1.
Aim
To accurately determine the d-spacing, crystallite size, unit cell dimension, and
degree of crystallinity of well-known polymers (i.e., nylon 6,6, LDPE, and HDPE) using
wangle angle X-ray diffraction (WAXD).
2. Introduction to Theory
X-rays are electromagnetic waves which, when incident on a material, interact
with electrons in the material and are scattered. X-ray waves scatter from different
electrons and interfere with each other. This interference gives the resulting diffraction
pattern, the positions of diffraction peaks and their relative heights, in which the
intensities vary with scattering angle. Upon analysis of the diffraction pattern,
information on the arrangement of atoms in a given lattice can be obtained: the shape and
size of the unit cell is obtained directly from diffraction peak positions while the positions
of atoms within the unit cell are related to the relative heights of the diffraction peaks. X-
rays are useful in determining lattice structures because their wavelengths are on the
order of 1 angstrom, the same order of magnitude as the interatomic distances in
condensed matter.
X-rays scattered from the periodic repeating electron density of a perfectly
crystalline material give sharp diffraction peaks at angles that satisfy the Bragg relation,
whether the crystal consists of atoms, ions, small molecules, or large molecules.
Amorphous materials will also diffract X-rays and electron, but the diffraction is a much
more diffuse, low frequency halo (the so called amorphous halo). Analysis of the
diffraction peaks from amorphous materials leads to information about the statistical
arrangement of atoms in the neighborhood of another atom.
In polymers, which are never perfectly crystalline, a superposition of both diffuse
and sharp scattering occurs. First, the crystals present in polymers are very small in size.
This leads to considerable broadening of the peaks as compared with fully crystalline
1
materials. Second, there exists some fraction of noncrystalline region in even the most
highly crystalline polymer. This results in a background of a broad, amorphous halo.1,2,3
2.1 Braggs law
When X-rays are scattered from a
crystal lattice, observed peaks of
scattered intensity, which correspond to
the angle of incidence, should be equal to
angle of scattering while the path length
difference is equal to an integer number
Figure 7.1. Schematic diagram for
of wavelengths (see figure 7.1). Bragg determining Braggs lawError:
derived Braggs law for the distance d Reference source not found
between consecutive identical planes of
atoms in the crystal:
n = 2d sin (7.1)
Where is the x-ray wavelength,
is the angle between the x-ray beam and
these atomic planes and n corresponds to
the order of diffraction. The condition for
maximum intensity contained in Bragg's
law above allow us to calculate details
about the crystal structure, or if the crystal
structure is known, to determine the
wavelength of the x-rays incident upon the
crystal.Error: Reference source not found,4 Figure 7.2 : An example of partically
crystalline polymer x-ray spectrum5
2.2 Percent Crystallinity in polymer
1
Sperling, L. H., An Introduction to Physical Polymer Science, John Wiley and Sons, Inc., 2001, 3rd Ed.
2
Bradly, R. F., Jr., Comprehensive Desk Reference of Polymer Characterization and
Analysis, American Chemical Society, Washington, D.C., 2003
3
Surman, D., Percentage Crystallinity Determination by X-Ray Diffraction,
http://www.kratos.com/XRD/Apps/pcent.html
4
Braggs Law, http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/bragg.html
2
A general polymer x-ray spectrum will have a broad amorphous peak, and if the
polymer has crystallinity, it will show up as sharp peaks on the top of large amorphous
peak (figure 7.2). The spectrum is the sum of crystalline peaks and an amorphous peak.
The true area of the crystalline peaks and the amorphous peak can be determined from a
computer software package performing a mathematical deconvolution of the peaks.5 The
percentage of the polymer that is crystalline can be determined from equation (7.2).
Area under crystalline peaks
%Crystallinity 100% (7.2)
Total Area under all peaks
The amount of crystallinity in a polymer depends on the following:
The secondary valence bonds (hydrogen bonding and Vander Waals forces)
which can be formed
The structure of the polymer chain (degree of order)
The physical treatment of the polymer (e.g. tensile pull)
The thermal history of the polymer (e.g., above Tm, it becomes amorphous while
if it is cooled slowly it will crystallize)
The molecular weight of the polymer. 5
2.3 Crystallite size in polymer
It is important to realize that crystallite size can be obtained via a simple approach
from peaks full width at half maximum (FWHM) measurement by using the Debye-
Scherrer equation,
L (7.3) 6
cos
in equation (7.3) is FWHM (in radians), is the x-ray wavelength, and is the
peak position in degree.Error: Reference source not found
3. Experimental Procedure
3.1 Initial Hardware Set-up and Sample Insertion
5
Alexander, X-ray Diffraction Methods in Polymer Science, Wiley, Interscience, New
York, 1969.
