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Nanocomposites & foams from cellulose nanofibrils

Published on April 1, 2016
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PhD Defense November 30, 2015

Florian Martoïa, LGP2's Ph.D. student, defended his Ph.D. thesis: "Nanocomposites and foams from cellulose nanofibrils: rheology during their processing and mechanical properties".

This University Grenoble Alpes doctoral thesis was prepared under the supervision of Naceur Belgacem, Professor (Grenoble INP-Pagora / LGP2) and the co-supervision of Pierre Dumont, Professor (INSA Lyon) and of Laurent Orgéas, CNRS Research Director (3SR, Grenoble).

This study focuses on the use of cellulose nanofibrils (NFCs) as bio-based nano-reinforcement in polymer composites and foams. These renewable materials can be used in place of traditional materials such as for instance to produce sandwich panels. This experimental, theoretical and numerical work aims at optimizing the processing of these NFC-based materials as well as their use properties.

In the first part of this work, the rheology of concentrated NFC suspensions, that behave as thixotropic yield stress fluids, is investigated at macro- and mesoscales using an original rheo-ultrasonic velocimetry (rheo-USV) setup allowing the local flow kinematic to be obtained. We show that the flow of NFC suspensions is highly heterogeneous and exhibits complex situations with the coexistence of wall slippage, multiple shear bands and plug-like flow bands. Using this experimental database, we develop an original multiscale rheological model for the prediction of the rheology of NFC suspensions. The model takes into account the anisotropic fibrous nature of NFC networks as well as colloidal and mechanical interaction forces occurring at the nanoscale. The model predictions prove that colloidal and hydrodynamic interaction forces together with the orientation and the wavy nature of NFCs play a major role on the yield stress and shear thinning behaviour of the suspensions.

In the second part of this work, NFC-reinforced polymer nanocomposite films are processed for a wide range of NFC contents. Using advanced microscopy techniques (AFM, SEM), X-ray diffraction and mechanical tests (tensile and DMA tests), we show that:
  • NFCs form highly connected nanofibrous structures with in-plane random orientation.
  • These connected NFC networks play a leading role on the mechanical behaviour of the nanocomposites.
  • The elastic properties of nanocomposite films are much lower than those predicted from the micromechanical models of the literature.
In light of these observations, we propose an alternative multiscale model in which the main involved deformation nano-mechanisms are those occurring both in the amorphous segments of the nanofibers and in the numerous nanofiber-nanofiber contact zones.

Finally, in a third part, we focus on the influence of the processing conditions, the suspension type and the NFC concentration on the microstructure (using X-ray synchrotron microtomography), the mechanical properties (using compression tests) and the deformation micro-mechanisms (using in situ compression test with X-ray microtomography) of various foams prepared from NFC suspensions by freeze-drying.

LGP2 Ph.D. thesis (2015)
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Date of update April 1, 2016

Université Grenoble Alpes