| 2. |
- Singh, Shikha, et al.
(författare)
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Crystallization of triethyl-citrate-plasticized poly(lactic acid) induced by chitin nanocrystals
- 2019
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Ingår i: Journal of Applied Polymer Science. - : John Wiley & Sons. - 0021-8995 .- 1097-4628. ; 136:36
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study was to gain a better understanding of the crystallization behavior of triethyl-citrate-plasticizedpoly(lactic acid) (PLA–TEC) in the presence of chitin nanocrystals (ChNCs). The isothermal crystallization behavior of PLA–TEC wasstudied by polarized optical microscopy, scanning electron microscopy, differential scanning calorimetry, and X-ray diffraction (XRD).Interestingly, the addition of just 1 wt % ChNCs in PLA–TEC increased the crystallization rate in the temperature range of 135–125 C.The microscopy studies confirmed the presence of at least three distinct types of spherulites: negative, neutral, and ring banded. TheChNCs also increased the degree of crystallinity up to 32%, even at a fast cooling rate of 25 C min−1. The XRD studies further revealedthe nucleation effect induced by the addition of ChNCs and thus explained the faster crystallization rate. To conclude, the addition of asmall amount (1 wt %) of ChNC to plasticized PLA significantly affected its nucleation, crystal size, and crystallization speed; therefore,the proposed route can be considered suitable for improving the crystallization behavior of PLA.
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| 3. |
- Singh, Shikha, et al.
(författare)
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Effect of Chitin Nanocrystals on Crystallization and Properties of Poly(lactic acid)-Based Nanocomposites
- 2020
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Ingår i: Polymers. - : MDPI. - 2073-4360. ; 12:3
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Tidskriftsartikel (refereegranskat)abstract
- The crystalline phase of poly(lactic acid) (PLA) has crucial eects on its own propertiesand nanocomposites. In this study, the isothermal crystallization of PLA, triethyl citrate-plasticizedPLA (PLA–TEC), and its nanocomposite with chitin nanocrystals (PLA–TEC–ChNC) at dierenttemperatures and times was investigated, and the resulting properties of the materials werecharacterized. Both PLA and PLA–TEC showed extremely low crystallinity at isothermal temperaturesof 135, 130, 125 ºC and times of 5 or 15 min. In contrast, the addition of 1 wt % of ChNCs significantlyimproved the crystallinity of PLA under the same conditions owing to the nucleation eect ofthe ChNCs. The samples were also crystallized at 110 ºC to reach their maximal crystallinity,and PLA–TEC–ChNC achieved 48% crystallinity within 5 min, while PLA and PLA–TEC required 40 min to reach a similar level. Moreover, X-ray diffraction analysis showed that the addition ofChNCs resulted in smaller crystallite sizes, which further influenced the barrier properties and hydrolytic degradation of the PLA. The nanocomposites had considerably lower barrier propertiesand underwent faster degradation compared to PLA–TEC110. These results confirm that the additionof ChNCs in PLA leads to promising properties for packaging applications.
