The material we chose is cigarette filter, whic is made of cellulose diacetate fiber. This material is a flexible and porous polymer. From chemical coposition to surface texture, this material is characterised by these methods below.

1. The chemical composition of this material is characterised by FTIR.
2. The crystallinity of the CDA fiber is characterised by X-Ray diffraction.
3. The size of the fiber and the morphology of the material is characterised by SEM.  

• What FTIR can tell

FTIR is an effective way to detect chemical composition and functional groups of CA.

• Principle

FTIR uses infrared radiation to excite molecules and generate stretching or bending variation of molecules. Different functional groups in molecules can only absorb IR with specific wavenumbers, which can be used to characterize the appearance of these functional groups. Each functional group has its own characteristic frequencies, which can be used to analyze the spectra.

• Information, Precision and Accuracy of FTIR

FTIR can get qualitative information about the functional groups in the molecules based on the position of the peak in spectra. Quantitative information (i.e. the concentration of functional groups) can also be delivered by FTIR. Peak height (or peak area), sample thickness and molar absorption coefficient can be used to calculate the concentration of functional groups.

FTIR can obtain spectra with high precision (less than 0.1%). Its resolution can be determined to 1cm^-1 and the spectral detection range is large (4000cm^-1 to 400cm^-1 wavenumber in figure 2). However, FTIR may get inaccurate qualitative result because peak position is hard to be determined sometimes and easy to be influenced by the intrinsic structure of the sample or the external environment that the sample stays in^[2]. Besides, the peak with low intensity may not be observed (it may be covered by the peak with high intensity), which leads that some functional groups may not be observed. In addition, molar absorption coefficient is also easy to be influenced by external environment, which also leads to inaccurate quantitative result. To improve the accuracy of this method, internal and external factors should be considered carefully.

• Example Data and Analysis

                                        Figure 1. Chemical composition of CA                 Figure 2. FTIR spectra of CA (synthesized by different material) and CP^[1]

Table 1. Functional groups and related wavenumbers^[1]

Figure 2 is the FTIR spectra of CA synthesized by different raw material and CP (a kind of raw material used to synthesize CA). The functional groups that the peaks represent are shown in table 1. Some key peaks show clearly in the figure, such as the peaks whose wavenumbers equal to 1740, 1165 cm^-1, meaning the C=O groups of saturated carboxylic esters and C-C-O of saturated carboxylic esters. The analysis results in Table 1 fits the composition of CA (Figure 1) well, which proves that the chemical composition of the sample is quite similar to CA. However, the position of the characteristic peak of hydroxyl groups is hard to be determined, which may lead to inaccurate result.

The intensity of characteristic peak of -OH of CA is much lower than that of CP, which means that most of them are substituted by -OCOCH₃. It also can be regarded as an evident to prove the sample has high purity.

• What X-Ray diffraction can tell

X-ray powder diffraction (XRD) is mainly used for phase identification of crystalline materials, crystal structure, preferred orientation, specific phases, and other properties such as average grain size, percent crystallinity and phase quantification ^[5].

• Principle

In XRD, X-rays are generated in a sealed tube under vacuum. Electric current is applied to heat the filament in the tube, thereby emitting electrons. A high voltage is applied inside the tube to accelerate the electrons, and then the electrons hit the target and generate X-rays ^[3]. The detector detects the X-ray signal and then electronically processes it to convert the signal to a count rate. When the X-ray beam hits the sample and diffracts, we can measure the distance between the atomic planes that make up the sample by applying Bragg's law nλ = 2 d sinθ ^[3]. The conversion of diffraction peaks to the distance between adjacent planes of atoms d-spacing can identify substances, because each substance has a unique set of d-spacings. Usually, this is achieved by comparing the d-spacing with the standard reference pattern.

• Discussion about XRD

XRD certainly is a good method to measure the crystallinity property of CDA material used as filters in the cigarette industry because of the significant differences in X-Ray diffractogram for different kinds of CA materials shown in figure 3 curve a) wood pulp; b) CTA; c) hydrolyzing CDA; d) CDA fibre ^[4]. 

XRD is very precise. The detection limit of XRD method for multiphase analysis is around 0.5% to 1%. The sampling depth is between ~20Å to ~30µm ^[5]. The uncertainty of measuring size of unit cell can be reduced to less than 0.1%. But XRD may be inaccurate because CDA is not a crystalline material. When use XRD testing CDA, the crystalline diffraction peaks can be partially superimposed due to the diffusion and both the edges of crystallized and non-crystallized peaks can coincide completely or mostly, the peak-fit process can be very hard and produces large errors ^[6].

This method could identify and analyze the crystallinity from both quantitative and qualitative prospects. XRD can qualitatively identify the property of crystallinity for CA by analyzing the different peaks in X-Ray diffractogram and quantitatively calculate the degree of crystallinity and grain size by the intensity of diffraction and the angle of incidence of the X-Ray beam. Here an example of data is listed as follows.

• Example Data and Analysis

 The curve d in figure 3 is the X-Ray Diffraction curve for CDA fibre ^[4]. It can be seen that there are only two peaks in this curve around 10° and 18°. This shows that the CDA is not a crystalline material, the amorphous structure predominates. By applying peak-fit processing to this curve, the related lattice parameters of CDA fibre can be got (here the detailed data is missing due to the authors did not list them in the easy for curve d but only for b) ^[4]. The amount of degree of crystallinity can be obtained and cell size in crystallinity area can be calculated out, which are shown in table 2. These data can verify that the crystallinity of this material is poor.

