Eur. Phys. J. E (2015) 38: 56
https://doi.org/10.1140/epje/i2015-15056-9

Regular Article

Experimental evidence of ultrathin polymer film stratification by AFM force spectroscopy

¹ Institut des Molécules et Matériaux du Mans (IMMM) - UMR CNRS 6283, Université du Maine, Avenue Olivier Messiaen, 72000, Le Mans, France
² Laboratoire d’Ingénierie des Matériaux de Bretagne (LIMATB), Université de Bretagne Sud, Rue Saint Maudé, 56321, Lorient, France
³ Institut Charles Sadron (ICS), Université de Strasbourg, 67034, Strasbourg, France

^* e-mail: nicolas.delorme@univ-lemans.fr

Received: 3 February 2015
Revised: 25 March 2015
Accepted: 11 May 2015
Published online: 22 June 2015

Abstract

By performing Atomic Force Microscopy measurements of pull-off force as a function of the temperature, we were able to probe the dynamic of supported thin polystyrene (PS) films. Thermal transitions induce modifications in the surface energy, roughness and surface modulus that are clearly detected by AFM and related to PS chain relaxation mechanisms. We demonstrated the existence of three transition temperatures that can be associated to the relaxation of polymer chains located at different depth regions within the polymer film. Independently of the film thickness, we have confirmed the presence of a region of high mobility for the polymer chains at the free interface. The thickness of this region is estimated to be above 7nm. The detection of a transition only present for film thicker than the gyration radius R_g is linked to the dynamics of polymer chains in a bulk conformation (i.e. not in contact with the free interface). We claim here that our results demonstrate, in agreement with other techniques, the stratification of thin polymer film depth profile in terms of relaxation behavior.