Synthesis and characterization of carboxymethylcellulose grafted with thermoresponsive side chains of high LCST: The high temperature and high salinity self-assembly dependence

Abstract

Graft copolymers based on carboxymethylcellulose (CMC) and thermosensitive polyetheramines (ethylene oxide/propylene oxide = 33/10 and 1/9) were prepared in water, at room temperature, by using a carbodiimide and N-hydroxysuccinimide as activators. SLS was applied to obtain M, A and R of CMC and its derivatives. Amide linkages were evidenced by FTIR and grafting percentage was determined by H NMR. TGA demonstrated that copolymers were thermally more stable than their precursors. DLS, UV-vis and rheological measurements revealed that properties were salt- and thermo-responsive and linked to the polysaccharide/polyetheramine ratio and the hydrophobicity of the graft. None of the copolymers showed cloud point temperature (Tcp) in water, but they turned turbid in saline media when heated. Copolymers exhibited thermothickening behaviour at 60 °C (>Tcp) in saline media. Below their Tcp, they showed the ability of keeping constant viscosity or even slight increase it, which was interpreted in terms of intermolecular hydrophobic associations.

HPAM‐g‐PEOPPO: Rheological modifiers in aqueous media of high temperature and high ionic strength

“…The 1 H NMR spectrum of PEOPPO and its chemical structure are shown in Figure (a). The peak at 0.96–0.97 ppm (doublet, 3H) corresponds to the methyl protons adjacent to the terminal amine; the peak at 1.07–1.08 ppm (doublet, 6H) corresponds to other methyl protons of PO units; 2.97–3.08 ppm (multiplet, 1H) corresponds to CH adjacent to nitrogen; 3.17–3.80 ppm (overlapped, 100H) corresponds to the hydrogen of the CHCH 2  groups of the propylene oxide and CH 2 CH 2  of the ethylene oxide, and the peak at 3.30 ppm (singlet, 3H) can be attributed to the hydrogens of the methoxy unit (OCH 3 ) . The integrations are equivalent to an EO/PO molar ratio of 19/3, with a corresponding molar mass of 1130 g.mol −1 .…”

“…The peak at 2868 cm −1 can be attributed to the stretching of CH bonds. The large band at 1097 cm −1 can be ascribed to the stretching vibration of the ether CO bonds.…”

“…It was an expected behavior for HPAM, because it is not a thermoresponsive macromolecule . The constant transmittance for the two copolymers can be attributed to the high hydrophilicity ratio of PEOPPO (EO = 19 and OP = 3), combined to the presence of negative charges on the backbone (unreacted carboxylate groups), which contributes to increase in the hydrophilicity of the copolymer and extends the macromolecules, inhibiting or hampering intermolecular associations between the grafts, as also observed in the literature for CMC‐ g ‐PEOPPO in water . However, the solutions of the copolymers had a higher turbidity (lower transmittance) than the HPAM solution.…”

“…The synthesis of the graft copolymers was performed by a coupling reaction between HPAM and PEOPPO, using EDC/NHS as coupling agents, according to conditions given in Table . About 1 g of HPAM was dissolved in 250 mL of distilled water under magnetic stirring for 24 h, at room temperature (~25 °C).…”

“…Then, for example, amino‐terminated PEOPPO can be coupled to carboxylate groups of a polymer backbone by the addition of carbodiimides, to obtain graft copolymers . Literature has extensively shown the use of polysaccharides as the backbone, such as carboxymethylcellulose, sodium alginate, pullulan, dextran, and even carboxymethyl tamarind and carboxymethyl guar . In a few cases, synthetic backbones such as poly(acrylic acid), and HPAM were investigated.…”