3.1. FTIR and XPS
Figure 5 shows the FT-IR spectra of AF before and after modification. The peaks of the AF untreated (AF0) appeared at 3316 cm
−1 (–NH, derived from hydrogen bond association state), 1636 cm
−1 (stretching vibration of –C=O, Amide I band), 1540 cm
−1 (curved vibration of –N–H), 1307 cm
−1 (bending vibration of –N–H) [
17,
18,
19]. The peak of hydrogen bonds nearby 3310 cm
−1 was broader and moved to a lower wave-number, which made it clear that the hydrogen bond between the surface molecular chains was weaker after cleaning and coating during scCO
2 processing. These indicated that scCO
2 can destroy the surface structure of AF to some extent [
11,
12]. The peaks around 2900 cm
−1, which derive from –CH– and–CH
2– indicated a good dispersion of glycidyl-POSS on the AF surface. In addition, the Si–O–Si stretching peak may be masked by AF peaks between 1095 cm
−1 and 1118 cm
−1, which exhibited that glycidyl-POSS was well-consumed in the surface of AF after scCO
2 processing.
Figure 6 and
Figure 7 show the mechanism of grafting reaction. Under the action of scCO
2, the amino and carboxyl groups on the surface of aramid fiber were released. Carboxyl and amino groups attacked the oxygen and carbon atoms of epoxy groups respectively, so that glycidyl-POSS were opened and grafted (
Figure 6). Firstly, for 2-ethyl-4-methylimidazole, the third nitrogen atom on the imidazole ring opened the epoxy group, the hydrogen atoms which connect with para-nitrogen atoms caused hydrogen proton transfer and then reacted with the epoxy group to form a 1:2 addition production. Then, the oxygen anion, generated by the epoxy ring opening, continued to catalyze the ring opening polymerization of the epoxy group. This effect does not only promote the self-polymerization of POSS, but also improves the reaction rate of epoxy group with amino and carboxyl groups. (
Figure 7).
To confirm the success of surface modification, XPS tests were conducted. The changes in the chemical structure of the specimens, induced by the scCO
2, can be observed by analyzing the XPS spectra. The wide scan and C1s core-level spectra of AF before, and after, modification represented in
Figure 8, and the results of the analysis were shown in
Table 2 and
Table 3. As shown in
Figure 8a,c,e,g, the wide scan spectrum of AF before, and after modification, showed the same peak components of C 1s, N 1s, and O 1s ascribed to the existence of C, N, and O elements on the surface. As shown in
Figure 8(b-1), compared with the theoretical value of N/O for untreated AF (AF0), the oxygen content increased after high temperature processing (O/N = 1.34, AF0 but treated at 150 °C for 1 h), which is consistent with the reported in literature. [
20] The content of oxygen increased after scCO
2 processing (O/N = 1.20,
Table 2). The O/C value of CO
2 was much higher than in AF0, indicating that after scCO
2 processing, CO
2 infiltrated into the surface molecular chain of AF in some way [
11,
12]. The oxygen content of AF2 increased after coating treatment (O/N = 2.54), which proves that glycidyl-POSS has been successfully coated onto the surface of AF. In the same conditions, the oxygen content of AF3 (O/N = 6.08) further increased after using 2E4MZ, which proved that a mass of glycidyl-POSS has been successfully coated onto the surface of AF.
Presented in
Figure 8b,d, the C1s core-level spectrum of AF can be curve-fitted with four peak components, whose binding energies located at 288 eV for C=O species, 284.6 eV for C=C species, 286.3 eV for C–N species and 285.3 eV for C–C species [
3,
9]. In the C1s spectra of AF2 and AF3 there were two more species at the binding energy of about 286 eV and 289 eV assigning to C–O and –COO–, as reported in the literature [
14,
21]. From the perspective of molecular structure, COO– is only derived from the reaction of glycidyl-POSS with terminal carboxyl groups and the SCCO
2 processing. C–O not only comes from POSS molecules, but is also derived from the reaction of glycidyl-POSS with terminal amino groups. By further comparing the C1s core-level spectrum of each specimen
Figure 8b,d,f,h, a significant change occurred at 286–288 eV. This suggested that the infiltration of the scCO
2, on the fiber surface, is not only a physical permeation but may also be accompanied by a chemical reaction [
12]. Meanwhile, the data in
Table 3 indicated that, after using 2E4MZ, the content of C–O increased to 21.74%, but the change of –COO– was not obvious. This phenomenon further confirms that the grafting reaction and self-assembly of glycidyl-POSS occur simultaneously.
3.2. XRD Results
To clarify the effect of grafting treatment on the surface structure of fibers, XRD measurements were carried out and the X-ray diffractograms in the 2θ range 10° to 40° are shown in
Figure 9. It was found that two distinct characteristic diffraction peaks appeared nearby 2θ = 20.0°, 23.8°, which correspond to [110] and [200], indicating that the crystal type of AF does not change after surface cleaning and surface grafting treatment with glycidyl-POSS solution [
9,
12].
In order to obtain the effect of the treatment process on the structure and crystallinity of AF, the XRD pattern of every specimen was determined in
Table 4 by using the curve fitting and normalization method.
