Nanocomposites are a class of materials which are used in a huge variety of applications due to the ability to engineer the mechanical, electrical, thermal, optical, electrochemical, catalytic properties with exceptional precision.
Because of their versatility, they are found in computer components, batteries, fuel tanks, food packaging, tissue engineering, in-situ drug delivery and many other diverse situations.
While the particular properties of these amazing materials are determined by interactions between the base materials and nanoparticles, it has been difficult to analyse the precise nature of material structures and hence understand these interactions. Until now, that is.
Materials scientists in the UK, China and Romania have developed a technique to image the morphology of semicrystalline polymer nanocomposites in three dimensions, helping to observe the distribution of nanoparticles, understand how the materials achieve their properties, and how the structure is influenced by the solidification process.
Previously, imaging techniques have given results in 2D, with the structure beneath being implied rather than observed. Attempts to render images in 3D have had some success, but the output is of low contrast and relies on specialized equipment, so accuracy and availability of data is limited.
The latest technique, developed by researchers at The University of Sheffield (UK), Xi’an Jiaotong University (China), Soochow University (China), Coriolan Dragulescu Institute of Chemistry (Romania) and Zhejiang Sci-Tech University (China), uses fluorescence microscopy to show the internal morphology of semicrystalline polymers and their nanocomposites.
By adding ultraviolet absorber to the polymer melt and controlling the cooling rate of the materials, the researchers have used confocal laser microscopy to observe how microstructural features within the solid form.
This study revealed a number of surprising results. Firstly, the way in which microstructural constituents known as spherulites aren’t always truly spherical, but can elongate perpendicular to the film surface. Secondly, nanoparticles aggregate at spherulite boundaries (shown by light areas in the video below).
Finally, and most surprisingly is the observation that during solidification the top surface of the nanocomposite (in places) is almost an exact copy of the bottom one. This means that the spherulites nucleated on opposite surfaces within seconds of each other.
As solidification progresses, the spherulites form a connection giving them the appearance of a goblet – a structure formed when two half spherulites formed at opposite surfaces meet. This phenomenon occurs due to the formation of one surface spherulite creating a negative pressure around it and at the opposite surface. The second spherulite is therefore stress induced. As the spherulites grow closer, a connected stress field forms which encourages the growth of the ‘stem’ of the goblet.
This process is shown in the video below.
The imaging technique described has enabled this growth mechanism to be observed for the first time with some amazing images captured showing this previously undiscovered treasure trove of cups and goblets.
The researchers are now looking for further applications of their imaging technique to help grow the knowledge of polymers and nanocomposite morphologies and microstructural features.
Full details of the research can be found in the paper published in Nature Communications (https://www.nature.com/articles/s41467-021-25297-w), and in a blog (https://chemistrycommunity.nature.com/posts/polymer-morphology-in-3d) written by Professor Ungar.