## 1 Introduction
More specifically, cellulose has a crystalline structure compared to the other constituents of natural fibers and offers both strength and stiffness to the fibers via linkages (e.g., hydrogen bonds) [18; 29; 30; 31]. Furthermore, increasing the crystallinity of cellulose leads to decreased elongation, lower water intake and fewer sites available to chemically react with other materials, since tightly packed structures reduce reactive sites' accessibility [32]. On the contrary, hemicellulose has an amorphous structure and among all the fiber's constituents exhibits the greatest sensitivity to thermal effects [18; 33; 34]. The presence of hemicellulose in lignocellulosic fibers increases the hydrophilic nature of the fibers [18; 25], leading to moisture absorption, while it is responsible for biodegradation processes and for reduced thermal stability, leading to thermal degradation at high temperatures [18; 29; 30; 31]. Lignin, on the other hand, is the most hydrophobic of all the fiber's elements/components, and it likewise exhibits an amorphous structure [35], leading to greater elongation loss and increased thermal stability of natural fibers [18; 25]. According to [PERSON] et al. [24], it is quite difficult to objectively define the concept of quality regarding natural fibers employed in specific applications, leading to a perpetual search of the adequate raw materials for each target utilization. Furthermore, the lack of standardized methods for the determination of the characteristics of natural fibers prevents the suitable parameter selection from being studied in order to achieve an estimation of certain properties, used for further exploitation in the lignocellulosic-polymer-composites industry. The aim of this study is to establish an alternative standardized procedure, which will be capable of providing a comparative prediction or/and estimation of the performance assessment of the lignocellulosic-based polymer composites, based on the study of individual plant fibers. It is known that various wet-chemical analytical methods are employed for determining the chemical composition of lignocellulosic fibers, but they are time-consuming and expensive, preventing them from being applied in commercial applications. Furthermore, NIR (Near-Infrared reflectance) Spectroscopy is also a useful tool for extracting chemical information about fibers, but that technique is not commercially friendly [36]. So, this study deals with an alternative standardized method for fibers' analysis based on Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), and Thermogravimetric analysis (TGA), and the way these procedures can optimize the fiber-selection process for various applications as alternative routes toward more commercially friendly methods. Bast fibers--fibers collected from the stem of the plant--used in the present study are from hemp, kenaf, ramie, and flax. In the literature, their physical and mechanical properties dramatically vary over a large range of values, since inherent variability, as well as the factors mentioned above, lead to an enormous heterogeneity of reported values. Table 1 presents an overview of some of the characteristics of the fibers studied according to a series of studies. These fibers differ in terms of their structure, the number of main building blocks, the content of cellulose and other constituents, water content, etc. It is worth noting that it is well known that the mechanical performance of plant-fiber-based composites is governed by the efficiency of the stress transfer between the fibers and the polymer matrix through the interface, but the hydrophilic nature of plant fibers, combined with the hydrophobic polymer matrices, end up in ineffective interfacial adhesion [16]. Although, no treatment is applied to natural fibers in present study, since the degree of crystallinity of the fibers can be altered [37], and fibers may be separated into microfibrils, leading to misleading outcomes since the chemical variety involved in the process may affect the fibers in different ways.