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1. Thermoresponsive, Recyclable, Conductive, and Healable Polymer Nanocomposites with Three Distinct Dynamic Bonds

   https://doi.org/10.1021/acsapm.2c00790  

   Dodo, Obed J.; Petit, Leilah; Dunn, Derrick; Myers, Camryn P.; Konkolewicz, Dominik (September 2022, ACS Applied Polymer Materials)

   Integration of multiple types of dynamic linkages into one polymer network is challenging and not well understood especially in the design and fabrication of dynamic polymer nanocomposites (DPNs). In this contribution, we present facile methods for synthesizing flexible, healable, conductive, and recyclable thermoresponsive DPNs using three dynamic chemistries playing distinct roles. Dynamic hydrogen bonds account for material flexibility and recycling character. Thiol-Michael exchange accounts for thermoresponsive properties. Diels–Alder reaction leads to covalent bonding between polymer matrix and nanocomposite. Overall, the presence of multiple types of orthogonal dynamic bonds provided a solution to the trade-off between enhanced mechanical performance and material elongation in DPNs. Efficient reinforcement was achieved using <1 wt % multiwalled carbon nanotubes as nanocomposite. Resulting DPNs showed excellent healability with over 3 MPa increase in stress compared to unreinforced materials. Due to multiple responsive dynamic linkages, >90% stress–relaxation was observed with self-healing achieved within 1 h of healing time. Increased mechanical strength, electrical conductivity, and reprocessability were achieved all while maintaining material flexibility and extensibility, hence highlighting the strong promise of these DPNs in the rapidly growing fields of flexible compliant electrodes and strain sensors. 

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2. Development of Nanocomposite Thermoelectric Generators for Body Heat Harvesting

   Chloe Flack, Baiyu Zhang (January 2019, Undergraduate Research Symposium)

   This work evaluates wearable thermoelectric (TE) devices consisting of nanocomposite thermoelectric materials, aluminum nitride ceramic headers, and a flexible and stretchable circuit board. These types of wearable systems are part of a broader effort to harvest thermal energy from the body and convert it into electrical energy to power wearable electronics. Thermoelectric generators are made of p-type (Bi,Sb)2Te3 and n-type Bi2(Te,Se)3. The nanocomposite thermoelectric materials investigated in this research address the two fundamental challenges for body heat harvesting. The first challenge is related to the unavailability of high zT n-type materials near the body temperature. The second challenge is related to the thermoelectric power factor. To improve the zT, one has to increase the power factor simultaneously while reducing the thermal conductivity. Our nanocomposites result in enhancement of the TE power factor along with the reduction of the thermal conductivity. The fundamental reason is a nanoscale effect that happens only when the energy distribution function of the carriers does not relax to that of the bulk material in the crystallites. Ten p-type and ten n-type nanocomposite ingots were synthesized and characterized in this research. All ingots were characterized versus their thermoelectric properties and they all showed similarly enhanced properties. Our nanocomposites, compared to commercial materials, have better zT and thermal resistivity by 40% and 75% for p-type, respectively, and 15% and 140% for n-type. Compared to the state-of-the-art materials, our nanocomposites produce significantly higher power due to their optimized properties for the body temperature. 

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3. Interfacial effects on the electrical behavior of elastomer nanoparticulate composites

   https://doi.org/10.1117/12.2515364  

   Meddeb, Amira; Ounaies, Zoubeida; Lopez-Pamies, Oscar (March 2019, SPIE 10968, Behavior and Mechanics of Multifunctional Materials XIII)

   Polymer nanocomposites exhibit unique effective properties that do not follow conventional effective media approaches. The nanoparticle-polymer interphase has been shown to strongly influence the nanocomposites behavior due o its significant volume when the particles are nano-sized, affording an opportunity to tune the dielectric response of the resulting nanocomposite. In this study, we investigate the effects of TiO2 nanoparticles on the electrical properties and the charges distribution and transport in polydimethylsiloxane (PDMS) nanocomposites. Impedance spectroscopy shows suppression of interfacial Maxwell-Wagner-Sillars (MWS) polarization accompanied by a reduction in the low frequency dielectric permittivity and loss at high temperatures in the presence of the TiO2 nanoparticles. Thermally stimulated discharge current measurements confirm that the suppression of the interfacial polarization relaxations happens by redistributing or depleting the charges through the composite and hindering their mobility, potentially resulting in lower electrical conduction and higher breakdown strength. Although the model materials investigated here are TiO2 nanoparticles and Sylgard 184 PDMS, our findings can be extended to other nanoparticulate-filled elastomer composites to design lightweight dielectrics, actuators and sensors with improved capabilities. 

