Introduction to Materials - Sample Preparation Topic for Transmission Electron Microscopy - page 13

methyl polymethacrylate are used below their glass transition temperature in the vit-
reous state. Their T g values are well above room temperature, on the order of 373 K.
Elastomeric rubbers such as polyisoprene and polyisobutylene are used above their
glass transition temperature, i.e., they are used in the rubbery state and are both soft
and flexible. Flexible plastics such as polyethylene and polypropylene are also used
above their glass transition temperatures. If the temperature is high compared to
the glass transition temperature, the polymer chains can move easily. Below T g ,the
chains are not able to move into new positions in order to reduce the stress. A chain
that can move easily will result in a polymer with a low glass transition temperature,
while a chain that hardly moves will result in a high- T g polymer.
Glass transition is produced in amorphous polymers. Fusion is produced in semi-
crystalline polymers. Fusion is the passage of polymer chains from an ordered
crystalline state to a disordered liquid state, but even semi-crystalline polymers con-
tain an amorphous portion, which generally represents between 30 and 60% of the
polymer's mass. This is why a semi-crystalline polymer material has both a high
glass transition temperature and a high fusion temperature. The amorphous portion
only undergoes glass transition and the crystalline portion only undergoes fusion.
For biological materials, generally only the T g of water is taken into account. At
room temperature (298 K), water is always above T g (138 K) and is therefore in a
soft phase. But a biological organism is in fact a heterogeneous compound; certain
portions can have an oriented texture (proteins in particular) that makes the whole
organism more rigid. Therefore, T g is not the only parameter that needs to be taken
into account. For all composite materials, there will be several T g values and several
components in the overall result of the rigidity or tensile strength.
Conclusion
A material can be both hard and brittle (e.g., some metal alloys, ceramics, or dia-
mond). The hardness of these two cases is different: One is associated with the
significant formation of dislocations that form dense lattices (structural hardening
of alloys) and makes the material brittle; the other is associated with the nature of
the chemical bonds. A material can also be ductile or soft and resistant like cer-
tain alloys (shape memory alloys), polymers, and composites. The characteristics
of these materials will be important to the use of mechanical thinning techniques
on samples. Difficulty will arise when the compound to be studied is composed of
different types of materials with substantially different mechanical properties. This
is the case with mixed-composite materials.
3 Microstructures in Materials Science
3.1 Problems to Be Solved in Materials Science
Regardless of the formation method used (such as classical sintering or sintering
under charge for polycrystalline materials, cathode pulverization, laser ablation for