Article pubs.acs.org/Macromolecules
Dynamics of Hybrid Polyacrylamide Hydrogels Containing Silica Nanoparticles Studied by Dynamic Light Scattering Séverine Rose, Alba Marcellan, Dominique Hourdet, Costantino Creton, and Tetsuharu Narita* Laboratoire SIMM-PPMD, UMR7615, UPMC-ESPCI ParisTech-CNRS, 10 Rue Vauquelin, 75231 Paris Cedex05, France ABSTRACT: Nanohybrid gels based on polyacrylamide (PAAm) and colloidal silica nanoparticles were synthesized by free radical polymerization. The influence of chemical cross-links and radical initiators on the dynamics of the network and the nanoparticles were studied by dynamic light scattering (DLS). In ensemble averaged autocorrelation functions we observed two decay modes in the hybrid gels, a gel mode and a Brownian diffusion mode of the silica nanoparticles particles at a concentration higher than 0.03%. An increase in the chemical cross-linking ratio did not influence the gel mode while the silica nanoparticles diffusion mode was slowed down. At lower silica concentrations, we observed (1) a higher scattered light intensity than for a silica suspension and for a gel without silica, (2) an increased plateau value, and (3) no silica diffusion mode in the autocorrelation function. These results indicate that the hybrid gels were more heterogeneous. The gelation time decreased in the presence of silica at low concentration compared with the corresponding gel without silica, suggesting that the transfer of radicals from persulfate to silica surface occurs and that PAAm chains grow from the surface creating increased heterogeneity in the gels.
1. INTRODUCTION Hydrogels are cross-linked polymer networks highly swollen by water that are used nowadays in many bioapplications such as contact lenses, drug delivery systems, superabsorbent, etc. While hydrogels can be classified as soft solids due to the rubber-like elasticity of the cross-linked network, diffusion of small solute as well as collective diffusion of polymer and water can also occur through hydrogels. Because of the high water content and special heterogeneity of the cross-link, synthetic hydrogels are usually very brittle. The reinforcement of the poor mechanical properties of hydrogels by keeping their high water content is a challenge for the numerous applications. Over the past 10 years, several reinforcement strategies have been developed all over the world, such as the elaboration of ideal networks,1,2 the design of double networks3,4 and the combination of organic and inorganic components in nanocomposite hydrogels. This last strategy, that has been intensively studied by Haraguchi,5−7 aims at mixing permanent and reversible cross-links by introducing inorganic nanoparticles in the polymer network. Our group developed a system based on poly(N-alkylacrylamide) hydrogels with silica nanoparticles.8,9 Reinforcement due to strong but reversible adsorption of polymer chains onto the surface of silica nanoparticles has been demonstrated. The dynamic character of the polymer/silica interactions is believed to be responsible for internal energy dissipation upon deformation and leads to a simultaneous increase in elastic modulus and enhancement in fracture toughness through an increase in strain at break. In these nanocomposite gels, prepared with and without chemical cross-linker such as N,N′-methylenebis(acrylamide), © XXXX American Chemical Society
inorganic particles behave as physical cross-linker. Different mechanisms for forming the unique network structure have been suggested. Haraguchi et al. proposed the formation of “clay-brush particles”.10 As a matter of fact, the interactions between clay surfaces and a mixture of N-isopropylacrylamide monomer, potassium persulfate and a common catalyst (TEMED) lead to the growth of polymer brushes on the exfoliated clay platelets. Abdurrahmanoglu et al. proposed adsorption of N-alkylacrylamide monomers on the clay particle surfaces, leading to depletion-induced particle aggregates.11 What is the dynamic behavior of the polymer and the particles in these nanohybrid systems formed by the proposed mechanisms? The dynamics of these nanocomposite hydrogels have not been well studied. The emerging questions are multiple and concern the dynamics of nanoparticles in hybrid hydrogels. Different kinds of interaction between the polymeric matrix and the nanoparticles can affect the dynamics and have to be precisely investigated. Systematic studies on the characteristic relaxation times of the nanohydrid hydrogels as a function of physicochemical parameters such as chemical cross-linker concentration, physical interaction strength, inorganic particle concentration, etc., have not been reported. Here, we report on the dynamics of hybrid polyacrylamide (PAAm) hydrogels containing colloidal silica particles in order to study separately the effects of chemical cross-links, physical cross-links and radical polymerization on the dynamics. Received: March 25, 2013 Revised: April 27, 2013
