Morphology Evolution of Stimuli-Responsive Triblock Copolymer Modulated by Polyoxometalates

Co-Solutes Induced Changes in the Properties of Polymeric Solution and Water Dynamics

In this paper we are addressing the co-solute-induced changes in the properties of an aqueous solution of a block copolymer. Due to the preferential interaction of different co-solute with different regions of the block copolymer, the changes were observed in both the physical properties and water dynamics. The modulation of both the physical properties and water dynamics was monitored using different spectroscopic techniques. Different co-solutes affect micellar properties of copolymer to a different extent signifying their interactions with different regions within the copolymer. The solvent relaxation dynamics were also modulated with the additions of different co-solutes. The change in free-energy (ΔGbf) and rate constant for bound to free water interconversion (kbf) in a copolymeric micelle was calculated which gets affected by the addition of co-solutes. The calculated kbf suggests that betaine, sarcosine, TMAO, and GnHCl favor the ordering of water molecules around the micelle and are excluded from the micellar surface whereas, urea favors the formation of free-water molecules rather than the structurally ordered bound water molecules around the micelle by accumulating at the micellar surface. Among the added methylamines trimethylamine N-oxide affected the water dynamics and its kinetics most profoundly. The protective property of GnHCl was revealed.

How Kosmotropic and Chaotropic Osmolytes Perturb the Properties of an Aqueous Solution of a Pluronic Block Copolymer?

Poloxamer 407 (P-407) composed of a poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) unit has two distinct microenvironments: the interior core formed by the PPG unit and the exterior shell formed by the PEG unit. In this work, we have used two fluorescent molecules coumarin-153 and 8-anilino-1-naphthalene sulfonic acid (ANS) of contrasting natures to characterize and probe the water dynamics in the core and corona regions of the copolymer by means of spectroscopic techniques, namely, absorption, fluorescence, and time-resolved fluorescence emission spectroscopy and Fourier transform infrared (FTIR) spectroscopy. Changes in the surface morphologies were characterized by using microscopic techniques. Further, two classes of osmolytes kosmotropic (betaine and sarcosine) and chaotropic (urea) known to perturb the water structure were added to aqueous solutions of P-407. Our studies reveal that the addition of kosmotropes decreases the critical micelle temperature (CMT) of the copolymer, whereas the chaotropic osmolyte increases the CMT. Steady-state studies reveal that the addition of the osmolytes to the copolymer increases the polarity of the micelle formed and hence results in the red shift in the ANS absorbance maximum. FTIR spectroscopy reveals that kosmotropes interact with the PEG moiety of the copolymer, whereas the chaotrope interacts with both the PEG and PPG moieties of the copolymer. Solvent relaxation studies produced less changes upon the addition of the kosmotropes, whereas a greater change in the relaxation time was observed in the presence of the chaotrope.

Polyoxometalates as chemically and structurally versatile components in self-assembled materials

Polyoxometalates (POMs) are anionic molecular metal oxides with expansive diversity in terms of their composition, structure, nuclearity and charge. Within this vast collection of compounds are dominant structural motifs (POM platforms), that are amenable to significant chemical tuning with minimal perturbation of the inorganic oxide molecular structure. Consequently, this enables the systematic investigation of these compounds as inorganic additives within materials whereby structure and charge can be tuned independently i.e. [PW12O40]3- vs. [SiW12O40]4- while also investigating the impact of varying the charge balancing cations on self-assembly. The rich surface chemistry of POMs also supports their functionalisation by organic components to yield so-called inorganic-organic hybrids which will be the key focus of this perspective. We will introduce the modifications possible for each POM platform, as well as discussing the range of nanoparticles, microparticles and surfaces that have been developed using both surfactant and polymer building blocks. We will also illustrate important examples of POM-hybrids alongside their potential utility in applications such as imaging, therapeutic delivery and energy storage.

