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Tunable Order in Alginate/Graphene Biopolymer Nanocomposites

Vilcinskas, Karolis, Norder, Ben, Goubitz, Kees, Mulder, Fokko M., Koper, Ger J. M., Picken, Stephen J.
Macromolecules 2015 v.48 no.22 pp. 8323-8330
X-ray diffraction, barium, biopolymers, calcium alginate, crosslinking, deformation, drying, graphene, graphene oxide, models, nanocomposites, scanning electron microscopy, sodium alginate
We report on highly aligned graphene oxide or graphene sheets inside an alginate matrix and their structure obtained for various compositions. The order of the platelet particles with respect to one another has been verified by environmental scanning electron microscopy (ESEM) and 2-dimensional X-ray diffraction (2D XRD). The microscopic order within the platelet particles has been analyzed by X-ray diffraction (XRD) measurements in the Bragg–Brentano reflection configuration as well as in Debye–Scherrer diffraction mode. The azimuthal angle intensity profiles obtained from 2D XRD analysis have been fit to Maier–Saupe and affine deformation model predictions, and the affine deformation model proved to be the most reliable to quantify the order parameter ⟨P₂⟩ values of graphene oxide/sodium alginate and graphene/calcium alginate composites with different weight fractions of the filler. The ⟨P₂⟩ values for graphene oxide/sodium alginate composites were found to show little dependence on the concentration of graphene sheets above ∼10 wt %, with a maximum ⟨P₂⟩ value of 0.8 at 25 wt % graphene oxide inside the sodium alginate matrix. The alignment of graphene sheets inside the calcium alginate matrix has been observed to be lower, with an average ⟨P₂⟩ value of 0.7. We have not observed preferred orientation of graphene sheets inside the barium alginate matrix. The formation of a highly aligned graphene oxide/sodium alginate composite structure has been explained by the affine deformation model, whereupon drying the developed yield stress causes sheets to align in-plane with the polymer matrix. The impaired orientation of graphene sheets inside the calcium alginate matrix and absence of orientation in the barium alginate matrix have been explained by the structure development in the polymer matrix itself due to metal-ion-induced cross-linking.