Conformational and Thermal Phase Behavior of Oligomethylene Chains Constrained by Carbohydrate Hydrogen-Bond Networks

Abstract

The oligomethylene spacer conformation and phase transition behavior of a series of N,N‘-bis(β-d-glucopyranosyl)alkane-1,n-dicarboxamides [1(n), n = 6−14, 16, 18, and 20] in a self-assembled solid state has been investigated using DSC, XRD, light microscopy, and FT-IR measurements. Both the chain conformation and thermal behavior of the assemblies have been used to classify them into three categories, short (6 ≤ n ≤ 8, and n = 10), medium (n = 9, and 11 ≤ n ≤ 13), and long chain (14 ≤ n ≤ 20). Thus, the bolaamphiphiles with a short-chain spacer self-assembled in water to form a gauche-including monolayered self-assembly, while the medium- and long-chain homologues formed monolayered assemblies with an all-trans oligomethylene conformation. The short- and long-chain bolaamphiphiles also exhibited a polymorphism on heating which proceeded with retention of the gauche chain conformation, with the long-chain homologues transforming from the K1 crystalline phase to the K2 crystalline phase (keeping an all-trans conformation) and eventually into the K3 crystalline phase with a gauche-including spacer. In addition, the long-chain, 1(n), and medium-chain, 1(13), bolaamphiphiles formed a thermotropic mesophase (smectic A).