Seeing is believing - optical techniques for measuring nanoparticle content and dispersion in polymeric matrices
Bill Broughton and Triantafillos Koukoulas from the UK’s National Physical Laboratory in Teddington highlight optical techniques for measuring nanoparticle content and dispersion in polymeric matrices.
Small concentrations of nanoparticles in polymeric matrices (typically one to five weight per cent) can have a beneficial impact on the material’s performance. Such concentrations have been observed to improve stiffness, strength, impact resistance, electrical conductivity of thermoset resins and thermal stability. A small addition of carbon nanotubes, say 0.5wt%, can increase electrical conductivity by six orders of magnitude and thermal conductivity by two orders of magnitude. However, poor or non-uniform particle dispersion (clustering) in polymer nanocomposites (PNCs) limits the realisation of performance improvements.
The challenge is measuring particle content (loading) and distribution (degree of dispersion) in PNCs. Measurement techniques such as transmission electron microscopy (TEM) and X-ray diffraction (XRD), can provide a wealth of information on the structural, physical and chemical nature of materials. Transmission electron microscopy is an immensely powerful, although destructive, technique, capable of providing high-resolution images at the nanoscale. Such images involve minuscule sample quantities (10-18kg), and therefore a large number of images are required to provide, sufficient information on the material’s loading and dispersion. X-ray diffraction can provide information relating to material structure, for example the extent of intercalation (interlayer spacing) and exfoliation of nanoclay particulates.
A major drawback is the difficulty in interpreting data, which can be influenced by factors such as peak broadening, resulting from the superposition of multiple peaks from incomplete intercalation, or signal attenuation due to extremely poor dispersion.
The dielectric and flow characteristics of polymeric melts can also indicate the levels of particle loading and dispersion, but data interpretation tends to be difficult as different combinations of loading and dispersion can produce similar results. Ultrasonic resonance spectroscopy, scanning acoustic microscopy, laser ultrasound, infrared spectroscopy, Raman spectroscopy and light transmission have also been used with limited degrees of success to characterise nanoscale structures in PNCs.
Looking at optics
A more direct and rapid approach is required that provides accurate, repeatable quantitative measurements and is cost-effective. The National Physical Laboratory (NPL), UK is exploring a number of optical techniques using static and dynamic light scattering for determining loading and dispersion in semi and fully transparent polymeric solids to meet industrial requirements for on-line and service inspection of PNC materials and products.
Contrary to common perception, optical techniques can be used to examine PNCs, as these materials often contain particulates with lengths in excess of a few microns. This coincides with wavelengths in the visible and near-infrared spectrum. According to the definition of a nanoparticulate, only a single dimension is required to be in nanometres.
The NPL project is considering frequency-domain optical coherence tomography (FD-OCT), Fraunhofer wavefront correlation (FWC) and oscillatory photon correlation spectroscopy (Os-PCS). The first two are essentially static light scattering techniques, while the third is a dynamic light scattering technique. The success of all three techniques is dependent on a distinct difference in refractive index existing between the nanoparticles, which act as light scatterers, and the surrounding polymer matrix.
Optical coherence tomography provides a powerful tool for 3D imaging, by acquiring individual 2D information resulting from refractive index boundaries within the sample. Frequency-domain optical coherence tomography based on a Michelson Diagnostics system, operates in a swept frequency mode with a centre wavelength of 1,305nm and a bandwidth of 150nm. It has been used to image 2D layers within samples with a four-micron-metre spacing between scans (layers), penetrating up to a depth of approximately one millimetre.
Depending on the level of loading and dispersion, each layer will appear to have a specific grainy texture, as shown in the OCT image obtained for nanoclay-reinforced polymethyl methacrylate (PMMA). It is important to be able to quantify this appearance and relate it to the particle loading and dispersion.
To achieve this, each layer is segmented and the standard deviation between segments is plotted so that every individual layer is numerically mapped, as shown for images of nanoclay epoxy with poor dispersion (at 1wt%) and good dispersion (at 4wt%). The standard deviation is a measure of the combined effect of particle loading and dispersion. Combining the numerically mapped depth information from all individual layers produces an accurate representation of particle distribution through the material.
Fraunhofer wavefront correlation works on the concept of optical diffraction and diffusion. When a coherent laser beam passes through a transmission grating, the pattern generated is essentially a diffracted 2D spatial function. If this function passes through the neat resin, it will remain largely unchanged as the polymer has a high level of homogeneity and a uniform refractive index distribution. If the original function passes through a polymer embedded with nanoparticles, it will become diffused depending on the level of particle loading and dispersion.
It is important to quantify this appearance and relate it to loading and dispersion. Typical optical wavefront images modulated by PNCs captured in the far field (Fraunhofer plane) are shown below:
The resulting optical wavefronts are captured using a charge-coupled device camera. The wavefront resulting from the neat resin acts as the reference function, while the modulated wavefronts from the PNCs act as the input functions. These are then cross-correlated to extract the contribution of the signal due to particle loading and dispersion.
Photon correlation spectroscopy is a well-established technique for measuring particle sizes in fluids (of known viscosity) as well as velocities in media, by studying the characteristics of the experimental auto-correlation function (ACF). The medium’s physical characteristics can be measured through computation of the ACFs.
Solid materials are stationary media, and so to simulate the fluid motion required for PCS analysis, the solids are mechanically oscillated at particular frequencies and amplitudes and the clarity of the resultant motion-dependant ACFs is studied.
The experimental system at NPL is a custom-built dual-beam PCS set-up (see main article image above) operating in the spectrum’s green part.
The system is intended to extend the capability of the optical techniques to include opaque materials, such as those containing carbon nanotubes and nanographene platelets, as well as transparent and semi-transparent samples. To achieve this goal, the light source will need to be shifted to the spectrum’s infrared region, operating from continuous modes down to pulse durations of tens of nanoseconds. Confirmation that this can be achieved has been seen from the preliminary results of analysing carbon nanotube-embedded polymers by FD-OCT.
Bill Broughton and Triantafillos Koukoulas, National Physical Laboratory, Teddington, Middlesex, TW11 0LW, UK. Tel: +44 (0)20 8943 6834/6316. Email: email@example.com or firstname.lastname@example.org Website: www.npl.co.uk