Current Position: Research Engineer, Heavy Oil & Mining, ExxonMobil Upstream Research Company
Advisor: Prof. R.A. Register
Undergraduate Institution: Penn State University
Ph.D. Thesis Research
Block copolymers are polymers that are covalently linked together. Like polymer blends, most block copolymers phase separate, but due to the covalent bond between each block, the length scale of phase separation is on the order of nanometers. The specific morphology of the phase separation is dependent on the relative volume fractions of each block. When the block copolymers are confined to thin films (thicknesses less than 1 micrometer), the effect of the substrate can orient the microdomains depending on surface interactions each block has with the substrate and air interface. For example, cylindrical microdomains will typically lie down parallel to the substrate.
There is much interest in block copolymer thin films to be used as nanolithographic masks since they can create periodic structures on the order of tens nanometers. A drawback to having applications like this realized is that the orientation of the microdomains is very difficult to control. Cylinders will form fingerprint like structures whereas straight oriented cylinders would be desired (for use as nanowires or polarizers). Spheres form polygranular structures with the size of the grains growing with a weak power with time. Many techniques have been developed to orient the microdomains with varying degrees of success. One technique that was developed by researchers here at Princeton uses a stress field to orient the microdomains in a preferred direction. The stress can be transmitted to the film using a PDMS pad or flowing a viscous fluid over the film. Advantages of this technique are that it can be done quickly (~15-30 minutes) and over areas on the order of square centimeters.
There are still restrictions on the degree of order that can be obtained through shearing and the mechanism through which shearing induces order is unclear (since only a narrow range of systems was analyzed). My research looks into both of these problems. Work has been done to evaluate how the topological defects (dislocations) destroy order in shear-aligned thin films. In the future, investigations will be done to look into the time dependence of shearing. Also, different molecular weight block copolymers will be researched to see how the parameters for shear-alignment are affected. After this is all understood, different polymer chemistries will be analyzed to determine the effect of wetting conditions, since the interaction the substrate has with the block copolymer could affect the shearing parameters.
I form the thin films by spin coating a dilute solution of the block copolymer onto a silicon substrate. Shearing can be done in one of three ways: 1) dragging a PDMS pad over the thin film, 2) using a rheometer and transmitting a stress through a viscous overlayer, or 3) following a viscous fluid over the film when the fluid is confined to a channel. Imaging of the thin film is done using tapping mode atomic force microscopy. Further analysis is done on the real space images obtained using custom software to quantify the orientation of the films and locate topological defects.