Atomic force microscopy evidence of patterning urethane/urea copolymers

The elastomeric film (60-mum 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 (lambda = 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(l)(l) ranging from 4 to 5 mum and alternating peak-to-valley heights of h(l)(1)(l) 65 +/- 12 nm and h(l)(l)2 = 140 +/- 50 nm. In contrast, the AFM height profile along d2 reveals a complex periodic structure characterized by a well-established spatial periodicity l(2)(I) = 8.47 +/- 0.12 mum. 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(2)(II)= 4.5 +/- 01 mum. The height difference between subsequent pairs is Delta(h)(22)(II) = 42 +/- 10 nm with a constant peak-to-valley height h(22)(II) = 290 +/- 30 nm. The profile along d1 reveals the imprint of state I. (C) 2003 Elsevier B.V. All rights reserved.