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 ℓI1 ranging from 4 to 5 μm and alternating peak-to-valley heights of hI11=65±12 nm and hI12=140±50 nm. In contrast, the AFM height profile along d2 reveals a complex periodic structure characterized by a well-established spatial periodicity ℓI2=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 ℓII2=4.5±01 μm. The height difference between subsequent pairs is ΔhII22=42±10 nm with a constant peak-to-valley height hII22=290±30 nm. The profile along d1 reveals the imprint of state I.