Rheol Acta (2010) 49:315–321 DOI 10.1007/s00397-009-0425-1 ORIGINAL CONTRIBUTION Thermo-thickening during melting of dibenzylidene sorbitol fibre networks Martin Kühne · Jonathan Wurth · Christian Friedrich Received: 12 June 2009 / Accepted: 16 December 2009 / Published online: 27 January 2010 © Springer-Verlag 2010 Abstract Gels of dibenzylidene sorbitol (DBS) in poly(propylene oxide) (PPO) show an increase in vis- cosity in a certain temperature range during melting. This so-called thermo-thickening behaviour is quite uncommon. In the few already known cases, the thermo-thickening is caused by simple thermodynamic processes that result in the formation of structures. We show for the first time that, in the case of PPO- DBS gels, experimental parameters like sample volume and heating rate play an important role and indicate non-equilibrium behaviour. It turns out that the pecu- liar thermo-thickening is the result of a reformation of the fibre network structure, which was destroyed earlier due to a strong temperature gradient within the sample. Keywords Rheometry · Dynamic mechanical properties · Thermoreversible gelation · Temperature effect · Rate dependence Introduction A promising route to new and tailor-made polymers is the modification of already existing ones, either by M. Kühne · J. Wurth · C. Friedrich (B ) Freiburger Materialforschungszentrum (FMF) and Institute of Macromolecular Chemistry, Albert-Ludwigs-Universität, Stefan-Meier-Str. 21, 79104 Freiburg, Germany e-mail: chf@fmf.uni-freiburg.de mixing certain polymers, thus creating a blend, or by using additives to make a compound. If the additive self-assembles into fibres, a network structure might be formed, which leads to strong modifications of the polymer properties. Thereby one has to distinguish between covalent in- teractions, which result in chemical networks, and phys- ical interactions (like H-bonds), which result in physical networks. Because of the characteristics of physical interactions such networks are thermoreversible, i.e. they melt above a characteristic temperature and form again during cooling. One example of additives that are able to form phys- ical networks are organogelators. These substances of low molecular weight build up network structures in certain organic liquids and polymers. The formation of the network induces a gelation of the surrounding matrix—hence the name organogelators. Such organogelators can be amino acid or steroid derivates or even two-component systems, for example (Terech and Weiss 1997). Another example of an organogelator is diben- zylidene sorbitol (DBS) (Fig. 1), which was used in this work. DBS can gel organic liquids like p-xylene or ethylene glycol (Yamasaki and Tsutsumi 1994, 1995) and certain polymers like poly(dimethyl silox- ane) or poly(propylene oxide) (PPO) (Ilzhoefer and Spontak 1995; Ilzhoefer et al. 1995; Nunez et al. 1996; Fahrlaender et al. 2000; Mercurio et al. 2001; Mercurio and Spontak 2001). In other polymers like polypropy- lene (PP), DBS and its derivates are used as a nucleat- ing agent (Jin et al. 2007; Dobreva et al. 2008). Gels of DBS in polymers have been investi- gated extensively (Wilder et al. 2002). Concerning the temperature-dependent properties of such gels,