Electrospinning Triboelectric Laminates: A Pathway for Scaling Energy Harvesters.

Herein, a new paradigm of triboelectric polymers-the triboelectric laminate-a volumetric material with electromechanical response comparable to the benchmark soft piezoelectric material polyvinylidene difluoride is reported. The electromechanical response in the triboelectric laminate arises from aligned dipoles, generated from the orientation of contact electrification in the laminates bulk volume. The dipoles form between sequential bilayers consisting of two different electrospun polymer fibers of different diameter. The loose interface between the fiber bilayers ensures friction and triboelectric charging between two polymers. The electric output from the electrospun triboelectric laminate increases with increasing density of the bilayers. This system design has clear benefits over other flexible devices for mechanical energy harvesting as it does not require any poling procedures, and the electromechanical response is stable over 24 h of continuous operation. Moreover, the electromechanically responsive electrospun laminate can be made from all types of polymers, thus providing ample room for further improvements or functionalities such as stretchability, biodegradability, or biocompatibility. The concept of a triboelectric laminate can be introduced into existing triboelectric nanogenerator form factors, to dramatically increase charge harvesting of a variety of devices.

Low-density PDMS foams by controlled destabilization of thixotropic emulsions.

In the current study we demonstrate a method of preparation of low-density polydimethylsiloxane (PDMS) foams from emulsions by using water-based thixotropic fluids as porogens. Aqueous dispersions of synthetic hectorite clay and nanocellulose were used as thixotropic fluids, enabling the preparation of fine emulsions in bulk form with the droplet size down to few tens of microns by simple hand mixing. Contrary to conventional emulsion templating where stabilization of emulsion is required, a strategy was developed for obtaining foams by using controlled destabilization of an emulsion, induced during the curing of the PDMS matrix phase by adding a carefully selected surfactant in optimized concentration. This strategy enables the preparation of bulk PDMS foams with interconnected porosity in a range of density values, fast and deformation-free drying and uniform porous structure with a range of mechanical properties. Clay microplatelet with clearly defined shape and with mass in the nanogram range is retained in spherical pores as the porogen is removed by evaporation. Foams with density down to 0.353 g/cm3 and thermal conductivity of 0.0745 W/m * K were prepared. Elastic modulus of the prepared foams ranged from 0.156 to 0.379 MPa, a reduction of 94.3-86.3% as compared to pure nonporous PDMS.

Thermal, Mechanical, and Acoustic Properties of Polydimethylsiloxane Filled with Hollow Glass Microspheres.

Polydimethylsiloxane (PDMS) is the most widely used silicon-based polymer due to its versatility and its various attractive properties. The fabrication of PDMS involves liquid phase cross-linking to obtain hydrophobic and mechanically flexible material in the final solid form. This allows to add various fillers to affect the properties of the resulting material. PDMS has a relatively low Thermal Conductivity (TC), in the order of 0.2 W/mK, which makes it attractive for thermal insulation applications such as sealing in construction. Although a further decrease in the TC of PDMS can be highly beneficial for such applications, most research on the thermal properties of PDMS composites have focused on fillers that increase the TC rather than decrease it. In the present work, we propose a simple and reliable method for making a PDMS-based composite material with significantly improved thermal insulation properties, by adding hollow glass microspheres (HGMs) to the mixture of the liquid base and the cross-linker (10:1 ratio), followed by degassing and heat-assisted crosslinking. We obtained a 31% reduction of thermal conductivity and a 60% increase in the elastic modulus of samples with HGM content of 17% by weight. At the same time, the sound insulation capacity of the PDMS-HGM composite is slightly decreased in comparison to pure PDMS, as a result of its lower density. Finally, the wettability of the samples had no dependence on HGM content.