This research project focused on cellulose crystals as building blocks for the design and development of new low-density functional polymer composites. Although the piezoelectricity of cellulose has been predicted from its non-centrosymmetric crystal structure, it has not been measured accurately, nor has cellulose been properly exploited as a piezoelectric material. If it is proven, it could enable novel low-density electroactive materials capable of surpassing the performance of the best synthetic piezoelectric polymer such as polyvinylidene difluoride (PVDF).

To fulfill this objective, a systematic study was required to judiciously manipulate dipoles of individual cellulose nanocrystals to produce polar ordering at the macroscale and accurately characterize the nano-to-mesoscale structural ordering and electromechanical properties of the produced materials.

Based on its noncentrosymmetric crystal structure, nanocrystalline cellulose has been assumed to be piezoelectric; but it has not been properly or convincingly characterized in the literature. The key obstacle is that nanoscale piezoelectric properties of individual crystals cancel each other when cellulose crystals are dispersed randomly, or when they are assembled into simple uniaxial structures in which most cellulose particles are packed in the antiparallel fashion in average. Overcoming this obstacle could accelerate the realization of cellulose-based piezoelectric materials and related applications.

With this premise, a team with multidisciplinary experimental backgrounds explored various ways of inducing polar ordering (i.e., parallel packing) of nanocrystalline cellulose domains. The methods tested included: (a) chemical derivatizations followed by Langmuir-Blodgett (LB) film deposition, (b) electric field (both DC and AC) induced orientation of cellulose dipoles, and (c) magnetic field-induced orientation using the diamagnetic property of cellulose.

With the unique capability of spectroscopically determining the degree of polar ordering of cellulose in the sample using vibrational sum frequency generation (SFG) spectroscopy and other complementary techniques, it was found that none of these methods, which were previously used and reported for preparation of cellulose-containing piezoelectric materials in the literature, really produces any polar ordering. Thus, what was reported as a piezoelectric response of cellulose in the literature cannot be attributed to piezoelectricity; what was reported falls under electromechanical coupling originating from other mechanisms that are not intrinsic to the non-centrosymmetric cellulose crystal structure. Those mechanisms were found to originate from extrinsic factors such as electrochemical or interfacial polarization within the composite samples containing cellulose nanocrystals.

Thus, the question regarding the intrinsic piezoelectricity of cellulose remains to be answered; the intrinsic piezoelectric coefficient of cellulose appears to be very small, although the absolute value could not be determined in this study. While exploring the directional alignment of cellulose, it was demonstrated that (a) SFG is indeed a quantitative tool to determine the polar ordering of molecular functional groups in the sample, (b) in the LB trough, cellulose nanoparticles can topographically corrugate liquid/surfactant/air interface, (c) the capillary flow of the fluid phase during the LB transfer can improve the uniaxial alignment of anisotropic nanoparticles onto a substrate, and (d) magnetic-field induced ordering of cellulose suspended in liquids is a function of dielectric constant, ion conductivity, and viscosity of the suspension solution. These research findings further expand potential applications of cellulose nanoparticles in various engineering applications.

This work was done by Seong H. Kim of Pennsylvania State University for the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) below. AFRL-0298

This Brief includes a Technical Support Package (TSP).
Design of New Piezoelectric Composites Using Nanocellulose

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This article first appeared in the December, 2020 issue of Aerospace & Defense Technology Magazine.

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