This journal is © The Royal Society of Chemistry 2015 Soft Matter, 2015, 11, 3559--3565 | 3559 Cite this: Soft Matter, 2015, 11, 3559 Interface-enforced complexation between copolymer blocks† Alexander A. Steinschulte, a Weinan Xu, b Fabian Draber, a Pascal Hebbeker, a Andre Jung, a Dimitri Bogdanovski, a Stefanie Schneider, a Vladimir V. Tsukruk b and Felix A. Plamper* a Binary diblock copolymers and corresponding ternary miktoarm stars are studied at oil–water interfaces. All polymers contain oil-soluble poly(propylene oxide) PPO, water-soluble poly(dimethylaminoethyl methacrylate) PDMAEMA and/or poly(ethylene oxide) PEO. The features of their Langmuir compression isotherms are well related to the ones of the corresponding homopolymers. Within the Langmuir-trough, PEO-b-PPO acts as the most effective amphiphile compared to the other PPO-containing copolymers. In contrast, the compression isotherms show a complexation of PPO and PDMAEMA for PPO-b-PDMAEMA and the star, reducing their overall amphiphilicity. Such complex formation between the blocks of PPO-b- PDMAEMA is prevented in bulk water but facilitated at the interface. The weakly-interacting blocks of PPO-b-PDMAEMA form a complex due to their enhanced proximity in such confined environments. Scanning force microscopy and Monte Carlo simulations with varying confinement support our results, which are regarded as compliant with the mathematical random walk theorem by Po ´ lya. Finally, the results are expected to be of relevance for e.g. emulsion formulation and macromolecular engineering. Introduction Amphiphilic copolymers are well known and understood for their self-assembly in bulk solution. 1 The formation of star-like (core-corona), worm-like or vesicular morphologies depend on e.g. the respective block lengths and the interfacial tension between the insoluble component and the solvent. 2,3 At the same time, many amphiphilic block copolymers exhibit surface activity, i.e. the polymers can be anchored at water–air or water– oil interfaces. One important tool to investigate the properties of interfaces is the Langmuir-trough experiment. Compression and expansion isotherms give information on the spatial demands of adsorbed polymers along with their ability to stabilize the interface. In previous studies amphiphilic diblock copolymers, 4–6 colloidal particles like microgels, 7,8 thermoresponsive polymers 9,10 and star-shaped molecules have been investigated. 11–14 Interfaces also operated as templates for complexation between different components. 15–19,50 In most reported studies, the dominant attractive forces would also act in common solvents, allowing complexation not only at interfaces. 19,20 Even without such attractive forces, immiscible branched polymers can intermingle at the bottom and top of fluid interfaces due to steric effects upon compression (though they are still immiscible on local chain level). 21 Hence, interfaces provide platforms for unexpected properties of surface-active entities, being unparalleled in bulk. 22,23,49 We have reported on the prevention of self-assembly of so-called miktoarm stars by architecture-induced complexation in bulk water. 24 Within one molecule, 25 different poly(propylene oxide) (PPO) and poly(dimethylaminoethyl methacrylate) (PDMAEMA) arms interacted due to the increased mutual segment density close to the center of these star-shaped polymers. The com- plexation led to a reduction of interfacial tension between the complex and bulk water: the rather hydrophilic PDMAEMA shields the more hydrophobic (PPO-rich) interior of the PPO– PDMAEMA complex. 26 In contrast, linear PPO-b-PDMAEMA block copolymers exhibit hardly any complexation in bulk water but initiate ‘‘conventional’’ micellization toward constructs possessing hydrophobic PPO cores and hydrophilic PDMAEMA coronas. 24 The connectivity as such is not sufficient to promote complexation but chain segregation dominates. This was also found for other copolymers: statistical copolymers show a synergistic depression of cloud points whereas block copolymers are unaffected. 27 In sum, there is a prerequisite of high mutual segment density and spatial proximity between weakly interacting partners. Only by aid of certain a Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany. E-mail: plamper@pc.rwth-aachen.de b School of Materials Science and Engineering, Georgia Institute of Technology, Georgia 30332, USA † Electronic supplementary information (ESI) available: Details about the synthesis, the matching of the compression isotherms, the expansion isotherms and the interactive simulation snapshots of selected systems. See DOI: 10.1039/c5sm00242g Received 28th January 2015, Accepted 17th March 2015 DOI: 10.1039/c5sm00242g www.rsc.org/softmatter Soft Matter PAPER Published on 17 March 2015. Downloaded by Rheinisch Westfalische Technische Hochschule Aachen on 02/02/2017 10:03:37. View Article Online View Journal | View Issue