Effect of Network Elasticity on Nematic Liquid Crystal/Cross-Linked Polymer Phase Diagram
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ABSTRACT The influence of elasticity on the phase behavior of a mixture of nematic liquid crystal (LC) and in-situ cross-linked polymer has been investigated. Unlike the phase diagram of the LC/pre-cured polymer, the experimental diagram of LC/cross-linked polymer phase supported by theoretical calculation showed no critical point. Instead, the binodal curve exhibits an upward asymptotic behavior as the LC volume fraction approaches unity due to domination arising from network elasticity, particularly at high LC volume fraction. An examination of the effect of cross-links segment length and network functionality on the phase diagram of LC/cross-linked polymer showed that the segment length between cross-links exerts a greater influence. INTRODUCTION Blends of liquid crystals (LC) and polymer, known as polymer-dispersed liquid crystals (PDLC) and polymer-stabilized liquid crystals (PSLC) are of interest for various electro-optical displays and control applications [1]. Photo-initiated polymerization-induced phase separation (PIPS) of mixtures of LC and highly reactive monomers is the preferred method of fabricating these composites since it offers flexibility, simplicity, and ease of fabrication. It is well known that the use of multi-functional monomers leads to the formation of cross-linked polymer network [2]. The presence of cross-links in the polymer may affect the phase behavior and ultimately the morphology development of LC/cross-linked polymer blends. Early models [3, 4] of LC/polymer phase behavior assumed that the polymer chains are linear for simplicity. When the polymer is made of cross-linked chains, the elastic free energy density of the network must be incorporated in the theoretical establishment of the phase diagram. In this paper, we examined the role of elastic free energy in the thermodynamics of nematic LC/cross-linked polymer phase diagram. We show both theoretically and experimentally that blends of nematic LC and cross-linked polymer exhibit different phase behavior when compared with blends of nematic LC and pre-cured polymer. Finally, the effects of chain segment between cross-links and network functionality on LC/cross-linked polymer phase diagram are examined. The theory is tested experimentally using blends of liquid crystal E7 and a cross-linked polymer. THEORETICAL SCHEME A convenient starting point in the theoretical description of a phase diagram of a binary mixture of low molar mass nematic LC and fully cross-linked polymer network is to express the total free energy density, g in terms of a simple summation of three independent free energy densities according to the following equation [5]:
=L InOL rL
+ X+LýP + I(L
r
lnz+ v*s2 )+
-•
2r
2
-QD2/ 344' 3
- op )+• R e
r,
117 Mat. Res. Soc. Symp. Proc. Vol. 559 01999 Materials Research Society
n
(1)
where the first two terms represent the isotropic free energy density, gi (assuming insoluble network), the next term represents the nematic ordering free energy density, g', while the last two terms represent elastic free ener
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