摘要:

Independent LCP melt rheological measurements were performed using both capillary and slit rheometer die equipment. The latter was attached to the end of a laboratory extruder so that the attending rheological measurements were obtained under actual processing conditions. The viscosity–shear rate behavior determined by these two methods for the subject LCP melts was significantly different. For a specified melt temperature and shear rate, the viscosity measured by the slit rheometer was as much as 10× lower than that measured by capillary rheometry. The above measured viscosity differences appear real since corresponding viscosity measurements on conventional polymers such as LLDPE and HDPE by these two methods produced equivalent results. Thus, the rheology of these LCP melts is strongly dependent upon preshear history, die geometry, and residence time. This dependence is attributed to the highly anisotropic nature of the LCP melt, the tendency of the polymer ‘‘domains’’ or chains to orient when processed and a long relaxation time for the oriented structure. Knowledge of LCP rheological behavior under actual processing conditions is of paramount importance to properly design extruder or injection molding screws, as well as corresponding dies and molds.

ISSN:0148-6055    

DOI:10.1122/1.550082

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

The two‐dimensional model introduced in a previous paper to describe the nematic phase of rodlike polymers is used to obtain predictions of transient rheological response at high shear rates. First, the transient behavior of a single domain is considered. It is found that, between the tumbling and nontumbling regimes, an intermediate range of shear rates exists where the director continuously oscillates between two orientations close to the shear direction, a result independently obtained in a recent work by Larson, who has considered the full three‐dimensional case. Polydomain structure, typical of polymeric liquid crystals, is then considered. In this paper, results for this case are given for the nontumbling regime only, whereas the tumbling and oscillating regimes are considered in the next paper.

ISSN:0148-6055    

DOI:10.1122/1.550083

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

The two‐dimensional model previously considered is here used to obtain predictions in the complex situation of the tumbling regime which prevails at low shear rates. Although each domain is permanently in a time‐dependent, periodic regime, the macroscopic response can be stationary in time because of the well‐known polydomain structure. First, the average steady rheological response for this situation is calculated by taking a polydomain structure which neglects interdomain interactions. Although the steady state predictions thus obtained favorably compare with the experimental results throughout the range of shear rates, the transient start‐up responses do not, because of the crucial role played by the interactions in such a case. These interactions, due to Frank elasticity, are then introduced in the model in the simplest possible way, i.e., by use of a mean field potential. Under this assumption, also the predictions of transient behavior in the tumbling regime show the correct qualitative features.

ISSN:0148-6055    

DOI:10.1122/1.550084

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

The effects of extrusion processing variables (such as melt tempeature, screw speed, and moisture content), Amylopectin/Amylose ratio as well as extent of starch conversion on the apparent viscosity of starch dough have been studied through the application of a capillary die and a slit die viscometers in conjunction with a single screw extruder, covering a shear rate range of at least two orders of magnitude. The results suggested that increasing melt temperature or moisture content reduced the viscosity. Both shear force and thermal energy contribute to starch conversion. Higher temperature and lower moisture content gave rise to a higher extent of starch conversion. 70% Amylose starch (Hylon 7) gave a higher overall viscosity and was gelatinized less than 98% Amylopectin starch (Amioca), which was attributed to the structural difference between these two corn starches. A simple linear modification of the temperature‐dependent constant and the moisture‐dependent constant in Harper’s viscosity model was shown to fit the data better. Expansion of this model including a power function dependence of viscosity on the extent of starch conversion further improved the prediction of viscosity, significantly in the case of high Amylose corn starch.

ISSN:0148-6055    

DOI:10.1122/1.550085

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

A study of the flow characteristics of a deflocculated, shear thickening, coal–water suspension indicated a significant antithixotropic behavior. The effect of applied shear rate, particle concentration, and shear zone dimensions were examined using a Couette flow viscometer. The dispersion was composed of refined coal particles with mass median diameter of 2.9 μm suspended in water with sufficient anionic dispersant. The rheological properties of eight dilutions of this suspension, from 47.7 to 54.5 vol %, were studied in detail. Shear stress was observed as a function of shear rate in ramp tests in which γ̇ was increased from 10 to 1000 s−1, and as a function of time at several constant shear rates. All tests were conducted using three rotors to test the effect of annular gap width. The characteristics of the reversible, shear thickening with time phenomenon (antithixotropy) that occurs when sufficient shear rate is applied, is described in terms of three regions. Quantitative relationships that allow the determination of stress as a function of shear rate, particle concentration, and shearing time are developed for one of the rotors. An examination of the anisotropic nature of this phenomenon is also presented.

ISSN:0148-6055    

DOI:10.1122/1.550086

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

Cone and plate rheological measurements have been performed on aqueous solutions of hydroxypropylcellulose in the liquid crystalline phase. The time‐ dependent experiments (start‐up of shear and oscillatory shear flow) suggest the presence of a supramolecular structure which is modified by the flow. The characteristic time of structure evolution is estimated, and a tentative explanation of such a process is suggested in terms of polydomain rearrangement. Steady‐state measurements confirm some peculiar properties of liquid crystalline polymers, such as the presence of a shear thinning regime at the lowest shear rates and the occurrence of negative values of the first normal stress difference. Finally, an anomalous, yet reproducible, behavior of the dynamical properties is reported and discussed.

ISSN:0148-6055    

DOI:10.1122/1.550139

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

When blending incompatible polymers in the melt, the resulting morphology is strongly dependent on the interfacial tension. One stage of the mixing process is now used to determine this interfacial tension: in the absence of an overall flow field, extended liquid threads in a liquid matrix exhibit sinusoidal disturbances which cause the threads to disintegrate into lines of droplets. From the growth rate of these disturbances, the interfacial tension between the thread phase and the matrix phase is calculated. For molten polymers, this so‐called ‘‘breaking thread method’’ is relatively fast and simple since it does not require density data for the two phases. Upon addition of a diblock copolymer to the thread phase, a considerable decrease in interfacial tension is measured.

ISSN:0148-6055    

DOI:10.1122/1.550087

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550088

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.550089

出版商:The Society of Rheology    

年代:1990

数据来源: AIP

摘要:

A theoretical analysis is developed for flow of lubricants in a dynamically loaded short journal bearing. The upper convected Maxwell model is chosen to study the influence of lubricant elasticity on the motion of the shaft center within the bearing. An order of magnitude analysis reveals that the fluid relaxation time does not influence bearing performance for steady loads. For time varying loads, however, numerical calculations show that the pressure distribution, the minimum oil film thickness, the attitude angle, and the shaft orbit are all significantly affected by the presence of fluid elasticity.

ISSN:0148-6055    

DOI:10.1122/1.550090

出版商:The Society of Rheology    

年代:1990

数据来源: AIP