Temperature-Dependent Self-Assembly and Mixing Behavior of Symmetrical Single-Chain Bolaamphiphiles

Abstract

The temperature-dependent self-assembly and the mixing behavior of symmetrical single-chain bolaamphiphiles with different polymethylene chain lengths and different headgroup structures were investigated in water by differential scanning calorimetry (DSC), cryo transmission electron microscopy (cryo-TEM), and small angle neutron scattering (SANS). The even-numbered polymethylene-1,ω-bis(phosphocholines) (PC-Cn-PC) are known to form nanofibers composed of stretched molecules with an all-trans alkyl chain conformation (Drescher, S.; Meister, A.; Blume, A.; Karlsson, G.; Almgren, M.; Dobner, B. Chem.—Eur. J. 2007, 13, 5300−5307). The odd-numbered analogues were synthesized to study a possible even−odd effect of these bolaamphiphiles during their aggregation in water. In addition to these bolaamphiphiles with phosphocholine headgroups, a new series of polymethylene-1,ω-bis(phosphodimethylethanolamines) (Me2PE-Cn-Me2PE) with smaller headgroup sizes was synthesized. These bolaamphiphiles show an additional fiber−fiber transition when the alkyl chain length exceeds 26 carbon atoms. The mixing behavior of both types of bolaamphiphiles indicates that differences in the alkyl chain length up to six carbon atoms are tolerated within the fiber structure. The mixing of two Me2PE-Cn-Me2PE or PC-Cn-PC type bolaamphiphiles with different alkyl chain lengths offers the possibility to adjust the temperature, where the cross-linking of the fibers is disrupted and where the fibers break apart. As a consequence, temperature switchable hydrogels are obtained that can be fine-tuned for drug delivery applications. The comparison with dotriacontane-1,32-diyl-bis[2-(methylammonio)-ethylphosphate] (MePE-C32-MePE), a new bolaamphiphile with even smaller phosphomonomethylammonio headgroups, illustrates the importance of the headgroup size for the aggregation behavior. This bolaamphiphile self-assembles exclusively into lamellar structures, and this aggregate type persists in mixtures with the fiber forming Me2PE-C32-Me2PE.