Nonequilibrium Colloids: Temperature-Induced Bouquet Formation of Flower-like Micelles as a Time-Domain-Shifting Macromolecular Heat Alert

Climate change requires enhanced autonomous temperature monitoring during logistics/transport. A cheap approach comprises the use of temperature-sensitive copolymers that undergo temperature-induced irreversible coagulation. The synthesis/characterization of pentablock copolymers (PBCP) starting from poloxamer PEO130-b-PPO44-b-PEO130 (poly(ethylene oxide)130-b-poly(propylene oxide)44-b-poly(ethylene oxide)130) and adding two terminal qPDMAEMA85 (quaternized poly[(2-dimethylamino)ethyl methacrylate]85) blocks is presented. Mixing of PBCP solutions with hexacyanoferrate(III)/ferricyanide solutions leads to a reduction of the decane/water interfacial tension accompanied by a co/self-assembly toward flower-like micelles in cold water because of the formation of an insoluble/hydrophobic qPDMAEMA/ferricyanide complex. In cold water, the PEO/PPO blocks provide colloidal stability over months. In hot water, the temperature-responsive PPO block is dehydrated, leading to a pronounced temperature dependence of the oil–water interfacial tension. In solution, the sticky PPO segments exposed at the micellar corona cause a colloidal clustering above a certain threshold temperature, which follows Smoluchowski-type kinetics. This coagulation remains for months even after cooling, indicating the presence of a kinetically trapped nonequilibrium state for at least one of the observed micellar structures. Therefore, the system memorizes a previous suffering of heat. This phenomenon is linked to an exchange of qPDMAEMA-blocks bridging the micellar cores after PPO-induced clustering. The addition of ferrous ions hampers the exchange, leading to the reversible coagulation of Prussian blue loaded micelles. Hence, the Fe2+ addition causes a shift from history monitoring to the sensing of the present temperature. Presumably, the system can be adapted for different temperatures in order to monitor transport and storage in a simple way. Hence, these polymeric “flowers” could contribute to preventing waste and sustaining the quality of goods (e.g., food) by temperature-induced bouquet formation, where an irreversible exchange of “tentacles” between the flowers stabilizes the bouquet at other temperatures as well.


Synthesis:
The synthesis of PDMAEMA-b-PEO-b-PPO-b-PEO-b-PDMAEMA pentablock copolymers was performed according to the instructions of Steinschulte et al. 1 for the synthesis of PEO-b-PDMAEMA diblock copolymers and Bütün et al. 2 for the selective quaternization of poly(dialkyl amino)ethyl methacrylate blocks (Figure S1).
The product was analyzed by 1 H-NMR spectroscopy and SEC.
After purification and freeze drying, the polymer (0.68 g; yield 33 %) was characterized by 1 H-NMR spectroscopy and SEC.According to 1 H-NMR of the non-purified product solution (comparison of the integrals of signals of PDMAEMA and monomeric DMAEMA), the conversion was 40 % (implying a theoretical PDMAEMA block degree of polymerization as 85), the corresponding number average degree of polymerization of each PDMAEMA block is 77 (determined by NMR endgroup analysis taking the PEO signal as reference; Fig. S2), and the resulting total number average molar mass is Mn=41 Kg mol -1 .The SEC curve shows an apparent molecular mass of Mn=36 Kg mol -1 (Ð=1.26), and this value seems reasonable in the light of the used PEO calibration.The shift of the molecular mass between the macroinitiator and PBCP is clearly observable (see Figure S3), which confirms the success of the block polymerization.The yield of the reaction was 0.68 g (33 %).
For the methylation of the PDMAEMA blocks to obtain the quaternized pentablock copolymer qPDMAEMA-b-PEO-b-PPO-b-PEO-b-qPDMAEMA (PBCP), the polymer was dissolved in tetrahydrofuran (THF) and 1.5 molar equivalents (regarding the number of amino groups) of methyl iodide were added under stirring at room temperature.After 15 min, a precipitate developed.The reaction continued for 24 h before it was diluted by water, dialyzed versus aqueous KCl solution (1 molꞏL -1 ) for two days exchanging the solvent three times and another two days versus MilliQ water before the colorless solution was freeze dried over two days.The product was characterized by 1 H-NMR spectroscopy in D2O.Under the assumption of a full conversion to the trimethyl ammonium chloride group, the number average molar mass is expected be at 49.8 Kgꞏmol -1 (Yield: 0.42 g, 60 %).Structural Elucidation -Small angle X-ray scattering: Small Angle X-Ray Scattering (SAXS) data was recorded to trace the solution behavior before and after the increase of temperature. 3nce, small angle X-ray scattering curves of a PBCP/ K3[Fe(CN)6]/ KCl solution at 25 °C and at 35 °C (below and above the threshold temperature of 27 °C) are depicted in Figure 5.The scattering intensity I(q) was fitted by different models of the type  q  q  q background, Equation S 1 where P(q) (= F(q) 2 ) is the form factor and S(q) is the structure factor.
At 25 °C, a form factor for core-shell particles 4 (see Equation S 2) was used under the assumption of flower-like micelles consisting of a qPDMAEMA/ferricyanide core and a PEO/PPO shell.
where VS is the volume of the whole particle, VC the volume of the core, rS the radius of the particle, rC the radius of the core, and ρC, ρS, and ρsolv the scattering length densities of the core, the shell and the solvent.
Additionally, a sticky-hard-sphere structure factor 5 was used for the interaction of the PPO blocks.
The fit parameters are listed in Table S1.Since scattering length densities of core and shell are not known, they were initially fitted together with the scale factor and the background and fixed at the obtained values (ρC = 14.121ꞏ10 -6 nm -2 , ρS = 9.59ꞏ10 -6 nm -2 , fixed ρsolv = 9.452ꞏ10 -6 nm -2 ).Since scattering length densities of core and shell are not known, the values of the flower fit above were used also for the bouquet fit.It is to mention that scattering data are distorted by Xray fluorescence at the used wavelength.Hence, the evaluation shows a tendency of the temperature dependent structural development, but are not assumed to be absolutely quantitatively accurate.

