Miktoarm stars and stimuli-sensitive copolymers: complexation and aggregation in dispersion

Amphiphilic polymer systems contain components with different solubility. Thus, they form so-called micellar structures in aqueous solution in which water-insoluble parts aggregate and are surrounded by the water-soluble polymer segments. For the applicability of such systems – e.g., for uptake and release of active substances in the biomedical sector – it is often advantageous if the shape and properties of the micelles can be altered. For this purpose, intelligent polymers are introduced which react to external influences (e.g., temperature, pH, additives) and change their behaviour in solution accordingly. A straightforward and rapid way to prepare polymeric micelles is complexing oppositely charged polymers, so-called polyelectrolytes. Interpolyelectrolyte complexes (IPECs) form spontaneously in aqueous solution and represent the water-insoluble part of the micelle. The complexation is favoured entropically by the release of counterions of low molecular weight. In the presence of salt (e.g., NaCl) in the solution, exchange processes and thus restructuring between individual IPECs can take place, yielding defined equilibrium structures. For the applicability of micellar polymer systems fundamental research on morphological control as well as targeted switching between different morphologies is essential. For this purpose, a multi-component polymer model system is introduced. In addition to a central IPEC, which exhibits salt-dependent micellar dynamics, the system includes a water-soluble component as well as an “intelligent” polymer segment, which sensitively reacts to an increase in temperature over a critical transition temperature (∼ 32 °C) by partly dehydrating and aggregating. The positively charged polyelectrolyte is part of a star-shaped polymer containing a hydrophilic poly(ethylene oxide) (PEO) arm, whereas the negatively charged polyion is part of a diblock copolymer and covalently linked to the temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) block. The diblock copolymer is expected to form spherical micelles above the transition temperature with aggregated PNIPAM in the core and a negatively charged corona. This micelle serves as a template for complexation with the star polymer. By means of scattering experiments and imaging procedures, size and structure of the formed micelles can be determined. Complexation at moderate salt concentrations leads to the formation of equilibrium structures. The micelles obtained below and above the transition temperature differ in their size and shape and can be reversibly converted into each other. Complexation at low temperatures (20 °C) results in small spherical micelles with an IPEC core and a water-soluble mixed corona. At 60 °C, a spherical core-shell-corona micelle (PNIPAM-IPEC-PEO) is formed by the templating effect, which over time is transformed into worm-like micelles as near equilibrium structure. Experiments on the kinetics of complexation have shown that it can hardly be resolved by means of stopped-flow measurements because of its rapidity. However, the investigations demonstrate a clear difference in the formation of micelles below and above the transition temperature. While the equilibrium structure at 20 °C is reached after a few milliseconds, for the complexation at 60 °C multiple distinct intermediate structures and a two-step process are observed. The low molecular weight salt is responsible for such restructuring processes toward the equilibrium structure. If the salt is subsequently removed from the solution or the salt concentration is significantly reduced, rearrangements are prevented and access to kinetically trapped morphologies is thus enabled. This principle is used to generate different structures apart from the equilibrium at the same final conditions. The worm-like micelles can be frozen at 60 °C and thus also preserved at 20 °C. The reduction of the salt content at 20 °C in turn leads to the freezing of small spherical-like micelles. This process of transforming equilibrium to non-equilibrium structures is reversible

Thermoresponsive Segments Retard the Formation of Equilibrium Micellar Interpolyelectrolyte Complexes by Detouring to Various Intermediate Structures

The kinetics of interpolyelectrolyte complexation involving architecturally complex (star-like) polymeric components is addressed. Specifically, the spontaneous coupling of branched cationic star-shaped miktoarm polymers, i.e., quaternized poly­(ethylene oxide)₁₁₄-(poly­(2-(dimethylamino)­ethyl methacrylate)₁₇)₄ (PEO₁₁₄-(<i>q</i>PDMAEMA₁₇)₄), and temperature-sensitive linear anionic diblock copolymers poly­(vinyl sulfonate)₃₁-<i>b</i>-poly­(<i>N</i>-isopropyl­acrylamide)₂₇ (PVS₃₁-<i>b</i>-PNIPAM₂₇) and further rearrangements of the formed complexes were investigated by means of stopped-flow small-angle X-ray scattering (SAXS). Colloidally stable micelles were obtained upon mixing both polymers at a 1:1 charge molar ratio in saline solutions. The description of the time-resolved SAXS data with appropriate form factor models yielded dimensions for each micellar domain and detailed the picture of the time-dependent size changes and restructuring processes. A fast interpolyelectrolyte coupling and structural equilibration were observed when mixing occurs below the lower critical solution temperature (LCST) of PNIPAM, resulting in small spherical-like assemblies with hydrated PNIPAM coronal blocks. Above the LCST, the collapsed PNIPAM decelerates equilibration, though temperature as such is expected to boost the kinetics of complex formation: after a fast initial interpolyelectrolyte coupling, different nonequilibrium structures of spherical and worm-like shape are observed on different time scales. This study illustrates how a thermoresponsive component can modulate the influence of temperature on kinetics, particularly for rearrangement processes toward equilibrium structures during interpolyelectrolyte complexation

Self-Templated Generation of Triggerable and Restorable Nonequilibrium Micelles

Conditional variations can lead to micellar transformations resulting in various (equilibrium) morphologies. However, creating differently shaped assemblies under the same final conditions (same ingredients, composition, temperature, etc.) is challenging. We present a thermoresponsive polyelectrolyte system allowing a pathway-dependent preparation of kinetically stable spherical star-like or cylindrical micelles. In more detail, a temperature-induced structure switch is used to generate equilibrated interpolyelectrolyte complex (IPEC) micelles of different morphologies (templates) below and above the lower critical solution temperature in the presence of plasticizer (salt). Then, lowering the salt concentration at a specific temperature kinetically freezes the formed IPECs, keeping the respective microstructural information encoded in the frozen IPEC also at other temperatures. Hence, different nonequilibrium morphologies at the same final conditions are provided. The salt-triggered transition from nonequilibrium to equilibrium micelles can be repeated for the same sample, highlighting a system with an on-demand changeable and restorable structure