Status: 5th year graduate student
Undergraduate Institution: North Carolina State University
Block copolymers are a unique set of materials in which two chemically distinct polymer chains (or blocks) are covalently bonded at a point. If the two chains want to phase separate or demix they can only do so locally because of their covalent link. This gives rise to the phenomenon called microphase separation, in which the block copolymer domains will phase separate into a variety of periodic structures (with period sizes of ~10-100 nm) such as spheres or cylinders of one block dispersed in a matrix of the second block.
The small size of these structures makes block copolymers an attractive candidate material as etch guides for nanopatterning. This process, termed block copolymer nanolithography, utilizes thin (10-100 nm) films of block copolymers and has the potential to pattern at sizes which are smaller than what is currently accessible via traditional photolithographic methods. Some applications of block copolymer nanolithography include the creation of light polarizers, high density memory storage, and superhydrophobic surfaces (to name a few).
In order to be a practical avenue for nanoscale patterning, we must better understand the physics and morphological behavior of block copolymer thin films (it has been observed that when confined to a thin film, a block copolymer’s properties and phase behavior can be substantially altered from its bulk behavior). This includes understanding ways to order the microdomains (i.e. controlling in-plane and out-of-plane orientation of cylindrical microdomains). One way to orient microdomains is through a process pioneered by our group known as shear alignment. When applying a shear stress to a block copolymer thin films, the microdomains will often orient in the direction of applied shear, creating a high degree of long range orientational order.
My work in the group focuses on better understanding the shear alignment process. This will include synthesizing the desired block copolymers, examining their thin film behaviors pre- and post-shear, and then utilizing these films for device nanofabrication. Some specific problems I intend to look at are:
- The effect of film thickness on thin film morphology pre- and post-shear
- The effect of block copolymer molecular weight and composition on alignment quality
- The influence that the substrate and the presence of a brush-like wetting layer has on the alignment process
- The effect block copolymer size and architecture has on the rate of alignment
- The idea of whether the viscoelastic contrast between blocks is important in setting the critical alignment parameters
- Creation of a mesoscale alignment process using a channel flow
- Using all the above information to optimize alignment conditions to then create high performance nanofabricated devices
In addition to my work on shear alignment I also intend to explore a project, supported by Xerox, in which I will study the synthesis and properties of model thermoswitchable surfaces. This will include synthesizing a new set of block copolymers which can disorder at relatively low temperatures and then examining their wetting conditions upon cooling under various environmental conditions. If successful, this work could lead to a novel printing method which can exploit these block copolymers' phase behavior to control surface wettability.