A Stretchable Expanded Polytetrafluorethylene-Silicone Elastomer Composite Electret for Wearable Sensor.

Soaring developments in wearable electronics raise an urgent need for stretchable electrets. However, achieving soft electrets simultaneously possessing excellent stretchability, longevity, and high charge density is still challenging. Herein, a facile approach is proposed to prepare an all-polymer hybrid composite electret based on the coupling of elastomer and ePTFE membrane. The composite electrets are fabricated via a facile casting and thermal curing process. The obtained soft composite electrets exhibit constantly high surface potential (-0.38 kV) over a long time (30 days), large strain (450%), low hysteresis, and excellent durability (15,000 cycles). To demonstrate the applications, the stretchable electret is utilized to assemble a self-powered flexible sensor based on the electrostatic induction effect for the monitoring of human activities. Additionally, output signals in the pressure mode almost two orders of magnitude larger than those in the strain mode are observed and the sensing mechanism in each mode is investigated.

Combining 'grafting to' and 'grafting from' to synthesize comb-like NCC-g-PLA as a macromolecular modifying agent of PLA.

The surface modification of nano particles is very important in nanotechnology. Grafting from (GF) and grafting to (GT) are two main methods to prepare surface modified nanoparticles like nanocellulose crystalline (NCC) grafted with polylactic acid (PLA) chains. In the GF method, the NCC can get high grafting degree but short side chains to improve its compatibility with the polymer matrix. The GT method can help obtain long side chains to increase the chain entanglements but owns low grafting density. To take the advantage of both methods, a mixed modification method combining GT and GF methods was put forward to synthesize comb-like NCC-g-PLA (NP) as a macromolecular modifying agent of PLA. Firstly, GT Method was used to obtain long side-chain NP to improve chain entanglement. Secondly, the GF method was applied to obtain NP-g-PLA (NPL) and NP-g-PDLA (NPD) with additional short side chains to improve its dispersion and compatibility in the PLA matrix. The products showed an enhanced nucleation effect, the degree of crystallinity (Xc) of PLA composites increased almost four times with only 1 wt% NPD or NPL. What's more, the storage modulus and loss modulus of the composite melts also increased with 1 wt% NPL or NPD. The NPD/PLA shows a higher effect than NPL/PLA owning to stronger interaction originated from the stereocomplex (SC) network of PLA matrix with PDLA short chains in NPD.

Distinct positive temperature coefficient effect of polymer-carbon fiber composites evaluated in terms of polymer absorption on fiber surface.

In this article, the positive temperature coefficient (PTC) effect was studied for high-density polyethylene (HDPE)/carbon fiber (CF) composites. All of the samples showed a significant PTC effect during the heating processes without a negative temperature coefficient (NTC) effect, even at a temperature much higher than the melting point of the polymer matrix. An ever-increasing PTC intensity with increasing thermal cycles was observed in our study that had never been reported in previous research. The absence of a NTC effect resulted from the increased binding force between the matrix and fillers that contributed to the very special structure of CF surface. We incorporated thermal expansion theory and quantum tunneling effects to explain PTC effect. From the SEM micrographs for the HDPE/CF composites before and after the different thermal cycles, we found that the surface of CF was covered with a layer of polymer which resulted in a change in the gap length between CF and HDPE and its distribution. We believed that the gap change induced by polymer absorption on the fiber surface had a great effect on the PTC effect.