Page 130 - Materials Chemistry, Second Edition

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2.4. The Amorphous State

Figure 2.80. Relationship between a siloxy precursor and its control over the three-dimensional shape of
a gel. Reproduced with permission from Chem. Rev. 2004, 104, 3893. Copyright 2004 American
Chemical Society.

its pores. The product at its gel point is often termed an alcogel, recognizing the
trapped solvent. At this stage, the gel is typically removed from its original con-
tainer, but remains saturated with the solvent to prevent damage to the gel through
evaporation. It is worthwhile to note that at the gel point, the –O–Si–O– framework
still contains a number of unreacted alkoxide moieties. Hence, sufficient time
(typically 48 hþ) must be allotted to allow for complete hydrolysis and polycon-
densation, so the network is suitably strengthened to prevent cracking – a process
referred to as aging,or syneresis.