The influence of the multi-level structure under high drawing on the preparation of high strength Lyocell fiber

In order to research the multi-level structure of Lyocell fiber at different draw ratios and to reveal the limiting factors for preparing the high strength Lyocell fiber, the paper reports on the effect of draw ratio including low drawing (1–5), high drawing (6–11) and excessive drawing (12–20) on the multi-level structure and the mechanical properties of Lyocell fiber. The structure was determined by wide-angle X-ray diffraction, small-angle X-ray scattering and fibrillation test, and the result showed that, at low draw ratio stage, the breaking strength, yield strength and modulus of the fiber increased with the draw ratio owing to crystallinity as well as orientation increased while the micropore decreased, and there were almost no microfibrils on the fiber surface. At high draw ratio stage, the orientation of amorphous region increasing was the principal reason for the increase of fiber mechanical properties, and the micropores continued to decrease and a few short and thick microfibril was formed. At excessive draw ratio stage, the breaking strength remained constant mainly due to the basically unchanged crystallinity and orientation of the fibers, the yield strength and modulus decreased due to the slip of the highly crystallized and oriented elementary fibril. Meanwhile, the micropores still decreased and became more slender, the number of microfibrils increased and the microfibrils showed tenuous structure. It can be summarized that Lyocell fiber has the characteristics of multi-level structure, and the fundamental reason limiting the improvement of mechanical properties with draw ratio increasing is the slip of elementary fibril.


Introduction
Lyocell fiber is known as one of the most representative green fiber in the twenty-first century due to its natural and renewable resource advantages, clean processing technology and degradable environmental friendliness. Lyocell fiber has the advantage of antistatic, hygroscopicity and comfortable wearing, and it is widely used in clothing, home textile and medical field (Medronho et al. 2012;Chen et al. 2015;Melani et al. 2020). Furthermore, Lyocell fiber has much higher strength and modulus, especially the outstanding wet strength and wet modulus compared to ordinary viscose fiber since its invention due to its dry-jet-wet spinning forming process (Röder et al. 2013;Zhang et al. 2018;Sayyed et al. 2019).
It can be inferred that Lyocell fiber could have made a further strength in mechanical properties, however, there is no Lyocell fiber product with much more outstanding properties than high-strength viscose fiber up to now, which seems to be a subject worthy of further discussion.
There has been considerable research in Lyocell fiber to improve the mechanical properties during the past decade by using high degree of polymerization cellulose pulp, adding modifier to the solution and carrying thermal treatment on the fiber et al. (Su-jin et al. 2013;Fink et al. 2014;Zhang et al. 2010a, b;Moriam et al. 2021;Zhang et al. 2021). However, cellulose pulp with high degree of polymerization makes dissolving and spinning difficult, and the addition of modifier may affect the recovery of NMMO (Zhang et al. 2010;Rosenau et al. 2021;Jin et al. 2021;Jadhav et al. 2021).With regard to spinning process conditions, although there are many factors (such as air gap length, temperature and concentration of coagulation bath, spinning speed and so on) affecting properties of Lyocell fiber (Mortimer et al. 1996a, b;Guizani et al. 2020), the most significant factor is the draw ratio under the premise of other process optimization. It is a common understanding of fiber preparation technology to obtain high crystallinity and high orientation fibers by increasing the draw ratio, so as to achieve the goal of high strength and high modulus (Silva et al. 2016). Is it the same for Lyocell fibers? Many researchers have studied the relationship between the draw ratio and the structure behaviors of Lyocell fiber (Mortimer et al. 2008;Jiang et al. 2012;Hauru et al. 2014;Silva et al. 2016;Michud et al. 2016). Mortimer and his coworkers (2008) have made great effort to investigate the influence of physical process parameters on the structure formation of Lyocell fiber. It was found that the breaking strength and modulus increase to a plateau region with draw ratio increasing and they attributed this change to the maximum crystallization and orientation, which was the same as Hauru' results (2014). However, the structure of Lyocell fiber is extremely complex, it is difficult to explain the evolution of fiber mechanical properties only according to their crystallization and orientation. It has long been known that different types of regenerated cellulose fibers (such as viscose fiber, modal fiber and Lyocell fiber) are composed of elementary fibrils that consisted of a succession of crystallites and less ordered cellulose molecules in the form of string of beads, and the microfibrils are comprised of elementary fibrils and the micropores between the elementary fibrils (Schurz et al. 1994;Moss et al. 2010;Sharma et al. 2021), the size of fibrils ranges from tens of nanometers to several microns, and the interconnertions between fibrils is easily broken and separated due to weak interaction (Schuster et al. 2003;Mao et al. 2019); it means that there are some other structure variations at different levels besides crystallization and orientation (Miyamoto et al. 2009;Sharma et al. 2019), which have a very remarkable impact on the mechanical properties in the process of spinning.
