Acknowledgements
AFM height image of one drop of the dispersion after drying
SEM image of one drop of the dispersion after
drying
AFM phase image of the cross section of
composite film
In-situ PVAc/CNC
Mixed PVAc/CNC
Characterization of mechanical properties What is the dispersion of nanomaterials?
Conclusion
Characterization of structure
Characterization of stability How did we improve it?
Why is it important?
Single-step method for producing cellulose based nanocomposites with outstanding dispersion
Ph.D student: Shiyu Geng Supervisor: Kristiina Oksman
Division of Materials Science, Department of Engineering Sciences and Mathematics, Luleå University of Technology, Luleå, Sweden
Matrix Polyvinyl acetate (PVAc)
Reinforcement Cellulose nanocrystals (CNC) Plasticizer Glyceryl triacetate (GTA) Crosslinker Sodium tetraborate (Borax)
Sample Particle size (nm)
PVAc 62
Mixed PVAc/CNC 69 In-situ PVAc/CNC 77 In-situ XPVAc/CNC 315
The dispersion of a nanomaterial describes the level of agglomeration in a matrix. Nanomaterials have strong tendency to agglomerate due to van der Waals force, and hydrogen bonding is another main factor for nanocellulose. Thus, obtaining good dispersion of nanocellulose is still challenging for producing nanocomposites.
What always happen is the nanocellulose materials form aggregates and generate poor dispersion in the matrixes, the high surface area is compromised and the aggregates can act as defects, which limit properties of the composites.
What we expect is nanocellulose materials are well-dispersed that means they are much smaller than the critical crack size for the matrixes and do not initiate failure. Thus, they provide a bright avenue for simultaneously strengthening and toughening the matrixes. Good dispersion is a key for achieving this.
The components of the nanocomposites we are using:
Method 1: In-situ polymerization in the presence of CNC
CNC dispersion
Adding vinyl acetate monomer
Emulsion polymerization
In-situ PVAc/
CNC dispersion
In-situ PVAc/
CNC film
Method 2: Crosslinking CNC with PVAc chains by borax
Reference: Direct mechanical mixing
In-situ PVAc/
CNC dispersion
Adding borax (1.5 wt%)
Tuning pH to 11
In-situ XPVAc/
CNC dispersion
In-situ XPVAc/
CNC film
“X” refers to “Crosslinked” in this poster.
CNC dispersion
Mixing with PVAc dispersion
Mixed PVAc/CNC dispersion
Mixed PVAc/CNC film
The stability of the pure PVAc and composite dispersions was characterized by zeta potential and size measurements. The ratio of PVAc to CNC in each composite sample is 80/20.
The dispersion of CNC in the in-situ and mixed nanocomposites were characterized by AFM and SEM. The ratio of PVAc to CNC in each sample is 80/20.
Raman spectroscopy characterized the crosslinks between PVAc chains and between CNC and PVAc. The crosslinked samples contain 1.5 wt%
of borax and the in-situ composites contain 18.7 wt% of CNC.
The mechanical properties of the composite films were measured by tensile testing. The in-situ XPVAc/CNC composites contain 1.5 wt% of borax, and the ratio of polymer to GTA in every sample is 95/5.
Both in-situ and mixed composite dispersions are electrostatic stable.
The size of the particles was increased significantly due to crosslinking.
The in-situ polymerization method generated much better dispersion after drying compared to the direct mechanical mixing. Raman spectroscopy shows that the crosslinks could be formed between CNC and PVAc because the heavy atoms (CC and CO) in cellulose experienced more stretching in the crosslinked composite.
The mechanical properties of the in-situ composites are better than the mixed composites and were additionally improved by crosslinking reaction. The samples with 15 wt% of CNC have the highest strength.
The authors appreciatively acknowledge KAW Knut and Alice Wallenberg Foundation for the financial support of this research.
