4.15.
CROSS-LINKED METHACRYLATE EMBEDDING
121
stable during electron bombardment so that the specimen is protected
from collapse. For this reason, also, naked sections without membrane
support can be examined. In our own experience, cross-linked methacrylate blocks are easier to section than any other embedment excepting
only ordinary methacrylate. This implies that, all other things being
equal, larger sections can be obtained from such blocks than when other
embedments are used. Another virtue of cross-linked methacrylate is an
exceptionally easy "stainability" with heavy metals (Fig. 28). In this
respect also it behaves as ordinary methacrylate.
Considering the benefits of these excellent characteristics, the reader
will wonder why this embedment is not more popular than it has been
so far. It must be admitted that there remain some difficulties in polymerization which perhaps have discouraged people who have given it
a cursory try. There still is not enough information on the quality of
fine-order preservation to justify full confidence that this is at least the
equal of Araldite or Vestopal. But clearly it deserves a full test.
It is extremely simple to embed in a mixture which will produce a
cross-linked methacrylate. Kushida (1961b) demonstrated that all one
need do is add 5% of the cross-linking agent divinyl benzene 55 (Dow
Chemical Co., Midland Div., Midland, Michigan) to an ordinary methacrylate mixture. The resulting plastic will be somewhat harder than it
otherwise would have been, but not excessively so. In practice, a mixture
with pure butyl methacrylate may prove sufficiently hard to section well,
or no more than 10 or 20% methyl methacrylate need be added to harden
it effectively.
Kushida (1961b) recommended using 1% benzoyl peroxide as the
catalyst. In our own experience the amount of catalyst is quite critical.
The difficulty in polymerizing cross-linked methacrylate seems to be that
severe strains are set up in the plastic during its gelation, which result
in much cracking and checking of the plastic mass. This does not particularly involve the specimen, so that unless a crack passes through it by
chance, even severely damaged blocks can be trimmed and used without
worry. The only way to reduce these defects in the block seems to be to
limit the rate of polymerization by using a minimum amount of catalyst.
In a graded series of mixtures when varying amounts of catalyst were used,
it has been our experience that about 0.8% has been best. Anything less
than that concentration proved inadequate, anything more promoted
defects. However, since divinyl benzene as supplied by the manufacturers
is far from a pure and standardized product, it may prove necessary to
CROSS-LINKED METHACRYLATE EMBEDDING
121
stable during electron bombardment so that the specimen is protected
from collapse. For this reason, also, naked sections without membrane
support can be examined. In our own experience, cross-linked methacrylate blocks are easier to section than any other embedment excepting
only ordinary methacrylate. This implies that, all other things being
equal, larger sections can be obtained from such blocks than when other
embedments are used. Another virtue of cross-linked methacrylate is an
exceptionally easy "stainability" with heavy metals (Fig. 28). In this
respect also it behaves as ordinary methacrylate.
Considering the benefits of these excellent characteristics, the reader
will wonder why this embedment is not more popular than it has been
so far. It must be admitted that there remain some difficulties in polymerization which perhaps have discouraged people who have given it
a cursory try. There still is not enough information on the quality of
fine-order preservation to justify full confidence that this is at least the
equal of Araldite or Vestopal. But clearly it deserves a full test.
It is extremely simple to embed in a mixture which will produce a
cross-linked methacrylate. Kushida (1961b) demonstrated that all one
need do is add 5% of the cross-linking agent divinyl benzene 55 (Dow
Chemical Co., Midland Div., Midland, Michigan) to an ordinary methacrylate mixture. The resulting plastic will be somewhat harder than it
otherwise would have been, but not excessively so. In practice, a mixture
with pure butyl methacrylate may prove sufficiently hard to section well,
or no more than 10 or 20% methyl methacrylate need be added to harden
it effectively.
Kushida (1961b) recommended using 1% benzoyl peroxide as the
catalyst. In our own experience the amount of catalyst is quite critical.
The difficulty in polymerizing cross-linked methacrylate seems to be that
severe strains are set up in the plastic during its gelation, which result
in much cracking and checking of the plastic mass. This does not particularly involve the specimen, so that unless a crack passes through it by
chance, even severely damaged blocks can be trimmed and used without
worry. The only way to reduce these defects in the block seems to be to
limit the rate of polymerization by using a minimum amount of catalyst.
In a graded series of mixtures when varying amounts of catalyst were used,
it has been our experience that about 0.8% has been best. Anything less
than that concentration proved inadequate, anything more promoted
defects. However, since divinyl benzene as supplied by the manufacturers
is far from a pure and standardized product, it may prove necessary to