SEM & TEM in Polymer Characterization

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

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

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

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

Scanning Electron Microscopy

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

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

SEM Sample Interactions

SEM Signals / Detectors Secondary Electrons Back-Scattered Electrons

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

SEM Micrographs Crystalline Latex Particles Polymer Hydrogel Surface

SEM Micrographs SEM Images of PVEA Comb-like Terpolymers

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

Back-Scattered Electron Detector BSE Micrograph Showing Crystalline Lamellae

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

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 50-100nm thick for effective TEM analysis with good resolution

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

Transmission Electron Microscopy

Isotactic Polypropylene Particle TEM Micrographs Isotactic Polypropylene Particle

TEM Micrographs Large Ceria Nanoparticles Small Ceria Nanoparticles

Microcrystalline Cellulose TEM Micrographs Microcrystalline Cellulose

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

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

The End http://www.mrs.org/s_mrs/doc.asp?CID=1803&DID=171434