Therefore, this paper focuses on some other structural levels of Lyocell fiber such as the evolution of micropores, elementary fibrils and microfibrils with draw ratio increasing and their relationship with crystallization and orientation according to wide angle X-ray scattering (WAXD) and small angle X-ray scattering (SAXS) combined with fibrillation test, especially under high and excessive draw ratio. Meanwhile, it proposes the structure evolution mechanism in terms of low drawing, high drawing and excessive drawing, which can provide a basis for understanding the change of mechanical properties in terms of structural features. The investigation on the multi-level structure change of Lyocell fiber with draw ratio increasing in this paper is of great importance to understand the essential reason that limit the mechanical properties improvement, and it can provide guidance for the improvement of Lyocell preparation process and fiber properties.

Materials
Wood pulp (7.15 wt% water content, DP = 550, acellulose content was 91%) was provide by COSMO Specialty Fibers, Inc, and N-methylmorpholine-N-oxide (NMMO) aqueous solution was bought from Amines & Plasticizers Limited with initial water content of 50 wt%. The propyl gallate (PG) was used as a stabilizer and was bought from Aladdin Industrial Corporation.

Preparation of cellulose solution
The NMMO aqueous solution was mixed with cellulose pulp and propyl gallate (PG), after swelling at 50°C for one hour, the mixture was dissolved at 90°C in a planetary stirred tank with a vacuum pressure was 2-5 kPa. The final solution was composed of 10 wt% cellulose, 78 wt% NMMO and 12 wt% water.

Preparation of regenerated cellulose fibers
A customized spinning equipment was used to prepare the Lyocell fibers and the schematic diagram of spinning process is shown in Fig. 1. The spinning temperature was 90°C , the specification of spinneret was 60 holes * 0.1 mm, and the air gap length was 50 mm with a relative humidity of 65% and temperature of 25°C. The concentration of NMMO in coagulation bath was 20 wt% and temperature of coagulation bath was 25°C. Then, the solvent in filament was washed out in 60°C hot water bath and 100°C boiling water bath, and finally dried in 105°C for 30 min to obtain fiber samples. 20 samples were prepared with different draw ratios by keeping the extrusion speed (6 m/min) constant, adjusted the speeds of the first drawing roller.

Measurements
The diameter of single filaments of all samples was measured by optical microscope (8XB-PC, China), and the breaking strength, yield modulus and initial modulus of Lyocell fibers were measured on a monofilament strength tester at least 30 measurements for each sample, and the drawing rate was 20 mm/min.
The fiber cross-sections of different Lyocell fibers were observed using a HITACHI UHR SU-8000 N scanning electron microscope, and the cross-sections of the fibers were observed after gold spraying treatment.
The WAXD measurements of the fibers were performed on the BL14B1 beam line of Shanghai Synchrotron Radiation Facility. The energy of the X-ray radiation was 10 keV, the wavelength is 0.124 nm, the distance between sample and detector of WAXD is 329.4 mm, and the exposure time was 60 s (Yang et al. 2015). All data were analyzed by the software package X-polar (Precision works NY, Inc., USA). (Jiang et al. 2012;Yuan et al. 2018;Chen et al. 2019).
