Physical and Chemical Mechanisms of Preparation Techniques - Sample Preparation Topic for Transmission Electron Microscopy - page 109

For materials in liquid solution, infiltration follows dehydration. The liquid or
gaseous phase of the sample is gradually replaced by a solvent and resin mixture.
Since it is difficult to make a more or less viscous liquid penetrate cavities or porosi-
ties, several mixing baths are carried out with an increasing ratio of resin to solvent.
This has the advantage of decreasing resin viscosity and facilitating penetration.
The proportions of the mixture and the time required between each bath depend on
several factors: the type of resin, its viscosity, the size and density of the material,
and the size of the “cavities” the resin must penetrate. To ensure proper penetration,
the volume of the solvent/resin solution must be at least 10 times greater than the
sample volume.
In the final phase, infiltration under a primary vacuum has the advantage of
quickly eliminating the solvent or the gaseous phase, while promoting penetration
of the resin into the material. This also helps to degas the resin, as air may have been
introduced during preparation of the mixture. Slow lateral or rotary agitation also
improves penetration while reducing the time required between each bath.
The last step in the infiltration procedure corresponds to the embedding of the
sample.
Infiltration (and embedding) polymers are mixtures in the liquid state, more or
less viscous, which hardens through polymerization. Polymerization results from
chemical bonds that form between the monomer and one or more hardeners resulting
in an intertwining (cross-linking) of the molecules forming a solid network. Once
polymerization occurs, the polymer is of a variable hardness, depending on the type
of polymer or the percentage of the compound mixture.
The infiltration resin is selected based on several parameters essential to the
preparation and observation of samples in the TEM:
- There must not be any shrinkage (reduction of volume) or expansion of the resin
following polymerization; this would result in either a resin/sample separation
during preparation or a change in the morphology of the material.
- The hardness of the resin must be as close as possible to the hardness of the
material, so that the polymer's response to the different types of preparation
mechanisms is identical or similar to the material.
- The polymerized resin must be stable under the electron beam.
- It must not present texture under observation.
- In the case of infiltration, the resin mixture must, in the liquid state, be of low
viscosity in order to properly penetrate the samples.
There are many infiltration resins available, but here we will concentrate only
on those most commonly used for preparation in electron microscopy: epoxy and
acrylic resins.
Epoxy resins are generally composed of one or more monomers, one or more
hardeners, a reaction accelerator, and sometimes a plasticizer or a flexibilizer that
acts as a hardener or a plasticizer. Epoxies are generally chemical substances con-
taining oxygen bridged onto a carbon-carbon bond. They are also referred to as
oxacyclopropanes (systematic nomenclature) or also to oxiranes. Epoxy resins are