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SEM & TEM in Polymer Characterization

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Presentation on theme: "SEM & TEM in Polymer Characterization"— Presentation transcript:

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

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