O ur research is focused on the fluid-phase assembly of cylindrical nanomaterials across multiple length scales.  Cylindrical nanomaterials such as single-walled carbon nanotubes (SWNTs) and inorganic nanorods and nanowires exhibit fascinating mechanical, electrical, thermal, sensing and antimicrobial properties.  However, in order for these materials to realize their full potential they must be assembled across multiple length scales with controlled spacing and orientation.   Our goal is to understand structure-processing-property interrelationships in these systems.  The chemical and physical structure of the nanomaterial - for example whether it is a single-walled carbon nanotube or a silver nanowire results in intrinsic properties which clearly affect the properties of the final macroscale assembly.  However, processing of the material is equally important in determining final properties.  Processing includes the steps in dispersing  the nanomaterial as well as the application of shear forces during the production of films, fibers and other bulk materials.   We use rheology to understand the interrelationships between structure, processing and final properties such as mechanical strength, electrical conductivity and thermal properties.  Rheology, the study of how things flow, tells us a great deal about the degree of nanomaterial dispersion and how the system responds to changes in both shear and temperature.  We can use rheology in determining whether the nanomaterials are dispersed as individual species or bundles, whether changing the temperature affects the  degree of dispersion, whether or not the rods align with shear, and whether or not the dispersion is liquid crystalline.  Some of our current research projects are listed below.

 

 

  COUPLED SELF-ASSEMBLY AND FLOW ALIGNMENT OF INORGANIC NANORODS

Developing applications for nanorods requires the ability to manipulate and organize them. The combination of nanorod self-assembly into liquid crystalline phases with flow alignment is a promising route for the production of highly aligned macroscopic arrays of nanorods. Self-assembly, particularly the ability of solutions of stiff anisotropic materials to form liquid crystalline phases, is already well established means to produce high performance polymeric materials such as bullet-proof vests and liquid crystalline displays. However, understanding the liquid crystalline phase behavior of nanorod dispersions is a nascent field formed by the intersection of nanotechnology, liquid crystalline science and colloid science. There is a need to develop fundamental understanding about the impacts of nanorod concentration, nanorod aspect ratio, and nanorod-nanorod and nanorod-solvent interactions on liquid crystalline phase behavior. In addition, the field of polymer liquid crystals has shown that phase behavior (self-assembly) often must be coupled with flow alignment to achieve defect free monodomain liquid crystals on reasonable time scales. Our research focuses on the fundamental liquid crystalline phase behavior of nanorod solutions and the impact of shear on domain size and the number of defects.

 

SWNT- POLYMER NANOCOMPOSITES:

Single-Walled Carbon Nanotubes (SWNTs) are an outstanding material that has the potential to have major impacts on diverse fields including aerospace, homeland security, medicine and even sporting goods. SWNTs have thermal conductivity and mechanical strength that exceeds all benchmark materials. In addition, both semiconducting and metallic forms of SWNTs can be produced. Incorporating SWNTs into polymers has the potential to radically improve the properties of the polymer matrix in terms of strength, toughness, electrical conductivity, thermal conductivity and/or flame retardancy. There two significant obstacles to achieving this potential: 1) SWNT dispersion and 2) separation of SWNTs from the polymer matrix during processing or use. A total system approach is needed to overcome these obstacles. Our research involves investigating the inter-relationships between SWNTs and polymer structure, processing conditions and composite properties. Functionalization of SWNT sidewalls can markedly improve dispersion and improve compatibility with the polymer matrix thereby reducing phase separation. In addition, the impact of processing conditions, particularly extruder design, shear rates, and temperatures can not be underestimated. We are investigating the combined impact of  structure and processing parameters on SWNT-polymer nanocomposite properties.
 

DISPERSION OF NANOMATERIALS IN BIOPOLYMERS

Biopolymers such as DNA  and proteins are proving to be excellent dispersants for  single-walled carbon nanotubes and other nanomaterials.  The use of biopolymers also enables the creation of multi-functional and smart materials.  For example, we have produced antimicrobial films that combine the antimicrobial properties of lysozyme, a natural antimicrobial enzyme found in tears with the outstanding mechanical properties of  SWNTs.  This work was done in conjunction with the Simonian Group in Materials Science.  Other systems are also being explored.
    

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Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.