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Fluorescent films that change their properties when stretched


Imagine a material both luminous and soft. Imagine that through polarized glasses its luminosity changes when turned in all directions… This is the kind of objects we have developed at the LPS. Funny isn’t it ? And useful ! Indeed, many applications can be imagined, from flexible and anti-reflective screens to light devices with varying intensity.

In order to develop such materials, we have used fluorescent particles that are synthesized in our laboratory : semi-conducting CdSe nanoplatelets. Many advantages arise from this flat geometry, compared to more classical spherical quantum dots. First, fluorescence emission is sharp and strong. Second, the emitted light is highly polarized, in the plane of the platelets.

When dispersed in a solid matrix, a composite (hybrid) material is obtained. We have established that an SBS (polystyrene (PS) – polybutadiene (PB) – polystyrene) polymer matrix is highly suitable for a controlled polarization of such materials. SBS is a triblock copolymer consisting of alternating domains of PB and PS. This thermoplastic elastomer is easy to process and to stretch. As the ligand covering the platelets is well compatible with PB, we have shown that the platelets are confined in PB domains as stacks. Macroscopic aggregation, which often appears in such hybrid materials, is avoided here.

We have prepared hybrid films that could be stretched up to a strain of 100%. We have shown that, under stretching, both block copolymer nanodomains and platelet stacks are oriented (see Figure). Under such conditions, the whole film emits polarized light. This can be seen in the image on the right, in which the film is observed through a polarizer respectively parallel and perpendicular to the film. This alignment is reversible and disappears instantaneously when strain is stopped. In conclusion, we are able to induce and control fluorescence anisotropy in hybrid films.

 

Reference :
Strain-controlled fluorescence polarization in a CdSe nanoplatelet / block copolymer composite
E. Beaudoin, B. Abecassis, D. Constantin, J. Degrouard and P. Davidson.
Chem Commun 51, 4051-4054 (2015).

Contact :
Emmanuel Beaudoin