Cellulose nanofiber-reinforced polymer biocomposites
The interest to incorporate cellulose nanofibers (CNFs) as reinforcing agents to enhance the properties of polymer matrices has recently grown considerably, as these nanoparticles are bio-based and have good physical and mechanical properties, comparable to those of inorganic fillers. However, one f...
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| Format: | Dissertation |
| Language: | English |
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ProQuest Dissertations & Theses
01-01-2017
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| Online Access: | Get full text |
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| Summary: | The interest to incorporate cellulose nanofibers (CNFs) as reinforcing agents to enhance the properties of polymer matrices has recently grown considerably, as these nanoparticles are bio-based and have good physical and mechanical properties, comparable to those of inorganic fillers. However, one faces important challenges associated with their large aspect ratio, fibrillated nature, high flexibility and hydrophilicity that make CNFs highly entangled, which render their dispersion/distribution in polymers quite difficult, especially in hydrophobic matrices. This thesis aims at developing polymer/CNF biocomposites with well-dispersed microstructure and enhanced properties. We chose two polymers that need improvements in thermomechanical properties: polylactide (PLA), which is a commonly used bio-based hydrophobic polymer, and poly(ethylene oxide) (PEO), which is a biocompatible/biodegradable hydrophilic polymer. The biocomposites were prepared via simple mixing methods, either using N,N-dimethylformamide (DMF) or water as solvents. For comparison, some composites were prepared in the molten state. High-performance composites comprising PLA and CNFs (0.25–5 wt%) were first prepared via solvent casting without any compatibilization. The complex viscosity and storage modulus of the PLA/CNF biocomposites were increased by up to two and five orders of magnitude, respectively, at low frequencies compared to the neat PLA. An improvement of up to 50% for the Young modulus of PLA was achieved by incorporating CNFs. Similarly, the tensile strength was raised by 31%. The flexural storage modulus in dynamic mechanical thermal analysis (DMTA) was increased by up to 51 and 264% at room temperature and 70 °C, respectively. Good transparency was retained for the biocomposite films in the visible light range comparable to that of neat PLA. The second part of this project was related to the preparation of enhanced PEO/CNF biocomposites with different nanofiber loadings, i.e., 1–3 wt%, via aqueous solution mixing. At low frequencies, increases of up to two and three orders of magnitude were obtained for the complex viscosity and storage modulus, respectively, of PEO/CNF biocomposites relative to the neat PEO. The Young modulus and the room temperature DMTA storage modulus were improved by ca. 48% by adding 3 wt% CNFs to PEO; the tensile strength also increased by 35%. Moreover, the transparency of the composite films in the visible range was similar to that of the neat PEO film. For the melt-prepared biocomposites, however, the CNFs were poorly dispersed and consequently no property enhancement was observed. In the last part, poly(ethylene glycol) (PEG) was used as a compatibilizer for the PLA/CNF system. Initially, a CNF/PEG masterbatch (ratio of 1/2) was prepared in an aqueous solution. Then, the biocomposites were prepared using a solvent-casting method. Different microscopic techniques showed that CNFs were better dispersed/distributed within PLA when the compatibilizer was employed. The complex viscosity and storage modulus were increased by one order of magnitude for the compatibilized composites compared to those of the uncompatibilized composites. With compatibilization, the DMTA storage moduli of the PLA containing 2 wt% nanofibers at room temperature and 80 °C were enhanced by up to 42 and 553%, respectively, compared to the neat PLA. The nucleation effect of the CNFs on PLA crystallization was more pronounced relative to the uncompatibilized samples. Also, better light transmittance was measured for the PLA/CNF/PEG composite films relative to the PLA/CNF composite films. On the other hand, no property enhancement, compared to the matrix, was observed for the biocomposites prepared in the melt. In conclusion, the morphology, rheology, mechanical properties and light transmittance results confirmed that, using simple solution methods, good dispersion and distribution of CNFs within the host polymer matrices, PLA, PEO and PLA/PEG, was achieved. Substantial enhancements in the thermomechanical properties of the composites were obtained at low nanofiber contents. |
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| ISBN: | 9798209560630 |