Monte Carlo Studies Of Liquid Crystalline Systems

Author : Kiran Khanal
Publisher :
ISBN 13 :
Total Pages : 128 pages
Book Rating : 4.:/5 (911 download)

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Download or read book Liquid-crystalline Ordering in Semiflexible Polymer Melts and Blends written by Kiran Khanal and published by . This book was released on 2013 with total page 128 pages. Available in PDF, EPUB and Kindle. Book excerpt: Semiflexible polymers of sufficient stiffness exhibit liquid crystalline order at low temperature and high polymer concentration. Blends of liquid crystalline and flexible polymers have interesting physical properties and important applications in organic electronics. We investigate melts and blends of flexible and semiflexible polymers with the aid of Monte Carlo simulations of an extension of Shaffer's bond-fluctuation model. To control chain stiffness we include a bending term in the Hamiltonian and investigate two models for semiflexibility that differ in the range of penalized bond angles. A study of structural, dynamic and thermodynamic properties of the first model shows that it describes melts of semiflexible chains that do not undergo a transition to a liquid crystalline state. Simulations of the second model reveal orientational order without positional order at high density and low temperature. The transition from the isotropic high-temperature phase to the nematic low-temperature phase, the IN transition, is accompanied by discontinuous changes in structural and thermodynamic properties. This agrees with mean-field theories and experimental observation that show that the IN transition is a discontinuous transition. To characterize our system fully, we determine the phase diagram and find that the IN transition temperature increases with increasing filling fraction, which agrees qualitatively with predictions by Onsager and Flory. Since pair distribution functions give insight into structure and morphology of polymers, we construct same-chain and different-chain distributions that we further differentiate by flexible and rod-like chain conformations. A study of same-chain pair distributions shows that the rod-like chains in our model align with a face diagonal in the nematic phase. Results for different-chain pair distribution functions show that a melt phase separates into a dense ordered region and a low-density disordered region when undergoing the IN transition at constant density. We study blends of stiff and flexible polymer chains to explore the effect of blending and investigate the morphology of blends that have undergone the IN transition. Our simulations show that the stiff component of the blend undergoes an IN transition and that, beyond the transition, ordered domains of rod-like chains are embedded in mixed regions of stiff and flexible chains. From the study of blends of different composition, we find that the IN transition temperature is always lower than in melts of the same density and increases with increasing fraction of stiff chains. We calculate same-chain and different-chain pair distribution functions to investigate the structure and find that the blends are well mixed at high temperatures and phase separate on ordering. In order to control the structure and morphology of the ordered phase, we investigate the effect of an ordering field on the liquid crystalline state. Our studies show that the isotropic to nematic transition temperature for both polymer melts and blends increases by applying an electric eld. The field also increases the size of ordered domains and, when applied along the preferred alignment direction of our model, it leads to very uniform ordered regions. In summary, we have developed and investigated a lattice model for semiflexible polymers that allows us to control the formation of liquid crystalline regions in melts and blends. The configurations generated in this way will be used in the future to study charge transport properties of liquid crystalline polymer blends.