4.9.
LIMITATIONS OF METHACRYLATE
105
thors believe it to be contra-indicated with solid tissues unless block sizes
are kept very small, for they have had difficulty achieving uniform cutting
qualities in the blocks.
Ward (1959) advocates the use of small quantities of uranyl nitrate
added to the methacrylate monomer to prevent polymerization damage.
He suggests using 0.75-0.37%. The effect is not entirely specific to the
uranyl ion for there is some evidence that other heavy metals behave
similarly. Unfortunately the cutting qualities of the blocks are altered by
these additions. The polymerized plastic becomes harder than one would
anticipate, and it is sometimes necessary to add a plasticizer. Dibutyl
phthalate can serve in this respect, as much as 10% being added in some
cases.
The reader is referred to Gettner and Ornstein (1956) for a further
account of polymerization damage and of efforts made to minimize it,
and to Moore and Grimley (1957) for a theoretical account of polymerization which bears on this problem.
4.9. Limitations of Methacrylate
In the introduction of this chapter allusion was made to an artifact
associated with methacrylate sections which is apt to destroy detail of
fine order. One of the principal virtues of methacrylate is that it decomposes under electron bombardment so that much of its substance disappears leaving a background of relatively low electron density (Fig. 9). Unfortunately, as this occurs the methacrylate apparently melts, and surface
tension forces come into play. Two membranes that are close together
may fall upon each other so that subsequently they are observed as a
single line. A cylinder may collapse. Two tiny granules that are close together may fuse into a single mass. Structures of complex angular geometry may be rounded. When one is primarily concerned with morphology
at the macromolecular level these artifacts may indeed be serious (although to a large extent "sandwiching" methacrylate sections between
two supporting films protects them from this damage, see Chapter 6.10).
Examples from other families of plastics have been developed as embedding media for ultrathin sectioning, and, in a large measure, eliminate the methacrylate artifact. The first of these historically, an epoxy
resin "Araldite," decomposes little, if at all, when bombarded by electrons at normal operating intensities of an electron microscope. Thus,
LIMITATIONS OF METHACRYLATE
105
thors believe it to be contra-indicated with solid tissues unless block sizes
are kept very small, for they have had difficulty achieving uniform cutting
qualities in the blocks.
Ward (1959) advocates the use of small quantities of uranyl nitrate
added to the methacrylate monomer to prevent polymerization damage.
He suggests using 0.75-0.37%. The effect is not entirely specific to the
uranyl ion for there is some evidence that other heavy metals behave
similarly. Unfortunately the cutting qualities of the blocks are altered by
these additions. The polymerized plastic becomes harder than one would
anticipate, and it is sometimes necessary to add a plasticizer. Dibutyl
phthalate can serve in this respect, as much as 10% being added in some
cases.
The reader is referred to Gettner and Ornstein (1956) for a further
account of polymerization damage and of efforts made to minimize it,
and to Moore and Grimley (1957) for a theoretical account of polymerization which bears on this problem.
4.9. Limitations of Methacrylate
In the introduction of this chapter allusion was made to an artifact
associated with methacrylate sections which is apt to destroy detail of
fine order. One of the principal virtues of methacrylate is that it decomposes under electron bombardment so that much of its substance disappears leaving a background of relatively low electron density (Fig. 9). Unfortunately, as this occurs the methacrylate apparently melts, and surface
tension forces come into play. Two membranes that are close together
may fall upon each other so that subsequently they are observed as a
single line. A cylinder may collapse. Two tiny granules that are close together may fuse into a single mass. Structures of complex angular geometry may be rounded. When one is primarily concerned with morphology
at the macromolecular level these artifacts may indeed be serious (although to a large extent "sandwiching" methacrylate sections between
two supporting films protects them from this damage, see Chapter 6.10).
Examples from other families of plastics have been developed as embedding media for ultrathin sectioning, and, in a large measure, eliminate the methacrylate artifact. The first of these historically, an epoxy
resin "Araldite," decomposes little, if at all, when bombarded by electrons at normal operating intensities of an electron microscope. Thus,