End Groups of Functionalized Siloxane Oligomers Direct Block-Copolymeric or Liquid-Crystalline Self-Assembly Behavior

Monodisperse
oligodimethylsiloxanes end-functionalized with the hydrogen-bonding
ureidopyrimidinone (UPy) motif undergo phase separation between their
aromatic end groups and dimethylsiloxane midblocks to form ordered
nanostructures with domain spacings of <5 nm. The self-assembly
behavior of these well-defined oligomers resembles that of high degree
of polymerization (N)–high block interaction
parameter (χ) linear diblock copolymers despite their small
size. Specifically, the phase morphology varies from lamellar to hexagonal
to body-centered cubic with increasing asymmetry in molecular volume
fraction. Mixing molecules with different molecular weights to give
dispersity >1.13 results in disorder, showing importance of molecular
monodispersity for ultrasmall ordered phase separation. In contrast,
oligodimethylsiloxanes end-functionalized with an O-benzylated UPy
derivative self-assemble into lamellar nanostructures regardless of
volume fraction because of the strong preference of the end groups
to aggregate in a planar geometry. Thus, these molecules display more
classically liquid-crystalline self-assembly behavior where the lamellar
bilayer thickness is determined by the siloxane midblock. Here the
lamellar nanostructure is tolerant to molecular polydispersity. We
show the importance of end groups in high χ–low N block molecules, where block-copolymer-like self-assembly
in our UPy-functionalized oligodimethylsiloxanes relies upon the dominance
of phase separation effects over directional end group aggregation.