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This is the home page of Paul Heiney's research group at the University of Pennsylvania . Shown below are some of images from our recent work--click on the image to get more information.
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The image shows schematically how wedge-shaped molecules self-assemble into cylinders or spheres, which in turn self-organize into liquid crystals.
Dendrimers are branched molecules. In collaboration with Virgil Percec, we are studying the relationship between the molecular architecture, which can be precisely designed and controlled, and the liquid crystalline structure. We are particularly interested in the relationship between molecular chirality, chirality of the self-organized units, and macroscopic helicity.
The image shows a Brewster Angle Microscopy image of a discotic liquid monolayer crystal at the air-water interface.
Liquid crystals are structurally intermediate between liquids and solids. The most commonly studied thermotropic liquid crystals are composed of organic molecules with rod-shaped backbones and one or more flexible aliphatic tails. At temperatures intermediate between that of the high temperature isotropic liquid and that of the low temperature crystalline phase(s), one may observe nematic phases, in which the molecules are oriented along a common direction but have only liquid-like positional order, and smectic phases, which consist of fluid layered structures. The primary technological importance of liquid crystals arises from their application in display devices; liquid crystalline polymers are also playing an increasingly large role in nonlinear optics. The fundamental properties of liquid crystals have also attracted wide interest in the condensed-matter-physics community. Because of the restricted nature of the positional order, fluctuations of both the molecular orientation and position qualitatively change the character of thermodynamic transitions, such as that from a smectic to a nematic phase.
Discotic liquid crystals are typically composed of molecules with a disk-like core and 6-8 aliphatic tails. Depending on the temperature and molecular geometry, discogenic molecules can form either discotic nematic phases (ND) or columnar phases with long-range intercolumnar order and only short-range intracolumnar order. The most commonly observed "hexagonal disordered" (Dhd) phase consists of a triangular array of columns with only fluid-like intracolumn order and consequently no long-range column-column correlation of the molecular heights.
These phases represent a new ordering of dimensionalities for a condensed state of matter: rather than being ordered in one dimension and disordered in two, like smectic liquid crystals, they are ordered in two dimensions and disordered in one. This new symmetry can have a dramatic effect on phase transitions. For example, the crystal-to-Dhd transition is allowed theoretically to be continuous rather than being first order.
In collaboration with Amos Smith, Helmut Ringsdorf, and other groups, we have used employed x-ray diffraction (XRD) and other tools such as Fourier Transform Infrared (FTIR) spectroscopy, Differential Scanning Calorimetry (DSC) and Atomic Force Microscopy (AFM), to study the long-range order and short-range disorder in a variety of discotic liquid crystal systems. We developed a "strand oven," which uses a pin-and-cup arrangement to pull single-orientation discotic strands suitable for x-ray diffraction analysis.
In our first discotic study, we used XRD to study well-oriented, free-standing columnar phase strands of truxene derivative HATX. We found that it was possible to distinguish between the cores and tails of discotic molecules in free-standing strands. Within the columns, the cores are positionally disordered, but have parallel orientations as in a nematic or smectic phase. By contrast, the tails are highly disordered, producing a nearly isotropic distribution of scattering. The decrease in tail orientational order with increasing temperature suggests that the temperature dependence of the conformational degrees of freedom of the hydrocarbon chains may play an important role in the thermodynamics of the columnar phases.
The organization of amphiphilic molecules such as lipids and soaps into monolayers at the air-water interface is a well-known phenomenon. Such monolayers are known as "Langmuir films;" when a Langmuir film is transferred to a solid substrate it is known as a "Langmuir-Blodgett" (LB) film. Surprisingly, it has recently become clear from work done in our laboratory and elsewhere that Langmuir monolayers can also be formed from disc-shaped molecules, even though they are not manifestly amphiphilic. In particular, thin films of the disk-shaped molecules comprising discotic liquid crystals show promise for applications as pressure sensors, anisotropic conductors, and display devices, due to their capability for self organization in highly anisotropic structures at the air-water and air-solid interfaces. We used surface pressure vs. molecular area isotherms to show that some discogenic molecules adopted an "edge-on" conformation with respect to the water surface, while others adopted a "face on" conformation . Langmuir films of disc-shaped molecules provide an interesting model system for melting of a two-dimensional system with no imposed substrate periodicity and highly anisotropic in-plane interactions. Models for the dislocation-mediated melting of such a system predict a new quasi-smectic phase between the quasi-long-range-ordered low temperature phase and the high-temperature isotropic phase. We used X-ray reflectivity (XR), grazing-incidence x-ray diffraction (GID) and Brewster Angle Microscopy (BAM) to study the structure of these films. Typical BAM images are shown below: the top image was collected at low temperature, and the bottom at high temperature. Corresponding GID patterns before heating, at high temperature, and after recooling are shown to the right.
At low temperatures, BAM images showed grainy birefringent domains as expected for solid monolayers. In response to air flow above the surface, the films exhibited the typical yield stress response of a solid. BAM images collected at higher temperatures were uniform, as expected for an isotropic fluid, and the response to air currents was also that expected for a freely flowing fluid. Similarly, at low temperatures, a single GID peak was observed, indicative of columnar order with a 14 Angstrom column spacing. At higher temperatures, the peak disappeared completely, indicating loss of columnar order. Finally, after re-cooling, a 20% sharper peak appeared at the same position, indicating a significant increase in the structural correlation length. All of these data are consistent with a reversible, first-order melting transition from a columnar crystalline or liquid-crystalline ordered phase to an isotropic solid. Furthermore, the increased sharpness of the peak upon recooling indicates that it is possible to improve the structural order by thermal annealing.
Advanced Photon Source: We have performed GID experiments at the Advanced Photon Source (APS), using the liquid surface spectrometer at the CMC-CAT .
The image below shows a scanning electron micrograph of a dried silica colloidal dispersion.
The storage modulus of the gel (reflecting the long range structure) continued to evolve long after changes in the local structure could no longer be distinguished, but ultimately it also converged to a quasi-stationary state. Disruption of the gel network by shear had no detectable effect on the local structure, but caused a dramatic reduction in the moduli which subsequently recovered following different kinetics than in the initial growth process.
Carbon Nanotubes: Carbon nanotubes are currently the focus of intense interest due to their unique thermal, mechanical, and electronic properties. In collaboration with J.E. Fischer (Materials Science) and A. Yodh (Physics) we have used neutron and x-ray scattering to probe the properties of nanotubes dissolved in superacid (anhydrous 102% sulfuric acid) and nanotube-surfactant solutions.
Click here for the complete list of Paul Heiney's publications.