Gas chromatography (GC) is useful for identifying volatile reaction products during cure. High performance liquid chromatography (HPLC) is particularly useful in determining the average molecular weights and molecular weight distributions of polymeric materials; particularly suited to analysing thermoplastic resins. This equipment is generally supplied with automated instrumentation, which is relatively simple to operate and maintain. Liquid chromatography technique separates molecules according to their size in solution and employs various detectors to monitor concentrations and identify chemical components. The technique requires calibration with standard polymers.
Nuclear reaction analysis can be used to directly measure localised moisture content through the thickness of polymeric materials. Specimens are conditioned in D2O instead of H2O and the resultant deuterium concentrations in the specimen are measured. The surface is bombarded with a finely collimated 3He beam, which reacts with the deuterium in the sample, releasing high-energy protons. The proton yield is directly proportional to deuterium in the reaction volume. Moisture profiles with a resolution of 12 mm have been measured.
Scanned Probe Microscopy (SPM) is a widely used technique to measure topography and/or localised material properties. A sharp probe is rastered over a surface, typically using a piezo tube scanner, whilst recording a signal (i.e. height (topography), lateral force, conductance, magnetic attraction or even electrochemical response). Alternatively the probe can be stationary and the sample rastered.
AFM surface map—anodised aluminium
Atomic Force Microscopy (AFM) is a subset of SPM and measures surface topography on the atomic scale. It has a spatial and depth resolution in the order of a few nanometres. Maximum scan range is typically 100 µm in the X- and Y-directions, and 7 µm in the Z-direction. A small tip mounted on a flexible cantilever attached to the tube scanner is used to analyse the vertical movements of the probe as it travels over the contours of the surface of the sample (similar principle of operation to the stylus profilometer). The surface deflection of the cantilever, caused by changes in topography, is detected using a laser and photo-detector. There are two main modes of operation: contact and dynamic (non-contact), which maintain a constant force or separation to the sample surface respectively. Almost all materials can be studied without the need for sample preparation. In contact mode, the probe is brought into contact with the surface with a low force (~µN) and rastered across the surface, alternatively the tip can be oscillated for tapping mode and non-contact mode imaging for soft materials. AFM provides a powerful tool for surface analysis. High-resolution scans can take some time, and effects from thermal drift and probe shape de-convolution must be considered.
For conducting materials, scanning tunnelling microscopy (STM) can be used, but this closely related technique can confuse between electrical and structural features. There is usually high scatter in roughness data, but significant differences are easily observed. Typical spatial resolution is 3 nm with a vertical feature resolution of < 0.1 nanometre. Investigated areas are limited to approximately 10 nanometre square. As well as quantifying surface roughness, this method can produce a 3-D image of a real surface for visual inspection and can also measure relative surface stiffness giving local hardness and modulus information (i.e. contact mode).
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