Structural changes in liquid crystal polymer vesicles induced by temperature variation and magnetic fields

Abstract

The structural changes in liquid crystal (LC) polymer vesicles triggered by changes in temperature and the application of a magnetic field were investigated by SANS, cryo-TEM, SEM and high sensitivity DSC. PEG-b-PA444 and PEG-b-PMAazo444, two block copolymers with side-on nematic hydrophobic blocks, were used to form vesicles with a radius of 450–500 nm and bilayer membrane thickness of 10–15 nm at room temperature. Upon heating the membrane thickness, d, started to increase dramatically from a temperature (∼55 °C) above T_g but below T_NI of the LC polymer block, and reached up to 120 nm at T > T_NI. The vesicles transformed into thick-walled capsules. The thickness of the membrane was inconsistent with a bilayer structure and surprisingly the capsules were stable even for temperatures above T_NI. As the PEG chains should partially dehydrate with temperature, we propose that the membrane reorganized into a structure consisting of microphase separated LC and PEG domains. Analysis of changes in structural parameters such as the internal aqueous volume and the polymer membrane volume suggest that capsule scission and fusion also occurred during this transition. The polymersomes were also sensitive to applied magnetic fields and for some temperatures the application of a strong magnetic field (1.4 T) substantially increased the membrane thickness due to the induced mesogen alignment.