摘要:

An understanding of the flow behavior of liquid crystalline polymers (LCP's) is of immense practical value because of the potential to form high modulus materials from these polymers. These fluids exhibit a high degree of structure even in the quiescent state, as evidenced by their ability to transmit polarized light. In an effort to understand how the structure changes during flow, we have carried out a study on the transient shear flow properties of two thermotropic copolyesters of 60‐ and 80‐mole % para‐hydroxybenzoic acid (PHB) and polyethyleneterephthalate (PET) and a lyotropic system of poly‐p‐phenyleneterephthalamide (PPT) in 100% sulfuric acid. In one of the first theories concerned with the flow behavior of liquid crystalline fluids, which was proposed by Ericksen, the transient flow properties are all predicted to be due to changes in orientation of a director which describes the orientation of packets of rod‐like molecules. Stress growth, interrupted stress growth, and stress relaxation experiments are carried out on the three LCP's and at first sight are in qualitative agreement with the predictions of Ericksen's theory. However, wide angle X‐ray scattering analysis of quenched samples subjected to shear flow along with annealing experiments on oriented samples indicate that these materials do not orient readily in shear flow. Furthermore, orientation generated during extensional flow relaxes at a rate much faster than is indicated by the interrupted stress growth experiments. It is concluded that the stress growth response of LCP's is due to a disruption of a domain structure which exists within the fluid rather than to orientation changes of the domains of rod‐like molecules.

ISSN:0148-6055    

DOI:10.1122/1.549863

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

摘要:

Cellulose triacetate (CTA) films have been prepared by extrusion of various liquid crystalline and isotropic CTA solutions in a mixture of dichloroacetic acid (DCA) and formic acid (FA) through an annular die. The extrudate was either given a uniaxial drawdown into a coagulation bath or biaxially deformed via expansion with recirculating coagulant to the inside of the extrudate. Thus solutions with CTA concentrations ranging from 23 to 30% were extruded as uniaxially‐ and biaxially‐oriented films. A survey of solvent systems to produce liquid crystalline CTA showed DCA or DCA/FA to be the only reasonable choices from among the solvents tried. Rheological characterization showed that CTA/DCA/FA could be processed in the 23 to 30% concentration range. A maximum in the viscosity vs. concentration curve, plastic flow stresses, and viscosity‐shear rate behavior typical of liquid crystalline systems were observed. At 60°C, the system reverts to isotropic and provides for some comparisons between isotropic and anisotropic behavior. The CTA films were characterized by small angle x‐ray scattering (SAXS), mass density, scanning electron microscopy (SEM), permeation studies, differential scanning calorimetry (DSC), wide angle x‐ray diffraction (WAXS), and birefringence. The latter two methods were used to characterize the molecular orientation produced with various processing conditions. The volume fraction of microvoids was computed by SAXS invariant analysis and found to vary from 0.01 to 0.18%. This is low compared to results from mass density measurements. Density and SAXS measurements indicate a void content decreasing with decreasing solution concentration and heat treatment. SEM locates micron‐size voids in the cross‐section of films produced from high solution concentrations. Fibrillar structure was observed for “peeled” uniaxial films. This is increased with an increase in solution concentration. Biaxial films could not be peeled.

ISSN:0148-6055    

DOI:10.1122/1.549864

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

摘要:

The theoretical framework of Doi and Marrucci for predicting the rheological properties of polymeric liquid crystals is considered in some detail. Previous limitations of the analysis, which restricted the theory to asymptotically low deformation rates, have been removed. The resulting predictions were compared with experimental data using both shearing and extensional flow configurations; for the latter a new fiber spinning apparatus was constructed with an order‐of‐magnitude increase in sensitivity over that of the best previously reported extensional devices. The agreement between theoretical prediction and experimental result is excellent for steady state deformations over the full range of conditions studied for both extensional and shearing modes of deformation; this is especially remarkable in view of the fact thatallrheological properties are given by equations which contain only two experimentally adjustable parameters. However, this excellent agreement between theory and experiment does not appear to extend to transient behavior, nor is it expected at very high deformation rates. At solution concentrations just beyond that of the isotropic‐anisotropic transition, the viscositydecreaseswith increasing polymer concentration. These fluids are of surprisingly low viscosity; in steady shear the power law index or slope of the shearing stress—shear rate curve decreases progressively as the shear rate increases over the range studied. The first normal stress difference islinearin shear rate, at low shear rates, and is predicted to saturate at high shear rates. The first normal stress difference is larger than the shearing stress at a given shear rate, yet the fluids are so inelastic that they exhibit a die swell which is less than that of a Newtonian fluid. The Trouton ratio (ratio of the extensional to shearing viscosities) does not appear to be equal to three at low deformation rates as it is for all other fluids, and the fluid is predicted to become extension‐thinning at high deformation rates. Polymeric liquid crystals are thus observed to be a class of materials rheologically distinct from Newtonian fluids and from solutions or melts of flexible polymer molecules.

ISSN:0148-6055    

DOI:10.1122/1.549865

出版商:The Society of Rheology    

年代:1986

数据来源: AIP