Polymers, Colloids & Interfaces, Short talk
PI-016

Quantitative mechanochromism of polydiacetylenes at the nanoscale

L. Juhasz1, R. D. Ortuso1, K. Sugihara1*
1University of Geneva

Polydiacetylenes (PDA) are mechanochromic polymers, that show a colour change (e.g. blue to red)  upon external stimuli such as heat, stress or changes in the chemical environment [1]. However, how these stimuli change their structure and how that relates to their optical properties is still not completely clear to this day. The overwhelming part of the research hitherto has focused on activation of the colour change of PDA by heat[2] or solvents[3], and little attention has been devoted to the mechanosensitivity of PDA in its true sense, i.e. activation of the blue-to-red colour change by forces. Furthermore, these studies were either only qualitative[4] or macroscopic[5]. The only study on the mechanosensitivity of polydiacetylenes at the nanoscale has been the work of Polacchi et al.[6], however, they correlated the applied vertical force to the fluorescence increase of polydiacetylenes. In contrast, it has been established that lateral forces are needed to trigger the movement of the side chains of PDA and thus trigger the colour change.[4a] Due to experimental difficulties, no quantitative study has been published on the fluorescence response of polydiacetylenes against locally exerted lateral forces.

 To address this question, we deposited 10,12-tricosadiynoic acid and 10,12-pentacosadiynoic acid onto plasma-activated glass substrates with Langmuir-Blodgett technique, exposed them to UV for polymerisation, and applied forces to the polymerised film by an atomic force microscope (AFM), while simultaneously measuring the emitted fluorescence. The standard AFM is not able to measure lateral forces directly, thus we needed a calibration method to convert the registered lateral deflection to lateral force.[7] The registered fluorescence is characteristic of the transformed red phase of polydiacetylenes, thus enabling us to follow the phase transition, while the atomic force microscope registers the forces exerted to the polydiacetylene chains. Combining the two techniques could open a path to the fluorescence – lateral force calibration of the colour change, contributing to a deeper understanding of polydiacetylenes and pointing towards potential force sensing applications.

[1] R. D. Ortuso, U. Cataldi and K. Sugihara, Soft Matter 2017, 13, 1728-1736.
[2] a) S. Wacharasindhu, S. Montha, J. Boonyiseng, A. Potisatityuenyong, C. Phollookin, G. Tumcharern and M. Sukwattanasinitt, Macromolecules 2010, 43, 716-724; b) B. Yoon, H. Shin, E. M. Kang, D. W. Cho, K. Shin, H. Chung, C. W. Lee and J. M. Kim, ACS Appl Mater Interfaces 2013, 5, 4527-4535.
[3] a) H. Jiang, Y. Wang, Q. Ye, G. Zou, W. Su and Q. Zhang, Sensors and Actuators B: Chemical 2010, 143, 789-794; b) Y. Xu, S. Fu, F. Liu, H. Yu and J. Gao, Soft Matter 2018, 14, 8044-8050.
[4] a) R. W. Carpick, D. Y. Sasaki and A. R. Burns, Langmuir 2000, 16, 1270-1278; b) A. R. Burns, R. W. Carpick, D. Y. Sasaki, J. A. Shelnutt and R. Haddad, Tribology Letters 2001, 10, 89-96.
[5] H. Terada, H. Imai and Y. Oaki, Adv Mater 2018, 30, e1801121.
[6] L. Polacchi, A. Brosseau, R. Metivier and C. Allain, Chem Commun (Camb) 2019, 55, 14566-14569.
[7] R. D. Ortuso and K. Sugihara, The Journal of Physical Chemistry C 2018, 122, 11464-11474.