Abstract
The interfacial bonding and structure at the nanoscale in the polymer–clay nanocomposites are essential for obtaining desirable material and structure properties. Layered nanocomposite films of cellulose nanofibrils (CNFs)/montmorillonite (MTM) were prepared from the water suspensions of either CNFs bearing quaternary ammonium cations (Q-CNF) or CNFs bearing carboxylate groups (TO-CNF) with MTM nanoplatelets carrying net surface negative charges by using vacuum filtration followed by compressive drying. The effect of the ionic interaction between cationic or anionic charged CNFs and MTM nanoplatelets on the structure, mechanical properties, and flame retardant performance of the TO-CNF/MTM and Q-CNF/MTM nanocomposite films were studied and compared. The MTM nanoplatelets were well dispersed in the network of TO-CNFs in the form of nanoscale tactoids with the MTM content in the range of 5–70 wt %, while an intercalated structure was observed in the Q-CNF/MTM nanocomposites. The resulting TO-CNF/MTM nanocomposite films had a better flame retardant performance as compared to the Q-CNF/MTM films with the same MTM content. In addition, the effective modulus of MTM for the TO-CNF/MTM nanocomposites was as high as 129.9 GPa, 3.5 times higher than that for Q-CNF/MTM (37.1 GPa). On the other hand, the Q-CNF/MTM nanocomposites showed a synergistic enhancement in the modulus and tensile strength together with strain-to-failure and demonstrated a much better toughness as compared to the TO-CNF/MTM nanocomposites.
Highlights
Cellulose nanofibrils (CNFs), called as microfibrillated cellulose (MFC) or nanofibrillated cellulose (NFC), prepared from wood pulp are 3 orders of magnitude smaller than the intact wood fiber cells
The tensile fractured surface of the QM70 sample showed a layered microstructure but with an obvious pull out of MTM aggregates or tactoids (Figure 5d). This is due to the reason that the Q-CNF/MTM nanocomposites contained both an intercalated structure and a large content of MTM aggregates when the MTM content was higher than 50 wt %, as revealed by the light transmittance of the dilute suspension (Figure 3c) and X-ray diffraction (XRD) analysis of the nanocomposite film (Figure 4a)
The structure of TEMPO-oxidized CNFs (TO-CNF)/MTM nanocomposites has obvious advantages in hindering the transport of oxygen and diffusion of decomposition products, which lead to the resistance of the material against burning even if the MTM content is as low as 10 wt %
Summary
Cellulose nanofibrils (CNFs), called as microfibrillated cellulose (MFC) or nanofibrillated cellulose (NFC), prepared from wood pulp are 3 orders of magnitude smaller than the intact wood fiber cells. The hybrids of CNFs with inorganic clay nanoplatelets such as the MTM have been extensively studied previously and have demonstrated an excellent mechanical performance together with other functionalities such as low oxygen permeation, thermal shielding, and fire retardancy.[16−27] Our first reported multilayered CNF/MTM nanocomposite with a weight ratio of 50:50 showed a lower modulus and tensile strength as compared to the neat CNF nanopaper due to the aggregation of the native CNF and MTM.[16,17] Later, CNF/MTM nanocomposites with different MTM contents (10−80 vol %) were prepared by carefully and completely exfoliating the MTM in water before mixing with the native CNF.[27] The synchrotron radiation scattering analysis confirmed that the MTM platelets were well dispersed in the form of a single platelet and loose stacks of 2−3 platelets when MTM contents increased up to 35 vol %, resulting in an increase in the modulus for the nanocomposites. The effects of the dispersion and intercalation structure of the MTM in the CNF matrix on flame retardant properties were studied
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