Article pubs.acs.org/Langmuir
(Quasi‑) 2D Aggregation of Polystyrene‑b‑Dextran at the Air−Water Interface Wouter T. E. Bosker,† Martien A. Cohen Stuart,† and Willem Norde*,†,‡ †
Laboratory of Physical Chemistry and Colloid Science, Wageningen University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands ‡ Department of Biomedical Engineering, University Medical Center Groningen and University of Groningen, Antonius Deusinglaan 1, 9713 AV Groningen, The Netherlands ABSTRACT: Polystyrene-b-dextran (PS-b-Dextran) copolymers can be used to prepare dextran brushes at solid surfaces, applying Langmuir−Blodgett deposition. When recording the interfacial pressure versus area isotherms of a PS-b-Dextran monolayer, time-dependent hysteresis was observed upon compression and expansion. We argue that this is due to (quasi-) 2D aggregation of the copolymer at the air−water surface, with three contributions. First, at large area per molecule, a zero surface pressure is measured; we ascribe this to self-assembly of block copolymers into surface micelles. At intermediate area we identify a second regime (“desorption regime”) where aggregation into large patches occurs due to van der Waals attraction between PS blocks. At high surface pressure (“brush regime”) we observe hysteretic behavior attributed to H-bonding between dextran chains. When compared to hysteresis of other amphiphilic diblock copolymers (also containing PS, e.g., polystyrene-b-poly(ethylene oxide)) a general criterion can be formulated concerning the extent of hysteresis: when the hydrophobic (PS) block is of equal size as (or bigger than) the hydrophilic block, the hysteresis is maximal. The (quasi) 2D aggregation of PS-b-Dextran has significant implications for the preparation of dextran brushes at solid surfaces using Langmuir−Blodgett deposition. For each grafting density the monolayer needs to relax, up to several hours, prior to transfer.
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INTRODUCTION
However, while characterizing the interfacial behavior of PSb-Dextran at the air−water interface prior to the LB deposition, time-dependent hysteresis was observed:2,3 the interfacial pressure versus area per molecule isotherms obtained by compression and expansion do not coincide. In some studies on amphiphilic diblock copolymers a similar type of hysteretic behavior was observed. Polystyrene-b-polyvinylpyridine (PS-bPVP) displayed large hysteresis when compressed to high interfacial pressure,10 comparable to our experiments. It was concluded that “the highly compressed film has undergone some rearrangement process whose kinetic barrier to return to the initial state conformation is rather large”.10 The nature of this “rearrangement process” was not clarified. Polystyrene-bpoly(ethylene oxide) (PS-b-PEO) showed rather complicated interfacial properties. The recorded hysteresis depends on the block size of PEO, gradually disappearing with increasing block size of PEO (and constant block size of PS).11−13 Moreover, the hysteresis for short PS-b-PEO becomes less pronounced with an increasing number of compression and expansion cycles.12,13 Gonçalves da Silva et al. explain their observations by an aggregation of the PEO chains of the block copolymer while in the water subphase.12 In our study on PS-b-PEO, we suggested that upon compression some kind of 2D-aggregation
With the aim of preparing dextran brushes at solid surfaces for antifouling purposes,1,2 we synthesized polystyrene-b-dextran (PS-b-Dextran) copolymers.2−4 These copolymers were utilized in Langmuir−Blodgett deposition (LB) to obtain reproducible dextran brushes with tunable grafting densities (σ).1,2 Oligo- and polysaccharide brushes have been prepared using various methods: physical adsorption and chemical grafting to and grafting from. Marchant et al. used SAMs of comb-like surfactants containing end-grafted dextran sidechains that produce a dense dextran layer.5 Ö sterberg and coworkers prepared a dextran brush using a grafting to method.6 However, they only studied a very dilute brush. Amylose brushes at silica particles were synthesized by Loos7 and Breitinger8 by an enzymatic grafting from method. Breitinger observed a broad chain length distribution due to reduced accessibility of the immobilized primers.8 Recently, van der Vlist and co-workers9 grafted amylose and amylopectine-like brushes to silica surfaces, intended as antifouling surfaces, using an enzymatic tandem polymerization related to the procedure of Loos.7 They were able to form dense brushes at the surface. Yet, the length of the polysaccharides was limited, due to a hindered accessibility of the primer and a difficulty to reach the active site of the enzymes.9 In our research we chose the physical adsorption method, applying PS-b-Dextran copolymers and the LB method, to be able to control both the dextran chain length as well as the grafting density. © 2013 American Chemical Society
Received: December 4, 2012 Revised: February 4, 2013 Published: February 4, 2013 2667
dx.doi.org/10.1021/la304797r | Langmuir 2013, 29, 2667−2675
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temperature, the PS layer becomes glassy. This treatment results in block copolymers that are irreversibly attached to the surface. AFM. Tapping mode AFM was applied to investigate the structure of the PS-b-Dextran monolayers in water. A Nanoscope III, Veeco Instruments Inc. was used, equipped with a fluid cell filled with ultra pure water (Seralpur Pro 90C, conductivity
