engleski

The role of water in formation of different siloxane structures

As a part of our study of polymerization process of aminopropylsilanetriol (APST) we investigated the behavior of APST in water solution (original concentration nearly 25 % ) and after the pumping under low pressure (at nearly 5 mbar) by Raman spectroscopy. It has been already noticed that the polymerized structure of APST depends significantly on the temperature at which the condensation takes place. At the same time the rate of evaporation of water could also play important role in forming different polymer structures. In order to resolve between these two effects, the glass tubes with solution were pumped on the vacuum line exposing samples to the low pressure of ~ 5 mbar for one to three hours. The samples with different starting concentrations of water were then left to polymerize under atmospheric conditions. The initial water content was determined by weighting the samples at two instances: 1) after being exposed to the low pressure and 2) at the end of the condensation process. Raman spectra of solutions and of condensed samples were recorded. Structure developments were also monitored under mid-magnification optical microscope. The spectra of solutions show the change in distribution of water clusters present in different solutions. Concentration of higher H2O agglomerates (tetramers, pentamers, hexamers) is increasing with decreased water content. The same circumstances favor formation of gauche conformation of propyl chain in APST molecules. Simultaneously, the accelerated decrease of water content hinders the formation of siloxane ladder structure in APST polymer. We have also examined mechanism of the solidification by precise weighting of the original solution (25%), that was left to evaporate at room conditions in the grey-light for 66 days. The experimental points can be nicely fitted to the two exponential functions. One of them describes the evaporation of water, while the other describes the creation of water molecules during the process of hydrolysis and consequently the formation of siloxane bridges.

siloxane ; conformations ; vibrations ; water

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