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4. Interphase-assisted suppression of electrode polarization in nanoparticulate-elastomeric composites

   https://doi.org/10.1063/5.0141243  

   Barhoumi Meddeb, A.; Ounaies, Z. (April 2023, Journal of Applied Physics)

   The electrical properties of polymer nanocomposites are governed by the behavior of the internal charges. In particular, the interphase around the nanoparticles strongly influences the distribution and mobility of charge carriers within the nanocomposites, which, in turn, impacts the performance of these materials. In this work, we probe the internal charge behavior in the presence of nanoparticles with a focus on the low-frequency regime using a suite of techniques. By investigating the depolarizing currents and the dependence of the dielectric properties on the frequency and temperature, we demonstrate that the interphases redistribute the space charges, increase their trap depth, and suppress the electrode polarization in an elastomeric nanocomposite. Additionally, we study the effect of the nanoparticle content on the dielectric behavior by comparing the internal charge behavior of 1, 2, and 4 vol. % nanocomposites. At only 4 vol. % loading, the mobility of charge carriers is effectively limited, leading to lower dc conductivity compared to the unfilled elastomer, and 1 and 2 vol. % nanocomposites. These findings are based on the model materials used in this study, TiO2 nanoparticles and polydimethylsiloxane, and can be extended to other nanoparticulate-filled elastomer composites to design lightweight dielectrics, actuators, and sensors with improved capabilities. Judicious manipulation of interfacial phenomena in polymer nanocomposites—especially those with a dilute content of nanoparticles—provides a promising path forward for the design of materials with exceptional electrical and other physical properties. 

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5. Data centric nanocomposites design via mixed-variable Bayesian optimization

   https://doi.org/10.1039/d0me00079e  

   Iyer, Akshay; Zhang, Yichi; Prasad, Aditya; Gupta, Praveen; Tao, Siyu; Wang, Yixing; Prabhune, Prajakta; Schadler, Linda S.; Brinson, L. Catherine; Chen, Wei (January 2020, Molecular Systems Design & Engineering)

   With an unprecedented combination of mechanical and electrical properties, polymer nanocomposites have the potential to be widely used across multiple industries. Tailoring nanocomposites to meet application specific requirements remains a challenging task, owing to the vast, mixed-variable design space that includes composition ( i.e. choice of polymer, nanoparticle, and surface modification) and microstructures ( i.e. dispersion and geometric arrangement of particles) of the nanocomposite material. Modeling properties of the interphase, the region surrounding a nanoparticle, introduces additional complexity to the design process and requires computationally expensive simulations. As a result, previous attempts at designing polymer nanocomposites have focused on finding the optimal microstructure for only a fixed combination of constituents. In this article, we propose a data centric design framework to concurrently identify optimal composition and microstructure using mixed-variable Bayesian optimization. This framework integrates experimental data with state-of-the-art techniques in interphase modeling, microstructure characterization and reconstructions and machine learning. Latent variable Gaussian processes (LVGPs) quantifies the lack-of-data uncertainty over the mixed-variable design space that consists of qualitative and quantitative material design variables. The design of electrically insulating nanocomposites is cast as a multicriteria optimization problem with the goal of maximizing dielectric breakdown strength while minimizing dielectric permittivity and dielectric loss. Within tens of simulations, our method identifies a diverse set of designs on the Pareto frontier indicating the tradeoff between dielectric properties. These findings project data centric design, effectively integrating experimental data with simulations for Bayesian Optimization, as an effective approach for design of engineered material systems. 

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