Processable Oligomeric and Polymeric Precursors to Silicates Prepared Directly from SiO 2 , Ethylene Glycol and Base.
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Processable Oligomeric and Polymeric Precursors to Silicates Prepared Directly from SiO 2 , Ethylene Glycol and Base.
CLINT BICKMORE, MARTIN L. HOPPE, and RICHARD M. LAINE. Departments of Materials Science and Engineering, and Chemistry, Univ. of Michigan, Ann Arbor, MI 481092136
ABSTRACT Bimetallic glycolato silicates of the form M2 Si 2 (OCH 2CH 2 0) 5 where M is an alkali metal, can be synthesized directly from SiO 2 , ethylene glycol and MOH. Further reaction with HO(CH 2CH 2 O)4 H, PEG 4 , transforms these compounds into rheologically useful polymers. These polymers offer considerable potential for processing novel silicate glass shapes, e.g. thin films, fibers and membranes, from relatively inexpensive precursors materials. In this paper, we describe pyrolysis studies on the model compound Li 2 Si2 (OCH 2 CH 20) 5 as a prelude to determing the utility of the polymeric precursors. These studies delineate the effects of temperature on the decomposition processes whereby the glycolato silicate transforms into almost phase pure, crystalline Li2 O-2SiO 2 . This work is contrasted with previous pyrolysis studies on K2 Si 2 (OCH 2 CH 2 0) 5 . INTRODUCTION We have recently developed an inexpensive method of converting sand, fused silica or silica gel into novel oligomeric and polymeric precursors to novel, pentacoordinate silicates[l] as shown in the following reaction:
2S10
2 +
2MOH + 5HOCH2 CH2OH
41120
2
L
SINO
-
7>
I-M
130 0 C/vacuum HOCI CH2OH
11-M
(1)
When M is an alkali metal such as Li, Na, K or Cs; monomeric compounds, I or dimeric compounds II are readily isolated. Reactions with alkaline metals such as Mg, Ca and Ba; rather than alkali metals, give hexacoordinate monomeric glycolato silicates, e.g.
Mat. Res. Soc. Symp. Proc. Vol. 249. @1992 Materials Research Society
82
BaSi(OCH 2 CH 2 0) 3 , as will be discussed elsewhere[2]. The OCH 2 CH 2 0 groups on crystalline I or II, are readily exchangable with other diol species such as tetraethylene glycol, HO(CH 2 CH 2 0) 4 H [PEG 4 ], reaction (2). Because the PEG 4 ligand is unable to
o-si{'i1 sX. •/ "-I"
t~~[LC L62
+ HOWOH 0
SI 2H
~ON.L 1 -0
_•J
-S 02"iii
0r
(2) function as a bidentate ligand, it provides crosslinks. Thus, as reaction (2) is driven to the right, an amorphous, polymeric material forms whose rheology can be adjusted by controlling the extent to which ethylene glycol is displaced. Reaction (2) permits the two step (from SiO 2) synthesis of type III-M (M = Li, Na, K, etc.) polymers that, because of their useful rheology, may offer access to thin films, fibers and membranes of alkali silicate glasses and ceramics. The bimetallic nature of these precursors is relatively novel and deserves detailed study. In particular, we are interested in developing non-hydrolytic routes to glasses and ceramics that may avoid the segregation problems inherent in typical sol-gel processing of multimetallic glasses and ceramics. In addition, because we can prepare, via reaction (3), a neutral, water-soluble alkoxysilane M2
0
2
i•
O-Si
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