Tunable micro and nano periodic structures in a free standing flexible urethane: experiments and modelling

 
M. H. Godinho1, A. C. Trindade1, J. L. Figueirinhas2,3, L. V. Melo3, 
P. Brogueira3,4, A. M. Deus4,5 and P. I. C. Teixeira6,*
 
1 Departamento de Ciencia dos Materiais and CENIMAT Faculdade de Ciencias e Tecnologia, 
Universidade Nova de Lisboa P-2829-516 Caparica, Portugal
2 Centro de Fisica da Materia Condensada, Universidade de Lisboa
P-1649-003 Lisbon, Portugal 
3 Departamento de Fisica, Instituto Superior Tecnico P-1049-001 Lisbon, Portugal
4 ICEMS, Instituto Superior Tecnico P-1049-001 Lisbon, Portugal
5 Departamento de Engenharia de Materiais, Instituto Superior Tecnico
P-1049-001 Lisbon, Portugal
6 Faculdade de Engenharia, Universidade Catolica Portuguesa 
Estrada de Talaide P-2635-631 Rio de Mouro, Portugal 
*Presenting author
 

The control and manipulation of tuneable structures that develop at rest in a free-standing urethane/urea elastomer film have been studied by means of atomic force microscopy AFM), small-angle light scattering (SALS) and polarizing optical microscopy (POM). The urethane/urea film was produced by means of a shear-casting technique, by extending a poly(propylenoxide) based triisocyanate terminated prepolymer PU) with poly(butadienediol) (PBDO) with a weight ratio of 60% PU/40% PBDO. UV irradiation results in latent micro and nano periodic patterns which can be `developed' by applying a uniaxial strain, or by immersing the elastomer in an appropriate olvent and then drying it. For this elastomer we describe six pattern states, how they are related and how they can be manipulated. The morphological features of the UV exposed ilm surface can be tuned, in a reproducible and reversible manner, by switching the direction of the applied mechanical field. Elastomers extracted in toluene exhibit different surface patterns depending upon the state in which they were developed. Sress strain data collected for the films before and after UV irradiation, allowed the detection of orientational order induced by the shear casting direction and enhanced by the mechanical field.

We have attempted to model the strain induced textures by a ssuming the film to consist of a thin, stiff surface layer (`skin') atop a thicker, softer substrate (`bulk'). The skin's higher stiffness is hypothesised to be due to the crosslinking of chains near the surface by UV radiation. Patterns thus arise as a competition between the effects of bending the skin and stretching the bulk. This model has been simulated using the finite element package ABAQUS; some preliminary results are presented which show promising agreement with experiment.