| The techniques offered by Malvern Instruments’ Material Characterization Services are shown below with a brief explanation as to the basis of the technique and its applicability.
Dynamic Light Scattering:
The Dynamic Light Scattering technique measures the time-dependent fluctuations in the intensity of scattered light which occurs because the particles are undergoing random, Brownian motion. Analysis of these intensity fluctuations enables the determination of the distribution of diffusion coefficients of the particles, which are converted into a size distribution using established theories.
The upper size limit of the technique is sample density dependent; as dynamic light scattering requires that particles be randomly diffusing, this places the upper size limit as the point where sedimentation of the particle dominates the diffusion process. The lower size limit will depend upon the excess scattered light the sample generates compared to the suspending medium. Many factors will contribute to this lower size limit including the sample concentration, the relative refractive index (i.e. the particle refractive index compared to the medium refractive index), laser power, laser wavelength, sensitivity of the detector and optical configuration of the instrument. The lowest particle size measured on an instrument containing non-invasive backscatter (NIBS) is 0.6 nanometres.
Traditionally dynamic light scattering experiments were performed at an angle of 90 degrees. This meant that samples had to be very dilute to avoid multiple scattering phenomena. With the advent of NIBS it is now possible to measure at higher particle concentrations (up to 40% w/v). Though it is often necessary to dilute the sample to lower concentrations in order to understand any effect that any particle - particle interactions are having on the reported size.
Static Light Scattering - Absolute Molecular Weight:
The intensity of scattered light that a macromolecule produces is proportional to the product of the weight-average molecular weight and the concentration of the macromolecule. The technique used to measure absolute molecular weight is static light scattering (SLS), which makes use of the time-averaged intensity of scattered light as a function of sample concentration.
For small molecules, which show isotropic scattering, accurate molecular weight measurements can be made at a single angle. For larger molecules, it may be necessary to measure over a range of concentrations and a range of angles for accurate molecular weight determination.
Laser Diffraction:
The technique of laser diffraction relies on the fact that particles passing through a laser beam scatter light at an angle that is inversely proportional to their size (small particles scatter light at high angles whereas large particles scatter light at low angles). It is therefore possible to calculate particle size distributions if the intensity of light scattered from a sample are measured as a function of angle. This angular information needs to be compared with a scattering model (Mie theory) in order to calculate the size distribution. The technique has a very large dynamic range, from 3.5mm to below 100nm, as defined by the range of angles over which the scattering pattern is collected and the instruments optical configuration.
The laser diffraction technique is flexible in terms of the type of samples that can be measured. Particles can be dispersed in a liquid medium (wet laser diffraction) or as solid particles dispersed in an air stream (dry laser diffraction). Measurements are also possible on aerosol-based systems such as liquid atomizers and pharmaceutical inhalers. For wet analysis samples require dilution to be measured. The sample concentration over which samples can be measured is approximately 50 - 1000 ppm.
Zeta Potential:
Zeta potential is an important parameter in understanding electrostatic colloidal dispersion stability. Zeta potential is the charge a particle acquires in a particular medium. It is dependent upon the pH, ionic strength or concentration of a particular component. The mobility of the particles undergoing electrophoresis is measured by the technique of laser Doppler electrophoresis. This measured electrophoretic mobility is then converted to zeta potential using established theories.
Typically, the particle size of the sample needs to be less than 10 microns and the sample should be dilute. Therefore, for samples that require dilution prior to measurement, it is important to specify the dilution medium. The effect on the zeta potential of changes in pH, ionic strength and concentration of an additive can be automated to provide information such as the iso-electric point of a sample.
Automated Vision Systems:
With Malvern’s range of automated image analysis systems it is possible to analyze thousands of particles and gain not just particle size information, but also particle shape information. Particle shape is being seen as an increasingly important factor in governing how materials process and perform and documenting samples for regulatory authorities.
Samples can be presented as dry powders, or suspensions in aqueous, oil or organic solvent media.
Image analysis can be used to measure particle size from 0.7 micrometers to 2 mm. By its very nature its dynamic range is more limited than with the other, ensemble, techniques. But, the range can be extended by combining data obtained at several magnification intervals.
Rotational Viscometry and Rheology:
Rheology measures the mechanical characteristics of a sample. Flow properties, viscoelastic and normal stresses are all properties that can be characterized. Bohlin rheometers can apply either a stress or a strain, making viscoelastic measurements such as creep/recovery and stress relaxation possible on one, integrated instrument.
A wide range of measuring geometries enables samples from as low a viscosity as water, to high modulus solids to be measured on one instrument. The wide range of temperature controllers allows measurements to be made from -150°C to 550°C. Accessories are also available which allow measurement of the effects of external stimuli, examples of which are Ultra-Violet irradiation, high pressures or high voltage electrical fields.
Rheological measurements can be used as quality tools as well as for development and trouble shooting purposes. The data obtained can help improve processes and final product quality as well as determine areas where formulation changes can lead to cost savings and increased productivity.
Capillary rheology:
Historically, capillary instruments were used mainly within the polymer industry for modelling of processes such as extrusion, moulding and fibre spinning. Nowadays they are also used widely within the food and coatings industries to simulate processes such as pumping and spraying.
The dual die system of the Rosand Rheometers allows both shear and extensional properties to be directly measured and accounted for.
A wide range of accessories are available so that other important extrusion parameters such as extrudate swell, melt rupture and fracture, and melt strength can be assessed. Processing variables, (for example degradation profiles) can be determined, which can help to increase productivity or decrease wastage.
Capillary rheometers have an advantage over the traditional polymer Melt Flow Index test, by being able to measure over a wide range of shear rates, and are therefore able to reproduce the kind of forces a sample will actually be subjected to within a process.
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