Mechanical and thermal properties of UV‐curable polyurethane and polyurea acrylates

Soft‐segment molecular weight, soft‐segment crystallization, hard‐segment‐type, and reactive diluent level were examined in urea and urethane acrylates. Increasing soft‐segment molecular weight led to a decreased tensile modulus, increased strength, and increased elongation at break. Soft‐segment crystallization increased the modulus below the melting temperature, but decreased the modulus above the melting temperature because crystallization interfered with cross‐linking. Materials with hard segments that allowed for greater phase separation showed a higher modulus, higher ultimate stress, and higher ultimate elongation. Materials with reactive diluents 2‐hydroxyethyl methacrylate (HEMA) and N‐vinyl pyrrolidinone (NVP) showed qualitatively different behavior. HEMA caused the modulus and strength to increase with little effect on elongation up to concentrations of 50 wt %; at higher concentrations, the elongation decreased. NVP also increased the modulus and strength; however, at 40 wt % NVP, ultimate elongation increased in a well‐phaseseparated system and decreased in a poorly phase‐separated system. In tensile tests, samples with 40% NVP also showed substantial sample necking and drawing.