Biomedical Engineering Reference
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B
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Figure9.19 ConceptualizedentanglementsinpureBC(A)andinBC/xyloglucansnanocom-
posites (B) explaining the difference in tensile properties. (Reprinted fromWhitney, S.E.C.;
Gothard, M.G.E.; Mitchell, J.T.; Gidley, M.J., Roles of cellulose and xyloglucan in deter-
mining the mechanical properties of primary plant cell walls. Plant Physiology, 1999,
121, 657-63, Copyright (1999), with permission from the American Society of Plant
Biologists.)
Dynamic FTIR spectroscopy further shed light on the morphology and molecular
orientation that can take place during linear stretching of these nanocomposites (61).
During stretching of neat BC, cellulose fibers molecularly reoriented along the loading
direction, a reorientation that was eased by the lubricating power of water (61). In the
BC/xyloglucan nanocomposites prepared in vitro, where cellulose fibers were already
oriented, xyloglucan bridges experienced less reorientation upon stretching. With 2D
FTIR spectroscopy the authors confirmed that the xyloglucans and the cellulose moved
collectively.
Bi-axial tensile loading revealed yet another difference between neat BC and in vitro
BC/xyloglucan nanocomposites. Under equi-biaxial tensile loading, the BC alone was
found to behave as a linear elastic material (59). The BC/xyloglucan nanocompos-
ites showed a markedly different behavior with much greater extensibility (threefold
higher than that of BC), a low stiffness, and most noticeably a nonlinear elastic behavior
(Figure 9.20). These drastic differences in behavior were confirmed by creep mea-
surements where neat BC did not exhibit any time dependent behavior whereas the
BC/xyloglucan nanocomposites displayed a large time-dependent and viscoelastic behav-
ior (Figure 9.20) (59).
While these initial studies (57-59, 65, 66) were all conducted on Tamarind xyloglu-
cans, there exists a large diversity of xyloglucans that could translate in a wide range
of interactions with BC and thus nanocomposite performance. To test that possibility,
Whitney et al . also examined BC/xyloglucan nanocomposites prepared in vitro with
eight different xyloglucans that systematically varied in terms of molecular weight and
substitution pattern including galactose content, fucose content and degree of acetyla-
tion (67). For all nanocomposites, significant amounts of xyloglucans remained in the
composites after washing demonstrating that they all interacted strongly with the cellulose
(19-98%). Morphological characterization with SEM and CP/MAS NMR showed that
for all the xyloglucans except the one with a 60% galactose depletion of the I α /I β ratio
was significantly reduced hence revealing that most xyloglucans perturbed the molec-
ular assembly of BC. The deep etch freeze fracture images also revealed a different
ultrastructure based on the characteristics of the xyloglucan. For high molecular mass
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