At room temperature, most segmented polyurethane (SPU) copolymers molecules consist of alternate rigid (hard) and flexible (soft) segments. The hard segments contain the urethane groups, usually linked by a so-called chain extender. The Tg of the hard segment is generally well above room temperature. The soft segment is highly flexible at room temperature and typically has a Tg between -50°C and -20°C. The result is a phase-separated morphology, consisting of a soft segment rich rubbery matrix reinforced by relatively hard pseudo-crystalline domains. The size of individual hard segment domains is commonly in the range 10 nm - 25 nm. The morphology of SPUs is difficult to characterise by electron microscopy, because, even after staining, there is little contrast in electron density between the different domains. PFM has, however, proved to be a very useful and convenient technique for obtaining high contrast images of such materials. The pull-off force signal, in particular, has proved to be very sensitive to differences in surface adhesion properties between the phases. In order to carry out a full spatial characterisation, we can again utilise a variable temperature stage, this time to cool the material below the Tg of the soft segment material. Some typical results are shown below:

The room temperature pull-off force image reveals occluded low adhesion (dark) domains, 100 nm - 400 nm in size, in a relatively high adhesion (light) continuous phase. The corresponding room temperature indentation image shows less contrast. At -50°C the continuous phase has undergone a marked decrease in adhesion, resulting in a loss of contrast in the pull-off force image. M-TDSC showed that the only transition in this temperature range was the glass transition of the soft segment. The continuous phase must, therefore, be dominated by the properties of soft-segment material. The two topographic images show that the same area was scanned at the two temperatures. They also prove that topographic effects did not produce the adhesion-dependent contrast, although the hard segment domains are higher than the surrounding continuous phase. The domain size is an order of magnitude larger than that of individual domains and could be indicative of fractal morphology.