1. Introduction
Polyurethane material is synthesized by the polycondensation of low-molecular-weight polyether/polyester and polyisocyanate. The main molecule chain comprises soft and hard segments. The different types and contents of two-component raw materials lead to differences in the number and structure of soft and hard chains, which causes the material to show various properties and mechanical characteristics [
1,
2]. Therefore, polyurethanes are polymers available for many technical applications requiring flexibility, durability, and impact resistance [
3,
4,
5]. Studies showed that polyurethane elastomers synthesized by macromolecular MDI and polyether have good hydrolysis resistance and low temperature flexibility, and are widely used in the construction field [
6,
7].
Mechanical properties determine the use of polyurethane elastomers. However, due to the complex composition and reaction mechanism of two-component polyurethane, current research on the mechanical properties of polyurethane elastomers is mainly based on macroscopic experiments. Researchers often determine the effects of two-component content, additives, and the external environment on polyurethane elastomers’ mechanical properties by testing the mechanical properties of polyurethane elastomer specimens [
8,
9]. In addition, the two-component polyurethane reaction’s degree of polymerization is also an important factor affecting its mechanical properties. Polyurethane synthesis is mainly the reaction between isocyanates and active hydrides (hydroxyl) in polyether polyol. The nucleophilic center of active hydride collides with the electrophilic center of isocyanate, causing the polymerization of two additional molecules [
10]. Therefore, the size of intermolecular interaction energy and the diffusion behavior of two-component molecules in the system affect the collision probability between molecules and the adequacy and rate of the polymerization reaction. However, studying the compatibility of two-component materials by macroscopic methods is difficult because the two-component polyurethane reacts very quickly after mixing. Molecular dynamics (MD) simulation technology provides a new way to solve this problem. It can accurately scale the time scale to the picosecond level and analyze the influence of component content and the external environment (temperature, pressure, etc.) on material properties at the nano-micro level [
11,
12].
Previous studies never predicted the mechanical properties of polyurethane materials in terms of the compatibility of the two-component blend. In addition, uniformly characterizing polyurethane is difficult on a molecular scale due to its complex composition. Performing molecular simulation calculations for two-component polyurethane is also difficult. Since polyether polyols and MDI are the main components of polyurethane elastomers, and the two are comparable on a nanoscale level, this paper selects the polyether polyol molecule–MDI molecular system as the research object. It uses Materials Studio molecular simulation technology and macroscopic experiments to study the two-component system at multiple scales. By analyzing the solubility parameters, binding energy, and molecular diffusion coefficient of the polyether polyol-MDI system at different temperatures and ratios, we judged the influence of temperature and ratio on the mixing effect of the two components. We used the Perl scripting language to calculate the mechanical properties of the two-component system and the relationship between stress and strain. Finally, we verified the microscopic mechanism obtained by molecular simulation using macroscopic experiments and analyzed the influence of temperature on the mechanical properties of two-component polyurethane molding specimens. These measures guide the formulation design of two-component polyurethane and the setting of each component parameter in the processing and production processes.
5. Results and Analysis of Direct Tensile Tests
Figure 9 shows the effects of the A:B ratio and temperature on the tensile strength of polyurethane elastomers. As shown in
Figure 9a, the elastomers’ tensile strength increased significantly with the A:B ratio decrease. At 20, −15, and −30 °C, the tensile strength of PB3 was 7.8, 5.42, and 3.11 times that of PB1, respectively. At 60 °C, the tensile strength of polyurethane elastomers increased slowly with the decrease in the A:B ratio. The material’s tensile strength was <1 MPa because polyurethane elastomers have difficulty releasing internal heat at high temperatures, resulting in a decreased degree of physical cross-linking of chain segments. Therefore, it is difficult to demonstrate high tensile strength [
20].
