摘要:

A surface force apparatus has been adapted to operate as a rheometer on a molecular scale. The way it works and the new possibility given to study dynamic properties of very thin films (<1 μm) squeezed between solid surfaces are discussed. Three kinds of tests, enabling the measurement of the viscoelastic properties of confined media in addition to static interactions between the surfaces, are presented. These are:draining—the surfaces are brought together at constant speed;relaxation—the displacement is stopped and the force is decreasing toward a static force;oscillations—the surfaces are submitted to small axial sinusoidal displacements around an average position. Results obtained with dilute polymer solutions moderately confined are presented. Variations of parameters characterizing the terminal zone such as the zero shear viscosity, the average relaxation time, and a parameter connected to the dispersion of relaxation times are presented and qualitatively discussed as a function of the separation between the solid surfaces.

ISSN:0148-6055    

DOI:10.1122/1.550588

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

This paper presents a new experimental approach for determining both normal‐stress differences using a modification of the Weissenberg rheometer. The method eliminates both wall slippage and rim fracture that so far have been major obstacles in attaining steady‐state flow conditions with highly consistent fluids in rotational instruments. This is achieved by extending the upper plate with a separately moving exterior ring that is closed on the outside. For determination of both normal‐stress differences with this modification, the wall pressure at the radius of separation is to be measured in addition to the integral force on the inner plate, yielding a set of linear differential equations inN2with linear terms inN1. Combining two geometrical configurations from the cone‐and‐plate, cone‐plate‐distance, and parallel‐plates systems, a differential equation inN2only is obtained whose solution can be determined from a quadrature of the measured mechanical quantities.N1results from the equations after reintroduction of the solution for the second normal‐stress difference.

ISSN:0148-6055    

DOI:10.1122/1.550589

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

Traditionally, the complex modulus is determined by Fourier analysis of steady state oscillatory data. However, steady state is not obtained immediately and data from the first period of oscillation must therefore be discarded. In the present work a recursive analysis algorithm for the determination of complex moduli from oscillatory data of the first period, which includes a transient response alongside the steady state response, is derived. The algorithm is based on Boltzmann’s principle of superposition. At any given time, the analysis algorithm provides the best possible estimate of the complex modulus on the basis of the information available at that time, i.e., the stress and strain history. The analysis algorithm has been tested on simulated data from a mathematical model of an amorphous polymer. The tests show that the new analysis algorithm can determine the dynamic mechanical properties with very good accuracy from oscillatory data of the first period, where Fourier analysis fails. Thus, use of the new algorithm allows a reduction of the experiment time by a factor of 2 for experiments with a sufficiently high signal‐to‐noise ratio to avoid averaging over several periods. This reduction of the measurement time is particularly important for experiments at very low frequencies where the long measurement times seriously limit the number of runs per day.

ISSN:0148-6055    

DOI:10.1122/1.550590

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

A high‐frequency torsional rod apparatus (HFTRA) has been developed for measurements of moderately high‐frequency viscoelastic properties of dilute, low viscosity polymer solutions. It employs a long cylindrically shaped resonating element driven by an X‐cut (torsional mode) quartz piezoelectric crystal. Up to 11 different normal modes are employed, resulting in 11 discrete frequencies, ranging from approximately 20 to 500 kHz. Measurements are made in free decay; a key feature of the apparatus is the method for high precision measurements of damping coefficients and eigenfrequencies. The instrument is suitable for liquids with 0.003P<‖η*‖<5Pand η’≥1.33η‘. Typically, η’has a relative uncertainty of ±2% or less, with the relative uncertainty for η‘being about 4% for liquids with η‘greater than 0.01P. Values of η’obtained in the HFTRA for low viscosity liquids with small η‘show excellent agreement with the known steady flow viscosities. Measurements of the viscoelastic properties of polymer solutions employing the HFTRA and two other instruments capable of higher precision measurements show excellent agreement, demonstrating that the HFTRA is sufficiently precise for dilute‐solution viscoelasticity studies; the working frequency range is such that most of the polymer relaxation time spectrum can be probed for many polymer/solvent combinations.

ISSN:0148-6055    

DOI:10.1122/1.550608

出版商:The Society of Rheology    

年代:1994

数据来源: AIP

摘要:

A new rheometrical technique has been developed to determine the mechanical relaxation spectra of viscoelastic materials by measurement of shear wave group and phase velocities in a virtual gap rheometer. The technique’s sensitivity and accuracy in providing relaxation spectra directly from measurements arises from exploitation of marked changes in wave dispersion characteristics in the vicinity of stress relaxation mechanisms. The new technique is compared with two conventional, wave‐based rheometrical techniques (based on pulse‐propagation and resonance measurements), which are adversely affected by wave dispersion. The superior accuracy and sensitivity of the new technique over conventional rheometry is demonstrated with reference to the Maxwell model and published relaxation spectra. Experimental results obtained on a concentrated colloidal dispersion serve to demonstrate the measurement principles.  

ISSN:0148-6055    

DOI:10.1122/1.550591

出版商:The Society of Rheology    

年代:1994

数据来源: AIP