New thermo-responsive polymer materials based on poly(2-ethyl-2-oxazoline) segments Darinka Christova a , Rumiana Velichkova a , Wouter Loos b , Eric J. Goethals b , Filip Du Prez b, * a Institute of Polymers, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria b Department of Organic Chemistry, Polymer Chemistry Division, Ghent University, Krijgslaan 281 S4-bis, 9000 Gent, Belgium Received 26 September 2002; received in revised form 30 January 2003; accepted 5 February 2003 Abstract First, the solution behavior of poly(2-ethyl-2-oxazoline) (PEtOx) in water has been investigated. The dependence of the cloud points on the molecular weight and concentration indicates a typical Flory – Huggins (Type I) demixing behavior with a lower critical solution temperature (LCST). Secondly, the synthesis and properties of temperature-responsive hydrogels and segmented polymer networks, based on PEtOx bis- macromonomers, are reported. PEtOx hydrogels have been prepared by UV-induced radical polymerization of the corresponding a,v-bis- acrylates. The networks exhibited a continuous shrinkage with increasing temperature. Series of segmented networks with LCST-behavior have been obtained by free radical copolymerization of PEtOx bis-macromonomer with the comonomers 2-hydroxyethyl methacrylate (HEMA), 2-hydroxypropyl acrylate (HPA) or methyl methacrylate (MMA). The LCST-behavior of the materials is controlled by varying the philicity of the comonomer and the fraction of PEtOx in the networks. PHEMA – PEtOx and PHPA – PEtOx hydrogels exhibited a large and reversible swelling – deswelling process, whereas the volume changes in PMMA – PEtOx swollen networks were small and occurred in a broad temperature interval. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Poly(ethyl oxazoline); Thermo-responsive hydrogels; Segmented polymer networks 1. Introduction Polymer materials that respond to external stimuli such as temperature, pH, solvent quality, pressure, light etc. by changing their physical or chemical properties have become a topic of major interest in the last few decades [1–3]. Among them, temperature-responsive polymers were most comprehensively studied due to the possible applications in fields such as drug delivery [4,5], enzyme activity control [6,7] and membrane separation [8]. Three types of so-called lower critical solution temperature (LCST) behavior have been distinguished recently by a phenomenological analysis of the critical miscibility behavior. The critical points of the corresponding polymer solutions in water differ in their dependence on concentration and polymer chain length [9]. Furthermore, it was shown that the swelling behavior of the corresponding cross-linked polymers could be directly related to the type of demixing. While the classical Type I demixing results in a temperature induced continuous swelling–deswelling behavior of the networks, the Type II and Type III demixing results in a discontinuous swelling behavior. Representatives of each class respectively, are poly(N-vinyl caprolactam) (Type I), poly(N-isopropyl acrylamide) (Type II) and poly(methyl vinyl ether) (Type III). PEtOx is an amorphous non-ionic tertiary polyamide, soluble in water as well as in a wide range of organic solvents. It possesses a unique combination of properties, leading to the proposal of several applications [10–12]. Chiu et al. reported that aqueous PEtOx solutions exhibit a cloud point temperature around 62 – 65 8C depending on the molecular weight [12]. In subsequent studies by Lin et al. the influence of the nature of the solvent and added salts on the cloud point temperatures of PEtOx was studied [13]. In a light scattering investigation of water solutions by Chen et al. the second virial coefficient of PEtOx was found to decrease with increasing temperature, indicating LCST behavior due to hydrogen bonding [14]. Only very recently, PEtOx has been used for the first time to design 0032-3861/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0032-3861(03)00139-3 Polymer 44 (2003) 2255–2261 www.elsevier.com/locate/polymer * Corresponding author. Tel.: þ 32-9-264-49-72; fax: þ 32-9-264-45-03. E-mail address: filip.duprez@rug.ac.be (F.D. Prez).