Amir Ameli

Highly stretchable conductive thermoplastic vulcanizate/carbon nanotube nanocomposites with segregated structure, low percolation threshold and improved cyclic electromechanical performance

We investigated electrically conductive nanocomposites made of thermoplastic vulcanizates (TPVs) and multiwalled carbon nanotubes (CNTs) that exhibit highly enhanced stretchability, low electrical percolation threshold, and improved electromechanical durability after cyclic loading. The TPV/CNT nanocomposites were fabricated by compounding pre-vulcanized rubber (PVR) fine particles with a maleic anhydride grafted polyethylene (MA-g-PE)/CNT compound. Our microstructural and morphological investigations showed that using PVR particles, rather than their more common virgin elastomer counterparts, locked the carbon nanotubes in the MA-g-PE phase. This guaranteed the formation of a segregated structure. Furthermore, it was confirmed that the chemical bonding forms between the PVR particles and the MA-g-PE matrix produced an excellent interfacial adhesion between the two phases. This engineered structure increased the TPV/CNT nanocomposites’ stretchability by 300%. Meanwhile their electrical percolation threshold was decreased by ∼50%, when compared with their MA-g-PE/CNT counterparts. Interestingly, the cyclic electromechanical properties were also improved, suggesting the nanocomposites’ great potential for flexible and stretchable electromechanical applications. The mechanisms linking the microstructure and their consequent characteristics were also discussed. Such property combinations can be extremely beneficial in flexible electronics, soft robotics, and health monitoring devices.

Functional Polymers and Nanocomposites for 3D Printing of Smart Structures and Devices

Three-dimensional printing (3DP) has attracted a considerable amount of attention during the past years, being globally recognized as one of the most promising and revolutionary manufacturing technologies. Although the field is rapidly evolving with significant technological advancements, materials research remains a spotlight of interest, essential for the future developments of 3DP. Smart polymers and nanocomposites, which respond to external stimuli by changing their properties and structure, represent an important group of materials that hold a great promise for the fabrication of sensors, actuators, robots, electronics, and medical devices. The interest in exploring functional materials and their 3DP is constantly growing in an attempt to meet the ever-increasing manufacturing demand of complex functional platforms in an efficient manner. In this review, we aim to outline the recent advances in the science and engineering of functional polymers and nanocomposites for 3DP technologies. The report covers temperature-responsive polymers, polymers and nanocomposites with electromagnetic, piezoresistive and piezoelectric behaviors, self-healing polymers, light- and pH-responsive materials, and mechanochromic polymers. The main objective is to link the performance and functionalities to the fundamental properties, chemistry, and physics of the materials, and to the process-driven characteristics, in an attempt to provide a multidisciplinary image and a deeper understanding of the topic. The challenges and opportunities for future research are also discussed.

Effects of uniaxial and biaxial orientation on fiber percolation in conductive polymer composites

A Monte Carlo simulation was built to estimate the percolation threshold of fibers in a system under different fiber orientations. A 3-D model was built. The orientation effect was modeled by introducing a degree of alignment in the randomly generated fibers via appropriate mathematical relationships and various degrees of uniaxial strain were applied. The critical volume fraction was then analyzed in both normal direction (through-plane) and parallel direction (in-plane) to that of the cross-section plane. The effect of uniaxial orientation was modeled by measuring the through-plane percolation threshold under tensile strain. The effect of biaxial orientation was modeled by measuring the in-plane percolation threshold under compressive strain. The results indicated that the introduction of fiber alignment changed both through-plane and in-plane threshold values, albeit with different trends. With the introduction of slight uniaxial orientation, the through-plane percolation threshold reached a minimum va...

Modelling of Rod-Like Fillers’ Rotation and Translation near Two Growing Cells in Conductive Polymer Composite Foam Processing

We developed a simple analytical model to describe the instantaneous location and angle of rod-like conductive fillers as a function of cell growth during the foaming of conductive polymer composites (CPCs). First, we modelled the motion of the fillers that resulted from the growth of one cell. Then, by taking into account the fillers located at the line that connected the centres of the two growing cells, we found the final filler’s angle and location. We identified this as a function of the corresponding cell size, filler size, and the filler’s initial angle and location. We based the model’s development on the assumption that a polymer melt is incompressible during cell growth. The two-cell growth model is better than the one-cell growth model because it describes the filler’s movement in the cell wall between the two growing cells. The results revealed that the fillers near the cell were the ones most affected by the cell growth, while those at the midpoint between the two cells were the least affected. As a cell grows, its affected polymer area also increases. A dimensionless factor η was introduced to demonstrate the effects of the cell size and the filler length on the filler’s interconnectivity in the CPC foams. It is vital to keep the filler length comparable to the cell size when preparing CPC foams with the desired electrical conductivity. Our research provides a deeper understanding of the mechanism through which foaming influences the filler connections in CPC foams.