Synthesis, micellar structures and emission mechanisms of an AIE and DDED-featured fluorescent pH- and thermo-meter

A new nanoprobe, the luminescent diblock copolymer PNIPAM(MAh-4)-b-P4VP (PN4P), with pH- and thermo-responsive deprotonation-driven emission decay (DDED) and aggregation-induced emission (AIE) features was designed and synthesized. The nanoprobe PN4P can form micellar structures in water with reversible dual-responsive fluorescence (FL) behavior within a wide pH range of 2–11. The critical solution temperature was found at about 32, 30 and 27 °C as the pH switched from 2, 7 to 11. The critical pH value of the probe was about 4.0, and the micelles showed a core–shell inversion in response to pH and thermal stimuli, accompanied by a desirable emission tunability. P4VP as the micellar shell at pH = 2 was more easily dehydrated with the increase in temperature as compared to PNIPAM as the micellar shell at pH > 4. The strongest dehydration of the P4VP shell would make PN4P the most strongly aggregated and the most AIE-active, which supports the 2.10-fold most distinguished thermal-responsive emission enhancement at pH = 2. Moreover, a dramatic acidochromic redshift of the emission band from 450 (pH > 4) to 490 nm (pH = 2) was observed, and the maximum emission at pH = 2 was enhanced by about 2.07-fold as compared with that at pH = 7. Therefore, the probe displays the desired dual responses and good reversibility. AIE and DDED are the two major mechanisms responsible for the dual-responsive emission change, with AIE playing a more important role than DDED. This work offers a promising approach to interpreting temperature (range from 28 to 40 °C) and pH changes (range from 2 to 7) in water.

A nanoprobe in water features pH- and thermal-responsive micellar/clustering structures, deprotonation-driven emission decay (DDED) and aggregation-induced emission (AIE).

Modulation of thermodynamic and kinetic inverted phase behavior of block copolymers by inorganic polyoxometalates

The Keggin polyoxometalates (POM) H₃PW₁₂O₄₀ (PW) electrostatically complexed with poly(styrene-block-2-vinyl pyridine) (PS-b-P2VP) in DMF, and ordered microphase separation occurred through solvent evaporation. The phase behaviors of PS-b-P2VP/PW in bulk were systematically investigated by using SAXS and TEM to discover the effect of POM content and molecular weight of the block copolymers. Computational simulation was also performed to reveal the same phase transition sequence as the experimental results. As the POM content increases, the PS-b-P2VP/PW complex with a low molecular weight changed from lamellar phase (LAM) to hexagonal cylindrical phase (HEX), and finally transited into spherical phase (SPH). Unexpectedly, PS-b-P2VP/PW complexes with a high molecular weight were inclined to form a kinetic-trapped intermediate phase (inverted HEX). The mechanism of formation of inverted phases was proposed based on simulation that asymmetric swelling in the concentrated DMF solution would result in the ultimate kinetic-trapped nanostructure in the bulk.

Flexible and enhanced multicolor-emitting films co-assembled by lanthanide complexes and a polymerizable surfactant in aqueous solution

Lanthanide complex doped lyotropic liquid crystals (LLCs) are soft materials which are impressive due to their excellent luminescence efficiencies and stabilities. The introduction of lanthanide complexes into polymerizable LLCs, however, may produce organized films with better optical and mechanical properties through in situ photopolymerization. An environmentally friendly strategy to fabricate flexible multicolor-emitting films has been developed through co-assembling red-/green-emitting trisdipicolinate lanthanide complexes [choline]3[Ln(DPA)3] (Ln-DPA, Ln = Eu, Tb) into LLC matrices mainly via electrostatic interaction and further photopolymerization. The LLCs were constructed from a polymerizable surfactant, 3-dodecyl-1-vinylimidazolium bromide (C12VIMBr), in aqueous solution. The maintenance of the well-defined LLC nanostructures in the luminescent films was validated by small-angle X-ray scattering (SAXS) as well as scanning electron microscopy (SEM) measurements. Remarkably, the lifetime and absolute luminescence quantum efficiency of such films have been improved significantly compared with those of the corresponding solid Eu-DPA complex or in aqueous solution and LLC matrices. Through tuning the molar ratio of Eu-DPA to Tb-DPA complexes, the emission color of the films could be finely-tailored between red and green in the CIE chromaticity diagram. Furthermore, the films also possessed certain mechanical strength and stability against pH, metal ions, and temperature, indicating their potential application as robust luminescent materials.