Hybrid nanoparticles obtained from mixed mesoglobules Barbara Trzebicka a, * , Emi Haladjova b , Lukasz Otulakowski a , Natalia Oleszko a , Wojciech Walach a , Marcin Libera a , Stanislav Rangelov b , Andrzej Dworak a a Centre of Polymer and Carbon Materials, Polish Academy of Sciences, Marie Curie-Sklodowskiej 34, 41-819 Zabrze, Poland b Institute of Polymers, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl. 103A, 1113 Sofia, Bulgaria article info Article history: Received 24 March 2015 Received in revised form 29 April 2015 Accepted 30 April 2015 Available online 11 May 2015 Keywords: Poly(2-isopropyl-2-oxazoline) Poly(N-isopropyl acrylamide) Mixed mesoglobules abstract This article describes the preparation and characterization of mixed mesoglobules from two thermor- esponsive polymers: poly(N-isopropyl acrylamide) (PNIPAM) and poly(2-isopropyl-2-oxazoline) (PIPOX) and their coating with a crosslinked shell to obtain hybrid nanoparticles. The variations in dimensions and stability of the mixed mesoglobules prepared with different compositions (20/80, 50/50 and 80/20 PIPOX-to-PNIPAM weight ratios) and using different heating protocols (gradual and abrupt heating) indicate their sensitivity to both parameters. Unlike neat PIPOX particles as well as PIPOX-rich ones, the mixed particles with higher PNIPAM content (20/80 and 50/50) exhibit a fully reversible coil-to-globule phase transition implying that in these cases PNIPAM effectively prevents the crystallization of PIPOX. The mesoglobules were covered with a cross-linked polymeric shell by seeded radical copolymerization of NIPAM and N,N-methylenbisacrylamide. The resulting hybrid nanoparticles retain their stability and do not disintegrate at low temperatures. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Thermoresponsive polymers have attracted considerable inter- est because of their unique property of undergoing a reversible soluble-to-insoluble state transition in response to small changes in temperature [1,2]. They are water-soluble at low temperatures but start to precipitate upon heating above a certain temperature, referred to as the cloud point temperature (T CP ). Above the phase transition in dilute aqueous solution, thermoresponsive polymers form colloidally stable, well-defined and narrowly distributed spherical particles called mesoglobules [3e6]. The mesoglobules' size ranges from tens to hundreds of nanometres depending on various parameters such as polymer molar mass and its initial concentration, the heating protocol and the presence of additives. Upon cooling below the T CP , the mesoglobules dissolve into indi- vidual macromolecules. The reversible formation and disintegra- tion of mesoglobules is an attractive feature because it can be employed for loading and controlled release of active molecules [7e9]. The most widely investigated thermoresponsive polymer is poly(N-isopropyl acrylamide) (PNIPAM), which exhibits a phase transition at approximately human body temperature [10,11]. Once formed, PNIPAM mesoglobules neither precipitate nor disintegrate upon dilution. Their periphery is presumably composed of small loops of partially hydrated PNIPAM sequences [12]. Both the heating rate and initial polymer concentration are size-determining factors for the PNIPAM mesoglobules. Dilute solutions, i.e., below the overlapping concentration, are most suitable for the preparation of mesoglobules. Non-equilibrium, rapid heating typically leads to the formation of small aggre- gates because intrachain contractions dominate over interchain associations. In contrast, upon slow and gradual heating, the chains associate with each other before their collapse, which fa- vours the formation of larger mesoglobules. The rate of remixing of PNIPAM mesoglobules is always less than the rate of demixing [5]. Therefore, hysteresis in the heatingecooling cycles has typi- cally been observed [12]. Poly(2-isopropyl-2-oxazoline) (PIPOX) is structurally similar to PNIPAM and has been considered as its structural isomer [13,14]. PIPOX is a thermoresponsive polymer that exhibits a phase tran- sition at approximately 40  C [15,16]. Unlike PNIPAM, PIPOX shows a sharp and reversible transition devoid of hysteresis unless it crystallizes. The dehydration of PIPOX chains facilitates dipolar interactions between amide groups, leading to the forma- tion of crystalline structures that were first reported by Schlaad et al. [17e20]. These authors found that long-term annealing of * Corresponding author. Tel.: þ48 322716077; fax: þ48 322712969. E-mail address: btrzebicka@cmpw-pan.edu.pl (B. Trzebicka). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer http://dx.doi.org/10.1016/j.polymer.2015.04.085 0032-3861/© 2015 Elsevier Ltd. All rights reserved. Polymer 68 (2015) 65e73