Síntese, caracterização e estudos em solução aquosa de copolímeros termoassociativos de poli(N-isopropilacrilamida) enxertada em carboximetilclulose (CMC-g-PNIPAM)

Abstract

This work presents the synthesis, characterization and studies in aqueous media of copolymers based carboxymethylcellulose (CMC) as backbone grafted witth poly (N-isopropylacrylamide). For this purpose, initially amino-terminated PNIPAM (PNIPAM-NH2) was prepared in a glove box, by using the redox pair 2-aminoethanethiol hydrochloride(AET.HCl)/potassium persulfate (KPS) as initiators. The reaction products were characterized by infrared spectroscopy (IR), thermogravimetric analysis, potentiometric titration and rheology at different temperatures. Through the infrared spectra, it was possible to observe the presence of PNIPAM-NH2 characteristic peaks. The thermogravimetric analysis showed that the polymers have different degradation temperatures, indicating different size of the chains prepared with different amounts of AET.HCl. The number of terminal amino groups per gram of PNIPAM was greater for the polymer synthesized with a greater amount of chain transfer agent (AET.HCl). Rheological measurements showed that the smaller the amount of AET.HCl added to the reaction mixture, the greater the solution viscosity and the lower is the temperature needed to polymer precipitation. The CMC used for the copolymerization was characterized, and the infrared spectrum showed characteristic absorption bands. By potentiometric titration, it was possible to determine the average number of carboxymethyl groups per number of CMC, which was equal to 1. Through capillary viscometry was possible to determine the viscosimetric-average molecular weight of the polysaccharide, which was equal to 7.1 x 104 g.mol-1, and the critical concentration, that was equal to 5.95 g.L-1. Rheological measurements for CMC showed that: (i) the higher the CMC concentration, the higher is the viscosity; (ii) the higher the temperature, the lower is the viscosity; and (iii) viscosity is decreased in the presence of salts when compared with water. The copolymers were synthesized by a coupling reaction between CMC and PNIPAM-NH2 with the aid of 1-(3-(dimethylamino)propyl)-3-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) as condensing agents. Infrared spectra of the copolymers exhibited the characteristic bands of CMC and PNIPAM, showing the chemical modification was realized. Thermogravimetric analysis showed that contrary to CMC and PNIPAM-NH2, which had only one DTG peak, the thermal decomposition of the graft copolymers occurred at two steps, being the first DTG peak higher and second one lower than the corresponding precursors..The rheological studies for the copolymers showed that: (i) the copolymers showed thermosensitive behavior; (ii) the viscosity is raised with increasing copolymer concentration; (iii) the viscosity was higher for the copolymer synthesized with the largest chain PNIPAM-NH2; (iv) the only copolymer that exhibited highest viscosity at 60 °C than at 25 °C was the one synthesized with PNIPAM-NH2 of longest chain (CMC-g-PNIPAM 3). This study reflects the importance of the synthesis parameters of amino-terminated PNIPAM on the thermoassociative properties of copolymers of CMC-g-PNIPAM