Atomic force microscopy evidence of patterning urethane/urea copolymers

The elastomeric film (60-μm thickness) was prepared from polypropylene oxide-based isocyanate-terminated triol prepolymer (PU) and polybutadiene diol (PBDO; 40% by weight). The sample was cut from the cross-linked film in the direction (d2) perpendicular to the casting rate (d1). The sample was subject to a uniaxial deformation along d2 equal to 1.3, at room temperature, under ultraviolet radiation (λ = 254 nm) for 4 days. After removing the mechanical field, a periodic pattern was obtained in the modulated polymer surface with a wave vector parallel to d2. A bi-stable behaviour of the elastomer is revealed by sequentially applying and removing mechanical shear stress along d1 (state I) and along d2 (state II). State I is characterized by two spatial periods along d1 and d2, respectively. The AFM height profile along d1 shows variability of the wave vector value with spatial periodicities ℓ^I₁ ranging from 4 to 5 μm and alternating peak-to-valley heights of h^I₁₁ = 65 ± 12 nm and h^I₁₂ = 140 ± 50 nm. In contrast, the AFM height profile along d2 reveals a complex periodic structure characterized by a well-established spatial periodicity ℓ^I₂ = 8.47 ± 0.12 μm. The peak-to-valley height varies between 30 and 170 nm within one spatial period in a reproducible sequence. State II is characterized by a clear spatial periodicity along d2. Within this periodicity peaks are grouped in pairs. Each pair is characterized by a spatial periodicity of ℓ^II₂ = 4.5 ± 01 μm. The height difference between subsequent pairs is Δh^II₂₂ = 42 ± 10 nm with a constant peak-to-valley height h^II₂₂ = 290 ± 30 nm. The profile along d1 reveals the imprint of state I.