene) block is significantly high. The molecular form of the alkyl/oligo(oxyethylene)/alkyl triblock compounds in the solid state is determined by the balance of the intramolecular conformational restoring force in the central oligo(oxyethylene) block and the intermolecular packing force in the end alkyl blocks. The conformational restoring force results from a tendency of the oligo(oxyethylene) chain to resume its intrinsic helical structure.

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

“…Accordingly, the conformational transformation is anticipated for C 8 E m C 8 to occur at a value of m larger than 5, at which the transformation occurs for C 6 E m C 6 . 16 The conformational transformation for C 8 E m C 8 actually takes place at m ¼ 6 as expected (Table 1). Although this shoulder is assignable to a solid-solid transition, we consider the composite peak for these compounds as a single endothermic peak associated with the melting, as we could not separate the shoulder from the main peak even at a slower rate of heating of 0.3 K min À1 .…”

“…The previous study 16 has indicated that the conformation of the C n E m C n compounds in the solid state is closely related to the length of the oligo(oxyethylene) block relative to the length of the alkyl block in the molecule and that the planar oligo (oxyethylene) structure becomes less stable with increasing fraction of the oligo(oxyethylene) block. Accordingly, the conformational transformation is anticipated for C 8 E m C 8 to occur at a value of m larger than 5, at which the transformation occurs for C 6 E m C 6 .…”

“…The dependencies on the relative molecular mass (M r ) of the melting point (T m ) and the solid-solid transition temperature (T t ) of C 8 E m C 8 are shown in Fig. 4, where T m of C 6 E m C 6 16 and the melting point of the rotator phase (T m(ro) ) and the crystalline-rotator phase transition temperature (T t(cry-ro) ) of n-alkanes [22][23][24] are also shown for comparison. Note that, although n-C 16 H 34 (M r 226.4), n-C 18 H 38 (M r 254.5) and n-C 20 H 42 (M r 282.5) do not experience the rotator phase before melting unlike other nalkanes used for comparison, we will still use, for the sake of convenience, the notation of T m(ro) to mean their melting point in Fig.…”

“…13 Our recent thermal and infrared spectroscopic study on C 6 E m C 6 (m ¼ 1-7) with high purity (499.7%) has shown the relationship between the molecular conformation and the thermal behaviour, which depend on the number of oxyethylene units. 16 The relevant results are as follows: (1) the melting points of the g-form C n E m C n are significantly lower than those of the corresponding n-alkanes despite the same molecular form; (2) the planar structure of the central oligo(oxyethylene) chain in the g-form is stabilized by the force of the magnitude that maintains the rotator phase of n-alkanes; (3) the melting points of the g-form C 6 E m C 6 exhibit the odd-even alternation with respect to the number of oxyethylene units (m); (4) the bform C 6 E m C 6 melt via a partially melted phase where the alkyl blocks are melted; and (5) conformational polymorphism takes place for C 6 E 5 C 6 depending on solidification conditions.…”

“…In the present work, we have extended the conformational and thermal studies of triblock compounds to C 8 E m C 8 (m ¼ 1-8), the compounds with longer alkyl chains than in C 6 E m C 6 , 16 aiming at acquiring a comprehensive understanding of chain-length-dependent conformational transformation and melting behaviour of alkyl/oligo(oxyethylene)/alkyl triblock compounds. We used the samples of C 8 E m C 8 with high purity of 99.49-99.96% to perform precise measurements of infrared spectroscopy and DSC.…”