M. E. Mrvosh and K. E. D ~ u g h e r t y ' . ~ University of Pittsburgh Pittsburgh, Pennsylvania 15260
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he LOW cost Construction of inorganic Polymer Models Using Polyurethane
Inorganic polymer models used to describe heteropoly anions and silicate structures are particularly effective when constructed from solid tetrahedral and octahedral units. Models constructed from the solid polyhedra readily depict the interconnected array of corner- and edgesharing central ions through oxygen, a feature which is characteristicof these fascinating structures. As a teaching aid, the three-dimensional models have a decided advantage over the printed form representations. The students can readily follow the pattern of repeating units and locate the annular spaces within the model, often indicative of the chemical and physical properties of the polymers. In order to achieve the proper representation of the polymeric structure, a relatively large number of polyhedral units are often reauired for their construction. Constructing the polyhedra from cardboard, paper or wood (1-3) can be tedious and time consumine. ". if manv such units are required. Manufactured polyhedra are the ultimate for constructing these models; however, the cost may he prohibitive. Inorganic polymers can he easily and inexpensively constructed using rigid polyurethane, the type used for commercial insulation. The riaid is similar in - ~olvurethane . density to styrofoam. However, unlike styrofoam, the polyurethane can easily he cut with a motorized saw without tearing and can he painted with quick drying lacquer hase paints without dissolving the polymer's matrix. Rigid polyurethane may be purchased in sheets of various lengths and thicknesses direct from the manufacturers or through distributors of insulation. Often, a manufacturer will sell, at a reduced price, sheets of polyurethane which do not meet manufacturer's specifications known as "off spec" grade.
For the construction of our models, we used "off spec" 4 5-ft sheets of Celthanem, ranging in thickness from 1% in. to 2% in., obtained from the Dacar Chemical Company. X
Constructing the Polyhedra 1) Initially, the sheet of rigid polyurethane is cut into 2 X 2-ft pieces with a hand saw to afford easier manipulation. 2) A motorized saw is used capable of having its blade set at an angle of 55" with respect to the cutting plane. A band saw may. also be used providing an inclined base is used with it to attain the proper angle (Fig. 1). 3) The saw guide is adjusted to provide the proper width of the cut. The width cut determines the base an one half of an octahedron and the length of an edge of a tetrahedron. In order to attain the proper base width, the height or thickness of the polyurethane stock is measured, and its value related to the hase through the form: 1.414 X height = base (Fig. 1). Once this adjustment is made, it may be used without further adjustment for a particular thickness of stock. 4) The stock is cut through in one direction a d then inverted and .. cut through a second time, yielding a triangular prism (Fig. '4. 5 ) The prism in turn is cut widthwise in the same manner as the sheet stock, yielding a series of alternating tetrahedra and octahedral halves (Fig. 3). 6 ) Once the entire stock has been cut, the octahedral halves may be cemented together using a rubber hase glue. Typically, a 4 X 5-ft sheet of rigid polyurethane will yield approximately 30 octahedra and 70 tetrahedra.
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Address correspondence to this author. 2Present address: General Portland, h e . , 4241 Sigma Road, Dallas, Texas 75240.
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Figure 1. Initial adjustment of saw blade with respect to the polyurethane sheet stock (sideview).
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Figure 2. Formation of triangular prism from the polyurethane sheet stock after a series of two cuts: S = scrap, 0 = 'h of an octahedron. T = tetrahedron.
Figure 3. lar prism.
Figure 5.
The mineral Beryl: Be3A12Sis0,s.
Formation of the individual polyhedral units from the triangu-
Construction of Polyhedral Models
There are many papers and hooks which depict solid polyhedra in their representations of polymeric structures, which may he used as a guide for the construction of many types of models. Several sources which we feel are particularly noteworthy are listed in the references (4-6). The models are most easily constructed by first painting the individual polyhedra by spraying, dipping or brushing the polyhedra with the desired color of any base paint. After the paint has dried, the polyhedra may he joined with toothpicks to obtain the desired pattern of a basic unit. The basic unit may now be permanently honded by using a quick drying cement of the type used for constructing wooden models. Finally, several basic units may be joined to give the larger repeating unit.
Figure 6. Zeoliteof thetype: N a ~ 2 ( S i 7 2 A l , ~ 0 4 e ) . 2 7 H ~ 0
Figures 4, 5, and 6 show the results of several types of inorganic polymeric structures which were constructed using the methods described in this paper. Figure 5 depicts a heteropoly anion of the type [Xm+M12040]'8-"'-, The octahedra represents the (Mo06) species. The intercavity tetrahedron is representative of the (Xm+On)species. Figure 4 depicts the mineral beryl Be3AlzSi6018. The large annular rings represent the anionic species Sis018~2-. The lighter shaded tetrahedra represent the (BeOa) species and the dark shaded octahedra represent the (Aloe) species. Figure 6 depicts the structure of zeolite NalniSi~zAl~zO48.27H20. The (Si,AI)O4 tetrahedra are linked into 3dimensional frameworks in which the 4, 6 and 8 membered rings can he seen. Literature Cited I l l Sheppard, W. J.. J.CHEM.EDUC..11.683 11967).
". ~ i g m 4. Heteropoly anion of the type: [ X " ' + M , ~ O ~ O I ' ~ ~ "Where p+= p s t , As'+. Si4+. Ge4+. Sn4+. Ti4+. Zr4+ and M = Mo. W.
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I21 Canesan, L.R.,andRenganathan, S., J. CHEMEDUC.,18.59 (19711. (31 Freoiand. B. H., andO'Bnen. R.J.J.CHEM.EDUC., 48.771 (19711. (1)Clark. Grahm M., "The Strnefure. ol Non-Molecular Solids." John Wliey and Sons, NewYork. 197:i. (5) Wells. A. F., "Structural Inownic Chemistry..' 3rd Ed.. Oxford Univeniry Reis. Oxford, 1962. (6) Wells. A. F.. "Models k Sfrnefvrsl Inorganic Chemiafry," Oxford University Press, Orfordand New Yark, 19711.
