Page 447 - Carrahers_Polymer_Chemistry,_Eighth_Edition

410 Carraher’s Polymer Chemistry

FIGURE 12.2 General silicate structures. From left to right the figures are (top row) A, B, C, (second row) D,

E, (third row) F, G, (last row) H, and I with the letters corresponding to Figure 12.3 third column.

In the structures cited in Table 12.3, except for pure SiO , metal ions are required for overall
2
electrical neutrality. These metal ions are positioned in tetrahedral, octahedral, and so on positions
in the silicate-like lattice. Sometimes they replace the silicon atom. Kaolinite asbestos has alumi-
num substituted for silicon in the Gibbosite sheet. Further, sites for additional anions, such as the
hydroxyl anion, are available. In ring, chain, and sheet structures neighboring rings, chains, and
sheets are often bonded together by metal ions held between the rings. In vermiculite asbestos, the
silicate sheets are held together by nonsilicon cations.
For sheet-layered compounds, the forces holding together the various sheets can be substantially
less than the forces within the individual sheets. Similar to graphite, such structures may be easily
cleaved parallel to the sheets. Examples of such materials are mica, kaolinite, and talc.
Bonding occurs through a combination of ionic and covalent contributions just as are present in
organic polymers except that the ionic character is a little higher. “Back-bonding” from electrons
associated with the oxygen to vacant orbitals in the silicon (or other tetrahedral metal atom) occurs
giving the silicon–oxygen linkages some double or pi-bond character.
As noted before, cations other than silicon may occupy the tetrahedral centers. A major factor
in predicting which cations will be found to substitute for silicon is ionic size. In general, cations
+4
whose size is about 0.03–0.1 nm are the best candidates. Si has an ionic radius of about 0.041 nm.
+3
+2
+2
+2
Cations such as Fe (ionic radius = 0.07 nm), Al (0.05 nm), Ca (0.1nm), and Mg (0.065 nm) are
most often found in silicate-like structures and meet this requirement.
Most silicate-like polymers can be divided into three major classes—the network struc-
tures based on a three-dimensional tetrahedral geometry (such as quartz), layered geometries
with stronger bonding occurring within the “two-dimensional” layer (such as talc), and linear
structures.

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