This work aims to demonstrate a facile method for the
controlled
orientation of nanostructures of block copolymer (BCP) thin films.
A simple diblock copolymer system, polystyrene-block-polydimethylsiloxane (PS-b-PDMS), is chosen to
demonstrate vacuum-driven orientation for solving the notorious low-surface-energy
problem of silicon-based BCP nanopatterning. By taking advantage of
the pressure dependence of the surface tension of polymeric materials,
a neutral air surface for the PS-b-PDMS thin film
can be formed under a high vacuum degree (∼10–4 Pa), allowing the formation of the film-spanning perpendicular cylinders
and lamellae upon thermal annealing. In contrast to perpendicular
lamellae, a long-range lateral order for forming perpendicular cylinders
can be efficiently achieved through the self-alignment mechanism for
induced ordering from the top and bottom of the free-standing thin
film.
Herein, this work aims to directly visualize the morphological
evolution of the controlled self-assembly of star-block polystyrene-block-polydimethylsiloxane (PS-b-PDMS)
thin films via in situ transmission electron microscopy (TEM) observations.
With an environmental chip, possessing a built-in metal wire-based
microheater fabricated by the microelectromechanical system (MEMS)
technique, in situ TEM observations can be conducted under low-dose
conditions to investigate the development of film-spanning perpendicular
cylinders in the block copolymer (BCP) thin films via a self-alignment
process. Owing to the free-standing condition, a symmetric condition
of the BCP thin films can be formed for thermal annealing under vacuum
with neutral air surface, whereas an asymmetric condition can be formed
by an air plasma treatment on one side of the thin film that creates
an end-capped neutral layer. A systematic comparison of the time-resolved
self-alignment process in the symmetric and asymmetric conditions
can be carried out, giving comprehensive insights for the self-alignment
process via the nucleation and growth mechanism.
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