Alternative Mechanism of Irreversible Coagulation:
It is known that non-quaternized PDMAEMA and PPO in spatial vicinity (neighboring arms in miktoarm stars) can form complexes. 6,7 This possible complexation might contribute to the noteworthy solution behavior of the pentablock copolymers.As a possible scenario, the originally spherical (flower-like) micelles could generate precipitating networks of worm-like micelles upon backfolding and complexation of the PPO block with the qPDMAEMA core, which would increase the hydrophobic domain being suitable for larger morphology changes.Even under the assumption that qPDMAEMA in presence of ferricyanide ions shows a similar complexation behavior toward PPO, the fraction of complexed PPO should be reduced because of the length of the PEO spacer block between PPO and qPDMAEMA, which separates the two blocks making a complexation less favorable. 8The plausibility of this mechanism was checked by 1 H-NMR spectroscopy in D2O of the PBCP solution with reduced amounts of potassium ferricyanide (0.25 charge equivalents considering the ammonium groups; Fig. S6).Because of the paramagnetism of ferricyanides, the resolution of the spectra is significantly reduced in contrast to the pure spectra without ferricyanide.The fact that the PPO (at 1.2 ppm) and PEO (at 3.7 ppm) peaks are least reduced indicates a spatial distance to the ferricyanide ions located in the immobilized core, which confirms the flower-like structure of the micelles.In contrast, signals of the qPDMAEMA blocks vanish even at small amounts of ferricyanide.The integral ratio between the PEO protons (at 3.7 ppm) and the most prominent PPO protons (at 1.2 ppm) changes only slightly from 8 to 7 after heating, Since hardly any reduction of the intensity of the PPO signal (compared to the PEO signals) was detected after increasing the temperature and cooling back to room temperature, consequently inducing a precipitation of the system, the PPO/qPDMAEMA complexation mechanism seems unlikely to explain the special thermoresponsive behavior of the investigated system.Additionally, the comparison with the water signal (at 4.7 ppm) points to only minor changes of the "NMR-availability" of both PEO and PPO protons before and after heating, despite the resulting limited colloidal stability reflected in a further line broadening.Hence, thermo-induced complexation between PPO and qPDMAEMA does not seem to be a major reason for the irreversible transition.polymer and the ferrous ions.Apparently, a part of polymer is still linked to the inorganic PB material, providing a blue/transparent supernatant.