Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol Thermoresponsive behavior of sodium alginate grafted with poly(N- isopropylacrylamide) in aqueous media Oana-Nicoleta Ciocoiu a , Georgios Staikos a, ⁎ , Cornelia Vasile b a Department of Chemical Engineering, University of Patras, GR-26504 Patras, Greece b “P. Poni” Institute of Macromolecular Chemistry, 41A Gr. Ghica Voda Alley, 700487 Iasi, Romania ARTICLE INFO Keywords: Sodium alginate Poly (N-isopropylacrylamide) Graft copolymers Thermothickening Thermoresponsive polymers ABSTRACT Thermoresponsive graft copolymers were synthesized. Thermally induced properties were introduced by grafting poly (N-isopropylacrylamide) (PNIPAM) side chains onto a sodium alginate backbone. The resulting graft copolymers cover a broad range in PNIPAM composition and molecular weight. Remarkable thermo- thickening behavior in water and 0.1 M NaCl is observed by viscometry when the temperature, concentration, weight ratio of PNIPAM side chains to backbone, and PNIPAM molecular weight are higher than certain critical values. This behavior is reversible and could have applications in oil rigs and drug delivery systems. 1. Introduction Thermothickening aqueous formulations, i.e., systems whose visc- osity increases with increasing temperature, have been thoroughly studied during the last two decades. They are mostly based on graft copolymers, prepared by grafting side chains that precipitate upon heating on a hydrophilic backbone. (De Vos, Möller, Visscher, & Mijnlieff 1994; Durand & Hourdet 1999; Hourdet, L’Alloret, & Audebert, 1994; Hourdet, L’Alloret, & Audebert 1997; L’Alloret, Hourdet, & Audebert, 1995). Polymers in solution usually precipitate upon cooling, but some water-soluble polymers precipitate upon heating, showing lower cri- tical solution temperature (LCST) behavior. (Heskins & Guillet 1968; Saeki, Kuwahara, Nakata, & Kaneko, 1976). Such LCST polymers, ex- hibiting inverse solubility behavior, have been used for the synthesis of the above-mentioned thermothickening graft copolymers, and include poly (ethylene oxide), (Hourdet et al., 1994; L’Alloret et al., 1995) ethylene oxide-propylene oxide random copolymers, (De Vos et al., 1994) and poly(N-isopropylacrylamide) (PNIPAM), the most well-stu- died LCST polymer, phase separating in water by increasing tempera- ture at approximately 32 °C. (Heskins & Guillet, 1968; Schild, 1992). The hydrophilic backbone of the thermothickening graft copolymer consists of a water-soluble polymer of relatively high molecular weight, such as poly(acrylic acid), (Durand & Hourdet 1999; Hourdet et al., 1994; Hourdet et al., 1998) partly hydrolyzed polyacrylamide, (De Vos et al., 1994) or 2-acrylamido-2-methyl-propane sulfonic acid. (Hourdet et al., 1997; L’Alloret et al., 1995) In many cases, polysaccharide backbones such as carboxymethylcellulose, (Bokias, Mylonas, Staikos, Bumbu, & Vasile, 2001; Hourdet et al., 1997; Karakasyan, Lack, Brunel, Maingault, & Hourdet, 2008) chitosan, (Bhattarai, Ramay, Gunn, Matsen, & Zhang, 2005) hyaluronan, (Kitazono, Kaneko, Miyoshi, & Miyamoto, 2004) dextran, and sodium alginate (NaAlg) (Karakasyan et al., 2008) have been successfully used. The thermoresponsive behavior of all these graft copolymers, obeys to the following general scheme. As the temperature increases, the side chains tend to precipitate and form aggregates, which function as thermo-responsive stickers interconnecting the hydrophilic backbones of the graft copolymers, finally leading to the formation of a physical reversible network. This thermoresponsive behavior could be exploited when improved rheological properties above a given temperature are required, and its study is of great interest for bioengineering (Rzaev, Dinçer, & Piskin, 2007) and biomedical (Klouda & Mikos, 2008) ap- plications, with drug release (Bhattarai et al., 2005) and tissue en- gineering (Drury & Mooney, 2003) being the most studied. Alginic acid is a biopolymer broadly used in food and beverage, pharmaceutical, and medical industries, (Tønnesen & Karlsen, 2002) and it has attracted a broad interest due to its biocompatibility and biodegradability. Moreover, due to the carboxylic acid unit contained in its repeating unit, it can be functionalized with chemical modifica- tions that could endow it with new properties and uses in novel ap- plications. In a previous study, it was shown that graft copolymers of NaAlg with PNIPAM (Cheaburu, Ciocoiu, Staikos, & Vasile, 2013) present a strong thermothickening effect, which was studied by steady shear, oscillatory shear, and step-strain measurements. After fluores- cence measurements, this behavior was attributed to the formation of hydrophobic microdomains consisting of micro-phase separated https://doi.org/10.1016/j.carbpol.2017.12.059 Received 22 October 2017; Received in revised form 20 December 2017; Accepted 20 December 2017 ⁎ Corresponding author. E-mail addresses: oananicoleta@chemeng.upatras.gr (O.-N. Ciocoiu), staikos@chemeng.upatras.gr (G. Staikos), cvasile@icmpp.ro (C. Vasile). Carbohydrate Polymers 184 (2018) 118–126 Available online 23 December 2017 0144-8617/ © 2017 Elsevier Ltd. All rights reserved. T