UPR 5301

Fangbo Lin’ s thesis defense on November 10th, 2020

This thesis is entitled "Regioselective modification of cellulose nanocrystals: synthesis, assemblies and functional properties" is co-supervised by M. Bruno JEAN (CNRS researcher and team leader at CERMAV).

Abstract :
“Cellulose nanocrystals (CNCs) have been identified as highly attractive building blocks for the design of innovative biosourced materials. These nanoparticles are indeed derived from an abundant and renewable source, cellulose fibers, and possess exceptional properties such as a very large surface area, a low density, non-toxicity, biocompatibility and mechanical properties comparable to those of Kevlar. An interesting feature of these nano-objects, which has not been widely exploited yet, is their chemical polarity. Indeed, the biosynthesis leads to different extremities of the rods from a chemical point of view, which makes it possible to generate asymmetrically functionalized CNCs. Alternatively, CNCs made of the allomorph II of cellulose (CNC-II) can also be produced. CNC-II exhibit a rather similar geometry as CNCs but both rod ends are amenable to chemical modification. This thesis project has thus focused on new strategies to efficiently modify in a regioselective manner CNCs and CNC-II particles in order to generate innovative and functional assemblies. First, an in situ growth strategy was developed to drastically optimize the regioselective labelling of CNCs and CNC-II with gold nanoparticles when compared to literature data (labelling yield increased from about 15 to 80%). This development allowed us to get insight into fundamental morphological features by confirming the antiparallel packing of cellulose chains in CNC-II and by showing that CNCs derived from cotton are made of a parallel assembling of chemically polar elementary crystallites. Secondly, both types of nanocellulose particles were successfully regioselectively functionalized with thermosensitive polymer chains using a two-step oxidation and peptide coupling strategy. In the case of CNCs, the resulting hybrid particles underwent a thermally induced-aggregation into star-shaped aggregates composed of 3 to 6 nanocrystals attached by their ends. Using CNC-II particles, a reversible temperature-triggered association into supra-micronic networks could be obtained through end-to-end attachment of the cellulose rods. The structural features of these new objects and their assemblies were characterized by transmission electron microscopy, dynamic light scattering and small angle X-ray or neutron scattering. Rheology measurements were used to show that in both cases, above the LCST of the grafted polymer chains, a gel-like behavior is obtained but the network structure led to stronger effects than the star-shaped complexes. Finally, the optimization of the grafting process was investigated and the use of DCC/DMAP or 4-PPY as catalysts of the peptide coupling and DMF as the solvent turned out to be the best conditions. The use of N-Methylmorpholine N-oxide (NMMO) to induce a swelling of the CNC ends and favor the reaction was also studied. However, no swelling could be detected but the treatment with NMMO had a noticeable effect of separating the elementary crystallites forming the CNCs. Since the undertaken modifications concern a very reduced fraction of the available anhydroglucose units, a quantitative direct characterization of the regioselective derivatization of CNCs remains challenging, even if the use of advanced techniques such as scattering methods give fruitful information. However, the present work shows that such a site-selective functionalization coupled with the use of biosourced particles allows a fine tuning of stimuli-sensitive assembling into innovative structures that give rise to new macroscopic properties.”