Size-Scale Sensitivity in Multiphase Systems with a Liquid Crystalline Phase

Cuny City College, New York NY

Investigators

Abstract

Abstract/ CTS-0112358 / Denn, Morton M / CUNY City College Size -Scale Sensitivity in Mutliphase Systems with a Liquid Crystalline Phase Liquid crystalline systems contain an intrinsic length scale, the correlation length over which the liquid crystalline order is preserved; this scale is of the order of a few micrometers in nematic liquid crystalline polymers (LCPs). The liquid crystalline order can affect the interphase structure in a multiphase system, which in turn can affect macroscopic properties. Thus, the system behavior depends on the interaction between the different length scales: the molecular nanoscale characterizing the interfacial region, the mesoscales characterizing the correlation length and the phase dimensions, and the macroscopic scale on which properties are determined. Blends containing small amounts of a LCP in a matrix of a flexible thermoplastic are of considerable technological interest. It is possible to develop "self-reinforced" composites that exploit the outstanding tensile properties of fibers made from LCPs by creating a fibrillar morphology in the LCP dispersed phase. In addition, LCPs can act as "flow modifiers" for conventional thermoplastics, effecting a reduction in extrusion pressure at low concentrations. Our prior research has shown that the linear viscoelasticity of LCP blends is insensitive to droplets smaller than the nematic correlation length, and that the dynamics of LCP droplets do not follow the same scaling as droplets of flexible polymers. Furthermore, we have shown in preliminary Monte Carlo calculations that the interfacial tension between a LCP and a flexible polymer depends on the far-field nematic orientation in the droplet. The proposed research comprises three complementary approaches to elucidate the effect of multiple length scales on the mechanics of blends containing LCPs: continuum theory, Monte Carlo calculations, and experiments. Continuum theory. The LCP orientation distribution in the droplet will be a major factor in the deformation mechanics, and transitions between radial and bipolar orientations, which will affect the interfacial tension, are expected with droplet deformation. The Principal Investigator plans initially to use a perturbation analysis of a Leslie-Ericksen (LE) material with equal Frank elastic constants to analyze the first-order effect of droplet deformation. The LE fluid is the most elementary nematic model, and it is the asymptotic limit of more complete models for LCPs that are extensions of the Doi theory. Then the Principal Investigator will undertake a full three-dimensional finite-element solution of the equilibrium distribution in a deformed sphere for a LE material with unequal Frank coefficients. The purpose of this computation is to understand the likely orientation distributions in droplets after shearing. The time scale for droplet response defines the frequency range in which interfacial effects can be observed in blend rheology. The Principal Investigator will initially determine the response of a LE fluid, using an expansion in spherical harmonics. This is an important calculation in light of the experimental observations for blend linear viscoelasticity and droplet dynamics. The principal investigator expects the transient response of the droplet in the linear regime to depend on the Frank elasticity when the elastic stresses in a "domain" become comparable to the Laplace pressure. Available data do not show this dependence, but they do show a dependence on initial strain that is absent for droplets of a flexible polymer. This behavior needs to be rationalized by an analysis of the type proposed here. The principal investigator will repeat the Palierne analysis of viscoelastic blends for a monodomain liquid crystalline droplet, first using the LE theory for simplicity. The computed rheological properties are likely to depend on the anchoring conditions imposed on the nematic phase at the boundary, which will define the nematic order in the domain interior. The principal investigator expects the response to be sensitive to the Frank elasticity terms in the LE equation in the neighborhood of singularities. The LE theory is structurally similar to multi-domain theories for LCPs, so these calculations should provide a framework for interpreting experimentally observed differences between multidomain and monodomain droplets. The principal investigator anticipates a dependence of the storage modulus G' on a new dimensionless group involving the correlation length. The principal investigator will then carry out the comparable calculation for the Doi theory and relevant extensions. Monte Carlo Calculations. The Bond Fluctuation Model (BFM) will be used to study the effect of nematic order on the interfacial tension between a LCP droplet and an amorphous matrix, exploring chain length, energy parameters, and polydispersity. Modification of the BFM by developing a rational method for placing interpolated points on the lattice near the interface is required to account for the large size differences between LCP and flexible chain monomers. The effect of shear flow on the chain orientation near the interface and on the interfacial tension will be explored using a pseudopotential model for self-avoiding lattice chains. Experiments. Linear viscoelastic measurements intended to isolate the effect of the interface and Frank orientational elasticity from macromolecular effects will be carried out on blends containing low molar mass liquid crystals (LMMLCs). The principal investigators preliminary measurements suggest that the linear viscoelastic behavior of the LMMLCs may be unusual (negative G') and shear history dependent. Thus, the first step will be to elucidate the linear viscoelastic behavior and to analyze the results in the context of the predictions of LE and Doi theories. The principal investigator will then study the rheology of the blends, employing LMMLCs that exhibit a phase transition to obtain nematic and isotropic dispersed phases with equal viscosities and identical chemical compositions.