Solvent sensitivity of smart 3D-printed nanocomposite liquid sensor

Fused deposition modeling (FDM) is one of the 3D printing methods that has attracted significant attention. In this method, small and complex samples with nearly no limitation in geometry can be fabricated layer by layer to form end-use parts. This work investigates the liquid sensing behavior of FDM printed flexible thermoplastic polyurethane, TPU filled with multiwalled carbon nanotubes, MWCNTs. The sensing capability of printed TPU-MWCNT was studied as a function of MWCNT content and infill density in response to different solvents, i.e., ethanol, acetone and toluene. The solvents were selected based on their widespread use and importance in medical and industrial applications. U-shaped liquid sensors with 2, 3 and 4wt.% MWCNT content were printed at three different infill densities of 50, 75 and 100%. Solvent sensitivity was then characterized by immersing the sensors in the solvents and measuring the resistance evolution over 25s. The results indicated a sensitivity order of acetone > toluene > ethanol, which was in agreement with the predictions of FloryHiggins solubility equation. For all the solvents, the sensitivity was enhanced as the infill density of the printed samples was decreased. This was attributed to the increased surface area to volume ratio and shortened diffusion paths. The MWCNT content was also observed to have a profound effect on the sensitivity; in samples with partial infill, the sensitivity was found to be inversely proportional to the MWCNT content, such that the highest resistance change was obtained for nanocomposites with the lowest MWCNT content of 2wt.%. For instance, a resistance increase of more than 10 times was obtained in 25 s once TPU-2wt.%MWCNT with 50% infill was tested against acetone. The results of this work reveals that highly sensitive liquid sensors can be fabricated with the aid of 3D printing without the need for complex processing methods.

3D printed thermoplastic polyurethane with isotropic material properties

Additive Manufacturing (AM) is an emerging field with rapid growth. Fused Deposition Modeling (FDM), as an AM method, is becoming increasingly popular. With the ability to create parts from a wide range of thermoplastics, it is necessary to understand the effects of FDM process on the printed part’s mechanical properties for a given material. This paper investigates the mechanical properties of 3D printed TPU parts created by a typical low cost desk-top FDM machine. TPU was first extruded into filament suitable for FDM and printed into samples for tensile tests according to the ASTM 3039 standard. The effects of raster orientation, nozzle temperature, and air gap on the mechanical properties were investigated. The compression-molded samples were used as the baseline. While the printed samples had an overall lower ultimate tensile strength (UTS) compared to the molded samples, the printed samples with a negative air gap showed nearly isotropic material properties, irrespective of raster orientation and nozzle temperature. For samples with a positive air gap, raster orientation had a large influence on the overall UTS. Nozzle temperature did not have much effect on the UTS. When compared to rigid thermoplastics TPU has a much lower glass transition temperature (Tg) at -40° C. This allows for much better interlayer bonding between print lines as TPU is above Tg for the entire printing process.

Electrical conductivity and piezoresistive response of 3D printed thermoplastic polyurethane/multiwalled carbon nanotube composites

Additive manufacturing (AM) is an emerging field experiencing rapid growth. This paper presents a feasibility study of using fused-deposition modeling (FDM) techniques with smart materials to fabricate objects with sensing and actuating capabilities. The fabrication of objects with sensing typically requires the integration and assembly of multiple components. Incorporating sensing elements into a single FDM process has the potential to significantly simplify manufacturing. The integration of multiple materials, especially smart materials and those with multi-functional properties, into the FDM process is challenging and still requires further development. Previous works by the authors have demonstrated a good printability of thermoplastic polyurethane/multiwall carbon nanotubes (TPU/MWCNT) while maintaining conductivity and piezoresistive response. This research explores the effects of layer height, nozzle temperature, and bed temperature on the electrical conductivity and piezoresistive response of printed TPU/MWCNT nanocomposites. An impedance analyzer was used to determine the conductivity of printed samples under different printing conditions from 5Hz-13MHz. The samples were then tested under compression loads to measure the piezoresistive response. Results show the conductivity and piezoresistive response are only slightly affected by the print parameters and they can be largely considered independent of the print conditions within the examined ranges of print parameters. This behavior simplifies the printing process design for TPU/MWCNT complex structures. This work demonstrates the possibility of manufacturing embedded and multidirectional flexible strain sensors using an inexpensive and versatile method, with potential applications in soft robotics, flexible electronics, and health monitoring.

