# Experimental Study on Flexural Properties of Polyurethane-Cement Composites under Temperature Load
## 3 Synthesis Mechanism of Polyurethane Cement Composites
### Chemical Structure and Reaction Mechanism of Polyurethane
#### 3.1.1 Surface Potential Analysis of Isocyanate and Polyether Polyol  
Two-component polyurethanes are formed by the cross-linking reaction of isocyanates and polyols. Isocyanates refer to compounds containing isocyanate groups divided into aliphatic isocyanates and aromatic isocyanates. Polyols are oligomeric compounds containing two or more hydroxyl groups (-OH) in the molecular structure, usually polyether polyols, Polyester polyols, or mixtures. The main component of WANNATE(r) PM-200 used in this paper is aromatic 4,4\({}^{\prime}\)-diphenylmethane diisocyanate (MDI), and the main component of ES305 type of polyol used is polypropylene glycol (PPG). The molecular structure is shown in Figure 3.  
Figure 2: Molecular structure of polyurethane.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Serial Number** & **Project** & **Index** \\ \hline
1 & Exterior & Off-white powder \\
2 & Molecular weight & 198.2 \\
3 & Melting point/\({}^{\circ}\)C & 98-102 \\
4 & Density (110 \({}^{\circ}\)C)/(g/cm\({}^{3}\)) & 1.15 \\
5 & Viscosity (110 \({}^{\circ}\)C)/mPa-s & 15 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Physicochemical properties of HQEE.  
Based on the B3 LYP general function in quantum chemical density generalization theory, the structures of isocyanate and polyether polyol molecules were analyzed using Materials Studio software to obtain stable structures and wave function files Dmol3 electrostatic potential was selected. The wave function files were analyzed for surface electrostatic potential, and average localized ionization energy and visualization results were generated. The electrostatic potential distribution on the whole molecular surface is shown in Figure 4. The blue area indicates that the electrostatic potential value is small; the electronegativity is large, and the electron-attracting ability is strong; the red area indicates that the electrostatic potential value is considerable, that the electronegativity is small, and the electron-absorbing ability is poor.  
Figure 4: Surface electrostatic potential of isocyanate and polyether polyol: (**a**) Surface electrostatic potential of MDI isocyanate; (**b**) Surface electrostatic potential of PPG polyether polyol.  
Figure 3: Molecular structures of isocyanates and polyether polyols: (**a**) the molecular structure of MDI; (**b**) the molecular structure of PPG.  
From Figure 4a, it can be seen that the oxygen and nitrogen atoms with large electronegativity in the MDI molecule exhibit blue areas, electrostatic potential minimal value points appear at both ends of the molecule, and -NCO (isocyanate group) is the active electron-absorbing group. In the potential diagram of the PPG molecular structure, as shown in Figure 4b, except for the hydrogen atoms at the two ends of -OH (hydroxyl group), which is a significant red region, the hydrogen atoms at the remaining positions are almost all electrically neutral and exhibit a white-gray region, with the leading reactive group being the hydroxyl group.