X-Ray Diffraction Analysis of Polymers Jia Liu Executive Summary The objective of this experiment is to understand the principle of X-Ray Diffraction (XRD) and its application in characterizing polymer samples. In this experiment we compare the XRD pattern of two nylon 66 samples with different cooling methods so as to discuss the crystalline conditions and parameters. Introduction Have a knowledge of crystal structure is a prerequisite to understand phenomena such as plastic deformation, alloy formation, and phase transformation. XRD method is a powerful tool for investigating orderly arrangements of atoms or molecules through the interaction of electromagnetic radiation to give interference effects with structures comparable in size to the wavelength of the radiation.[1] X-rays are produced in an x-ray tube consisting of two metal electrodes enclosed in a vacuum chamber. Electrons are produced by heating a tungsten filament cathode. The cathode is at a high negative potential, and the electrons are accelerated toward the anode, which is normally at ground potential. The electrons, which have a very high velocity, collide with the water-cooled anode. The loss of energy of the electrons due to the impact with the metal anode is manifested as X-rays. Actually only less than 1% of the electron beam is converted to x-rays; the majority is dissipated as heat in the water-cooled metal anode. The components of an X-ray Diffract meter include the X-ray source (in this experiment, Cu-K, = 1.542 A), the specimen (Nylon66 films by compression molding), the optics and the detector. By considering crystals as reflection gratings for X-rays, Bragg derived his equation: λ=2 d sinθ ,Where λ is the X-ray wavelength, θ is the angle between the X-ray beam and these atomic planes.[2] Besides, Scherrer derived an expression for broadening of X-ray diffraction peaks due only to small crystallite sizes: D=Kλ/(Bcosθ),where λ is the wavelength of the X-ray used, θ is the Bragg angle, B is the average crystallite size measured in a direction perpendicular to the surface of the specimen, and K is a constant.[2] Experiment Procedure The samples prepared are two nylon 66 films endured different cooling process: slowly cooled (3C /min) and quench. The instrument was set Voltage: 30 kV, Current30 mA and Power 0.9 kW with Cu-K radiation in the 2θ angle range of 5∘ to 35∘. The samples are tested one after another following the operation manual. Analysis and Discussion
1
20000 17500 15000
Slowly cooled down
Intensity
12500 10000
Quenched (amorphous)
7500 5000 2500
0
5
10
15
20
25
30
35
40
45
Diffraction angle(2)
Figure1. X-Ray diffraction Pattern of slowly cooled and quenched nylon 66 TABLE.1 Indexing the Diffraction Pattern of nylon66 (slowly cooled) Peak
2θ
θ
θ (radians)
B
B (radians)
d (Å)
D (Å)
d1 d2
20.24 23.72
10.12 11.86
0.1761 0.2064
0.79 1.32
0.0137 0.0230
4.39 3.75
1.76 1.06
% Crystallinity 18.24
Calculations: 1. d-pacing: d=λ/ (2sinθ), for Cu-Kα, λ=1.542Å 2. Crystallinity: %Crystallinity =Ic /(Ic+Ia)* 100 Where Ic =Intensity of Amorphous region=113920.857 Ia =Intensity of Crystalline region=25412.453 3. Crystallite size: D=Kλ/ (Bcosθ) Where K=0.89, λ=1.542Å, B is the Breadth of the peak at half height (in crystal region), θ is Bragg’s angle in radians. For slowly cooled nylon 66, the pattern consists of two peaks which combined together. The peak intensity is plotted on the ordinate and the measured diffraction angle, 2θ, along the abscissa. Each peak, in the diffraction pattern corresponds to X-ray diffracted from a specific set of planes in the specimen, and these peaks are of different heights. The intensity is proportional to the number of X-ray photons of a particular energy that have been counted by the detector for each angle 2θ. The intensities of the reflections depend on several factors, including structure factor, incident intensity, slit width, and values of current and voltage. The position of the peak depends on the crystal structure of the 2
materials as well as the wavelength of the X-rays. The difference between slowly cooled nylon 66 and quenched nylon 66 is by rapidly quenching it from the melt phase, it can be made almost amorphous; then, by annealing the polymer at a temperature above the recrystallization temperature, the crystallinity will recover. [1]As amorphous polymers do not contain any crystalline regions, “crystalline” polymers generally are only semi crystalline, so we can calculate the crystallinity by integrating the total area of the curve (both crystalline and amorphous) and the halo parts (the amorphous) as shown in table 1. Conclusions X-ray diffraction is an effective method to study the polymer crystal properties; it can tell kinds of lattice and show details of crystal structures. According to this experiment, we get to know the D-spacing, crystal size and crystallinity from the diffraction pattern. The longer the cooling time is, the bigger the crystal size is. Both crystallinity and crystal Size follow the same trend: Quench < Slowly cooled
Reference [1] C.Suryanarayana, M. Grant Norton. X-Ray Diffraction – A practical Approach. Plenum Press, 1998 [2] B.D.Cullity, Elements of X-Ray Diffraction. Addison-Wesley Publishing Company, Inc, 1967 [3] L.H.Sperling, Introduction to Physical Polymer Science. John Wiley& Sons, Inc. 2006
3
