Page 48 - ILL Annual Report 2019

SOFT CONDENSED MATTER
Nico Carl. German
The ILL and the University of Paderborn, Germany
‘My work focuses on the self-assembly of polymers in aqueous solutions. Neutrons are a unique tool with which to study such structures, as they permit us to highlight certain parts of the self-assemblies by deuteration and contrast variation.’
Micellization of block copolymers with two anionic blocks
Small-angle scattering instrument D11 SAXS beamline ID02, ESRF
Polyelectrolytes are macromolecules that carry an ionic group in the monomer
unit. They are water soluble and highly relevant for industrial applications such
as wastewater treatment or as rheology modifiers for concrete. We developed
a block copolymer composed of two negatively charged polyelectrolytes, forming a new type of block copolymer micelles
in the presence of Ca2+. Micelle formation arises from selective complexation of Ca2+ with one of the blocks, as elucidated using small-angle neutron scattering (SANS).
AUTHORS
N. Carl and Klaus Huber (ILL & University of Paderborn, Germany) R. Schweins and Sylvain Prévost (ILL)
ARTICLE FROM
Soft Matter (2019)—doi: 10.1039/c9sm01138b
REFERENCES
[1] R. Schweins, P. Lindner and K. Huber, Macromolecules 36 (2003) 9564
[2] M. Hansch, H.P. Kaub, S. Deck, N. Carl and K. Huber, J. Chem. Phys. 148 (2018) 114906
[3] H.B. Stuhrmann, J. Appl. Crystallogr. 7 (1974) 173
[4] C.G. Sinn, R. Dimova and M. Antonietti, Macromolecules 37
(2004) 3444
Sodium polyacrylate (PA) and sodium polystyrene sulfonate (PSS) are two negatively charged polyelectrolytes. Ca2+ cations bind to the anionic moieties of PA, which results in an aggregation and eventually precipitation of the polymer above a critical concentration [1]. This is commonly referred to as specific interaction. In contrast, Ca2+ does not bind to PSS and therefore no precipitation is observed [2]. Our study employed these considerably different interaction patterns by synthesizing a block copolymer of PA and PSS (figure 1a). In order to differentiate between the two polymer blocks in a neutron scattering experiment, the PA block was deuterated. This allows a contrast- variation SANS experiment to be performed [3] where the ratio of light H2O and heavy water D2O is systematically varied.
Figure 2 shows the SANS profiles of one of the polymers in the presence of Ca2+ at varying ratios of H2O/D2O. The contrast variation technique allows the scattering length
of the solvent to be adjusted to that of the core and the corona, as well as to the average value of the polymer. The scattering curves from these measurements show
that PA is complexed by Ca2+ and forms the core of the micelles. In contrast, PSS is located in the corona of the self-assemblies and stabilises them electrostatically. The
a)DD b)
D - - 25°C - -
--- Ca2+ --- NaO - - O----
-- --
---EDTA 60°C --- --- ---
PA-b-PSS
ANNUAL REPORT 2019
SO Na 3
-- --
Figure 1
a) Chemical structure of the PA-b-PSS block copolymer.
b) Schematic representation of the micelle formation and its reversibility.