Glass formation and glassy behavior remain areas of investigation in soft matter physics with many aspects which are still not completely understood, especially at the nanometer size-scale and close to the glass transition temperature. In the present work, we show an extension of the “nanobubble inflation” method developed by O’Connell and McKenna [Rev. Sci. Instrum. 78, 013901 (2007)] which uses an interferometric microscope (white light scanning interferometry method) to measure the surface topography of a large array of 5 μm sized nanometer thick films. These so-called free-standing films are subjected to constant inflation pressure during which the nanobubbles grow or creep with time. Measurements of multiple bubbles in real time are possible via the technique of Phase Shifting Microscopy (PSM) thanks to the fast acquisition and processing of interferometry. This has been implemented using in-house developed LabVIEW based software combined with the IMAQ Vision module. Moreover this technique has the advantage, over the AFM method of O’Connell and McKenna, to be a true non-contact technique. Using this optical configuration, there is no substrate interaction to affect the polymer chains. Here we demonstrate the method using ultra-thin films of both poly(vinyl acetate) (PVAc) and polystyrene (PS) and discuss the capabilities of the method in comparison to AFM, with its advantages and disadvantages. The viscoelastic responses of the nanobubbles are determined by measuring their time-dependent diameters and then by extracting both the stress and strain time-dependent components (here the history of the polymer films has to be taken into account). We show that the results from experiments on PVAc are consistent with the prior work on PVAc. However high stress results with PS show signs of a new non-linear response regime that could be related to the plasticity of the ultra-thin film. Our homemade setup used to apply stresses on the films is also described. This first allows the control of both temperature, using a Peltier ring which surrounds the sample, and pressure, using gas flow linked to a manometer. Then major improvements of our setup in order to solve small experimental issues are described. Finally, plans for further improvements to the cell are explained for future experiments.

Date Published: 24 May 2018
PDF
Proc. SPIE 10677, Unconventional Optical Imaging, 106770U (24 May 2018); doi: 10.1117/12.2306441

Published in SPIE Proceedings Vol. 10677:
Unconventional Optical Imaging
Corinne Fournier; Marc P. Georges; Gabriel Popescu, Editor(s)