Figure 9b shows that the polyurethane elastomer’s tensile strength increases with the temperature decrease. When the temperature is <20 °C, the growth rate of the elastomer’s tensile strength increases gradually. The tensile strengths of PB1, PB2, and PB3 increased by 45.1, 5.7, and 0.8%, respectively, when the temperature decreased from 20 to −15 °C. The tensile strengths of PB1, PB2, and PB3 increased by 81.1, 11.1, and 3.9%, respectively, when the temperature decreased from −15 to −30 °C because the crystallization rate between the molecules inside the polyurethane gradually increased as the temperature decreased, resulting in difficult movement between the segments. Therefore, the tensile strength of the elastomer gradually increases.
Figure 10 shows the effects of the A:B ratio and temperature on the elongation at the break of polyurethane elastomers.
Figure 10a shows that the elongation at the break of polyurethane elastomers decreases with the decrease in the A:B ratio. At 60 and 20 °C, the elongation at the break of PB3 decreased by 127.1 and 88.72%, respectively, compared to PB1.
Figure 10b shows that the elongation at the break of polyurethane elastomers decreases with the decrease in temperature. When the temperature decreased from 60 to −15 °C, the elongation at the break of PB1, PB2, and PB3 decreased by 225, 136, and 98.6%, respectively. At −15 °C, with the increased tension, the specimen had no obvious deformation. When the tension reached a certain value, the specimen suddenly broke, and the failure section was neat (
Figure 11). When we reduced the temperature to −15 °C, the properties of the elastomer changed, and the material exhibited brittle characteristics. At this time, the elongation at the break of the elastomer was minor. The elongation at the breaks of PB1, PB2, and PB3 were 2.13, 2.04, and 1.63%, respectively. When the temperature decreased from −15 to −30 °C, the elongation at the break of the elastomer decreased slowly and tended to the horizontal line. At this time, the elongation at the break of the elastomer was about 0.6%.
Figure 12 shows the influence of the A:B ratio and temperature on Young’s modulus of polyurethane elastomers. As shown in
Figure 12a, Young’s modulus of polyurethane elastomers continues to increase as the A:B ratio decreases. Young’s modulus of elastomer increased more significantly with the increase in isocyanate content at low temperatures than at room temperature. At 20, −15, and −30 °C, Young’s modulus of PB3 increased by 25.4, 356, and 872.3 MPa, respectively, compared with PB1. As shown in
Figure 12b, Young’s modulus of polyurethane elastomers continues to increase as the temperature decreases. When the temperature decreased from 20 to −30 °C, Young’s modulus of PB1, PB2, and PB3 increased by 363.2, 635.4, and 1210.1 MPa, respectively. The decrease in temperature and A:B ratio enhances the polyurethane elastomers’ ability to resist deformation.
In summary, the polyurethane elastomer’s tensile strength and Young’s modulus increased with increased isocyanate content, whereas the elongation at the break decreased. This result is mainly because the hard segments of polyurethane elastomers are mainly provided by isocyanate. With the increase in hard segment content, the hard segments are connected, and the number of hydrogen bonds increases, leading to increased interaction and physical cross-linking between the segments [
21]. Therefore, the elastomer exhibits a higher Young’s modulus and mechanical strength at the macro level. However, increases in the degree of physical cross-linking between the segments hinder the movement of the macromolecular chain of the elastomer material. Thereby, the deformation of the elastomer material is inhibited, and the elongation at the break decreases. In addition, the soft segment can provide certain flexibility and elasticity to the elastomer. However, due to the increase in hard segment content, the proportion of soft segment gradually decreases, and the soft segment’s effect gradually weakens, which is also a reason for the decrease in elongation at the break of the elastomer.
In addition, with the decrease in temperature, the tensile strength and modulus of polyurethane elastomers increased, and the elongation at break decreased. This result is because the glass transition temperature of polyurethane elastomers is often lower than room temperature [
22]. The decrease in temperature increases the crystallinity of polyurethane elastomers and makes the chain segments difficult to move. Therefore, the elastomer exhibits a brittle-hard state macroscopically. This finding is consistent with Young’s modulus of polyurethane elastomers, which increase with the increase in isocyanate content and the decrease in temperature.