Small-angle light scattering of polymer liquid crystals under shear flow

Takebe, Toshihiko; Hashimoto, Takeji; Ernst, B.; Navard, Patrick; Stein, Richard S.

doi:10.1063/1.458150

Cited by 54 publications

(33 citation statements)

References 21 publications

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“…Figure 6 shows birefringence as a function of shear strain following step increases in shear MaCrOmO~eCUkS, Vol. 26 transmitted intensity spectra (although in normalized form) observed at intervals following cessation of shear flow for three of the solutions we have studied. In all cases, the previously applied shear rate was in the range of the low shear rate plateau in steady-state birefringence discussed in the preceding paper.…”

Section: Resultsmentioning

confidence: 99%

Molecular alignment of polymer liquid crystals in shear flows. 2. Transient flow behavior in poly(benzyl glutamate) solutions

Hongladarom¹,

Burghardt²

1993

Macromolecules

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Flow birefringence is used to study the degree of molecular orientation in transient shear flows of liquid crystalline solutions of poly(benzy1 glutamate) (PBG). Experiments are confiied to the low shear rate regime in which it is known that such solutions exhibit director tumbling. Our optical results exhibit characteristics common to PBG solutions, including oscillatory responses that scale with shear strain and relaxation processes that scale inversely with the previously applied shear rate. The measured degree of orientation is compared with the transient predictions of the linear Larson and Doi tumbling polydomain model [Larson, R. G.; Doi, M. J. Rheol. 1991,35,539], which is known to qualitatively reproduce transient streas behavior well. Our results reveal some ~hortcomings of the model regarding its predictions of molecular orientation. Our ma& startling observation is a significant increase in molecular orientation upon cessation of shear flow. This increase in orientation provides a straightforward explanation for the gradual decrease in dynamic moduli that is observed following cessation of shear flow. This result is in direct contradiction with conclusions drawn by Asada and co-workers on similar PBG solutions [Aaada, T.; On&, S.; Yanm, H. Polym. &ng. Sci. 1984,24,355]. We attribute this discrepancy to ambiguity in birefringence measurements resulting from multiple retardation orders; we have employed a spectrographic birefringence technique that eliminates this source of uncertainty. This work is an extension of an accompanying paper on steady-state flow behavior.

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Section: Resultsmentioning

confidence: 99%

Molecular alignment of polymer liquid crystals in shear flows. 2. Transient flow behavior in poly(benzyl glutamate) solutions

Hongladarom¹,

Burghardt²

1993

Macromolecules

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“…It should be mentioned that SALS of LCPs has been previously studied by Hsiao et al, 38 Takebe et al, 40 Picken et al, 41 and Walker et al 42 with the goal of elucidating the influence of shear on microstructure. Figure 6 gives digital snapshots of CSALS patterns for γ ) 0.5 s -1 at times following shear start-up of 0, 4, 100, and 240 s, corresponding to strains of 0, 2, 50, and 120 strain units.…”

Section: Microstructural Evolution During Shear Start-upmentioning

confidence: 99%

Morphological and Rheological Responses to Shear Start-up and Flow Reversal of Thermotropic Liquid-Crystalline Polymers

et al. 2000

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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.

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“…For example, the scattering curve cannot be solely described in terms of a single rod theory. The main contradictions with experiments of this simple model are the appearance of oscillations in the vertical streak and a drastic growth of the intensity at small values of scattering angles with increased shear rate as observed in (20,21).…”

mentioning

confidence: 83%

“…Flow light scattering patterns had a bright vertical streak parallel to the vorticity axis with four lobes and was similar to the patterns developed in references (20)(21)(22). The intensity in the vertical streak was processed using numerical image analysis.…”

Section: Methodsmentioning

confidence: 99%

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Light Scattering from Lyotropic Textured Liquid-Crystalline Polymers Under Shear Flow

Патлажан

Riti

Navard

1995

ACS Symposium Series

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Small-angle light scattering (SALS) experiments performed on lyotropic liquid crystalline polymer (LCP) solutions under steady-state shear flow are discussed. The structure of scattering objects is described in terms of a simplified model assuming linear defects located in different levels and oriented along the flow direction. A light scattering pattern appearing as a bright streak with strong intensity oscillations is theoretically calculated. From the comparison of the theoretical and experimental scattering curves for the streak the following conclusions were made: 1) defects created under steady state shear flow in cholesteric LCP solutions are grouped into sets of assemblies; 2) an increase in shear rate is accompanied by a decrease of the average domain size.It is well-documented now that the rheology of main-chain polymer nematics at low shear rates strongly depends on coupling between the director orientation and the structure of texture (1). This has been shown in a series of key experiments dealing with transient and steady-state flow. In these experiments, such effects were observed as oscillatory responses in stress and conservative dichroism after sudden reverse or step increase in shear rate (2-5), slow elastic recoil after removal of shear stress (6), and dependence of moduli on the mechanical history of the sample (3,7,8).Peculiarities in the rheological behaviour of the liquid crystalline polymers (LCP) are the result of an instability of the director at low Deborah number (tumbling regime) (1,(9)(10)(11)(12)(13) caused by hydrodynamic torque exerted by shear flow. This instability is thought to cause spatial variations in the director field and to give rise to the formation of defects (13,14). The area of the sample per one defect line is referred to as a "domain". Its average spatial scale, a", was estimated by Marrucci (15) from the balance between Frank elastic distortion and viscous energies. It was derived that this scale decreases with an increase in the shear rate, γ, as γ 1/2

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Small-angle light scattering of polymer liquid crystals under shear flow

Cited by 54 publications

References 21 publications

Molecular alignment of polymer liquid crystals in shear flows. 2. Transient flow behavior in poly(benzyl glutamate) solutions

Molecular alignment of polymer liquid crystals in shear flows. 2. Transient flow behavior in poly(benzyl glutamate) solutions

Morphological and Rheological Responses to Shear Start-up and Flow Reversal of Thermotropic Liquid-Crystalline Polymers

Light Scattering from Lyotropic Textured Liquid-Crystalline Polymers Under Shear Flow

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