Rheological Study on the Thermoreversible Gelation of Stereo-Controlled Poly(N-Isopropylacrylamide) in an Imidazolium Ionic Liquid

Abstract

The thermoreversible sol-gel transition for an ionic liquid (IL) solution of isotactic-rich poly (N-isopropylacrylamides) (PNIPAMs) is investigated by rheological technique. The meso-diad content of PNIPAMs ranges between 47% and 79%, and molecular weight (Mn) is ~35,000 and ~70,000 g/mol for two series of samples. PNIPAMs are soluble in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide ([BMIM][TFSI]) at high temperatures but undergo a gelation with decreasing temperatures. The transition temperature determined from G’-G” crossover increases with isotacticity, consistent with the previous cloud-point result at the same scanning rate, indicating imide groups along the same side of backbones are prone to be aggregated for formation of a gel. The transition point based on Winter-Chambon criterion is on average higher than that of the G’-G” crossover method and is insensitive to tacticity and molecular weight, since it correlates with percolation of globules rather than the further formation of elastic network (G’ > G”). For the first time, the phase diagram composed of both G’-G” crossover points for gelation and cloud points is established in PNIPAM/IL mixtures. For low-Mn PNIPAMs, the crossover-point line intersects the cloud-point line. Hence, from solution to opaque gel, the sample will experience two different transitional phases, either clear gel or opaque sol. A clear gel is formed due to partial phase separation of isotactic segments that could act as junctions of network. However, when the partial phase separation is not faster than the formation of globules, an opaque sol will be formed. For high-Mn PNIPAMs, crossover points are below cloud points at all concentrations, so their gelation only follows the opaque sol route. Such phase diagram is attributed to the poorer solubility of high-Mn polymers for entropic reasons. The phase diagram composed of Winter-Chambon melting points, crossover points for melting, and clear points is similar with the gelation phase diagram, confirming the mechanism above.

Self-Assembly of Nucleoside-Derived Low-Molecular-Weight Gelators: A Thermodynamics and Kinetics Study on Different Length Scales

“…These values are consistent with the power law behaviors previously observed (Figure 4 bottom panels) and with temperature sweep experiments at different frequencies that are reported in ESI (Figure S13-S17). This concordance between different ways to access the phase transition was previously reported in other works on thermally triggered and ionic gels 43,44 , but as their authors have acknowledged, and for the sake of rigorous definition, the point of G' and G'' drop in values in temperature sweep experiment will be then referred as G'-G'' crossover point.…”

Experimental and numerical simulation study on the flow characteristics and optimization of structural parameters of displacement agent in partial pressure tools

“…However, with the increase of the flow space, the flow state of the solution is diffusion‐like. Due to the elasticity of the molecular chain, it can generate an elastic restoring force after stretching, causing the molecular chain to shrink and the overall structure to become compact, with a macroscopic increase in viscosity . From the viscosity curve, it can be seen that in the periodic shearing process, the shearing effect is greater than the elastic recovery effect, which leads to a viscosity reduction of the polymer solution.…”

“…Due to the elasticity of the molecular chain, it can generate an elastic restoring force after stretching, causing the molecular chain to shrink and the overall structure to become compact, with a macroscopic increase in viscosity. [61][62][63][64][65][66][67][68] From the viscosity curve, it can be seen that in the periodic shearing process, the shearing effect is greater than the elastic recovery effect, which leads to a viscosity reduction of the polymer solution. After the accumulation of the shearing effect of the throttling unit, the viscosity of the polymer solution decreases by 6.69%.…”

Biocompatible thermoresponsive N-isopropyl-N-(3-(isopropylamino)-3-oxopropyl)acrylamide-based random copolymer: synthesis and studies of its composition dependent properties and anticancer drug delivery efficiency

“…31 Among all these, PNIPAM is widely studied as a model polymer with interesting results. [25][26][27][28] It exhibits an LCST around body temperature (32 1C) in aqueous solution and shows dependence of its LCST on its concentration, 32 molecular weight, 33 and tacticity. 34 However, PNIPAM has several limitations.…”