Rheological analysis of thermo-responsive alginate/PNIPAAm graft copolymers synthesized by gamma radiation

Abstract

Focused on biomedical applications of thermo-responsive polymers, low-doses of gamma radiation from a 60Co source were applied in a simple one-pot method to synthesize graft copolymers of alginate and poly(N-isopropylacrylamide) (PNIPAAm) with different compositions. The molar percentage of grafted NIPAAm (% molar NIPAAm) was determined by thermogravimetric analysis (TGA) and elemental analysis (EA), being the copolymer structure-property relationship studied in terms of thermo-associative and rheological behavior in aqueous solutions. The addition of more NIPAAm monomer in the initial mixture of reaction, as well as, increasing absorbed dose lead to a greater grafting. However, increasing radiation dose produces copolymers with diminished viscoelastic properties caused by the alginate backbone scission. From rheological curves, two transition temperatures, Ta and Tgel, were determined as a consequence of the thermo-responsiveness of PNIPAAm side chain. Storage (G′) and loss (G″) modulus curves undergo a slope inversion at Ta temperature, where both moduli begin to increase caused by an associative behavior of PNIPAAm domains. While, Tgel temperature is related to the onset of the gelation process at the G′/G″ crossover. In order to design samples with liquid-gel transitions close to the human body, able to form gels in situ once inoculated, it was possible to tailor both transition temperatures selecting copolymers with an appropriate PNIPAAm content and optimizing the copolymer concentration in the aqueous solution. A good agreement between transition temperatures and viscoelastic properties was achieved for 5 wt% aqueous solutions of copolymers with low NIPAAm content synthesized at the lowest absorbed dose (0.5 kGy).