Tuneable micro- and nano-periodic structures in a free-standing flexible urethane/urea elastomer film

Abstract

We have studied the control and manipulation of tuneable equilibrium structures in a free-standing urethane/urea elastomer film by means of atomic force microscopy, small-angle light scattering and polarising optical microscopy. The urethane/urea elastomer was prepared by reacting a poly(propyleneoxide)-based triisocyanate-terminated prepolymer (PU) with poly(butadienediol) (PBDO), with a weight ratio of 60% PU/40% PBDO. An elastomer film was shear-cast onto a glass plate and allowed to cure, first in an oven, then in air. Latent micro- and nano-periodic patterns are induced by ultra-violet (UV) irradiation of the film and can be "developed" by applying a plane uniaxial stress or by immersing the elastomer in an appropriate solvent 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 film surface can be tuned, reproducibly and reversibly, 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. Stress-strain data collected for the films before and after UV irradiation reveal anisotropy induced by the shear-casting conditions and enhanced by the mechanical field. We have interpreted our results by assuming the film to consist of a thin, stiff surface layer ("skin") lying atop a thicker, softer substrate ("bulk"). The skin's higher stiffness is hypothesised to be due to the more extensive cross-linking of chains located near the surface by the UV radiation. Patterns would thus arise as a competition between the effects of bending the skin and stretching/compressing the bulk, as in the work of Cerda and Mahadevan (Phys. Rev. Lett. 90, 074302 (2003)). We present some preliminary results of a simulation of this model using the Finite Element package ABAQUS.

Swelling‐Induced Surface Patterns in Hydrogels with Gradient Crosslinking Density

“…We would like to point out that our system differs from the UVcured coatings where a hard elastic skin is generally formed above a softer elastic or viscous layer. [24] We created a soft layer that gradually stiffened with depth ( Fig. 1).…”

Soft Janus, wrinkles and all

“…Of course, things are more complicated than this, e.g., the film composition has to be just right. For full details, see [1].…”

First Curl, Then Wrinkle

“…In the past few years, the excellent properties of urethane/urea elastomers have been exploited to trigger these elastic instabilities, more recently in geometries other than planar . These elastomers are lightly cross‐linked and contain unreacted UV‐functional groups.…”

“…In this reversible process, the UV‐exposed areas of the elastomer tend to wrinkle, producing the patterns described earlier. These patterns may be further enhanced, or attenuated, by the application of external loads . The reversibility of elastomer deformations makes them ideal materials for the development of self‐operating devices with switchable functionalities.…”