All-cellulose composites (ACCs) is a new generation of biocomposites with both the matrix and reinforcing fibers being based on cellulose. This thesis explores various designs of ACCs using either macro- or nano-scale cellulose fibers as raw materials and ionic liquids (ILs) as cellulose solvents. Prior to ACCs fabrication, the swelling and dissolution kinetics of different cellulose fibers (natural fiber flax, viscose-type Cordenka and two Ioncell fibers with one containing lignin and hemicelluloses) in solvent power tuned ILs (1-ethyl-3-methylimidazolium acetate, [EMIM][OAc] and N-methyl-1,5-diazabicyclo[4.3.0]non-5-enium dimethyl phosphate, [mDBN][DMP]) was studied. High performance unidirectional flax-based ACCs, isotropic filter paper-based ACCs and nanopaper were then produced involving controlled dissolution and/or swelling of cellulose fibers. The fundamental investigation on fibers swelling and dissolution kinetics revealed that the rate of fiber dissolution in [EMIM][OAc] depended on fiber accessibility and solvent viscosity. The fastest dissolution (in [EMIM][OAc] and [EMIM][OAc]-5% water) or swelling (in [EMIM][OAc]-15% water) was recorded for Ioncell fibers and the slowest for Cordenka. An anomalous two-step dissolution-swelling behavior of flax was observed in [mDBN][DMP] and [mDBN][DMP]-water. ACCs from either unidirectional flax or isotropic filter paper were produced via selective dissolution strategy. For flax-based ACCs, it was shown that the mechanical properties were controlled by the sufficient amount of matrix and the non-dissolution of the inner, mechanically strong cell walls in the fiber. The interface between the matrix and the fibers was further artfully re-modelled by utilizing two-step dissolution-swelling phenomenon in [mDBN][DMP], resulting in increased tensile strength in both transverse direction and fiber direction. For isotropic paper-based ACCs, a digital image correlation technique was explored for accuracy enhanced analysis of mechanical properties. A 30-60 min impregnation allowed increasing tensile strength, Young's modulus and toughness of a filter paper in almost 10 times, 5 times and 25 times, respectively. Isotropic ACC constructed from highly disordered and entangled cellulose nanofibrils (CNFs) through their self-binding exhibited an exceptional ductility (up to 35%) while combining high strength (up to 260 MPa) and toughness (up to 51 MJ/m3). This was realized by swelling of hemicelluloses in-between the nanofibrils in [EMIM][OAc]–water below the dissolution limit of cellulose and hemicellulose, thus allowing delamination of thick CNF bundles into thinner ones without influencing the length. The results obtained demonstrate that a fundamental understanding of cellulose swelling and dissolution promotes various possibilities to design ACCs with engineered performance. The findings bring new perspectives to the design of macroscale and nanoscale cellulosic materials.
|Julkaisun otsikon käännös||Designing High Performance All-Cellulose Composites by Dissolution/Swelling of Macro- and Nano-scale Cellulose Fibers|
|Tila||Julkaistu - 2021|
|OKM-julkaisutyyppi||G5 Tohtorinväitöskirja (artikkeli)|