A multiscale survey of the “Buckle Gum”: Hydrostatic response of elastomers filled with hollow thermoplastic microspheres

• Post-buckling of hollow thermoplastic microspheres is visualised under pressure . • Largest microspheres dominate the composite volume change under pressure. • Microspheres largely recover their shape, exhibiting high reversibility. • Microspheres volume change correlates with macroscopic hydrostatic measurements. • Pre- and post-buckling simple models partially reproduce experimental observations. “ Buckle Gum ” refers to elastomer matrix composites filled with hollow thermoplastic microspheres (HTM). These particles are attracting particular attention for applications involving pressure, since they are capable of reversible deformation, unlike their glass counterparts usually used in syntactic foams. This work provides direct evidence of this deformation under in situ conditions, using an experimental hydrostatic compression set-up embedded in a tomograph. The HTMs buckle at a certain critical pressure, beyond which shrinkage and loss of sphericity are observed, characteristic of a post-buckling regime. The resulting volume change across the microsphere population appears independent of volume fraction up to 10 bars, and is predominantly governed by the response of the largest particles. This observation corroborates hydrostatic compression tests conducted on the composite, analysed using an alternative imaging technique. Upon return to ambient pressure, nearly all HTMs recover their initial shape and volume without exhibiting damage. Our inverse modelling approach, supported by both tomographic and macroscopic experiments, reproduces the microspheres response for pre- and post-buckling regimes thanks to two complementary model branches based on the Composite Spheres Assemblage theory. Even though they cannot yet be unified, they provide a consistent framework for future modelling investigations.