Structural Variation in Homopolymers Bearing Zwitterionic and Ionic Liquid Pendants for Achieving Tunable Multi-Stimuli Responsiveness and Hierarchical Nanoaggregates

Abstract

A series of monomers comprising units bearing both imidazolium bromide ionic liquid (IL) and zwitterionic imidazolium alkyl carboxylate moieties with different alkyl spacer groups are designed and synthesized. RAFT polymerization of these monomers produces a new class of ionic homopolymers, named poly­(zwitterionic ionic liquid)­s (PZILs), which behave like poly­(ionic liquid)­s as well as poly­(zwitterion)­s depending on the solution pH. Such PZILs exhibit an isoelectric point (pI) at pH 5.7, where they exist in their zwitterionic form, making them dual responsive to both pH and temperature. Above pH 5, the aqueous transparent solution of PZIL transforms into turbid suspension due to the formation of insoluble hierarchical nanoaggregates (NAs) of various morphologies such as small spheres, large spheres, flower-like, dendrite-shaped, and dendritic fibril-like depending upon the solution pH and PZILs’ structures. The dissolution of aggregates upon heating and reaggregation upon cooling suggests existence of reversible upper critical solution temperature (UCST)-type phase transition above pH 5. Below pH 5, owing to the presence of cationic IL moieties, aqueous PZIL solution exhibits transparent-to-turbid transition due to the formation of anion-induced NAs of various dendritic morphologies upon addition of various chaotropic anions within the Hofmeister series. Upon heating, this colloidal turbid suspension becomes transparent, showing a distinct UCST-type phase transition, and the process is reversible. It is easily possible to fine-tune the cloud point and morphologies of the NAs by changing various parameters such as molecular weight, concentrations, structure of PZILs, nature and concentration of anions, and solution pH.

Interplay of Solvation and Size Effects Induced by the Counterions in Ionic Block Copolymers on the Basis of Hofmeister Series

“…For example, Clˉ and Brˉ counterions are considered as highly ionized ions for fixed polycations, while PF 6ˉ and bis(trifluoromethane)-sulfonimide (ˉNTf 2 ) are able to form tight ion pairs and turn cationic blocks insoluble in water but soluble in common organic solvents. [37][38][39][40][41][42] On the basis of this factor, the morphological structures of cation-containing polymers could be manipulated by simple anion exchange, which results from the tunable solubility of the ionic blocks. [43][44][45][46][47][48][49][50][51] For example, Vijayakrishna and co-workers found that the morphology of aggregates could be altered by exchanging of ˉN(SO 2 CF 3 ) 2 for Brˉ in imidazoliumbased ionic blocks.…”

Counterion‐Mediated Self‐Assembly of Ion‐Containing Block Copolymers on the Basis of the Hofmeister Series

“…[35] In particular, Cl − and Br − counterions are highly ionized in the presence of imidazolium-based ionic blocks, while PF 6 − and bis(trifluoromethane)-sulfonimide ( − NTf 2 ) are able to form tight ion-pairs and turn the ionic blocks insoluble in water. [36][37][38][39][40][41] On the basis of this factor, the self-assembly morphologies of ion-containing polymers could be triggered by simple anion exchange, which is a result of the tunable solubility of the ionic segments. [42][43][44][45][46][47][48][49][50] For example, Vijayakrishna et al found the size and shape of the self-assembled aggregates could be altered by exchanging of Br − counterions of ionic liquid blocks for − N(SO 2 CF 3 ) 2 .…”

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

“…It is, therefore, expectedt hat these ZIL would show some kind of responses toward foreigna nions as many cationic poly(ionic liquids) (PILs) also do due to intra-and/or inter-chain ionic cross-linkingb etween the neighbouring PIL chainst hrough the excessf oreigna nions. [64][65][66] Unfortunately, we did not observea ny turbidity formation upon addition of variousi norganic ions, such as NO 3 À ,C l À ,B r À ,I À ,S CN À ,C lO 4 À and SO 4 2À up to ac oncentration of 5.0 m into the aqueous ZIL solutions( 1wt%). This observation may suggest ap oor stability of the small molecular ZIL aggregates through ionic bridging in the presence of foreign anion;w hereas ionically bridged PIL aggregates in the presence of foreigni ons are stable enough to give turbid suspension in water.F urther,i tw as noticed that upon addition of large hydrophobic ion, such as NTf 2 À (Tf = triflate), the ZIL molecules get coagulated and phase separated out from their aqueous solution and remained precipitated upon heatingu pt oa pproximately 90 8C.…”

“…In this context, it is understood that there should not be any deprotonation of attached carboxylic acids in the acidic pH region, that is, below the pI, thus the ZIL molecules can be considered as ILs containing imidazolium cations and Br − counterions. It is, therefore, expected that these ZIL would show some kind of responses toward foreign anions as many cationic poly(ionic liquids) (PILs) also do due to intra‐ and/or inter‐chain ionic cross‐linking between the neighbouring PIL chains through the excess foreign anions . Unfortunately, we did not observe any turbidity formation upon addition of various inorganic ions, such as NO 3 − , Cl − , Br − , I − , SCN − , ClO 4 − and SO 4 2− up to a concentration of 5.0 m into the aqueous ZIL solutions (1 wt %).…”

“…Preparation of different alkyl/vinyl/aryl imidazoles:I nt he first step, six different substituted imidazoles (Scheme 1) were synthesised by an ucleophilic substitution reaction of imidazole with various alkyl/vinyl/aryl halides by using as light modification of ar eported protocol. [66,67] The synthesis procedure was as follow: six two-neck RB flasks (250 mL) were separately charged with imidazole (16.9 g, 248 mmol), NaHCO 3 (6.52 g, 77.6 mmol) and aw ater/ acetone mixture (124 mL, 1:2, v/v). The mixtures were stirred at 25 8Cf or 1h under continues purging of N 2 for complete dissolution.…”