Next Generation Aerospace Composites Through Nanotechnology
D. Yebassa, S. Balakrishnan, and D. Raghavan
Howard University, US
Keywords: Materials, Surfaces and Interfaces
Polymeric nanocomposites are a relatively new class of materials obtained by dispersing inorganic layered silicates in polymer matrix. Generally well-dispersed exfoliated morphology of clay platelets in nanocomposite is sought with the expectation that this would improve the properties of nanocomposites. The objective of this study is to control the clay surface response to the host matrix by using reactive surfactant that promote host-matrix adhesion while facilitating clay platelet exfoliation and dispersion in the matrix.
Surfactants with reactive functional group (olefinic group) at different locations along the backbone chain were synthesized for the possibility of forming covalent crosslinks between the reinforcing clay and vinyl ester resin. For example, by subjecting long chain alkenol to a series of reactions that include bromination, azide formation, and azide reduction, surfactant with reactive functional group at terminal position was synthesized (refer synthetic scheme in Fig.1). 1H and 13C NMR and FTIR data support the formation of high purity novel nanomer. The 1H NMR spectrum of reactive surfactant (refer to Fig. 2) clearly indicates that the vinyl group (peaks at 4.93 d & 5.78 d) has been preserved during azide reduction and is available for further reaction with the vinyl ester resin during the formulation of nanocomposites.
Na-Montmorillonite clay was ion-exchanged with reactive and non-reactive surfactant. Both the surfactant modified clay (nanomer) and vinyl ester resin was soaked in reactive diluent for predetermined time interval. The swollen modified clay and vinyl ester resin were mixed, ultrasonicated, precured, and postcured to synthesize vinyl ester nanocomposite. For the aerospace relevant Apotech Q6055 vinylester resin, a loading of 3.5 wt% of this nanomer into the matrix leads to marked improvement in transparency of nanocomposites compared to unmodified clay filled nanocomposite. With X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM), we were able to characterize the exfoliation and dispersability of clay in matrix. For example, by studying the breadth, position, and intensity of the basal reflection of XRD, the relative extent of intercalation/exfoliation of the organoclay-polymer hybrid nanocomposite was determined. We have shown series of XRD spectrum for different vinyl ester nanocomposites in Fig. 3., using reactive and non-reactive C11 and C18 surfactant modified clay. As can be seen in the Fig. 3, the shift of the basal spacing (001) towards lower angle (2q) was observed for octadecyl amine salt treated nanoclay filled vinyl ester nanocomposite, compared to undecyl amine salt treated nanoclay filled vinyl ester nanocomposite. This indicates as the surfactant chain length increases the clay layer spacing increases. As expected, no peaks were observed for C18 reactive surfactant treated nanoclay filled vinyl ester nanocomposites and little hump was seen for C11 reactive surfactant treated nanoclay filled vinyl ester nanocomposites. As there is no reflection peak in the basal spacing (001) region for reactive surfactant treated nanoclay filled nanocomposite, we can attribute to the partially exfoliated morphology to improved compatability of clay and vinyl ester nanocomposites. We also observed the layer spacing of organoclay in vinyl ester matrix to be a strong function of the processing conditions. Studies are underway to obtain information about the mechanical properties of nanocomposites.
NSTI Nanotech 2003 Conference Technical Program Abstract