| 1. |
- Singh, Shikha, et al.
(författare)
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Orientation of Polylactic Acid–Chitin Nanocomposite Films via Combined Calendering and Uniaxial Drawing: Effect on Structure, Mechanical, and Thermal Properties
- 2021
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Ingår i: Nanomaterials. - : MDPI. - 2079-4991. ; 11:12
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Tidskriftsartikel (refereegranskat)abstract
- The orientation of polymer composites is one way to increase the mechanical properties of the material in a desired direction. In this study, the aim was to orient chitin nanocrystal (ChNC)-reinforced poly(lactic acid) (PLA) nanocomposites by combining two techniques: calendering and solid-state drawing. The effect of orientation on thermal properties, crystallinity, degree of orientation, mechanical properties and microstructure was studied. The orientation affected the thermal and structural behavior of the nanocomposites. The degree of crystallinity increased from 8% for the isotropic compression-molded films to 53% for the nanocomposites drawn with the highest draw ratio. The wide-angle X-ray scattering results confirmed an orientation factor of 0.9 for the solid-state drawn nanocomposites. The mechanical properties of the oriented nanocomposite films were significantly improved by the orientation, and the pre-orientation achieved by film calendering showed very positive effects on solid-state drawn nanocomposites: The highest mechanical properties were achieved for pre-oriented nanocomposites. The stiffness increased from 2.3 to 4 GPa, the strength from 37 to 170 MPa, the elongation at break from 3 to 75%, and the work of fracture from 1 to 96 MJ/m3. This study demonstrates that the pre-orientation has positive effect on the orientation of the nanocomposites structure and that it is an extremely efficient means to produce films with high strength and toughness.
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| 2. |
- Butylina, Svetlana, et al.
(författare)
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Cellulose Nanocomposite Hydrogels : From Formulation to Material Properties
- 2020
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Ingår i: Frontiers in Chemistry. - : Frontiers Media S.A.. - 2296-2646. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- Poly(vinyl alcohol) (PVA) hydrogels produced using the freeze-thaw method have attracted attention for a long time since their first preparation in 1975. Due to the importance of polymer intrinsic features and the advantages associated with them, they are very suitable for biomedical applications such as tissue engineering and drug delivery systems. On the other hand, there is an increasing interest in the use of biobased additives such as cellulose nanocrystals, CNC. This study focused on composite hydrogels which were produced by using different concentrations of PVA (5 and 10%) and CNC (1 and 10 wt.%), also, pure PVA hydrogels were used as references. The main goal was to determine the impact of both components on mechanical, thermal, and water absorption properties of composite hydrogels as well as on morphology and initial water content. It was found that PVA had a dominating effect on all hydrogels. The effect of the CNC addition was both concentration-dependent and case-dependent. As a general trend, addition of CNC decreased the water content of the prepared hydrogels, decreased the crystallinity of the PVA, and increased the hydrogels compression modulus and strength to some extent. The performance of composite hydrogels in a cyclic compression test was studied; the hydrogel with low PVA (5) and high CNC (10) content showed totally reversible behavior after 10 cycles.
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| 3. |
- Butylina, Svetlana, et al.
(författare)
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Properties of as-prepared and freeze-dried hydrogels made from poly(vinyl alcohol) and cellulose nanocrystals using freeze-thaw technique
- 2016
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Ingår i: European Polymer Journal. - : Elsevier BV. - 0014-3057 .- 1873-1945. ; 81, s. 386-396
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Tidskriftsartikel (refereegranskat)abstract
- Poly(vinyl alcohol), PVA hydrogels are potential materials for biomedical and biotechnogical applications. However, their low mechanical properties restrict their use. In this study, the effect of PVA concentration, addition of nanocrystalline cellulose, CNC, number of freeze-thaw cycles and freeze-drying stage on properties of resulting hydrogels were investigated. The results showed that increase in PVA concentration and the addition of CNC improved the compressive properties of the hydrogels. Overall, increase in number of freeze-thaw cycles from 3 to 5 did not show any improvements in properties of hydrogels. Concentration of PVA had great effect on morphology of freeze-dried hydrogels. The CNC reduced crystallinity of PVA/CNC hydrogels as compared to PVA hydrogels. Rehydrated PVA and PVA/CNC hydrogels had higher compressive characteristics than their as-prepared analogues. In general, an improvement of compressive properties of hydrogels was achieved via reduction of their water content. In case of 5% PVA hydrogel, an addition of CNC was found to be beneficial because it increased degree of swelling and water content on rehydration.
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| 4. |
- Geng, Shiyu, et al.
