The goal of this collaborative project is to develop smart polymeric materials that can report critical material deformation in the 10 to 500 nm range through an embedded electrically conductive network. Nano-structured building blocks will be synthesized to generate such network. The building block has a brush-like structure with electrically conducting polymers as the bristles, and carbon nanotubes (CNTs) as the backbone of the brush. When the nano-brushes are blended into the host matrix, the conducting polymers will be entangled with the host polymers. The brush structure is designed to enhance the signal in response to nano deformations. The electrical signals obtained from the conductive network will be far richer in details than traditional plots of mechanical stress-strain behavior. The details will be used to verify or refute the mechanisms previously proposed to explain the mechanical stress-strain curves under various types of quasi-static and dynamic loading conditions.
The outcome of this project can be used as smart sensors that include structural health monitoring in bridges, hydropower plants, aircrafts, vehicle crumple zones and smart body-armor responsive systems. This collaborative research project will greatly benefit both graduate & undergraduate students by interacting with faculty and students in science and engineering from two different universities. The students will be asked to visit local high-schools for demonstrating simple experiments on proposed conductive systems under various loads. The results from this project study will also be posted regularly on web pages of principal investigators for speedy dissemination of information.
This project aimed to understand the details of non-linear deformation, damage initiation and growth in polymer particulate composites using carbon nanotube (CNT) networks under slow rate and impact loading. Due to their excellent electrical conductivity and high aspect ratio, CNTs are an ideal filler material for generating good conductive networks within polymer composites. Under mechanical loads, the CNT networks change due to deformation and damage of the matrix material. Two material systems were used in the investigation: (1) CNT-reinforced polymers and (2) Carbon Nanobrush (CNB) reinforced polymers. Intellectual Merits I. CNT- Reinforced Polymer Composites Electro-mechanical response of CNT-Polymer Composites Due to limited space available for this project outcome report, we discuss three cases of electro-mechanical response of CNTs embedded epoxy composites for understanding the capability of CNTs in detecting damage: (a) rubber toughened epoxy under static tensile loading, (b) CNTs embedded epoxy under impact loading, and (c) dynamic loading. Based on a rigorous percolation study, 0.1 wt.% CNTs were used for all specimens. (a) Rubber toughened epoxy under slow rate tensile loading: Fig. 1 shows the electro-mechanical response of 20 parts of rubber in epoxy composite. The large deformation that comes from addition of rubber provided the ability of CNTs network to respond to various mechanisms that happen inside the composite. During the initial loading, the increase in resistance is attributed to the axial separation of CNTs in the network due to deformation of flexible rubber particulate along with epoxy in the composite. As the specimen approaches maximum stress and induces a considerable amount of elongation of the entangled epoxy clusters, the distance between CNTs is decreased and therefore increases the electrical conductivity of the specimen. Further increasing strain causes damage to occur within the matrix, thus causing an increase in resistance. (b) CNT-embedded epoxy under impact loading conditions: Fig. 2 shows the electrical response of CNTs embedded epoxy under impact loading using a drop weight tower. It can be seen that the sample undergoes uniform compression from 200 µs to approximately 750 µs. At 800 µs, the right and left side of the specimen demonstrate an expansion. The expansion induces damage initiation and propagation in the form of a void or crack within. From 800 µs to 950 µs damage is propagating throughout the sample, causing an increase in resistance. (c) CNTs embedded epoxy subjected to dynamic compressive loading: Fig. 3 shows the electrical response of CNTs embedded epoxy subjected to dynamic compressive loading (2000/s). During compression, more efficient electrical pathways are created by decreasing the inter-tube gaps between the CNTs present within the matrix and increasing the number of new contacts which is shown as a decrease in resistance. As the compressive strain increases, the radial expansion of the matrix decreases the efficiency of the CNT network. It can be seen that the rate of resistance change in turn changes drastically, showing a less overall decrease in resistance due to the combinative effect of both axial compression and radial expansion. II. CNB- Reinforced Polymer Composites To further improve the sensitivity of the CNT-polymer composites, we explored ways to synthesize a nano-structured and electrically conductive material called Conductive Nano Brush (CNB). The structure of CNB consists of a core of a CNT grafted with an electrically conductive polymer (shown in Fig. 4). When embedded within a polymer, the conductive polymer in the brush can become entangled with the polymer of the host matrix, providing higher sensitivity to changes in the matrix. Fig. 5 shows a transmission electron microscope image of the CNB. The CNB also helped in fully dispersing MWCNT in aqueous solution or organic solvent. The dispersion appears to be indefinitely stable in water. The major conclusion of the project is that CNTs can act as excellent sensory materials at very low loadings inside highly insulating polymer composites. The electrical response can be used to monitor damage in real-life structures to prevent catastrophic failure under uncertain loads. Broader impacts This research program trained and graduated six Master’s degree students and supported two doctoral students currently pursuing their degree at URI. These Master’s students are now working in US industries (Philips, Caterpillar, and General Dynamics). Fourteen undergraduate students (4 under represented, 2 international) were trained and participated in various aspects of the research. Two high school students were trained and many other high school students visited our research labs during open houses. Researchers from several industries visited and discussed nanocomposites applications with our research team. An invention disclosure was filed for a patent on carbon nanotube brush. As a result of this research, a successful new grant was obtained from the state of Rhode Island and another proposal is currently pending with NSF. In coming years, this new knowledge will lead to designing smart sensors for monitoring structural health of various infrastructure systems.
