Structure Formation of Adsorption Layers of Ionic ... - ACS Publications

Downloaded by UNIV OF DELAWARE MORRIS LIB on June 15, 2012 | http://pubs.acs.org Publication Date (Web): September 19, 2011 | doi: 10.1021/bk-2011-1070.ch008

Chapter 8

Structure Formation of Adsorption Layers of Ionic-Amphiphilic Copolymers on Inorganic and Organic Pigment Surfaces As Studied by ESA Claus D. Eisenbach,1,* Nikolay Bulychev,1,2 Klaus Dirnberger,1 Bart Dervaux,3 Filip E. DuPrez,3 and Vitali Zubov4 1Institute

for Polymer Chemistry, Universitaet Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany 2N.S. Kurnakov Institute of General and Inorganic Chemistry of Russian Academy of Sciences, 119991, Leninsky avenue, 31, Moscow, Russia 3Department of Organic Chemistry, Polymer Chemistry Research Group, Ghent University, Krijgslaan 281 S4, B-9000 Ghent, Belgium 4Lomonosov Moscow State Academy of Fine Chemical Technology, pr. Vernadskogo, 86, 117571, Moscow, Russia *Corresponding author: Prof. Dr. Claus D. Eisenbach, tel.: +49 711 68564441, fax.: +49 711 68564396, E-mail: [email protected]

The solid-liquid interface behaviour and the structure of adsorption layers of ionic-amphiphilic acrylic acid (AA)/ isobornyl acrylate (iBA) based copolymers on hydrophilic titanium dioxide (TiO2) and hydrophobic copper phthalocyanine (CuPc) has been studied by the electrokinetic sonic amplitude (ESA) method. It was shown that the polymer gel layer theory can be applied to polyelectrolytes, giving detailed information not only about the polymer-particle interaction but also the thickness and structure of the coating layer around the particles. CuPc was found to be covered by a relatively thin copolymer layer irrespective of the copolymer architecture, i.e., for both PAA-b-PiBA block copolymers with sharp block boundary and PAA-b-P(AA-co-iBA) block-like copolymers with tapered block-transition and isolated AA units in the hydrophobic block. In terms of the gel layer theory, PiBA blocks form the inner dense layer, and the PAA blocks represent © 2011 American Chemical Society In Amphiphiles: Molecular Assembly and Applications; Nagarajan, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

the outer, permeable region of the adsorbed layer. Contrarily, different adsorption mechanisms of block and block-like copolymers were observed for the TiO2. In case of block copolymers, a multilayer coating consisting of PAA anchor blocks, an interphases layer of PiBA blocks, and an outer PAA shell is obtained, whereas a solloid monolayer is formed for block-like copolymers.


Introduction Aqueous colloidal dispersions can be effectively stabilized by the addition of amphiphilic polymers (1–4). In this context, polyelectrolyte-type copolymers are of particular interest because of their electrosteric stabilization properties (5–7). For ionic-hydrophobic poly(styrene)-poly(acrylic acid) block and graft copolymers, specific polymer – titanium dioxide (TiO2) pigment surface interactions in combination with self-assembly processes result in a special pigment coating characterized by a polyelectrolyte shell (7). With the example of a series of well-defined amphiphilic block and block like-copolymers of isobornyl acrylate (iBA) and acrylic acid (AA), it was shown (8) that the interaction and layer formation of polyelectrolyte copolymers on, e.g., TiO2 (hydrophilic) and copper phtalocyanine CuPc (hydrophobic) particles dispersed in aqueous media strongly depend on the composition and structure of the copolymers. As to the adsorption of poly(isobornyl acrylate)-block-poly(acrylic acid) block copolymers PiBAx-b-PAAy and poly(isobornyl acrylate)-blockpoly(isobornyl acrylate-co-acrylic acid) block-like copolymers PiBAx-b-(PiBAyco-PAAz) on TiO2 or CuPc particles, in-situ analysis of the adsorption process by applying the electrokinetic sonic amplitude (ESA) method under constant low frequency (cf. (9)) allowed to establish models of the interaction of the copolymers with the pigment surface, and to determine the thickness of the resulting polymer adsorption layer as a whole (8). However, insights into the dimensions of the inner structure of the adsorbed polymer layer as related to the copolymer architecture were not accessible in these experiments. Detailed information about the inner structure and the thickness of the polymer adsorption layer can only be obtained by means of ESA measurements under variation of the applied alternating electrical field as demonstrated for non-ionic polymers (10, 11). Here we report on the investigation of the adsorption of ionic amphiphilic block and block-like copolymers PiBAx-b-PAAy and PiBAx-b-(PiBAy-co-PAAz), respectively, onto TiO2 and CuPc particles in aqueous dispersion by applying frequency dependant ESA measurements (cf. (12)).