(författare)
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Aligned biodegradable cellulose-reinforced nanocomposites with high strength and toughness
- 2017
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Konferensbidrag (refereegranskat)abstract
- Cellulose, as the most abundant component in wood, has attracted a lot of attention for utilizing it in environmentally-friendly applications to replace the fossil-based materials. Nanocellulose materials with high stiffness and strength, large surface area and biodegradability, are promising reinforcement in polymers. However, the energy consumption of nano-scale isolation of cellulose and the dispersion of nanocellulose materials in the polymers are still challenging for obtaining low-cost and ultra-strong nanocomposites. To overcome these, we focus on investigating the aligned nanocomposites reinforced by a very low cellulose nanofibers (CNF) content (0.1 wt%), and grafting polyethylene glycol (PEG) on CNF was performed to improve the dispersion of them. We found that the alignment can improve mechanical properties of the polylactic acid (PLA)/CNF composites dramatically. With a draw ratio of 8, the strength of the aligned composite reached 320 MPa and the toughness was 30 times enhanced compared to the isotropic material. Much better dispersion of the CNF grafted with PEG in PLA matrix was confirmed by scanning electron microscopy (SEM) compared to the ungrafted CNF, and further supported by the mechanical testing results. Furthermore, the aligned nanocomposites exhibited light scattering behavior indicating they have the potential to be used in optical applications.
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| 5. |
- Geng, Shiyu, et al.
(författare)
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Aligned polylactic acid based nanocomposite reinforced using a tiny amount of functionalized cellulose nanofibers
- 2018
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Konferensbidrag (refereegranskat)abstract
- Due to the challenges of the cost of nanocellulose materials and the dispersion of them in polymer matrix, small amount and well-dispersed nanocellulose materials are desired as reinforcement to achieve environmentally-friendly nanocomposites with high performance. In this study, an aligned polylactic acid (PLA) based nanocomposite reinforced by 0.1 wt% of functionalized cellulose nanofibers (CNFs) was investigated. The CNFs were covalently grafted by polyethylene glycol (PEG), which improves the dispersion of the CNFs in the PLA significantly compared to the native CNFs. The improved dispersion was examined by scanning electron microscopy (SEM), polarized optical microscopy (POM) and mechanical testing. Furthermore, it was found that the alignment can improve mechanical properties of the nanocomposite dramatically. The strength of the aligned nanocomposite reaches 343 MPa with a draw ratio of 8, meanwhile the toughness is about 30 times enhanced compared to the isotropic material. The aligned nanocomposite also exhibits light scattering behavior, indicating that it has the potential to be used in optical applications.
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| 6. |
- Geng, Shiyu
(författare)
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Cellulose-based Nanocomposites – The Relationship between Structure and Properties
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nanocellulose materials extracted from various types of biomass have recently attracted significant attention. Due to their remarkable mechanical properties, large surface area and biodegradability, they can be promising reinforcements in nanocomposites. Cellulose-based nanocomposites constitutive of nanocellulose reinforcements and biodegradable polymer matrices have great potential to be used in environmentally friendly applications to replace fossil-based materials. However, the challenge of controlling their nanoscale structure, especially achieving good dispersion of nanocellulose in hydrophobic polymer matrices, still poses significant obstacles to producing high-performance nanocomposites. Therefore, this thesis reports several methods for structural modification of cellulose-based nanocomposites toward the objectives of improving the dispersion of nanocellulose and enhancing the properties of the nanocomposites. The methods include in situ emulsion polymerization in the presence of nanocellulose, crosslinking of polymer matrix, grafting of polymer brushes to nanocellulose and drawing of nanocomposites to obtain aligned structures. The resulting mechanical, thermal and other related properties are investigated, and the relationship between structure and properties of the nanocomposites are discussed.To address the challenge of achieving good dispersion of nanocellulose in hydrophobic matrices, in situ emulsion polymerization of vinyl acetate monomer in the presence of cellulose nanocrystals has been developed. Microscopy results show that the in situ method improves the compatibility between nanocellulose and hydrophobic polymers, which consequently improves the dispersion of nanocellulose in the nanocomposites. Compared with direct mixed polymer/nanocellulose composites, the in situ synthesized nanocomposites exhibit higher stiffness and strength arising from their superior interphase volume, which is confirmed theoretically and experimentally. Crosslinking of partially hydrolyzed poly(vinyl acetate) by borate additives under different pH conditions has been studied to further enhance mechanical properties of the nanocomposites. Moreover, the “grafting to” modification method also helps to overcome this challenge. It is revealed that poly(ethylene glycol)-grafted cellulose nanofibers disperse better in poly(lactic acid) matrix than unmodified cellulose nanofibers, which is attributed to the improved compatibility and steric effect provided by the covalently grafted poly(ethylene glycol) brushes.To substantially enhance the unidirectional mechanical properties of cellulose-based nanocomposites, a highly aligned structure in the materials is obtained through the drawing process. Drawing conditions including temperature, speed and draw ratio show considerable effects on the mechanical and thermal properties of the nanocomposites. Furthermore, the aligned nanocomposites consisting of poly(lactic acid) matrix and ultra-low weight fraction of poly(ethylene glycol)-grafted cellulose nanofibers demonstrate competitive strength, superb toughness and interesting optical behaviors compared with other aligned nanocellulose-based materials reported in the literature, indicating their potential to be further developed for large-scale environmentally friendly applications.
