摘要:

The rigid rod model has been proven successful in predicting the rheological response of rodlike polymers in the nematic phase. The model can be reduced to constitutive equations written in terms of a set of first‐order ordinary differential equations, provided that a mathematical approximation required to close the problem is introduced. These constitutive equations are attractive as they are suitable for fast numerical processing. The required closure approximation, however, may have profound effects on the capabilities of the constitutive equation to retain the features of the original model. A method based on the bifurcation analysis is proposed as an effective test of constitutive equations. As an example, the bifurcation analysis of a particular constitutive equation is presented, and the results are compared with the bifurcation pattern of theexactmodel.

ISSN:0148-6055    

DOI:10.1122/1.550559

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

A banded texture develops upon cessation of flow in polymeric liquid crystals. In addition to the well‐known lower critical shear rate, an upper limit for band formation is observed. The kinetics of the band evolution is investigated on a series of PBG solutions as a function of previous shear rate, concentration, temperature, equilibrium structure (nematic versus cholesteric), and gap size. It is concluded that the band texture occurs along two different routes depending on the shear rate range under investigation. At relatively low shear rates both the lateral and the width dimensions increase with time. The shear, however, affects the kinetics in these two directions differently, which explains the existence of a lower critical shear rate. At higher shear rates, on the other hand, the bands immediately reach their final length when the flow stops. Hence the evolution can be considered as one dimensional. After long rest periods the banded structure disappears. This phenomenon depends on the material under consideration. In PBG solutions the bands widen before they disappear, while for HPC solutions the parallelism of the bands is lost without considerable widening. Existing models for band formation are compared with the present data.

ISSN:0148-6055    

DOI:10.1122/1.550560

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

A capillary rheometer with quenching capability was used to study the formation of microstructure in mesophase pitch—a discotic nematic liquid crystal—under the flow conditions of fiber spinning. We report here some observations by reflected polarized‐light micrography on polished sections of specimens quenched during shear and elongational flow, or quenched during relaxation after cessation of flow. A prominent feature of mesophase flow in a capillary is the formation of ‘‘rippled’’ structures that appear to be due to the instability of shear flow in an anisotropic liquid. The ripples display a regular pattern of misorientation from the flow direction. The fully developed microstructures near the capillary exit consist of three microconstituents distributed in a concentric pattern: a fibrous core, surrounded by fine concentric bands with the mesophase layers in zig‐zag radial arrays, and a fine‐textured rim. Upon relaxation, the fine microstructures rapidly decay to disclination arrays, and coarsening continues by disclination‐annihilation reactions. The draw‐down cone, from which a filament is spun, showed no evidence of rippled structures, but instead a tendency to reorient mesophase layers to the axial direction. The rheological memory of mesophase pitch was observed in spinning experiments in which an orthogonal gridlike pattern is imposed by a screen placed above the spinneret. The ripples formed upon passage through the screen dissipate within seconds after flowing past the shear field. In contrast, the orthogonal pattern is much longer lived; its persistence upon reduction to extruded rod or drawn filament is dependent upon the viscosity determined by the spinning temperature.

ISSN:0148-6055    

DOI:10.1122/1.550561

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

The dynamics of oscillatory shear‐induced alignment in side‐group liquid‐crystalline polymers (SGLCPs) are investigated by measuring transmittance and viscoelasticity during and after the alignment process. We compare the dynamic moduli and transmittance of shear‐aligned samples with those of magnetically aligned samples. Relative to an unaligned nematic, magnetic alignment doesnotappreciably alter the dynamic moduli. This suggests that, at small strains, distortion of the polymer backbone dominates the linear viscoelastic response so that the moduli are insensitive to macroscopic director alignment. With increasing strain, deviations from linear behavior are manifested by anincreasein the dynamic modulus. This strain hardening may be due to the tendency of the mesogens to orient perpendicular to the backbone in the present SGLCP, since the mesogens attached to a given strand may be forced to adopt an intermediate orientation between that dictated by the backbone segment and that dictated by the local orientation of the director. As strain increases, this competition could reduce the order parameter from its equilibrium value; the associated increase in the free energy may contribute significantly to stress, resulting in the observed strain hardening. Furthermore, the coupling between the mesogen and backbone provides a means for the bias in the orientation distribution of the backbone to bias the orientation of the mesogens, which may drive the observed flow‐induced alignment of SGLCPs.

ISSN:0148-6055    

DOI:10.1122/1.550606

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

We report oscillatory shear and steady‐shear measurements on a model main‐chain thermotropic polyether in its nematic and isotropic phases, and in the nematic–isotropic biphase. The nematic phase is viscoelastic on all time scales measured. Since these time scales are longer than the molecular time scale of diffusion, we conclude that the nematic viscoelasticity is controlled by the large‐scale defect structure. The isotropic phase approaches the response of a viscoelastic liquid, but there are much longer relaxation times present in the isotropic phase than would be expected for a flexible polymer. Viscoelasticity of the nematic and isotropic phases show very weak temperature dependences, but the viscoelastic response of the biphase is extremely temperature sensitive due to the first‐order phase transition. The Cox–Merz rule was tested and found to apply in the isotropic phase and fail in the nematic unless the sample is sheared just prior to the dynamic analysis.

ISSN:0148-6055    

DOI:10.1122/1.550562

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550563

出版商:The Society of Rheology    

年代:1994

数据来源: AIP