Modification Of Nanocellulose And Its Enhancement For Environmental Response Nanocomposites

Cellulose is the most abundant organic polymer in nature, with many advantages including low-cost, degradability and environment-friendly. Nanocellulose, obtained from natural cellulose materials, has been attracting a great deal of interest as promising candidates for bio-nanocomposite due to their appealing intrinsic properties, such as low density, high surface area and superior mechanical strength. However, because of the hydrophilicity, nanocellulose needs modification before used as the reinforcement of polymer matrices. In this work, nanocellulose was modified via grafting and magnetizing. Then, the modified nanocellulose was implemented as nanofillers in the polyurethane matrix, to prepare thermoand magnetic- responded nanocomposites. This study will provide a new way for preparing high-value nanocellulose materials.First, two kinds of nanocellulose were prepared using acid hydrolysis and TEMPO-mediated oxidation, named as CNW and TONC, respectively. There were some significant differences between them in surface structure, crystallinity and thermal stability. Both of the nanocelluloses preparations were rod-like nanowhiskers. TEMPO-mediated oxidation was more efficient in preparing nanocellulose with higher aspect ratio, while acid hydrolysis resulted in higher crystallinity(77.8%) of nanocellulose. Meanwhile, the thermal stability of CNW decreased as the presence of residual sulfate groups after sulfuric acid hydrolysis.Then, polycaprolactone(PCL) was grafted onto the surface of both freeze-dried and solvent-exchanged CNWs by ring-opening polymerization(ROP). The efficiency of surface grafting was in fact much different for the two protocols. The freeze-dried samples had a significantly reduced PCL grafting density. The surface of solvent-exchanged CNW tended to have more loose and porous, resulting in a higher sensitivity toward the polymerization. Because of the differences in surface structure and chemical accessibility between the two species, it was found that their dispersibility, aggregation, and thermal stability were different after the modification. In general, compared with freeze-drying, the solvent-exchanged CNW was more efficient to the grafting of PCL. The modified CNW was more hydrophobic, with a higher PCL content of 41.6%.The polymerization conditions were optimized using the response surface methodology and the optimum grafting conditions were 14:1 of the ε-caprolactone to CNCs ratio(monomer content), 130°C of the polymerization temperature, and 26.5 h of the polymerization time. In this condition, the maximum predicted grafting ratio was138.57%.Shape memory polyurethane(CLPU) was synthesized using hexamethylene diisocyanate(HDI), PCL-diol(PCL) and 1, 4-butylene glycol(BDO). The PCL-grafted CNW(PCNW) was incorporated into CLPU. It was found that PCNWs could disperse very well in the CLPU matrix with an improved interfacial adhesion. There was a co-continuous phase forming between PCNW and CLPU as well as new hydrogen-bonds, which could promote the microphase separation of CLPU matrix. With the addition of PCNW, the storage modulus of the nanocomposites increased, while the loss factor went to the opposite. Meanwhile, the Young’s modulus of was significantly enhanced with low level loadings of PCNWs, which was 159.7% higher than CLPU, while decrease of the elongation at break was only 11.8%. Compared with CLPU, the addition of PCNW obviously enhanced the shape fixity ratio of the nanocomposite, with little impact on the shape recovery ratio. When the content of PCNW was 10%, the shape recovery ratio could be maintained above 80%. Thus, the PCNW/CLPU nanocomposites exhibited preferable shape memory properties.TONC was also modified with PCL. It was found that carboxyls on the surface of TONC could also initiate the ROP. The PCL-grafted TONC(PTONC) was more hydrophobic. When composited with CLPU, different contents of PTONC would significantly vary the storage modulus, glass transition temperature and hydrogen structrue of the nanocomposites. The increase of PTONC content led to improvements for Young’s modulus and tensile strength, which were 2.43 and 2.04 times of the neat CLPU, respectively, with a lower decrease of elongation at break. Meanwhile, due to the hydrogen-bonds and the co-crystallization between the matrix and filler, although the shape recovery ratio of the nanocomposites was reduced from 90.6 to 80.8% with the PTONC content range from 0 to 5%, the shape fixity was improved as shown by the change from 80.5 to 90.5%.A novel magnetic cellulose whiskers(MGCNW) bearing Cu-Co ferrite was prepared in-situ by the controlled hydrothermal method. The immobilization of ferrite nanoparticles were carried out with ammonia on the surface of nanocellulose with smaller sizes and better dispersibility. Due to the substantial hydroxyls on the surface, CNWs in such system acted as the matrix and template to promote the size distribution of ferrite particles. In order to maintain the crystalline structure of nanocellulose, ferrite with a content lower than 50% was perfect. The magnetic properties of the materials could be controlled by changing the composition of ferrites. At a low ferrite content, the best composition was Cu0.5Co0.5Fe2O4 with a small crystal size of 11.3 nm and a maximum saturation magnetization of 10.95 emu/g.Incorporation of MGCNW into CLPU via a casting/evaporation technique endowed the nanocomposites with thermo- and magnetic- response. The addition of MGCNW promoted the hydrogen-bond formation in the crystal and amorphous regions. XRD and SEM analysis demonstrated the homogeneous dispersion of MGCNW in CLPU matrix. Although the ductility of the composites was negatively affected, both of the Young’s modulus and storage modulus increased with the increasing content of MGCNW. The nanocomposites exhibited favourable shape memory properties in both thermal and alternating magnetic stimulus. The shape fixity ratio and magnetic induced shape recovery ratio increased 87.2% and 72.9%, respectively, with the MGCNW content of 10%. Meanwhile, the thermal induced shape recovery ratio decreased with the addition of MGCNW, with a final value of about 75%.