6
Cullity, B. D., Elements of X-ray Diffraction. Prentice Hall, 2001, 3rd Ed.
3
1. Sign in at the log book: Lab name, date and time of use.
2. Make sure that the Peltier detector power is on (Power Switch lit). If it is not turn
it on and wait 45 minutes before operation.
3. Press Control Power On on the front of the Scintag. The switch will be lit when
the machine is on.
4. Open door and insert the sample into the sample holder by pressing down on both
sides of the metal plates as shown with arrows in figure 7.3 below. Make sure that
the sample is centered in the holder.
5. Write down the angular positions of both the detector and the source arms using
the scale and the OM and TH micrometers above the arms (see Figure 7.3).
D E
Figure 7.3: Detector Configuration. A: Source. B: Detector. C: Sample holder. D: TH
micrometer. E: OM micrometer.7
6. Close the door and press the red Interlock button; the button should not be lit
when the interlock is reset. This button is above the Control Power On button.
The Interlock must be reset each time the door is opened.
7. Press and hold the green X-ray Off button for at least five seconds. Check the
current and voltage of the system. The current should register as 2 A and the
voltage should read 10 V.
7
Percharsky, V. K., MSE 535 Class Notes, ISU, Ames, IA, Fall 2003
4
8. Press down the X-ray On button. The button will be lit when it is on. The x-ray
shutter will remain closed although the generator will be powered up to produce
x-rays.
3.2 Computer Set-up
1. On the desktop, click on the icon for the x-ray software. A small dialog box will
open up. Use your login and password. Under Account Number, place
MatE 453 and for advising professor, place Dr. Michael Kessler.
2. Choose OK and the software will open up. This will include a dialog box as
seen in Figure 7.5 that needs filled for running the XRD. The window will consist
of two parts, a column on the left and an event data part on the right.
Figure 7.5: EventList.evt Window. 8
3. Under General, there is a command line entitled Raw File Name. This
command line shows you the directory you are in. You can now name your data
file. Use the Browse button open a window called Set Raw File Path. The name
of the last data file used will appear highlighted in the File Name box. Type a
name for your file and it will replace the highlighted text. For the filename, use
the abbreviation of the polymer and you initials (e.g., if you are using polystyrene
and your name is Joe Smith your filename should be PSJS.) The file will
automatically be given a .raw suffix. Make sure the file is saved under
C:\XRAYDATA\MatE_453
8
Scintag User Manual
5
4. Hit OK.
5. Type in the ID box to give the file a short description that will appear on each
printout. Be as specific and brief as possible with this ID.
6. Type in the Comments box for a longer, more detailed description that is saved
along with the file. The program will not let you print these comments out!
7. If you have changed anything you can click on Save Event now, or you can wait
until you are finished all setup pages.
8. Select Slit. See Figure 7.5 on next page. Verify that the slits dimensions have not
2
been changed and press the Save Event button if it is lit. The numbers should be 2
for divergence Slit Width and 4 for Scattered Slit Width.
2 4
Figure 7.5: Slits Window. 8
9. Select Scan. This page is show in Figure 7.6. This page is used to set the step size,
start and stop angles and the scan rate. The scan settings should be as follows:
0.02 for step size; Normal for Type; 5 for Start Angle; 35 for Stop Angel; 2 in
Time/Step; and 2.00/min in rate. If the Set button on the lower right side is
highlighted you MUST press it. Press Save Event when done modifying this
page.
10. Press Go From Top. The
software will ask if you want to
initialize hardware. Press OK.
You will be shown a page that
has the current positions of both
6
arms of the X-ray machine. If these numbers correspond to what those from Step
6 of Initial Hardware Setup, hit OK. If they
Figure 7.6. Normal Window. 8
are not the same as those that are given, you will have to change them. Enter the
correct values into the Set Axis Position box for both detector and tube. Hit Set
after entering each true value.
11. Verify that X-ray On button is lit and hit OK. This will initiate the scan. The
detector and tube will move to the appropriate positions, the shutters will open,
and the scan will begin.
12. A Small window called Scan Status filename.raw will appear noting the
progress. Click on the Realtime Display button to watch the progress of the scan.
