Side-Chain Liquid Crystalline Polymers (SCLCPs)
So...here's my work in a nutshell.
Side-chain liquid crystalline polymers (SCLCPs) are a relatively new class
of polymeric materials. These polymers consist of some type of liquid crystalline
molecule (called a mesogen) attached to the polymer backbone though a spacer
chain that is typically a few carbon atoms long (Scheme 1). The possible
uses for these materials take advantage of their physical properties. Among
the potential uses are: non-linear optical devices (fiber optics), optical
data storage, ferroelectric organic compounds, and the like.
Scheme 1. Schematic diagram of SCLCPs.
First, some background on SCLCPs. The type of SCLCPs that we are examining
are known as thermotropic liquid crystalline polymers. That is, they exhibit
their properties when heated. Unlike most organic compounds which melt
from an ordered crystalline solid directly to a completely disordered liquid,
SCLCPs undergo several phase changes between the crystalline solid and
liquid states. These "between" phases are known as mesophases. The molecular
arrangements in these mesophases varies with the type of mesophase. The
mesophases are more ordered than the liquid state, but they flow, unlike
the crystalline state. The variety of ordering types of these mesophases
is shown in Scheme 2.
Scheme 2. Diagrammatic Representation of Liquid Crystalline Mesophases.
Polymer backbone has been eliminated for clarity--only mesogens are shown.
Temperature decreases from left to right.
For several reasons, the mesophase labeled "smectic C" is particularly
interesting to us. Most of these reasons center around its usefulness in
the applications named previously. These applications depend on the mesogen's
ability to change alignment. In the compounds we are examining, we are
interested in trasitions from the nematic mesophase to the smectic mesophases.
Notice that the difference between the nematic mesophase and the smectic
C mesophase is that the smectic C mesophase has layered ordering in addition
to the ordering of the mesogens in the nematic phase.
So what am I specifically doing with SCLCPs? My interest is in modifying
the behavior of the polymer shown in Scheme 3. In "polymer chemist notation"
the parenthesis show the repeat unit of the polymer. You can imagine that
the unit that is shown in Scheme 3 is one boxcar of a long train of identical
units just like it, all connected at the parenthesis. The subscript "n"
denotes the number of "boxcars" in this train. Why am I interested interested
in this system? The properties of this system are very well understood,
so it is easier to work with. This particular system which was examined
extensivly by Dr. R. Schrock, et. al., at MIT shows only a nematic mesophase.
In fact, it is widely believed that this laterally attached type of system
is only capable of forming a nematic mesophase. It is particularly challenging
then, to attempt to force this system to modify its behavior, that is,
form smectic mesophases upon heating.
Scheme 3. Model Compound Upon Which Our SCLCP System is Based.
How will I accomplish layering? Well, if you examine the pictures of the
mesophases in Scheme 1, you might see that if we were to make the mesogens
interact in some way, they might be inclined to form layers. This is the
approach we are taking. We intend to form copolymers of electron donating
and electron accepting derivatives of the system shown in Scheme 3, thereby
increasing the interactions between the mesogens and perhaps forcing them
into a smectic allignment. The use of electron donor-acceptor pairs to
modify phase behavior is known in SCLCPs, but has not been used in a system
such as this. The electron donors and acceptors that I am synthesizing
are shown in Scheme 4.
Scheme 4. Electron Donor and Acceptor Mesogens.
A good link on Polymer Liquid Crystals can be found here.
I am VERY interested in knowing how this page is received, especially
what is/isn't easily understood. Please send
me your comments!
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