IONOMER GLASSES: DESIGN AND CHARACTERIZATION - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 412

fi eld gradient around the site. In MAS-NMR spectroscopy, the resolution and
sensitivity when quadrupolar nuclei are studied, is often limited because of the
inherent distribution of the quadrupolar coupling interactions, PQ, in disordered
materials. It is well known that most anisotropic interactions to which non-
quadrupolar nuclei are subject are effectively eliminated by MAS. However,
MAS alone is incapable of completely narrowing the peaks of half integer quad-
rupolar nuclei such as 27 Al (I = 5/2). The effect of the second order quadrupolar
coupling is a broadening of the peak to lower frequency relative to the isotropic
chemical shift. Therefore, by employing high magnetic fi eld there is an increase in
Larmor frequency,

0 , which provides greater frequency dispersion in chemical
shifts and decreases the second order quadrupolar effect on the nuclei [31].
In the case of ionomer glasses, the useful information obtained from a MAS-
NMR experiment is mainly information on the coordination states of the ele-
ments in study, the local bonding environment and the information on the next
nearest neighbours to the elements in study. The authors will refer only to infor-
mation obtained from 27 Al, 31 P, 29 Si and 19 F MAS - NMR experiments. MAS - NMR
studies [29] of a series of glasses based on the composition 2SiO 2 - Al 2 O 3 - (2 - x)
CaO - xCaF 2 showed that the increase in the fl uorine content resulted in the for-
mation of F-Ca(n) species (Figure 12.5) around

ν

150 ppm where n is the number
of Ca around a fl uorine atom. These species bonded ionically with only one NBO
in the glass compared to the fl uorine free glass where Ca 2+ ionically bonded with
two or possibly more NBOs. Consequently, the glass network became disrupted
and at high fl uorine contents replacement of NBOs by non-bridging fl uorine
occurred. In this case aluminium in fi ve- and six-fold coordination state appeared,
as reported by Stebbins et al. [32] and Stamboulis et al. [31], and that can be
related to the appearance of signifi cant amount of Al-F-Ca(n) species in the 19 F
MAS - NMR spectra.

−

19 F

LG99

LG96

LG26

LG115

LG120

50 0 -50 -100 -150 -200 -250

ppm

Chemical shift

Figure 12.5. 19 F MAS-NMR spectra for a series of ionomer glasses. Spinning side bands are
indicated by • [35].