Architected Multiscale Polymer Foams

Polymeric foam materials continue to gather commercial and research interests due to their unique physical characteristics and emerging applications in a wide variety of industries. This research work viewed the polymer foam industry from three different perspectives, namely materials, processes and applications. Accordingly, technical challenges were carefully selected to make contributions towards each segment by expanding materials choice, proposing architected foam fabrication process and exploring multifunctional applications of foam sensors. Thermoplastic elastomers are thermally processable yet rubberlike materials which experience shrinkage when operated between the glass transition temperatures of soft rubbery phase and hard phase. This brings a challenge in making microcellular foams using batch foaming process where the materials are not fully melted to generate cellular structure. The issue was addressed in this research by incorporating a second phase (i.e. a polymer blend system) to perform as a shape fixing unit. Thus, a series of thermoplastic polyurethane (TPU) elastomeric foams were prepared by blending polylactic acid (PLA) as the shape fixing unit. The morphological, thermal and rheological behavior of the blend system was studied prior to foaming. The blends that contained PLA as the minor phase resulted in foams with high expansion ratio. These blend foams were compared to TPU foams in terms of shape fixity ratio. The results were fitted with Kohlrausch-Williams-Watts (KWW) function to estimate foam relaxation times. Foam relaxation time and shape fixity ratio increased with increasing PLA content. The glass transition temperature of PLA performed as the anchor point to stabilize the foam structure. Architected polymeric materials when designed for specific application could satisfy design requirements with desired unit cell design for having controllable pore size, pore density and pore connectivity. With development of additive manufacturing, fabrication of macro, micro and even nano porous structures have become a possibility. In this research, a new route to fabricating architected multiscale polymer foam is proposed and consequently studied in detail with a view to realizing its potential as a near net-shape process. The fabrication process utilized the synergy of additive manufacturing and batch foaming process to induce macro and microporosity (i.e. structural hierarchy). The results suggested that the process can generate foams with more than 95% expansion uniformity with significantly reduced saturation time. The process is also capable of handling a variety of thermoplastics which also includes polymer blends. Traditional applications of polymeric materials include insulation, energy absorption, floatation, packaging and so on. Though a relatively new concept, multifunctional foams have attracted the research community to develop and explore applications of materials that utilize foams as the skeleton. Such materials demonstrate sensing capability for having piezoresistive characteristics induced by conductive nanomaterials. Piezoresistive auxetic foams sensors coated with silver nanowire were prepared in this research to demonstrate their application as pressure sensors, 3D strain sensors, smart filtration and human motion interface. The auxetic foam sensors reported herein demonstrated improved piezoresistive properties compared to conventional counterpart and showed repeatable and reliable sensing performance for a variety of deformation modes.