Yajnaseni Biswas

Tunable doubly responsive UCST-type phosphonium poly(ionic liquid): a thermosensitive dispersant for carbon nanotubes

A phosphonium poly(ionic liquid) (PIL), poly(triphenyl-4-vinylbenzylphosphonium chloride) (P[VBTP][Cl]), of varying and controllable molecular weights is synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization to afford a doubly responsive PIL that responds to both halide ions and temperature. The addition of halide ions transforms the transparent aqueous PIL solution into a turbid two-phase solution, forming insoluble microgel aggregates owing to the screening of the positively charged phosphonium groups of PILs and eventually forms intra- and/or inter-chain cross-linking among the PIL chains through halide ion bridges. Further, this turbid solution exhibits a distinct upper critical solution temperature (UCST)-type phase transition and transforms into a one-phase transparent solution due to the disruption of ion bridges upon heating. The rate of aggregation of P[VBTP][Cl] increases sharply with an increase of the size of the added halide ions in this order I− > Br− > Cl−. The cloud point temperature (Tcp) increases linearly with increasing halide ion concentration. The Tcp also increases with increasing molecular weights of the PIL. The phase diagram of aqueous PIL solution shows the highest Tcp at 6 wt%. Interestingly, the Tcp of the P[VBTP][Cl] in water decreases sharply with addition of small but increasing amounts of organic cosolvents. This PIL exhibits very good stabilizing ability for carbon nanotubes in water, whose dispersion state can be switched from dispersed to agglomerate and vice versa by adding halide ions and increasing the temperature respectively. The cross-linked hydrogel of P[VBTP][Cl] also shows dual responsiveness towards both halide ions and temperature.

Methionine-based cationic polypeptide/polypeptide block copolymer with triple-stimuli responsiveness: DNA polyplexation and phototriggered release

This study demonstrates the synthesis of positively charged 2-nitrobenzyl functionalized polymethionine (P[MetNB][Br]) and its block copolymer of pseudopeptidic poly(2-ethyl-2-oxazoline) (PEtOx-b-P[MetNB][Br]) by firstly employing ring-opening polymerization (ROP), followed by post-functionalization through nucleophilic substitution. The as-prepared cationic P[MetNB][Br] and PEtOx-b-P[MetNB][Br] are readily soluble in water, exhibiting responsiveness towards multiple stimuli such as ions, temperature and light. The aqueous solutions of both of the cationic polypeptides become cloudy in the presence of anions like I−, ClO4− and SCN− due to the formation of insoluble anionically cross-linked colloidal aggregates of cationic polypeptides. These cloudy solutions of the polypeptide and polypeptide block copolymer both show reversible upper critical solution temperature (UCST)-type phase transitions due to the dissolution of the aggregates upon heating. The parameters, polypeptide concentrations and the nature of the anion and its concentration affect the cloud points in aqueous solutions. Upon irradiation with UV light (λ = 350 nm), the pendent nitrobenzyl sulfonium moiety of both of the cationic polypeptides undergoes photocleavage to a neutral polypeptide/polypeptide block copolymer. This study further demonstrates the binding of a cationic polypeptide/polypeptide block copolymer with calf thymus DNA (ctDNA) through electrostatic complexation and its phototriggered release as monitored by fluorescence spectroscopy and gel electrophoresis.

Responsive Polymer Nanostructures

Here in this review, we describe recent advances and challenges toward the design and development of stimuli-responsive polymeric materials that are self-assembled to from nanostructured building blocks. A short introduction describing non-materials in general along with the different synthetic methodologies for making polymeric materials that are responsive toward several physical, chemical, or biochemical stimuli such as temperature, light, pH, redox reaction, ionic strength, glucose, CO2, enzyme. A general discussion on the cause of thermoresponsivenes in polymers having either lower critical solution temperature (LCST) or upper critical solution temperature (USCT) transition is described. This followed by the detailed description of the nanostructured polymer materials, which are responsive to temperature. pH-responsive polymers are important materials in terms of their diverse range of applications, such as drug delivery, diagnostics. Thus, we describe the details of synthesis of different polymer and copolymer systems and their pH-responsive assembly into several types of nanostructures such as micelles, vesicles followed by their pH-triggered disassembly in aqueous solution. The polymeric nanomaterials responsive to light in particular has attracted much attention since light can be localized in time and space and can be applied from outside of the system. Therefore, it is indeed important to discuss the nanostructured polymer materials that are responsive to light followed by their potential applications. Polymeric nanomaterials that are responsive to other different stimuli such as redox reaction, CO2, sugar are also described in this review. Finally, we also describe different multi-stimuli-responsive nanostructured polymer systems. The main goal of this chapter is to serve as a guideline to inspire future researchers toward the design and synthesis of stimuli-responsive materials so that novel applications and new generations of smart materials can be realized.

