## 3 Results and Discussions
### Thermal Property and Crystallinity
Once the polyols were mixed with the hard segments, the transition temperature decreased for both sample series, and the decrease of Tg was larger for the samples with higher MDI concentrations; with further decreasing the molar ratio to \(\gamma=0.5\) or 0.325, the transition temperature slightly increased as summarized in Table 2. As can be seen, similar to the low molecular weight polyol SMPUs developed, the high molecular weight polyol PEG-SMPUs have transition temperatures lower than 50 \({}^{\circ}\)C, which are close to human body temperature and hence are suitable for implants and other medical applications [25,26]. Figure 2: Differential scanning calorimetry (DSC) results of the PEG-6000 polyol **(a)** and PEG-SMPUs **(b)**. Table 2 also summarizes the DSC measurement results for the PEG and PCL-based SMPUs and corresponding pure polyols. Comparison between the pure polyol PEG-6000 and PEG-SMPUs revealed a dramatic decrease in \(\Delta\)H\({}_{f}\) from 822 to less than 100 once the hard segment was introduced. It is known that low heat of fusion is an indication of low crystallinity and high phase compatibility between the hard and the soft segments [9]. Decrease of \(\Delta\)H\({}_{f}\) in the PEG-SMPUs is more drastic than in the PCL-SMPUs; comparison of the \(\Delta\)H\({}_{f}\) of the PEG-SMPUs with those of the PCL-SMPUs with the same \(\gamma\) revealed that the heat of fusion is always higher for the PEG-series than those of the PCL-series, implying that the PEG-series may have higher crystallinity than the PCL-series. It is known that the crystallinity of polyols can influence the phase segregation between the soft and the hard segments, and high crystallinity is related to low phase compatibility and poor SME [13]. On the other hand, \(\Delta\)H\({}_{f}\) increases gradually with IPDI concentration, indicating that the crystallinity of the SMPUs increases, in agreement with our previous work on SMPUs with low molecular weight polyols [20]. It is believed that the reactivity between the polyols and MDI molecules is higher than that between the polyols and IPDI molecules. This results in low crystallinity in the soft segment with a high MDI concentration, and enhances the phase compatibility between the soft and the hard segments, which is better for SME. An increase in IPDI concentration results in a reduction of chemical reactions between MDI and polyol, leading to co-existence of the soft and hard segment macrophages, poor polymerization and poor shape memory effect [20]. Although the transition temperatures are similar to the PCL-SMPUs, the PEG-SMPU samples showed \(\Delta\)H\({}_{f}\) values that were two to three times those of the PCL-SMPUs, indicating that the high molecular weight polyols tend to crystallize more easily than those with low molecular weight. This was further confirmed by the X-ray measurements as shown in Figure 4, where the PEG-SMPUs have higher intensities of the X-ray diffraction peaks than those of the low molecular weight polyol SMPUs. The major peaks in the WAXD curves of both the sample series remained unchanged compared to those of pure polyols. The degree of the crystallinity decreased with the increase in \(\gamma\) value, which is in agreement with the conclusion that the polyols determine the crystalline structure of SMPUs, whilst introduction of hard segments only changes the crystallinity of the copolymers, as found in our previous work [20], which affects the phase compatibility. In brief, the PEG-SMPUs have higher crystallinity and lower cross-link density at soft segment sites than those of the PCL-SMPUs. The results indicate that the PEG-SMPUs may be stiffer with lower elongation at break and possibly poorer shape memory properties. Figure 4: WAXD curves of the PEG (**a**) and PCL (**b**) SMPUs.