## 3 Results and Discussions  
### Thermal Property and Crystallinity  
Figure 1 shows the TGA curves for the PEG-SMPUs [23]. These SMPUs show three thermal transitions with corresponding weight loss clearly seen. The initial weight loss is around 60 \({}^{\circ}\)C,  
corresponding to the evaporation of the water and solvents entrapped. The other two starting temperatures for material to decompose are around 250 and 300 \({}^{\circ}\)C for all the samples. The weight loss at 250 \({}^{\circ}\)C is rather small, and as the temperature increases to \(\sim\)300 \({}^{\circ}\)C, it triggers another decomposition process with a large reduction in the weight. It is believed that these two decompositions correspond to soft and hard segments, respectively. As it shown in Figure 1, PEG-H1 has a slightly better stability than the others. The remaining weight above 410 \({}^{\circ}\)C is the residual materials such as carbon _etc_. The total weight losses up to 410 \({}^{\circ}\)C for sample PEG-H1, PEG-H2, PEG-H3, PEG-H4 and PEG-H5 are approximately 86, 88, 91, 90 and 94% respectively, indicating the thermal stability decreases with increasing the IPDI contents. The total weight loss for the PCL-SMPUs as listed in Table 2 is 94 \(\sim\) 98%, higher than those of the PEG-SMPUs with the same molar ratios of MDI to IPDI. The PEG-SMPUs showed a slightly better thermal stability than the PCL-SMPUs, implying that the chemistry of polyol and the cross-link between the soft and the hard segments play important roles in thermal stability of the SMPUs. On the \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Sample** & **Weight Loss at (410 \({}^{\circ}\)C)** & \(\mathbf{T_{m}}\) & \(\Delta\mathrm{H_{f}}\) & **Stress at Break** & **Strain at Break** \\ \hline  & **\%** & \({}^{\circ}\)C & \(\mathbf{Jg^{-1}}\) & _MPa_ & **\%** \\ \hline
**PEG-6000** & N/A & **65.0** & **822.0** & N/A & N/A \\ PEG-H1 & 86 & 43.6 & 59.5 & 4.49 & 425 \\ PEG-H2 & 88 & 46.3 & 75.6 & 5.15 & 170 \\ PEG-H3 & 92 & 50.0 & 79.4 & 5.40 & 110 \\ PEG-H4 & 90 & 46.0 & 71.6 & 6.37 & 50 \\ PEG-H5 & 94 & 47.0 & 83.4 & 8.20 & 12 \\
**PCL-2000** & N/A & **51.6** & **76.6** & N/A & N/A \\ PCL-S1 & 94 & 40.3 & 8.6 & 6.99 & 930 \\ PCL-S2 & 94 & 41.3 & 18.8 & 6.24 & 869 \\ PCL-S3 & 94 & 44.3 & 23.4 & 5.64 & 790 \\ PCL-S4 & 98 & 42.6 & 20.5 & 4.39 & 334 \\ PCL-S5 & 98 & 43.3 & 27.2 & 3.06 & 192 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Critical values of weight loss, \(\mathrm{T_{m}}\) and heat of fusion of PEG and PCL-SMPUs. Figure 1: Thermogravimetric analysis (TGA) curves of the PEG-SMPU samples. other hand, the weight loss was found to increase with the decrease of the molar ratio for both sample series, indicating that higher MDI concentration is better for thermal stability of the shape memory polymers. Crystallization and melting of the soft segments in SMPUs are important in controlling the shape memory effect and deformation of the polymers, as the transition temperature of the SMPUs is normally dominated by the melting temperature, Tm [24]. As shown in Figure 2(a), a pure PEG-6000 polyol has a transition (melting) temperature from 58 to 68 \({}^{\circ}\)C with a narrow transition region of \(\sim\)10 \({}^{\circ}\)C. Whilst a PCL polyol has a much broader transition region from 35 to 56 \({}^{\circ}\)C with a broad shoulder at about 40 \({}^{\circ}\)C (Figure 3(a)). The heat of fusion (\(\Delta\)H\({}_{\rm f}\)) of the PEG-6000 polyol is about 822 J/g, more than one order of magnitude higher than 77 J/g for the PCL-2000 polyol, implying that the PEG-6000 polyol has a high degree of crystallinity. Figures 2(b) and 3(b) show the DSC curves of the SMPUs with PEG and PCL polyols at different molar ratios of \(\gamma\).