Influence of Migrating Ionic Groups on the Solubility of Polyelectrolytes: Phase Behavior of Ionic Poly(N-isopropylacrylamide) Copolymers in Water

Dual Responsive Regulation of Host-Guest Complexation in Aqueous Media to Control Partial Release of the Host

The regulation of the concentration of a wide range of small molecules is ubiquitous in biological systems because it enables them to adapt to the continuous changes in the environmental conditions. Herein, we report an aqueous synthetic system that provides an orchestrated, temperature and pH controlled regulation of the complexation between the cyclobis(paraquat-p-phenylene) host (BBox) and a 1,5-dialkyloxynaphthalene (DNP) guest attached to a well-defined dual responsive copolymer composed of N-isopropylacrylamide as thermoresponsive monomer and acrylic acid as pH-responsive monomer. Controlled, partial release of the BBox, enabling control over its concentration, is based on the tunable partial collapse of the copolymer. This colored supramolecular assembly is one of the first synthetic systems providing control over the concentration of a small molecule, providing great potential as both T and pH chromic materials and as a basis to develop more complex systems with molecular communication.

Conformation-Dependent Affinity of Thermoresponsive Biodegradable Hydrogels for Multifunctional Ligands: A Differential Scanning Calorimetry Approach

We investigated energetics of binding of multifunctional pyranine ligands to hydrogels of the cross-linked poly(methoxyethylaminophosphazene) (PMOEAP) from data on the thermotropic volume phase transition of the gels by means of high-sensitivity differential scanning calorimetry. Dependences of the transition temperature, enthalpy, and width on the concentration of pyranines were obtained, and the excess transition free energy as a function of the pyranine concentration was calculated. We found that the affinity of the gels for the pyranine ligands increased very significantly upon the gel collapse. The intrinsic binding constants and free energies of binding of the ligands to the gels in the collapsed state were estimated from the DSC data. They revealed a significant increase in the hydrogel affinity for pyranines proportional to the number of anionic groups in the ligand structure. The affinity of the PMOEAP hydrogels for the multifunctional ligands was not affected by an increase in the cross-linking density of the gels and only slightly reduced by physiological salt concentrations.

Architecture, Assembly, and Emerging Applications of Branched Functional Polyelectrolytes and Poly(ionic liquid)s

Branched polyelectrolytes with cylindrical brush, dendritic, hyperbranched, grafted, and star architectures bearing ionizable functional groups possess complex and unique assembly behavior in solution at surfaces and interfaces as compared to their linear counterparts. This review summarizes the recent developments in the introduction of various architectures and understanding of the assembly behavior of branched polyelectrolytes with a focus on functional polyelectrolytes and poly(ionic liquid)s with responsive properties. The branched polyelectrolytes and poly(ionic liquid)s interact electrostatically with small molecules, linear polyelectrolytes, or other branched polyelectrolytes to form assemblies of hybrid nanoparticles, multilayer thin films, responsive microcapsules, and ion-conductive membranes. The branched structures lead to unconventional assemblies and complex hierarchical structures with responsive properties as summarized in this review. Finally, we discuss prospectives for emerging applications of branched polyelectrolytes and poly(ionic liquid)s for energy harvesting and storage, controlled delivery, chemical microreactors, adaptive surfaces, and ion-exchange membranes.

Specific counterion repercussions on the thermal, pH-response, and electrochemical properties of side-chain leucine based chiral polyelectrolytes

Effects of counterions of side chain amino acid based polyelectrolytes (PEs) on the solubility in aqueous medium, pH responsiveness, thermal properties, and ionic conductivities have been appraised. Deprotection of the tert-butyl carbamate (Boc) group from poly(Boc-l-leucine methacryloyloxyethyl ester) [P(Boc-l-Leu-HEMA)] was carried out to produce PE with trifluoroacetate as an associative counteranion (1a). PEs with bis(trifluoromethylsulfonyl)imide and hexafluorophosphate counteranion were prepared through anion exchange reactions of 1a. Protonation of the neutralized polymer (2) obtained from 1a, followed by anion exchange, leads to the production of miscellaneous PEs bearing different counteranions, such as tetrafluoroborate, trifluoromethanesulfonate, chloride, and nitrate. Differential scanning calorimetry traces of the PEs reveal that the comparatively larger and weakly coordinated counteranions require less thermal energy to dissociate, and thus, the glass transition temperature (Tg) of the PEs fall off with an increase in the size of the counteranion. A remarkable conductivity of 2.1 mS/cm was obtained in deionized water when Cl(-) acted as the counteranion. Steric and electronic factors of the counteranion induce a change of transition pH in different PEs, although the chiroptical nature was retained, as confirmed by circular dichroism spectroscopy.

