Effects of molecular geometry on the self-assembly of giant polymer-dendron conjugates in condensed state

A series of giant polymer-dendron conjugates with a dendron head and a linear polymer tail were synthesized via "click" chemistry between azide-functionalized polystyrene (PS_N, N: degree-of-polymerization) and t-butyl protected, alkyne-functionalized second generation dendron (tD), followed by a deprotection process to generate a dendron termini possessing nine carboxylic acid groups. The molecular structures were confirmed by nuclear magnetic resonance, size-exclusion chromatographic analyses, and matrix-assisted laser desorption ionization time-of-flight mass spectra. These well-defined conjugates can serve as a model system to study the effects of the molecular geometries on the self-assembly behaviour, as compared with their linear analogues. Four phase morphologies found in flexible linear diblock copolymer systems, including lamellae, bicontinuous double gyroids, hexagonal packed cylinders, and body-centred cubic packed spheres, were observed in this series of conjugates based on the results of small angle X-ray scattering and transmission electron microscopy. All of the domain sizes in these phase separated structures were around or less than 10 nm. A 'half' phase diagram was constructed based on the experimental results. The geometrical effect was found not only to enhance the immiscibility between the PS_N tail and dendron head, but also systematically shift all of the phase boundaries towards higher volume fractions of the PS_N tails, resulting in an asymmetrical phase diagram. This study may provide a pathway to the construction of ordered patterns of sub-10 nm feature size using polymer-dendron conjugates.