By combining experiment and molecular simulation, in this work we have systematically elucidated the fundamental mechanism of the significantly improved damping property of nitrile-butadiene rubber (NBR) contributed by the introduction of hindered phenol (AO-80) small molecules. At the molecular level, through FTIR, ¹H-NMR and temperature-dependent FTIR, it is observed that hydrogen bonds (H-bonds) interaction exists between AO-80 small molecules and NBR polymer chains, leading to the formation of a H-bonds network structure. Meanwhile, positron annihilation lifetime spectrometer (PALS) and molecular dynamics simulation were also employed to characterize the fractional free volume for different NBR/AO-80 mixtures and it reached the minimum at the blending mass ratio of 100/60, which also possesses the largest number of H-bonds and the greatest binding energy through quantitative comparison. All of these microscopic analyses just rationalize the maximum dynamic loss factor. Therefore, it was indicated that there was an optimum ratio of rubber to hindered phenol molecules for achieving the maximum damping property. These fundamental studies are expected to provide some useful information to design and fabricate the high-performance polymeric damping materials.

Partially aligned polyacrylonitrile (PAN)-based nanofibers were electrospun from PAN and PAN/single-walled carbon nanotubes (SWNTs) in a solution of dimethylformamide (DMF) to make the nanofiber composites. The as-spun nanofibers were then hot-stretched in the oven to enhance its orientation and crystallinity. With the introduction of SWNTs and by the hot-stretched process, the mechanical properties will be enhanced correspondingly. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray scattering (XRD), differential scanning calorimetry (DSC), and the tensile test were used to characterize the microstructure and performances of the nanofibers. The orientation and crystallinity of the as-spun and hot-stretched nanofibers confirmed by X-ray have increased. Differential scanning calorimetry showed that the glass transition temperature of PAN increased about 3 °C by an addition of 0.75 wt% SWNTs indicating a strong interfacial interaction between PAN and SWNTs. The tensile strength and the modulus of the nanofibers increased revealing significant load transfer across the nanotube-matrix interface. For PAN nanofibers, the improved fiber alignment, orientation and crystallinity resulted in enhanced mechanical properties, such as the tensile strength and modulus of the nanofibers. It was concluded that the hot-stretched nanofiber and the PAN/SWNTs nanofibers can be used as a potential precursor to produce high-performance nanocomposites.

Molecular dynamics (MD) simulations are used to investigate the fundamental damping mechanism of the AO-60/nitrile-butadiene rubber (AO-60/NBR) composites at the molecular level in this study. The hydrogen bonds (H-bonds), binding energy, and fractional free volume (FFV) of the AO-60/NBR composites were obtained. The AO-60/NBR composite with an AO-60 content of 36 phr had the largest H-bonds, highest binding energy, and smallest FFV, all indicating a good compatibility between NBR and AO-60 and good damping performance of the AO-60/NBR composite. The experimental FTIR, and ¹H-NMR results also showed that two types of H-bonds exist between the AO-60 small molecules and NBR polymer chains. Moreover, DSC and DMA were employed to characterize the compatibility between NBR and AO-60 in the composites. Phase separation between NBR and AO-60 appeared as the AO-60 content exceeded 36 phr. We hope the present study provides theoretical guidance for the design of optimum damping properties of polymer composites.