Responsive Liquid Crystal Polymers

The goal of this project is to produce and characterize a series of responsive side chain (SC) liquid crystal (LC) block co-polymers (BCP) in an effort to produce anti-chemical weapon coatings for the Army through the Institute for Soldier Nanotechnologies (ISN). A number of temperature responsive end-on liquid crystal polymers (LCP) and photo-responsive end-on LCP’s have been produced and studied by Verploegen, and this project will be a continuation moving toward chemo-responsive LCBCP’s.

As with any material, these LCP’s change their phases and properties in response to temperature. Specifically, the LC’s change from a smectic A phase to an isotropic disordered phase above a given temperature, and the BCP’s change from a nano-phase segregated system to a disordered system above another given temperature. These temperatures are inherent to the material and the environment and have been extensively measured and characterized.

In an effort to make the materials respond to other external stimuli, Verploegen synthesized an azobenzene LC and attached it to a polyvinylmethyl siloxane (PVMS) homopolymer. Upon irradiation with a certain wavelength of UV light, the azobenzene changes from a trans to a cis conformation which disrupts the LC phase and causes a sharp decrease in the mechanical properties of the material. These materials are where this project will start. The next step is to attach the photo-responsive LC to a BCP for added mechanical integrity as well as an introduction to the necessary synthetic and characterization techniques.

After the photo-responsive LCBCP’s have synthesized and tested, the next step in the project is to improve upon the response of the LCP’s so that it can translate to a suitable coating for the Army. There are two major ways to do so: a change in diffusivity or a change in porosity. Changes in diffusivity come automatically with the smectic to isotropic transition; however, disruption of the smectic phase would increase diffusivity. On the other hand, a change in porosity could be achieved through a mechanical response whereby the pores in the thin film shrink because of the disruption of the LC. One way to achieve such a mechanical response would be to change the LC from end-on to side-on which tends to contract the polymer when the LC changes from smectic to isotropic.

Finally, and most importantly, the LCP’s must be made chemo-responsive. The primary goal of the project is to produce coatings that can block chemical weapons. As a first step and proof of a diffusion controlled mechanism, molecular imprinting will be used to make a LCBCP thin film that will be diffusive to small molecules but bind and block the chemical that was used in the molecular imprinting. As a second step and proof of a porosity control mechanism, the current LC’s can bind iodine vapor that could sterically disrupt the smectic phase. Once a functioning chemo-responsive system has been constructed, the concepts can be applied to other chemicals relevant to the Army, such as chemical weapons or toxic industrial compounds.