  The mechanical response of syntactic foams comprising hollow thermoplastic
microspheres (HTMs) embedded in a polyurethane matrix were experimentally
examined under uniaxial compressive strain. Phenomenological strain energy
models were subsequently developed to capture both the axial stress-strain and
transverse strain response of the foams. HTM syntactic foams were found to
exhibit increased small-strain stiffness with reduced density, revealing a
highly tuneable and extremely lightweight syntactic foam blend for
applications. The foams were also found to become strongly compressible at
large strains and possess a high threshold for plastic deformation, making them
a robust alternative to hollow glass microsphere syntactic foams. The
non-standard transverse strain relationship exhibited by HTM syntactic foams at
high filling fractions was captured by Ogden-type strain energy models. The
thermal characteristics of these syntactic foams were also explored with
Differential Scanning Calorimetry testing which showed that HTMs have a
negligible impact on the thermal characteristics of the matrix.
