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

Abstract

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.

Photoswitchable Nanomaterials Based on Hierarchically Organized Siloxane Oligomers

“…Small angle X-ray scattering (SAXS) confirmed the existence of lamellar morphology with 5.1 nm periodicity (Figure 2A). Since previous studies suggest that the length of the ODMS16 spacer is ≈2.5 nm, [30,31] this periodicity suggests that the azobenzene domain consists of a single tilted trans-4-methoxyazobenzene monolayer. Sharp scattering peaks in the wide angle region (4.97 nm −1 < q < 27.8 nm −1 ) further suggests that trans-4-methoxyazobenzene end groups are highly organized, with the peak at 3.8 Å possibly corresponding to the stacking distance between H-aggregated planar trans-azobenzene.…”

“…[30] Notably, utilization of discrete length oligomers played an important role in obtaining these ordered structures. These findings further suggest that phase segregation can facilitate monolayer crystallization of aromatic motifs within lamellar domains.…”

“…Thus, to design photoswitchable materials with well-organized molecular arrangements, we created a series of oligodimethylsiloxanes end-functionalized with azobenzene motifs (Scheme 1). Perfectly monodisperse ODMS with 8 or 16 Si atoms were synthesized using a stepwise procedure described previously by our group [30][31][32] and olefin-terminated AZO motifs were subsequently attached via platinum-catalyzed hydrosilylation.…”

“…Recently, we reported on the synthesis of a class of well-defined and discrete end-functionalized siloxane oligomers that form highly ordered multidomain nanomaterials with sub-5 nm periodicities. [30] We demonstrated that siloxane-based materials with different molecular arrangements can be obtained by balancing the effects of directional end-group association and phase segregation. In particular, we found a synergistic effect between phase segregation and end-group crystallization that resulted Photoisomerization…”

“…[30,31] Here, we utilize these self-assembly principles to direct azobenzene motifs into spatially ordered domains as well as promote a high degree of molecular organization. The resulting nanomaterials can exhibit rapid and reversible solid-liquid athermal phase transitions with UV and blue light, thus showing photoswitching between adhesive properties due to crystallization of the azobenzene and lubricant properties due to the oligodimethylsiloxane (ODMS) spacers.…”