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Proposal Objectives
 The present proposal is aimed at elucidating spatio-temporal growth of gigantic azobenzene chromophore (AC) single crystals in photo-curable systems while subjected to trans-cis photoisomerization. Photo-curing and photoisomerization are decoupled by selecting irradiated wavelengths:  using UV to switch AC to the cis-state and blue light to induce azobenzene crystallization in the trans-state.  Selecting photo-initiators provides a handle on wavelengths required to photopolymerize the matrix. Photo-switching azobenzene between the crystallizable trans and cis state affords a unique on-and-off crystallization switch. The effect of cis-trans isomerization on the dynamics of nucleation and growth of crystalline azobenzene chromophores will be determined and the confinement effect in the holographically patterned regions will be elucidated.  The primary goal is to control phase behavior of mesomorphic liquid crystals and gigantic azobenzene single crystal mixtures by photo-switching between the crystallizable trans-state and cis-state. Chromophore crystal growth will be investigated in the context of phase field theory of solidification coupled with the photopolymerization rate. This theory of solidification and subsequent simulation will shed light on the confinement effect of crystal and mesophase structures.  Fundamental knowledge thus acquired will guide holographic photolithography of photo- or field-responsive AC/polymer composite films. 

Project Activities and Findings
Research and Education Activities:
 Prior to investigating the trans-cis photoisomerization effects, we found gigantic azobenzene pyramid single crystal capable of swimming in triacrylate monomer solution. We established the solid-liquid phase diagram of AC and triacrylate (TA) 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 TA solvent is rejected from the growing faceted fronts, a concentration gradient forms at the crystal-solution interface.  This induces spatial variability of surface tension known as the Marangoni effect contributes to the observed shooting of AC single crystals in TA solution. Judging from the sink or swim behavior of the azobenzene crystal, we hypothesize a solution-air interface nucleation model.  This model was developed and simulated in collaboration with Professor Dmitry Golovaty.
 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.

Students Involved and/or Graduated:
 Namil Kim, Ph. D. graduated August 14, 2010 with the dissertation supported by this ACS-PRF grant: “Photoisomerization and photopolymerization induced phase transitions in mixtures of photoresponsive chromophores and reactive mesogens”. Dr. Kim is currently a post-doctoral research fellow at Vanderbilt University under Professor Peter Pintauro.  Neelakandan Chandrasekaran, Ph.D. developed polarized video microscopy for monitoring crystal motion as student.  He also graduated August 14, and is currently working at 3M. Luke Stetzik, an undergraduate Biochemistry student from the College of Wooster, worked 6 weeks determining photopolymerization rates of triacrylate/diacrylate networks. He is now a graduate biology student at University of Akron. Grant Riley, an undergraduate Physics student from Miami University (OH) studied AC crystal motion in diacrylate during summer 2010 (10 weeks) and was jointly supported by NSF-REU. Mr. Tom Sutter, a Ph. D. candidate, is currently exploring the azobenzene/E7 liquid crystal system and developing a photolithography setup to complete the second focused area of the grant.

Major Findings:
1) The present project is the first to demonstrate the swimming, shooting, and sinking of diamond AC crystals during crystallization in solution.
2) We unambiguously demonstrate nucleation of AC single crystal occurring at the solution-air-crystal interface. This finding challenges the general perception of substrate nucleated polymer single crystal growth.
3) The phenomenon of crystal swimming is attributed to the unbalanced surface forces resulting from solvent rejection from growth fronts and a solvent concentration gradient. Solvent rejection exerts thrust resulting in observed buoyancy effects (sinking and floating). The novel surface tension mechanism is the first to be reported in the context of crystal growth.
4) Stratified surface layering (ripple formation) signifies photoisomerization even in the solid state.
5) 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).
6) AC single crystals in diacrylate solutions are capable of cascading nucleation events, tumbling while shooting, and varied single crystal shapes (hexagonal or pyramidal). This kind of crystal rotation and tumbling motion is certainly new.

Proposed Study in the second year
 It is proposed to investigate photo-switching of the azobenzene chromophore crystals between the trans-state and cis-state under the photolithographic illumination. The spatio-temporal crystal growth will be investigated in the context of phase field theory of solidification by coupling with the photopolymerization rate. The influence of photopolymerization and photoisomerization on the crystal self-motion will be explored.