*3.4. Bulk Density*

The visual image of the packed native PeaF and ISMS-treated PeaF powers was shown in Figure 4A. ISMS treatment observably increased the packed volume of pea fiber in varying degrees in light of the treatment intensity. Figure 4B illustrated bulk density values of fibers versus the extent of ISMS treatment. The bulk density of native PeaF was 2.20 mL/g, and pre-pulverizer treatment increased the bulk density of fiber by about 3.07 fold. While the fiber was subjected to ISM treatment at 60 MPa, the bulk density of fiber rose, reaching 8.48 mL/g. Intriguingly, in comparison with ISM-60 PeaF, applying an ISM pressure of 90 MPa resulted in a smaller increase in bulk density (6.79 mL/g). Moreover, subsequent strengthening treatment induced a continuous increase in the bulk density, which was 7.59 and 8.21 mL/g for ISM-120 PeaF and ISM-120-T2 PeaF, respectively. Indeed, the bulk density of fiber was related to their morphological characteristics, as mentioned by Wang, Sun, Zhou and Chen [17] and Wang, et al. [23]. It was universally acknowledged that the compact structure occupied a small space, thus native PeaF with compact and thick blocks had a low value of bulk density. As revealed in Figure 3, ISM-60 PeaF was a large dimensional flat plate in a distorted and stacked state, which produced its high bulk density. With regard to pea fibers treated by high intensity, there was more compressible

space between fibers with loose flakes, thus ISM-90, ISM-120 PeaF and ISM-120-T2 PeaF behaved at a lower value of bulk density than the ISM-60 PeaF.

**Figure 4.** The graph of appearance volume (**A**) and bulk density (**B**) of ISMS-treated PeaF. Different letters in (**B**) indicated significant differences (*p* < 0.05) of bulk density between samples.

### *3.5. XRD Analysis*

The fiber was mainly composed of cellulose, lignin and hemicellulose, and the crystalline structure was dominated by cellulose [24]. The X-ray patterns and crystallinity of native PeaF and ISMS-treated PeaF are determined and displayed in Figure 5. There was one strong diffraction peak near the 2θ diffraction angle of 22.40◦ and two weak ones at about 16.5◦ and 35.2◦ in the XRD pattern of native PeaF. It was indicated that pea fiber belonged to cellulose type I, where both crystalline and amorphous regions coexist [25]. No significant difference in the peak position between native PeaF and ISMS-treated PeaF was observed. Nevertheless, the intensity of the peak near 2θ at 16.5◦ was visibly attenuated, and the peak near 2θ at 35.2◦ even disappeared with the increasing of ISMS treatment intensity, implying that the crystalline structure of pea fiber was perturbed. The crystallinity of fiber was gradually decreased from 28.60% to 21.99% along with the treatment intensity, which also suggested that the fiber was decrystallized after ISMS treatment. The weakened crystalline structure was accompanied with a reduction in the crystallinity, and this is most likely because of the destruction of the original ordered cellulose structure as analyzed by Sun, et al. [26]. Decrystallization could activate the cellulosic fiber for functionalization [18]. Therefore, the functional properties of pea fiber after ISMS treatment would be altered.

**Figure 5.** X-ray diffraction patterns of ISMS-treated PeaF.
