# Synthesis of Shape-Memory Polyurethanes: Combined Experimental and Simulation Studies
## 3 Results and Discussion  
### Characterization of PCUUs  
In this work, we performed computer simulations of PCUU synthesis. The detailed characterization of the structural, thermal, mechanical, and thermomechanical properties of obtained samples was also performed. Furthermore, shape-memory properties of PCUUs were investigated.  
The average molar masses of soluble fractions of PCUUs were determined using GPC. The results reported in Table 1 suggest that samples were physically cross-linked, which is indicated by the large dispersity index (D) and high weight average molar mass (M\({}_{w}\)). The molar masses of final PCUUs were much higher than in the case of OCDs, which confirms the chain extension of synthesized and cured structures.  
Our experiments suggest that we cannot rely on the accuracy of the GPC measurement with polystyrene-based calibration. That is why we relied on the results obtained with the use of NMR and MALDI-TOF techniques. For instance, the number average molar mass  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **M\({}_{\mathbf{a}}\)** & **M\({}_{\mathbf{w}}\)** & **D** \\ \cline{2-4}  & **g \(\times\) mol\({}^{-1}\)** & **g \(\times\) mol\({}^{-1}\)** & **-** \\ \hline E\_PCUU3000\_3 & 23,550 1  & 2.008 \(\times\) 10\({}^{6}\)1  & 85.3 1  \\ \hline E\_PCUU3000\_2 & 56,290 1  & 1.674 \(\times\) 10\({}^{6}\)1  & 29.7 1  \\ \hline E\_PCUU3000\_2 & 43,750 1  & 2.665 \(\times\) 10\({}^{6}\)1  & 60.9 1  \\ \hline E\_PCUU3000\_1 & 64,590 1  & 1.169 \(\times\) 10\({}^{6}\)1  & 18.1 1  \\ \hline E\_OCD\_5000 & 7610 1  & 19,220 1  & 2.5 1  \\ \hline E\_OCD\_3000 & 5950 1  & 13,850 1  & 2.3 1  \\ \hline \hline \end{tabular}
\end{table}
Table 1: Molar mass and dispersity index.  
Figure 7: Schematic representation of the base unit participating in the curing process.  
(M\({}_{\rm{n}}\)) of E_OCD_3000, estimated based on \({}^{1}\)H NMR, was equal 3000 g \(\times\) mol\({}^{-1}\) and was in agreement with the one obtained by means of the MALDI-TOF technique (2910 g \(\times\) mol\({}^{-1}\), Table S1 and Figure S29, Supplementary Materials).  
In the case of samples E_PCUU5000_3, E_PCUU5000_2, and E_PCUU3000_3.5, the proposed experimental methodology led to physically cross-linked structures, while the remaining samples were soluble in chloroform. Gel fraction (G\({}_{\rm{f}}\)) and the degree of swelling (Q\({}_{\rm{m}}\)) were determined in this solvent (Table 2). Presented results show that a high excess of diisocyanate compound is not solely responsible for the creation of a polymeric gel. In the case of discussed PCUUs, volumetric deformation leading to gel creation seems to also be strongly related to the length of the OCD chain. E_PCUU3000_3.5 had a high capacity to absorb chloroform with Q\({}_{\rm{m}}\) of approximately 8400%. Samples based on E_OCD_5000 had significantly lower swelling ratios: Q\({}_{\rm{m}}\) = 4480% for E_PCUU5000_3, and 5840% in the case of E_PCUU5000_2. We have observed a higher content of gel fraction in the case of higher IPDI excess, which leads to a decrease in chain mobility. Therefore, the swelling ability decreased with decreasing cross-linking of the material, resulting in the general rule that higher values of Q\({}_{\rm{m}}\) are connected with a lower G\({}_{\rm{f}}\)[55]. PCUUs based on E_OCD_3000 prepared with a molar excess of IPDI below 3.5 were soluble in the chloroform, or G\({}_{\rm{f}}\) was insignificant enough to be undetected using the described method.  
On the other hand, hard segment (HS) content in the sample is an important factor when analyzing the solubility of PCUUs; results indicate that the OCD chain length plays a significant role as well. It seems possible that these results are due to a higher amount of carbonyl groups present in the soft chain segment, which can form hydrogen bonds. Therefore, carbonyl-carbonyl interactions among hard and soft segments are possible in greater intensity along the length of the chain.