3D printing of highly elastic strain sensors using polyurethane/multiwall carbon nanotube composites

As the desire for wearable electronics increases and the soft robotics industry advances, the need for novel sensing materials has also increased. Recently, there have been many attempts at producing novel materials, which exhibit piezoresistive behavior. However, one of the major shortcomings in strain sensing technologies is in the fabrication of such sensors. While there is significant research and literature covering the various methods for developing piezoresistive materials, fabricating complex sensor platforms is still a manufacturing challenge. Here, we report a facile method to fabricate multidirectional embedded strain sensors using additive manufacturing technology. Pure thermoplastic polyurethane (TPU) and TPU/multiwall carbon nanotubes (MWCNT) nanocomposites were 3D printed in tandem using a low-cost multi-material FDM printer to fabricate uniaxial and biaxial strain sensors with conductive paths embedded within the insulative TPU platform. The sensors were then subjected to a series of cyclic strain loads. The results revealed excellent piezoresistive responses of the sensors with cyclic repeatability in both the axial and transverse directions and in response to strains as high as 50%. Further, while strain-softening did occur in the embedded printed strain sensors, it was predictable and similar to the results found in the literature for bulk polymer nanocomposites. This works demonstrates the possibility of manufacturing embedded and multidirectional flexible strain sensors using an inexpensive and versatile method, with potential applications in soft robotics and flexible electronics and health monitoring.

The impact of nozzle and bed temperatures on the fracture resistance of FDM printed materials

Additive manufacturing refers to a new technology in which physical parts are directly produced from a computer model by incremental addition of the constituent materials. Fused deposition modeling (FDM) is one of the most common types of additive manufacture processes. The ultimate mechanical performance of FDM printed parts is a function of the interlayer bond quality. Current literature however focuses only on the phenomenological evolution of standard mechanical properties (such as tensile and bending) as a function of printing conditions. Such studies do not provide direct information about the interlayer adhesion and in-practice failure characteristics. In this work, a fracturemechanics- based methodology was used to characterize the fracture resistance of FDM 3D printed Acrylonitrile Butadiene Styrene (ABS) samples as a function of nozzle and bed temperatures. Double cantilever beam (DCB) specimens was printed in such pattern that the applied load exerted only tensile opening stresses at the crack front. This facilitated the measurement of crack growth under pure mode-I condition. A finite element model was then used to obtain the J-integral strain energy release rate values, as a measure of the fracture resistance. Since the crack propagated at the interlayer in all the cases, the fracture resistance was a direct indication of the interlayer adhesion. The results revealed that the critical crack growth load, the actual fracture surface area (governed by printed mesostructure) and the apparent fracture resistance all increased when the nozzle or bed temperature was increased; the nozzle temperature showed a much stronger effect. The layer-to-layer adhesion, as reflected by the interlayer fracture resistance, did not show monotonous increase with the temperatures and appeared to level off at higher temperatures, indicating that complete interlayer fusion was achieved. This work provides insight into and characterizes the relationships between the 3D printing conditions, the resultant mesostructure, the apparent fracture resistance and the interlayer adhesion in FDM 3D printed materials.

Expanded polylactide bead foaming - A new technology

Bead foaming technology with double crystal melting peak structure has been recognized as a promising method to produce low-density foams with complex geometries. During the molding stage of the bead foams, the double peak structure generates a strong bead-to-bead sintering and maintains the overall foam structure. During recent years, polylactide (PLA) bead foaming has been of the great interest of researchers due to its origin from renewable resources and biodegradability. However, due to the PLA’s low melt strength and slow crystallization kinetics, the attempts have been limited to the manufacturing methods used for expanded polystyrene. In this study, for the first time, we developed microcellular PLA bead foams with double crystal melting peak structure. Microcellular PLA bead foams were produced with expansion ratios and average cell sizes ranging from 3 to 30-times and 350 nm to 15 µm, respectively. The generated high melting temperature crystals during the saturation significantly affected the ex...