From figure3 curve b and d comparison, it can be seen that the degree of crystallinity of CDA fibre are lower than the other CA materials. Cellulose acetate samples of x - ray diffraction analysis in D/Max - 2550 PC 18 kw designed target determination on X-ray diffractometer. Test condition: Ni filter, Cu target Kα Ray, 40 kV tube voltage, tube flow 40 mA, scanning speed is 2degree per min, and 2θ Angle scanning range is from 5 to 50.

Figure 3. X-Ray diffractogram of CA materials ^[4]

 Table 2. Crystalline properties of CDA fibre ^[4]

• What SEM can tell

SEM tells the microscopic-scale information on the size, shape, composition, crystallography and orientation of the fiber in Cellulose acetate (CA) ^[7].

• Principle of SEM

A finely focused beam of energetic electrons is emitted. The electron beam is modified by the components of the SEM and the diameter of the beam is reduced. The beam sequentially scans the sample but in discrete locations. Two outgoing electron products, which are backscattered electrons and secondary electrons, are produced when the beam interacts with the sample. These outgoing electron signals are measured using one or more electron detectors and the images are produced ^[7].

• SEM Image Imformation and Resolution

In the image, the morphology of the material is presented directly. The size, shape, texture and crystallinity of the material can be seen. To characterize the morphology of CA fiber, SEM is an appropriate method. The configuration of CA fiber in a microscopic scale, the size of the cavity and fiber can be known. Both quantitative and qualitative information can be known.

SEM has a very high resolution. The resolution can be adjusted by controlling the energy and diameter of the electron beam. The scanning electron microscope, JSM-6390, is used in the test of CA fiber in the paper, which has a resolution of 3.0 nm ^[8][9]. The diameter of the CA fiber is around 1-4μm. (show in Figure 5.) The resolution is only 0.03% -  0.075% of the diameter. The measurement can be really precise and accurate.

• Example of SEM Image for CA fiber and Analysis

 Figure 4. Overall view of fiber (a) and fiber morphology (b). The scales are 200 μ m and 2 μm respectively.

From figure 4(a), it can be seen that CA fibers pack randomly and there are cavities between fibers. Fibers entangle with each other, forming a loosely packed network. This form of structure gives CA material the property of being flexible and porous. This material has very high surface/volume ratio, making it a good choice for filter. (Figure 4. (a)) In a more detailed view in figure 4b, several fibers are aligned together, forming a strand. The surface of the fiber in its longitudinal direction is smooth and relatively uniform. There are not many obvious grooves. (Figure 4. (a))  The diameter of the fiber is around 1 - 4μm.

• FTIR provides clear information about the ratio of -OH and -OCOCH₃ in the micro structure of CA. It shows that more -OCOCH₃ than -OH, meaning weak intermolecular interaction and poor aggregation behaviour^[9]. This makes CA chains flexible and soft. 
• XRD shows the low crystallinity of the material. Low crystallinity means lower Tg and higher flexibility of the molecules. Besides, low crystallinity leads to low aggregation and higher porosity. 
• SEM shows that the fibers entangle loosely, leaving large empty space. This gives CA material high surface area (good adsorption property) and high flexibility.

The high flexibility and adsoption properties of CA make it a good choice for filters, especially in cigarette industry. It can also be used in other applications that require the filter of smokes.

[1] Jinghuan Chen, Jikun Xu, Kun Wang, Xuefei Cao, Runcang Sun, Cellulose acetate fibers prepared from different raw materials withrapid synthesis method, Carbohydrate Polymers, 137 (2016) 685–692.

[2] Rui Yang, Analytical Method for Polymer Characterization, CRC Press, 2017.

[3] USGS. X-Ray Powder Diffraction. May 1997. Retrieved from https://pubs.usgs.gov/info/diffraction/xrd.pdf. [Accessed on 2020/6/4]

[4] He Jianxin, Tang Yuyuan, Cui Shizhong, & Wang Shanyuan. (2013/5/19) Crystalline structure and thermal property of cellulose acetate. TS 182 A. Retrieved from https://wenku.baidu.com/view/6a45f90052ea551810a6876f.html [Accessed on 2020/6/4]

[5] “X-Ray Powder Diffraction (XRD) Services Laboratory.” H & M Analytical Services, 5 Oct. 2017, Retrieved from https://h-and-m-analytical.com/wp/xrd/ [Accessed on 2020/6/9].

[6] Gu Hubo. (2018/6/30). A brief description of the principles and implementation methods of WAXD, DSC, IR and density testing methods for the determination of polymer crystallinity. Retrieved from https://wenku.baidu.com/view/fdea37c2aa00b52acfc7ca2e.html [Accessed on 2020/6/4]

[6] Joseph I. Goldstein, et al (2017) Scanning Electron microscopy and X-ray Microanalysis (fourth edition), Springer, p. 7

[7] JEOL - Model JSM-6390 - Scanning Electron Microscope, Jeol USA Inc, Retrieved from  https://www.environmental-expert.com/products/jeol-model-jsm-6390-scanning-electron-microscope-426298 [Accessed on 2020.6.4]

[8]Lee A. Goetz, et.al (2016) Superhydrophilic anti-fouling electrospun cellulose acetate membranes coated with chitin nanocrystals for water filtration,

[9] Mei Jie, Ou Yifang, Chen Jianan, High Temperature Synthesis and Properties of Cellulose Acetate[J], Journal of Cellulose Science and Technology, 2000(01):9-16.