WARNING: DO NOT PRESS HOLD OR ABORT AFTER STARTING THE
SCAN! THE X-RAY MACHINE WILL STILL BE TAKING DATA EVEN
THOUGH THE SOFTWARE HAS STOPPED COLLECTING IT! DO NOT
USE THE DATA FITTING OPTIONS WHILE SCANNING.
13. When the scan is done you will get the message, Event list execution complete.
Hit OK.
3.3 Analyzing and Printing Data
1. Open the file you just collected by choosing Open from the File menu and
select your saved data file. An opened scan may appear similar to the one if
Figure 7.7 below. You can zoom on any particular area of the scan by using the
right mouse button. Using the Print option from the File menu, print out your
diffraction pattern. Choose landscape view when printing all plots.
A B C
Figure 7.7: A: Background Curve Fitting. B: Peak Curve Fitting. C: Insert a New Peak.8
1. Press the Background Curve Fitting button shown in Figure 7.7. This will bring
up the window shown in Figure 7.8 below. Select the Manual Spline Curve Fitting
Button and Click on Edit Points.
7
Figure 7.8: Example scan of PS. 8 Figure 7.9: Background window. 8
2. A line will be automatically drawn for you. If this line does not fit the data, (see
the black line in Figure 7.8) click on the end that does not fit the data and move
the line until it fits the end points of the graph. Click on Calculate (Figure 7.9)
when done.
3. Press on the Insert Peaks button on the menu (see Figure 7.7 above) and draw a
triangle to represent each peak, starting from the bottom left to bottom right and
then pull the point up until it fits your peak. See Figure 7.10 below for a sample
triangle. Repeat this step until all peaks that are three times greater than the
background have been fit. You will also need to fit an amorphous peak for your
data. When fitting this peak make sure that its point does not correspond to that of
any other peak in the plot.
4. Now press the Curve
Fitting button (see
Figure 7.7). A
window called
Profile Fitting will
come up. See Figure
7.11
Figure 7.10: Sample Triangle8
8
5. Choose the Lorenzian option from the Type of Curve dropdown box (see
Figure 7.11). Press Calibrate twice. Use the Print option on the page to printout
the peak information in the box.
6. Now using the Print option from the File menu, print the plot with the fitted
curves on it. If you close the Profile Fitting window before you print, your
peaks will be lost.
7. Export the data to a file. Choose Export from the File menu and a window like
that seen in Figure 7.12 will appear. Under the Raw tab, choose CPS under the
Units heading. Under the Format heading choose text. Choose Export to file
from the Destination option.9
Figure 7.11
Figure 7.12
9
Scintag Applications DMSNT Software
9
7. Assignments
7.1. Calculate the d-spacing, crystallite size, unit cell dimension, and degree of
crystallinity of nylon 6,6, LDPE, and HDPE.
WAXD radiation: Cu K
Wavelength: 1.540562
Scanning range: 5 -35 degrees
Note: c for PE is 2.55 angstroms for both HDPE and LDPE
7.1.1 Peak Positions
Nylon 6,6
Peak # Amp.(counts Pos.( 2) FWHM Area
per second
CPS)
LDPE
Peak # Amp.(CPS) Pos. ( 2) FWHM Area
HDPE
Peak # Amp.(CPS) Pos. ( 2) FWHM Area
10
4.1.2 Interplanar spacing (i.e., d-spacing)
Braggs Equation: = 2dsin ()
Nylon 6,6
() Pos. ( 2) Sin() d ()
LDPE
() Pos. ( 2) Sin() d ()
HDPE
() Pos. ( 2) Sin() d ()
7.1.3 Unit Cell Dimensions
7.1.3a Triclinic crystal: nylon 6,6
7.1.3b Orthorhombic crystal: LDPE
7.1.3c Orthorhombic crystal: HDPE
11
7.1.4 Crystallite Size
Scherrers Formula: L = /(*cos)
Nylon 6,6
Pos. ( 2) cos() L
(i.e., FWHM)
LDPE
Pos. ( 2) cos() L
HDPE
Pos. ( 2) cos() L
7.1.5 Degree of Crystallinity
Xc = Crystalline Area / Total Area
Crystalline Area Amorphous Area Xc
Note: using the software Origin or other software to deconvolute the major diffraction
peaks
7.2 Based on the experimental results, discuss the differences between LDPE and HDPE
and Nylon 6,6.
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