|The objective of this research is to valorize lignocellulosic materials obtained from kraft pulp and paper industry for functional applications through design, synthesis, and processing of their nanocomposites. The lignocellulosic nanomaterials that are mainly focused are cellulose nanocrystals (CNC), cellulose nanofibers (CNF), and kraft lignin (KL). Developing CNC, CNF, and KL based transparent and UV protection nanocomposite films and CNF based conducting nanocomposite films are two main aspects of functional applications in this research.
Simple, novel, and green approach is designed to produce transparent, homogeneous CNC-KL based UV protection films through optimal addition of electrolytes. First, the effect of electrolytes (NaOH, NaCl, CsCl, CaCl2, AlCl3) which varies in size, valency, and pH on CNC self-assembly, structural, optical, and mechanical properties was studied. The chiral nematic self-assembly of CNC could be tuned through addition of electrolytes. Nematic CNC films with maximum transparency and mechanical properties were prepared through optimized electrolyte addition. Further, CNC/KL nanocomposites were designed by incorporating the KL in NaOH containing CNC suspension and casting the mixture. NaOH facilitated uniform dispersion of KL in CNC film producing transparent, homogeneous, UV blocking nanocomposite films. To further improve the transparency of CNC/KL films, acetylation or oxidative peroxide bleaching treatment were applied to KL. The peroxide bleached KL obtained under optimized bleaching conditions resulted in improved transparency without significantly affecting its UV blocking properties. CNC nanocomposites using peroxide bleached KL showed superior performance in terms of UV blocking and transparency compared to 4-amino benzoic acid (4-ABA), a commercial organic UV absorbent and most of the lignin based transparent, UV blocking nanocomposites in literature. This work shows for the first time that CNC aqueous suspensions with and without containing lignin could be tuned through the addition of electrolyte to produce transparent, homogenous films, providing a facile approach in engineering CNC/lignin transparent, UV-protection nanocomposite films.
To further expand the scope of the nanocellulose and KL, multifunctional nanocomposites were synthesized using KL, CNF, and polyvinyl alcohol (PVA). First, KL grafted CNF was produced via amidation reaction which was further treated with hydrogen peroxide bleaching treatment to reduce the lignin color. This modified nanofiber was introduced to aqueous PVA solution to cast nanocomposite films. Transparent, UV blocking PVA nanocomposites with high flexibility, mechanical strength, and thermal stability were obtained. Water vapor barrier properties of PVA were retained in these nanocomposites. Heat treatment of these nanocomposites rendered them water resistance and prevented lignin leaching. PVA and modified nanofiber nanocomposites also showed superior performance in terms of transparency and UV blocking properties compared to most lignin based transparent and UV blocking nanocomposites reported in literature.
Finally, a papermaking like method for the fast preparation of uniform cellulose nanopaper (CNP) was developed. This standardized method could also be used for comparing the CNF properties produced from different sources and processes. CNP produced using this method was developed as a flexible and biodegradable substrate for synthesizing the polypyrrole based conducting nanocomposite films with improved physical and electrical properties. A new approach based on in-situ CNP polymerization was developed. Physically cross linked PVA coated CNP (PVA-CNP) provided water stability, reduced porosity, and resulted in smooth deposition of PPy. As a result, PPy/PVA-CNP showed enhanced conductivity, dry and wet tensile strengths compared to widely used in-situ nanofiber polymerization approach in literature. The applicability of these nanocomposite films in EMI shielding applications was shown. PPy/PVA-CNP exhibited higher shielding effectiveness (SE) compared to nanocomposites produced using in-situ nanofiber polymerization approach. These nanocomposites have applicability as an electromagnetic interference (EMI) shielding, and anti-static coating/packaging material for electronic devices.
In summary, this research established new routes to valorize the kraft pulping process derived lignocellulosic nanomaterials through their chemical modifications, synthesis, and processing into functional nanocomposites.