The one-dimensional integral curve of Lyocell fiber was fitted by adding crystallization peak and amorphous peak by using peak-fit software, and then the crystallinity is determined by the Eq. (1): where X c , S c and S a represent the crystallinity, the peak areas of crystalline phases and the peak areas of amorphous phases respectively. The Scherrer equation was used to calculate the size of crystallites in Lyocell fibers (Jiang et al. 2012).
where L hkl represents the crystal size perpendicular to the plane with Miller indices hkl, k is 0.124 nm, h is one half diffraction angle (2h), b is the integral width corresponding to the diffraction peak of crystal plane Fig. 1 Schematic diagram of Lyocell fiber spinning process and when calculating b is expressed in radians, K is 0.9. The orientation of crystal region was determined from the Eq. (3) (Klug and Alexamder 1954): where \cos 2 u c;Z [ is orientation parameter of crystal axis (c) relative to the fiber axis (Z). The orientation parameter \cos 2 u c;Z [ was obtained according to the Wilchinsky Model (1959) and crystal symmetry of regenerated cellulose fibers (Kolpak and Blackwell 1976). For Lyocell fiber, the reflections in equatorial (110), (020) and (-110) were used to calculate the orientation parameter \cos 2 u c;Z [ , and for each reflection (hkl), the orientation parameter \cos 2 u hkl [ can be determined by Eq. (4) and (5) (Klug and Alexamder 1954): where u hkl is the azimuthal angle and I (u hkl ) is the diffraction intensity along the reflection plane (hkl). The birefringence (Dn) was determined on polarizing microscope (type SSY-C).
The orientation factor (f a ) of amorphous region was calculated by Stein Eq. (6): where Dn represents the birefringence of Lyocell fibers, X c represents the crystallinity, Dn co is the birefringence index of the crystalline regions with a value of 0.0545, and Dn ao is the birefringence index of amorphous regions as 0.0545 (Peng et al. 2003).
The SAXS measurements were determined at BL16B beamline of Shanghai Synchrotron Radiation Facility. The energy of the X-ray radiation was 10 keV, the wavelength is 0.124 nm, the distance between sample and detector of SAXS is 1920 mm, and the exposure time was 200 s (Zeng et al. 2017). The detailed structure parameters of the fibers were obtained by the software package X-polar referred to the previous methods (Murthy and Grubb 2010;Jiang et al. 2007;Colombe et al. 2011).
For Lyocell fiber, it is generally accepted that the striae on the equator in the SAXS are caused by the scattering of the micropores along the fiber axis (Chen et al. 2007;Sharma et al. 2019Sharma et al. , 2021. Therefore, Guinier functions can be used to calculate the transverse dimension of micropore as shown in Eq. (7) (Guinier et al., 1955): where R is the radius of micropores, q (q = 4p sin h/k) represents the scattering vector, h is half of the scattering angle and k is 0.124 nm in this experiment. The average micropores length (L) and orientation deviation angle (B U ) which relative to fiber axis were obtained according to the method of Ruland (2010).
The real density of the fiber is calculated by the crystallinity of Lyocell fiber (the crystal region density of Lyocell fiber is 1.585, the amorphous region density is 1.483), and the real density of the fiber is calculated from Eq. (8): where X c is the crystallinity of the fiber. The apparent density of the fiber is calculated Eq. (9): where D is the linear density of the fiber, and d is the fiber diameter. The fibrillation behavior was determined by observing the morphology of the treated Lyocell fibers with an optical microscope after radiated the fiber surface for 30 min by ultrasonic wave (Yuan et al. 2018). The average length of fibril is obtained by processing the image and quantitatively calculated by Eq. (10): where L f is the average fibril length, l i is the fibril length, n is the total number of fibrils.

Results and discussion
Mechanical properties of Lyocell fibers with different draw ratios As we all known, the diameter of the fiber decreases as the draw ratio is increasing during spinning, which helps to improve the mechanical properties of the fiber. It can be seen from Fig. 2a that the diameters of the fibers decrease non-linearly, and the data can be fitted according to the model proposed by Mortimer (1996a), and a theoretical prediction of fiber diameter (d) can be calculated according to the draw ratio (D R ) as follows: where d 0 is the diameter of the spinneret, and the value of a is 0.50. The model is consistent with the data of this paper, and gives the value of d 0 and a is 41 lm and 0.51 respectively, which is basically the same as the value measured by Mortimer. Additionally, it can be seen that the diameter decreases obviously at first and then slowly with draw ratio increasing.
