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Download fileUsing Deposition Rate and Substrate Temperature to Manipulate Liquid Crystal-Like Order in a Vapor-Deposited Hexagonal Columnar Glass
journal contribution
posted on 2021-03-08, 16:05 authored by Camille Bishop, Zhenxuan Chen, Michael F. Toney, Harald Bock, Lian Yu, M. D. EdigerWe
investigate vapor-deposited glasses of a phenanthroperylene
ester, known to form an equilibrium hexagonal columnar phase, and
show that liquid crystal-like order can be manipulated by the choice
of deposition rate and substrate temperature during deposition. We
find that rate–temperature superposition (RTS)the equivalence
of lowering the deposition rate and increasing the substrate temperaturecan
be used to predict and control the molecular orientation in vapor-deposited
glasses over a wide range of substrate temperatures (0.75–1.0 Tg). This work extends RTS to a new structural
motif, hexagonal columnar liquid crystal order, which is being explored
for organic electronic applications. By several metrics, including
the apparent average face-to-face nearest-neighbor distance, physical
vapor deposition (PVD) glasses of the phenanthroperylene ester are
as ordered as the glass prepared by cooling the equilibrium liquid
crystal. By other measures, the PVD glasses are less ordered than
the cooled liquid crystal. We explain the difference in the maximum
attainable order with the existence of a gradient in molecular mobility
at the free surface of a liquid crystal and its impact upon different
mechanisms of structural rearrangement. This free surface equilibration
mechanism explains the success of the RTS principle and provides guidance
regarding the types of order most readily enhanced by vapor deposition.
This work extends the applicability of RTS to include molecular systems
with a diverse range of higher-order liquid-crystalline morphologies
that could be useful for new organic electronic applications.