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| 7. |
- Geng, Shiyu, et al.
(författare)
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Cellulose-based nanocomposites with outstanding dispersion produced by in-situ polymerization
- 2016
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Konferensbidrag (refereegranskat)abstract
- Cellulose-based nanocomposites are promising materials to replace the fossil-based polymers since they are biodegradable and produced from renewable resources. However, achieving good dispersion of nanocellulose in the matrix is one of the main obstacles because nanomaterials tend to form aggregates and lose their merits. In this study we developed an in-situ polymerization method to produce cellulose nanocrystals reinforced polyvinyl acetate, and the method of direct mechanical mixing was used as reference. The stability of in-situ and mixed nanocomposite aqueous dispersions was investigated by zeta potential measurements, and the results show that both of them were electrostatic stable at pH 4. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to characterize the dispersion of cellulose nanocrystals in the in-situ and mixed nanocomposites after drying, and better dispersion could be seen in the in-situ samples compared with the mixed ones. Tensile testing showed that the in-situ nanocomposites with same cellulose content had higher strength and longer elongation at break compared to the mixed nanocomposites. Furthermore, crosslinking the cellulose and partially hydrolyzed polyvinyl acetate with sodium tetraborate was also performed to further improved the reinforcing efficiency. The results from Raman spectroscopy illustrate that the heavy atoms (CC and CO) in cellulose experienced more stretching in the crosslinked nanocomposites, and the tensile testing indicated the elastic modulus and ultimate strength of them were increased significantly than those without crosslinking.
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| 8. |
- Geng, Shiyu, et al.
(författare)
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Crosslinked polyvinyl acetate (PVAc) reinforced with cellulose nanocrystals (CNC) – structure and mechanical properties
- 2016
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Ingår i: Composites Science And Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 126, s. 35-42
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Tidskriftsartikel (refereegranskat)abstract
- The structure of cellulose-based nanocomposites significantly influences their final mechanical properties. However, obtaining a good dispersion of hydrophilic nanocellulose materials in a hydrophobic polymer matrix is challenging. In this study, two unique methods were developed to improve the dispersion of cellulose nanocrystals (CNC) in a poly(vinyl acetate) (PVAc) matrix. One method was the crosslinking of PVAc by sodium tetraborate (borax), which is expected to prevent agglomeration of CNCs during the drying process, and the other method was the in-situ polymerization of vinyl acetate in the presence of CNCs to generate good compatibility between CNC and PVAc. The results showed that the crosslinking degree of PVAc could be varied by tuning the pH. The atomic force microscopy images illustrate that after drying, the in-situ polymerized PVAc/CNC composite was much better dispersed than the composite produced using mechanical mixing. The mechanical and thermo-mechanical characterizations indicate that the in-situ nanocomposite with 10 wt% of CNC had a higher strength and storage modulus compared with the mixed composite with the same CNC concentration. Further investigations of the restriction effect caused by the crosslinker are required.
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| 9. |
- Geng, Shiyu, et al.
(författare)
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Crosslinked polyvinyl acetate reinforced with cellulose nanocrystals – : Characterization of structure and mechanical properties
- 2015
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Konferensbidrag (refereegranskat)abstract
- The structure of cellulose based nanocomposites influences their final mechanical properties significantly. Obtaining good dispersion of hydrophilic nanocellulose materials in hydrophobic polymer matrix is challenging. Here two unique methods were developed to improve the dispersion of cellulose nanocrystals (CNC) in polyvinyl acetate (PVAc) matrix. One is in-situ polymerization of vinyl acetate in the presence of CNCs, and the other one is crosslinking of PVAc by sodium tetraborate (Borax), which restricts the movement of CNCs during the drying process. The results from atomic force microscopy (AFM) show that the in-situ CNC/PVAc emulsion has much better dispersion than the one produced by stirring. Moreover, the mechanical characterization indicates that the in-situ composite with 10 wt% CNC has higher strength compared to the stirred composite with the same CNC concentration. The mechanical properties of crosslinked PVAc materials can be varied by changing the pH and may be attributed to differences of the crosslinking degree. Further investigations of the restriction effect caused by borax are needed.
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| 10. |
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