W. Richard Bowen and Nidal Hilal 4

6.4 MICRO- ANd NANOTHERMAL CHARACTERISATION wITH SPM 187
form to another under storage conditions. While cimetidine is known to
have seven polymorphs, only type A and type B are used in tablet and
suspension formulations, respectively [53]. SThM images were obtained
using a probe temperature of 50°C and a scan rate of 1 Hz. Figure 6.7
shows topographical (images a and c) and SThM images (images b and d)
of a 50:50 mixture of polymorph type A and type B from two different
areas. The contrast highlighted by arrows in images b and d is a result of
the differences in surface thermal conductivity in the two different polymorphs.
LTA on discs of pure samples of polymorphs type A and type B
were used to help elucidate that the lighter and darker regions observed
in the topographical data (Figure 6.7a, c) were of polymorphs type A and
FIgure 6.7 50 �m � 50 �m images of a pressed disc comprising a 50:50 mixture of the A
and B polymorphs of cimetidine (scale bar: 10 mm): (a) and (c) show topographical images of
two different areas of the disc and (b) and (d) show the corresponding thermal conductivity
images. Features highlighted with arrows show contrast as a result of the differing thermal
behaviour of the two polymorphic forms of cimetidine. Reproduced with permission [52].