Free radical polymerisation of MMA with thermal initiator in brick and Grimsel granodiorite

Abstract

The aim of this work was to come up with a reliable thermally initiated free radical polymerisation method to replace the existing irradiation initiated free radical polymerisation method in low permeability granitic rock porosity measurements using C-14-polymethylmethacrylate (C-14-PMMA) impregnation technique. Different chemical initiator concentrations and reaction conditions were tested in vials in order to find the optimum conditions, which were to be used for the polymerisations inside the matrices. The thermally initiated polymerisation was facilitated by the use of a thermal initiator, benzoyl peroxide (BPO). The matrices tested in this project were brick, and Grimsel granodiorite. The results of the thermally initiated polymerisation were compared to those from irradiation initiated polymerisation. The polymerisation conversion of polymethylmethacrylate (PMMA) was studied with Nuclear Magnetic Resonance spectroscopy (NMR) and the molar mass distributions with Gel Permeation Chromatography (GPC).The optimum reaction conditions were determined to be 55°C with a reaction time of 16h. Conversions and molar masses showed no significant correlation to the initiator concentration or matrix. Conversions were almost universally above 90% and molar masses in the range of 500,000–1,200,000g/mol. The conversions were in good agreement with the ones from irradiation polymerisation. On the other hand, significantly higher molar masses were obtained with thermally initiated polymerisation.Once optimum reaction conditions had been established, porosity measurements were performed and the porosity results of thermally initiated polymerisation were compared to those from irradiation polymerisation. Porosities were studied using Thermogravimetry (TGA) and Autoradiography, and parallel measurements with Water gravimetry on non impregnated samples were made for comparison. Porosities for the brick were in the range of 31–32% and the porosities in the Grimsel granodiorite samples were in the range of 0.8–1.0%. The porosity values given by the three methods were in good agreement with each other. The results were similar to the values obtained previously using irradiation polymerisation method.