Nine samples were characterized for each clay proportion. Figure 6.9 shows WAXS pattern of PP and PP nanocomposites with different amounts of PEGM, prepared by melt intercalation method. Figures 6.10 - 6.13 show TEM images of PP and PP nanocomposites. Clay ratios (1 and 2 phr) do not show any appreciable peak in WAXS pattern, indicating that exfoliation has occurred in the nanocomposites. This could be due to the low filler ratios, to melt intercalation and to compatibilizer present in the nanocomposites. However, complete exfoliation cannot be ascertained for 1 and 2 phr ratios. WAXS pattern of treated clay filled polymer did not show any characteristic peaks, which indirectly implied the exfoliation of clay layers. Strong interaction between PP molecules and PEGM treated clay would be expected to be due to the presence of the long alkyl chain of PEGM and PP-g- MA added. A lower (001) diffraction signal intensity was related to a higher extent of platelet exfoliation that occurred during the synthesis and processing of the nanocomposite.
2.5 4 5.5 7 8.5 10 Scattering angle (2θ°) R ela tiv e I nt en sit y PP PP1 PP2 PP5 PP7
Moad et al suggested that the clay layers dispersed well in the presence of PEO based surfactants in the preparation of PP nanocomposites [8, 9]. Shearing action of the extruder supported exfoliation of clay layers without use of compatibilizer. Melt intercalation leads to exfoliation at 5 phr clay ratios for researchers elsewhere. Authors suggested that in melt intercalation, polymer chains snaking through the gallery of a stack of clay nanoplates [8, 9]. The gallery spacing is sometimes observed to increase slightly in the presence of polymer melts; however, the increase may be a hydrostatic effect originating from the distribution of the clay particles with the melt. Melt exfoliation begins when the interlaminar gallery becomes large enough to allow significant diffusion of the molecular chains into the opening between plates. This is the mechanism behind any melt intercalation process. In this research, melt intercalation improved dispersion of clay layers in PP matrix dramatically, which are clear from WAXS and TEM patterns. With 5 phr loading of clay; the diffraction pattern is featureless except at 3 degrees, indicating exfoliation and intercalation of clay platelets. It can be concluded that the loading of clay directly affects the dispersion of clay layers within the polymer matrix. At 7 phr clay ratios, the peak moved slightly towards the right indicating less intercalation compared to 5 phr clay ratio reinforced nanocomposite. There is also a peak around 5.5 (in X-axis) indicating the presence of aggregated or clay undisturbed by extruding force (Figure 6.9). The reason could also be that rather small swelling of the gallery may result from the partial intercalation, i.e. not all the silicate crystallites are intercalated. Same results can be observed in TEM images of corresponding clay ratios. Similar results were observed by authors who used PEO, crown ethers as intercalants [12, 21, 24, 26, 111]. The advantage of the monolaurate alkyl chain could be the reason for near complete exfoliation at 1, and 2 phr clay ratios. However, as discussed previously complete exfoliation of organoclay is possible only with 40 % PP-g-MA. WAXS pattern of all nine samples were in close proximity.
Of all nine data, the data with slight peak is shown here in Figure 6.9. For instance, five samples out of seven samples of 5 phr clay ratio showed exfoliation (diffraction pattern is featureless). Remaining 2 samples were showing peak in the range of 2 to 3 degrees. One
of them is shown here in Figure 6.9. From WAXS and TEM patterns, it can be inferred that choice of intercalant and mode of preparation of nanocomposite is a critical step. The above mentioned factors validated the notion that the shear force applied in melt intercalation leads to delamination of clay layers. It is apparent that PP chain diffused into the clay gallery of the clay. The non polar chains of PP would be compatible with PP-g- MA, and alkyl chain of the intercalant. Hence, intercalation occurred. PP nanocomposites do not have distinct (001) plane peaks up to 5 phr indicating that the nanocomposites might have an exfoliated morphology. However, the nanocomposites with 7 phr showed a weak shoulder around 5, meaning that the nanocomposite may contain small amount of clay intercalated. In the ultimate platelet configuration, the clay is completely dispersed and exfoliated, the specific surface is at its maximum and the greatest advantage can be obtained from the nanocomposite.
The size of clay lamella observed for PEO dispersed PP nanocomposite is a reasonable match for the size dimension obtained previously in TEM studies of nanocomposites. In case of 1 and 2 phr clay ratios, exfoliation was observed by both TEM and WAXS pattern.
It can be suggested that the weakened d001 peak in WAXS pattern may result from possible
exfoliation of some layers from the silicate stacks. However for 5 and 7 phr clay ratio, it might be a combination of intercalation, exfoliation and incompatibility. Figures 6.10- 6.12 show the exfoliated lamellae, tactoids composed of variable numbers of lamella and aggregates of tactoids. Figures 6.10 – 6.12 also show that more homogeneous distribution of silicate layer without clay aggregates. Higher magnification (100 nm) clearly shows that the clay layers are uniformly dispersed in the PP matrix, except for 7 phr clay ratios. The aspect ratio of clay inclusions can be inferred from the length and thickness of the dark lines in TEM micrographs at different magnifications. Typical layers are 100 to 300 nm in length and 1 nm in thickness. The silicate layers are well dispersed in the matrix. The results agree with that of WAXS pattern. It can be clearly noticed that better dispersion was achieved in the case of polyether treated clay. There are more polymer chains intercalated into the clay galleries in nanocomposites. The silicate crystallites (tactoids) consisting of several tens of silicate layers are dispersed in the polymer matrix, which is consistent with the result of WAXS. Individual silicate layers of about 1 nm thickness can
be thin silicate sheets composed of several layers can be seen, which does further support the result of the WAXS measurement. It should also be noted that driving force of the intercalation comes from the strong hydrogen bonding between the maleic acid groups generated from the hydrolysis of the MA groups and the oxygen groups of the silicates. Figure 6.13 shows the clay tactoids of 7 phr clay ratio nanocomposites. It shows the clay layers sheared apart into small galleries due to force applied during extrusion of nanocomposites. Figure 6.14 clearly shows the image of cleaved clay tactoids, due to shearing action of extruder. Figure 6.15 schematically represents the diffusion of polymer chains in clay layers due to shear force by melt intercalation. The main advantage of nanocomposites is the use of nanofiller, which has maximum surface area. The properties can be fully inferred only if the clay layers are dispersed completely in the matrix; however, rate of exfoliation can be improved only by increasing compatibilizer percentage. Mass fraction of compatibilizer in preparing nanocomposite, was stepped up from 5 to 10 phr gradually, after few optimization batches. At lower concentrations, there were more agglomerations of clay tactoids which was observed from TEM images and WAXS patterns. Selected WAXS patterns of optimization batches (5 and 7.5 phr PP-g-MA) are shown in appendix 1. Hence, 10 phr compatibilizer was at optimum level for the set of conditions used. This results correlate with other researchers in selecting the proportion of compatibilizer in preparing nanocomposites [87, 171, 196-199].
Figure 6.11 TEM images of PP-PEGM(MB)nanocomposites – 2 phr PEGM
Figure 6.13 TEM images of PP-PEGM(MB)nanocomposites – 5 phr PEGM
Figure 6.15 Schematic representation of clay tactoids cleaved apart due to shear in melt blending / melt intercalation