Resumen de Properties of poly(lactic acid) in presence of cellulose and chitin nanocrystals
Plastic based materials are widely used for packaging applications. However, disposal of such petroleum-based materials e.g. polyethylene (PE) and polypropylene (PP) has become a huge threat to the environment due to its non-biodegradable behavior and complexity for waste management. 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 option due to its low toxicity, biodegradability, and eco-friendly behavior. Moreover, PLA has good optical and mechanical properties, which are similar or comparable to the some of the petroleum-based materials. However, pristine PLA possess few challenges such as slow crystallization rate, low crystallinity, poor toughness, and poor barrier properties, 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. First segment of the research was aimed to understand and gain an insight about the role of ChNCs on the crystallization behavior of PLA and optimize the isothermal crystallization conditions. ChNCs, due to large surface area, acted as better nucleating agent and improved the overall crystallization rate of PLA by reducing the crystallization time and size of the spherulites. In second part of the research, knowledge gained about crystallization behavior was further explored to produce isothermally crystallized films at larger scale. The effect of crystallinity on the thermal, optical, barrier, and hydrolytic degradation properties of the nanocomposites were investigated. Noticeably, at 110ºC, the highest rate of crystallization achieved within 5 min. Furthermore, homogenous crystallization and smaller spherulite size (7 nm) of PLA achieved with ChNCs, significantly improved the crystallinity, thermal, barrier, and hydrolytic degradation properties. 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. These oriented PLA 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. 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 the dispersion of CNC into PLA matrix and increased the elastic modulus of the PLA/CNC nanocomposites. Knowledge and results gained in this thesis demonstrate the potential path for the development of the PLA nanocomposites with higher properties for packaging applications.
