Nicolet is10 fourier infrared spectrometer

The chemical shifts of the dinitro compounds and diamines were measured by AVANCE 500 MHz nuclear magnetic field resonator (Bruker, Swiss), and the deuterated reagent was dimethyl sulfoxide (DMSO). The characteristic functional groups of the compounds and polymers were tested by Nicolet IS10 Fourier infrared spectrometer (Thermo Fisher Scientific,USA) in the range of 500–4000 cm⁻¹. Pyris 1 TGA thermal analyzer (PerkinElemer, USA) was used to investigate the thermal stabilities of the polyimides, with a heating rate of 20 °C min⁻¹ and nitrogen flow rate of 20 mL min⁻¹ in the range of 30–600 °C. Based on the maximum tan δ peak, the glass transition temperature (T_g) of the polyimides was determined. The UV-visible absorption spectra of the film samples were tested by Lambda 900 UV/Vis/NIR spectrophotometer (PerkinElmer, USA), scanning from 250 to 800 nm. The dielectric constant and dielectric loss of polyimide film were tested by the AET (high frequency cavity resonator) microwave (high-frequency 10 GHz) dielectric constant tester (AET Corporation, Japan), with the sample size of 10 cm × 1 cm. The samples were tested by Guilin Saimeng Testing Technology Co., Ltd.

Field emission scanning electron microscopy (S8100, Hitachi, Japan) was utilized to characterize the morphologies of the original PU foam and MWCNTs/PU composite foam. The infrared spectra of PU foam and MWCNTs/PU foam were measured using a Thermo Fisher Scientific Nicolet iS10 Fourier infrared spectrometer. The electrical conductivity of the composite foam was measured using a U3402A multi-meter from Agilent Technologies and Dual-electricity digital four-probe tester (ST 2263) from Suzhou Lattice Electronics Co., Ltd, Suzhou, China. The flexible sensor test system (TP-550) developed by Tianjin Zhirou Technology Co., Ltd, Tianjin, China was used to test the mechanical properties and sensing properties of the MWCNTs/PU piezoresistive sensor.