The mechanical data obtained from stress-strain curves (as shown in supporting information (Fig. S1)) of Lyocell fibers with different draw ratios are shown in Fig. 2b-d. It can be seen that the breaking strength of Lyocell fiber gradually increases to a plateau area with draw ratio increasing to 12, while the modulus and yield strength increase firstly and then decrease slightly. Combined with the results of fiber diameter, the decrease of fiber diameter means the improvement of orientation and density, the mechanical properties of fibers should increase, but the actual result is obviously not like this. Normally, the mechanical properties of fibers are directly related to the structure, the difference between breaking strength, modulus and yield strength reflects the diversity of fiber structure, it can be said that there are some uncertain structures different from previous studies, which result in diameter of the fiber decrease with draw ratio increasing, while the breaking strength reach a plateau, the modulus and yield strength increase firstly and then decrease. This paper will make a detailed study of its structural changes in the following. The crystal structural parameters of Lyocell fibers prepared at different draw ratio were calculated by analyzing the 2D WAXD patterns and the result is shown in Fig. 3. It can be seen that Lyocell fiber shows typical cellulose II crystalline structure and the diffraction pattern shows the characteristic reflections of (-110), (110) and (020) crystal plane on the equator and the (002) crystal plane reflection in meridian (Jiang et al. 2012). From Fig. 3, the diffraction arcs become shorter as draw ratio is increasing which illustrates that the orientation degree of crystal region relative to fiber axis get higher with draw ratio increasing. Subsequently, in order to further analyze the change of microstructure, one-dimensional integral curves were obtained by processing the twodimensional WAXD patterns as shown in supporting information (Fig. S2). The information of crystal and amorphous structural parameters as depicted in Fig. 4 were determined by fitting the one-dimensional intensity curves with six crystal peaks with the strongest diffraction and an amorphous peak according to the cellulose II crystalline structure which is shown in supporting information (Fig. S3). The crystallinity and crystal size of the fibers as shown in Fig. 4a and b indicated that the crystallinity of fibers increased with draw ratio increasing at low drawing stage, but remained constant at high and excessive drawing stage. The crystal size of (-110), (110) and (020) crystal plane on the equatorial line does not change significantly, while the crystal size of (002) crystal plane on the meridian direction increases significantly at low drawing stage, which is the principal reason for the rise of crystallinity.
The orientation factor of crystal region (f c ), orientation factor of amorphous region (f a ) and birefringence index (Dn) were also obtained and are shown in Fig. 4c. The orientation of crystal region and amorphous region of Lyocell fibers as well as the birefringence index also increase with the draw ratio increasing. However, the increase of crystal orientation mainly occurs at low drawing stage which is consistent with the change of crystallinity, while the increase of amorphous orientation and birefringence occurs at low and high drawing stage.
Evolution of fibril and micropore structure of Lyocell fiber In addition to the crystalline and orientation structure, the microstructure (such as hollows, micropores and microfibrils) in Lyocell fiber also has a very important impact on the mechanical properties (Moss et al. 2010;Sharma et al. 2021Sharma et al. , 2019. The morphological structure of the cross-sections of fibers was presented in Fig. 5. It can be observed that there are a few hollows with micron size in the fiber at low drawing stage, and the size of hollows gradually decreases at high and excessive drawing ratio, leading to the formation of the dense cross-section. Besides, there are also some nano-scale micropore in Lyocell fibers, which locates between fibrils and fibrils according to the basic fiber model of cellulose fiber proposed by Schurz (Schurz and Lenz 1994). It is difficult to quantitatively analyze the nano-size variation by electron microscopy. Therefore, density method, SAXS technology and fibrillation test were used to study the evolution of micropore of Lyocell fiber during drawing.
The real density and apparent density of Lyocell fiber with draw ratio increasing were calculated and shown in Fig. 6. It can be seen that apparent density is smaller and increases slower compared to the real density with the draw ratio increasing, which indicates the existence of micropores in the fiber. The percentage of micropores volume was obtained by further calculating the apparent density and real density additionally shown in Fig. 6. It can be seen that the change trend of the percentage of micropores volume is contrary to the apparent density.
