Cononsolvency of poly-N-isopropyl acryl amide (PNIPAM): Microgels versus linear chains and macrogels Christine Scherzinger a , Annett Schwarz b , André Bardow b , Kai Leonhard b , Walter Richtering a, ⁎ a RWTH Aachen University, Institute of Physical Chemistry, Landoltweg 2, 52074 Aachen, Germany b RWTH Aachen University, Institute of Technical Thermodynamics, Schinkelstrasse 8, 52064 Aachen, Germany abstract article info Article history: Received 26 February 2014 Received in revised form 12 March 2014 Accepted 12 March 2014 Available online xxxx Keywords: Microgels Poly-N-isopropyl acryl amide Cononsolvency Methanol Computer simulation Poly-N-isopropyl acryl amide (PNIPAM) is swollen in both pure water and pure methanol but collapses in mix- tures of these solvents. In this review, this cononsolvency of PNIPAM in water/methanol mixtures is discussed. Experimental studies of linear PNIPAM chains and macrogels are compared to microgels. Theoretical studies are presented based on molecular dynamics simulation and quantum mechanical calculations as well as semi- empirical models. The different explanations for the cononsolvency available in the literature are introduced. Ex- periments show that all PNIPAM species collapse and re-swell at comparable methanol fraction in the mixture. Cross-linker density of macrogels and microgels has only slight influence on cononsolvency, whereas chain length of linear chains has a significant influence. Microgels provide advantages to study cononsolvency by en'abling a broader experimental approach. Furthermore, multi-sensitive microgels can be prepared, which con- tain compartments sensitive to different stimuli. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Stimuli-sensitive polymers are an important class of materials. Espe- cially water-based systems are of great interest for various applications (e.g. in catalysis, sensors, enzyme encapsulation, drug delivery) [1,2]. Aqueous polymer solutions often show phase separation in response to changes of temperature or pH [3–5]. Linear polymers precipitate, whereas macroscopic gels and microgels shrink and expel the solvent; this transition is usually referred to as volume phase transition [1]. The most prominent example of temperature sensitive polymers in aqueous solution is poly-N-isopropyl acryl amide (PNIPAM) with a transition temperature of ~32 °C [1,4–7]. Thus, water is a good solvent below 32 °C and a poor solvent at higher temperatures. The phase tran- sition of PNIPAM (linear chains, gels and microgels) in water was exten- sively studied experimentally and theoretically throughout the years. The volume phase transition is entropy-driven and hydrogen bonding plays an important role [1,5,7–15]. The phase transition temperature depends on molecular weight [16–18 •• ], polymer concentration [19], tacticity [20–22] as well as on the topology (cyclic, linear or branched) [23,24]. Also the pressure de- pendence was studied [25–27]. Recently, microgels have been investigated increasingly [4,6,8,14]. This is because microgels can be prepared with a very narrow size dis- tribution and they can be colloidally stable even above the volume phase transition temperature (VPTT) [8]. Furthermore, more complex architectures such as e.g. core–shell particles can be prepared [28,29]. 1.1. The cononsolvency effect Some polymers are not only sensitive to temperature or pH but also to the composition of solvent mixtures. In special cases, a mixture of two good solvents for a polymer can be a bad solvent for the same polymer [30]. This effect is named cononsolvency [31,32 • ]. For example, methanol (MeOH) and water are both good solvents for PNIPAM. But certain mixtures of both are unfavourable. A PNIPAM microgel particle collapses upon the addition of MeOH to an aqueous dispersion at room temperature [33]. Linear polymers precipitate from solution [31] and macrogels [15] also collapse. Adding further MeOH leads to re-swelling of microgels and macrogels respectively and re- solvation of linear chains [33]. The cononsolvency behaviour of linear polymers [24,31,34] gels [15,35 • ] and microgels [33] received some attention in recent years. Cononsolvency is mainly studied for the case of PNIPAM [15,24,33, 36–39 • ] but other systems have been reported as well [40–43]. Linear poly-N,N-diethyl acryl amide (PDEAAM) [44,45 • ] and copolymers of PNIPAM and a second monomer [45 • ] were studied regarding cononsol- vency. Cononsolvency of PNIPAM was found in water/MeOH [31,46–49] mixtures as well as in other water/alcohol [33,50–53] mixtures and in water/organic solvent mixtures [54–57]. Cononsolvency of a PNIPAM microgel in water/MeOH is schematically depicted in Fig. 1. Current Opinion in Colloid & Interface Science xxx (2014) xxx–xxx COCIS-00900; No of Pages 11 ⁎ Corresponding author. http://dx.doi.org/10.1016/j.cocis.2014.03.011 1359-0294/© 2014 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Current Opinion in Colloid & Interface Science journal homepage: www.elsevier.com/locate/cocis Please cite this article as: Scherzinger C, et al, Cononsolvency of poly-N-isopropyl acryl amide (PNIPAM): Microgels versus linear chains and macrogels, Curr Opin Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cocis.2014.03.011