Phase-separated microstructures in “all-acrylic” thermoplastic elastomers

Abstract

Atomic Force Microscopy (AFM) is used to study the phase separation process occurring in block copolymers in the solid state. Measuring simultaneously the amplitude and the phase of the oscillating cantilever in tapping-mode operation provides the surface topography along with the cartography of microdomains with different mechanical properties. This in turn allows to characterize the organization of the various components at the surface in terms of well-defined morphologies (e.g., spheres, cylinders, or lamellae). Here this approach is applied to a series of symmetric triblock copolymers made of a central elastomeric segment (polyalkylacrylate) surrounded by two thermoplastic sequences (polymethylmethacrylate). The occurrence of microphase separation in these materials and the resulting microscopic morphology are essential factors for determining their potential applications as a new class of thermoplastic elastomers. This paper describes how the surface morphology can be controlled by the molecular structure of the copolymers (volume ratio between the sequences, molecular weight, length of the alkyl side group) and by the experimental conditions used for the preparation of the films. The molecular structure of the chains is fully determined by the synthesis of the copolymers via living anionic polymerization while the parameters that can be modified when preparing the samples are the nature of the solvent and the thermal annealing of the films. Finally, we report on a systematic comparison between images and approach-retract curve data. We show that this experimental comparison allows the origin of the contrast that produces the image to be straightforwardly evaluated. The method provides an unambiguous quantitative measurement of the contribution of the local mechanical response to the image. We show that most of the contrast in the height and phase images is due to variations in local mechanical properties and not in topography.