Smart organic–inorganic nanohybrid stars based on star-shaped poly(acrylic acid) and functional silsesquioxane nanoparticles Manuela Schumacher a , Markus Ruppel a , Joachim Kohlbrecher b , Markus Burkhardt a , Felix Plamper a , Markus Drechsler a , Axel H.E. Mu ¨ ller a, * a Makromolekulare Chemie II and Bayreuther Zentrum fu ¨r Kolloide und Grenzfla ¨chen, Universita ¨t Bayreuth, D-95440 Bayreuth, Germany b Laboratory for Neutron Scattering, ETH Zu ¨rich & Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland article info Article history: Received 25 November 2008 Received in revised form 2 February 2009 Accepted 4 February 2009 Available online 20 February 2009 Keywords: Polyelectrolyte stars Poly(acrylic acid) Hybrids abstract pH- and salinity-responsive organic–inorganic nanohybrid stars based on poly(acrylic acid) (PAA) stars and N,N-di(2,3-dihydroxypropyl)3-aminopropylfunctional silsesquioxane nanoparticles are readily formed by mixing of aqueous solutions of the components. The interaction between stars of two different arm lengths, (PAA 100 ) 21 , (PAA 200 ) 24 , with water-soluble silsesquioxane nanoparticles is studied according to changes in pH and salt concentration. The original size of the stars is conserved during complexation according to dynamic light scattering (DLS) measurements and light scattering (LS) titration experi- ments, which exclude star–star aggregation or crosslinking during the interaction. The proposed inter- action mechanism is based on hydrogen-bonding and Coulomb interactions. Cryogenic transmission electron microscopy measurements demonstrate the formation of nanohybrid stars. Small-angle neutron scattering experiments enable a quantitative determination of the fraction of bound nanoparticles and indicate an equilibrium between free and bound nanoparticles. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Organic–inorganic hybrid materials have found great interest, in particular in the areas of biomaterials, optical and mechanical applications. New materials like those hybrids with unique prop- erties are formed through the combination of organic and inorganic material properties [1–5]. An important class of hybrid materials contains silica or silsesquioxanes as the inorganic component. The organic and inorganic components can be simply mixed, e.g. in nanocomposites [5–14], they can be attached in a covalent way [5,9,10,15–22] or they can form defined complexes [9,10,23–27]. The preparation of polymer–nanoparticle assemblies remains a tedious task. Nanoparticles are commonly not readily miscible with polymers [28,29] due to entropic reasons combined with chain stretching. Only strong enthalpic interactions may overcome the entropic penalty and may promote the mixing of nanoparticles with polymers. One important driving force is Coulombic interaction. Smart materials, i.e. materials that react on external stimuli like pH, salinity, or temperature offer new applications, in particular sensors, membranes, drug delivery, emulsifiers, foam stabilizers, detergents, nanocontainers, catalysis and biohybrid materials [2– 4,30,31]. In particular, weak polyelectrolytes like poly(acrylic acid) (PAA) or poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) have been shown to react by structural and solubility changes on pH, salinity or on the presence of multivalent counterions [3,31– 44]. Complexes of weak polyelectrolytes and inorganic materials offer the chance to develop a new class of smart nano-structured organic–inorganic materials [1,2,32]. In various publications we reported the pH-dependent inter- action of N,N-di(2,3-dihydroxypropyl)3-aminopropylfunctional silsesquioxane nanoparticles (Chart 1) with PAA of various topologies. These nanoparticles possess about 14.2 Si atoms and secondary amino functions on average. They have irregular, cage- like structures [45]. Aqueous solution of the nanoparticles and linear PAA showed a pH-dependent turbidity [24,25]. The strongest turbidity was found between pH 2.5 and 5.7. Similarly, planar PAA brushes grafted onto a gold surface showed the strongest interac- tion at pH ¼ 5.3 [27]. Thus, these nanoparticles penetrate into the PAA brush in the pH range from 5 to 6 but can be liberated at higher or lower pH. Similarly, the interaction of the PAA corona of block copolymer micelles composed of amphiphilic poly(n-butyl acry- late)-block-poly(acrylic acid) showed a dependence on pH and salinity, the strongest interaction being found at pH w 7.5 [26]. We * Corresponding author. Tel.: þ49 921 55 3399; fax: þ49 921 55 3393. E-mail address: axel.mueller@uni-bayreuth.de (A.H.E. Mu ¨ ller). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2009.02.010 Polymer 50 (2009) 1908–1917