Dynamics of amphiphilic diblock copolymers at the air–water interface

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Abstract

Two polyisoprene–polyethyleneoxide diblock copolymers with different block length ratios adsorbed to the water surface were investigated by multiple angle of incidence ellipsometry, evanescent wave light scattering, and surface tension experiments. In a semidilute interfacial regime, the transition from a two-dimensional to a “mushroom” regime, in which polymer chains form loops and tails in the subphase, was discussed. A diffusion mechanism parallel to the interface was probed by evanescent wave dynamic light scattering. At intermediate concentrations, the interfacial diffusion coefficient D scales with the surface concentration Γ, as D  Γ0.77 in agreement with the scaling observed for polymer solutions in a semidilute regime. At relatively high concentrations a decreasing of D is discussed in terms of increasing friction due to interactions between polyisoprene chains.

Graphical abstract

The interfacial diffusion and the structure of two polyisoprene-polyethyleneoxide diblock copolymers at the water surface was studied in a semidilute interfacial regime.

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Research highlights

► Amphiphilic diblock copolymers adsorbed at the interface. ► In plane diffusion measurements by evanescent wave dynamic light scattering. ► Conformational changes in a semi dilute regimes.

Introduction

The dynamics of polymer chains at the interface is an important research topic in physics, biology and chemistry. The physical understanding of the diffusion and polymer conformations close to an interface has also a great impact on the study of biological systems such as proteins and membranes and on controlled drug release. Many processes occurring close to a biological membrane could be studied using polymers at liquid–fluid interfaces as model systems. Polymers can adopt at the interface different conformations such as brush-, mushroom-, or pancake-like morphologies [1] and hence, their diffusion mechanism differs significantly from that in solution. In dilute and concentrate solution the dynamics of flexible polymers is well described by the Rouse and the Reptation model, respectively [2].

Furthermore, liquid interfaces are frequently the loci for chemical reactions. In this view, the interfacial region might be considered almost as 2D-reactor with width of only a few nanometers where reactions are governed by the particular interfacial properties (pH, lyophilic–lyophobic interactions, electrical charge, etc.) that significantly differ from the bulk values [3].

Up to date, macromolecules at the air–water interface were investigated extensively mainly regarding adsorption kinetics and chain conformation transitions [4], [5], [6]. The kinetics of adsorption from the bulk phase to the interface could be followed, for instance, by tensiometry and ellipsometry [6], [7]. The surface tension changes as a result of a diffusion process followed by conformational reorganizations at the interface [6] and the resulting layer is usually referred as “Gibbs layer”.

The knowledge of the interface concentration is an essential parameter for studies of the conformation of adsorbed polymers. This concentration is usually controlled by varying the interfacial area in a Langmuir trough, in which a non-aqueous polymer solution is spread on the top of the water surface. These layers are known as “Langmuir layers”and their structural conformations have been studied by tensiometry, rheology, neutron/X-ray reflectivity, capillary wave scattering, electrocapillary waves and oscillating barrier experiments [8], [9], [10], [11], [12], [13], [14], [15].

What remained almost unexplored, due to experimental limitations, is the in-plane diffusion at the air–water interface. Only very few techniques are capable to measure the mobility of polymer chains at the interface. Among them, fluorescence recovery after photobleaching (FRAP) and single molecule fluorescence imaging were used to study the interfacial diffusion of phospholipids [16], [17]. For non-fluorescing adsorbed molecules, evanescent wave light scattering is a proper tool [18], [19], [20].

The structure and dynamics of polymers attached to an interface are different compared to the bulk [21]. Considering the case of amphiphilic diblock copolymers, different regimes and phases can be encountered in the solution when the concentration is changed. In these different states the polymer changes both the conformation and the diffusion mechanism, which can be usually expressed by a different scaling law [2]. In a common solvent for the two blocks, a micro phase separation is encountered above a characteristic concentration which affects several diffusion and relaxation mechanisms [22], [23], [24], [25], [26], [27]. In a selective solvent, on the other hand, polymer micelles are formed with a core built up by the insoluble block. The diffusion of polymeric micelles is similar to colloidal particles, although the friction is significantly altered by the polymer corona formed by the soluble block segments [28], [29].

Here, we investigated two amphiphilic neutral biocompatible diblock copolymers composed of polyisoprene and polyethyleneoxide at the air–water interface. In order to study the polymer dynamics in different regimes, we prepared “Gibbs layers” by changing quasi-statically the concentration of the block copolymers in the aqueous phase. The experimental system is well defined and consists only of block copolymers distributed between the bulk water and the vapor–water interface.

The paper is organized as follow: Section 2 describes the chemical system and the methods used for the interfacial characterization; Section 3.1 recalls the properties of the solutions in the range of concentration considered here. After presenting the ellipsometric results in Section 3.2, interfacial regimes (Section 3.3) and the corresponding dynamics (Section 3.3) are discussed before the Conclusions section (Section 4).

Section snippets

Amphiphilic diblock copolymers

Polyisoprene–polyethyleneoxide (PI–PEO) diblock copolymers were synthesized by anionic polymerization using high vacuum techniques, as described elsewhere [30], [31]. Two block copolymers, characterized by different degrees of polymerization N, were used: PI111PEO201 and PI88PEO334. For those systems, in Table 1 the values of molecular weight Mw and polydispersity Mw/Mn (obtained by GPC analysis), and the weight percent of polyisoprene (evaluated by NMR) are reported. The sizes of the

Results and discussion

In the first section of the this part, the properties of polymer solutions as a function of the bulk concentration will be described. Whereas in the following sections, the investigation of equilibrium or pseudo-equilibrium states of adsorbed polymers at the interface will be presented. For each bulk concentration studied, these interfacial states are reached after completing the adsorption from the bulk to the interface.

Conclusions

The adsorption behavior of polyisoprene–poly(ethylene oxide) diblock copolymers at the air–water interface is governed by the hydrophobic and the hydrophilic block in specific ways. If the surface pressure is determined by the hydrophilic block PEO [13], the adsorption to and the anchorage at the interface is determined by the polyisoprene blocks over the whole concentration range. The relative size of the hydrophilic poly(ethylene oxide) block determines also the mode and intensity of

Acknowledgment

Financial support of the Max Planck Society is gratefully acknowledged.

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