Bo Nyström, Department of Chemistry, University of Oslo, Norway

Scattering Studies of Complex Block Copolymers

Amphiphilic block copolymers provide a versatile platform for achieving desired nanostructures by self-assembly. This type of copolymers can undergo conformational changes in response to changes in temperature, pH, magnetic field, electrical field, or the wavelength of light, and they can form stable structures of interest across a range of technologies, including targeted drug delivery, sensors, imaging, catalysis, and lubrication.1-4 The self-assembly process usually lead to the formation of micelles and intermicellar complexes. Depending on the composition of the copolymers, temperature and pH the self-assembled moieties may assume different structures. These structures can be characterized by using light scattering and SAS (small-angle neutron scattering and small-angle X-ray scattering).

In this presentation, I will discuss copolymers of different natures that are both charged and uncharged. Scattering experiments on dilute aqueous solutions of copolymers of the type PEO-PPO-PEO (Pluronic, F127), with short (PCL(5)) or long (PCL(11)) PCL blocks at both ends were conducted. The dynamic light scattering results clearly show that both micellization and inter-micellization can be controlled by polymer concentration, temperature, and length of the PCL block. SANS results revealed interesting structural changes of the polymer from spherical to cylindrical shape as a function of the length of the PCL-block and temperature. It is shown that that PCL-modification of Pluronic has a large influence on the self-assembly process and on the final structure of the assemblies.5

SAXS experiments on triblock copolymers composed of MPEG-PNIPAAM-charged block with a PNIPAAM block of various lengths; reveal some interesting temperature–induced structural changes as result of the self-assembling process. For triblock copolymers with a short PNIPAAM block no self-assembling is observed. The copolymers with medium and long PNIPAAM block length self-assemble and the structure of the micelles varies with the PNIPAAM block length. The polymer with medium PNIPAAM block length forms spherical micelles, whereas the polymer with a long PNIPAAM block length forms cylindrical micelles.6

The triblock copolymer methoxyl-poly(ethylene glycol)-block-poly(2-acrylamido-2-methyl-1-propanesulfonic sodium)-block-poly(N-isopropyl acrylamide) (MPEG-b-PAMPS-b-PNIPAAM) was synthesized via two different atomic transfer radical polymerization (ATRP) methods, namely “one-pot” and “two-pot”. These two approaches are frequently employed in the synthesis of various copolymers. In the “two- pot” method, the first block is isolated and purified and then used as a macroinitiator, whereas in the “one-pot” method the synthesis proceeds through sequential monomer addition. As a consequence, the latter procedure leads to a copolymer where the PNIPAAM block is contaminated with a minute quantity of AMPS residuals and this sample does not form micelles over the considered temperature region. By using scattering methods, it was demonstrated that the “two-pot” method, with isolated blocks without any contamination, is more reliable in the synthesis of this three block copolymer and that a small amount of residual AMPS groups has a crucial impact on the self-assembly of the copolymer in solution. The findings from this investigation suggest that the infection of the PNIPAAM block with AMPS residues neutralizes the sticking power of the PNIPAAM block and thereby prevents self-assembly and micellization.7

References:
1. G.S. Kwon, K. Kataoka, Adv. Drug Delivery Rev. 1995, 16, 295.
2. A. Agarwal, R. Unfer, S.K. Mallapragada, J. Control. Release 2005, 103, 245.
3. M Block Copolymers in Nanoscience; M. Lazzari, G. Liu, S. Lecommandoux, Ed.; Wiley-VCH: Weinheim, Germany, 2006.
4. T. Schnitzler, A. Herrmann, Acc. Chem. Res. 2012, 45, 1419.
5. N Gjerde, K. Zhu, B. Nyström, K.D. Knudsen, Phys. Chem. Chem. Phys. 2018, 20, 2585.
6. M.A.Behrens, M. Lopez, A.-L. Kjøniksen, K. Zhu, B. Nyström, J.S. Pedersen, Appl. Cryst. 2014, 47, 22.
7. V.F. Motlaq, L. Momtazi, K. Zhu, K.D. Knudsen, B. Nyström, J. Polym. Sci. Part B. 2019, 57, 524.

The lecture is part of the symposium Recent progress in small-angle scattering from soft matter and biological systems on the occasion of Professor Jan Skov Pedersen's 60th birthday.