1. SEM & TEM in Polymer Characterization

2. Outline Scanning Electron Microscopy (SEM)
Uses
Sample Preparation
Instrument
Principles
Micrographs
Transmission Electron Microscopy (TEM)

3. What can you see with an SEM?
Topography
Texture/surface of a sample
Morphology
Size, shape, order of particles
Composition
Elemental composition of sample
Crystalline Structure
Arrangement present within sample

4. SEM Sample Preparation
Samples must be small enough to fit in sample chamber
Most modern microscopes can safely accommodate samples up to 15cm in height
Samples must be electrically conductive
Polymer samples typically need to be sputter coated to make sample conductive
Ultra-thin metal coating
Usually gold or gold/palladium alloy
Coating helps to improve image resolution

5. Scanning Electron Microscopy
Once sample is properly prepared, it is placed inside the sample chamber
Once chamber is under vacuum, a high voltage is placed across a tungsten filament to generate a beam of high energy electrons (electron gun) and serves as the cathode
The position of the anode allows for the generated electrons to accelerate downward towards the sample
Condensing lenses “condense” the electrons into a beam and objective lenses focus the beam to a fine point on the sample

6. Scanning Electron Microscopy

7. Scanning Electron Microscopy
Scanning coils move the focused beam across the sample in a raster scan pattern
Same principle used in televisions
Scan speed is controllable

8. Scanning Electron Microscopy
As electron beam strikes sample, secondary electrons are emitted from the sample
In addition, backscattered electrons are also emitted from the sample

9. SEM Sample Interactions

10. SEM Signals / Detectors
Secondary Electrons
Back-Scattered Electrons

11. Secondary Electron Detector
Electrons strike the sample surface
As a result, some electrons “splash” out from the sample (secondary electrons)
A detector with a strong positive charge attracts these electrons, however depending on the surface topography, not all electrons will be attracted
Electrons on high “peaks” will be attracted to the positively charged detector
Electrons in low “valleys” will not be attracted to the detector

12. SEM Micrographs
Crystalline Latex Particles
Polymer Hydrogel Surface

13. SEM Micrographs
SEM Images of PVEA Comb-like Terpolymers

14. Back-Scattered Electron Detector
Electrons from high-energy beam strike the sample
Some electrons pass close to a nucleus and are deflected by the positive charge
These back-scattered electrons return to the sample surface moving at high speed
Back-scattered electrons is dependent on atomic number of sample
Can provide elemental composition information about a sample

15. Back-Scattered Electron Detector
BSE Micrograph Showing Crystalline Lamellae

16. What can we see with a TEM?
Morphology
Shape, size, order of particles in sample
Crystalline Structure
Arrangement of atoms in the sample
Imperfections in crystalline structure (defects)
Composition
Elemental composition of the sample

17. TEM Sample Preparation
Samples need to be extremely thin to be electron transparent so electron beam can penetrate
Ultramicrotomy is a method used for slicing samples
Slices need to be nm thick for effective TEM analysis with good resolution

18. Transmission Electron Microscopy
Instrument setup is similar to SEM
Instead of employing a raster scan across the sample surface, the electron beam is “transmitted” through the sample
Material density determines darkening of micrograph
Darker areas on micrograph indicate a denser packing of atoms which correlates to less electrons reaching the fluorescent screen
Electrons which penetrate the sample are collected on a screen/detector and converted into an image

19. Transmission Electron Microscopy

20. Isotactic Polypropylene Particle
TEM Micrographs
Isotactic Polypropylene Particle

21. TEM Micrographs
Large
Ceria Nanoparticles
Small
Ceria Nanoparticles

22. Microcrystalline Cellulose
TEM Micrographs
Microcrystalline Cellulose

23. SEM Pros and Cons Cons Pros
Maximum magnification is lower than TEM (500,000x)
Maximum image resolution is lower than TEM (0.5nm)
Sputter coating process may alter sample surface
Pros
Easier sample preparation
Ability to image larger samples
Ability to view a larger sample area

24. TEM Pros and Cons Cons Pros Sample preparation
Sample structure may be altered during preparation process
Field of view is very narrow and may not be representative of the entire sample as a whole
Pros
Higher magnifications are possible (50,000,000x)
Resolution is higher (below 0.5Å)
Possible to image individual atoms

25. The End