Page 21 - Neutrons For Science - Annual Report 2015

SCIENTIFIC HIGHLIGHTS
Figure 2
Monitoring the elastic signal as a function of temperature allows us to determine the onset of proton mobility by noting points of inflexion in the elastic scattering response. These were detected at 205 K (-68 °C) for the Aqua (a) and at 200 K (-75 °C) for the Poly (b) and on the nanosecond time scale. Additionally, the drop in intensity at the lowest value of Q suggests differences in the diffusivity of the liquid in both materials for the 5-day-old Poly and Aqua samples. No changes in the temperature at which motions were activated was noticed for the Poly (d) after 23 days of ageing, while a significant reduction in the amount of mobile protons was detected for the Aqua (c). This implies that important structural changes occurred in the structure of the latter.
We used both neutron and X-ray imaging to investigate
the highly complex pore structure within these cements, and neutron spectroscopy to assess hydrogen mobility [3]. We compared two commercial glass ionomer cements, both based on fluoroaluminosilicate glass powders, but set using different liquids. In the cement denoted “Poly” the liquid is composed of an aqueous polyacid solution. In contrast, in the cement denoted “Aqua” the liquid used was water, since the powder already contains the freeze-dried polyacid.
Using the ILL’s backscattering instrument IN10 we evaluated the evolution of the energy distribution of neutrons scattered by each sample, the IN10 instrument providing information about proton mobility on the nano-second time scale. In particular our experiment was interested in monitoring
the intensity of only those neutrons scattered elastically (i.e. without a change of energy), to obtain information about the number of immobile protons. Such analysis, performed as either a function of time or temperature, is known as the elastic fixed window (EFW) method [4]. When using the EFW approach as a function of setting time, more immobile protons were observed in the Aqua on the nanosecond time scale [3]. When using the EFW
approach as a function of temperature, we observed the onset of proton mobility by noting the inflection points in the elastic scattering response of both cements upon ageing (figure 2). While no differences in proton mobility were observed for Poly (figure 2b and 2d), ageing reduces proton mobility for Aqua (figures 2a and 2c). At the same time, the increase in the activation temperature for the Aqua with ageing means that less energy is necessary to produce proton mobility in this material. The latter suggests that perhaps the motions under investigation are faster
than the time window of IN10 or that a different proton population is being probed.
Our results indicate that the development of the pore structure plays an important role in the strength of the cements investigated. Findings from the neutron scattering experiments further suggest that the lower mechanical strength of Aqua (figure 1) results not only from its increased pore volume, but also, apparently, from the influence of hydrogen mobility within the material. These results open up new opportunities for research in dental cements, with neutron scattering providing a unique tool with which to better understand how to improve restorative dental material.
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