Article pubs.acs.org/Macromolecules
Rheology of Poly(N‑isopropylacrylamide)−Clay Nanocomposite Hydrogels Di Xu,† Divya Bhatnagar,†,‡ Dilip Gersappe,† Jonathan C. Sokolov,† Miriam H. Rafailovich,† and Jack Lombardi*,† †
Materials Science and Engineering Department, Stony Brook University, Room 314 Old Engineering, Stony Brook, New York 11794-2275, United States ‡ N.J. Center of Biomaterials, 145 Bevier Road, Piscataway, New Jersey 08554, United States S Supporting Information *
ABSTRACT: We used molecular dynamics simulations and experiments to study the rheology of polymer−clay nanocomposite hydrogels. The molecular dynamics simulations studied the formation of physical cross-linked networks as a function of the clay concentration. Simulations showed that while the local structure changed from isolated polymer−clay clusters to a percolating network with increasing clay concentration the networks were only able to sustain stress at concentrations of roughly 1.5 times the percolation transition. Experiments using poly(N-isopropylacrylamide) (PNIPA)−clay nanocomposites at different clay concentrations were compared with simulation results. Experiments showed a transition from viscous to gel like behavior at a clay concentration of 15.24g/L, in good agreement with simulations. The modulus, G′ and G″, prior to yield was observed to increase exponentially with clay concentration and G′ at yield could be fitted to the modified Mooney’s equation.
I. INTRODUCTION Polyacrylamide (PA) has long been known for its ability to swell in water to make soft clear gels. Since the discovery by Heskins and Guillet in 1968 that PNIPA, a variant of PA, exhibits a lower critical solution temperature attributed to a coil to globule transition,1,2 study of this material has intensified.3−5 Unfortunately it was found to exhibit relatively brittle behavior compared with its PA predecessor, thus limiting its use for biological and industrial applications. Haraguchi and others6,7 later demonstrated that it was possible to initiate free radical polymerization of NIPA from the clay surface, thus making a strong nanocomposite (NC) hydrogel that was thermally responsive within a physiological temperature range. This enabled numerous applications for PNIPA, such as drug delivery systems,8,9 rapid release cell culture substrates,10,11 flocculation additives,12 separation devices,13 and woundhealing dressings.14 The specific applications chosen for the PNIPA−clay gels have also necessitated fine-tuning the mechanical properties, which was found possible via changes in clay concentration6 and solvent quality.4 However, previous studies of polymer− clay gels have focused on regimes above the gelation point (or did not explicitly distinguish gelation threshold). We note that in previous studies the mechanics were mostly tested using strain or compression techniques,1,2,10 which do not provide sufficient rheological information subjected to shear forces. Furthermore, the subtle mechanical changes and structure at the gelation point have not been studied. Such studies will yield valuable information into the formation of these networks and © XXXX American Chemical Society
the factors that control their formation. Our goal in this paper is to use theoretical studies to examine the onset of gelation in PNIPA−clay gels and the mechanisms that control the properties of these gels. We then plan to initiate a series of experimental studies probing the rheology of these gels and also to develop a theoretical framework to characterize the rheological response of these gels. We use both theoretical and experimental methods to understand the fundamental phenomena that govern the properties of PNIPA−clay hydrogels. Molecular dynamics (MD) simulations have been used as a means to analyze the onset of gelation and the mechanics of the gels. These are combined with a set of rheology measurements on PNIPA− clay NCs, where we use a range of clay concentrations, from clay−polymer solutions to very stiff hydrogels. The large amount of data characterizing the PNIPA−clay NCs and the molecular details from the simulations make this system an excellent platform for the development of new NC gels with their rheological response tailored for specific applications.
II. MOLECULAR MODELING In principle, MD model is an ideal tool to study molecular structure and dynamics of NC gels. However, polymer interactions at clay surface have not been well-defined, and the large aspect ratio of clay has made it difficult to compute a Received: October 15, 2014 Revised: December 23, 2014
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DOI: 10.1021/ma502111p Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
system was equilibrated for 5.0 × 104 τ in the NPT ensemble (fixed number of particles, pressure, and temperature) with pressure set as 0.02ε/σ3 and temperature at 0.8ε/kb. A cutoff distance of 1.12σ was used for clay−clay interactions that are purely repulsive, while the cutoff is 2.5σ for other pairwise interactions. Using these parameters simulated a good solvent condition and resulted in fully exfoliated clay platelets and uniformly distributed polymers, as shown in Figure 1; this exfoliated clay states is what is supposed to happen in the our experiments. The ε between stickers and clay particles εfp was then changed to 5.0ε (6.25kbT) to trigger physical bonding. Another 2.5 × 104τ simulation time was executed before the system was switched to the NVT ensemble (fixed number of particle, volume, and temperature). All results shown here were averaged over five initial configurations of the system under NVT condition. The volume fraction of clay platelets was defined as (19n)((4/3)πσ3)/V, where n is the number of clay platelets and V is volume at equilibrium. (For spherical fillers, the volume fraction is simply n((4/3)πσ3)/V, where n is the number of sphere fillers.) It should be noted that the equilibrated volume for the whole system changed only slightly for different simulations; the volume varied
