Influence of multiple stimuli on the lower critical solution temperature of new cationic poly(N‐acryloyl‐N′‐ethylpiperazine‐co‐N‐isopropylacrylamide) solutions

Abstract

New multi-stimuli responsive cationic copolymers based on N-acryloyl-N 0 -ethyl piperazine (AcrNEP) and N-isopropylacrylamide (NIPAM) were prepared by thermal free-radical solution polymerization in dioxane at 75 C. The chemical composition of the copolymers was determined by 1 H NMR spectroscopy and was found that the copolymers were slightly rich in NIPAM content than that of AcrNEP. The reactivity of the two monomers for the copolymerization reaction was evaluated by the extended Kelen-T€ ud€ os method. The distribution of monomer sequence in the copolymer chain was estimated using the terminal copolymerization model. The maximum tendency to alternation ( 70%) was at 60 mol % of AcrNEP in the monomer feed. The copolymers were readily soluble in water at room temperature at all compositions and exhibited well-defined lower critical solution temperature (LCST) phenomenon. The influence of various stimuli such as pH, temperature, simple inorganic salts, and surfactants on the LCST of the copolymers was studied in detail. Simple inorganic salts such as sodium chloride, sodium bromide, and sodium sulfate showed a salting-out effect while sodium iodide showed a salting-in effect. The salting-out coefficient of the salts were calculated using the Sestchenow method, and the salting trend followed the order SO 4 22 > Cl 2 > Br 2 > I 2 . The divalent salt was more effective in lowering the LCST than the monovalent salts. The cationic surfactant hexadecyl trimethylammonium bromide at concentrations above the critical micelle concentration caused a gradual increase in the LCST of the copolymer solutions. The intrinsic viscosity and light scattering behavior of the copolymers in water and in sodium chloride solutions were studied in detail.

A NIPAM–Zwitterion Copolymer: Rheological Interpretation of the Specific Ion Effect on the LCST

“…Previous reports have shown that some salts or metal ions, the chaotropic agent, increase LCST (“salting‐in” effect), whereas others salts, the kosmotropic ones, decrease LCST (“salting‐out” effect) . The thermosensitivity of the copolymer is derived from the hydrophilic/hydrophobic interaction related to the copolymer chains and water molecules .…”

Thermothickening Behavior of Graft Copolymers Containing Poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) Side Chains in Aqueous Solution

“…Secondly, it becomes obvious that T ass is shifted towards the lower temperature with increasing NaCl concentration and this trend originates from the salting-out effect of NaCl on the grafted side chains, which has been widely reported for other LCST polymers (30)(31)(32)(33)(34). As a matter of fact, the addition of NaCl reduces the quality of the solvent for the grafted side chains and consequently the hydrophobic association occurs at a lower temperature compared to pure water (17).…”

Thermogelling Copolymers for Medical Applications

“…The rapid coil-to-globule transition of PNIPAM chain in aqueous solution gives rise to the lower critical solution temperature (LCST) at around 32 C. 58 The main mechanism of the aqueous phase separation of PNIPAM is the thermally induced release of water molecules bound to the polymer isopropyl side groups, which results in increasing intra-and intermolecular hydrophobic interactions between the isopropyl groups above its LCST. The LCST can also be easily adjusted by copolymerizing of NIPAM with other functional segments through the control of di®erent hydrophilic/hydrophobic balance.…”