Rotary viscometer / Rotary rheometer RHEOTEST® RN and Capillary Viscometer RHEOTEST® LK - Chemistry of Polymers


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Measuring task in research and development

Solutions of polymers are used in different branches of industry, e.g.:

In food industry as bonding agents and thickeners
In pharmaceutics as medium in production of pills
In processes of coating applying to reduce resistance to the flow
In water handling facilities for waste water treatment
In mineral oils production to improve viscosity index of motor oils

Range of application stretches over very wide range of molecular weights from about 103 bis 108 g/mol.

One of the most important property of plastic materials is fitness to easy and diverse processing in melted liquid state. It is used especially wide in putting into shape, such as injection casting, extrusion or film blowing. Solutions and melts of polymers have pronounced non-Newtonian flow properties. First of all, they become apparent in viscosity that depends very much on shear rate at stationary shear flow. Polymer solution or polymer melt behave as Newtonian liquid in the range of very small shear rates. It means that viscosity at quiescent state (zero or initial viscosity - viscosity at zero rate of shear) does not depend on shear rate. Viscosity decreases at shear rate increase in conditions higher than critical shear rate. Along with viscous properties polymer solutions and polymer melts demonstrate elastic flow properties also. In general, the latter define properties of polymers during processing.

On the one hand, conditions of load and temperature have an influence on viscoelastic properties. On the other hand, molecular composition of dissolved and melted polymers have a big influence on viscoelastic characteristics. By using of rheometers the flow properties are recorded. Following tests give important information to the polymer structure:

stationary measurements of shear to determine viscosity at rest and viscosity fall at shear rate higher than critical one
non-stationary measurements of shear to characterize process of acceleration and damping
mechanical oscillation measurements to determine complex oscillation viscosity, memory module and loss module

Measuring task in the area of quality control

Quality of polymers depends essentially on their molecular composition. In other words, it depends on molecular weight, molecular weight distribution and chemical structure of molecules. Hence, finally targets in the area of quality control are related to this. Molecular weight determination is often carried out by method of Staudinger. For that, one measures viscosity of a number of dissolved polymer's concentrations (diluted solutions) and of a solvent. It is possible to carry out precise measurements of viscosity with the help of our patented capillary series RHEOTEST® LK viscometer, which, in addition, is distinguished also by measuring system made of high quality steel, simple maintenance, electronic temperature compensation of viscosity and interface RS 232 C.
For concentrated solutions and melts of polymers it is necessary to carry out measurements of flow curves in the wide range of shear rates. Zero viscosity determined from the flow curves, correlates with molecular weight. In order to take off flow curves one should use rheometer with technical specifications of which allow to take off flow curves in the actual range of loads on polymer, that is, in conditions of it practical use or further processing. It is possible, e.g., with our series RHEOTEST® RN rheometer.

Possible measurement methods:

• Measurement method to determine flow curves
Controlled Rate Tests - CR-Tests:
• Measurement method to determine acceleration and damping processes
Controlled Rate Tests - CR-Tests:
  Controlled Stress Tests - CS-Tests
• Measurement method for determination of molecular weight
Flow curves measurement in very wide range of shear rates


Figure 1: Shear viscosity h against shear rate D at different concentrations of polymers (polystyrene in toluene)


Viscosity function of polymers, especially zero viscosity and transition from shear rate-independent viscosity to shear rate-dependent viscosity depends upon molecular weight, molecular weight distribution and chemical structure of molecules and for polymers solutions depends additionally also upon concentration and quality of solvents. Consumer properties of end polymer products or properties of semi-products during processing depend on, first of all, load ranges encountered in praxis.

Notes:

High temperature constancy and time of measurements have decisive significance to determine viscosity as function of shear rate. Time of measurements should be selected in such a way that for each shear rate the equilibrium must set in between the latter and resultant shear stress. It means that longer measurement time is necessary for small shear rates and shorter measurement time is necessary for big shear rates.

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Measurement method for determination of acceleration and damping processes


Figure 2: Shear stress change in time at different step-change of shear rate



Figure 3: Creep-recovery measurement

Shear modulus and shear viscosity could be determined directly using shear stress acceleration characteristic. Molecular weight of structure net and time of relaxation are calculated on the basis of these parameters. From here one could obtain data about molecular composition of a polymer, consumer properties and properties during processing. Measurement results for measurement of creep and reverse deformation tests give possibility to distinguish exactly the viscous and elastic component of deformation. Moreover, one obtains detailed notion concerning molecular structure of a polymer. Values calculated from here for such parameters as modulus, viscosity, relaxation time and retardation time or for the whole their spectrum give clear idea about netlike bonds of molecules that form macromolecules due to bond forces. Besides, one obtains important information concerning the influence of additives and filling agents on flow properties of a polymer. From here it is possible to determine exactly the molecular weight on the basis of viscosity values measured.

Notes:

Prerequisite for measurement of processes of acceleration and damping is implementation of high quality rheometer, measurement results obtained using the latter reflect real properties of a material but not inertia characteristic of the instrument only.

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Molecular weight determination with the help of capillary viscometer


Figure 4: Relationship of reduced viscosity hred against concentration for different molecular weights


The basis for determination of molecular weight according to Staudinger method is the fact that relative viscosity of suspensions depends on volumetric proportion of solid particles. One could consider solid state particles in somewhat ideal form as balls. According to Staudinger it is possible to transfer to solutions of polymers also, since polymers in solutions form ball-like structure also and could be seen as solid substance. Polymers increase viscosity of solutions in comparison with the pure solvent the greater, the higher is their molecular weight.
From these theoretical notions one could come to a conclusion that it is possible to determine Staudinger index [h] with the help of simple viscosity measurements. There is relationship [h] = K * Ma between Staudinger index and molecular weight. Constants K are listed for many polymer-solvent systems in "Polymer-Handbook" by Brandrup, J.; Immergut, E.H..

Notes:

Relative viscosity hrel = h0 / hLM is calculated on the basis of measured solution viscosity h0 and solvent viscosity hLM . Reduced viscosity hred = (hrel -1) / c is calculated on the basis of relative viscosity. It is presented as function of concentration c and is extrapolated at c = 0. Determined in such a way value corresponds to the Staudinger index [h]. One could measure viscosity of diluted polymer solution and of a solvent very simply, precisely and quickly with the help of our capillary series RHEOTEST® LK viscometer.

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