Thein Kyu, Ph D, University of Akron
Research and Education Activities:
Prior to investigating the trans-cisphotoisomerization effects, we found gigantic azobenzene pyramid single crystals shooting through monomer and nematic liquid crystal melt phases. The unusual shooting phenomena have been observed in trimethylolpropane triacrylate (TMPTA), 1,6-hexanediol diacrylate (DA), and E7 nematic liquid crystals. We established the solid-liquid phase diagram of AC and each of the solvating components above, showing isotropic, pure crystalline, and coexistence of crystal + isotropic regions bound by solidus and liquidus lines. Upon thermal quenching from the isotropic melt to the crystal + liquid region, AC crystals nucleate and form faceted rhomboidal single crystals. As solvent is rejected from a growing front, a concentration depletion zone develops at the crystal-solution interface. This composition gradient induces spatial variability of surface tension, a relation known as the Marangoni effect. If single crystals nucleate in this surface tension gradient, they are propelled away from the growing front by unequal surface tension forces. Such crystal rapid motion (hereafter called crystal shooting) has prompted development of a model for the motion based on the momentum (or force) balance between surface tension and opposing drag forces.
As a continuation, photoisomerization-induced phase transition of neat liquid crystalline azobenzene chromophore (LCAC) and its mixtures with reactive mesogenic diacrylate monomer (RMDA) have been investigated experimentally and theoretically. Phase transition temperatures and corresponding morphologies of the blends have been examined by means of differential scanning calorimetry and optical microscopy. Theoretical phase diagram of binary nematic and crystalline system was constructed by self-consistently solving the combined free energy densities of Flory-Huggins, Maier-Saupe, and phase-field theory. It displayed various coexistence regions such as nematic + isotropic (N1 + I2), crystal + isotropic (Cr1 + I2), crystal + nematic (Cr1 + N2), and crystal + crystal (Cr1 + Cr2) as well as pure nematic (N1 , N2). The calculated liquidus lines were in good accord with the mesophase transition points. Upon irradiation with the UV light, the nematic phase of LCAC transformed to isotropic, while the crystal phase showed the stratified layering on the surface (i.e., ripples) due to buckling. Of particular interest is the trans-to-cis isomerization of LCAC has led to suppression of nematic + isotropic (N1 + I2) coexistence regions and complete disappearance of the nematic phase (N2) of LCAC. This part of the project was undertaken in collaboration with Professor Quan Li, Liquid Crystal Institute, Kent State University.
Crystals which move quickly through the solution also appear to take on shape selection rules based on solution undercooling. A fourfold rhomboidal symmetry is exhibited at shallow undercoolings, while the rhomboidal shape becomes truncated at the acute tips, creating a six-fold crystal symmetry transforming to hexagonal crystals at deeper solution undercoolings. A phase field model for crystal solidification is being developed to simulate crystal shape selection rules; the phase field model is advantageous in that the thickness of the interface affects the front propagation speed and shape. Also, the surface energy anisotropy of growing single crystals is incorporated into the model so as to preferentially grow the crystal along certain crystallographic planes. We hypothesize the relative growth rates along crystallographic planes changes with undercooling depth, much like the shapes of snowflakes depend on saturation conditions in the clouds they grow in. Once shape selection behavior is characterized in relation to the phase diagrams, we plan to use these guiding principles during photo-patterning experiments to select the preferred crystal interface topologies. A paper based on the above finding is being written for journal submission shortly.
Crystals nucleated during deep quenches showed polymorphic character – taking on a pleochroic single crystalline phase among others. Several polymorphs were grown on a single glass slide using a spray-quench technique wherein a compressed liquid is propelled towards the sample, creating a thermal gradient with spots of lower temperature where droplets strike the slide and evaporate. The thermal history is captured in the crystal polymorph forms left behind after quenching – dendritic growth occurs in the coldest regions, whereas single crystal forms in the warmer regions, and a third form rich in AC that grows following a liquid-liquid separation that enriches small droplets in the crystalline (AC) component. These diverse shapes of single crystals are reminiscent of pharmaceutical crystals (Metformin HCl) that crystallize into polymorphic forms during solidification under concentration and/or thermal gradients; determining the underlying science of crystal form is of utmost importance to pharmaceutical scientists. A paper is being written for journal submission.
Students Involved and/or Graduated:
1 Ph. D. student graduated with a Ph. D. and another student is expected to graduate in 2013. 5 undergraduate students received research training under the present sponsorship in the form of chemical and supplies, but they received their stipends from the NSF-REU program at UA.
Professor Quan Li, Liquid Crystal Institute, Kent State University
Professor Dmitry Golovaty, Department of Mathematics, University of Akron
Professor Wiley Youngs, Department of Chemistry, University of Akron
1) The present research is the first to demonstrate the swimming, shooting, and sinking of rhombus AC crystals during crystallization in solution.
2) The phenomenon of crystal motion is attributed to the unbalanced surface forces resulting from solvent rejection from growth fronts and the resulting solvent concentration gradient leading to surface tension gradient. This is the first report of the surface tension mechanism in the field of crystal growth.
3) Stratified surface layering (ripple formation) was identified by AFM and POM, this mesoscopic ordering signifies photoisomerization even in the solid state.
4) LCAC/RMDA blends showed various phase coexistence regions in accordance with a theoretical phase diagram. Upon UV irradiation nematic LCAC phase transformed to isotropic, while the trans-cis isomerization also suppressed nematic + isotropic (N1 + I2) coexistence regions and caused disappearance of the nematic LCAC phase (N2).
5) AC single crystals in diacrylate solutions are capable of undergoing cascading nucleation events while moving through the melt.
6) A thermal imaging technique was developed to match the thermal field during thermal quenching with the crystal polymorph that developed under the optical microscopic view.