Poly[oligo(2-ethyl-2-oxazoline)acrylate]-Based Poly(ionic liquid) Random Copolymers with Coexistent and Tunable Lower Critical Solution Temperature- and Upper Critical Solution Temperature-Type Phase Transitions

The synthesis of a series of dual thermosensitive nonionic-ionic random copolymers with varying compositions by reversible addition-fragmentation chain transfer polymerization is described. These copolymers contain oligo(2-ethyl-2-oxazoline)acrylate (OEtOxA) and either triphenyl-4-vinylbenzylphosphonium chloride ([VBTP][Cl]) or 3- n-butyl-1-vinylimidazolium bromide ([VBuIm][Br]) ionic liquid (IL) units. The copolymers having low content of ionic poly(ionic liquid) (PIL) (P[VBTP][Cl]/P[VBuIm][Br]) segments show only lower critical solution temperature (LCST)-type phase transition with almost linear increase of their cloud points with increasing percentage of ionic PIL segments. Furthermore, LCST-type cloud points ( TcLs) are found very sensitive and tunable with respect to the nature and concentration of halide ions (X- = Cl-, Br-, and I-) and copolymer compositions. However, copolymers with high content of ionic PIL segments show both LCST-type followed by upper critical solution temperature (UCST)-type phase transitions in the presence of halide ions. Dual LCST- and UCST-type phase behaviors are prominent and repeatable for many heating/cooling cycles. Both types of cloud points are found to be sensitive to copolymer compositions, concentration, and nature and concentration of the halide ions. The phase behaviors of both types of copolymers with a very high ionic content (>90%) are exactly similar to that of P[VBTP][Cl] or P[VBuIm][Br] homopolymers showing only UCST-type phase transition in the presence of halide ions. The inherent biocompatibility of the P(OEtOxA) segment along with the interesting dual thermoresponsiveness makes these copolymers highly suitable candidates for biomedical applications including drug delivery.

Chemical Tuning of Zwitterionic Ionic Liquids for Variable Thermophysical Behaviours, Nanostructured Aggregates and Dual-Stimuli Responsiveness

The design and synthesis of a series of zwitterionic ionic liquids (ZILs) to understand the structure-property relationship towards an increase of the thermal stability, a variation of the glass transition temperature, the shape-tuning of nanostructured aggregates and the tuning of the stimuli responsiveness are demonstrated. The substitution reaction of imidazole with various aliphatic and aromatic bromides followed by the reaction of the corresponding substituted imidazoles with bromoalkyl carboxylic acids of varying spacer length produces the ZILs. In aqueous solution, a ZIL molecule either exist in its ionic liquid (substituted imidazolium bromide) form or its zwitterionic (substituted imidazolium alkyl carboxylate) form with an isoelectric point (pI) depending on the pH value of the solution. Upon changing the pH to near or above the pI, the aqueous ZIL solution undergoes transition from a transparent to a turbid phase due to the formation of insoluble hierarchical nanostructured aggregates of various morphologies, such as spheres, tripods, tetrapods, fern-like, flower-like, dendrites etc. depending on the pH of the solution and the nature of the alkyl/vinyl/aryl substituents. Upon heating the solution a phase transition occurs from turbid to transparent, exhibiting a distinct reversible upper critical solution temperature (UCST)-type cloud point (Tcp ). It is observed that the cloud point varies with the nature of the substituent, an increase of the concentration of the ZIL as well as with changes of the pH of the solution.