Cellulose Crystallinity In Development Of Cotton Based On XRD And Raman Spectroscopy

As a natural fiber crop,cotton is not only an important textile raw material,but also an essential strategic material in China.In mature cotton,cotton fiber is composed of more than 90%cellulose.The physical,mechanical and chemical properties of cellulose are significantly affected by its crystallinity（the degree of crystallization which is a parameter describing the relative content of crystallized cellulose in cellulose materials）.For example,with the increase of crystallinity,the tensile stress,dimensional stability and density of biomaterials increase,while the chemical reactivity and swelling degree decrease.Up to now,researches on the crystallinity of cellulose mainly focus on wood and various bacterial cellulose materials,while there is few researches with regard to cotton fiber.Therefore,the study on accumulation of cellulose crystallinity in the development stage and after maturity of cotton fiber is great significance not only to cotton research but also to the processing and utilization of cotton fiber.In this study,XRD and Raman spectroscopy were used to study the variation of cellulose crystallinity of cotton fibers during the development stage,and a new evaluation method of cellulose crystallinity of cotton fibers was proposed,which was used to evaluate the cellulose crystallinity of cotton fibers during the development stage.At first,the length,width and length-width ratio of boll of 38 cotton varieties（lines）at different development stages of 10-40 DPA were analyzed.The statistical results show that the change trend of length and width are similar during the growth and development of cotton bolls.The surge of boll mainly occurs at 10-14 DPA,then the growth of 14-20 DPA slows down.After 20 DPA,the size of cotton bolls remained unchanged,while the length-width ratio has no obvious pattern with the development period.Secondly,the 24-40 DPA of cotton fibers of six varieties（lines）were scanned by Raman spectrometer,and the crystallinity of cellulose was evaluated by using four existing Raman crystallinity models（X93-Raman、X380-Raman、XRFS-100和Xc Raman）.It was found that X_93-Raman、X_380-Raman、X_RFS-100 all showed a decreasing trend with the development stage,and the trend was not too obvious,while Xc_Raman showed an increasing trend with the development stage.Therefore,Xc_Raman method is suitable for evaluating the crystallinity of cotton cellulose at the development stage.Then,XRD and Raman spectroscopy were performed on the mature fibers of the other 10 varieties（strains）,and the correlation analysis between the scanning results and the fiber quality detection results was carried out.It was found crystallinity calculated by XRD（XRD-CI）have good correlation with fiber length,fiber strength,fiber uniformity,micronaire value and elongation.Among the four Raman crystallinity models,X_93-Raman method also showed a good correlation with the results of fiber quality detection.Finally,the XRD and Raman spectral of cotton fibers at different developmental stages of three existing cotton lines（Upland cotton 0-153,9708,and Island cotton S-6）were reanalyzed,and established a new evaluation model for cotton cellulose crystallinity by Raman spectra.XRD crystallinity algorithm was used to analyze the crystallinity variation trend of these three cotton fibers at different development stages.It was found that the relative intensity of the shoulder peaks at 2850 and 2920 cm^-1gradually weaken and then disappear in the high wave number range of 2800-3000 cm^-1 of Raman spectra.However,the relative intensity of peak at 2900 cm^-1 becomes stronger,which is similar to the variation trend of the crystallinity of cotton fiber cellulose obtained by the XRD crystallinity algorithm with the development period.The peaks at 2850 and 2920 cm^-1 comes from Wax,while the peak at 2900 cm^-1originate from cellulose.According to this,a new method named Wax crystallinity index（WCI）was used to evaluate the crystallinity of cotton fiber.The peak intensity or peak area ratio of three characteristic peaks(2920,2900 and 2850 cm^-1)within 2800-3000 cm^-1 were used to represent the crystallinity of cotton fiber at the developing stage.The characteristic peaks at 2920,2900 and 2850 cm^-1 were extracted from the complete spectral results by means of peak separation fitting method.Through calculation and correlation analysis,it was found that the correlation determination coefficient R² all above 0.75.Most of the linear fitting regression correlation determination coefficient R² was greater than 0.86,with the highest reaching 0.98.Subsequently,the method was used to evaluate the crystallinity of mature cotton fibers,and the results showed that WCI also performed well in mature cotton fibers.It shows that the new method is not only useful for evaluating the crystallinity of cotton fibers at the developing stage,but also suitable for mature cotton fibers.In summary,the evaluation of cellulose crystallinity of immature and mature cotton fiber by Raman spectroscopy has a high correlation with XRD results.The results of this paper show that the evaluation of cotton fiber crystallinity by Raman spectroscopy combined with XRD technology is not only of guiding significance to the breeding of cotton varieties,but also of great value to the later processing and utilization of mature cotton fiber materials.