A large piezoelectric response in highly-aligned electrospun poly(vinylidene fluoride/trifluoroethylene) nanofiber webs for wearable energy harvesting.

In this study, highly-aligned and molecularly oriented poly(vinylidene fluoride/trifluoroethylene) [P(VDF/TrFE)] nanofiber webs were fabricated and their piezoelectric response was investigated. Using systematic post-treatments under appropriate conditions, a significant enhancement of the piezoelectric response in the P(VDF/TrFE) nanofiber webs was observed for the first time. The high-quality nanofibers exhibited a large output voltage of 0.4 V. The short-circuit current of post-treated nanofibers was found to be 731.25 µA, which increased about 330 times and the surface electric charge density was found to be 0.64 nC cm-2, which was about 2.7 times higher than those of the as-spun nanofibers. The large enhancement of piezoelectric response of the nanofibers is attributed to the additional stretching, annealing and poling of the as-spun nanofibers under the appropriate post-treatment conditions. The results highlight the potential of the high-quality P(VDF/TrFE) nanofibers to be used as wearable piezoelectric energy harvesters and other flexible self-powered portable devices.

Verification of the Influence of Processing History through Comparing High-Speed Melt Spinning Behavior of Virgin and Recycled Polypropylene.

A 'model' material of recycled polypropylene (PP) was prepared through the injection molding process, and the effect of processing history on the polymer characteristics was investigated through the high-speed melt spinning of virgin and recycled PP. On-line measurement of the thinning behavior of the spin-line revealed the downstream shift of solidification point for the recycled PP at the take-up velocity of 1.0 km/min, indicating the suppression of flow-induced crystallization. The difference was not clear at higher take-up velocities of up to 5 km/min. For any identical take-up velocity, no clear difference in the stress-strain curves and birefringence of the fibers from virgin and recycled PP could be observed, whereas the detailed investigation on the variation of relative amount of c-axis and a*-axis oriented crystals in the fibers prepared at varied take-up velocities suggested the deterioration of flow-induced crystallization at 1.0 km/min. It was speculated that the processing history induced the lowering of the entanglement density, which affected the melt spinning and crystallization behavior. An undistinguishable difference between the virgin and recycled PP at increased take-up velocities suggested the existence of an optimum elongational strain rate for the detection of the different states of molecular entanglement.

Effect of Elevated Temperatures on the States of Water and Their Correlation with the Proton Conductivity of Nafion.

For the first time, we report the effects of elevated temperatures, from 80 to 100 °C, on the changes in the states of water and ion-water channels and their correlation with the proton conductivity of Nafion NR212, which was investigated using a Fourier transform infrared spectroscopy study. Experimentally, three types of water aggregates, protonated water (H+(H2O) n ), nonprotonated hydrogen (H)-bonded water (H2O···H2O), and non-H-bonded water, were found in Nafion, and the existence of those three types of water was confirmed through ab initio molecular dynamics simulation. We found that the proton conductivity of Nafion increased for up to 80 °C, but from 80 to 100 °C, the conductivity did not increase; rather, all of those elevated temperatures showed identical conductivity values. The proton conductivities at lower relative humidities (RHs) (up to 50%) remained nearly identical for all elevated temperatures (80, 90, and 100 °C); however, from 60% RH (over λ = 4), the conductivity remarkably jumped for all elevated temperatures. The results indicated that the amount of randomly arranged water gradually increased and created more H-bonded water networks in Nafion at above 60% RH. From the deconvolution of the O-H bending band, it was found that the volume fraction f i (i=each deconvoluted band) of H-bonded water for elevated temperatures (>80-100 °C) increased remarkably higher than for 60 °C.

A comparison between highly crystalline and low crystalline poly(phenylene sulfide) as polymer electrolyte membranes for fuel cells.

Sulfonation-induced changes in the crystalline structures of highly crystalline (HC) and low crystalline (LC) poly(phenylene sulfide) (PPS) electrolyte membranes and relative humidity-induced changes in the morphology of those sulfonated PPS (SPPS) membranes were characterized by wide-angle X-ray scattering, small-angle X-ray scattering, and atomic force microscopy. The correlations between the hydrated morphology and the development of proton channels in both kinds of membranes are discussed in the paper. For HC-PPS membranes, crystallinity decreased steeply up to an ion-exchange capacity (IEC) of 1.6 meq/g during sulfonation, but in the case of LC-PPS membranes, crystallinity decreased up to IEC = 0.9 meq/g, but not as steeply as for HC membranes. In the varied hydration conditions of the membranes, water molecules were not predominately located within the ionic aggregation but, rather, were gradually dispersed and rearranged through the whole membrane with increasing hydration level. For both kinds of membranes, HC-SPPS and LC-SPPS, Porod plots showed a "positive deviation" that revealed that the polymer/water interface under varied hydration conditions was not smooth, but diffused, and well-developed proton channels did not form in the membranes. LC-SPPS membranes showed about a 10% rougher (from α value) polymer/water interface than HC-SPPS membranes.

Effect of water on the changes in morphology and proton conductivity for the highly crystalline hydrocarbon polymer electrolyte membrane for fuel cells.

The effects of water on the changes in morphology of sulfonated poly(phenylene sulfide) (SPPS) hydrocarbon polymer electrolyte membranes (PEM) with an ion exchange capacity (IEC) of 0-2.0 mequiv/g are investigated using small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM). Wide-angle X-ray scattering (WAXS) was used to characterize the effect of direct sulfonation on the changes in membrane crystalline structure, and it was found that the crystallinity and crystallite domain size decrease and the volume of the amorphous region in the SPPS membranes increases with increasing IEC. The experimental data have been fitted to the Porod law for approaching the analysis of the sharpness of the polymer/water interface, development of the proton channel, or dispersion of water in the hydrated membranes. Porod plots showed positive deviation which revealed that the polymer/water interface in the hydrated SPPS membrane is not smooth but diffused and a well-developed proton channel does not form in the membrane.