The effect of the catalyst on the formation of RF xerogels

• Main Author: Anderson, Lynsey
• Published: University of Strathclyde 2014
• Subjects: 660
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.665220

The ability to alter the nanostructure of resorcinol formaldehyde (RF) gels by varying the synthesis conditions, along with the obtainable properties, makes them valuable for a number of applications. Consequently, optimising the synthesis process is important to produce high quality gels that possess desired properties for the desired application. In order to control the formation of RF gels, it is vital to understand the specific role of the catalyst (C), with previous studies suggesting that its function is to alter the initial sol pH [1, 2], enabling gel formation to take place. It has been reported that controlling sol pH, allows the nanostructure of the final gel to be controlled [3], with lower initial pHs resulting in gels with higher surface areas, pore volumes and wider pore size distributions, where the inverse is true for higher pHs. This theory, however, does not explain why gels prepared from sols with equivalent initial pH possess significantly different characteristics, for example average pore diameters and pore volumes. Furthermore, studies into the effect of initial sol pH alter the alkalinity of the sol by modifying the molar ratio of R to C, subsequently, making it difficult to differentiate between the effects of catalyst and sol pH. In this study organic xerogels were prepared using the pre described poly-condensation method, however, a wide range of base catalysts (Na₂CO₃, K₂CO₃, NaHCO₃, KHCO₃, NaOH, KOH, CaCO₃, SrCO₃, BaCO₃ and (NH₄)₂CO₃) were employed, at varying concentrations, with all other experimental variables kept constant. Through the use of HPLC, titration and DLS the RF polymerisation reaction was monitored allowing effects of R/C ratio, initial sol pH, cation size, charge and presence, as well as base type and deprotonating ability to be identified, differentiated and comprehensively studied. It was found that, in general, for each individual catalyst, increasing R/C ratio caused an increase in average pore diameter and total pore volume, and a decrease in specific surface area. When all catalysts were considered together it became apparent that, rather than initial sol pH influencing the final properties, the xerogel nanostructure was dependent on the potential deprotonation ability (DPA) of the catalyst. Comparing RF gels prepared with Group I and Group II metal catalysts confirmed that cation size and charge affects the polymer stability and aggregation, subsequently, influencing overall structure. Monitoring the reaction of gels catalysed with no catalyst or (NH₄)₂CO₃ demonstrated that the presence of a metal cation is vital for base catalysed sol-gel polymerisation of R and F. Identifying and distinguishing the specific role of the basic catalyst, will allow RF polymerisation to be studied in full, using Design of Experiments (DoE). This approach will not only determine the effects of individual variables, but will also establish the combined influence of experimental conditions. Understanding the polymerisation fully will offer complete control of RF structure, permitting gels to be prepared with precise desired properties.