Insight in the phase separation peculiarities of poly(dialkylaminoethyl methacrylate)s

The thermoresponsive and pH-sensitive behavior of poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA), poly(N,N-diethylaminoethyl methacrylate) (PDEAEMA), and poly(N,N-diisopropylaminoethyl methacrylate) (PDiPAEMA) is compared by use of different techniques. We employed temperature- and pH-dependent turbidimetry, fluorescence spectroscopy (of the polarity indicator 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran, 4HP, which is sometimes also abbreviated as DCM), and IR spectroscopy (of the carbonyl band). Within specific pH windows, all polymers showed phase separation at elevated temperatures (showing a lower critical solution temperature behavior, an LCST behavior). By increasing the hydrophobicity of the dialkylaminoethyl substituent, the phase separation is shifted to lower pH (at constant temperatures; pH(PDMAEMA) > pH(PDEAEMA) > pH(PDiPAEMA)) or to lower temperatures (at constant pH; T(PDMAEMA) > T(PDEAEMA) > T(PDiPAEMA)). While PDMAEMA does not exhibit pronounced changes in polarity upon phase separation (as seen by fluorescence spectroscopy), PDEAEMA and PDiPAEMA provide a nonpolar surrounding for the 4HP uptake above their collapse. In addition, PDiPAEMA causes the sharpest transition (as seen by the 4HP probe), although the carbonyl hydration experiences a more gradual (sigmoidal) transition for all polymers (as seen by IR). These observations allow a distinction of the phase separation mechanisms. While the LCST properties of PDMAEMA are mainly caused by backbone/carbonyl interactions, its rather polar dimethylaminoethyl group does not inflict pronounced hydrophobicity, but promotes a higher water content within the phase-separated polymer. In contrast, the phase separation of PDEAEMA and PDiPAEMA is mainly influenced by the less polar dialkylaminoethyl groups, leading to drastic changes in the hydrophobicity around the cloud points. Further, the IR data suggest that the diisopropylaminoethyl groups of PDiPAEMA tend to backfold to the carbonyl groups/backbone to minimize water-polymer contact already in its soluble state. Finally, this study might lead to advanced lasing applications of the laser dye 4HP.

ABC triblock terpolymers exhibiting both temperature- and pH-sensitive micellar aggregation and gelation in aqueous solution

Two poly(ethylene-alt-propylene)-b-poly(ethylene oxide)-b-poly(N-isopropylacrylamide-co-acrylic acid) (PEP-PEO-P(NIPAm-co-AA)) triblock terpolymers were synthesized by a combination of anionic and RAFT polymerizations, followed by acid hydrolysis. Micellar aggregation and gelation behavior in aqueous solutions were studied by dynamic light scattering (DLS) and rheology, respectively. DLS measurements on dilute solutions revealed that the triblock terpolymers form micelles with PEP cores and PEO-P(NIPAm-co-AA) coronae at room temperature and undergo a micelle to micellar aggregate transition upon heating. Rheological measurements showed that micellar aggregation manifests itself as gelation at higher concentrations (~4 wt %). The observed thermoresponsive aggregation and gelation is due to the intermicellar association of P(NIPAm-co-AA) blocks in the coronae above the lower critical solution temperature of the P(NIPAm-co-AA) block. The critical micellar aggregation and gelation temperatures are controlled by the mole fraction and degree of acrylic acid (AA) ionization in the P(NIPAm-co-AA) block, and therefore they can be modulated as functions of both pH and AA content in the polymer.

Phase diagram of the system sodium alginate/water: A model for biofilms

Sodium alginate is a polyelectrolyte consisting of the monomer units beta-D-mannuronate and alpha-L-guluronate. Mainly based on the theory of Khokhlow et al., the state diagram of the binary system alginate/water has been calculated using different sets of parameters like degree of ionization, degree of polymerization and interaction function. The calculations comprise miscibility gaps, liquidus curves, eutectic points and the behaviour at temperatures below the melting point of water. Also gel and swelling curves have been treated, where gels are physically crosslinked. The DSC diagram of a 0.5 by wt.% polymer sol shows a double melting peak, which is explained by a heterogeneity above 0 degrees C. The crystallization of water seems to concentrate the gelled system irreversibly.

Effect of the mobility of charged units on the microphase separation in amphiphilic polyelectrolyte hydrogels

The effect of the migration of charged units on the structure of hydrophobically modified polyelectrolyte gels swollen by D(2)O was studied by small-angle neutron scattering on an example of gels of terpolymers of acrylic acid, n-dodecylacrylate, and 2-acrylamido-2-methyl-1-propanesulfonic acid (quenched charged units) and gels of copolymers of partially neutralized acrylic acid and n-dodecylacrylate (annealed charged units). The content of charged units did not exceed 20 mol %, so that the electrostatic repulsion was too weak to disrupt the hydrophobic domains formed by self-assembled n-dodecyl chains, which was evidenced by NMR data. It was shown that upon increasing the charge content both types of gels undergo microphase separation with the formation of hydrophobic clusters consisting of several densely packed hydrophobic domains and hydrophilic regions swollen by water, where most of the charged repeat units and counterions are located. The dimensions of the nanostructure of the gels with quenched and annealed charged groups were compared. It was shown that the size of clusters in the gels with annealed charged units is much bigger than that in the gels with the same fraction of quenched charged units. This effect was attributed to a much weaker electrostatic repulsion in the corona of the hydrophobic clusters in the gels with annealed charged groups, because the charged units repelling each other are able to move farther apart.