Polymers, Colloids & Interfaces, Short talk

Mechanochemical Activation at Interfaces Driven by Swelling of Polymer Brushes

J. N. Bleich1, M. M. Marcinek1, H. A. Klok1*
1Institut des Matériaux et Institut des Sciences et Ingénierie Chimiques, Laboratoire des Polymères, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland

Surface-initiated controlled radical polymerization (SI-CRP) is a powerful technique to grow densely graft chain-end tethered polymers from surfaces. These polymer assemblies are usually referred to as polymer brushes. SI-CRP allows to control grafting density and chain conformation. Immersed in a good solvent, polymer brushes swell and individual chains are forced into an extended conformation. This extended chain conformation is an important contributor to the non-fouling and low friction properties, which were reported for a number of hydrophilic polymer brushes. While for a long time surface grafted polymer brushes were considered very stable thin films, an increasing number of papers has been published within the last decade that report degrafting of hydrophilic brushes in aqueous media. Hence, the stretched chain conformation is not only an important factor for the brush properties, but also influences the chemical reactivity at the substrate-brush interface. A fundamental understanding of this phenomenon, however, is still lacking. In the field of polymer mechanochemistry external force fields are usually applied to alter the polymer reactivity using techniques such as ultrasound sonication, and turbulent or elongational flow fields. Mechanochemical activation by swelling of a polymer brush may offer an alternative tool with the advantage that no external stimulus is needed.

With the aim to better understand and predict the degrafting of polymer brushes, hydrophilic polymer brushes are prepared via surface-initiated atom transfer radical polymerization (ATRP) from silicon substrates modified with a dimethylchlorosilane-based ATRP initiator. Subsequently the polymer brush modified substrates are incubated in aqueous media and their degrafting behavior is monitored by ellipsometry. Apparent initial rate constants are determined from the degrafting profiles assuming pseudo first-order kinetics. We describe and discuss the analysis of these apparent rate constants and our attempts to correlate these with the swelling behavior of polymer brush films of different polymer molecular weights and grafting densities in different media. Moreover, strategies to investigate reactions other than the ester or siloxane hydrolysis mentioned above (e.g. primary sulfonate hydrolysis or reductive disulfide cleavage) are developed. The challenge herein lies in the development of hydrolysis stable platforms, which only undergo the reaction of interest. Next to hydrophobic-hydrophilic diblock polymer brush structures, hydrophobic polymer brushes incubated in dry organic media are investigated with the advantage that background hydrolysis of the present ester and siloxane functionalities is avoided.