Using polyethylene adipate diol (PEA), 1,5-naphthalene diisocyanate (NDI) and diphenylmethane diisocyanate (MDI) as raw materials, and 1,4-butanediol (BDO) as chain extender, polyurethane elastomers (PUE) were synthesized by prepromerization method.The influence of NDI/MDI mass ratio (1/9 to 9/1) on material properties was systematically studied, and the results showed that：
Mechanical properties: When NDI/MDI=1/1, the comprehensive mechanical properties are optimal (tensile strength 55.2MPa, tear strength 127.2kN/m, high temperature tensile strength 12.5MPa) Thermal properties: With the increase of the proportion of NDI, the glass transition temperature (Tg) drops from -40.0℃ to -48.3℃, and the thermal stability is significantly improved (weightlessness 5% temperature*T_{5%}*increased by 10.8℃) Dynamic performance: The increase in the proportion of NDI leads to a 37% decrease in the loss factor, The energy storage modulus is increased to 1200MPa (room temperature)
1.1 Raw materials and formula Soft section: PEA (Mn=2000) Hard section: NDI, MDI chain extender: BDONDI/MDI ratio：1/9,3/7,5/5,7/3,9/1
1.2 Preparation process PEA dehydration (100℃, vacuum)→ Mixing with NDI/MDI to make a prepomer (NCO%=7%) and adding BDO to expand the chain→ pouring vulcanization (60℃)→ maturation (100℃, 16h)
1.3 Properties Characterization Mechanical properties: Hardness (GB/T 531), tensile/tear strength (GB/T 528) Thermal properties: Tg (DSC, -80~100℃, 10℃/min), heat loss (TGA, 20~ 600℃) Dynamic mechanics: energy storage modulus/loss factor (DMA, -80~110℃, 3℃/min) Structural analysis: FT-IR spectrum (4000~400cm⁻1)
2.1 The hump law of mechanical properties
The influence of NDI/MDI mass comparison on the mechanical properties of PUE
| NDI/MDI | Hardness ShoreA | Tensile Strength(Mpa) | Tear Strength(kN/m) | Tensile Strength in High Temperature(Mpa) |
| 1/9 | 87 | 37.9 | 98.7 | 11.9 |
| 3/7 | 88 | 47.1 | 103.5 | 12.3 |
| 5/5 | 94 | 55.2 | 127.2 | 12.5 |
| 7/3 | 95 | 35.3 | 116.4 | 12.0 |
| 9/1 | 96 | 18.3 | 83.2 | 9.0 |
Mechanism：
Tg is reduced by 8.3℃: NDI promotes the crystallization of the hard segment and reduces the restraint on the soft segment (PEA).
Improved thermal stability: The symmetrical naphthalene ring of NDI enhances the molecular stacking density and improves the decomposition energy barrier