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46070-AC7
Design and Synthesis of Liquid Crystalline Polymer Brush Films
Daniel J. Dyer, Southern Illinois University
Our project involves the synthesis of liquid crystalline
monomers and chiral moities that will be incorporated into side chain
polymers. Furthermore, the
polymers will be synthesized by a grafting-from approach from silicon oxide
substrates. The polymer brush
films should exhibit liquid crystalline behavior, particularly the chiral
nematic, or so-called cholesteric phase.
These LC brushes are expected to respond to external stimuli, such as
the binding of a guest molecule.
Figure 1 illustrates our strategy for creating a host-guest
LCP brush film that could respond to the binding of a guest molecule. First, a SAM is deposited onto a
substrate such that the terminal group is a controlled free radical initiator
(ATRP or RAFT). This substrate is
then used to synthesize an LCP brush by the grafting-from (GF) technique, which
is capped by a short block containing a host monomer. Previously LCP brushes exhibited LC textures, which were
visible by polarized optical microscopy in films less than 40 nm. The resulting LCP brush (Figure 1b)
could be aligned to form a homogeneous planar film by shear alignment or by
surface rubbing; homeotropic alignment could be imparted by modifying the SAM
or other surface treatments.
Furthermore, a ligand could be incorporated into the brush so that it
will bind to a specific analyte, such as a metal ion or biological
molecule. Upon binding of the
guest molecule, the surface of the brush is perturbed and this causes a
realignment of the bulk film, resulting in a change in color and/or texture (Figure
1c). Since polymer chains may
respond slowly, these brush films could be swelled with low molar mass LCs to
increase the amplification of the surface perturbations.
Figure 1. Design strategy for an end-capped liquid crystalline
polymer brush sensor: (a) Controlled
free radical initiator is chemisorbed to substrate by a self-assembled
monolayer; (b) ATRP or RAFT polymerization
is initiated in the presence of an LC monomer to create an LC brush. (c) A host monomer is added to synthesize a short
block that caps the LCP brush; and (d) The guest molecule binds to the host and perturbs
the interface, the disorder is propagated to the bulk film and results in a
change in texture or color. (Note: a random copolymer could also be
synthesized where the host is distributed throughout the chain; this would be
easier than a controlled polymerization.)
The first stage of this program involves the design of
monomers based on known examples of LC side chain polymers. Specifically, we are focusing on
nematic LCPs that have transition temperatures near room temperature. These polymers will be tethered to
glass, silicon, and quartz substrates via surface initiated polymerization
techniques. In particular, we will
utilize GF free radical initiators that have been described in the literature
and previously synthesized in our laboratory (see below). The second stage of the project will
involve a thorough characterization of the LC properties of the polymer
brushes. We will align the brushes
and compare the properties of the tethered brush to that of an untethered LCP,
which is merely physisorbed to the substrate. The third stage will involve the incorporation of functional
groups (hosts) that will respond to a change in pH by altering the texture
after the LCP brush is immersed into an aqueous solution. In the fourth stage we will incorporate
chiral units into the LCP in order to create a film that reflects visible
light. The fifth stage will
examine the switching properties of these cholesteric brushes after adding
hosts specific for metal ions and the protein avidin. The response of these
films will be monitored by POM, UV-vis, FT-IR, and ellipsometry as a function
of time, temperature, and analyte concentration.
Currently we are in the initial stages of the project and
have succeeded in synthesizing a relatively large amount of our first monomer (7), which is described in Scheme 1a. The proton NMR spectrum is displayed in
Figure 2 and confirms the synthesis.
In addition, we have nearly completed the synthesis of our first chiral
monomer (10) as illustrated in Scheme
1b. We are now in the process of
synthesizing random copolymers of the two monomers and examining the LC
properties of the polymers. Our
fist experiments will be in the bulk and then we will initiate polymerizations
from quartz substrates.
Scheme 1 (a)
Synthesis of liquid crystalline monomers and (b) chiral side-chain moieties for incorporation into random copolymer
brushes.
Figure 2. 1H NMR
spectrum of acrylate monomer 7.
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