Abstract
In this work, cellulose nanocrystal (CNC) was modified by an ureido-pyrimidinone (UPy) system based on quadruple hydrogen bondings, and CNC-UPy was obtained. Then, this powder was added into polyvinyl alcohol (PVA), and PVA/CNC-UPy composite membranes and hydrogels were prepared. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), polarizing optical microscopy (POM) and particle size distribution (PSD) were used to characterize CNC-UPy. From the FTIR results, the characteristic peaks of NCO group sat 2270 cm−1 disappeared, indicating the successful synthesis of CNC-UPy. XRD results showed that the modification by UPy may change the structure of CNC and its degree of crystallinity was increased. PSD analysis showed that the particle size of CNC was increased and its size distribution became narrower after modification by UPy groups. The structure and properties of the composite membranes and hydrogels were studied by differential scanning calorimeter (DSC), thermogravimetric analysis (TGA) together with investigation of swelling, sustained release and self-healing performances. DSC curves depicted that the glass transition temperature, Tg, of different PVA membranes was increased with addition of different proportions of CNC-UPy. TGA data showed that the temperature of maximum weight loss rate was increased, which illustrated the enhanced thermal stability of PVA/CNC-UPy composites. Meanwhile, it was also revealed that the PVA/CNC-UPy composite hydrogels possess good self-healing and better sustained release behavior for the soil conditioner, fulvic acid (FA).
Highlights
In recent years, many problems, such as soil salinization, nutrient and microbial imbalance, have seriously restricted the intensive development of urban agriculture and become an urgent problem to be solved in modern facility agriculture in China
As shown in Scheme 1c, when CNC-UPy and Polyvinyl alcohol (PVA) aqueous solutions were combined and self-assembled, hydrogen bonds may be formed between the –NH/–OH in CNC-UPy and –OH in PVA, and PVA/CNC-UPy composite membranes may be obtained after drying
PVA aqueous solution, CNC-UPy, and fulvic acid (FA) were combined with borax, and a composite hydrogel was formed from the crosslinked PVA molecules
Summary
Many problems, such as soil salinization, nutrient and microbial imbalance, have seriously restricted the intensive development of urban agriculture and become an urgent problem to be solved in modern facility agriculture in China. As a type of soil conditioner, fulvic acid (FA) may be used in regulation of plant growth, which can play an important role in drought resistance, improve plant stress resistance, increase yield and improve quality. Hydrogels area type of stretchable, compressible and cross-linked hydrophilic polymer with three-dimensional networks, adjustable water content and switchable mechanical structures, while maintaining their structural integrity during deformation [6,7] They are considered to be powerful material platforms, providing novel ways to design soft bioelectronics and flexible energy storage/conversion devices, such as wearable devices and implantable devices, release devices and electronic skins. The results exhibited that with the addition of low amounts of CNC into PVA composites, their degree of crystallinity and water sorption ability were reduced, and it was shown that the amount of hydroxyl groups in the new hydrogen bonded systems were increased. The intermolecular interaction force was strong, making it possess better thermal stability, higher mechanical strength and sustained release behavior of the soil conditioner
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