posted on 2021-05-10, 22:29authored byUtsav, Wei Bu, Binhua Lin, Rupak Banerjee
We report on the surface ordering
and crystallization sequences
in differently organic-substituted amphiphilic polyhedral silsesquioxane
(POSS) variants induced by regulated compression at the air–water
interface. Such molecular systems floating at the interface serve
as a model system to study dynamic crystallization mediated by weak
interactions. In situ grazing incidence X-ray scattering (GIXS) measurements,
performed at a synchrotron X-ray source using a liquid surface diffractometer
and corroborated with Brewster angle microscopy, revealed transformations
for the different POSS variants (viz. trisilanol isobutyl POSS (TBPOSS),
trisilanol cyclohexyl POSS (TCHPOSS), disilanol octaisobutly POSS
(DOBPOSS), and trisilanol isooctyl POSS (TOPOSS)) from a weakly correlated
monolayer structure to appreciably different structural and crystalline
phases in various packing schemes. GIXS measurements revealed a stable
nature of the crystallization of DOBPOSS, varying degrees of metastable
crystallization for TCHPOSS and TBPOSS, and complete absence of crystalline
phase in TOPOSS molecules. Incidentally, for all POSS variants showing
crystalline phases, the motifs always assembled in a triclinic lattice
with P1̅ symmetry. For the metastable crystals, preferential
surface ordering of the crystallites promotes selective crystalline
planes to exhibit preferred tilt angles with respect to the interface.
The structural transformations of the differently substituted POSS
molecules and their variations therein are attributed to the changing
balance of the hydrophobic vs hydrophilic interaction in the layers,
which is determined by the anisotropic shape and distribution of substitutional
groups over the nanosized core cage of the monomer, steric interaction
between nearest dimeric neighbors, as well as the in-plane and out-of-plane
assembly of the overlayers.