2.4. Characterisation

The content of carbon, hydrogen and nitrogen in ND and ND-PEI samples was quantified by a Thermo Scientific^TM FLASH 2000 CHNS/O Analyzer. X-ray photoelectron spectrometer (XPS) was applied by a Kratos Axis Ultra XPS equipped with a 165 mm hemispherical electron energy analyser and a monochromatic Al Kα (1486.6 eV) radiation at 150 W (15 kV, 10 mA). The C1s main peak position was fixed at 284.8 eV and taken as an internal standard. The loading of PEI on the ND particles was calculated by the Perkin Elmer Instruments STA 6000 Thermo Gravimetric Analyser. The ND-PEI sample was heated under an air atmosphere at a uniform heating rate of 5 °C min⁻¹ from 40 to 800 °C. Fourier-transform infrared (FTIR) spectra of ND, ND-PEI and PEI were obtained by using a Perkin Elmer Spectrum 100 spectrometer equipped with an attenuated total reflection (ATR) objective.

Transmission electron microscopy (TEM) was performed on a Tecnai 20 FEG TEM with the accelerating voltage of 200 kV. A JEOL JSM7100 scanning electron microscope (SEM) was applied for the morphologies of the MMM cross-sections. Focused ion beam scanning electron microscopy (FIB-SEM) was applied to evaluate the dispersion of nanodiamond in the Pebax matrix by using an FEI SCIOS FIB/SEM dual beam system. The description of the method is shown in our previous studies [36,37,38,39]. A hole was milled on the surface of the MMM by Ga⁺ FIB (Figure S1). Slices with a thickness of 10 nm were removed from the sample up to a depth of 10 μm by the Ga⁺ FIB at 30 kV and 3 nA. Back-scattered electron (BSE) imaging mode was applied to capture the exposed cross-section images automatically. A set of cross-section SEM images was captured during this slice-and-view section with resolution of 10 nm. The stack of these SEM images was aligned. The segmentation of different phases was identified by image thresholding. The ND fillers phase is the brightest as there is a trace of heavier elements (Na, Si, S) in NDs than Pebax. The dark phase corresponds to the polymer matrix. The stack of obtained SEM images was reconstructed in three-dimensions by using the software Avizo. The volume fraction of each phase was also calculated by the software.