In this section, WAXS and TEM patterns of PP – PEG-ML (Mn=600 g/mol) treated clay
nanocomposites were discussed. The hypothesis proposed was that higher molecular weight intercalant in presence of high shear mixing will lead to better dispersion of clay layers in polymer composite. It was inferred from clay treatment with different molecular weight intercalants that higher molecular weight with low / no shear will have much less impact on d-spacing compared to lower molecular weight intercalants [9, 18, 19]. The reason being more resistance towards higher molecular weight chains to snake between clay layers in conventional boiling technique of solution blending. The results were in correlation with results from other researchers [9, 18, 19].
The added advantage PEG monolaurate of higher molecular weight is the presence of more ethoxy groups to react with sodium ions and alkyl chain that readily reacts with PP. The effect of reinforcement by layered silicate in PP is determined by at least two important
factors: clay dispersion (exfoliation or intercalation) and interaction between clay and polymer. WAXS patterns demonstrated exfoliated structure for 1, 2 phr ratios of clay. WAXS patterns for the clay sample used and PP clay composites are shown in Figure 6.5. The disappearance of the organo-clay peak is correlated with the PP insertion into the silicate galleries that delaminates the periodical layered structure of the clay. In 5 phr clay nanocomposites, there was a small peak observed in the range of 3° and 5.7° (Figure 6.5). WAXS results of PEGM reinforced PP nanocomposites also demonstrated exfoliated structure for lower clay ratios 1 and 2 phr ratios of treated clay. WAXS patterns PP clay composites are shown in Figure 6.5. The disappearance of the organo-clay peak is correlated to the PP molecules insertion into the silicate galleries that delaminates the periodical layered structure of the clay. In 5 phr clay nanocomposites, there was a small peak observed in the range of 6.5° (Figure 6.5). WAXS pattern of 5 phr PP nanocomposite showed a slight shift of the peak towards lower angles, thereby indicating that the distance between clay platelets had become greater. The shift of the first peak is related to the increase of distance between clay layers while a possible coalescence of clay platelets can be inferred from the position of the second peak. Moreover, also an increase in disorder of the silicate layered structure, related to the presence of an intercalated-delaminated configuration, was revealed by the decrease in intensity of the diffraction peak.
2 3 4 5 6 7 8 9 10 11 12 Scattering angle (2θ°) R el at ive I nt ens ity PP PP1 PP2 PP5
Figure 6.5 WAXS pattern of PP and PP-PEGM(SB) nanocomposites – 1,2 and 5 phr clay Lee et al prepared exfoliated PP based nanocomposites by solution blending [195]. The same author suggested that exfoliation of clay layers in nanocomposites occurred only at 1 and 3 wt % of clay loading, not at 5 wt % clay, where clay layers collapse leading an immiscible system. If the clay layers are aggregated into 'agglomerates', their behavior is no more different from an ordinary composite material. In the ultimate platelet configuration, the clay is completely dispersed exfoliated, the specific surface is at its maximum and the greatest advantage can be obtained from the nanocomposite. It should be noted that not all clay platelets can be fully treated due to the natural defects / charge heterogeneities that pre-exist [7, 10]. Therefore, some of the organic surfactant may not be ionically bound and only physio-sorbed onto the clay surface. This might be removed in washing the clay with water during clay treatment process. In Figure 6.5, the peak could be due to some untreated clay or aggregation of clay tactoids in the nanocomposite. This could be a reason for the peak in the WAXS pattern. The specimens prepared with volatile solvent (o-xylene) and ultra-sonication to facilitate in the clay gallery, since mass transport of highly viscous resin into the clay galleries is widely considered one of the limiting steps
in clay platelet separation. As mentioned previously, the relative amounts of clay platelet separation and aggregation depends strongly on the mixing technique and surfactant used for modifying clay platelets.
WAXS pattern of treated clay (1 and 2 phr) filled polymer does not show any characteristic peaks, which indirectly implies the exfoliation of clay layers. As previously discussed, MMT clay is composed of regularly arranged tactoids / galleries. The length of the organic tail (in this case: long alkyl chain of monolaurate - Figure 5.7, Section 5.2.3.3) and CEC are the two parameters that determine how the chain packs between the silicate layers [31]. No peaks were observed for all six samples studied by WAXS. Nanocomposites prepared using PEGC and PEGC in ratios of 1, 2 phr exfoliated well in the polymer matrix, with an exception of agglomerate formation for 5 phr clay loading in the PP matrix. Diffraction patterns suggested exfoliation of clay layers in the PP nanocomposites. The patterns themselves cannot be used to adequately describe the nanoscale dispersion of the layered silicate present in the PP nanocomposites. Interpretation of the structure and clay state was evident with associated morphological observation with TEM. The actual nanoscale dispersion of the clay in the polymer was revealed and nanocomposites with low loading (at 1 and 2 phr) formed exfoliated structure of clay gallery layers (Figure 6.6-6.8). The 5 phr clay composition revealed that most clay layers were dispersed homogeneously in the matrix polymer, although some clusters or agglomerated particles were detected leading to the diffraction peaks observed in WAXS spectra. These clusters could be due to clay undisturbed by dispersion through the Ultraturrax. At 5 phr, the conditions employed to prepare the nanocomposites was not sufficient to fully exfoliate and disperse the clay efficiently. At 5 phr, the presence of small aggregates in PEGC and PEGM reinforced PP nanocomposites is consistent with WAXS and TEM images, which validated the choice of intercalant. It is very clear from WAXS patterns that both PEGC and PEGM acted comparably well, however PEGM-PP nanocomposites show more exfoliation compared to PEGC-PP nanocomposites. The results were attested by TEM images, where clay dispersion is relatively better in PEGM-PP nanocomposites (Figure 6.6-6.8).
In the case of the 5 phr, a combination of effects may be giving rise to the observed structure, such as the limited intercalation at of a relatively high content of clay, shearing facilitation to break up larger agglomerates of clay tactoids, and incompatibility. The size of clay lamella observed for polyethylene oxide dispersed PP nanocomposite is a reasonable match for the size dimension obtained previously in TEM studies of nanocomposites [8, 9, 109]. Figures 6.6 and 6.7 show the exfoliated lamellae, tactoids composed of a variable number of lamella and aggregates of tactoids. Clay ratios (1 and 2 phr) show more homogeneous distribution of silicate layer without clay aggregates. Higher magnification clearly displays that clay layers are homogeneously dispersed in the PP matrix, except for 5 phr clay ratios. For 5 phr clay ratio, the intercalated silicate layers are locally stacked to a hundred nanometres in thickness and from several hundred nanometres to more than one micrometer in length. The aspect ratio of clay inclusions can be inferred from the length and thickness of the dark lines in TEM micrographs at different magnifications.The primary challenges of nanoparticles are particle growth and surface treatment. Due to their ready susceptibility to acquiring charge, and their low mass, nanoparticles tend to aggregate readily. Only through shear, aggregation can be minimized. This hypothesis was utilized to see how well melt intercalation improve PEGM reinforced PP nanocomposite.
Figure 6.7 TEM image of PP-PEGM (SB) nanocomposite -2 phr PEGM