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dx.doi.org/10.1021/ma4004874 | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
vertically translated steadily while time-averaging. Thus, g(2) coincides with the ensemble-averaged autocorrelation function, up to a cutoff delay time fixed by the sample rotation speed. The averaging duration was more than 3600 s depending on the slowest relaxation modes found in the autocorrelation functions. Gels samples for DLS measurements were prepared in test tubes (10 mm in diameter), prefiltered solutions were mixed in a test tube. Syringe filters of 0.8 μm were used. Gelation of PAAm samples containing various amounts of silica nanoparticles was also studied by DLS. Gelation time was determined as the time where the system becomes nonergodic, by measuring the scattered light intensity as a function of the elapsed time. 2.4. Microrheology by Diffusing-Wave Spectroscopy. Microrheological measurements based on diffusing-wave spectroscopy (DWS) were performed using a laboratory-made setup. The coherent source was a Spectra-Physics Cyan CDRH laser, operated at λ = 488 nm with an output power of 50 mW. The laser beam was expanded to approximately 1 cm at the sample. The diffused light was collected by an optical fiber placed in the transmission geometry. Gels samples for DWS measurements were prepared in cuvettes for fluorescence spectroscopy (4 mm in path length). Here, 1% of polystyrene microspheres (diameter: 500 nm) used as probes were dispersed in gels. The hybrid gel/microsphere mixtures were stable; no aggregation was observed for all gels tested in this work for more than 6 months. In order to obtain an ensemble-averaged autocorrelation function, a diffuser glass plate was placed in front of the optical fiber and rotated at a constant rate. The procedure to obtain viscoelastic moduli, G′ and G″, from the DWS autocorrelation functions can be found elsewhere.14 The field autocorrelation function and the scattered light intensity IT were measured in the transmission geometry. The transport mean free path of the scattered light, l*, which is the only unknown parameter, is obtained for each sample by comparing with the known value of l* and IT of water used as reference. The mean square displacement of the particles is then calculated from the expression given by Weitz and Pine,15 then G′ and G″ are obtained using the generalized Stokes− Einstein equation.
Different from poly(N-alkylacrylamides) which are known to be able to form hydrogen-bonds with silica surfaces, polyacrylamide shows less interaction (since it is simultaneously hydrogen-bonding donor and acceptor). In fact, the hybrid polyacrylamide hydrogels do not show any reinforcement property when synthesized in the presence of silica nanoparticles.12 Polyacrylamide-clay hydrogels have been studied by Okay and Oppermann.13 Although they observed a large increase in elastic modulus of the gels with increase in the clay concentration, they did not discuss the nature of the polymer− clay interactions. Clay platelets are known to strongly interact with salts and especially with potassium persulfate which is used as the initiator of radical polymerization for poly(Nalkylacrylamide) systems in most of the studies. This strong ionic interaction leads to the formation of three-dimensional structures called “house-of-cards”. In this paper we discuss the effects of (1) chemical cross-links on the silica nanoparticle dynamics in the gels and (2) radical polymerization, in PAAm/silica hybrid hydrogels. The originality of our work resides in the study of the dynamics of the polymer network combined to that of the silica nanoparticles. Relatively low silica concentrations were used in order to neglect interparticle interactions. Thus, the silica nanoparticles act as diffusion tracer probing the microenvironment in the PAAm hydrogel, giving information about the consequences of concentration parameters and cross-linking densities on the silica nanoparticles dynamics.
2. EXPERIMENTAL SECTION 2.1. Materials. Acrylamide (AAm, 99%, Sigma-Aldrich), N,N′methylenebis(acrylamide) (MBA, Fluka), and potassium persulfate (KPS, Acros Organics) were used as received without further purification. The silica particles (Ludox TM-50 from Dupont) were purchased from Aldrich, and used as received. The suspension was at a concentration of 52 w% in water, having conductivity of 5 mS/cm. The size and its polydispersity were characterized by dynamic light scattering using an ALV/CGS-3 compact goniometer (ALV, Langen, Germany). The hydrodynamic radius Rh was measured to be 17 nm with the polydispersity index of 0.2 in dilute suspension (