The micropores of Lyocell fiber at different draw ratio were measured by SAXS as shown in Fig. 7 in order to further investigate the microstructure evolution with the increase of draw ratio. The sharp and long striae on the equator in the SAXS is caused by the scattering of the micropores along the fiber axis. Additionally, the weak and short striae on the meridian in SAXS patterns is due to the orientation of micropore in Lyocell fibers (Sharma et al. 2019). Based on the quantitative analysis of the two-dimensional SAXS scattering pattern, the micropore parameters of Lyocell fibers were calculated, including micropore diameter, micropores length and micropore orientation deviation angle as shown in Fig. 8. The Guinier plot and Ruland plot of Lyocell fibers spun at different draw ratio were shown in supporting information (Figs. S4 and S5). It can be seen that the micropore in Lyocell fibers showed multi-level characteristics. The size of micropore decreased with draw ratio increasing, especially the larger micropore. The micropore length increased gradually while the orientation deviation angle of micropore decreased with draw ratio increasing especially at excessive drawing stage, indicating that the micropore get longer and slenderer during drawing.
The fibrillation of Lyocell fiber samples at different draw ratios was measured by radiated strong sonic wave on the fiber surface for 30 min. The fibrillation and the average fibril length of Lyocell fiber is shown in Fig. 9 and Table 1 respectively. In order to intuitively describe the differences in structure as well as the structural evolution of Lyocell fiber at different draw ratio, an appropriate structural model based on all the above tests was proposed, as shown in Fig. 10. It is generally accepted that regenerated cellulose fibers had multi-level structural characteristics: a succession of crystallites and less ordered cellulose molecules formed the elementary fibrils; elementary fibrils and the micropores between the elementary fibrils formed the microfibrils; microfibrils aggregate to form fibrils which can be observed after fibrillation treatment. At low drawing stage, there are transverse connections of cellulose molecular chains due to low crystallinity and low orientation degree of amorphous region, which increases the interconnections of the fibrils, therefore, there are no obvious fibrils on the fiber surface. With the increase of the drawing ratio at low drawing stage, the increase of crystallinity and degree of orientation lead to the increase of the breaking strength, modulus and yield strength of the fiber. At high drawing stage, a highly crystalline and oriented structure is formed, fibrils are easy to generate on the surface of fibers after fibrillation treatment due to the weak transverse interconnections of oriented cellulose molecules. Meanwhile, the increase of the fibril length is due to the increase of the orientation degree in amorphous region, which leads to the lengthening of the elementary fibrils and microfibrils, as shown in Fig. 10. It can also be proved by the variation of micropores structure from the results of SAXS. In addition, the increase of the breaking strength, modulus and yield strength of the fiber with the drawing ratio at high drawing stage is mainly owing to the increase of amorphous orientation. Moreover, it can be seen from Table 1 that the fibril length also increased with draw ratio especially when the draw ratio was above 15. But both crystallinity and orientation of the fiber remained unchanged with draw ratio increasing at excessive drawing stage while the length of micropores and microfibrils increased. This seems to be a contradictory problem, but the fact is that the elementary fibrils slip due to the weak interconnections between elementary fibrils, which leads to the increase of the length of micropores and microfibrils as well as the fibrils length ultimately. The structure of highly crystallized and oriented elementary fibrils keeps the breaking strength constant, but the slip of elementary fibrils leads to the decrease of modulus and yield strength. Fig. 9 The effect of draw ratios on fibrillation of Lyocell fibers

Conclusions
Summarizing the changes of mechanical properties and structure of Lyocell fiber with different draw ratio, it can be concluded that, at low drawing stage, the increase of the breaking strength, modulus and yield strength are attributed to the rise of crystallinity and orientation, and the rise of crystallinity is mainly due to the increase of crystal region and crystal size perpendicular to the (002) crystal plane; at high drawing stage, the mainly reason for the breaking strength, modulus and yield strength increasing was the improvement of amorphous orientation; and at excessive drawing stage, the highly crystallized and oriented structure kept the breaking strength constant, while the decrease of modulus and yield strength was due to the elementary fibrils slipping caused by weak interaction. Therefore, the above structural defects limited the increase of fiber strength with the increase of draw ratio, in other words, there is a limitation to improve the mechanical properties by increasing the draw ratio in the current production process of Lyocell fiber, this suggests that further improvements must be made in the processing of Lyocell fibers.