Physical and Chemical Mechanisms of Preparation Techniques - Sample Preparation Topic for Transmission Electron Microscopy - page 110

polyaryl ethers of glycerol with a terminal epoxy group. They can be polymer-
ized using a wide variety of hardening agents, which are anhydrous aromatics that,
through a chemical reaction, are added to the epoxy groups in order to form a 3D
structure. The macromolecules intertwine and form a solid network. These resins
are said to be cross-linked. The relative proportion of each component (monomers
and hardeners) can produce resins of different hardnesses. The reaction is triggered
by the addition (in a very small quantity) of an accelerator, often from the family of
aliphatic polyamines. Of all the embedding resins, epoxy resins provide the smallest
shrinkage rates (2% maximum). Polymerization is uniform. They are non-polar and
only polymerize in the presence of traces of water. Their adhesion to most materials
is excellent.
They are polymerizable under heat (from 333 to 393 K). Their polymerization
time is relatively long (from a few hours to several days).
Acrylic resins consist of autopolymerization components that are stabilized in
their commercial form. At the moment they are used, a destabilizer is added
(benzoyl peroxide) to trigger polymerization. Acrylic resins are usually polar and
polymerizable under heat or UV rays, at room temperature or to 277 K. Some
(Lowicryl resins) polymerize at low temperatures, from 203 to 273 K under UV
radiation. For these resins, a photoinitiator must also be added at very low temper-
atures. Acrylic resin hardens, is thermoplastic, and resists chemicals. Methacrylate
resins are composed of one or more methacrylates, such as methyl methacrylate,
butyl methacrylate, hydroxyethyl methacrylate (HEMA), a plasticizer, and a desta-
bilizer (benzoyl peroxide). Generally speaking, acrylic resins in the liquid state are
more fluid than epoxy resins. Their ability to polymerize under cold is ideal for infil-
trating or embedding heat-sensitive materials. The most commonly used resins and
their properties are listed in Table 5.2.
6.4 Embedding or Inclusion Principles
The purpose of embedding a material (also called inclusion), usually one with
small dimensions, using a polymer (a resin) is to enable the material to be manip-
ulated and/or to protect its surface from possible damage during the preparation
process. Embedding is used alone, i.e., without infiltration, in materials science
for non-porous materials and in life sciences for samples with very small volumes
(single-layered cells, for example). Regardless of whether they belong to materials
science or life science, porous or hydrated materials must be infiltrated beforehand.
Biological materials are always fixated beforehand, except for organic matrices
treated as minerals (bone, teeth, etc.). In this case, the embedding resin is the same
as that used for infiltration. Generally speaking, the types of embedding resins are
the same as those used for infiltration.
The sample must present an absolutely clean contact surface free of contami-
nants and grease or any other dirt which could prevent the adhesion of the polymer
around the sample. The process should not generate a chemical reaction between
the polymer and the material.