Selective Laser Sintering Fabricated Thermoplastic Polyurethane/Graphene Cellular Structures with Tailorable Properties and High Strain Sensitivity

Abstract

Electrically conductive and flexible thermoplastic polyurethane/graphene (TPU/GE) porous structures were successfully fabricated by selective laser sintering (SLS) technique starting from graphene (GE)-wrapped thermoplastic polyurethane (TPU) powders. Several 3D mathematically defined architectures, with porosities from 20% to 80%, were designed by using triply periodic minimal surfaces (TMPS) equations corresponding to Schwarz (S), Diamond (D), and Gyroid (G) unit cells. The resulting three-dimensional porous structures exhibit an effective conductive network due to the segregation of graphene nanoplatelets previously assembled onto the TPU powder surface. GE nanoplatelets improve the thermal stability of the TPU matrix, also increasing its glass transition temperature. Moreover, the porous structures realized by S geometry display higher elastic modulus values in comparison to D and G-based structures. Upon cyclic compression tests, all porous structures exhibit a robust negative piezoresistive behavior, regardless of their porosity and geometry, with outstanding strain sensitivity. Gauge factor (GF) values of 12.4 at 8% strain are achieved for S structures at 40 and 60% porosity, and GF values up to 60 are obtained for deformation extents lower than 5%. Thermal conductivity of the TPU/GE structures significantly decreases with increasing porosity, while the effect of the structure architecture is less relevant. The TPU/GE porous structures herein reported hold great potential as flexible, highly sensitive, and stable strain sensors in wearable or implantable devices, as well as dielectric elastomer actuators.

Recent Progress on Polymer Materials for Additive Manufacturing

“…Glass bead/PA12 [161a] Cu/PA12 [200] CF/PA12 [201] GF/PA12 [202] Carbon black/PA12 [203] OMMT/PP [140] GNP a) /PEEK [190b] • MWCNT/PA12 [189,191,204] MWCNT/TPU [190a,204a] SWCNT b) /TPU [205] Carbon black/PA12 [206] Graphene/TPU [192] OMMT/PP [140] GNP/PEEK [190b] GNP/PA12 [207] • Low cost • Moderate filler dispersion on powder particle surfaces • Good interfacial interaction possible • Solvent intensive Melt blending and mechanical grinding CNF/PA12 [194] OMMT/PP [140] BaTiO 3 /PA11 [162] • Good filler dispersion in powder particles • Good interfacial interaction possible • Irregular particle shape • Poor powder flowability • Particle rounding or other post-processing treatment required…”

On the Synergistic Effect of Multi-Walled Carbon Nanotubes and Graphene Nanoplatelets to Enhance the Functional Properties of SLS 3D-Printed Elastomeric Structures

“…All porous structures exhibited a robust negative piezoresistive behavior, with outstanding strain sensitivity. However, the obtained results showed that GE particles obstruct the polymer powder coalescence, thereby resulting in a porous structure that exhibits an imperfect percolative network and poor mechanical properties [27]. The analysis of the literature confirms that elastomer-based porous structures realized by SLS technology with powders modified with carbonaceous fillers have been exhaustively investigated as innovative materials for piezoresistive sensors.…”

“…The thermogravimetric curves of TPU-based samples are compared in Figure 3. TPU degradation occurs in two steps as already described in our previous research [27]. Briefly, the first degradation that starts at about 280 °C is attributed to the cleavage of urethane bonds of TPU [42] and shows a maximum rate at 309 °C, accounting for about 30% mass loss.…”

“…Thermoplastic polyurethane (TPU) is a soft and flexible elastomer widely used as building material for the SLS process [25]. Recently, TPU composites reinforced with nanoscale fillers drew great attention for their enhanced mechanical, thermal, and electrical properties [26,27]. In fact, combining a TPU matrix with conductive fillers and SLS manufacturing is a simple and effective approach to prepare nanocomposite materials with optimized thermal, anticorrosive, and electromagnetic shielding properties [11,[28][29][30].…”

Performance Assessment of a Three-Dimensional Printed Porous Media Produced by Selective Laser Melting Technology for the Optimization of Loop Heat Pipe Wicks

“…Hence, the use of permeability measurement devices [35][36][37][38][39] permits indirect determination of wick permeability by measuring the pressure loss (∆P) and flow rate through the medium. The flow at pore scale must be in the Darcy's regime, which means that the Reynolds number (Re) must be Re < 10 [40].…”