Flow���induced structure in a thermotropic liquid crystalline polymer as studied by SANS (original) (raw)
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Rheologica Acta, 2005
The properties of liquid crystalline polymers (LCPs) are strongly influenced by flow-induced changes in the degree of molecular orientation during processing . Thus, it is not surprising that much research has been devoted to unraveling the interrelationships between rheology, defect texture, and molecular orientation states in LCPs. These efforts have steadily yielded improved understanding, particularly for lyotropic LCP solutions . Progress in thermotropic LCP melts has come more slowly, but recent work on ''model'' mainchain thermotropic LCPs has led to new insights into the factors which control rheology and orientation development under flow in these materials . These model materials incorporate flexible spacers in the backbone, which leads to lower transition temperatures, accessible isotropic phases and improved thermal stability. These attributes facilitate detailed fundamental studies that have, to date, been impossible to conduct on commercial main-chain thermotropic LCPs, Unfortunately, it seems that the backbone flexibility which renders these materials attractive for research purposes strongly affects their dynamic behavior . As a result, scientific understanding still falls short for the one class of LCP materials where it
Orientation dynamics in commercial thermotropic liquid crystalline polymers in transient shear flows
Rheologica Acta, 2007
In situ X-ray scattering measurements of molecular orientation under shear are reported for two commercial thermotropic liquid crystalline polymers (TLCPs), Vectra A950® and Vectra B950®. Transient shear flow protocols (reversals, step changes, and flow cessation) are used to investigate the underlying director dynamics. Synchrotron X-ray scattering in conjunction with a highspeed area detector provides sufficient time resolution to limit the total time spent in the melt during testing, whereas a redesigned X-ray capable shear cell provides a more robust platform for working with TLCP melts at high temperatures. The transient orientation response upon flow inception or flow reversal does not provide definitive signatures of either tumbling or shear alignment. However, the observation of clear transient responses to step increases or step decreases in shear rate contrasts with expectations and experience with shear-aligning nematics and suggests that these polymers are of the tumbling class. Finally, these two polymers show opposite trends in orientation following flow cessation, which appears to correlate with the evolution of dynamic modulus during relaxation. Specifically, Vectra B shows an increase in orientation upon flow cessation, an observation that can only be rationalized by the assumption of tumbling dynamics in shear. Together with prior observations of commercial LCP melts in channel flows, these results suggest that this class of materials, as a rule, exhibits director tumbling.
Transient elongational flow behavior of thermotropic liquid crystalline polymers
Journal of Non-Newtonian Fluid Mechanics, 1992
The transient elongational viscosity (;iiz> behavior of two thermotropic liquid crystalline polymers, LCPs (hydroxypropylcellulose, HPC, and a copolyester of 2-hydroxy-6-naphthoic acid and para-hydroxybenzoic acid, Vectra A900), has been found to be similar to that determined for isotropic melts of linear polyolefins such as high-density polyethylene (HDPE). measurements were made on a newly constructed rotary clamp extensional rheometer designed to handle low-viscosity materials at high temperatures. The values of ij,' were observed to follow linear viscoelastic behavior at low strains and subsequently show mild strain hardening with increasing strain. The elongational flow behavior of HPC in the isotropic state was also determined and compared to that in the anisotropic state. Differences in the behavior appear to be related to differences in the structure of the melt and might indicate the influence of the melt state on the flow behavior of LCPs. However, the generality of these results to other LCP systems is uncertain because of the possible influence of residual crystallinity. The predictions of the Doi theory for elongational flow were also compared to the experimental behavior of the isotropic and anisotropic melts. It was found that the Doi theory could not predict quantitatively the elongational flow behavior of LCPs when the parameters in the model were
2000
Novel rheo-optical methods have been used to directly observe morphology evolution, during shear start-up and reversal flow, in semiflexible main-chain thermotropic liquid-crystalline polymers (TLCPs). Using a specially designed microrheometer allowing for simultaneous transient optical and mechanical observations, we observed band formation upon reversal of flow direction. It was seen that this band formation causes asymmetric light diffraction in HV small-angle light scattering mode, indicating a tilted arrangement of the regularly spaced bands. Also conducted were shear start-up and flow reversal experiments using a cone-and-plate rheometer under the same thermal and deformation histories as those in rheo-optical experiments for polymers of differing spacer lengths at equal temperature difference below the nematic-to-isotropic transition temperature (T NI). It was observed that, during both shear startup and flow reversal, the first normal stress difference N1 + (γ , t) exhibits a large overshoot followed by an oscillatory decay, while shear stress σ + (γ , t) exhibits a large overshoot followed by a monotonic decay. It was found that the higher the applied shear rate, the larger the overshoot of N1 + (γ , t) and σ + (γ , t), and the longer the persistence of oscillations in N1 + (γ , t). Similarity was found between the ratio N1 + (γ , t)/ σ + (γ , t) and flow birefringence ∆n + (γ , t) during shear start-up and flow reversal of the TLCPs investigated in this study. Further, we found that the ratio σ + (t,γ)/σ scales with γ t but the ratio N1 + (γ , t) /N1 does not, where σ denotes shear stress at steady state and N1 denotes first normal stress difference at steady state.
