Foams – gas trapped in solids or liquids – are common materials, found all around us. Liquid foams can be found in cosmetic formulations, home care products and food, whereas solid foams find use in mattresses, insulation and lightweight structures in automotive, space and aircraft applications. Solid foams are cellular materials which have low densities and thermal conductivities. These properties, together with high energy absorption and sound dampening, render them ideal for use as packing material, shock absorbers and acoustic isolators.
To model, predict and tailor the mechanical properties of these foams, thorough characterization of wall thickness, cell size, shape and arrangement are required. On smaller length scales, the manufacturing process itself also has an influence on the mechanical properties. Thermoplastics such as polystyrene, polyethylene, polypropylene, polyuretane and polylactic acid are often used as base materials for foams, and the thermal history and degree of crystallinity as well as crystal type and orientation have been shown to be of major influence on the reaction of the foam to mechanical stress and its failure behavior. The large range of length scales involved that needs to be probed, renders scattering techniques (Static and Dynamic Light Scattering as well as Diffusing-Wave Spectroscopy and Small and Wide-Angle Neutron and X-ray Scattering) essential for full characterization of foams.
In a publication1 from the Technical University of Dresden in collaboration with the Leibniz Institute for Polymer Research in Dresden, the researchers illustrate the importance of full understanding of the base materials and the modifications this undergoes in processing of polypropylene (PP) to a closed-cell bead foam. Using Wide Angle X-ray Scattering (WAXS) measurements, they were able to find the absolute amounts of α- β- and γ-phase of the PP. Different ratios of the crystalline phases were found for pristine beads as compared to moulded material. Furthermore, these measurements revealed different ratios as well as different percentages of crystalline component for scans across beads joined by the moulding process. Finally, 2D WAXS measurements revealed that the PP is oriented (right image in the picture below) at the joints of merged beads and at the outer skin of pristine beads, also characterized by lower transmissions (as illustrated in the middle graph), whereas it is non-oriented at inner areas (left image showing isotropic scattering). The degree of crystallinity was found to be a good predictor of the stress-strain curves for these materials.
Credit: International Journal of Engineering Science, 2019, doi: 10.1016/j.ijengsci.2019.103168
In a recent example of what SAXS (Small-Angle X-ray Scattering) and WAXS can reveal about foams, Ningbo Key Laboratory of Specialty Polymers at Ningbo university, reports on the influence of different temperatures on the crystallization of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) foams. Using both WAXS and SAXS they find that the crystal form remains the same across variations in processing temperature. There are however differences in the long period distance which represents the average distance between hard domains, especially between pristine and foamed PHBV. They reason that the foaming process reduces the crystal nucleation energy barrier, which results in thickened crystal lamellae.2
Finally, The College of Polymer Science and Engineering of Sichuan published a study3 detailing the effects of stretching on thermoplastic polyurethane (TPU). TPU exhibits microphase separation with soft segments forming a continuous phase and rigid, hard domains distributed uniformly which form the physical cross linking. They find that the long period distance of thermoplastic polyurethane (TPU) foams increases when the foams are stretched, while removal of the stretching force results in decrease of the long period distance. The increase of the distance between the hard domains upon stretching is explained by the extension of soft segment chains along the elongation direction. Removing the external force at temperatures significantly higher than the glass transition temperature of the soft segments results in chain relaxation of flexible domains.
These results demonstrate that SAXS and WAXS are efficient tools to study the crystalline properties of various types of polymer based foams under dynamic external conditions such as temperature and stress/strain variations.