Shear-induced structure in polymer–clay nanocomposite solutions

https://doi.org/10.1016/j.jcis.2004.03.039Get rights and content

Abstract

The equilibrium structure and shear response of model polymer–clay nanocomposite gels are measured using X-ray scattering, light scattering, optical microscopy, and rheometry. The suspensions form physical gels via the “bridging” of neighboring colloidal clay platelets by the polymer, with reversible adsorption of polymer segments onto the clay surface providing a short-range attractive force. As the flow disrupts this transient network, coupling between composition and stress leads to the formation of a macroscopic domain pattern, while the clay platelets orient with their surface normal parallel to the direction of vorticity. We discuss the shear-induced structure, steady-shear rheology, and oscillatory-shear response of these dynamic networks, and we offer a physical explanation for the mesoscale shear response. In contrast to flow-induced “banding” transitions, no stress plateau is observed in the region where macroscopic phase separation occurs. The observed platelet orientation is different from that reported for polymer–melt clay nanocomposites, which we attribute to effects associated with macroscopic phase separation under shear flow.

Introduction

Polymer–clay nanocomposites have attracted significant interest because they exhibit synergistic behaviors derived from the two disparate components. Unique or enhanced mechanical, electrical, optical or thermal properties have been observed for both bulk and solution nanocomposites [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Such improvements in physical properties with the addition of nanoclay platelets are often influenced by the polymer–clay interaction and the state of dispersion or exfoliation. In general, good adhesion and favorable interactions between polymer and clay are necessary to achieve good exfoliation, which has in turn been found to be largely responsible for property enhancement. Furthermore, incorporating nanoscale platelets into a polymer matrix, as opposed to larger more conventional reinforcing agents, may also increase the light transmittance, allowing the preparation of transparent nanocomposite materials.

Poly(ethylene oxide) (PEO) and the synthetic hectorite clay Laponite (LRD) form viscoelastic solutions in water [12], [13], [14]. In equilibrium, adsorption of PEO segments onto the LRD surface [13] leads to the formation of a transient network or gel, and the diffuse polymer chains serve as a way of “bridging” neighboring clay particles, which feel a short-range electrostatic repulsion [13], [15], [16]. The quiescent or equilibrium structure is thus disordered or miscible in the sense that the suspensions are macroscopically homogeneous with ideal clay dispersion, or model clay exfoliation. Such PEO–LRD solutions are transparent and have good interfacial adhesion between the polymer and clay, and these characteristics are maintained in bulk materials to the extent that transparent nanocomposite gels can be prepared from well-dispersed polymer–clay solutions. From a practical point of view, this suggests the possibility of developing strong yet transparent films, coatings, and membranes with enhanced barrier properties associated with alignment of the clay nanoplatelets.

As noted above, PEO–LRD suspensions can form physical gels via the dynamic adsorption–desorption of polymer segments onto the surface of the platelets, and these interactions greatly affect the viscoelastic properties and shear response of the mixtures, which in turn is relevant to the flow processing of clay nanocomposite melts and solutions. In the presence of polymer, the large aspect ratio of the clay may also lead to supramolecular organizations and structures under shear, reminiscent of other mesoscopic systems such as liquid crystalline polymers, surfactants, or block copolymers. The influence of shear on PEO–LRD solutions has been investigated via small-angle neutron scattering (SANS), from which the shear-induced anisotropy at the nanoscale can be determined [14]. Such measurements suggest that when subjected to sufficient shearing, the clay platelets orient with their surface normal along the vorticity direction. We have also used SANS measurements performed on contrast-matched samples and samples in pure D2O to deduce the shear-induced orientation of the polymer with respect to the clay platelets. With increasing shear rate, the clay colloids orient first, with the polymer chains starting to stretch along the direction of flow at higher shear rates.

The goal of the present study is to further probe macroscopic changes in these solutions under shear using rheometry, light scattering, and optical light microscopy. Simultaneous shear light scattering and optical microscopy allow for a direct comparison of the morphological structure observed in both real and reciprocal space [17]. Here, we focus on the shear response of PEO–LRD mixtures in which the polymer content is fixed at 2% and the amount of clay is varied, as well as the 2% polymer–3% clay solutions of primary interest in our previous study [14]. The combination of SANS, light scattering, optical microscopy and rheometry provides a more complete physical picture of the shear response of these intriguing suspensions, which in turn offers a clearer understanding of the shear-induced mesoscale structures that can arise in response to the highly nonequilibrium conditions encountered during flow processing. Small and wide-angle X-ray scattering are used to fully characterize the equilibrium structure and extent of exfoliation of these physical polymer–clay gels.

Section snippets

Materials

The synthetic hectorite type clay Laponite RD5 (LRD) was obtained from Laporte Industries Ltd. The poly(ethylene-oxide) (PEO), with a molecular

Equilibrium structure and rheology

Over the range of polymer and clay compositions studied, equilibrium transitions from liquidlike to gellike behavior can be achieved by adjusting the clay and polymer content. In the current study, we examine the interaction of PEO and LRD in solution and how this influences the observed shear-induced structural changes. Small-angle X-ray scattering measurements were used to verify the dispersion of polymer and clay in water, as shown in Fig. 1a. As expected, the low-angle intensity increases

Conclusions

We suggest that the response of the PEO2–LRD3 suspension can be qualitatively understood in terms of a critical shear rate, γ̇c. For γ̇<γ̇c, the shear-induced reorientation of clay platelets is sufficiently slow that the polymer chains have time to diffuse in a manner that accommodates yielding of the orientational glass [15], [19], and η thus resembles the viscosity of the pure clay. For γ̇>γ̇c, however, the platelets move faster than the polymer can diffuse, fragmenting the quiescent network

Acknowledgements

The authors thank P.D. Butler for assistance with the SANS measurements. One of us (G.S.) gratefully acknowledges the financial support of an Alexander von Humboldt Foundation Fellowship, and SLG acknowledges the financial support of a National Research Council Postdoctoral Fellowship. Research carried out at the National Synchrotron Light Source at Brookhaven National Laboratory was supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences,

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