Europhysics Letters (EPL), 2002
This letter describes the non-linear rheology of the isotropic phase of a thermotropic side chain liquid-crystal polymer (SCLCP), from which we infer a flow-induced isotropic-to-nematic (IN) phase transition above a critical shear stress and construct non-equilibrium phase diagrams. In contrast to the well-studied wormlike-micellar solutions and predictions for simple liquid-crystalline systems, the critical stress does not vanish as the equilibrium transition temperature is approached from the above. We postulate that this is due to: i) the coupling between mesogens and the polymer backbone, whose equilibrium oblate nematic backbone conformation contrasts with the prolate non-equilibrium conformation; and ii) the peculiar topological constraints in SCLCP melts, which have been previously postulated as leading to long-lived clusters.
Macromolecules, 1991
A novel cone-and-plate rheometer has been used to measure the shear rate dependence of the second normal stress difference (Ne) for various solutions of a rodlike polymer that exhibits liquid crystallinity. Remarkable differences are observed between measurements on isotropic and liquid crystalline phases of the same rodlike polymer. The rheology of the isotropic phase is more or less similar to that of a typical concentrated polymer solution. By contrast, the liquid crystalline phase exhibits a number of distinctive rheological features. Thermodynamic pressure is below atmospheric at most locations within the flowing liquid crystal, with the minimum value occurring near the tip of the cone. Nz is an oscillatory function of shear rate and is often comparable in magnitude to the primary normal stress difference (N1). Most surprisingly of all, the measured value of N2 is positive within certain narrow shear rate ranges. Although most of these experimental observations are without precedent in the rheological literature, a recently published version of the Doi molecular theory successfully describes the essential features of liquid crystal behavior in steady shear flow.
Macromolecules, 1993
We describe results of an experimental investigation into the orientation state of liquid crystalline solutions of poly(benzy1 glutamate) under shear flow and how the microscopic structure relates to the macroscopic mechanical rheological behavior. The technique of flow birefringence was used to study the degree of molecular orientation. A spectrographic flow birefringence apparatus is described that eliminates ambiguities associated with multiple orders of retardation in birefringence measurements. The birefringence observed in textured solutions under shear flow is always less than that measured in quiescent, defect-free monodomains of the solutions. At low shear rates, the birefringence is roughly constant and in the range of 5343% of that observed in a monodomain; there is no evidence of a low-orientation, "piled polydomain" structure. At high shear rates, the birefringence is again roughly constant and around 90% of the monodomain value. The transition between low-and high-orientation states as a function of shear rate is closely correlated with changes in sign of the first normal stress difference of these Solutions, leading us to identify it as a manifestation of a transition between regimes of director tumbling at low shear rates and flow alignment at high shear rates. These observations are compared qualitatively and quantatively with predictions of the nonlinear Doi molecular model for textureleas samples [
Polymer Engineering and Science, 1992
The shear and elongational viscosities of a thermotropic liquid crystalline polymer (LCP), polycarbonate (PC), and their 2O%LCP/8O%PC blend, were studied using a capillary rheometer. The investigation focused on experimental studies using two sets of capillaries. The first set comprised capillaries having a converging entrance followed by a cylindrical section. The second set, "zero length" set, included capillaries having only the converging section. In the two sets various entrance angles were used. Experimental results have shown that shear viscosities and entrance pressures are practically independent of the entrance angles. The entrance pressure drop was small in the case of PC and reached 50% of the total pressure drop for LCP. The elongational viscosities of the LCP were found to be higher than those of the PC in the elongational-rate range studied, while shear viscosities of the LCP were higher in the lower shear rate region and lower in the higher shear rate region compared to those of PC. This was attributed to the orientability of LCP in elongational and shear flows.
Polymers, 2018
In this study, we report on the visco-elastic response during start-up and cessation of shear of a novel bio-based liquid crystal polymer. The ensuing morphological changes are analyzed at different length scales by in-situ polarized optical microscopy and wide-angle X-ray diffraction. Upon inception of shear, the polydomain texture is initially stretched, at larger strain break up processes become increasingly important, and eventually a steady state texture is obtained. The shear stress response showed good coherence between optical and rheo-X-ray data. The evolution of the orientation parameter coincides with the evolution of the texture: the order parameter increases as the texture stretches, drops slightly in the break up regime, and reaches a constant value in the plateau regime. The relaxation of the shear stress and the polydomain texture showed two distinct processes with different timescales: The first is fast contraction of the stretched domain texture; the second is the ...