Table 4 showed the crystal angle value corresponding to the location of each characteristic diffraction peak of AF. After the scCO
2 processing, the 2θ value slightly shifted to a lower angle in
Table 4, indicating that the interplanar spacing of modified AF had increased, and the stacked density of microcrystals had decreased after scCO
2 processing [
12]. After scCO
2 processing, the crystallinity of the AF1 were 77.10% with a significant drop, which was associated with the destruction of the surface molecular chain. Whether coated or being grafted, the degree of disorder of molecular chains on the fiber surface would increase. Compared with the untreated AF0, the crystallinity of AF3 reduced to 74.54%, which was related to the coated glycidyl-POSS on the surface of fiber.
3.3. TG and DTG
Figure 10 showed the TG and DTG curves of aramid fiber before, and after, modification. Although TG curves in
Figure 10a showed that there are no distinct difference in AF0 and AF1, AF2 and AF3 had a pyrolysis characteristic in a lower temperature (nearby 150 °C). Meanwhile, both AF1 and AF2 showed an obvious second-order thermogravimetric mechanism, but AF3 showed only a first-order thermogravimetric mechanism and the thermal properties of the fibers improved after glycidyl-POSS (+2E4MZ) processing. These phenomena can be explained as follows [
12]: After scCO
2 processing, the surface structure of the fibers was loosened, the degradation mechanism changed, and the thermal properties decreased. The looser surface structures were favorable for the permeation of glycidyl-POSS, this penetration contributed to the grafting and coating of POSS.
In detail, the pyrolysis characteristic near 150 °C can be attributed to the evaporation of absorbed water and remained glycidyl-POSS (or 2E4MZ). There was almost no weight change within 100–450 °C for AF0 and AF1. According to the analysis of the mass residual rate, the mass residual rate of AF1 is 8.4% lower than in AF0, which may be the residual organic solvent in the processing. By comparing AF1 and AF2, we can draw a clear conclusion that, whether the attachment is a graft product or self-polymerization product, only a little of glycidyl-POSS can attach onto the fiber surface after scCO2 processing. Using 2E4MZ can promote the grafting and coating of glycidyl-POSS (AF3). 10.7 wt % residues were the glycidyl-POSS which grafted or coated on the surface of the fibers.
3.4. Surface Morphology of the Aramid Fibers
The surface morphologies of the specimens were observed by SEM, just as shown in
Figure 11 the AF0 displayed a smooth surface, and there was almost no obvious concave and convex structure. As shown in
Figure 11(a-T), after high temperature treatment (AF0 but treated at 150 °C for 1 h), there is almost no change on the surface of the fibers. Nevertheless, a significant change has taken place on AF1 after scCO
2 processing, even part of the surface structure of the fiber was obviously broken. For comparison, the SEM images of AF2 and AF3, which were without scCO
2 processing, are also listed in
Figure 11c,e. It can be clearly seen that a small amount of glycidyl-POSS adhered onto the surface before scCO
2 processing
Figure 11e. After using 2E4MZ, part of glycidyl-POSS penetrated into the fiber through the looser surface, while self-assembly and grafting reactions of glycidyl-POSS occurred on the surface and adhered onto the surface of the fiber. The surface of the fiber was densely coated with a layer of glycidyl-POSS, and the roughness of fibers changed.
3.7. The Mechanical Performance
To evaluate the influence of the grafted AF on the interlaminar shear strength (ILSS), the composites which were made of epoxy and AF treated with different type of processing were tested by inserting a grafted fiber layer in the AF/epoxy composites. The results of the micro-bond pull-out test and the measurement of the ILSS were both shown in
Figure 14. As shown, the ILSS of AF1 reduced in comparison with AF0 (from 65.6 to 58.5 MPa), and the weakening of interlaminar shear strength well-correlated with the IFSS (from 18.90 to 16.79 MPa). While AF2 exhibited a higher ILSS (70.85 MPa) than that of AF0, ILSS of AF3 reached a new plateau of 84.26 MPa using 2E4MZ, as observed in
Table 6. After scCO
2 processing, the epoxy resin (epoxy micro-droplet) can easily penetrate the surface molecular chain of the fiber, the abundant carboxylic acids and amine functional groups could provide many reactive anchoring sites for the cross-linking of the epoxy resin, thus increasing adhesion performance. Moreover, a large number of glycidyl-POSS grafted (or self-assembly) on the surface of the fiber, improved the level of roughness after using 2E4MZ, and was critical for the adhesion. Although the value of IFSS in our research was not better than that of the processing of macromolecules grafting on the aramid fiber (increased by 34%) [
1,
3], the ILSS also increased significantly with the increase of IFSS. The increment of ILSS (25.33%) was much higher than the reports in the literature (14.7%) [
23], which showed that the aramid fiber was modified according to our processing of aramid fiber-reinforced composites, which exhibited better properties.
To further evaluate the ILSS of fiber/epoxy composites, the impact fractured surfaces were examined under SEM. The fracture of AF0 is perfectly flat, as well as the fiber surface, and there was no adhesive epoxy resin on the fiber surface. Due to the action of scCO
2, the roughness of AF1 increased and the surface structure was destroyed. The fiber was torn under an external force impact
Figure 15b. This indicated that the decrease of ILSS was mainly caused by the decrease of single fiber strength, not adhesion [
21,
24,
25,
26,
27]. After glycidyl-POSS processing, the polarity of AF2 increased and more epoxy resin was adhered to the fiber surface
Figure 15c. On the fractured surface of AF3
Figure 15d, many epoxy debris still adhered, which confirms the strong interfacial adhesion between AF3 and the epoxy matrix.