BIOCOMPOSITES BASED ON CORE-SHELL CELLULOSE NANOFIBERS Preparation, Structure and Properties

Cellulose nanofibers (CNFs) are of interest as load-bearing components for polymer matrix nanocomposites. A wide range of nanostructured materials including nanopaper/films, foams, aerogels, and hydrogels can be prepared from CNFs. The material performance can be fine-tuned when CNFs are combined with different polymer matrices. The main idea of the present study is to test the hypothesis that the concept of Core-shell CNFs can provide processing and performance advantages for nanocomposites through improved nanostructural control. The problems of matrix distribution and interface structure at nanoscale are investigated. The first part of this thesis (Paper I-III) describes an alternative preparation procedure for biocomposites based on the Core-shell concept. Inspired by the structural framework and mechanical function of the primary cell wall in plants, Core-shell CNFs are formed by coating wood CNFs with a polysaccharide matrix, and are subsequently used for fabrication of biocomposite films. The nanostructure of Core-shell CNFs and their nanocomposites is characterized. Mechanical properties of the biocomposites at various hydration conditions are investigated. A study on molecular water mobility and moisture stability of the materials is conducted. In the present thesis, three different biological polysaccharides including amylopectin, xyloglucan and galactoglucomannan have been used for man-made nanocomposites based on Core-shell nanofibers. The later sections of the thesis (Paper IV-V) describe an alternative method to disintegrate holocellulose nanofibers from wood chips. These CNFs based on holocellulose possess Core-shell structure with native hemicelluloses as “shell”. Peracetic acid pretreatment is used for the preparation of holocellulose. This procedure is a promising single-step pulping as preparation for nanofibrillation. The nanostructure of the holocellulose nanofibers are characterized, and compared with CNFs prepared from enzymatic pretreatment. The holocellulose nanofibers are used for preparation of films and porous materials. The influence of nanostructural characteristics of two different nanofibers (CNFs from enzymatic pretreatment vs holocellulose nanofibers from peracetic acid pretreatment) on final properties of the nanocomposites is clarified. Favorable characteristics of Core-shell fibrils are reported in terms of colloidal stability, controlled matrix distribution, improved interface characteristics, and improved hygromechanical properties of the biocomposites. In both a scientific and industrial context, the Core-shell nanofiber concept thus offers great potential for the materials design of new cellulose nanomaterials with unique characteristics.