Application of functionalized cellulose nanocrystals to improve their dispersion in polymer matrix and enhance the thermo-mechanical properties of polymer composites

Abstract

In the past few years, Cellulose nanocrystals (CNC) and its derivatives are exploited as potential bio-based nanofillers for functional polymer composites. The main challenge for manufacturing CNC-based composites is to disperse CNC uniformly in the hydrophobic polymer matrix. The hydrophilicity of CNC hindered their uniform distribution in the matrix and reduced the overall thermal, mechanical, and physical properties of the composite system. To overcome this challenge, several methods have been developed, such as physical and chemical modification of CNC. Also, polymer-based composites are highly flammable in nature and can cause severe life-threatening incidence. The flame-retardants available commercially should be added to the composite for a very high add-on% and toxic in nature, which is not eco-friendly and can be dangerous for humans. In this study, two critical research gaps are addressed: 1) uniform dispersion of CNC in a hydrophobic matrix, and 2) improving fire-retardancy of polymer composites by using non-toxic inorganic oxides. In the first part, a chemical compound, aminosilane was used as a dispersing agent for CNC in polyethylene oxide (PEO) based composite films and their thermo-chemical properties were analyzed. In the next part, the silanated CNCs are coated with nano ZnO and nano B2O3 and added as a functional filler to manufacture high density polyethylene (HDPE) based composites. The ZnO coated composites showed a significant improvement in thermo-mechanical properties of the composites even at a low add-on%, due to the smaller size and higher surface area of nano ZnO. The lignin containing CNC (LCNC) was also used as a functional additive along with nano ZnO and B2O 3. The hydrophobic character of lignin could not improve the dispersion of LCNC in the HDPE matrix because of the reduction of the aspect ratio and aggregate formation of LCNC/inorganic oxides during freeze drying. The role of the new age material graphene quantum dots (GQD) as a dispersing agent for CNC was also exploited. The CNC/GQD inclusion complex showed excellent dispersion behavior in HDPE matrix along with the significant improvement in thermo-mechanical properties of the composites, even at very low add-on% of GQD. The preliminary idea of using CNC/GQD inclusion complex as an energy storage device was also developed. Overall, the current work addresses the two main challenges which hindered the functional application of CNC based polymer composites and developed safe, effective, and eco-friendly methods to overcome those challenges. CNC based polymer composites have strong potential to be used in various industrial applications from fire-retardant material to energy storage device.