Experimental Materials and Measurements Titanium dioxide TiO2 rutile pigment Kronos 2310 with a particle size of 0.3 μm and breadth of particle size distribution of 60 nm, and copper phthalocyanine 118 In Amphiphiles: Molecular Assembly and Applications; Nagarajan, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


(CuPc, BASF AG, Ludwigshafen) with a primary particle size of 0.1 μm and a particle size distribution breadth of 15 nm were employed as received. Block copolymers PiBAx-b-PAAy were synthesized via the macroinitiator strategy whereas block-like copolymers PiBAx-b-(PiBAy-co-PAAz) were prepared via a sequential monomer addition (Figure 1). More details about the synthesis and advantages of either route and the characterization of these tailored copolymers can be found in the literature (13). Electrokinetic sonic amplitude (ESA) measurements were carried out with an Acoustosizer 2 Instrument (Colloidal Dynamics, Sydney, Australia). 1 wt.-% aqueous dispersions of TiO2 of CuPc were employed in the copolymer adsorption studies. Frequency dependent ESA measurements (frequency range 1.0 - 18 MHz were carried out. In these measurements, the saturation concentration (SC (7)) which had been determined in separate experiments at fixed low frequency (1 MHz) was chosen as concentration of the added copolymers. At SC, all the added polymer is adsorbed to the pigment surface, i.e., practically none is remaining in solution. For the evaluation of the ESA data according to the polymer gel layer theory (10) (cf. (14)), the following theoretical parameters were used: dynamic viscosity η = 0.95 N·s·m-2 (viscosity of water at 22 °C), Debye-Hueckel parameter κ = 0.2 nm-1, drag coefficient α = 0.02 N·s·m-2·nm-2, and relaxation frequency ω0 = 0.85 s-1. For details of the experimental protocol and data analysis it is referred to earlier publications (8, 11).

Figure 1. Scheme of the synthesis (13) of isobornyl acrylate (iBA) and acrylic acid (AA) block and block-like copolymers, and copolymer structures. 119 In Amphiphiles: Molecular Assembly and Applications; Nagarajan, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

Results and Discussion


Methodology Electrokinetic sonic amplitude (ESA) measurements have been demonstrated to be a powerful method for providing information about the process of polymer adsorption onto particles dispersed in aqueous media (9, 10). In the ESA technique, an alternating electrical field of frequency ω is applied across a colloidal system. The alternating field exerts an electrical force on the charged colloidal particles which causes them to move back and forth with a sinusoidal velocity. It is this backwards and forwards motion that generates pressure waves which propagate as sound waves and are recorded as ESA signal. The ESA signal is directly related to the dynamic mobility μD, i.e. electrophoretic mobility of a particle in the alternating field (cf. (10)). The suitability of the ESA measurement to detect changes in the surface charge density of particles upon addition of polyelectrolytes which interact with the particle surface follows from the considerations of the limiting conditions from a theoretical point of view. This is shortly addressed below. For a dilute suspension (particle volume fraction φ

Structure Formation of Adsorption Layers of Ionic ... - ACS Publications

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