Overview

Our current research interests are nanocomposites and ion-containing polymers.  We design and fabricate new polymer nanocomposites containing carbon nanotubes and now metal nanowires with the aim of improving the mechanical, electrical and thermal properties.  With ion-containing polymers our focus is uncovering the nanoscale structure using a variety of state-of-the-art instruments for both static and dynamic experiments and then correlating the structure with the ion conduction as in batteries or fuel cells.  Additional research interests include block copolymers, particularly block copolymers that incorporate nanoparticles and charged blocks.

 

Ion-Containing Polymers
(Sponsors:  NSF-DMR, ARO-MURI, DOE)

Precisely-Spaced Acid Groups Produce Unique Morphologies (details)
      When polymer synthesis is controlled such that functional groups are evenly spaced along a linear polymer, the result is new morphologies.  We first reported this in collaboration with Prof. Ken Wagener's group, who used acyclic diene metathesis to produce linear polyethylene with carboxylic acid groups every 9th, 15th, or 21st carbon atom. Our X-ray scattering results found that the acid groups form dimers that drive the formation of acid-rich layers perpendicular to the polymer backbone within the PE crystallites.  The spacing between the acid layers correlates with the number of carbon atoms between acid groups.  This hierarchical structures might provide additional control over ion transport in polymers.  More recently we have found hierarchical structures 12, 12-ammonium ionenes.

 

X-ray Scattering & STEM Reconciled to Give Ionomer Morphology (details)
      X-ray scattering and STEM imaging are two powerful tools to for studying ionic aggregates in ion-containing polymers. In this study, we provide the first quantitative attempt to reconcile the number density of aggregates found by these two methods.  To accomplish this it is necessary to account to extensive overlap when imaging the ionic aggregates in high angle annular dark field STEM.  First, a 3D morphology was constructed using the parameters obtained by fitting the X-ray scattering data to a modified liquid-like packing model of spherical particles.  Second, a 2D projection was calculated to mimic a STEM image.  The number of features in the simulated STEM image relative to the number of objects in the 3D morphology followed a power law in thickness and thereby allows STEM images to be properly interpreted.  This quantitative analysis provides the best evidence to date that the liquid-like scattering is an excellent description of the nanoscale morphology in both poly(styrene-ran-methacrylic acid) and poly(styrene-ran-styrene sulfonate) based ionomers. Our comprehensive characterization methods provide a foundation for systematic studies of the influences on nanoscale morphology and macroscopic properties of ionomers.

 

 

Polymer Nanocomposites
(Sponsors:  NSF-MRSEC, Dupont)

Cellular Structures Redefine "BEST" Dispersion (details)
      The mantra in nanocomposites has become, "better dispersion leads to better properties."  There are a few in the field that are starting to counter this with, "a designed dispersion delivers the best properties."  This shift in approach is demonstrated by our work to form macroscopic cellular structures of carbon nanotubes.  We have developed a versatile method that coats polymer particles with carbon nanotubes and then hot presses them to form a solid part.  This processing method produces a cellular structure that is comparable to a cobblestone street wherein the polymer and carbon nanotubes are analogous to the stones and dirt between the stones.  This distribution of nanotubes was designed to reduce the weight percent of nanotubes needed to increase the electrical conductivity.  In contrast to composites with better dispersion, we found that this cellular structure maintained the viscoelastic properties of the polymer.  This approach to nanocomposite design provides improved electrical properties, better processability, and reduced materials costs.

 

Tuning the Mechanical Properties in Nanotube / Nylon Composites (details)
      Previous researchers have established that functionalizing carbon nanotubes can improve nanotube dissolution into various solvents.  Here, we establish that functionalization can be used to dramatically improve the mechanical properties in polymer nanocomposites.  We adapted our previously reported method of interfacial in situ polymerization of nylon, such that the functional groups on the nanotubes participate in the nylon polymerization and form covalent bonds. The length of the alkane spacer appears to influence the properties with a four-carbon and nine-carbon spacer yielding the highest modulus and toughness, respectively.

 

REVIEWS:  Polymer Nanocomposites
      We published our first review article about the state of polymer nanocomposite research in which the fillers are single-wall or multiwall carbon nanotubes in Macromolecules in 2006.  This provides a good entry into the field with a synopsis of carbon nanotube materials and nanotube suspensions.  Many of the challenges and opportunities noted in this early review are still pertinent.  In April 2007,  Rich Vaia and I edited an issue of MRS Bulletin that includes articles from academic and industrial scientists working on a wide range of polymer nanocomposites for a variety of applications, include aerospace and biomedical materials.  Our overview article summarizes the plethora of traditional and nanoscale fillers that are available along with their physical properties.  We also highlight the inherent implications of small particle size by plotting the interfacial volume normalized by the particle volume as a function of aspect ratio (figure below), as the relative interfacial thickness (delta) increases.