The integration of photoresponsive units into polymeric materials holds great potential, enabling precise modulation of material characteristics with spatial and temporal resolution via light-induced reactions. To date, the most common strategy employs photo- responsive units as pendant groups [1] in a polymer material or within a crosslinked hydrogel network [2].

There are limited examples of reversible photoresponsive linear polymers [3,4,5], as it is synthetically challenging to find conditions that yield linear polymers with sufficient molecular weight while maintaining the integrity of the chromophore unit. Still, we expect that incorporating these photo-responsive units directly into the polymer backbone will yield more significant effects on the material properties. Investigating the effect of photoinduced isomerization on the material properties, particularly via rheological characterization in the melt and the impact on thermal properties, requires that the photo-switchable unit exclusively undergoes isomerization upon irradiation, rather than being thermally induced. Among various families of photoswitches, hydrazones have recently emerged as a class of P- type photoswitches.

These hydrazones switch between two thermally stable E/Z isomers not only in solution but also in the solid state[ 6] which is a prerequisite for switching in solid polymer materials. In addition, the remarkably long half-life times extending over a millennium [7] enabling thermal polymer characterization methods without alteration of the material properties, and the occurrence of the switching in the visible and UV- A wavelength range which mitigates the risk of photoinduced degradation of the polymer, hydrazones appear as ideal candidates for use in photoresponsive polymeric materials. Thus, in this work we report the synthesis of linear step-growth polymers with photoswitchable repeating units and analyze how the photochemical E/Z isomerization within the polymer backbone influences the chain stiffness (persistence length) and the resulting chain mobility [8].

For an in-depth understanding, we compare also various regioisomers of the photoswitching unit incorporated in the polymer main chain. By comparison we identify which photoswitch induces the most significant changes in material properties upon isomerization, while maintaining both a good switching efficiency and a high conversion of the photoisomerization in the solid state. Indeed, the reversible modulation of glass transition temperature and modulus observed in our study, highlights the potential for customized lithographic applications. For instance, it would enable the reversible generation of 2D patterns consisting of spatially resolved hard and soft segments.

References

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Acknowledgments

H. M. acknowledges the University of Haute-Alsace for the financial support from the French National Research Agency with the reference "ANR-22-CPJ1-0077-01" and from the CNRS for the junior professorship contract.