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| 4. |
- 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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| 5. |
- Singh, Shikha
(författare)
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Properties of poly(lactic acid) in presence of cellulose and chitin nanocrystals
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Plastic based materials are widely used for industrial and domestic packaging application. However, disposal of such petroleum-based materials e.g. polyethylene (PE), polypropylene (PP), and polyethyleneterephthalate (PET) has become a huge threat to the environment. These materials are non-biodegradable and complex for waste management, which causes plastic-pollution in both land and marine eco-system. For a sustainable industrial and economic development, it is indeed an urgency to develop packaging materials, which are environmentally benign, easy for waste treatment and recycling, and less/non-toxic. However, developing suitable and efficient plastic-substituents needs multiple requirements to be fulfilled viz., logistics and cost-effectiveness, good mechanical, thermal, optical and barrier properties, structural integrity of the constituents and morphological properties of the films. In this regard, utilizing bio-based polymers such as poly(lactic acid) (PLA), which originates from the natural resources, can be a viable and practical due to its low toxicity, biodegradability, and eco-friendly behaviour. Moreover, it has good optical and mechanical properties, e.g. high stiffness (3-4 GPa) and strength (50-70 MPa), which are similar or comparable to the polymers used for packaging applications. However, pristine PLA poses few challenges to overcome before it finds real world applications. Especially, slow crystallization rate, low crystallinity, poor toughness (very brittle material) and, poor barrier properties (O2 barrier) of PLA are particularly important aspects, which need to be modified and fine-tuned. Utilizing nano-reinforcements, such as nanocellulose and nanochitin, is a promising approach for modifying PLA because of raw materials abundancy; easily obtainable from forest-based and bio-waste, hence, utilizing such materials also help the sustainable bioeconomy. Chitin nanocrystals (ChNCs) and cellulose nanocrystals (CNCs) possess unique properties, such as, low density, biodegradability, low toxicity, good mechanical, and barrier properties; therefore, can act as suitable nano-reinforcements for PLA.Homogeneous dispersion of the nano-reinforcements into the polymer matrix is crucial and challenging. To achieve good dispersion, primarily two methods were employed viz., (a) liquid–assisted extrusion of PLA with ChNCs in the presence of plasticizers, and (b) surface modification of the CNC via grafting. First segment of the research was aimed to understand and gain an insight about the role of nano-reinforcements on the crystallization behaviour of plasticized PLA e.g. crystallization kinetics including rate and temperature dependency, and morphology of the spherulites. ChNCs, due to large surface area, acted as better nucleating agent and improved the overall crystallization rate by reducing the crystallization time and size of the spherulites. Interestingly, rarely found neutral type of spherulites along with commonly occurring negative type, and multi ring-banded spherulites were observed at different crystallization time and temperature. Second part of the research was aimed to investigate the role of homogenously dispersed nano-reinforcements on the thermal, optical, barrier, and hydrolytic degradation properties of the nanocomposites. Noticeably, at a lower temperature (110 ºC), the highest rate of crystallization achieved within 5 min. Furthermore, homogenous crystallization and smaller spherulite size (7 nm) of PLA achieved due to the good dispersion of ChNCs significantly improved the crystallinity, thermal, barrier, and hydrolytic degradation properties. Faster crystallization at lower temperature resulted in a smaller spherulites sizes, which improved the oxygen and moisture barrier properties by hindering permeation path of the gases. On the other hand, the synergistic effect of isothermal crystallization and ChNCs improves the rate of hydrolytic degradation. It is noticeable that nanocomposites showed better optical properties than the plasticized PLA even at same crystallization conditions. As mechanical properties play an important role in packaging applications. So, the third part of the research involved the study of mechanical properties of oriented films (PLA/ChNCs) achieved by a combination of solid-state and melt-state drawings. Melt state drawing of relatively higher amount (5 wt%) ChNCs with PLA was prepared to obtain oriented films. These oriented nanocomposites films exhibited excellent mechanical properties. For example, a tensile strength with 360%, elongation at break with 2400%, and the toughness with 9500% increment achieved as compared to un-oriented nanocomposite films. The degree of crystallinity of highly oriented nanocomposite films increased from 8% to 53% with respect to the un-oriented nanocomposite films and smaller crystallites sizes were observed. Drawing conditions including drawing temperature and speed had a strong impact on the properties. By utilizing this knowledge, materials with high strength and toughness can be produced. Finally, in the fourth part, mechanical properties of the surface modified PLA/CNCs nanocomposites were investigated by a conventional tensile test and compared with the small punch test. Surface modification of CNC facilitated better dispersion of CNC into PLA matrix and increased the elastic modulus of the PLA/CNC nanocomposites. Grafting induced crazing effect, which induced better ductility. Knowledge and results gained in this study demonstrate the potential path for the development of the PLA nanocomposites with higher properties for packaging applications.
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