  Recently, hollow thermoplastic microspheres, such as Expancel made by
Nouryon, have emerged as an innovative filler material for use in
polymer-matrix composites. The resulting all-polymer syntactic foam takes on
excellent damage tolerance properties, strong recoverability under large
strains, and favourable energy dissipation characteristics. Despite finding
increasing usage in various industries and applications, including in coatings,
films, sealants, packaging, composites for microfluidics, medical ultrasonics
and cementious composites, there is a near-complete absence of statistical
geometrical information for Expancel microspheres. Further, their mechanical
properties have not yet been reported. In this work we characterise the
geometrical quantities of two classes of Expancel thermoplastic microspheres
using X-ray computed tomography, focused ion beam and electron microscopy. We
also observe the spatial distribution of microspheres within a
polyurethane-matrix syntactic foam. We show that the volume-weighted
polydisperse shell diameter in both classes of microsphere follows a normal
distribution. Interestingly, polydispersity of the shell wall thickness is not
observed and in particular the shell thickness is not correlated to the shell
diameter. We employ the measured geometrical information in analytical
micromechanical techniques in the small strain regime to determine, for the
first time, estimates of the Young's modulus and Poisson's ratio of the
microsphere shell material. Our results contribute to potential future
improvements in the design and fabrication of syntactic foams that employ
thermoplastic microspheres. Given the breadth of fields which utilise
thermoplastic microspheres, we anticipate that our results, together with the
methods used, will be of use in a much broader context in future materials
research.
